GB1560346A - Use of ethers of hydroxypropiosulphonate salts in recovering oil from oil-bearing formations - Google Patents

Description

(54) THE USE OF ETHERS OF HYDROXYPROPIOSULPHONATE SALTS IN RECOVERING OIL FROM OIL-BEARING FORMATIONS (71) We, ETHYL CORPORATION, a Corporation organised and existing under the laws of the State of Virginia, United States of America, of 330 South Fourth Street, Richmond, State of Virginia, 23219, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to the use of certain surfactant salts to enhance the recovery of oil from oil bearing formations. In particular, the invention is concerned with surfactant salts which are highly stable against hydrolysis at elevated temperatures and for prolonged periods and are especially suitable for use in systems which contain brine and alkaline earth metal ions.

The art is voluminous and involves the use of one, two, three or more surfactants, individually or in combination. Such art is typified, for example, by U.S. Patent Specifications Nos. 3,308,068; 3,508,612; 3,527,301; 3,714,062: 3,777,818; 3,782,472; and 3,827,497; and the art cited therein. It is evident from the foregoing typical art that an extremely wide variety of surfactant and auxiliary materials is known, it also being known that the effectiveness and longevity of various materials depend upon numerous factors such as temperature, and the fortuitous or planned presence or absence of salt and of metal ions contributed by the formation, or otherwise.

The compositions disclosed in U.S. Patent Specification No. 3,827,497 have limitations with respect to temperature since they are based on or include the salts of sulfated oxyalkylated alcohol. Salts which contain the C-O-S linkage are known to be hydrolytically unstable at elevated temperatures. The sulfonates of U.S. Patent Specification No. 3,827,497 are ethyl sulfonates which are difficult to produce.

In accordance with the teachings of the present invention salts of sulfated oxyalkylated alcohol are avoided.

The invention provides a method for obtaining oil from a subterranean oil bearing formation, which method comprises contacting the formation with a compound of the formula RO(CH2CH2O)nCH2CH2CH2SO3M wherein R is alkyl having from 14 to 24 carbon atoms, n is from 2 to 10 and M is an alkali metal, amine or ammonium cation and then recovering at least a part of the available oil.

By the use of the propiosulfonate salts, compositions are provided which are easily produced and which inherently have improved hydrolytic stability at elevated temperatures, especially in comparison to sulfated ethoxylated alcohols. As a result the present compositions are resistant to hydrolysis during the prolonged time the oil recovery surfactant compositions are required to be in the ground in contact with brine and alkaline earth metal ions. This property is of particular value in situations where the oil-bearing formations are located comparatively deep within the earth's crust.

Although useful in other ways, the surfactants used in the present invention are especially useful in combination with polyalkylene glycol alkyl ether or in ternary combination with a polyalkylene glycol alkyl ether and a surfactant salt or gn organic sulfonate, such as those described in U.S.

Patent Specification No 3,827,497. On the other hand, the surfactants are characterized by being useful in aqueous solutions in general.

Preferably R in the above formula has from 16 to 20 carbon atoms. Preferably n is from 3 to 5. Preferably M is sodium.

Preferably the salt is Cl6~,8H33~370(CH2CH20)3 CH2CH2CH2SO3Na or C20H41(CH2CH2O)5 CH2CH2CH2SO3Na The foregoing salts are in general used in aqueous media so that preferred compositions for use in the present invention are aqueous solutions of the foregoing propiosulfonate salts.

In a preferred aspect, the present invention includes a method of recovering oil from a subterranean oil-bearing formation which comprises: A) injecting into the formation a saline solution containing a mixture of (a) from 0.5 to 15 percent by weight based on said solution of a surfactant salt of an organic sulfonate, (b) from 0.25 to 10 percent by weight based on said solution of a polyalkylene glycol alkyl ether, and (c) from 0.25 to 10 percent by weight based on said solution of a salt of the formula RO(CH2CH20)nCH2CH2CH2SO3M wherein R is alkyl having from 14 to 24 carbon atoms, n is from 2 to 10, and M is an alkali metal, amine or ammonium cation; and B) flooding the formation with a saline solution to effect oil recovery therefrom.

Preferably, Component (a) is an alkali metal salt of a petroleum sultonate, Component (b) is diethylene glycol hexyl ether and Component (c) is an alkali metal salt.

Preferably, the amounts of Components (a), (b) and (c) are, respectively, 2-10 percent, 1--8 percent and 1--8 percent by weight.

Preferably, the saline solution in Step B) contains a polymer to improve the mobility thereof, such polymer preferably being a polysaccharide.

The compositions can be used in combination with salts of sulfated oxyalkylated alcohol or sulfonates thereof, such as those described in U.S. Patent Specification No. 3,827,497, as an extended Thus, in some instances it may be desired to include sulfated oxyalkylated alcohol salt in the present compositions with the expectation that, as the surfactant salt containing system progresses through the formation becoming more dilute due to combination with water or brine present in the formation itself, a progressive timewise deterioration of the less stable salt of sulfated oxyalkylated alcohol can result in a preferred overall process, especially in the presence of certain alkali metal ions such as calcium or magnesium ions.

It is noteworthy that the propionate salts contain only C-C linkages, C-O-C linkages and C-S-O linkages in the fundamental skeleton thereof. Notably absent from these compositions are the unstable ester linkage

and the C-O-S linkage of sulfated oxyalkylated alcohol. The salts of the present invention are characterized by the absence of linkages which are hydrolytically unstable at elevated temperatures and by the presence in the molecule of the propiosulfonate group -CH2CH2CH2SO3M.

An outstanding advantage of such structures is that the terminal group -CH2CH2CH2SO3M can be provided by the reaction of an oxyalkylated alcohol with propane sultone, a reaction which proceeds readily under comparatively simple conditions as described in further detail hereinafter.

In general, the salts are used in aqueous solutions so as to facilitate the introduction thereof into oil-bearing formations in the earth. Typical solutions contain from about 0.005 wt. percent to about a saturation amount of the preferred salt in water; however, these concentrations may vary depending upon the method chosen for making the solution and introducing them into the formation. Solutions may be introduced into the formation separately for each individual solute or in combinations such as with surfactant salts of organic sulfonates and of polyalkylene glycol alkyl ethers, such ternary systems in water frequently being especially preferred for injection. Such systems can be used with more or less pure water or with water which contains alkali metal halides such as sodium chloride in solution; viz, brine, as well as with water containing alkaline earth metal ions, either with or without the alkali metal halide of brine.

A preferred process in accordance with the present invention involves the recovery of oil from a subterranean oil-bearing formation by injecting into the formation a saline solution containing the propiosulfonate salt of the present invention, a surfactant salt of an organic sulfonate and a polyalkylene glycol alkyl ether and then flooding the formation with an aqueous medium to effect oil recovery therefrom. Preferred flooding aqueous medium is frequently a brine or saline solution to minimize costs inherent in obtaining non-saline aqueous solutions in the field.

The organic sulfonate components useful in connection with the present invention include those described in U.S. Patent Specification No. 3,827,497; however.

preferably those having unstable structures are avoided where utmost stability of the overall system is desired. Preferably, sulfonates are used which are readily available commercially such as "Bryton Chemical F467", "Witcon Chemical TRS 10". (Bryton and Witco are Registered Trade Marks) Suitable sulfonates are usually and preferably metal salts of alkaryl sulfonates, preferably alkali metal salts and alkyl benzene sulfonates containing 12 to 30 carbon atoms. Suitable sulfonates also can be aliphatic sulfonates, alkylated naphthalene sulfonates and the like, the essential requirement being that they have surfactant properties. The cationic portion of such sulfonates can be ammonium or amine as well as alkali metal but is usually and preferably sodium. The molecular weight of the organic sulfonate surfactant is usually from 300--600, preferably from 35W525. These materials can be prepared by well known procedures such as those described in U.S. Patent Specification No 3,308,068. They can be prepared synthetically or can be prepared from petroleum and are commonly known as petroleum sulfonates.

The polyalkylene glycol monoalkyl ethers are widely available commercially. The alkylene group is usually ethylene but can be propylene or others up to about five carbon atoms. It can repeat itself up to about 10 times (i.e. the "poly" can be up to about 10) but usually repeats itself 2-6 times, more usually twice, e.g. diethylene. It should also be understood that for any specified polyalkylene, the number of alkylene units is either exactly as specified or varies but the average is as specified. This same principle also applies to the alkyl group. Preferably the glycol portion is diethylene glycol. The alkyl group will normally contain 2-12 carbon atoms, preferably W10. In general the more alkylene units or the longer the alkylene unit, the longer the alkyl group should be.

The preferred component is diethylene glycol hexyl ether. These materials are available commercially or can be made by known procedures.

Propiosulfonate salts for use in the present invention typically are readily prepared by the reaction of ethoxylated alcohols with propane sultone. This introduces the propiosulfonate structure readily and at good yields. The ethoxylated alcohols may be produced by reacting an alcohol ROH with ethylene oxide. Preferably. the alcohol used is predominantly a primary alkanol, more preferably a svnthetic primary alcohol mixture containing up to about 50 percent of branched primary alcohols.

The compositions used in the present invention provide stable emulsions in various ways. As an example. an aqueous system containing lOwt. percent C1618H3337O(EO)3(CH2)3SO3Na, with 1 1/2wt. percent NaCI added, 2 cc of C6H13O(EO)2H added and 20 percent by volume of crude oil added was stable.

The invention will now be further described and illustrated with reference to the accompanying drawings and the following Examples. The various Figures of the drawings show stability characteristics for various compositions embodying the features of the present invention as well as for comparative compositions to show the superiority of the present compositions over the prior art.

Figure 1 shows the stability characteristics of compositions utilizing the salt C,6~,6H33~37OtEO)3(CH2)3SO3Na at 75"F; Figure 2 shows the stability properties of the salt of Figure 1 at a temperature of 150OF Figure 3 shows the stability properties of the salt C20+H41+O(EO)5(CH2)3SO3Na at 75OF; Figure 4 shows the stability properties of the composition of Figure 3 at 1500F: Figure 5 shows unsatisfactory stability properties of a comparative salt C20+H41 +O(EO)5(CH2)2SO3Na at 750 F: Figure 6 shows the solubility properties of the salt of Figure 5 at 1500F; Figure 7 shows the stability properties of the salt C20+H4,+O(EO),s(CH2)3SO3Na at 750F; Figure 8 shows the properties of the salt of Figure 7 at 150OF: Figures 9 to 14 show the stability properties of the salt of Figure 1 at 750F and 150OF with three additional petroleum sulfonates.

Propiosulfonates prepared as described in the following Examples and comparative samples were tested using the following procedure.

To a 4-ounce bottle was added 2.5 grams of petroleum sulfonate (Petronate L of Witco Chemical Company) which is a petroleum sulfonate of415 to 430 equivalent weight range.

Then 1 gram of hexanol ethoxylate averaging two ethoxy groups per molecule was added.

Next, 1.5 grams (active basis) of candidate propiosulfonate composition or comparative material was added preferably as an aqueous solution of 10--40 percent concentration, or as 100 percent active material.

Next the appropriate amount of sodium chloride was added preferably as a 10 percent aqueous solution to give the desired final concentration of salt (brine).

Similarly, the appropriate amount of calcium ion was added preferably as a 1 per cent solution of calcium ion prepared as calcium chloride.

The sample was then adjusted to 100 grams to provide the desired concentration of the ingredients. The sample was then heated and stirred vigorously in a sealed system to prevent evaporation of any of the components and when dispersion or dissolution was complete the samples were placed in an environment of the desired temperature and observed after 24 hours or more. Observations were made as to the stability or instability of the system.

Instability in general is a definite and easily visible phase separation so that a curd of water-insoluble material separates either at the bottom or the top depending on the brine concentrations. Generally, all solutions are opaque. Satisfactory conditions are stable dispersions. Rarely, clear systems are obtained and indicated by "C". Those which showed good results were tested further by dilution with the appropriate sodium chloride and calcium ion solutions at proportions of 1:1, 1:2, 1:5 and 1:10, and reexamined at the indicated temperatures for stability or instability.

Samples were tested with 1, 2, 3, 4 and 5 percent by weight of sodium chloride and with 200, 500, 1000 and 2000 parts per million of calcium ion. Data are shown on the Figures for the various contents of sodium chloride, calcium ion and for the various dilutions of O, 1:1 with brine and calcium ion solution, 1:2 with brine and calcium ion solution, etc.

In the following Examples, Examples III and IV are given for comparative purposes.

In the formulae in the following Examples a "+" sign indicates that whilst the compound is largely comprised of molecules of the formula given a small proportion of higher homologues is present.

Example I C1618H337O(CH2CH2O)2(CH2)3SO3Na To a 12-liter round bottom flask equipped with an agitator and an off-gas trap was charged 1312 grams of C,6- ,8H33~37O(CH2CH2O)3H averaging three ethoxy units per molecule. The average molecular weight of the alcohol prior to ethoxylation was 260.0. To the flask was also added 421 grams of sodium bicarbonate and 2 liters of xylene, the latter being added to azeotrope water.

The flask was heated to 1200C and feed of propane sultone started. 435 cc of propane sultone was added over a 3 hour period, the reaction temperature being held at about 140--1450C.

Agitation was continued at a temperature of 14" 145PC for an additional 3-1/2 hours. The heat was then turned off, the flask contents diluted with 5.2 liters of water, and the contents of the flask stirred for 1-1/2 days.

The contents of the flask were then stripped by passing in nitrogen at from 0.72 to 0.19 liters per minute for about I hour.

A vacuum was then applied, a vacuum of 15 to 18 millimeters of mercury being maintained for about 2 hours. The heat was turned off at about the midpoint of the two hours vacuum treating period.

The product was then diluted to 26 wt.

percent product in water.

The resulting Ct6~,8 propiosulfonate was then tested as described. Data are presented in Figs. 1 and 2 for 75"F and 150OF, respectively.

Example II C20eH41:O(CH2CH2O)6(CH2)3SO3Na 99 grams of C20+H41+O(CH2CH2O)H, containing 25 percent hydrocarbon, -QH value (wt. percent) 2.17, average molecular weight 783 (126 millimols) was charged to a l-liter flask equipped with a magnetic stirrer.

27 grams (221 millimols) of propane sultone was added.

18.7 grams (221 millimols) of sodium bicarbonate was added.

The mixture was stirred overnight at 130"C.

The crude product was dissolved in a mixture of 500 ml H2O and 500 ml methyl ethyl ketone and extraced three times with 150 ml portions of a 2:1 volume mixture of methyl ethyl ketone and hexane (100 ml methyl ethyl ketone +50 ml hexane). The mixture was heated to separate the phases.

The aqueous phase was evaporated overnight on a hot plate to dryness giving 64 grams of crude product.

The crude product was desalted with 1600 ml of 50 percent aqueous isopropanol and 224 grams Na2CO3. The solvent was evaporated and product dried in oven overnight. The product was analyzed by infrared and wet chemical analysis and found to be consistent for the product.

The resulting C20propiosulfonate was then tested as described. Data are presented in Figs. 3 and 4 for 75"F and 150OF.

respectively.

Example Ill (Comparative) C20 H41,O(CH2CH20)sCH2CH2SOlNa 70.7 grams (90.3 millimols) of the C20 H41. O(CH2CH2O)sH ethoxylated alcohol of Example II was placed in a flask and heated with 18.0 grams (77.4 millimols) of sodium methoxide as a 24 percent solution in methanol with stirring for 1.5 hours at 1300C and 200 mm pressure. This produces the ethoxy alcohol alkoxide.

17.7 grams (77.4 millimols) of bromoethane sulfonate (BrCH2CH2SO3Na) in water was azeotroped with toluene to remove the water.

The ethoxy alcohol alkoxide was then added to the bromoethane sulfonate. About 40 ml undecane was added to facilitate stirring and the mixture was heated to 200"C and stirred for 3 hours.

The reaction mixture was cooled to room temperature and dissolved in 700 ml of 50 percent aqueous methyl ethyl ketone and extracted twice with 200 ml portions of 3:1 methyl ethyl ketone:hexane.

The aqueous solution was evaporated to about 150 ml and then desalted with 900 ml of 50 percent aqueous isopropanol and 126 grams of Na2CO3. The system was then evaporated overnight to dryness in a 1200 oven.

The resulting C20: ethyl sulfonate was then tested as described. Data are presented in Figs. 5 and 6 for 75"F and 1500F, respectively and are seen to be inferior to the propiosulfonates of Figs. 14.

Example IV (Comparative) C2o+H4r, O(CH2CH20), s(CH2)3SO3Na 99 grams of C20. H41. O(CH2CH2O)1sH, containing 15 percent by hydrocarbon, --OH value (wt. percent) 1.39, average molecular weight 1223 (81 millimols) was charged to a l-liter flask equipped with a magnetic stirrer.

To the flask was added 14.8 grams (122 millimols) of propane sultone and 10.3 grams (122 millimols) of sodium bicarbonate.

The mixture was stirred for 8 hours at 135"C and allowed to stand overnight.

Product work-up was as described in Example II.

The resulting product was tested as described. Data are presented in Figs. 7 and 8 for 75"F and 150OF, respectively. Results with a propiosulfonate having 15 ethylene oxide units per molecule are shown to be inferior to those with the propiosulfonates of Figs. I4.

From the foregoing, it is evident that the features of the present invention may be employed in various ways. Although the C20.

and C,6~18 salt compositions are preferred mixtures, individual salts such as those wherein the alkyl group has 16, 18, 20 or 22, carbon atoms may be used.

Examples V--VII C1618H3337O(CH2CH2O)3(CH2)3SO3Na The above propiosulfonate as prepared in Example I was tested with different petroleum sulfonates with different proportions.

For each of the following examples, instead of 2.5 grams of the petroleum sulfonate, 3.0 gram was used. As before, one gram of the hexanol ethoxylate averaging two ethoxy groups per molecule was used.

Instead of 1.5 grams (active basis) of candidate propiosulfonate, 1.0 grams was used. Otherwise the procedure described previously was followed at 1, 3 and 5 percent by weight of sodium chloride and with 200, 500, 1000 and 2000 parts per million of calcium ion, the testing for the non-diluted sample of Example VI (figs. 11 and 12) being extended to include 6, 8, 10 and 12 percent by weight of sodium chloride and 5000 and 10,000 parts per million of calcium ion to show how far stability extended beyond the ranges of usual interest.

Example V The petroleum sulfonate used was Petrostep 420 of Stepan Chemical Company which is a 60 percent active material having 21 percent free oil, 17.5 percent water, 1.5 percent inorganic salts and an approximate equivalent weight of 420. Data are shown for 750F and 150OF by Figs. 9 and 10, respectively.

Example VI The petroleum sulfonate used was Petrostep 450 of Stepan Chemical Company which is a 60 percent active material having 13.5 percent free oil, 24.5 percent water, 2.0 percent inorganic salts and an approximate equivalent weight of 450. Data are shown for 75"F and 150OF by Figs. 11 and 12, respectively.

Example VII The petroleum sulfonate used was Petrostep 465 of Stepan Chemical Company which is a 60 percent active material having 13.1 percent free oil, 23.5 percent water, 3.6 percent inorganic salts and an approximate equivalent weight of 465. Data are shown for 75"F and 150OF by Figs. 13 and 14, respectively.

WHAT WE CLAIM IS: 1. A method for obtaining oil from a subterranean oil-bearing formation, which method comprises contacting the formation with a compound of the formula RO(CH2CH2O)CH2CH2CH2SO3M wherein R is alkyl having from 14 to 24 carbon atoms, n is from 2 to 10 and M is an alkali metal, amine or ammonium cation

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (18)

**WARNING** start of CLMS field may overlap end of DESC **. Example Ill (Comparative) C20 H41,O(CH2CH20)sCH2CH2SOlNa 70.7 grams (90.3 millimols) of the C20 H41. O(CH2CH2O)sH ethoxylated alcohol of Example II was placed in a flask and heated with 18.0 grams (77.4 millimols) of sodium methoxide as a 24 percent solution in methanol with stirring for 1.5 hours at 1300C and 200 mm pressure. This produces the ethoxy alcohol alkoxide. 17.7 grams (77.4 millimols) of bromoethane sulfonate (BrCH2CH2SO3Na) in water was azeotroped with toluene to remove the water. The ethoxy alcohol alkoxide was then added to the bromoethane sulfonate. About 40 ml undecane was added to facilitate stirring and the mixture was heated to 200"C and stirred for 3 hours. The reaction mixture was cooled to room temperature and dissolved in 700 ml of 50 percent aqueous methyl ethyl ketone and extracted twice with 200 ml portions of 3:1 methyl ethyl ketone:hexane. The aqueous solution was evaporated to about 150 ml and then desalted with 900 ml of 50 percent aqueous isopropanol and 126 grams of Na2CO3. The system was then evaporated overnight to dryness in a 1200 oven. The resulting C20: ethyl sulfonate was then tested as described. Data are presented in Figs. 5 and 6 for 75"F and 1500F, respectively and are seen to be inferior to the propiosulfonates of Figs. 14. Example IV (Comparative) C2o+H4r, O(CH2CH20), s(CH2)3SO3Na 99 grams of C20. H41. O(CH2CH2O)1sH, containing 15 percent by hydrocarbon, --OH value (wt. percent) 1.39, average molecular weight 1223 (81 millimols) was charged to a l-liter flask equipped with a magnetic stirrer. To the flask was added 14.8 grams (122 millimols) of propane sultone and 10.3 grams (122 millimols) of sodium bicarbonate. The mixture was stirred for 8 hours at 135"C and allowed to stand overnight. Product work-up was as described in Example II. The resulting product was tested as described. Data are presented in Figs. 7 and 8 for 75"F and 150OF, respectively. Results with a propiosulfonate having 15 ethylene oxide units per molecule are shown to be inferior to those with the propiosulfonates of Figs. I4. From the foregoing, it is evident that the features of the present invention may be employed in various ways. Although the C20. and C,6~18 salt compositions are preferred mixtures, individual salts such as those wherein the alkyl group has 16, 18, 20 or 22, carbon atoms may be used. Examples V--VII C1618H3337O(CH2CH2O)3(CH2)3SO3Na The above propiosulfonate as prepared in Example I was tested with different petroleum sulfonates with different proportions. For each of the following examples, instead of 2.5 grams of the petroleum sulfonate, 3.0 gram was used. As before, one gram of the hexanol ethoxylate averaging two ethoxy groups per molecule was used. Instead of 1.5 grams (active basis) of candidate propiosulfonate, 1.0 grams was used. Otherwise the procedure described previously was followed at 1, 3 and 5 percent by weight of sodium chloride and with 200, 500, 1000 and 2000 parts per million of calcium ion, the testing for the non-diluted sample of Example VI (figs. 11 and 12) being extended to include 6, 8, 10 and 12 percent by weight of sodium chloride and 5000 and 10,000 parts per million of calcium ion to show how far stability extended beyond the ranges of usual interest. Example V The petroleum sulfonate used was Petrostep 420 of Stepan Chemical Company which is a 60 percent active material having 21 percent free oil, 17.5 percent water, 1.5 percent inorganic salts and an approximate equivalent weight of 420. Data are shown for 750F and 150OF by Figs. 9 and 10, respectively. Example VI The petroleum sulfonate used was Petrostep 450 of Stepan Chemical Company which is a 60 percent active material having 13.5 percent free oil, 24.5 percent water, 2.0 percent inorganic salts and an approximate equivalent weight of 450. Data are shown for 75"F and 150OF by Figs. 11 and 12, respectively. Example VII The petroleum sulfonate used was Petrostep 465 of Stepan Chemical Company which is a 60 percent active material having 13.1 percent free oil, 23.5 percent water, 3.6 percent inorganic salts and an approximate equivalent weight of 465. Data are shown for 75"F and 150OF by Figs. 13 and 14, respectively. WHAT WE CLAIM IS:

1. A method for obtaining oil from a subterranean oil-bearing formation, which method comprises contacting the formation with a compound of the formula RO(CH2CH2O)CH2CH2CH2SO3M wherein R is alkyl having from 14 to 24 carbon atoms, n is from 2 to 10 and M is an alkali metal, amine or ammonium cation

and then recovering at least a part of the available oil.

2. A method as claimed in claim 1, wherein R has from 16 to 20 carbon atoms.

3. A method as claimed in claim I or claim 2, wherein n is from 3 to 5.

4. A method as claimed in any preceding claim, wherein M is sodium.

5. A method as claimed in claim 1, wherein the said compound has the formula C,~18H33~37O(CH2CH2O)3CH2CH2CHs2o N Sb3Na

6. A method as claimed in claim 1, wherein the said compound has the formula C20H4tO(CH2CH20)3CH2CH2CH2SO3Na

7. A method as claimed in any preceding claim, wherein the compound is contacted with the formation in the form of a composition containing water.

8. A method as claimed in claim 7, wherein the composition also contains a polyalkylene glycol alkyl ether.

9. A method as claimed in claim 8, wherein the polyalkylene glycol alkyl ether is diethylene glycol hexyl ether.

10. A method as claimed in any one of claims 7 to 9 wherein the composition also contains a surfactant salt of an organic sulfonate.

11. A method as claimed in claim 10, wherein the surfactant salt of the organic sulfonate is an alkali metal salt of a petroleum sulphonate.

12. A method as claimed in any one of claims 7 to 11, wherein the composition also contains sodium chloride.

13. A method as claimed in claim I, wherein the composition is a saline solution containing 0.5 to 15 percent by weight based on the solution of a surfactant salt of an organic sulfonate, 0.25 to 10 percent by weight based on the solution of a polyalkylene glycol alkyl ether and 0.25 to 10 percent by weight based on the solution of a compound as defined in any one of claims 1 to 6.

14. A method as claimed in any preceding claim, wherein oil recovery is effected by flooding the formation with a saline solution.

15. A method as claimed in claim 14, wherein the saline solution contains a polymer to improve the mobility thereof.

16. A method as claimed in claim 15, wherein the polymer is a polysaccaride.

17. A method as claimed in claim 1, substantially as hereinbefore described.

18. Oil or an oil-containing material which has been obtained from a subterranean oil-bearing formation by a method as claimed in any preceding claim.