GB2273484A

GB2273484A - Encapsulated acid composition and process for well stimulation using same

Info

Publication number: GB2273484A
Application number: GB9324145A
Authority: GB
Inventors: Valmore Rodriguez; Vincenzo Savino
Original assignee: Intevep SA
Current assignee: Intevep SA
Priority date: 1992-11-25
Filing date: 1993-11-24
Publication date: 1994-06-22
Anticipated expiration: 2013-11-24
Also published as: CA2102769A1; GB2273484B; CA2102769C; DE4339801A1; GB9324145D0

Description

2273484 ENUPSULATED ACID COMPOSITION AND PROCESS FOR PREPARING AND USING

SAME

The invention generally relates to the field of well stimulation and, more particularly provides an encapsulated acid composition for use, particularly, but not exclusively, in the acid treatment of wells, especially hydrocarbon producing wells, and a process for preparing such a composition.

Well stimulation refers generally to the treatment of wells in order to improve production from such wells. Numerous methods and techniques are known for stimulating such wells. one field of well stimulation is known as acid treatment. This technique generally involves pumping an acid solution into a formation at a rate which stresses the formation beyond its structural strength and thereby fractures the formation. The acid solution then flows into such fractures, where the acid "etches" the walls of the fracture. After pressure is returned to normal, the fracture closes but the walls no longer line up due to the acid etching. Thus, new channels are formed from the formation into the well bore and production is enhanced.

Acids typically used in such processes include hydrofluoric, hydrochloric, formic, acetic and mono, di and trichloroacetic acids in aqueous solution having an acid concentration typically in the range of fromabout 3 to 28%. It has been found, however, that increased formation temperatures combined with the relatively strong acid concentrations results in a large and unacceptable corrosion rate on the tubing strings of the well.

1 One known method for preventing this corrosion involves the use of corrosion inhibitors which can control the corrosion rate for tubing fabricated from low carbon steel, duplex steel and low chromium steel up to maximum temperatures of about 300OF (150OC). In wells which produce H2S and C02, however, high chromium tubing is typically used. Further, corrosion inhibitors do not protect such high chromium tubing from corrosion rates which are so high that acid treating such wells involves a significant risk of losing the well.

We have now found it possible:

to provide a composition for acid treating a formation without exposing tubing in the formation to increased corrosion from acid; to provide a process for preparing such a Composition; and to provide a process for using such a composition in an acid treatment process.

Other aims and advantages will appear hereinbelow.

According to one aspect of the present invention there is provided a composition comprising an aqueous solution of acid encapsulated in a polymeric material which is chemically resistant to the acid in the aqueous solution of acid and which Polymeric material is substantially impermeable to acid and water.

Preferably, the polymeric material is heat sensitive and 'is preferably formulated so as to provide a desired temperature at whi.ch the polymeric material ruptures so as to release the aqueous solution of acid. Such a desired temperature may be, for example, about 2000C. Such a composition can be mixed with any carrier fluid such as water or methanol and pumped into a formation. The acid solution remains encapsulated, and therefore cannot damage the tubing, unzil the desired temperature is reached, the polymeric material then ruptures, and the acid solution is released into the formation.

According to another aspect of the present invention, such a composition is prepared in a process comprising the steps of: forming an emulsion having a continuous phase of oil and a dispersed phase of an aqueous solution of acid; mixing said emulsion with a polymer which is soluble in said oil and insoluble in said aqueous solution of acid, said Polymer being chemically resistant to said acid in said aqueous solution of acid, and said polymer being substantially impervious to said acid and to water; and mixing said emulsion and said polymer with a cross-linking agent so as to encapsulate droplets of said dispersed phase of said aqueous solution of acid in a cross linked polymer encapsulant.

4 According to yet another aspect of the present invention there is provided a process for treating a subterranean formation, comprising the steps of: mixing an encapsulated acid composition, comprising an aqueous solution of acid encapsulated in a polymeric material which is chemically resistant to said acid in said aqueous solution of acid and which polymeric material is substantially impermeable to water and acid, with a carrier fluid; and injecting said fluid into said formation, whereby elevated temperatures of said formation cause said encapsulated acid composition to rupture, thereby contacting said aqueous solution of acid with said formation.

According to a further aspect of the present invention there is provided a process for neutralizing a basic solution, comprising the steps of: mixing said basic solution with an encapsulated acid composition, comprising an aqueous solution of acid encapsulated in a polymeric material which is chemically resistant to said acid in said aqueous solution of acid and which polymeric material is substantially impermeable to water and acid; heating said mixture so as to cause said encapsulated acid composition to rupture, thereby mixing said aqueous acid solution with said basic solution so as to neutralize said basic solution.

Various preferred features and embodiments of the present invention will now be described by way of non-limiting example.

The invention provides a composition for use in the acid treatment of subterranean formations, especially for stimulating the production of hydrocarbon wells drilled into such formations. Acid treatment generally induces new fractures into the formation and "etches" the new fractures so as to form new flow areas from the formation into the well bore, thus enhancing production.

The present invention is drawn to a composition comprising an encapsulated acid which prevents the acid from damaging tubing and other well equipment while being pumped into the formation, and which composition then releases the acid into the formation as desired.

According to the invention, such a composition comprises an aqueous solution of acid encapsulated in a polymeric material. The polymeric material is preferably chemically resistant to the encapsulated acid so as to prevent the acid from breaking through the polymeric material. The polymeric material is also preferably impervious to water and acid so as to prevent water from diluting the aqueous acid solution and to prevent the acid form escaping through the polymeric material.

The acid to be encapsulated should be selected based upon the composition of the formation to be treated. The acid should also preferably be oil insoluble for reasons which will become apparent during the following discussion of the process for preparing the composition. Suitable acids include mineral acids, preff-.-ably 6 hydrofluoric acid, hydrochloric acid, and mixtures thereof. The concentration of acid in the aqueous solution may preferably be based upon the desired amount of Oetching" and the susceptibility of the formation to the acid. The aqueous solution of acid may preferably have an acid concentration of up to about 15% by weight.

The polymeric material which encapsulates the acid is preferably a crosslinked polymeric material such as, for example, cured epoxy resin, vinyl ester, and polyester. Such cross-linked polymeric material may preferably be cross-linked with a cross-linking agent which may preferably be aliphatic amides, aromatic amides, and mixtures thereof. More preferably, the cross-linking agent could suitably be diethylenetriamine, triethylenetriamine, melamine, formaldehyde butylamine, and mixtures thereof. Styrene and divinylbenzene are preferred cross-linking agents for vinyl ester and polyester polymeric material.

According to the invention, the cross-linked polymer material is heat sensitive, that is, it ruptures at a particular temperature so as to release the aqueous acid solution. Thus, the cross-linked polymer is formulated so as to provide an appropriate rupture jemperature, that is, the cross-linked polymer will preferably have a rupture temperature higher than the temperature in the tubing, and lower than the formation temperature. By rupture temperature is meant the temperature at which the polymeric material decomposes or disintegrates or is otherwise broken so as to release the aqueous acid solution. The rupture temperature may, for example, suitably by about 2000C.

The composition may preferably have a particle size in the range of about 0.5 to 3000 gm, more preferably between about 1 to 100 gm, and more preferably still between about 2 to 30 gm.

such a composition may suitably be used in any acid treatment of a well where it is desirable to prevent contact between the acid and the tubing and pumping equipment. Such a composition also serves to preserve the acid solution and prevent its dilution or breaking down before contact with the formation.

The composition may preferably be used in an acid treatment by mixing the composition with a carrier fluid such as, for example, water, methanol, or mixtures thereof, and pumping the mixture into the formation to be treated. The acid is encapsulated and therefor does not contact the tubing or pumping equipment, thereby avoiding the aforesaid corrosion. Further, the forration temperature heats the mixture to the noint where the polymeric material ruptures to free the acid solution. After the fluid including composition has been pumped into the formation for the desired time, the fluid and composition is preferably pumped out of the formation and tubing, thus leaving the formation enhanced or stimulated, and capable of improved production.

It should be noted that in another embodiment the composition of the present invention may also suitably be used to neutralize a basic solution. In such a use, the composition may be mixed with the solution, suitably in an amount sufficient to provide a neutralizing amount of acid. The mixture can then be heated to a temperature exceeding the rupture temperature of the composition so as to release the acid solution, allowing same to mix with the basic solution and thereby neutralize the basic solution.

According to the invention, the encapsulated acid composition may be prepared as follows:

First, an emulsion may be formed having a continuous phase of oil and a dispersed phase of an aqueous acid solution. The emulsion may preferably be formed having a ratio of oil continuous phase to aqueous acid solution dispersed phase of between about 95:5 to 70:30.

The continuous oil phase may be any oil solution which forms a stable emulsion with the aqueous acid solution and which does not interfere with the subsequently described cross-linking step. A suitable oil may be, for example, any isomer of xylene.

The dispersed aqueous acid solution phase is an aqueous solution of the appropriate acid. The preferred type and concentration of the acid are as set forth above.

A surfactant may be used, if desired or necessary, in order to provide a stable emulsion. Such a surfactant may be selected from known surfactants.

The emulsion is preferably formed so as to provide a droplet size of the dispersed aqueous acid solution of between about 0.1 to 100 Am. Such a droplet size facilitates the overall desired particle size of the end, product.

The emulsion is preferably then mixed with a number of materials, as follows, so as to form encapsulating material in the continuous phase of the emulsion, surrounding the dispersed aqueous acid solution droplets.

The emulsion is preferably initially mixed with a polymer material. To facilitate the formation of encapsulated droplets of aqueous acid solution, the polymer is preferably oil soluble, particularly soluble in the oil used as the continuous phase. Further, the polymer material is preferably insoluble in the aqueous acid solution. Since the polymer material is to form the encapsulant of the aqueous acid solution, the polymer material is preferably chemically resistant to the acid of the aqueous acid solution, and is substantially impervious to acid and to water. Suitable polymer and cross-linked polymer material has been set forth above. The polymer material is preferably used in amounts sufficient to provide a ratio of polymer to acid in the emulsion of between about 0.1:1 to 4:1. More preferably, the weight ratio is between about 0.2.1 to 2:1.

Suitable cross-linking agents have been set forth above. The crosslinking agent is preferably added in amounts, by weight of the polymer, of between about 3 to 20%.

The cross-linking between the polymer and the cross-linking agent may be expedited or facilitated through the use of a promoter selected for such purpose. Suitable promoters include trimethylaminomethylphenol and dimethylaminomethylphenol, hereinafter referred to as DMP-30 and DMP-10, respectively. The promoter is preferably added in amounts, by weight of the polymer, of between about 2 to 10%.

The solution is then preferably allowed to settle. Spheres which contain the aforedescribed encapsulated aqueous acid composition may then be recovered.

The preparation of the composition of the present invention is further illustrated by the following non-limiting examples.

EXAMPLE 1

This example demonstrates the preparation of an aqueous acid solution in oil emulsion.

3 g of surfactant (Pluronic L122) were mixed and dissolved with 80 9 of xylene. 20 g of a 10% (wt.) hydrochloric acid aqueous solution was added to the xylene mixture and stirred to form a water in oil emulsion, wherein the water contains hydrochloric acid.

EXAMPLE 2

This example demonstrates the encapsulation of the aqueous acid solution of the emulsion.

1.1 8 g of epoxy resin were mixed with 0.78 g of diethylenetriamine (cross- linking agent) and 0.48 DMP-30 (reaction promoter) and dissolved in 20 g of xylene. This mixture was added to the emulsion of Example 1 under stirring and let react to form an encapsulating coating around the emulsion dispersed droplets to form microcapsules. After the reaction finished, the product collected consisted of spherical particles having a core of hydrochloric acid solution encapsulated by the epoxy resin.

EXAMPLE 3

This example demonstrates the temperature resistance of the encapsulated aqueous acidic solution.

A sample of dry powder (spherical particles) was analyzed by differential scanning calorimetry to determine the rupture temperature of the capsules. The heat was increased 25OCImin from 40 to 2500C. The capsules exhibited a rupture temperature of 2000C.

Thus disclosed is a novel composition which allows acid treatment of hydrocarbon producing formations without subjecting well tubing and pumping equipment to harmful corrosive effects of the acid, as well as a process for preparing and a process for using such a composition.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential t, i characteristics thereof. The present embodiments are therefore to be considered as in all respects to be illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

1.

Claims

1. An encapsulated acid composition, comprising an aqueous solution of acid encapsulated in a polymeric material which is chemically resistant to said acid in said aqueous solution of acid and which polymeric material is substantially impermeable to water and acid.

2. A composition according to claim 1, wherein said aqueous solution of acid has an acid concentration of up to about 15% by weight.

3. A composition according to claim 1 or claim 2, wherein said acid in said aqueous solution of acid is an oil insoluble acid.

4. A composition according to any preceding claim, wherein said acid in said aqueous solution of acid is a mineral acid.

5. A composition according to any preceding claim, wherein said acid in said aqueous solution of acid is selected from hydrofluoric acid, hydrochloric acid and mixtures thereof.

6. A composition according to any preceding claim, wherein said polymeric material is a cross-linked polymeric material.

7. A composition according to claim 6, wherein said cross-linked polymeric material is selected from cured epoxy resin, vinyl ester and polyester.

8. A composition according to claim 7, wherein said cross-linked polymeric material is cured epoxy resin, said cured epoxy resin being cured with a cross-linking agent selected from aliphatic amides, aromatic amides, and mixtures thereof.

a

9. A composition according to claim 8, wherein said cross-linking agent is selected from diethylenetriamine, triethylenetriamine, melamine and formaldehyde butylamine.

10. A composition according to any preceding claim, wherein said composition has a particle size of between about 0.5 to 3000PM.

11. A composition according to claim 10, wherein said composition has a particle size of between about 1 to 100pm.

12. A composition according to claim 10, wherein said composition has a particle size of between about 2 to 30pm.

13. A composition according to any preceding claim, wherein said polymeric material ruptures at a temperature of about 2000C.

14. A process for preparing an encapsulated acid composition, comprising the steps of:

forming an emulsion having a continuous phase of oil a..%d. a dispersed phase of an aqueous solution of acid; mixing said emulsion with a polymer which is soluble in said oil and insoluble in said aqueous solution of acid, said polymer being chemically resistant to said acid in said aqueous solution of acid, and said polymer being substantially impervious to said acid and to water; and mixing said emulsion and said polymer with a cross-linking agent so as to encapsulate droplets of the dispersed phase of said aqueous solution of acid in a cross-linked polymer encapsulant.

15. A process according to claim 14, wherein said forming step includes forming said emulsion at a ratio of said continuous phase to said dispersed phase of between about 95:5 to 70:30.

16. A process according to claim 14 or claim 15, wherein said continuous phase of oil is xylene.

17. A process according to any one of claims 14 to 16, wherein said dispersed phase of said aqueous solution of acid has an acid concentration of up to about 15% by weight.

18. A process according to any one of claims 14 to 17, wherein said acid is an oil insoluble acid.

19. A process according to any one of claims 14 to 18, wherein said acid is a mineral acid.

20. A process according to any one of claims 14 to 19, wherein said acid is selected from hydrofluoric acid, hydrochloric acid, and mixtures thereof.

21. A process according to any one of claims 14 to 20, wherein said emulsion forming step includes forming an emulsion wherein said dispersed phase of said aqueous solution of acid has a droplet size of between about 0.1 to 10011m.

22. A process according to any one of claims 14 to 21, wherein said step of mixing said emulsion with said polymer includes the step of mixing said polymer in a ratio to said aqueous solution of acid of between about 0.1:1 to 4:1.

23. A process according to claim 22, wherein said ratio of said polymer to said aqueous solution of acid is between about 0.2:1 to 2:1.

24. A process according to any one of claims 14 to 23, wherein said polymer is a cross-linking polymer.

1..

25. A process according to any one of claims 14 to 24, wherein said polymer is selected from epoxy resin, vinyl ester, polyester and mixtures thereof.

26. A process according to any one of claims 14 to 25, wherein said crosslinking agent is added in an amount by weight to said polymer of between 3 to 20%.

27. A process according to any one of claims 14 to 26, wherein said crosslinking agent is selected from aliphatic amides, aromatic amides, and mixtures thereof.

28. A process according to any one of claims 14 to 27, wherein said crosslinking agent is selected from diethylenetriamine, triethylenetriamine, melamine, formaldehyde butylamine, mixtures thereof, styrene and divinylbenzene.

29. A process according to any one of claims 14 to 28, wherein said step of mixing said emulsion and said polymer with said cross-linking agent further includes mixing said emulsion and said polymer with said crosslinking agent and a promoter for promoting a cross-linking reaction between said polymer and said cross-linking agent.

30. A process according to claim 29, wherein said promoter is selected f rom trimethylaminomethylphenol and dimethylaminomethylphenol.

31. A process according to any one of claims 29 or claim 30, wherein said promoter is added in an amount by weight to said polymer of between about 2 to 10%.

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32. A process for treating a subterranean formation, comprising the steps of: mixing an encapsulated acid composition, comprising an aqueous solution of acid encapsulated in a polymeric material which is chemically resistant to said acid in said aqueous solution of acid and which polymeric material is substantially impermeable to water and acid, with a carrier fluid; and injecting said fluid into said formation, whereby elevated temperatures of said formation cause said encapsulated acid composition to rupture, thereby contacting said aqueous solution of acid with said formation.

33. A process according to claim 32, wherein said carrier fluid is selected from water, methanol, and mixtures thereof.

34. A process for neutralizing a basic solution, comprising the steps of: mixing said basic solution with an encapsulated acid composition, comprising an aqueous solution of acid encapsulated in a polymeric material which is chemically resistant to said acid in said aqueous solution of acid and which polymeric material is substantially impermeable to water and acid; heating said mixture so as to cause said encapsulated acid composition to rupture, thereby mixing said aqueous acid solution with said basic solution so as to neutralize said basic solution.

35. An encapsulated acid composition according to claim 1 substantially as hereinbefore described.

36. A process for preparing an encapsulated acid composition according to claim 14 substantially as hereinbefore described.

37. A process for treating a subterranean formation according to claim 32 substantially as hereinbefore described.

38. A process for neutralizing a basic solution according to claim 34 substantially as hereinbefore described.