CA1185778A - Stable foams and methods of use - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Enhanced foams are described which comprise:
an aqueous medium, a surfactant, a gas, a heteropoly-saccharide, and a water soluble inorganic salt. For example, a foam fracturing fluid was prepared by foaming an aqueous solution of a xanthan gum, KCl and a commercial blend of surfactants with nitrogen. This foamed fracturing fluid had excellent proppant carrying capacity, low fluid loss, and excellent stability even at elevated temperatures.

Description

STABLE FOAMS AND METHODS OF USE

This invention pertains to novel foams and methods of use. This invention pertains in particular to novel ~oams having enhanced stability and their use in treatment of subterranean earthen formations (e.g.
fracturing).

Foams have a wide variety of commercial uses.
For example, foams have been used as cleaning mediums in industry and at home to clean hard surfaces, soiled linens, etc. Foams have also been used commercially to clean scale and other debris from boiler tubes and the like; these procedures capitalize on the foam's capacity to carry solids. See USP 3,037,887 and 3 212,762 which illustrate this utility.

The capacity of foam to carry solids has also been recently utili~ed in the oilfield. Hydraulic Eracturing of subterranean earthen formations with proppant laden foam has been described by Blauer et al.
in USP 3,937,283 and by Plummer et al. in USP 3~980,136.
The state of the art is represen~ed by Blauer et al, supra.
Foam fracturing is a substantial advance in fracturing technology because the low liquid content causes less form-ation damage and the high gas content supplies energy to return fracturing fluids to the surface. The net result is faster and better c]eanup.

29,267-F

- 2 - ~5~

The state of the art has been enhanced further by the recent discovery by Zingg of a slurry concentrator that can be used to make more concentrated proppant slurries prior to ~oaming. This slurry concentrator is described in a commonly owned, copending Canadian patent application, serial no. 391,517, filed December 4, 1981.
Another such device was described by Black in USP
4,126,181 which allegedly has a similar use in the treatment of wells.

The essential components of a foam are well known and comprise (a) a foamable liquid, and (b) a blowing agent. The foamable liquid is usually an aqueous liquid containing a surfactant or surfactant mixture.
In foam fracturing, the surfactant is usually a nonionic or cationic surfactant (or a mixture thereof), or an amphoteric surfactant. A wide variety of such surfactants are known and commercially available, as illustrated by -the above referenced materials and by the handbook of McCutcheorls, Combined Edition (published by McCutcheons' Division, MC Publishing Company, Glen Rock, N~Jo)~
The surEactants described by J.L. Thompson in USP 4,018,689;
USP 4,028,257; USP 4,108,782; and USP 4,113,631 are of particular interest i.n the present invention insofar as it pertains to treatment (e.g. foam fracturing) of subterranean formations.

Foam stability is an important property in most commercial applications. Various additives have been used to enhance this property. For example, certain alcohols have given excellent results in enhancing foamed aqueous acids (as shown by Scherubel in a co~unonly owned Canadian patent 1,138,309 entitled "Foamable Acid Compositions", granted December 28, 1982 and various polymers have also given good results (e.g. the hetero-polysaccharide marketed by Kelco and described in the Kelco product bulletin entitled "Xanthan Gum", Second Edition.

.r .
29,267~F - 2 -~B5~78 The blowing agents used to gene~ate foam are normally gases or compounds which genera-te gas in situ.
Most commonly, the blowing agent is a gas (e.g. air, nitrogen, carbon dioxide, etc.). Such gases are usually available from co~mercial sources in pressure cylinders or other cryogenic containers and in truck load quantities (e.g. as liquid nitrogen).

It is surprising that even in the fac~ of the extensive literature on foams and the commerical demand for foams that there is still a need for a foam having enhanced foam stability, yet that need exits, particularly for foams used in hydraulic fracturing.

A novel foam has now been discovered which has excellent foam stability. The novel foamable liquid composition comprising:
(a) an aqueous medium, (b) a viscosity increasing amount of xanthan gum, (c) a cationic surfactant, and (d) a minor Eoam stabi]izing amount o~ water-soluble inorganic salt.

The invention also resides in a foam suitable eor use in fracturing a subterranean earth formation comprls1ng:
(a) an aqueous medium, (b) a viscosity increasing amount o xanthan yum, (c) a cationic surfactant, (d) a minor, foamable stabilizing amount of a water-soluble inorganic salt, and (e) a blowing agent.

29,267-F - 3 -7~3 ~4--The new foam has excellent stability and proppant-carrying capaciky which makes it particularly useful in foam fracturing of subterranean formations.

The foamable liquid comprises an aqueous medium, which is usually water or dilute acid ~.g.
dilute HCl). Water is the medium of choice in most instances although dilute acid ~e.g. about 1-5 percent HCl~ can be advantageous in foam fracturing of some formations. The aqueous medium can also contain other addi~ives, such as lower alkanols (e y. methanol, ethanol isopropanol, etc.~ to aid dissolution, compatible inhibitors, and the like, if desired.

The heteropolysaccharides form a known cla5S
of polymeric compounds, any of which can be used herein so long as the compound(s) chosen uniformly thickens the agueous medium (i.e. increases i~s viscosity) and enhances khe stability of the foam. A simple experimental ~xocedure will be described below for evaluating such compounds. The biopolymers produced by the microorganism anthomonas campestris are preferred heteropolysaccharides and are commercially available (e.g. from Kelco Division of Merck & Co.). Such biopolymPrs may be used as they are formed in the fermentation process (containing live bacteria) or as they are commercially marketed. The commexcially marketed materials have been pasterurized with heat to kill the ~acteria, precipitated with a lower alkanol (e.g. isopropanol), and recovered by iltration.

The heteropolysaccharides are normally used in amounts of up to about 60 lbs/1300 gal. of a~ueous liquid medium. However, in most instances, 29,267-F ~4 ~5--~uantities of from 20 to 40 lbs/1000 yal~ are adequate, and preferred.

Other viscosifiers or gelling agents (e.g.
locust bean gums, guar gums, etc.~ can also be added to the foam~ble liquid if desired so long as they ax~
compatible in the foamable liquid and do not distabilize the foam.

The cationic surfactants likewise form a known class of compounds, any member (or mixture) of which can be used herein so long as a stable foam is generated. The cationic sur~actant can be used as such or as a blend with a nonionic surfactant. Such blends are advantageous and usually preferred. The nonionic surfactants are, of course, a known class of compounds.
When the foam is to be used in foam fracturing, it is customary for the operator to check the ~oam (and its components) for compatibility with the formation fluids.
Such compatibility screening is done routinely in the ~ield. Selection of a su~fac~ant is therefQre important, bu~ can be varied and is within the skill of the art.
The surackanks described by J. L. Thompson in USP
4,018,689; USP 4,028,257 and ~SP 4,108,782 are especially useul in foam fracturing applications and are pxeferred in such instances.

The water soluble inorganic salts are a known class of compounds, any member of mixture of which can be used herein so long as the salt(s) is compatible with the remaining components o~ the foam. Obviously, the salt will not be chosen in a manner tha~ would render the foam unacceptable for its intended purpose by virtue of the effect of the anion and/or cation on 29,267-F ~5-the substrate ~o be cleaned, fractured, etc. For example, one would normally tend to choose a salt having an anion other than chloride if the foam was to be used in cleaning a stainless steel surface. Similarly, some salts might be undesirable in foam fracturing because of potential formation damage and would be avoided. Such information and knowledge is within the skill of the art and is readily determined by routine experimentation. Examples of suitable such salts include: alkali and alkaline earth metal halides (such as NaF, NaCl, KCl, NaBr, KBr, CaC12) etc.), alkali metal carbonates (such as Na2C03, etc.), alkali metal bicarbonates (such as NaHC03, etcc), alkali metal nitrates (such as NaN02, etc.), and the like. NaCl and KCl are presently preferred salts for use in foam fracturing fluids, and KCl is most preferred.

Applicants were surprised to learn that minor amounts o~ dissolved inorganic salts substantially enhanced the foam stability of the presen~ foams. The ~alts are included in the foams in foam stabilizing amounts (as d~termined by the foam half-lie test below).
The salts are normally included in amounts of up to about 5 w~ight percent, based on the weight of the liquid aqueous medium, and are preferably included in amounts o~ from 2 to 3 weight percent, with amounts o from 2 to 2.5 weight percent being most preferred.

The ~Iblowing agent" used herein is an inert gas (i.e. not reactive with the constitutents in the foam) and can be added as such or generated in situ by adding a compound which reacts with water to evolve a gas (e.g. C02). Most commonly, the blowing agent is 29,267-F -6-~85~
_7, added as a gas. E~amples of such blowing agents include air, nitrogen, carbon dioxide, normally gaseous hydro-carbons (e.g. propane~, etc. Nitrogen is the blowing agent of choice because of economics, availability and handling safety. ~he ~oam "quality" is defined and calculated according to the procedure set forth in Blauer et al., supra. A Mitchell foam ~uality of from 0.60 to 0.85 (at fonmation temperature and pres-sure) is preferred when the foam is to be used in foam fracturing of subterranean formations.

The novel foams are conveniently prepared by blending the components necessary to make a foam~le liquid composition and ~hereafter adding the foaming agent. The foamable li~uid composition,can be prepared in one blending step or in multiple blending steps.
For example, the foamable liquid composition which is used in foam racturing is advantageously prepared by blending the hydratable heteropolysaccharide wi~h the aqueous medium containing dissolved salt(s) and subse-~uently blending in the surfactant and blowing agent tomake the ~oam. Product uniformity seems to be enhanced if the het~ropolysaccharide is introduced in this matter but different blending methods can also be used, with good results. For example, a dry blend of salt and heteropolysaccharide can be blended with the aqueous medium and then foamed by adding surfactant and a blowing agent. Other blending methods will be apparent to the skilled artisan.

It has been found advantageous but not critical to pressurize the foamable li~uid composition before adding the blowing agent if the resultant foam is to be used at elevated pxessures ~e~g. in fracturing)O In this manner, pumping is facilitated.

29,267-F -7~

Experimental The following experiments will further illustrate the invention:

Experiments 1-4 An aqueous stock solution containing 2 weight percent KCl and 0.80 volume percent of a blend of commercial surfactants.(0.48 percent F7~ and 0.32 percent F75N, products of The Dow Chemical Company) was prepared. Various polymeric gelling agents and inorganic salts were then addPd to 100 milliliter aliguots of the stock solution and the resulting mixture was stirred for 30 seconds at high speed in a commercial Waring Blender. The oamed contents were then transferred quickly to a graduated cylinder. The foam half-life was measured by recording the length of time required for 50 milliliters of liquid to separate out of the foam.
The longer the ~oam half~life, of course, the more stable the foam.

This series of experiments was conducted to show the performance of xanthan ~Im (a heteropolysac-charide) r~lative to other polymeric thickening agents.

TABLE I
Half-Life (minutes) at Experiment ~y~Polymer Conc. (lbs/1000 qal.) . 20 30 40 1 guar gum 14 20 41 2 hydroxy~ 7 14 28 propyl guar

3 hydroxy- 15 26 52 ethyl cellulose

4 xanthan 210 315 441.5 gum 29,267~F -8-_9_ A foam prepared from the stock solution without any thickening agent had a half-life of about 3 minutes.

Experiments 5-39 This series of experiments was designed to show ~he e~fect o~ various amounts of KCl in an aqueous stock solution containing xanthan gum (20, 30 or 40 lbs/lO00 gal. of solution), 0.80 weight percent of the blend of surfactants identified in Experiments l 4.

TABLE II
Xanthan Gum - 0 _ gal Initial Foam Fo~m Half-Life xperlment KCl (wt. %) Volume ~mll_ (minutes~
0 350 2.6 6 0.25 310 2.5 7 0.50 270 2.0 8 0.75 280 1.8 9 1.00 300 1.7 1.50 N.A. 50.7 20 11 2.00 290 77.5 12 2.25 350 86.5 13 2.50 295 81.0 14 2.75 350 92.5 3.00 385 87.5 25 16 3.25 360 28.5 * N.A. means data not acquired.

29,267-F -9-~10--TABLE I I ( continued ) Xanthan Gum - 20 lb~/1000 yal Initial FaomFoam Half~Life Experiment KCl (wt. ~O) Volume (ml ~ (minutes )_ 17 3 . 50 320 27 . 0 1~ 3 . 75 325 21 . 5 19 4. 00 385 2~ . ~
4 . 50 28~ 25 . 0 21 5 . 00 300 26 . 0 TABLE I I I
X~Atha~ 30 lb~1000 ~al Initial Foam Foam Hal~Life .KCl (wt. %) Volume (ml ) _(miIlutes ) 22 0 2~0 0 . 7 15 23 0 . 25 240 1 . 9 24 a . 5 280 10 . 5 0 . 75 325 13 . 1 26 1 . 0 325 17 . 5 27 2 . 0 325 191 . 0 20 28 3 . 0 N.A. 231. 0 29 4.0 N.A. 57.0

5 . 0 265 57 . 0 29, 267-F -10-7~

TABL~ IV

Initial Foam Foam Half-Life Experiment KCl (wt~_~ Volume lm].) (minutes) 31 0 240 15.0 32 0.25 300 40.5 33 0.5 300 55.5 34 0O75 300 77.5 1.0 300 90.5 36 2.0 300 441.5 37 3.0 N.A~ 410.0 38 4.0 N.A. 118.0 39 5.0 200 137.0 ~ lments 40-51 1$ This ser.ies of experiments was conducted in a manner essenti.ally identical to those above except NaCl was used in place of KCl.

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Experiments _5 - 6 0 This series o experiments was conducted in a manner essentially identical to those above except CaC12 was used in place of KCl.

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29, 267 F -14-7~3 Experiments 61-69 This series of experiments was conducted in a manner essentially identical to those above except ~gC12 was used in place of KCl.

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29, 267 F -16-Experiments 70-78 This series of experiments was conducted in a manner essentially identical to those above except NH4Cl was used in place of KCl.

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~ 8~i77~3 E~periments 79-85 This series of experiments was desiyned to show the effect of various ratios of cationic surfactants (F78) to nonionic surfac~ants (F75N). The li~uid in each instance contained 2 percent KCl, xanthan gum (20 lbs/lO00 yal.) and the surfactant (8 gal/lO00 gal) dissolved in water. In parallel experiments in which KCl was omitted, li~uids containing the cationic surfactant formed a gummy precipitate. The f3ams were, of course, generated and evaluated as shown above.

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Experiments 86-91 This series of experiments was designed to show the effectivene s of cationic surfactants in the present foam sys-tem relative to anionic surfactants (F52 -commercial product from The Dow Chemical Company) andnonionic surfactants (Igepal~ CO 730 ~ commercial product from GAF Corp.). The liquid in each instance contained xanthan gum (40 lbs/lOOO gal.), the indicated surfactant (5 gal/lOOO gal.) and KCl (in amounts as indicated~
dissolved in water. The foams were, of course, generated and evaluated as shown above.

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29, 267-F -22-~5~

Experiment 92 The slurry concentrator illustrated by Figures 1 and 2 in Zingg et al., supra, was used in a fracturing system as shown in Figure 3 of Zingg et al. in a foam 5 fracturing treatment of a well located in the Luling Branyon Field in Guadalupe County, Texas. The well was approximately 2,000 feet deep and it has a bottom hole static pressure of 700 psi and a permeability of 0.21 millidarcies (average).

In this trea~ment, S,000 gal. of an a~leous carrier fluid (2 percent KCl brine gelled with a commercial ~anthan gum, Dowell~ J312 (20 lbs/1000 gal.
brine) and 5,000 gal. of carrier fluid wexe blended with 5,000 lbs. of 100 mesh sand and then 2,500 gal. of carrier fluid wexe sequentially pumped through the slurry concentrator with the choke assembly valved off so that no li~uid was removed from the fluids passing through at a rate of 20 barrels per minute (bpm~i a blend of commercial surfactants [Dowell~ F78 t3 parts by volume) and Dowell~ F75N (2 parts by volume] dissolved in dilute HCl was added at rate of 0.5 bpm (i.e. 5 gal.
of surfactant concentrated per 1000 gal. of carrier ~luid), and the fluids were then foamed with gaseous nitrogen (13 bpm, liquid basis) to give a foam guality of 65 (i.e. a Mitchell valve of 65) and injected into the well. The valve on the choke assembly was then opened and a slurry of carrier fluid blended with increasing amounts 20 to 40 mesh sand was then pumped through the system (20 bpm) concentrated and foamed using the same blend of surfactants and nitrogen at the same rates shown above. The slurry was blended at a concentration designed to give 1 lb. sand/gal. of liquid in the foam initially and then to rise steadily 29,267-F -23-~577~3 to a final concentration of 5 to 6 lbs. sand/gal. of liquid in the foam over a short period of time (e.g. 3 to 4 minutes). This schedule was easily achieved. The slurry going into the concentrator thus varied from an absolute density of 9.4 to 13.1 lbs/gal. and the slurry discharged from the concentrator had an absolute density that varied correspondingly from 9.9 to 14~6 lbs/gal.
These dat are correlated in Table XI.
TABLE XI
.
Proppa-nt-conc. ~ luid3 Initial SlurryDischarge Slurry Foam g.a~ 9.9 10.4 11.1 2 11.0 12.~ 3 1516.6 13.0 4 12.2 13.7 5 13.1 14.6 6-~

To illustrate the data in another way commonly used in the ~ield, 12.2 and 13.7 lbs/gal. absolute density correspond to 8.8 and 14.3 lbs. of sand added per gallon of fluid, respectively. Without the slurry concentrator and the dynamics of the system, it would generally be thought impossible, for e~ample, for 1 gallon of this carrier fluid to hold 14.3 lbs. of sand, much less the 18.5 lbs. which was achieved during the course of this fracturing treatment. In total, 235,000 lbs. of proppant were emplaced using 89,000 gals. of carrier fluid and 647,369 standard cubic feet of nitrogen.
The well was shut in for a period and brought back 29,267-F -24-~5~8 slowly, according to s~andard procedures. The fracture treatmen-t was extremely successful according to initial production data.

29,267-F -25-

Claims (17)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A foamable liquid composition comprising:
(a) an aqueous medium, (b) a viscosity increasing amount of xanthan gum, (c) a cationic surfactant, and (d) a minor foam stabilizing amount of water-soluble inorganic salt.

2. The composition defined by Claim 1 wherein (a) is water or a dilute aqueous acid.

3. The composition defined by Claim 1 wherein (b) is a biopolymer produced by the organism Xanthomonas campestris.

4. The composition defined by Claim 3 wherein said biopolymer has been purified by precipitation with a lower alkanol, pasturized with heat, and recovered.

5. The composition defined by Claim 1 wherein (b) is present in amounts of up to 60 lbs/1000 gal. of foamable liquid composition.

6. The composition defined by Claim 5 wherein (b) is present in amounts of from 20 to 40 lbs/1000 gal.
of foamable liquid composition.

7. The composition defined by Claim 1 addition-ally comprising a nonionic surfactant.

8. The composition defined by Claim 1 wherein (d) is present in amounts of up to about 5 weight percent, based on the weight of (a).

9. The composition defined by Claim 8 wherein (d) is present in amounts of from 2 to 3 weight percent, based on the weight of (a).

10. The composition defined by Claim 1 wherein (a) is water or dilute HCl; (b) is a biopolymer produced by the microorganism Xanthomonas campestris, which biopolymer is present in amounts of from 20 to 40 lbs/1000 gal. of foamable liquid composition; (c) is a blend of a cationic surfactant and a nonionic surfactant; and (d) is NaCl and/or KCl in amounts of from 2 to 2.5 weight percent, based on the weight of (a).

11. A foam suitable for use in fracturing a subterranean earth formation comprising:
(a) an aqueous medium, (b) a viscosity increasing amount of xanthan gum, (c) a cationic surfactant, and (d) a minor, foamable stabilizing amount of a water-soluble inorganic salt, and (e) a blowing agent.

12. The foam defined by Claim 11 wherein (e) is air, nitrogen, carbon dioxide, or normally gaseous hydrocarbons.

13. The foam defined by Claim 12 wherein (e) is nitrogen.

14. The foam defined by Claim 11 wherein said foam has a Mitchell foam quality of from 0.52 to 0.99 at formation temperature and pressure.

15. The foam defined by Claim 14 wherein said foam quality is from 0.60 to 0.85.

16. The foam defined by Claim 15 wherein (a) is water or dilute HCl; (b) is a biopolymer produced by the organism Xanthomonas campestris, which biopolymer is present in amounts of from 20 to 40 lbs/1000 gal. of foamable liquid composition; (c) is a blend of a cationic surfactant and a nonionic surfactant; and (d) is NaCl and/or KCl in amounts of from 2 to 2.5 weight percent, based on the weight of (a).

17. In the method of initially fracturing or extending a fracture of a subterranean formation at elevated pressures using a foam fracturing fluid, the improvement comprising using the foam defined by Claim 11 as said fracturing fluid.