CA1187404A - Method for reducing the permeability of underground strata during secondary recovery of oil - Google Patents

Abstract

Abstract of the Disclosure The invention provides a method for the secondary recovery of oil from an oil-bearing stratum having a zone of high fluid permeability, in which this zone is plugged by introducing into the stratum a gelable solution comprising an alkaline material, a polyphenolic vegetable material such as mimosa tannin extract and formaldehyde, the last compound being present as free formaldehyde para-formaldehyde or a phenol-formaldehyde resole. The gelable solution has a pH of at least 9.5 and may optionally include a viscosifier. The gelable solutions are relatively insensitive to brines or temperature and gel to produce mechanically strong gels, even in the presence of oil-wet minerals. If desired, the alkaline material can be an alkali metal carbonate, and the use of such carbonates permits the permeability of the plugged high permeability zone to be adjusted after formation of the gel by treating the gel with acid.

Description

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METHOD FOR REDUCING TElE PERMEABILITY OF UNI)ERGROUND STRATA
DURING SECONDARY RECOVERY OE OIL

Background of the Invention .... . _ The invention relates to a method of reducing the permeability of underground strata during secondary recovery of oil.
When an oil well is first drilled3 oil will often flow from the well under the 05 natural pressure existing in oil-bearing strata. When this natural pressure becomes insufficient, further quantities of oil may be recovered from the well by a mechanical pump. However, it is well known to those skilled in the art that evenwhen no more oil can be recovered from the well simply by mechanical pumping, large quantities of oil often still remain in the oil-bearing stratum, especially if the lO oil is a heavy, viscous type of crude oil. To recover at least part of this residual oil, which will not flow naturally to the bottom of an oil well penetrating the oil-bearing stratum, so-called "secondary" recovery techniques have been developed. In such secondary recovery techniques, a flooding liquid (which may be, for example, water, brine, an aqueous solution of a polymer, an aqueous solution of a surfactant or a 15 hydrocarbon l'luid) is pumped down an injector well. The flooding liquid flows from the injector well through the oil-bearing s,trata and forces at least part of the residunl oil into a producer well. In some cases, it is advantageous to inject steam clown the injector well since the resultant hleating of the oil-bearing strata reduces tlle viscosity Or the oil present therein and assists flow of the oil to the producer 20 well.
Unfortunately, the various zones within an oil-bearing strata often differ greatly in i~uid permeability. Often it is found that there are fractures within the oil-bearing strata; these fractures may be natural or may occur because of the fracturing normally effected near the bottom of a producer well in order to assist 25 oil flow thereinto during the first phase of the recovery. Alternatively, no actual fractures may be present within the oil-bearing stratum but there may be zones of, for example, loosely packed sand of very high-permeability. In some cases even if no high-permeability zones are originally present in the oil-bearing stratum, such zones may be created by the aetion of the flooding liquid sweeping through the oil-30 bearing stratum. Where such high-permeability zones are present, almost all the flow of flooding fluid takes place along these zones with the result th;3t, after a short period of flooding, almost all the liquid recovered from the producer well ~74~)~

comprises ~ flooding fluid with only a small proportion of oil, while significant amounts of displaceable oil in zones of lower permeability within the oil-bearing ~' strata are no$ recovered. Thus~ the presence of such zones of high-permeability greatly decreases the efficiency of the secondary recovery process.
05 In order to overcome the aforementioned problems caused by the high-permeability zones in the oil-bearing stratum, it is known to inject into the stratum solutions which at least partially plug the high-permeability zones, thereby greatly decreasing the permeability of these zones9 so that flooding fluid injected thereafter is forced to traverse other zones in the oil-bearing stratum, thus leading to 10 increased oil recovery. The liquids used to plug the high-permeability zones are usually injected via the injector well, but may also be injected via the producer well if necessary. For example, U. S. Patent 3,396,790, issued August 13,1968 to Eaton, proposes a method of plugging high-permeability zones in which water is first injected into a well at a high rate, then a viscous solution comprising sodium 15 silicate, polyacrylamide and water is injected. After the injection of the viscous solution, water is again injected at a high rate and under high pressure, followed by injection of a less viscous solution containing ferrous sulfate and water. By carefully controlling the pressure and injection rates of the viscous and ferrous sulfate solutions, the two solutions react together to form plugs in the high-20 permeability zones.
U. S. Patent 3,749,172, issued July 31,1973 to Hessert et al, proposes a similarpro~edure for plugging high permeability zon~s, but in which the plugging solution contains a polymeric gel.
U. S. Patent 3,882,938, issued May 13, 1975 to Bernard. describes a plugging 25 technique involving the injection into the oil-bearing stratum of one or moreaqueous solutions of reagents that react within the oil-bearing str~tum to form a plugging material. One specific solution described is an aqueous solution of sodium silicate and a gelling agent such as an acid, an ammonium salt, a lower aldehyde, a polyvalent metal salt or an alkali metal aluminate.
- 30 U. S. Patent 3,897,827, issued August 5, 1975 to Felber et al, describes a gel forming solution consisting of a dichromate activator and a lignosulfonate solution containing an alkali metal or alkaline earth metal halide.
U. S. Patent 3,583,486, issued June 8, 1971 to Stratton, describes a plugging solution containing an ethoxylated condensation product of a phenol and formalde-35 hyde.

U. S. Patellt 49212,747, issued July 15,1980 to Swanson, proposes as a plugging solution a shear thickening polymer composition contairling a high-molecular weight polyalkylene oxide polymer with a phenol/aldehyde resin, the eomposition being alkaline.
05 U. SO Patent 4,246,1249 issued January 20, 1981 to Swanson, describes an aqueous plugging solution containing a water-dispersible polymer9 an aldehyde and a phenolic compound, which may either be a simple phenol or a tannin such as quebracho or sulfomethylated quebracho.
To be effec~ive in the wide variety of situations encountered during the 10 secondary recovery of oil, any composition intended for plugging zones of high fluid permeability within the oil-bearing stratum must meet numerous requirements. Theplugging solution must be suf ficiently heat stable to plug the high-permeability zones at the temperatures of 60C or more often encountered in oil-bearing strata.
In addition, since brine is present within many oil-bearing strata, the plugging15 solution must be able to gel in the presence of brine and the formed gel must not deteriorate chlring prolonged exposure to brine. Since the high-permeability zones to be plugged are often still wet with oil, the plugging solution must be able to gel in the presence of residual oil especially in the presence of oil-wet sandstone often encountered in oil-bearing strata, and the gel must be stable in the presence of such 20 oil. The formed gel must also be resistant to all conventional flooding liquids, some of which may be used at elevated temperatures, and to steam injected to recover viscous oil from the oil-bearing stratum; th;s steam may be heated to temperatures of at least 290C. In order that the plugging solution may be pumped down the well and a considerable distance into the high-permeability zMIe, the solution should have 25 low viscosity when first made up and should retain this low viscosity for the period (which may be several hours) necessary to pump the solution down a deep well and a substantial distance into the high-permeability zone. Once in position in the zone, the plugging solution should gel rapidly to a gel having Q high mechanicul strength.
It is very desirable that the plugging solution be of a type which permits the 30 operator, by vQrying the relative amounts of the various eomponents in the plugging solution, to vary the time lag before gelling of the solution begins. Since in some cases it may be desirable to produce only a reduction in permeability of the high-permeability zones, it is desirable that the operator be able to control the composition of the plugging solution in sueh a manner as to ~llour only partial 35 plugging of the high-permeability zones, thereby leaving some residual permeability therein. Furthermore, since it may sometimes be difficult to adjust the per-meability of the zones to precisely the right degree immediately, it is desirable thatit be possible to increase the permeability of the zones containing the plugging solution after the plugging solution has gelled. Fin~lly, since the plugging solution wi}l often be subjected to considerable shear forces as it is pumped into the porous 05 high fluid-per~neability zones, it is important that the plugging solution not be affected by such shear forces.
No prior art plugging solution meets all these exacting requirements. Many prior art solutions produce gels of insufficient mechanical strength or will not gel properly in the presence of brine or residual oil. MoreoYer, many prior art plugging 10 solutions are so viscous that it is difficult ~o pump them with sufficient speed and ~hey are susceptible at least partial gelling before t~ley have penetrated the higl~
permeability zones. In most cases, it is difficult to control the time lag before ge~ling of the plugging solutions occurs, so ~hat gelling may occur before the plu~ging solution has penetrated beyond the bottom of the well~ which may involve 15 the expensive process of sinking a new well into the oil-bearing stratum. Finally, as shown below9 prior art solutions based upon highmolecular weight polymers are susceptible to physical degradation by shear stresses, so that the shear stresses encountered during pumping of these plugging solutions into porous strata will greatly reduce the strength of the gel finally produced.
It will therefore be seen that there is a need for a method of plugging high-permeability zone in oil-bearing strata which fulfills all of the above-mentioned requirements, and the instant invention provicles su~h a plugging method.

Summar~of the nvention Aecordingly, the invention provides a method for the seeondary recovery of oil

2~ from an oil-bearing stratum, said stratum having at least one zone of greater fluid permeability than the surrounding zones, which method comprises:
injec~ing into said oil-bearing stratum via a well penetrating said stratum an aqueous, alkaline gelable solution, said solution comprising an alkaline material, a polyphenolic vegetable material selected from the group consisting of tannin 30 extracts, catechir~s and alkaline extracts of coniferous tree barks, said polyphenolic material being soluble in alkaline solution, and formaldehyde, said formaldehydebeing present as formaldehyde itself, paraformaldehyde or a phenol-formaldehyde resole, the total active solids content of said solution being from about 5 to about 33% by weight of said solution9 said solution having a pH of at least about 9.5, and 35 being formed by dissolving said alkaline material, said polyphenolic vegetable ~ . ~

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rnaterial and said formaldehyde in a brine containing not more than about 0.275% by weight of cations having a valency greater than l and forming insoluble hydroxides, the gelling time of said solution and ~he rate of injection thereof being such that said solution passes down said well by which it is injec~ed and achieves substantial 05 penetration into said high fluid permeability zone before substantial gelling of said solution occurs; and allowing said solution to gel within said high fluid permeability zone, thereby reducing the fluid permeability of said zone.
The term "total active solids content" used herein in relation to the gelable 10 solutions used in the instant method refers to the to~al content of alkaline material, polyphenolic vegetable ma~erial, formaldehyde (in whatever form the formaldehydeis present) and sodium chloride (if any) in the gelable solutions. As described below, the presence of sodium c~oride in the gelable solutions significantly affects their gel times and thus, although sodium chloride is not a reactant in the gel-forming 15 reaction, it is an active component in determining the gel times of the solutions.
The solutions used in the instant method may also contain various minor aàditives, such as preserv~tives and anti-dusting agents, but these additional minor additives have no significant effect on the gel times OI the solutions and are thus excluded when calculating the total active solids content of the solutions.
After the fluid perm eability of the high fluid perm eability zone has been adjusted by the me~ho~ of the invention, a flooding fluid may be injected into the oil-be~ring stratum, thereby enabling oil to be recovered from the oil-bearing stratum. This floodin~ fluid may be of any conventional type, for example water,brine, an aqueous solution of a polymer, an aqu00us solution of a surfactant or a 25 hydrocarbon fluid.

Brief Description of the Drawings Fig. 1 shows graphs of viscosity agair~t time for various gelable solutions usedin the instar~t invention at varying solids contents and at varying temperatures;
Fig. 2 shows graphs of gel time against temperature for various gelable 30 solutions used in the instant method and having varying solids contents;
Fig. 3 shows graphs of ge~ling time against brine concentration for various gelable solutions used in the instant method at varying solids contents and temperatures;
Fig. 4 is a schematic diagram of a sand-propped Berea core used to simulate 35 an oil-bearing stratum having a zone of high-fluid permeability, in certain ex-periments described below;

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Fiy. 5 shows -the proportlon of original oil recovered from the Berea core shown in Fig. 4 as a function of the volume of water injected thereinto as a flooding fluid;
Fig. 6 is a schematic diagram of a sand bed used in certain experiments described below to simulate an oil-bearing stratum having ~ones of hiqh fluid~permeability;
Fig. 7 is a graph showiny the proportion of original oil recovered from the sand bed shown in Fig. 6 as a function of the volume of flooding fluid injected thereinto;
Fig. 8 is a graph similar to Fig. 7 but showing the results of multiple injections of gelable solution into the sand bed shown in Fig. 6;
Fig. 9 is a graph similar to Fig~ 7 but showing the cE;Eects of -treating the sand bed shown in Fig. 6 with gelable so]utions containing a viscosifler, Fig. 10 is a graph showing gel time against solids concentration for various gelable solutions used in the instant mcthod and containing varyin~ amounts of sodium carbonate;
Fig. 11 is a graph showing the variation of gel time with temperature of various gelable solutions used in the instant method and containing varying amounts of sodium carbonate;
Fig. 12 shows the variation of viscosity with time for various prior art gelable solutions described in the afore-mentioned U.S. Patent 4,246~124; and Fig. 13 is a graph similar to Fig. 7 but showing comparative results obtained using a yelable solution used in the instant method and a prior art solution described in the afore-mentioned U.S. Patent 4,246,124.

:`' " sd~ ~6-Detailed Description of the Invention . ....
Gelable solutions similar to -those used in the instant process are described in U.S. Patents 3,686,872 and 3,884,861, both issued to Whitworth et al. ~he patents describe the use of compositions containing an accelerator for stabilizing soil.
It shvuld be noted that requirements for a soil-stabilizing composition are much less stringent than for a composition for reducing the permeability of high fluid-permeability zones in oil-bearing strata. Soil-stabilizing compositions are not ~0 required to gel in the presence of brine or residual oil, since such substances are not usually present in soil being stabilized.
Furthermore, the time lag before gelling is much less critical in soil-stabilizing compositions than in compositions for use in oil-bearing strata since the former do not have to be pumped lony distances, being ~, '7~3~

injected directly into the soil to be stabilized. Indeed, the compositions described in both Whitworth et al. patents are accelerated compositions containing a metal orsilicon gelling accelerator to ensure that the solution gels within a few minutes.
Such a rapidly-gelling composition is not suitable for treatment of the majority of ()5 oil-bearing strata, sinee it would gel in the well through which it was being injected long before it reached the desired high-permeability zones. Furthermore, the shape of the viscosity-against-time curve for a soil-stabilizing composition is of little consequence, since the composition is usually injected into the soil to be stabilized immediately after the composition has been formed9 so there is little or no need for lO the composition to remain non-viscous for any protracted period. As explainedabove, gelable solutions used to treat deep oil-bearing strata must remain sub-stantially non--viscous for a period of several hours in order to ensure that they can be pumped at an adequate rate into the deep strata. Again, because of the elevated temperatures found in oil-bearing strata, gelable solutions for use therein must be t5 capable of gell;ng at various elevated temperatures, whereas soil-stabilizing com-positions do not need to be gelable at elevated temperatures, since soil temp-erntures will normally be less than about 20C. Indeed, one of the advantages claimed for the accelerated composition~: described in the aforementioned Whitworth et al. patents ;s their ability to gel rapidly at temperatures just above 20 freezing point. Finally, since the fluid permeability of stabil;zed soil is of little or no consequence, so that a soil-stabilizing composition does not need to be capable of reducing but not completely eliminating the ~luid permeability of an originaLly high-permeability zone. E;or all these reasons, a composition used for soil-stabiliæing is not necessarily suitable for blocking high-prerneability zones in oil-bearing strata, 25 and indeed the great majority of the compositions used for soil-stabilizing are useless for reducing permeability in oil-bearing strata.
The alkaline gelable solution used in the instant method must3 as already stated, have a pH of at least about 9.5. Preferably the pH of the gelable solution is between about 10 and 11, sinee above about pH 11 gelation tends to be inhibited,30 vvhereas below pH 9.5 precipitation of the polyphenolie vegetable material may occur leaving a non-gelable solution. Obviously, after injection into the oil-bearing stratum, the gelable solution may be diluted by liquid already present in the stratum, which liquid may have a pH substantially different from that of the gelable solution. If it is thought likely that mixing of the gelable solution with fluid already 35 present in the oil-bearing stratum will reduce the pH of the gelable solution below about 9.5, an alkaline solution having a pH of above 9.5 may be injected into the oil-bearing stratum before the gelable solution in order to prevent such lowering of thepH of the gelable solution after injection.
The alkaline mater ial suitable for use in the gelable solution used in the instant method may be any alkaline material which will produce a sufficient pH and 05 which does not adversely effect the gelling properties of the polyphenolic vegetabIe material/formalc]ehyde mixture. Preferred alkaline materials include sodium hy-droxide, potassium hydroxide, sodium carbonate and potassium carbonate. ~or r easons explained below, where it is desired that the final permeability of theoriginally high-permeabili~y zone be capable of being adjusted after treatment by LO tl~e instant method, it is preferred that at least part of the alkaline material in the gelable solution be a carbonate.
The amount of alkali needed in the gelable solution to obtain the desired pH
depends on the particular polyphenolic vegetable material used. If, for example, the polyphenolic material is an alkaline extract, it will be largely in the form of sodium l5 snlts of the polyphenolic vegetable material and only relatively smaU amounts of alkali rnay be required to raise the pH of such a solution above 9.5.
Polyphenolic vegetable materials suitable for use in the instant method include vegetable tannins such as those extracted from mimosa, quebracho, mangrove and wattle; catechin and chatechu sucll as those extracted from ~cacia catechu and 20 ~cacia suma, mahogany wood and the like; the alkaline extracts of certain coni~elous tree barks includillg the barks of Weston Hemlock, Douglas Fir, WhiteEiir, Sitka Spruce ~md Southern Yellow PineJ particularly such extracts prepared as described in U. S. Patents 2,782,2419 2,819,295 and 2,823,223; and vegetable tannin extracts obtained from Eucalyptus crebra, Callitris calcarata and Callitris glauca.
25 The preferred polyphenolic vegetable material for use in the instant method is that readily available in commerce as Mimosa Tannin Extract, which is extracted from l~cacia mollissima.
The exact chemical composition of many of the commerciaUy available polyphenolic vegetable materials is not known, but the aforementioned materials are 30 known to possess three properties necessary in the gelable solutions used in the instant method, namely solubility in alkaline solutions, ability to combine withformaldehyde, and ability to form a gel with formaldehydeO l`he ability to combine with formaldehyde is conveniently~ measured as the number of grams of for-maldehyde which react in four hours with 100 grams of the dry polyphenolic 35 vegetable materi~l dissolved in an aqueous solution of pH 9.5. We prefer to use polyphenolic vegetable materials having a formaldehyde combining capacity of at least 5Ø

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g The ge1able solution used in the instant method has a total active solids content of from about 5 to about 33% by weight of the solution. AboYe about 33%
by weight of these materials, the solution tends to gel too rapidly to be praetically useful, while below about 5% by weight the solution is too slow to gel or may not gel 05 properly at all. In general, the higher the weight percent of these materials, the shorter the gel time and the higher the temperature at which the solution must gel3 the shorter the gel time. Where it is desired to reduce the permeability of highfluid-permeability zones, but not to completely eliminate all permeability thereof, it may be desirable to use a gelable solution containing from about 5 to about 10% by 10 weight of these materials. Preferably, the gelable solution contains from about 15 to about 25 parts by weight of formaldehyde per 100 parts by weight of the polyphenolic vegetable material on a dry basis.
We preïer that the formaldehyde used in the instant gelable solutions be present as paraformaldehyde, since this solid m~terial is easier to handle and store 15 than liquid solutions of formaldehyde (formalin solutions~.
The gelable solutions used in the instant method have relatively low viscosity when first prepared, in the range of about 2 to about 30 mPa.s depending upon the solids content of the solution and the temperature. These viscosities render thegelable solutions easy to pump and capable of rapid injection into the oil-bearing 20 stratum. Furthermore, as shown for example, by the data in Fig. 1, discussed in detail below, the viscosity of the gelable solultions remains substantially unchanged ~or fl relRtively long period, which may be up~ to nbout 100 hours, thus enabling the solutions to be pumped for a protracted period without risk of them gelling at an undesired location. At the end of this time, the viscosity of the gelable solutions 25 increases very rapidly and a gel of substantial mechanical strength is formed. The ability o~ the gelable compositions used in the instant method to remain non-viscous for protracted periods and then to form a strong gel very rapi~y is particularlyuseful for selective plugging of high-permeability zones at some distance from the bo~tom of the well through which they are injected into the oil-bearing stratumO30 Since it is usually desirable to reduce or eliminate the fluid permeability of a high-permeability zone lying s~me distance from the well through which the gelable solution is injected into the oil-bearing stratum, a non-gelable displacing ~luid should normally be injected into the oil-bearing stratum after the gelable solution has been injected and through the same well9 thus causing the gelable solution $o be displaced 35 from around the well through which it was injected and preventing excessive loss of permeability around the bottom of this well. The displacing fluid used in this 7~
-lO-procedure may be water or a viscous aqueous solution of a polymer; suitable polymers performing such viscous aqueous solutions are well-known to those sk~illed in the art.
As already mentioned, oil-bearing strata frequently contain brines, typically 05 comprising solutions of sodium~ calcium and magnesium chlorides. As is shown in more detail below, at least some of the gelable compositions used in the instantmethod will gel satisfactorily in the presence of up to about 5% of total dissolved salts, depending upon the concentration of the gelable solution. Since the gelable solution (initially containing little or no dissolved divalent ions) will dilute brine ~; tO present in the reservoir, ~ in practice the instant method can be used in oil-bearing strata containing brine having considerably more than 5% dlssolved salts.
Moreover, as described in more detail below, increasing the proportion of alkaline to polyphenolic vegetable material/parai`ormaldehyde increases the tolerance of thegelable solutions to brine salts.
t5 It is often ~ound that, in brine-containing oil-bearing strata, the concentration of brine is relatively low within the high-permeability zones of the strata, since most of the brine has been washed out of the high-permeability zones by the flooding fluid used during the first part of the secondary oil recovery process, while the concentration of brine remains relatively high in the zones of lower permeabil-20 ity. In the presence of the calcium and magnesium cations usually present in such brines, the gelable solutions used in the instant method form insoluble particles.
Accordingly, when such solutions are, by the instant method, injected into oil-bearing strata in which the concentration of brine is much lower within the zones of high permeability than within the surrounding zones, the gelable solutions enter the 25 high-permeability zones and cause the fsrmation of insoluble material at the interfaces between the high ~uid-permeability zones and the surrounding zones9 where the gelable solutions come into contact with the brine in the latter zones.
The resultant deposition of insoluble material at these interfaces limits the penetration of the gelable solution into the surrounding zones, thereby improving the 30 containment of the gelable solution within the high permeability zones and ensuring that the gelable solution does not substantially invade oil-bearing unswept zones.
If little or no ilooding o~ the high permeability zones has taken place prior touse of the instant method, fresll water may be injected into the oil-bearing stratum befo~e the gelable solution is injected and through the same well or wells to lower 35 the brine concentrati~n within the high permeability zones, thereby ensuring that, when the gelable solution is injected, the gelable solution will be con~ined to the high permeability zones by formation of the insoluble material, as described above.

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Because the gelable solutions are tolerant og brine, ii` it is desired to carry out the instant method in a location e.g. a desert where procuring sufficient fresh water to make up the gelable solutions is difficult or costly~ the gelable solutions may be prepared by dissolving the alkaline material, the polyphenolic vegetable material û5 and the formaldehyde in a brine (which might be a brine derived from the oil reservoir)g provided that the brine does not contain more than about 0.275 weight percent of cations, such as magnesium cations, having a valency greater than oneand forming insoluble hydroxides.
A5 is well-known to those skilled in the art, although the main salt component ~- ~ 10 in~P~P brines is sodium chloride, the monovalent sodium cation has little effect on the geling of gelable solutions and the change in gelillg of the solutions and the formation of insoluble material is mainly caused by the divalent cations~ such as calcium and magnesium present in the bines. The presence of (say) l - 2% by weight of sodium chloride in the instant gelable solutions is not deleterious, although it may 15 affect their gel times somewhat.
As indicated above, by careful adjustment of the concentrations of the various components in the gelable solution, the instant method may be c~rried out so that gels are formed which do not completely destroy the permeability of the treated part of an oil bearing strata but only reduce the permeability of this part. This is 20 advantageous, since such reduction but not elimination of the permeabililty of the treated part eliminates the possiblity of inadvertently and completely stopping the flow of flooding liquid through $he oil-bearing stratum. This is highly undesirable, since cornplete blockage of flooding liquid flow would necessitate the expensis~e drilling of a new injector well into an unblocked part of the oil-bearing stratum or 25 fracturing of the original injector well. To produce such incomplete blocking of the treated part of the oil-bearing straturn, in most cases it is desirable to use a gelable solution in which the total weight of the alkaline material, the polyphenolic vegetable material and the paraformaldehyde is from about 5 to about 10% of the weight of the gelable solution.
The sand or other minerals present in the high permeability zone treated by the instant method often contain oil. Fortunately, the gelable solutions used in the instant method are capable of achieving substantially complete gelation even in the presence of oil-containing minerals. The aqueous gelable solutions cannot wet oradhere to the oil~containing minerals, so that the presence of such oil-containing 3~ minerals prevents the gelable solutions from forrning a truly consolidated mass within the high permeability zone, the solution instead gelling in the interstitial ~8'7~

voids between the oil-containing grains of mineral. However, the permeability ofthe high permeability zones can be greatly redueed, in most cases by a factor ofabout 50, and such marked reduction in permeability is sufficient in most eases.The instant method also allows the permeability of the treated part of the oil-05 bearing stratum to be adjusted after the gelable solution has gelled. For thispurpose, at least part7 and preferably the major part, of the alkaline material in the gelable solution should be in the form of a carbonate. After such a gelable solution has geUed, a strong acid, preferably a mineral acid and most desirably hydrochloric acid, i5 injected into the treated part of the stratum, whereupon part of the l0 carbonate is dissolved from the gel7 thereby increasing the permeability of the treated part of the stratum. If necessary, the acid treatment may be repeated, and when the desired permeability has been achieved, the treated part of the stratum is flushed with water or some other Qooding liquid to remove the acid from the treated part, thereby preventing further reaction between the acid and the gelled 15 solution and consequent further increase in the permeability of the treated part.
In cases where the permeability of the high permeability zones in the oil-bearing stratum is very great, or where the ratio between the high permeability zolles is relatively low, it may be found that the use of a gelable solution containing only an alkaline material, a polyphenolic vegetable material and paraformaldehyde 20 causes an excessive rate of material dissipation within the oil-bearing stratum and, if brine is present in the stratum, excessive dilution of the gelable solution with the brine and consequent difficulty in obtaining proper gelling. To limit the dissipation of tlle gelable solution in such situations, it is advantageous to add to the gelable solution a small quantity of a viscosifier, preferably a polymer and most desirably ~5 poly(sodium acrylate) or its copolymers. From 0.l to 1% by weight of the gelable solutions of poly(sodium acrylate) usually gives satisfactory results; those skilled in the art will be aware of numerous other types o~ viscosifiers suitable for use in plugging solutions and the quantities in which such viscosifiers should be usedOOther possible viscosifiers include carboxymethylcellulose and o~her cellulose deriv-30 atives and polysaccharides. However, because of the difficulties associated withpumping hig~y viscous gelable solutions into the oil-bearing stratllm, we prefer to avoid the use of viscosifiers if possible.
As already mentioned, in most cases the gelable solutions used in the instant method should not contain any accelerators, since it is desirable to have a period of 35 several hours before the solutions gel, in order to allow for pumping the solutions down deep wells and substantial distances into the high permeability zones.

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However, in a minority of cases there may be very large fractures or channels, through which fluids can ~low very qu;ckly, within the oil-bearing stratum. Where sucll large fractures or channels are present9 the use of a non-accelerated gelable solution may result in such excessive dissipation of the gelable solution that no 05 substantial reduction in permeability of the large fractures or charmels occurs. In such cases, it may be nece~sary to carry out the instant method using an accelerated gelable solution. The accelerator used may be of any of the types described in the aforementioned U. S. Patents 3,686,872 and 3,884,861, that is to say a gelling agent ;~; soluble or colloidally dispersable in the gelable solution and ~ontaining a complexing l0 element, this element being silicon~ vanadium, molybdenum, manganese, titanium, copper, zinc or zireonium. The preferred accelerator is an ~lkali metal silicate~
preferably sodium si1icate, and such a silicate is preferably added to the gelable soluffon in an amount of from about 0.l to about 6.5% of the weight of the polyphenolic vegetable material in the solution. By the use of such accelerators, the 15 gelling time of the gelable solution can be reduced to as low as about l0 seconds.
Obviously, it is not practical to pump such rapiclly-gelable solutions over substantial distances, and thus in order to ensure that the gelable solutions reach the desired location in the short period before they gel, the gelable solution and a solution of the accelerator, are pumped separately down the or each well used for injecting the 20 gelable solution and allowed to mix within the well at Q point spaced from the upper end thereof to form the rapidly-gelling solution. Such a procedure avoids the waste of gelable solution which would be incurred if it were necessary to use a slowly-gelling solution to seal such large fractures and channels. When using such rapidly-gelling solutions9 it is often advantageous to add a viscosifier to one ~r other of the 25 solutions pumped down the well so that the accelerated gelable solution formed upon mixing the two solutions has high viscosity and flows in a piston-like manner through the high-permeability zones, since $his flow pattern results in a more effectiveblocking of the high-permeability zones after the solution gels.
Alternatively, instead of adding an accelerator to tlle gelable solution, itself, a 30 solution of an accelerator may be injected into the high-permeability zone~s~ either before or after the gelable solution is introduced into the zone(s).
As shown by the examples below, the gelable solutisns used in the instant method are capable of maintaining, at temperatures up to at least about 65C3 gel times sufficiently long to allow for handling and desirab1e penetra$ion into oil-35 bearing strata. A subs$antial proportion of oil-bearing strata do not exceed this temperature. The instant method can still be used in oil bearing strata above this temperature if a very short gel time is desirable. If the stratum to be treated is above 65C, either because of natural geological heat or because steam or other hot fluid has previously been injected into the stratum to effect secondary recovery of oil, it may be necessary to inject a supply of cooling fluid into the oil-bearing 05 stratum before the gelable solution is injected, in order to cool the stratum to a S ~ temperature at which~satisfactorily long gel time can be achieved. Once a gelled ~'` solutiorl has been achieYed by the instant method, the resultant ge]ls are capable of withstanding temperatures much higher than 65C and in fact can stand prolonged exposure to temperatures as high as 290&~ without substantial deterioration.
10 Accordingly, the instant method can be used to reduce the permeability of zones in oil-bearing strata where it is desired to subsequently inject steam into the oil-bearing stratum in order to recover heavy oil therefrom. This is a considerable advantage of the instant method, sinee the gels formed by most prior art gelablesolutions are not capable of withstanding the temperatures reached in oil-bearing 15 strata during steam recovery of heavy oils.

Example 1 This example illustrates the variation in the gel times of the gelable solutionsused in the instant rnetllod with temperature and with the total concentration of alkaline materiaI, polyphenolic vegetable material and paraformaldehyde in the 2() gelnble soluti ons.
The gelable so1utions used in this and subsequent Examples were prepared from a dry powder comprising:

Parts by Weig Mimosa tannin extract 75.2 Paraformaldehyde 13.2 Sodium chloride (filler) 10.4 Dowicide A 0.04 ~arnea oil 0 ~4 Dibutyl phthalate 0.92 7~

Dowicide A is a preservative and mildewicide while Carnea oil and dibutyl phthalate are used as antidusting agents; neither component significantly affects the geling of the solutions of the powder. The powder was prepared in accordance with the aforementioned U. S. Patents 3,686,872 and 3,884,861. To prepare a solution 05 designated X% below, 0.1X parts by weight of sodium hydroxide were dissolved in (100-l.lX) parts by weight of water and, after the sodium hydroxide had completely dissolved, X parts by weight of the dry powder were then slowly added to the alkaline solution with rapid stirring. After all the powder had been added, the solution was then stirred for 15 minutes to ensure ~t~, ~rought to the 10 temperature at which its gel time was to be tested and the gel time rneasured from the end of the 15 minute mixing period.
Because of the amount of Dowicide A, Carnea oil and dibutyl phthalate in the dry powder, the total active solids content (i.e. the total concentration of alkaline material, polyphenolic vegetable material, paraformaldehyde and sodium chloride) in 15 the various solutions was not prec;sely equal to the nominal percentage solution, though the differences are small enough to be ignored for practical purposes. For example, the nominally 15% solution comprised 83.5 parts by weight of water, 1.5parts by weight of sodium hydroxide, 11.28 parts by weight of mimosa tannin extract, 1.98 parts by weigh t of paraformaldehyde and 1.56 p~rts by weight of sodium 20 chloride, so that the total active solids content therein was 16.32% (i.e. the total concentration of mimosa tannin extract, paraformaldehyde, soclium chloride and sodium hydroxide) rather than 15%.
Nominally 5%, 8%, 12.5%, 15%, 20% and 25% solutions were made up by this method. The gel times of the various solutions was then determined at temp-25 eratures ranging from about 10C to 65C. Over the temperature range 25-4~C, the viscosity of the gelable solutions was determined using a commercially-available"Tecam" gel timer, while outside this temperature range the gel time was determined by visual observationO Fig. 1 shows viscosity against time curves for the 596 solution at 65C, the 8% and 12.5% solutions at 25C and the 20 and 25%
30 solutions at 10C. The eurves are cu~ off arbitrarily at 300 mPa.s because in the final stages of gelation the viscosity was changing so fast that the viscometer readings become unreliable. The viscosity indicated by the viscometer did, however, exceed several thousand mPa.s before complete gelation occurred. As will be seenfrom the curves in this Figure, the viscosity of the gelable solutions remains almost 35 unehanged for most o~ the gel time. At the end of the gel time, the originally low viscosity gelable solutions rapidly lost normal fluid characteristics and their viscosity increased very rapidly until they formed a strong gel. It will also been seen from this Figure that increasing temperature tends to decrease the gel time, while increasing the total concentration of alkali material, polyphenolic vegetable 05 material and paraformaldehyde also tends to decrease gel time. Gel times of about 30 minutes to about 100 hours can be produced, as shown in Fig. 1, without usinggelling accelerators. The long periods of tirme in which the viscosity of the gelable solut;ons remains virtually unchanged allows easy pumping of the solutions into deep wells and through substantial distances into high permeability zones within oil-lO bearing strata.
Fig. 2 illustrates the variations of gel time flgainst temperature for solutionsof constant composition. The curves in this Figure show that in all cases gel time increases with decreasing temperature, but that solutions containing lower totalconcentrations of alkaline material, polyphenolic vegetable material and para-LS formaldehyde are rnore sensitive to temperature variations than those with highertotal concentrations of these components. A separate experiment (not shown in Fig.
2) showed that a 5 weight percent solution had a gel time of about 100 hours at 65C.

This l~xample illustrates the effect of brines on the gel times of the gelable 20 solutions used in the instant method.
5%, 10%, 15~ and 20% (nominal) solutions were made up in the same way as in Example 1 except that the water was replaced by brines containing sodium chloride, calcium chloride dihydrate and magnesium chloride hexahydrate in a molar ratio of 2~:2:1, the total salt concentration in the brines varying from O to 2% by weight. To 25 make up the solutions, the gelable solution was made up in a reduced amount of fresh water and then the requisite amount of a 1096 by weight standard brine wasadded to produce a final solution having the desired total active solids content and brine salts concentration. The gel times of the 10%,15% and 20% gelable solutions were then determined at 25C, while the 5% solution was tested at 50C. The 30 results are shown in Pig. 3, in which the total dissolved solids percentages refer to the solids derived from the brine ~.
The curves in Fig. 3 show that, within certain limits, the gel time of the solutions decreases rapidly with increasing brine concentration~ This decrease in gel time with brine concentration continues until the brine content is such that 35 precipitation of magnesium hydroxide and calcium hydroxide lowers the pH of the 7~

gelable solution below the pH range in whieh the solutions gel properly, namely below about pH 9.5. When the pH falls below about 9.5, precipitation of the polyphenolic vegetable mater;al occurs and a non-gelling composition is produced.
The brine concentration at which this phenomena occurs is hereinafter referred to 05 as "no-gel point" and varies according to the total concentration of alkalinematerial, polyphenolic vegetable material and paraformaldehyde in the gelable solutiom The no-gel points are indicated by vertical bars in Fig. 3 and occur atabout 1.1% salt content for the 5% solution at 50~C, and about 1.8-1.9 percent for the 1096 and 15% solutions at 25C. At the same temperature, the 2096 solution does not 10 display an observable no-gel point since it continues to gel at brine concentrutions so large that its gel time falls effectively to zero. When the brine concentration in the 10% solution was increased above the no-gel point, the composition formed a wet, non-solidifying sludge. A similar experiment (the results of which are not shown in Pig. 3) with a 25% solution showed that this solution rapidly hardens even 15 at a brine concentration of 7% total dissolved salts to give a gel. EIowever, such a gel is not the strong gel normally formed in the instant method but is a solid of low mechanical strength, crumbling easily under relatively small compressive forces.As mentioned above, the tolerance of the gelable solutions to brine may be increased by increasing the concentration of alkaline material in the gelable 20 solutions. To illustrate this increase in tolerance of brine with increasing alkaline material content, a series of 10% nominal gelable solutions were made up from the dry powder described in Example 1, fl esh water, sodium hydroxide and the 10%
aforementioned standard experimental brine using the technique described in the second paragraph of this Example9 except that the concentration of sodium 25 hydroxide was increased by the molar equivalent of the calcium and magnesium ions in the final, brine-containing composition. The concentration of the experimental brine was varied to produce from 0 to 3% of the brine salts in the final composition.
The results of this e~periment are shown in Fig. 3 as the curve headed "increased alkaline component". As in the experiments previously described, the gel 30 time of this composition decreased with increasing brine concentration, but sat-isfactory gels were formed up to a brine concentration of about 3.0% total dissolved s~ilts. Thus~ by increasing the umount of the alkaline material relative to the other components of the solution, greater tolerance for brine is achieved.
Because of the decrease in gel time with increasing brine concentration, brines 35 of known composition may, if desired, be used to reduce the gel time of the gelable compositions used in the instant method. Furthermore, the foregoing results show 7~

that if it is necessary to use brines rather than water to make up the gelable solutions (for example, when operating in a desert area where salt-free water is not readily available) it may be necessary to adjust the amounts of alkaline material3 polyphenolic vegetable material and paraformaldehyde in the gelable solutions to05 compensate for the accelerating effect of the brine.
As wiU be seen from the composition given above, the curve denoted "Increased allcaline component" in Fig. 3 was derived from a 10% (nominal) gelable solution. Similar experiments carried out using a 25% (nominal) gelable solutionshowed a no-gel poin-t of 4.5% total dissolved salts. At brine concentrations above 10 the no-gel points for these increased ~lkaline component solutions, the pH of the solution is still in the proper range but the large mass of insoluble hydroxides formed dispersed throughout the solution prevents it from becoming consolidated into a proper gel. It should be noted that7 in all the above experiments, the no-gel point was determined as a function of total dissolved salts in the gelable solution itself.
15 In actual use, provided the gelable solutions are made up with water containing relatively little dissolved salts, the instant method can be carried out in oil-bearing strata containing brine considerably more concentrated than the no-gel points determined above would indicate. Firstly, the brine in the oil-bearing stratum would be diluted by the gelable solution so that the total dissolved solids content of the 20 resultant mixture will be considerably less th~n that in the brine. Furthermore, as mentioned above, since the brine concentrat;on in the high-permeability zones isusllally less thEIn that in the surrounding zones, precipitation of insoluble hydroxide material tends to occur only at the interfaces between the high-permeability zones and the surrounding zones, and the resultant precipitation of insoluble materW at 25 these interfaces tends to limit diffllsion of the brine into the gelable solution.
Accordingly, the above experiments show that in practice the instant method can be practiced in oil-bearing strata containing brine having a total dissolved solidscolltent of consiclerably more than 3% and still produce satisfactory gelling within the high-permeability zones. Accordingly, the instant method can be used in 30 practiee in most brine-containing oil-bearing strata.

This example illustrates the use of the instant method to reduce the permeability of an oil-containing high permeability zone in an e~perimental model.
The experimental model used is illustrated in 'Figo 4 and comprises two Berea 35 sandstone cores I and 2, each 28.2 x 2~47 x 2.73 cm. The two cores were separated by a 0.09 cm. gap 3 propped with 285-325 mesh sand and were enclosed within an impermeable skin 4 of fiberglass saturated w;th a cross-linked epoxy-based resin to prevent leakage of ~uid through the surfaces of the Berea cores. Both of the outer long faces of one of the Berea cores was provided with three transverse fluid-05 injection tubes 5, and a single longitudinal injection/withdrawal tube 6 is provided ateach end of the cores adjacent the sand-filled gap 3 (only one of these longitudinal injection/withdrawal tubes 6 is visible in Fig. 4).
The cores were first saturated with the standard experimental brine described above containing 6,6 total dissolved salts and then a 1.3 cps. miner~l oU was injected 10 through the transverse tubes 5 under a pressure of 50 psi. (3.52 kg/cm.2). Secondary recovery of the oil was then simulated by injecting water longitudinally into the core ViQ one of the longitudinal tubes 6 at rate of 42 cm.3 per hour (equivalent to a linear watel~flow veloci~y of 3.5 meters per day) until 1~ cm.3 of oil (about 33% of the oil present in the core) had been recovered, as shown by the lower curve in Fig.
15 5. As shown by this lower curve in Fig. 5, further flooding of the core with water produced substantially no further oil recovery.
The permeability, K of the core in millidarcies (md.) was calculated from the standard equation:

K = ~c lOoo where Q is l:he flow rate in cm.3 sec. 1 is the viscosity of the fluid in cps (mPa.s) L is the length of the zone being swept in centimeters, 25 ~P is the pressure drop through the zone in atmospheres, and A is the cross-sectional area of the zone being swept in square centimeters.
At the beginning of the experiment, the effective permeability of the core to brine was determined to be 228 md. Since the estimated effective permeability ofthe Berea cores was 10 md., the permeability of the sand-filled gap 3 ~synthetic30 fracture) was calculated as 13400 md.
After all the available oil had been removed by the ~irst water flooding, a 10%
(nominal) gelable composition produced by the technique described in Example 1~ but using a 1.1% total dissolved salts stand~rd experimental brine contQining a 20:201 by ~ mixture of sodium, calcium and magnesium ions as the aqueous vehicle, was 35 injected into the core and allowed to set for 22 hours. Although in theory the next il7~@~

step sllould have been over-displacement of the gelable solution with brine to remove the gelable solution from the injection face of the core, this was found to be unnecessary in view of the small volume of the model core and instead the face of the core through which the gelable solution had been injected was scratched with a 05 drill bit in order to overcome the effects of surface plugging of the core. The permeability of the core to brine following this scratching was found to be 19 md.
6% total dissolved salts standard experimental brine having the same salt ratios as the aforementioned 1.196 brine was then injected to displace the 1.1% brine in order to determine the influence of brine concentration on tlle perrneflbility reduction 10 effected by the gelled solution. After this flooding with 6% brine, the permeability of the core was found to be 20 md., showing that the 6% brine had almost no effect on tile permeability of the core. Next, the core was again saturated with oil, but this time the oil was injected via the longitudinal injection tube 6 and since the injection pressure was the same as in the first oil injection, but the length of the l5 core in the direction of injection was substantially greater, the pressure gradient in the directiorl of oil injection was lower, and consequently the resultant oil saturation was lower than in the first oil injection. Accordingly, the amount of oil required to saturate the core was about 3 cm.3 lower than in the first oil saturation. The core was then again flooded with water in the same manner as during the first water 20 flood and the percentage of oil recovered was plotted against the volume of water injectcd to give the upper curve shown in E~i~. 5. As shown in that Figure, the propoltion of oil recovered after plugging of the synthetic fracture by the instant rnethod w~s greater than the proportion recovered before treatment by the instant method. It is especially not~worthy that, during the second water flood, immediate-25 ly after water flooding began the oil was being recovered at a faster rate thanduring the first water flood, prior to treatment of the core by the instant method.
Although the improvement in oil recovery following plugging by the instant method is not great9 this relatively sm~ll improvement can be attributed ~o the fact that Berea sandstone becomes very strongly wet in contact with water, being subject to 30 strong water imbibition, and rock subject to such strong water imbibition can be expected to yield less improvement of oil recovery following plugging of the fracture than would a less strongly water-wettable rock.
The permeability of the plugged core to brine was found to be 46 md. 20 hours after the start of the second oil injection. l`o determine whether this increase in 35 permeability was due to a deterioration of the gel, the core was left`to age for 41 days from the injection of the gelable solution. At this time, the permeability of 3~

the core was found to be 34.5 md., indicating that no signifieant deterioration of the gel had taken place.

Example 4 This Example illustrates the ability of the gelable solutions used in the instant 05 invention to gel in the presence of residual oil in a simulated high-permeability zone.
An experimental model of a high-permeability zone was prepared by filling stainless steel tubes llaving a cross-sectional area of 0.71 cm. with Standard Ottawa Testing Sand, ASTM C-190, a river sand supplied by the Canlab Division o~ McGaw 10 Supply Limited. The sand was then saturated with a mineral oil having a viscosity of 5 mPa.s. The oil-saturated sand was then treated by the instant method with gelable solutic)ns preferred as described in ~xample 1 at concentrations of 10-25%
(nominal). The permeability of the sand was determined both before and after this plugging treatment and it was found that the plugging treatment reduced the 15 perrneability by a factor of about 50. In addition, when the tubes were flooded with water after plugging, the oil/water mixture leaving the tubes was only very slightly discolored, thus indicating that all the compositions had achieved substantiallycomplete gelling even in the presellce of the oil-saturated sund.

Example 5 This Example illustrates the high mechanical strength of the gels formed by the instant method.
An experimental model of a high-permeability zone plugged by the instant method was prepared by filling capillary tubes of 0.105-0.111 or 0.205-0.210 cm.internal diameter by the instant method using gelable solutions prepared RS
25 described in Example 1 and having concentrations of 10,15 and 25% (nominal). Air pressure was then applied to the gel in steps of 1 psi. (0.0704 kg. cm. 2) per minute.
Similar experiments were carried out using 5 and 7% (nominal) solutions in the aforementioned 1.1% standard brine whose composition is speci~ied in Example 3.
The maximum pressure drop sustained by the gel before ejeetion from the tubes was 30 measured and the yield streæ ~ calculated therefrom by the standard equation:

t = D ~P

where D is the internal diameter of the capillary, AP is the maximum pressure drop sustained before yielding, and L is the length of the gel segment in the capillary.
This equation assumes that the fluids involved are incompressible, but this assump-05 tion does not involve serious error.
The yield stresses observed in these experiments are shown in Table I below.

Table I
Yield Stress of Gelled Solutions Solution Yi eld Stress Concentration (kPa) ~ ~
0.011 0.0016 7 0.16 0.023 1~.45 0.066 1515 0.90 0.13 5.4 0.79 The data in the above table show that the yield stress of the gelled composition increases in proportion to the total concentration of allcaline material, polyphenolic vegetable material and formaldehyde in the gelable solution used. The 20 maxirnum yield stress values were achieved within two days of gelation except for a 5% gel forming solution in the aforementioned standard brine containing 1.1% total dissolved salts; the latter solution took about seven days to achieve maximum yield stress.

_xample 6 This Example illustrates the improvement in oil recovery from a hetero-geneous formation treated by the instant method.
The experimental model of a heterogeneous formation used in these exper-iments is shown in Fig. 6. The model comprised a square aluminum frame 10 havinga cut-out in its upper surface which was closed by a transparent organic glass plate 30 lL to permit visual observation of the flow patterns within the frame during the experiment. The combined aluminum frame and glass plate were used in two sizes, the larger size defining a square cavity having dirnensions of 20.3 x 20.3 x 1.27 cm.
and the smaller size defining a cavity having dimensions of 15.25 x 15.25 x 0.632 cm.

~137~0~

Each short edge of each frame was provided with a single tubular port; proceeding clockwise around the frame, these ports were a flooding fluid p~rt 12, used to introduce flooding fluid and connected to a flooding fluid pump 13, an oil injection port 14, a flooding fluid/oil outlet port 15 and a sand filling port 16.
05 To fill the eavity, ~he frame was held in a vertical plane with the sand-filling .~ port 16 ,~t the top. Measured quantities of sand were then poured into the unit to e - ~; e q e o ~
create he~e~g~ layers of sand within the cavity. In the model illustrated in Fig. 6, three layers of fine, crushed sandblasting sand, obtained from Bell and MacKenzie Company, Lld. a~ternated with two layers of the aforementioned Ottawa 10 ~iver sand, to form a~-b~ sancl layer in which the Ottawa River sand streaks acted as two high-permeabililty zones, since the sandblasting sand had abrine permeability of 770 md. and the Ottawa sand a perrneability of 65000 md.
after tapping and vibration to settle the sand layers. After the sand layers had been filled, the model was returned to a horizontal plane with the glass uppermost.
Water was then injected into the sand layer via the flooding fluid port 12 untilthe sand layer was saturated, thereby determining the pore volume of the sand layer. Following this w~ter ~looding, the zone was flooded via the oil inject;on port 14 with a 1:1 v/v mixture of Sunpar 107 H and Sunpar 2280 oils, the viscosity of this mix being 97 cps., as measured by Brookfield LVF ~2/60. The volume of water 20 displaced by the oil was measured, thereby determining the amount of oil required to saturat~ the sand layer.
Following this oil saturation of the sand layer, secondary recovery of oil from the sand layer was simulated by flooding the sand layer with water or brine using as the pump 13 either a Sage syringe pump 355 or a Milton Roy D 82-60 Reciprocating~5 Pump depending upon the required pumping pressure. The water flow rate and the pressure drop across the sand layer were recorded and the water flooding continued until the water:oil ratio of the mixture emerging from the flooding fluid/oil outlet 15 was about 40:1. The rate of water flow through the sand layer was typically about 60 cm. per hour.
When the water:oil ratio had risen to about 40, approximately 0.1 times the pore volume of the sand layer of various gelable solutions prepared in the manner described in E~ample 1 was injected into the sand layer via the flooding fluid port 12 and allowed to gel before water or brine flooding was resumed. Gellable solutions having concentrations of 10-20% (nominal) were used. After the solutions had 35 gelled, the water or brine flooding was continued in the same manner as before and the flooding fluid flow rate, the preæsure drop across the sand layer and the water:oil ratio in the liquid leaving the outlet port 15 were noted.

f~1~3~

Fig. 7 shows the percentage of oil recovery against volume of flooding liquid injected îor an experiment using m, 15 and 20% (nominal) gelablefsol~t~ It will be i~ seen that the proportion of original oil recovered increased substantial1y following the treatment of the sand layer by the instant method.
05 Fig. 8 shows the results of a further experiment in which mult;ple treatments by the instant method of a heterogeneous sand layer as shown in ~igure 6 were used to increase recovery of oil therefrom. The experimental technique used was the same as that described above with reference to Figure 7 exeept that the sequence of flooding and treatment steps was as follows:

tO (l) first flooding;
(2) 0.05 times the pore volume of the sand layer of a 2.5% sodium silicate pre-flush solution~
(3) O.lO times the pore volume of a 20% (nominal) gelable solution;

(4) 0.025 times the pore volume of the same solution as in step (2);
15 (5) second flooding;
(6) repeat of step (4);
(7) repeat of step (3);
(8) O.Ol times the poae volume of the same solution used in step (2);
(9) third flooding;
(lO) repeat of step (l);
(11) 0.075 times the pore volume of a 30% (nominal) gelable solution;
(12) O.OlS times the pore volume of the same solution used in step 2.
~ig. 8 clearly shows that repeated injections of the gelable solution at intervals during the secondary recovery process caused a greater increase in the proportion o~
25 oil recovered than a single injection. This is because the second and third injections of gelable solution bloclced relatively high permeability zones which were sweptclear by the water or brine flooding after the first injection of gelable solution had gelled. After the second and thirc injections of gelable solution, zones farther arld farther from the initialf~P~flow path of the flooding liquid were swept.
It was also established in these injections that the portion of oil recovered and the rate of oil recovery are not appreciably affected by the salinity of the displacing fluid~ thus proving that once the gelable solutions used in the instant me$hod have gel1ed, the resultant gels are not affected by prolonged exposure to brine.
Accordingly, when carrying out the instant method, the flooding fluid may, if 35 desired, be a brine, and the ability to use brine as a flooding fluid is important since when operating in desert or other locations where large supplies of fresh water are not readily available, the most convenient source of flooding ~luid may be brinepreviously extracted from a producer weU and separated from the accompanying oil.

~7 05 This E2~ample illustrates the use of gelable solutions containing a viscosifier in the instant method.
Where the permeability ratio between the high-permeability zones and the surrounding zones in an oil-bearing stratum is large (that is to say, where the surrounding zones are relatively "tight"), it is preferred to use a low-viscosity 10 solution in the instant method since such a low-viscosity solution ensures better penetration of the high permeability zones without substantial invasion of the less permeable zones. However, where the permeability ratio between the high-permeability zones and the surrounding zones is smaller, it is desirable to add viscosifiers to reduce the mobility of the gelable solution and to ensure that it 15 preferentially enters the region of highest permeability. Under low permeability ratio conditions, a non-viscosified gelable solution tends to diffuse from the high~
permeability zones into the surrounding zones, and this may result in dilution of the gelable solution to such an extent that gelation will not occur, or at least will not occur satisfactorily. This Example iUustrates the use of such a viscosified gelable 2n solution.
The experiments were conducted in the same manner as in Example 6, using 15% (nominal3 gelable solutions. However, in these experiments, 0.196 or 0.296 by weight of poly (sodium acrylate), in the form of a commercially available product Polyresin 5544, available from Bate Chemical, was added to each solution. The 25 results are shown in Fig. 9, together with the results of a control experiment in which no gelable sohltion was used. Comparing Fig. 9 with Fig. 7~ it will be seen that the inereases in the proportion of oil recovered were larger using the viscosified solutions used in Fig. 9 rather than the non-viscosified solutions used in Fig. 7, because the permeability ratios hetween the zones in the e~perimental model 30 were rather low.
,~ i Other known viscosifiers, such as carbo~ymethylcellulose or other ce11ulose derivatives~ polysaccharides, and copolymers of acrylamide, could be used with similar results. The improvement in the proportion of oil recovered at the rate of recovery produced by the addition of these viscosifiers is due to the more precise 35 placing and more complete gelation of the gelable solutions. However, the use of ~uch viscosified gelable solutions does have the disadvantage that pumping pressure has to be increased substantiaLly in order to achieve flow rates with the viscosified solutions comparable to those achieved with the low viscosity, non-viscosified gelable solutions.

05 ~
This ~xample illustrates the use o~ the instant method to reduce, but not completely elimin~te, the permeability o~ a high permeability zone.
In this Example, a Berea sandstone block having an initial brine permeability of about 116 md. was treated by the instant method with a 5% (nominal) gelable 10 solution prepared in accordance with Exarnple 1. After allowing the gelable solution to gel completely with;n the sandstone~ the brine permeabilitv of the sandstone was reduced to 0.1-0~3 md.

Example 9 This Example illustrates the use in the instant method of a gelable solution 15 containing flll allcali metal carbonate and the adjustment of permeability of a treated zone with acid after complete gelation has been effected, this acid treatment serving to adjust the permeability o~ the treated zone.
The gelable solutions used in these experiments were prepared in the same manner as in lSxample 1, and had total concentrations of alkaline material, 20 polyphenolic vegetable material and paraforrnaldehyde ranging from 7 to 25% by weight, except that the alkaline material used was sodium carbonate instead of sodium hydroxide and the sodium chloride was omitted from the dry powder used tomake up the solutions. When making up solutions containing sodium carbonate, it is undesirable to include sodium chloride in the powder since the presence of sodium 25 chloride reduces the maximum amount of sodium carbonate which can be in-corporated in the solutions and hence limits the extent to which the gelled solution can be redissolved by acid. Since sodium carbonate is a much weaker alkali than sodium hydroxide, it was necessary to use larger quantities of sodium carbonate,relative to the quantities of polyphenolic vegetable material and paraformaldehyde, 30 and in these experiments the amoun~ of sodium carbonate comprised from 30 to 70%
by weight of the total weight of sodium carbonate, polyphenolic vegetable material and paraformaldehyde. For example, the 20% solution used in these experiments contained 80 parts by weight of water, 13.94 parts by weight o~ sodium carbonate,

5.16 parts by weight of mimosa tannin extract and 0.90 parts by weight of paraformaldehyde, so that 69.7% of the total dissolved solids were sodium car-bonate. The gel times of the various compositions were determined in the same manner as in Example 1, the gel t;mes being determined at room temperature (20C), and the results are shown in Fig. 10. It will be seen that the sodium carbonate 05 compositions give a~ceptable gel times, although in the case of such compositions the total concentration of sodium carbonate, polyphenolic vegetable material andparaformaldehyde preferably does not exceed about 15% by weight, since otherwisethe gel times tend to be too short. Within the range of sodium carbonate of 30-70%
total so]ids, the higher the proportion of sodium carborlate, the longer the gel time.
A further series of experiments were conducted using carbonate-containing gelable solutions comprising 10% by weight total dissolved alkaline material, polyphenolic vegetable material and paraformaldehyde and in which the sodium carbonate content was either 30% or fi9.7% of the total weight of allcaline material, polyphenolic material and paraformaldehyde. The gelling times of these two 15 solutions were then determined in the same manner as in Example 1 over a temperature range of S-65C. The results are shown in Fig. 11, which shows that rensonable ~elling times can be achieved over the temperature range of 5-55C. As with the hydroxide containing solutions used to prepare the data in Fig. 2, the gel time increases substantially as the temperature decreases and, as in Fig. 10, the 20 greater the ratio of sodium carbonate to polyphenolic vegetable material and paraformaldehyde, the greater the gelling time at a given total solids content.
The ubility of the aforementioned composition cornprising 80 parts by weight of water, 13.94 parts by weight of sodium carbonate, 5.16 parts by weight of mimosa tannin extract and 0.90 parts by weight of paraformaldehyde to plug porous 25 formations was demonstrated in an experimental model comprising a stainless steel tube 20 cm. in length and having ar2 internal diameter of ~.95 cm ~illed with the aforementioned Ottawa sand having a permeability of 65,000 md. A quantity of thegelable solution equal to 1.4 times the pore volume of the tube was injected into the tube at room temperature and allowed to gel completely. After complete ge~ling, 30 the permeability of the sand was found to be ~50 md. A quantity of 20%
hydrochloric acid equal to twice the pore volume of the tube was injected into the plugged sand and allowed to remain in contac t with the sand for 24 hour~, whereafter the acid was flushed from the tube with water and the permeabi]ity again measured; the permeability following this first acid treatment was found to be 35 2400 md. A second acid treatment was then conducted in a similar manner, except that the acid was allowed to remain in contact with the sand for five days before .

water flushing; the permeability of the sand following this second acid treatment was found to be 3600 md. Thus, this experiment shows that when a high-permeability ~one is plugged by the instant method using a gelable solution containing a carbonate, the permeability of the plugged zone can later be sub-05 stantially increased by treatment of the plugged zone with acid.

Example 10 This Example illustrates the thermal stability of the gels formed in the instantm ethod.
The gelable solutiolls used in this experiment were the solution described in 10 Example 1, 1096 ~nominal) in tap water, and the solution described in Example 9, containing 80 parts by weight of water, 13.94 by weight of sodium carbonate, 5.16 parts by wei~ht of mimosa tannin extract and 0.90 parts by weight of formaldehyde.
Samples of both solutions were allowed to gel in separate glass tubes of 4 mm.
internal diameter and in capillaries as previously described in Example 5 and then 15 immersed in the 10% totaI dissolved solids standard expermental brine described in Example 2 at 290C for seven days. After this immersion, the ~el samples were tested and their yield s-tresses were found to show only a ~ ~ecrease as compared with the values before the brine immersion. These results show that theinstant method can be used to treat oil--bearing strata containing relatively 20 concentrated brine when it is desired to recover oil from the stratum using steam injectlon.

Example 11 This l~xample illustrates the advantages of the instant method over the plugging methods descr~bed in the aformentioned Swanson U. S. Patent 4,246,124.

25 Preparation The gelable compositions used in the instant method may be prepared by the technique described in Example 1 above in about 20-25 minutes. This preparation time does not increase when large batches, such as those necessary under actual field conditions, have to be prepared. In contrast, we have found that preparation of 30 the solutions described in Examples I and X of Swanson using Reten 420 as thepolyacrylarnide and resorcinol and mimosa tannin extract as the phenolic com-ponents, requires at least four to six hours, and to achieve proper solution preferably about 16-20 hours. The protracted preparation of the Swanson compositions is a considerable disadvantage under field conditions.

7~

The Swanson compositions were found to have a pH in the range of 7.5-7.8.
The relatively low pH of the Swanson compositions proves that their gelling ability is due to a differen~ mecharlism from that of the gellable solutions used in theinstant method since, as noted above, the gelable solutions used in the instant 05 method will not gel below about pH 9.5. In fact, the gelability of the Swanson compositions depends upon the polyacrylamide originally present therein, whereasthe gelling of the solutions in the instant method depends upon the copolymerization of the polyphenolic vegetable material and paraformaldehyde to form phenol/formaldehyde copolymers.

10 Viscosity Against Time Curves As shown in Fig. 1 and deseribed above in Example 1, the gelable solutions used in the instant method maintain a low viscosity, usually in the range of 2-30 mPas.
for at least 60% of their gel time and then rapidly form a rigid gel stable up to about 2(J0C. The gels formed in the instant method are kue, relatively rigid gels IS having an elasticity which can be measured in terms of their yield point. In contrast, when viscosity/time data taken from Tables IX, X and XI of the Swansonpatent were plotted, the curves shown in Figure 12 were produced. It should be noted that the data in the Swanson patent were obtained at temperatures of 205-325F (96.1-162.8C) as follows:

20 Phenolic ComponentTemperature tC) Swanson's Table ~,0, ~o, 1, 4-benzoquinone 162.8 IX
Resc,rcinol 96.1 ~
Catechol 135 X
25 Phenol 135 X
Hydroquinone 135 Sulfomethylated quebracho (103 162.8 SMQ (85) 162.8 X
SMQ (15) 162.3 X

The data in this Figure show that in many instances, a gradual increase in viscosity started immediately after the solutions were mixed. Moreover, the viscosity of some of the Swanson gels reached a maximum and then decreased, showing that the gels were not thermally stable at the temperatures used in those ~7~a3~
-3n-tests. Many of the Swanson solutions do not yield desirable "J'Lshaped curves produced by the gelable solutions used in the instant method, in which the viscosity remains low for a protracted period and then suddenly increases; rather7 many of the Swanson compositions increased slowly and relatively uniformly in viscosity over a 05 protracted period beginning shortly after the solutions were formed. Accordingly, pumping of the Swanson solutions into deep wells is likely to be difficult.
Finally, it should be noted that, because the gel times of Swanson's gelable solutions increase greatly with decreasing temperature cf. Example l above, the gel times of the Swanson solutions are likely to be excessively long when used in zones lU at temperatures below 40C, whereas the solutions used in the instant method can be made to have much shorter gel times at such relatively low temperatures.

Gel Strength When a 10% (nominal~ gelable composition usable in the instant method prepared as in Example l was tested at room temperature by the technique in 15 Example 1 of Swanson, after five days less than 0.5 ml. ol9 fluid was collected in 10 minutes (0-0.05 ml./min.) as a result of the slight shrinkage of the gel structure.
This compares most favorably with the Swanson solutions which tend to weaken with time, and which gave leakage rates of 0.04 to 0.33 ml./min. at 60~C:
The Swanson solu tions were also compared with those used in the instant 20 method by the technique of measuring gel strength described above in Example 5.
The instant solutions used were the 5, 7, lO, 15 and 25% (nominal) solutions described above in Example 1, and the resul~s are shown in Table I in Example 5 above. TheSwanson solutions that are shown in Table II below are those listed in Table I run l of the Swanson patent (913 ppm. of resorcinol solution) and similar solutions in which 25 the resorcinol concentration was increased to 1369 and 1847 ppm. respectively.
Although Swanson elaims that the results in his Table I produced most stable gels as determined by sand pack gel stability data3 our test results9 shown in Table II below, show that the yield stress of the Swanson gels is very low. It is seen that the solutions used in the instant method are capable of producing stronger gels than the 30 Swanson solutions.

TABL II
~"~

Resorcinol Re~en 420 I corr 05 (ppm) ~e (psi)*
913 ~SB7 0.056 1369 4565 O.O~L7 1847 4563 0.062 *The above T corr values represents the pressure at which the gel is actually 10 extruded from the tube.
l`he actual T values for all groups is 0.02-0.03 psi to a maximum of 0.07 psi.

Shear Stress Tests The crucial component of the Swanson solutions, ~n which their gelling depends, is a high molecular weight polyacrylamide, sueh as the aforementioned 15 R~ten ~20. Such high molecular weight polymers are known to be susceptible tophysical degradation by shear stresses, such as will be encountered when pumpingthese solutions down wells and into high permeability zones in oil-bearing strata. A
Swanson solution (the solution used in Example I, run (1), Table I of Swanson's patent) h~ving an initial viscosity of 25 cps. was subjected to five minutes of shearing in a 20 Waring blender; after this shearing by the blender, the viscosity o-f the solution was reduced to S cps., indicating substantial and irreversible damage to the solution from the shearing. While the unsheared solution attained a viscosity in excess o~
100,000 cps. when allowed to gel for five days, the sheared sample attained a viseosity of only 6 cps. after 10 days. This indicates that, although the Swanson ~5 solutions may give reasonably strong gels in laboratory e~p~riments, they areunlikely to produce very strong gels under field conditions because of the shearstresses to which they are exposed during placement in oil-bearing strata.
In contrast, when the 15% (nominal) solution described above in Example 1 was sheared under the same ccnditions, no substantial change in either the viscosity o~
30 the solution immediately after shearing, or in the viscosity o~ the resultant ~el, was observed.

_provement in O~ ecove~y ~ o /4 ~
The ability of the 15% ~nominal~ described above in Example 1 and a Swanson solution (the Reten ~20/formaldehyde/minosa tannin extract solution of Swanson'sExample X) to improve oil recovery from an experimental model of a sand bed were05 tested at 60-65C by the technique described above in Example 6. The results are shown in Fig. 13. The results shown in this figure indicate that the solution used in the instant method is slightly superior to the Swanson solution in improving oilrecovery. It should be noted, however, that this experiment was conducted at 60-65C and the superiority of the solutions used in the instant invention would belO expected to be much grcater at the elevated temperatures used in steam recovery of oil or in sealing very highly permeable zones in which the weak Swanson gels would degrade very quickly. As already indicated, the gels formed by the instantmethod are ~* heat-resistant, whereas the Swanson gels would degrade very quickly at the temperature used in steam recovery of oil.
Tt will be apparent to those skilled in the art that numerous changes and improvemerlts may be made in the instant method without departing from the scopeof the invention. Accordingly, the foregoing description is to be construed in an illustrative and not in a limitative sense, the scope of the invention being defined solely by the appended claims.

SlnPLE~ENTARY DIS~SURE
Summ~x~of the Invention . _ . .
It has now be.en discovered ~hat useful adjust~ents in the fluid perrneabili-ty of high Eluid permeability zones in oil-bear~ng strata can be achieved using solutions haviny a lower active solids content than those previously discussed. In addition, it has been discovered that, in addition to the formaldehyde sources discussed above, urea formaldehyde concentrate and hexame-thylenetetramine can be used ~I the gelable solutions used in the instant methods. Accordingly, the invention provides a method for adjusting the fluid permeability of a hiyh fluid perr~3ability zone in an oil-bearLng stratum, said zone having greater fluid penneability than the surrounding zones of said stratum/ which me-thod comprises.
injecting into said oil-bearing stratum via a ~ell penetrating said stratum an aqueous, alkaline gelable solution, said solu-tion comprising an alkaline material, a polyphenolic vegetable ma-terial selected from the g~oup consisting o~ tannin extracts, catechins and alkaline extracts of coniferous tree barks, said polyphenolic material being soluble in alkaline solution, and formalde~.yde, said formaldehyde being present as formaldehyde itself, pRra-fornkaldehyde, urea-formaldehyde concentrate, hex&methylenetetramine or a phenol-formaldehyde resole, the total active solids content of said solution being from about 1 to about 33% by weight of said solution, said solution having a pH of at least about 9.5 and being formed b~ dissolving said alkaline material, said polyphenollc vegetable material and said formaldehyde in a brine containing not more than about 0.275 percent by weight of cations having a valency g.rea~e.r than one and forming insoluble hydroxides, the gelling time oE said solution and the rate of injection thereo~ being such that saia solution passes down said well by which it i9 injected and achieves substantial penetration into said high fluid permeability zone before substantial gelling of said solution occurs; and allowing said solution to gel within said high fluid permeability zone, thereby reducing the fluid permeability of said zone.
The invention also provides a method for adjusting the fluid permea-bility of a high fluid permeability zo~e in an oil-bearing stratum, said zone haviny greater fluid permeabili.ty than the surroundiny zones of said s~ratum, which method comprises:
injecting in-tc said oil~bearing stratum via a well penetrating said stratum an aqueous, aIkaline gelable solution, said solution comprising an alkaline material, a polyphenolic vegetable material selected fron the group consisting of kh/~

37~

tannin extracts, catechins and alkaline extracts of coniferous tree barks, said polyphenolic material being soluble in alkaline solution, and he~amethylene-tetramine, the total active solids content of said solution being from about 1 to about 33% by weight of said solution, said solution having a pH of at least about 9.5, 05 the gelling time of said solution and the rate of injection thereof being such that said solution passes down said well by which it is injected and achieves substantial penetration into said high fluid perme~bility zone before substantial gelling of said solution occurs; and allowing said solution to gel within said high fluid permeability zone, 10 thereby reducing the fluid permeability of said zone.
The invention also provides a composition for use in adjusting the fluid permeability of a high fluid permeability zone in an oil-bearing stratum, said compostion being an aqueous gelable solution having a pH of at least about 9.5 and cornprising an alkalirle matèrial, a polyphenolic vegetable material selected from 15 the group consisting of tannin e~tracts, catechins and alkaline extracts of coniferous tree barks, said polyphenolic vegetable material being soluble in alkaline solution, and hexamethylenetetramine, the total weight of said alkaline m~terial polyphenolic vegetable material and hexamethylenetetramine being from about 1 to about 33% byweight of saicl solution.
2() Finally, the invention provides a àry composition for use in preparing a gelable composition this dry composition comprising a dry mixture of an alkaline material, a polyphenolic vegetable material sol~lble in alkaline solution selected from the group consisting of tannin extracts, catechins and alkaline extraets of coniferous tree barks, and hexamethylenetetramine9 the proportions of said alkaline material, said 25 polyphenolic vegetable material and said hexamethylenetetramine being such that, upon dissolution in water, said dry mixture forms a gelable soluti~n.
Brief Description of the Additional Drawings Fig. 14 is a schematic diagram (not to scale~ of an experimental apparatus for measuring water and brine permeabilities at temperatures of 25-170C;
Fig. 15 is a graph showing the final permeability obtained in simulated high permeability zones using the instant methoàs with varying concentrations of total active solids in the gelable solutions9 a~d ~ig. 16 is a graph of permeability against temperature for gelled form in artifieial high fluid permeability zones using the instant methods with relatively low 35 concentrations of total active solids.

Detailed Description of the Invention -As already mentioned! it has now been discoYered that the instant method can be practiced with solutions containin~ as little as 1% total active solids, so that the invention can be practiced using gelable solutions having a total active solids 05 content of from about 1 to about 33% by weight. Above about 33% total active solids content, the solution tends to gel too rapidly to be practically useful, while below about 1% by weight the solution is too slow to gel or may not gel properly at all. Gelable solutions having relatively low solids content, and particularly those containing less than 5% total active solids content, tend not to completely eliminate 10 aLI permeability from high fluid permeability zones in oil-bearing strata, but, as shown by the Examples below, can achieve great reductions in permeability which are often sufficient under field conditions.
The gel time of the solutions used in the instant methods depends upon a nl1mber of factors, including the total active solids content of the gelable solution, 15 the exact components and proportions of the various components in the gelablesolution, the temperature of the oil-bearing stratum in which gelling takes place and the quantity of ions which are either deliberately introduced into the gelable solution or which become admixed with the gelable solution because of the presence o~ brines in the oil-bearing stratum. ~lthough, as those skilled in the art are aware, 20 linal selection of the various parameters used in the instant method is to a certain extent dependent upon the skill and judgement of the operator, certain general principles for guidance in selection of these parameters can be set forth. In general, the higher the total active solids content of the gelable solution used, the shorter is the gel time and the lower the final permeability obtained in the originally high fluid 25 permeabili$y zone. }Iigher temperatures in the environment in whieh the gelling takes place i.e. in the oil-bearing stratum, the lower the gel times. Also, as explained in more detail below, the presence of polyvalent cations in the gelable solutions tends to reduce gel times, whether those polyvalent cations are introduced into the gelable solution by making the gelable solution up using a brine in 30 accordance with the first instant method9 or whether such polyvalent cations are introduced as the gelable solution becomes admixed with brine present in the oil-bearing stratum.
The urea-formaldehyde concentrate which can be used as a source of formal-dehyde in the gelable solutions used in the instant methods is commercially available 35 and is manufactured by adsorbing gaseous formaldehyde into an aqueous solution of urea. This urea-formaldehyde concentrate has the advantage that, whereas simple ~87~

aqueous solutions of formalàehyde containing more than about 50% formaldehyde are unstable and likely to polymerize, the urea-formlldehyde concentrate eont~in considerably greater concentrations of formaldehyde and still have long term storage stabilit~.
05 Using hexamethylenetetramine as the formaidehyde source in the gelable solutions normally produces solutions having longer gel times than similar solutions in which formaldehyde, p~raformaldehyde or a phenyl-formaldehyde resole is used as the source of formaldehyde. Moreover, it appears (although the invention is in no way limiled by this belief) that the way in which hexamethylenetetramine produces 10 gelling is somewhat different from that in which formaldehyde, paraformaldehyde, and phenyl-formaldehyde resoles produce gelling. Paraformaldehyde is unstable inwater and thus when the gelable solutions made up using paraformaldehyde the paraformaldehyde dissociates to form free formaldehyde in the solution. Accord-ingly, when either formaldehyde or paraformaldehyde is used as the formaldehyde 15 source, the solution as originally formed contains substantial concentration of formaldehyde and thus, in principle, begins to gel as soon as it is formed, although as illustrated in the examples below, no perceptible change in gelling properties takes place for a considerable period. Phenol-forrnaldehyde resoles do not hydrolyze in the gelable solutions, but in this case the reactive entity responsible for the gelling ~0 re~ction is the methylol ~roups of the resole and these reactive groups present from the forrmntion of the gelable solution. Thus, in flll cases at least in principle geling of the gelable solution begins as soon as this solution is formed.
However, although free formaldehyde produced by hydrolysis appears to be the effective gelling agent in gelable solutions containing hexamethylenetetramine, 25 hexamethylenetetramine is much less labile than paraformaldehyde and the rate at which hexamethylenetetramine is hydrolyzed to free formaldehyde and ammonia in aqueous solution is negligible at room temperature. For practical purposes, the gellable solutions containing hexamethylentetramine are stable indefinitely at room temperature since in the absence of free formaldehyde no gelling reaction 30 takes place. Hydrolysis of the hexamethylenetetramine to form~ldehyde and - ammonia to an extent sufficient to produce significant gelling reactions only takes place when the gelable solution is raised to a temperature substantially above room temperature, in excess of 40 and preferably more than about 50C, such as wouldnormally occur when the gelable solution is injected into oil-bearing stratum a 35 substantial distance below ground. Accordingly, the use of gelable solutions containing hexamethylene tetramine is not recommended where the oil-bearing 43~

stratum is relatively eool, fol example because large ~uantities of flooding liquid have previously been passed through the stratum. On the other hand, compositionscontaining hexamethylenetetramine may be highly desirable for use in a relatively hot oil-bearing stratum where other gelable solutions might tend to gel so quickly 05 that proper penetration of the gelable solution into the high fluid permeability zone would not be achieved before gelling occured.
We prefer that the formaldehyde used in the instant gelable solutions be present as paraformaldehyde, since this solid material is easier to handle and store than liquid solutions of formaldehyde (formalin solutions) or urea-formaldehyde 10 concentrate. However, for the reasons stated in the proceeding paragraph, gelable solutions based on hexamethylenetetramine as the formaldehyde source may be es-pecially useful under certain conditions.
As indicated above, by careful adjustment of the concentrations of the various cornponents in the gelable solution, the instant method may be carried out so that 15 gels are forrned which do not completely destroy ~he permeability of the treated part of an oil bearing strata but only reduce the permeability of this part. This is advantageous, since such reduction but not elirnination of the permeabililty of the treated part eliminates the possiblity of inaclvertently and completely stopping the flow of flooding liquid through the oil-bearing stratum. This is highly undesirable, 20 sirlce complete blockage of floocling liquid flow would necessitate the expensive drilling of a new injector well into an unblocked part of the oil-bearing stratum or fracturing of the original injector weU. To produce such incomplete blocking of the treated part of the oil-bearing stratum, in most cases it is desirable to use a gelable soluti~n in which the to1 al weight of the alkaline material, the polyphenolic 25 vegetable material and the paraform~ldehyde is from about 1 to about 10% of the weight of the gelable solution. ln some situations, however, gelable solutions containing as little as 1% total active solids may not produce sufficient blocking.
Accordingly, in generaly we prefer to use gelable solutions containing at least 2%
total active solids. Where substantially complete blocking of the high fluid 30 permeability zone is desired, it is generally necessary to use gelable solutions containing at least 5% total active solids, but obviously this depends upon the exact nature of the gelable solution and the properties of the high fluid permeability zone being treated.
It has already been mentioned that the gelable solutions used in the instant 35 method having a total active solids content of at least about 5% and using sources of formaldehyde other than hexamethylenetetramine are capable of maintaining, at temperatures up to at least about 65C, gel time sufficiently long to allow for handling unc~esirable penetration into oil-bearing strata, and thus that the instant method can be used in oil-bearing strata which are not above this temperature, either naturally or after cooling by passing cooling fluid inlo the oil-bearing stratum 05 before the gelable solution is injected. Also, gelable solutions having active solid contents below 5% and/or using hexamethylenetetramine as the formaldehyde source tend to be slower in gelling than the other instant gelable solutions, and hence it may be possible to use these gelable solutions at temperatures in excess of 65C.
The additional gelable solutions discussed in this Supplementary Disclosure can of course contain the same additives, and may be employed using the same techniques, as the solutions discussed in the original disclosure.
The following examples are now given to illustrate the use of solutions having relatively low total active solids contents.
Example 12 This Example ilustrates the ability of the instant methods to achieve useful reductions in the permeability of high fluid permeability zones using available solutions having relatively low total active solids contents. Nominally 8%, 5%, 3%
and 1% solutions of the powder described in Example I above were made up by the 2() procedule described in that Example. A model of a high fluid permeability zone was made by packing a stainless steel tube 15.24cm. long by 0.95cm. internal diameter with water-washed Athabasca tar sand having a residual oil saturation of 2.6%. The packing density of the sand was approxirnately 1600kg.m. with an inital permea-bility to a standard brine ~this brine contained 196 of a mixture of sodium, calcium 25 and magnesium chloride, in a 20:2:1::Na:Ca:Mg molar ratio) of approximately 2.~
Darcies. Separate tubes were then injected with 0.5 pore volumes of the gelable solution, followed by a 0.17 pore volume water after-tlush. The treated tubes where then maintained for 24 hours at 225C to ensure proper gelation and there permeabilities thereafter determined at 25C~. The results are shown in Fig. 15.It will be seen from the results plotted in Fig. 15 that the final permeability of the treated sand decreased as the percentage of total active solids in the solution increased; for all practical purposes, the sand treated with the 8% total active solids solution was impermeableO However, even with the 1% total active solids solution, useful reduction in permeability was achieved; the fluid permeability of approx-35 imately 150 millidarcies produced with the 1% solution represents an approximately 18-fold reduction in permeability as compared with the original permeability of the 7'~

sand, asld such a reduction in permeability may often be sufficient under field conditions.
Example 13 This Example illustrates the ability of the gels produced by gelable solutions 05 having relatively low total active solids contents to withstand high temperatures after gelling.
The apparatus used in these experiments is shown in Pig. 14. This apparatus comprises an eluent reservoir 20 supported and stirred by, and capable of bein heated by, a hot plate/stirrer 21. From the reservoir 20, a liquid supply line extends 10 via a metering pump 22, past a pressure gauge 23 and via a valve 2a to the inlet 25 of a test core 26, which is surrounded by an oven 27. From the outlet 28 of the test core 26, a line extends past a pressure gauge 29 and via a valve 30 to a pressurized collector cell 31 provided with a drain valve 32. Pressurization of the cell 31 is effected by means of a gas cylinder 33 which supplies gas via a pressure regulator 34 15 to the gas inlet 35 of the cell 31.
A differential pressure transducer 36 is arranged so as to measure the pressure drop across the v~lve 24 and the test core 26~ the pressure difference measured on the transducer 36 being displayed on a digital readout 37. The valve 24 and the test core 26 are also bridged via a bypass line 38 in which are connected in series a2D metering valve 39 and a shut-off valve 40.
The apparatus shown in Fig. 14 is used as follows. The eluent fluid in the reservoir 20 is rmaintained by the hot plate 21 at a temperature sufficient to degas the fluid and is suplied by the pump 22 via the valve 24 to the test core 26 which is maintained by the o~en 27 at the desired temperature. The pressure drop across the 25 test core is measured by the differential pressure transducer 36 and displayed on the display 37. The back-pressure necessary to prevent evaporation of the eluent liquid is measured on the gauge 29, this back-pressure being set by the pressure regulator 34. The collector cell 31 ~lso serves as a collector of the eluent liquid from the test core 26; the cell 31 can be emptied by opening the drain valve 32 without 30 depressurizing the system.
After the material in the test core 26 has been treated by one of the instant methods, the permeability thereof can become so low that even the minimum liquiddelivery rate capable of being produced by the pump 22 could produce so high a pressure gradient across the test core 26 that the gel formed therein would fracture.
35 Accordingly, if the pressure differential across the test core approaches a poten-tially excessive value, the valve 24 is closed and the valYe 40 (which is closed during 4~3~

normal operation of the apparatus) is opened. the metering valve 39 is then adjusted to provide a pressure differential across the core at or near the maximum to which the core should be exposed. The valve 24 is then opened and the eluent liquid allowed to flow through the core and the metering valve 39 simultaneously. This 1)5 opening of the valve 24 lowers the pressure differential because the total flow rate remains constant. The residual core permeability (K) is then calculated from thefollowing equation, which is derived from Darcy's equation:
K=Qn 1/ 1 1 ~
Al (,~P P2 J
wherein:
Al is the cross sectional area of the core in cm.2;
K is the permeability of the core to the liquid in Darcies;
11 is the length of the core in centimeters;
Q is the flow rate of the flooding liquid in cm s ;
a P is the pressure drop when the valves 40, 39 and 24 are opened, measured in 15 atmospheres;
QP2 is the pressure differential with the valve 24 closed, measured in atmospheres; and n is the viscosity of the liquid at the tesl: temperature in mPa.s.
~ s the core permeability approaches zero, the pressure drop, recorded after 20 the valve 24 is opened, becomes equal to the pressure drop recorded with the Yalves 39 and 40 open but with the vslve 24 closed.
Sand-packed stainless steel tubes were prepared in exactly the same manner as in Example 12 above9 except that the sand was packed to a density of approximately lSOOkg.m. 3, resulting in an initial permeability to the same standard brine of 25 approximately 8-10 Darcies. Two separate tubes were then treated by the instant method in the same manner as in Example 12 above, 0.5 pure volume of a nominal 2% or 3% solution identical (apart from concentration) to those used in ~xample 12 being injected into the sand, followed by a 0.17 pore volume water after-flush and maintenance of the treated tube for 24 hours at 225C to allow for gelling. After 30 the gelling had been completed, the pemeabilities of the treated sand were tested over the range of 25-170C using the apparatus shown in Pig. 14. The results areshown in Fig. 16.
The data plotted in Fig. 16 show that the 2% and 3,6 total active solids solutions used in these experiments did produce substantial reductions in the 35 permeability of the packed sand and that the resultant gels were temperature stable, as indicated by the fact that permeabilities were relatively stable over the entire temperature range of 25-170C.
Example 14 To test the effect of high rates of shear upon the gelable solutions used in theinstant methods, gelable solutions containing 5-15% total active solids were sub-05 jected to high shear on a Hercules ~Ii Shear Viscometer. The shear rates used rangefrom 0 to 18,000sec. 1 at 4400 rpm.
The results obtained from the gelable solutions used in the instant methods yielded viscosity against shear rate curves which were substantially parallel to those of water, although of course the curves of the gelable solutions were shifted towards 10 higher viscosities; the viscosity values for the 1596 solutions were typic~lly 1.5-2 times the corresponding water values. However, the fact that the gelable solutions used in the instant methods did yield curves substantially parallel to water shows that the gelable solutions do not break down under high-shear conditions as do many prior art polymer-based gelable solutions for use in plugging oil-bearing strata.

Claims (58)

1. A method for adjusting the fluid permeability of a high fluid perme-ability zone in an oil-bearing stratum, said zone having greater fluid permeability than the surrounding zones of said stratum, which method comprises:
injecting into said oil-bearing stratum via a well penetrating said stratum an aqueous, alkaline gelable solution, said solution comprising an alkaline material, a polyphenolic vegetable material selected from the group consisting of tannin extracts, catechins and alkaline extracts of coniferous tree barks, said polyphenolic material being soluble in alkaline solution, and formaldehyde, saidformaldehyde being present as formaldehyde itself, paraformaldehyde or a phenol-formaldehyde resole, the total active solids content of said solution being fromabout 5 to about 33% by weight of said solution, said solution having a pH of at least about 9.5 and being formed by dissolving said alkaline material, said polyphenolic vegetable material and said formaldehyde in a brine containing not more than about 0.275 percent by weight of cations having a valency greater than one and forminginsoluble hydroxides, the gelling time of said solution and the rate of injection thereof being such that said solution passes down said well by which it is injected and achieves substantial penetration into said high fluid permeability zone before substantial gelling of said solution occurs; and allowing said solution to gel within said high fluid permeability zone, thereby reducing the fluid permeability of said zone.

2. A method according to claim 1 wherein said formaldehyde is present as paraformaldehyde.

3. A method according to claim 1 wherein, after said gelable solution has been injected into said oil-bearing stratum, a non-gelable displacing fluid is injected into said oil-bearing stratum via said well, thereby causing said gelable solution to he displaced from around said well and preventing excessive loss of permeabilityaround said well.

4. A method according to claim 3 wherein said displacing fluid comprises water.

5. A method according to claim 3 wherein said displacing fluid comprises a viscous aqueous solution of a polymer.

6. A method according to claim 1 wherein said oil-bearing stratum contains cations capable of causing the formation of insoluble material when in contact with said gelable solution, whereby formation of said insoluble material occurs at the interface between said high fluid permeability zone and said surrounding zones, thereby limiting the penetration of said gelable solution into said surrounding zones.

7. A method according to claim 1 wherein steam has been injected into said stratum prior to said injection of said gelable solution and wherein, prior to said injection of said gelable solution, a cold liquid is injected into said high fluid permeability zone to cool said zone to a temperature low enough to increase the gel time of said gellable solution, sufficiently to permit pumping of said gelable solution down said well and substantial penetration of said gelable solution into said high fluid permeability zone before said gelable solution gels.

8. A method according to claim 1 wherein said gelable solution further comprises a high molecular weight viscosifier.

9. A method according to claim 1 wherein the concentrations of said alkaline material, said polyphenolic vegetable material and said formaldehyde are adjusted so that said gelable solution does not form a homogeneous gel completely blocking the flow of fluids through said high fluid permeability zone but sub-stantially and permanently reduces the permeability thereof.

10. A method according to claim 9 wherein the total active solids content of said solution is from about 5 to about 10 percent.

11. A method according to claim 1 wherein said high fluid permeability zone is oil-wet and said injection of said gelable solution substantially reduces the fluid permeability of said zone but does not completely block the flow of fluids therethrough.

12. A method according to claim 1 wherein said flooding liquid comprises water, brine, an aqueous solution of a polymer, an aqueous solution of a surfactant or a hydrocarbon fluid.

13. A method according to claim 1 wherein said gelable solution and a solution of an accelerator for accelerating the gelling of said gelable solution are piped separately down said well, said solutions being allowed to mix within said well at a point spaced from the upper end thereof to form a rapidly-gelling solution.

14. A method according to claim 13 wherein said accelerator is an alkali metal silicate.

15. A method according to claim 13 wherein at least one of said solutions further comprises a high molecular weight viscosifier.

16. A method according to claim 1 wherein, prior to or after said injection of said gelable solution, a solution of an accelerator is pumped into said high fluid permeability zone.

17. A method according to claim 16 where said accelerator is an alkali metal silicate.

18. A method according to claim 1 wherein at least part of said alkaline material comprises an alkali metal carbonate.

19. A method according to claim 18 wherein, after said gelable solution has gelled within said high fluid permeability zone, an acid is injected into said zone to dissolve at least part of said alkali metal carbonate.

20. A method according to claim 1 wherein the pH of said gelable solution is from about 10 to about 11.

21. A method according to claim 1 wherein said alkaline material comprises sodium hydroxide and/or sodium carbonate.

22. A method according to claim 1 wherein said polyphenolic vegetable material is mimosa tannin extract.

23. A method according to claim 1 wherein said gelable solution has a viscosity, when first formed, of from about 2 to about 30 mPa.s.

24. A method according to claim 1 wherein said gelable solution has a gel time at 25°C in excess of about 3 hours.

25. A method according to claim 1 wherein said gelable solution contains from about 15 to about 25 parts by weight of formaldehyde per 100 parts by weight of said polyphenolic vegetable material on a dry basis.

26. A method according to claim 1 wherein, after said solution has gelled within said high fluid permeability zone, a flooding fluid is injected into said oil-bearing stratum and oil is recovered from said oil-bearing stratum.

CLAIMS SUPPORTED BY THE SUPPLEMENTARY DISCLOSURE

27. A method for adjusting the fluid permeability of a high fluid perme-ability zone in an oil-bearing stratum, said zone having greater fluid permeability than the surrounding zones of said stratum, which method comprises:
injecting into said oil-bearing stratum via a well penetrating said stratum an aqueous, alkaline gelable solution, said solution comprising an alkaline material, a polyphenolic vegetable material selected from the group consisting of tannin extracts, catechins and alkaline extracts of coniferous tree barks, said polyphenolic material being soluble in alkaline solution, and formaldehyde, saidformaldehyde being present as formaldehyde itself, paraformaldehyde, urea-formal-dehyde concentrate, hexamethylenetetramine or a phenol-formaldehyde resole, the total active solids content of said solution being from about 1 to about 33% by weight of said solution, said solution having a pH of at least about 9.5 and being formed by dissolving said alkaline material, said polyphenolic vegetable material and said formaldehyde in a brine containing not more than about 0.275 percent by weight of cations having a valency greater than one and forming insoluble hydroxides, the gelling time of said solution and the rate of injection thereof being such that said solution passes down said well by which it is injected and achieves substantial penetration into said high fluid permeability zone before substantial gelling of said solution occurs; and allowing said solution to gel within said high fluid permeability zone, thereby reducing the fluid permeability of said zone.

28. A method according to claim 27 wherein said formaldehyde is present as paraformaldehyde.

29. A method according to claim 27 wherein, after said gelable solution has been injected into said oil-bearing stratum, a non-gelable displacing fluid is injected into said oil-bearing stratum via said well, thereby causing said gelable solution to be displaced from around said well and preventing excessive loss of permeabilityaround said well.

30 A method according to claim 29 wherein said displacing fluid comprises water.

31. A method according to claim 29 wherein said displacing fluid comprises a viscous aqueous solution of a polymer.

32. A method according to claim 27 wherein said oil-bearing stratum contains cations capable of causing the formation of insoluble material when in contact with said gelable solution, whereby formation of said insoluble material occurs at the interface between said high fluid permeability zone and said surrounding zones, thereby limiting the penetration of said gelable solution into said surrounding zones.

33. A method according to claim 27 wherein steam has been injected into said stratum prior to said injection of said gelable solution and wherein, prior to said injection of said gelable solution, a cold liquid is injected into said high fluid permeability zone to cool said zone to a temperature low enough to increase the gel time of said gelable solution sufficiently to permit pumping of said gelable solution down said well and substantial penetration of said gelable solution into said high fluid permeability zone before said gelable solution gels.

34. A method according to claim 27 wherein said gelable solution further comprises a high molecular weight viscosifier.

35. A method according to claim 27 wherein the concentrations of said alkaline material, said polyphenolic vegetable material and said formaldehyde are adjusted so that said gelable solution does not form a homogeneous gel completely blocking the flow of fluids through said high fluid permeability zone but sub-stantially and permanently reduces the permeability thereof.

36. A method according to claim 35 wherein the total active solids content of solution is not greater than about 10% by weight.

37. A method according to claim 27 wherein the total active solids content of said solution is at least about 2% by weight.

38. A method according to claim 27 wherein said high fluid permeability zone is oil-wet and said injection of said gelable solution substantially reduces the fluid permeability of said zone but does not completely block the flow of fluids therethrough.

39. A method according to claim 27 wherein said flooding liquid comprises water, brine, an aqueous solution of a polymer, an aqueous solution of a surfactant or a hydrocarbon fluid.

40. A method according to claim 27 wherein said gelable solution and a solution of an accelerator for accelerating the gelling of said gelable solution are piped separately down said well, said solutions being allowed to mix within said well at a point spaced from the upper end thereof to form a rapidly-gelling solution.

41. A method according to claim 40 wherein said accelerator is an alkali metal silicate.

42. A method according to claim 40 where at least one of said solutions further comprises a high molecular weight viscosifier.

43. A method according to claim 27 wherein, prior to or after said injection of said gelable solution, a solution of an accelerator is pumped into said high fluid permeability zone.

44. A method according to claim 43 wherein said accelerator is an alkali metal silicate.

45. A method according to claim 27 wherein at least part of said alkaline material comprises an alkali metal carbonate.

46. A method according to claim 45 wherein, after said gelable solution has gelled within said high fluid permeability zone, an acid is injected into said zone to dissolve at least part of said alkali metal carbonate.

47. A method according to claim 27 wherein the pH of said gelable solution is from about 10 to about 11.

48. A method according to claim 27 wherein said alkaline material comprises sodium hydroxide and/or sodium carbonate.

49. A method according to claim 27 wherein said polyphenolic vegetable material is mimosa tannin extract.

50. A method according to claim 27 wherein said gelable solution has a viscosity, when first formed, of from about 2 to about 30 mPa.s.

51. A method according to claim 27 wherein said gelable solution has a gel time at 25°C in excess of about 3 hours.

52. A method according to claim 27 wherein said gelable solution contains from about 15 to about 25 parts by weight of formaldehyde per 100 parts by weight of said polyphenolic vegetable material on a dry basis.

53. A method according to claim 27 wherein, after said solution has gelled within said high fluid permeability zone, a flooding fluid is injected into said oil-bearing stratum and oil is recovered from said oil-bearing stratum.

54. A method for adjusting the fluid permeability of a high fluid perme-ability zone in an oil-bearing stratum, said zone having greater fluid permeability than the surrounding zones of said stratum, which method comprises:
injecting into said oil-bearing stratum via a well penetrating said stratum an aqueous, alkaline gelable solution, said solution comprising an alkaline material, a polyphenolic vegetable material selected from the group consisting of tannin extracts, catechins and alkaline extracts of coniferous tree barks, said polyphenolic material being soluble in alkaline solution, and hexamethylene-tetramine, the total active solids content of said solution being from about 1 to about 33% by weight of said solution, said solution having a pH of at least about 9.5, the gelling time of said solution and the rate of injection thereof being such that said solution passes down said well by which it is injected and achieves substantial penetration into said high fluid permeability zone before substantial gelling of said solution occurs; and allowing said solution to gel within said high fluid permeability zone, thereby reducing the fluid permeability of said zone.

55. A method according to claim 54 wherein, after said solution has gelled within said high fluid permeability zone, a flooding liquid is injected into said oil-bearing stratum and oil is recovered from said oil-bearing stratum.

56. A composition for use in adjusting the fluid permeability of a high fluid permeability zone in an oil-bearing stratum, said compostion being an aqueous gelable solution having a pH of at least about 9.5 and comprising an alkaline material, a polyphenolic vegetable material selected from the group consisting of tannin extracts, catechins and alkaline extracts of coniferous tree barks, said polyphenolic vegetable material being soluble in alkaline solution, and hexamethyl-enetetramine, the total weight of said alkaline material polyphenolic vegetable material and hexamethylenetetramine being from about 1 to about 33% by weight ofsaid solution.

57. A composition according to claim 56 wherein said polyphenolic vegetable material comprises mimosa tannin extract.

58. A dry composition for use in preparing a gelable composition according to claim 56, said dry composition comprising a dry mixture of an alkaline material, a polyphenolic vegetable material soluble in alkaline solution selected from the group consisting of tannin extracts, catechins and alkaline extracts of coniferous tree barks, and hexamethylenetetramine, the proportions of said alkaline material, said polyphenolic vegetable material and said hexamethylenetetramine being such that,upon dissolution in water, said dry mixture forms a gelable solution.