US2966457A - Gelled fracturing fluids - Google Patents

Description

. GELLED FRACTURING FLUIDS Raymond W. Starmann, Evergreen Park, and Paul Gibson, Chicago, 111., as'signors to Swift & Company, Chicago, 111., a corporation of Illinois No Drawing. Filed May 8, 1956,Ser. No. 583,367

12 Claims. (Cl. 2528.55)

This invention in general relates to hydraulic pressure treatment of subterranean gas and/ or oil bearing formations. More particularly, the invention relates to means for providing gels of liquid hydrocarbons-said gels having improved consistency in viscosity-temperature relationships and adapted for use in high temperature wells where viscosity build-up has heretofore been a problem.

The process which has become known as hydraulic fracture, or hydrofracing as it is often called, of oil or gas bearing formations is achieved by exertion of increasing quantities of pressure on a liquid in the well bore until pressure is built up sufliciently to produce What is thought to be a fracture of the formation. It is generally ac- ;cepted that fracture usually occurs along horizontal bedding planes where the tensile strength of the formation is usually the weakest, but occasionally, and particularly in non-Stratified or unconsolidated formations, fracture is thought to occur along a vertical plane. In the process, fluid is pumped into the well at a rate which causes the pressure to rise until there is a sharp break in pressure (in theory the point of fracture) after which the pressure remains fairly constant at a low level as the fluid in theory penetrates the new crack. Thereafter, the pressure is relieved, and the well is put into production.

It has been established that the release of hydraulic pressure after the sharp break in pressure does not result in return of the formation to its original condition, but rather, the formation exhibits a substantial increase in permeability. However, it has been established that the permeability of the formation may be further increased ,bythe use of oil-inert small particles such as sand, metal chips, crushed shell, etc. These particles are suspended in the hydraulic fluid. Under the fracture theory, they enter the formation with the fluid at the point of fracture. When the pressure is released and the fracturing fluid flows from the formation, these materials remain to prop the formation apart to some extent at the point of fracture.

The hydraulic fluids which are generally used are miscible with the interstitial crude oil, e.g., oily liquids,-such as crude petroleum oils, a refined fraction of crude oils, e.g., gasoline, kerosene, diesel oil, naptha, etc., and, much less often than the above, animal, vegetable and fish triglyceride oils.

The viscosity of these fluids often is increased by the addition of other compositions to'the base fluid. In the case of petroleum oils or its fractions, gelling agents, which tend to thicken the oil, enable the suspension of the small particles in the oil. Soaps of fatty acids such as alkali metal soaps of red oil and tall oil fatty acids commonly are used as gelling agents in petroleum base fluids. In addition to alkali metal soaps, other 'fatty acid soaps of metals of the alkali metal and alkaline earth series, or polyvalent metals such as cadmium, mercury, cobalt, lead, nickel and aluminum have been suggested as gelling agents for petroleum base fracturing fluids. Recently, the use as gelling agents for liquid hydrocarbons of alkali metal soaps of marine oil fatty acids (such as menhaden oil, herring oil, whale oil, seal oil, tuna oil, etc.), preferably fully hydrogenated, and mixtures thereof with other higher fatty acids in ratios such that the C and above acid content is at least 10%, has been developed. This subject matter is disclosed and claimed in a copending application of Charles S. Steiner and Paul Gibson, Serial No. 583,352, filed concurrently herewith, for an invention entitled Liquid Hydrocarbon Gels and Uses Thereof."

We are aware that the theory of hydraulic fracture is not universally accepted. There are those who propose that the fluid jets issuing from perforations in the casing at the pay zone etch cuts through the blocked area immediately surrounding the bore hole to the more permeable areas more remote from the bore. Others have proposed that the hydraulic fluid drives the water block immediately surrounding the hole into the more remote areas of the formation Without actual fracture. Regardless of theory, it has been observed that the application of increasing pressure to a hydraulic fluid in the well bore results in a sharp break in pressure, after which the pressure levels off at a substantially constant lower level. This type of treatment of pay zones in general results in increased production of oil and/or gas. In View of the fact that the term hydraulic fracture has been widely accepted by those in the art to describe the foregoing process of well treatment, we shall employ the same terminology to describe the process of hydraulic pressure treatment with the understanding that We are not limited specifically to any particular theory.

The process of the instant invention comprises the addition of an inorganic ammonium salt to a gelled liquid hydrocarbon, the gelling agent comprising an alkali metal soap of a higher aliphatic monocarboxylic acid having between 14 and 24 carbons. Tall oil fatty acids, which contain some abietic acid, are considered to be a low grade of fatty acids for purposes of this invention.

We have noted that with the aforementioned gelling agents, particularly with the saturated acids, high gel viscosities develop at temperatures in the range of 220 to 250 F. as what appears to be a secondary gel formation occurs. When wells having high bottom hole temperatures are fractured with these gelled fluids, the high temperatures cause the gels to thicken to the point where they become difiicult to remove. This thickening can occur either in the formation or as the fracture is brought to the surface after completion of the fracture operation. In addition, the gels become markedly more fluid (less viscous) as the temperature increases up to the point of viscosity build-up as secondary gel formation begins, and sand suspension properties thereby diminish.

We have discovered that the addition of an inorganic ammonium salt to the gel improves the fluid characteristics of the gel with increasing temperature. The characteristic loss in viscosity is substantially lessened by the presence of the ammonium salt. In addition, the ammonium salt prevents the formation of the highly viscous secondary gel at temperatures in the range of 220-250 F.-'-a problem of serious proportions particularly in gels made with soaps of saturated fatty acids.

It is important in the perforamnce of the process of this invention that the ammonium salt is added after the gel is formed. The salt may be in either solid form or in concentrated aqueous solution. While we do not wish to be limited to any particular theory, it would appear that when the temperature of the gel is raised sufiiciently,

'base exchange occurs between the alkali metal and ammonium ions. When this occurs, the gel breaks because the ammonium soap is not temperature stable insofar as gel properties are concerned.

Thus, the invention in practical use involves the formation of the gelled petroleum oil, either crude or a refined fraction thereof, by the addition of caustic to the oil containing between about 0.5-3.0% by weight of the aforementioned acids. Sand or other propping agents are added either before or after the gelation. After the gel has formed, the ammonium salt, preferably in aqueous solution, is added to the tank and mixed therewith. About 1-3 equivalents of ammonium salt is added per equivalent of caustic used. This is roughly 1.5-5 .0 parts by weight ofammonium salt per part of caustic. In the final gelled composition, the ammonium salt is present in, about 1-3 equivalents per mol of alkali metal, the latter being present as the cation in the soap and in excess caustic. The fluidis then ready to be pumped into the well to exert pressure on the formation.

The ammonium salts are preferably salts of a mineral acid such as ammonium sulfate, ammonium chloride, ammonium nitrate or ammonium phosphate. However, other ammonium salts such as ammonium acetate, ammonium citrate, ammonium carbonate, ammonium oxalate, ammonium bromide, etc., may be used if desired.

The decomposition temperature of the inorganic salt has an effect on the temperature at which the gel breaks. With salts of strong acids, the gel breaks between about 190240 F.; whereas with the less stable ammonium carbonate, destruction of the gel occurs at much lower temperatures.

The following examples illustrate gel compositions falling within the scope of the present invention, it being understood that such examples are in nowise limitative of the scope of our invention. All parts are by weight.

Example I Kerosene 100.0 Gelling agent:

Red oil 2.0 Caustic 0.5 Ammonium sulfate 1.5

Example II Kerosene 100.0 Gelling agent:

Hydrogenated herring oil fatty acids 1.5 Caustic 0.5 Ammonium nitrate 1.2

Example III Low asphaltic crude oil 100.0 Gelling agent:

Hydrogenated menhaden oil fatty acids 2.5 Caustic 1.5 Ammonium sulfate 6.0

Example IV Naphtha 100.0 Gelling agent:

Tall oil fatty acids 3.0 Caustic 0.5 Ammonium acetate 2.5

Example V Gasoline 100.0 Gelling agent:

Hydrogenated menhaden oil fatty acids 2.5 Caustic 0.5 Ammonium phosphate 2.0

Obviously, many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope thereof, and there fore, only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. In a method of hydraulic pressure treatment of subterranean formations, the steps which comprise: mixing liquid hydrocarbon, aliphatic, monocarboxylic acids having 14-24 carbon atoms, and caustic to form a gelled liquid hydrocarbon, adding to the gel an ammonium salt capable of undergoing a base exchange with said caustic, and thereafter pumping said gelled fluid into the formation until there is a sharp break in pressure.

2. In a method of hydraulic pressure treatment of subterranean formations having high temperatures, the steps which comprise: mixing liquid hydrocarbon, aliphatic, monocarboxylic acids having 14-24 carbon atoms, and caustic to form a gelled liquid hydrocarbon, adding to the gel 1-3 equivalents of an ammonium salt per equivalent of caustic, and thereafter pumping said gelled fluid into the formation until there is a sharp break in pressure.

3. A gelled composition which comprises: a liquid hydrocarbon gelled with a gelling agent comprising an alkali metal soap of a mixture of aliphatic monocarboxylic acids having between 14 and 24 carbon atoms; 1-3 equivalents of an ammonium salt of a mineral acid per mol of alkali metal in said gel; and oil-inert small particles suspended therein.

4. A gelled composition which comprises: a liquid hydrocarbon gelled with a gelling agent comprising an alkali metal soap of higher aliphatic monocarboxylic acids; a solution of an inorganic ammonium salt containing about l-3 equivalents of ammonium per equivalent of alkali metal; and oil-inert small particles suspended there- 5. The composition of claim 4 wherein the ammonium salt is ammonium sulfate. 7

6. The composition of claim 4 wherein the ammonium salt is ammonium nitrate.

7. The composition of claim 4 wherein the ammonium salt is ammonium chloride.

8. The composition of claim 4 wherein the ammonium salt is ammonium phosphate.

9. A gelled composition which comprises: a liquid hydrocarbon gelled with 0.5-3.0% by weight of a gelling agent comprising an alkali metal soap of an aliphatic monocarboxylic acid having between 14 and 24 carbon atoms, dispersed in said gel; an aqueous solution of an inorganic ammonium salt in an amount to provide 1-3 equivalents of said salt per mol of alkali metal in said gel; and oil-inert small particles suspended therein.

10. The composition of claim 9' wherein said ammonium salt is a member of the group consisting of ammonium sulfate, ammonium nitrate, ammonium chloride, and ammonium phosphate.

11. The composition of claim 9- wherein said aliphatic acids are hydrogenated marine oil fatty acids having substantial amounts of C and above acids.

12. The composition of claim 11 wherein said ammonium salt is a member selected from the group consisting of ammonium sulfate, ammonium nitrate, ammonium chloride, and ammonium phosphate.

References Cited in the file of this patent UNITED STATES PATENTS 2,595,557 Worth et al. May 6, 1952 2,596,844 Clark May 13, 1952 2,667,457 McChrystal Jan. 26, 1954 2,753,364 Boner et al. July 3, 1956 2,779,735 Brown et al. Jan. 29, 1957

Claims (1)

1. 3. A GELLED COMPOSITION WHICH COMPRISES: A LIQUID HYDROCARBON GELLED WITH A GELLING AGENT COMPRISING AN ALKALI METAL SOAP OF A MIXTURE OF ALIPHATIC MONOCABOXYLIC ACIDS HAVING BETWEEN 14 AND 24 CARBON ATOMS, 1-3 EQUIVALENTS OF AN AMMONIUM SALT OF A MINERAL ACID PER MOL OF ALKALI METAL IN SAID GEL, AND OIL-INERT SMALL PARTICLES SUSPENDED THEREIN.