CA1187691A - Aqueous well servicing fluids - Google Patents

Abstract

Abstract of the Disclosure A method and composition for decreasing the fluid loss of aqueous well servicing fluids in which an additive comprised of a cross-linked hydroxyethyl starch and a hydroxyethyl cellulose is added, in effective amounts, to the aqueous fluid, the aqueous fluid preferably being one which contains at least one water soluble salt of a multi-valent metal ion.

Description

-2-Background of the Invention The present invention relates to a method and co~position for increasing the viscosity and reducing the fluid loss of aqueous systems used as well servicing fluids.
Aqueous mediums, particularly those contain;ng oil field brines, are commonly used as well servicing fluids such as drilling fluids, workover fluids, completion fluids, packer fluids, well treating fluids, subterranean formation treating fluids, spacer fluids, hole abandonment fluids~, etc. Such well servicing fluids, if they are to be effective and economically attractive, must exhibit low fluid loss. It is known to add to the well servicing fluid certain hydrophilic polymeric materials for fluid loss control. For example, it is known to use starch and cellulose products, e.g.corn starch and potato starch derivatives, as additives to well servicing fluids for fluid loss control. The degree of fluid loss control exhibited by such materials is largely dependent upon the composition of the well servicin~ fluid. Thus, it is known that the presence of high concentrations of calcium or zinc ions, common components of heavy brines, makes fluid loss control more difficul~t.
Viscosity enhancement of aqueous well servicing fluids is also necessary in many applications. Again, starch and cellulose derivatives have been used to achieve such viscosity enhancement, but their effectiveness is also affected by the presence, in certain heavy brines, of high concentrations of calcium or zinc ions.

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-3-Summary of the Invention It is, therefore/ an object of the present invention to provide a new co~position for synergistically increasing the viscosity and controlling the fluid loss of aqueous well servicing fluids.
~ further object of the present invention is to provide a new composition useful for synergistically increasing the viscosity and lowering the fluid loss of aqueous brine solutions used as well servicing fluids.
Still a further object of the present invention is to provide an improved method for decreasing the fluid loss of an aqueous well servicing fluid.
The above and other objects of the present invention will become apparent from the description given herein and the appended claims.
In accordance with one embodiment of the present invention, there is provided a method for decreasing the fluid loss of an aqueous well servicing fluid by adding and dispersing in the well servicing fluid an effective amount of a cross-linked hydroxyethyl starch and an effective amount o a hydroxyethyl cellulose.
In another embodiment of the present invention, there is provided a composition useful for increasing the viscosity and lowering the fluid loss of an aqueous medium comprised of a cross-linked hydroxyethyl starch and a hydroxyethyl cellu-lose.
In yet another embodiment oE the present invention~ there is provided a well servicing fl~id comprised of an aqueous medium, an effective amount oE a cross-linked hydroxyethyl starch and an effective amount of a hydroxyethyl cellulose.

Description of the Preferred Embodim_nts The two polymeric componen's of the novel composition of the present invention are cross-linked hydroxyethyl starch and hydrox~ethyl cel.l.ulose (HEC). The ~EC polymer.s are gener-a:Lly solid, particulate materials which are water soluble orwater dispersible and which upon solution or dispersion in an aqueous medium increase the viscosity of the system, HEC
polymers are generally high yield, water soluble, non-ionic materials produced by treating cellulose with sodium hydroxide followed by reaction with ethylene oxide. Each anhydroglucose unit in the cellulose molecule has three reactive hydroxy groups, The average number of moles of the ethylene oxide that becomes attached to each anhydroglucose unit in cellulose is called moles of substituent combined, In general, the greater the de~ree of substitution, the greater the water solubility, It is preferable to use HEC polymers having as high a mole substitution level as possible, The HEC polymers which are useful in the present inven-tion, depending upon the method of preparation of the well servicing fluidsl can either be in the form of a dry powder, essentially untreated, or can be an 'lactivated'l HEC. The term "activated" as used herein refers to an HEC polymer which will substantially hydrate or solubilize in a brine solution having a density greater than about 14,2 pounds per gallon (ppg) without the necessity for mixing, as by rolling, at elevated temperatures, Examples of such activated HEC
polymers are ~o be found in co-pending Canadian Patent App-lications Nos, 369,357, filed February 27, 1981; and 374,309, filed March 31, 1981, As disclosed in the aforementioned patent applications, HEC polymers which have been activated will solubilize in brine solutions without the necessity for rolling or other forms of mixing at elevated temperatures, In general, any HEC polymer which will solubilize in a brine having a density in excess of about 14.2 ppg at room temperature can be considered an "activated" ~EC, It is to be understood, however, that the present invention is not . ~

limited to the use of such activated HEC polymers. Depending upon the condition of mixing, and the composition of the a~ueous well servicin~ fluid, unactivated or dry powder HEC
uol.ymel s are compatible with the aqueous well servicing fluids used in the presellt invention, The term "compatible" as used hereill, means that the HEC polymer can be solvated or solu-bi:lizecl in a given aqueous solution with the use of mixing techniques such as rolling at elevated temperatures, Thus, an incompatible system is one in which the HæC polymer will 10 not solubilize in the brine regardless of the mixing conditions used, The other polymeric component of the compositions of the present invention is a cross-linked hydroxethyl starch, Such hydroxethyl starches are produced by introduction of non-15 ionic hydroxyethyl side groups onto the polymer chain of the starch followed by cross-linking techniques well known in the art such as, for example, those disclosed in U.S, Patents Nos, 2,500,950; 2,929,811; 2,989l521 and 3,014,901, all of which are herein incorporated by reference for all purposes, 20 Generally speaking, the cross-linked hydroxyethyl starches which are useful in the present invention are those in which the hydroxethyl side chain degree of substitution ~DS) is from about 0,15 to about 0.8, preferably from about 0,25 to about 0,6. A particularly useful cross-linked hydroxethyl 25 starch is known as BOHRAMYL CR~, a cross-linked potato starch derivative manufactured by Avebe (Veendam, Holland~, BOHRAMYL CR, which is a coarse white flaky material, has a bulk density (kg/m3) of approximately 325 and a DS of about 0,4, As noted above with respect to the HEC, the cross-linked hydroxethyl starch may be utilized either in the form of a dry powder or flake, essentially untreated, or can be an "activated" starch, wherein the term "activated" has the same meaning as used above with respect to the discussion of activ-35 ated HEC. Methods of activating the cross-linked hydroxyethyl starch are disclosed in Canadian Patent Application Serial No, 369,357, filed February 27, 19~1, It is to be understood that the present invention is not limited to the use of activated hydroxyethyl starch. Indeed, it is a feature of the present invention that both the HEC and the hydroxyethyl starch can be used in dry form to produce well servicing flulds which exhibit excellent rheological properties and low fluid los5. ~lowever, in certain brine solutions, activation or pre-~olvatlon of the HEC and/or the hydroxyethyl starch may be d~sirable to reduce mixing times and severity of~conditions oE
mixing.
:tO The polymer composition of the present inyention which can be used to decrease fluid loss and increase the viscosity of aqueous well servicing fluids is comprised of an effective amount of HEC and an effective amount of the cross-linked hydroxyethyl starch. It has been found that when these two lS polymeric materials are added to aqueous well servicing fluids, depending upon the nature of the fluid, synergistic enhancement of viscosity and/or fluid loss is achieved. The particular amount of each of the polymeric components present in the additive composition will vary depending upon the nature and composition of the aqueous well servicing fluid with which` the additive is to be adMixed. In general, the polymer composition of the present invention will contain a weight ratio of hydroxyethyl starch to HEC of from about 10 to 90 to about 90 ~o 10, preferably from about 33 to 67 to about 75 to 25. The polymeric cornposition of the present invention can be either in the form of a dry mixture of the HEC and the hydroxyethyl starch or, if preferred, it can be in the form of solvated or activated forms of the polymers. Thus, for example, the HEC and the hydroxyethyl starch can be activated and those activated solutions mixed together to provide the novel polymeric compositions used herein.
The novel well servicing fluid of the present invention comprises an aqueous medium and an effective amount of a cross-linked hydroxyethyl starch and an effective amount of a ~S hydroxyethyl cellulose. The relative amounts of the hydroxy ~ ethyl starch and the HEC admixed with the aqueous medium is such as to provide a synergistic decrease in the fluid loss of 37~3~

the aqueous medium. ~gain, the precise amount of each of the polymeric components used will depend upon the nature of the aqueous well servicing fluid In general, however, the weight ratio of the hydroxyethyl starch to the HEC in the well servicing fluid will be from about 10 to 90 to about 90 to 10, more preferably from about 33 to 67 to about 75 to 25.
In general, the well servicing fluids will contain t~e polymer components in amounts of from about 0.25 to about 5 ppb HEC and from about 0 5 to about 5 ppb hydroxyethyl starch.
The aqueous medium used in the well servicing fluids of the present invention can range from fresh water to heavy brines having a density in e~cess of 19 ppg Generally speaking, well servicing fluids as, for example, those used ln completion and workover opera~ions, are made from aqueous mediums containing soluble salts such as, for example, a soluble salt of an alkali metal, an alkali earth metal, a Group Ib metal, a Group IIb metal, as well as water soluble salts of ammonia and other cations. The mixed HEC/cross-linked hydroxyethyl starch compositions are particularlyuseful in the preparation of low fluid loss, heavy brines, i.e aqueous solutions of soluble salts of multi-valent ions, e.g 2n and Ca.
The preferred heavy brines useful in forming the well servicing fluids of the present invention are those having a density greater than about 11 ppg, especially those having a density greater than 15 ppg. Such heavy brines are comprised of water solutions of salts selected from the group consist-ing of calcium chloride, calcium bromide, zinc chloride, zinc bromide, and mixtures thereof.
We have shown in co-pending Canadian patent application Serial No. 380,104, filed June 18, 19~1, that in certain heavy brines containing zinc bromide in a concentration of less than about 20~ by weight, HEC is incompatible, i.e. it will not solvate in such brines to efficiently enhance the viscosity However, by adding the synergistic combination ,iYr `~` -~1~, ,L,~

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of HEC and the cross-linked hydroxy-ethyl starch, brine solutions wherein the content of æinc bromide is from about 0.5 to about 20% by weight, and the density is from about 14.2 ppb to about 16.25 ppb can be viscosified.
If desired, bridging agents may be added to the well servicing fluids to aid in fluid loss control. Indeed, somewhat low filtrates are obtained with their use. However, it is a distinct and unexpected feature of the invention that a bridging agent is not necessary to achieve low fluid loss values in aqueous brines. Thus, using the present invention, it is possible to obtain clear, heavy brines having low fluid loss characteristics and, at low concentrations of HEC, having low rheological characteristics.
In the method of the present invention, the mixed HEC/
cross-linked hydroxyethyl starch can be added to the aqueous well servicing medium either in the dry form or in the activated form as discussed above. ~n the method, the polymeric components are dispersed in the aqueous medium by suitable mixing techniques To more fully illustrate the present invention, the following non-limiting examples are presented. Unless other-wise indicated, all physical property measurements were made in accordance with testing procedures set forth in STANDARD
PROCEDURE FOR TESTING DRILLING MUD API RP 13B, Seventh Edition, April, 1978. The HEC polymer employed, unless otherwise indicated, was an HEC marketed by Hercules, Inc., uner the trademark NATROSOL 250 HH ~. The cross-linked hydroxyethyl starch employed, unless otherwise indicated, was BOHRAMYL CR~ marketed by Avebe IVeendam, Holland).

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Ex ample 1 To show the effect on viscosity and fluid loss achieved by mixing HEC and cross-linked hydroxyethyl starch, 2 ppb of HEC and either 0, 2 or 4 ppb of BOHRAMYL CR were added to an aqueous brine containing 30 ppb calcium chloride and ~ixed for 30 m;n~tes on a Multimixer. Thereafter, t:he samples were rolled at 150F for 16 hours, cooled, stirred for 5 minutes and the API rheology and fluid loss determined. Th~data obtained . ~nd given in Table 1 below indicate that thP BOHRAMYL CR
-io eEEiciently decreased the fluid loss in the-brine in the ~ presence of HEC.

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Table 1 - - ppb. BOHRAMYL CR
API Properties O 2 4 Apparent Viscosity 40 57 82 Plastic Viscosity 20 25 32 Yield Point 40 65 101 .
10 Sec. Gel Strength 3 7 12 PH . 7.6 7.8 7.8 API Fluid Loss 50 14.2 7.3 . .

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The example demonstrates the use oE using solvated HEC
compositions, The following samples were prepared:
Sample_A
L24,5 parts of isopropanol and 0,5 parts of CAB-O-SIL M5 ~colloidal silica) were mixed 10 minutes on a Multimixer, There was then added 50 parts by weight of NEC and mixing COIl-tinued for an additional 3 minutes, Following this, 75 parts by weight of ethylene glycol were added and an additional 5 minutes of mixing conducted, Sample B
A solution of 0,5% by weight Rlucel ~ ~hydroxypropyl cellulose) in isopropanol was prepared, To 55 parts by weight of this isopropanol solution were added 20 parts by weight of HEC and 25 parts by weight of ethylene glycol, The brine solution used to evaluate the samples was a 16,0 ppg CaBr2/ZnBr2 solution, The brine samples were prepared and evaluated using the following procedure:
1, The amounts of HEC, BOHRAMYL C ~ and BARACARB~
~CaCo3 bridging agent) indicated in Tables 2, 3 and 4 were added to the 16,0 ppg brine and mixed on a Multimixer for 15 minutes, 2, The API rheology was then obtained.
3, The samples were then aged overnight at room temperature and the API rheology and fluid loss obtained,

4, The samples were then rolled overnight at 150F and the API rheology and fluid loss obtained after the samples had cooled to roo~ temperature, Table 2 gives the data obtained using Sample A. Table 3 gives the data obtained using Sample B, Table 4 gives the date obtained for BOHRAMYL CR alone, L~
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Table 4 - Effect of BOHRAMYL CR on the Properties of a 16.0 ppg CaBr2/ZnBr2 Solution .. . . _ .. .
19 20 ~1 22 23 24 25 ~ _ _ _ _ _ _ BOHRAMYL CR, ppb. 1 2 3 4 1 2 3 BARAC~RB CaC03, ppb 0 0 0 0 3 3 3 Initial Proper-ties After 15 Minutes on a Multimi~er A.V. 6.5 6.5 7 - 6.5 - 6.5 7 P.VO 7 6 7 - 7 ~6 7 Y . P . -1 1 0 -- -1 1 0 10-Sec. Gel Str. 0 0 0 -Properties After Hydrating Overnight at 74F
A.V. 10 13 1820.5 10 13 18.5 P.V. 10 13 18 20 10 13 18 Y.P. O O 0 1 0 0 10-Sec. Gel Str. 0 0 0 1 0 0 API Fluid Loss89 68 2810.8 57 20 13 Properties After Rolling Overnight at 150F
A.V. 10 13.5 1923.5 10 13.5 20 P.V. 10 14 19 23 10 14 20 Y.P. O ~1 0 1 0 -1 0 10-Sec. Gel Str. 0 0 0 0 0 0 API Fluid 1Oss107 61 30 20 50 35 15 7~

The data obtained and shown in Tables 2, 3 and 4 indicate that the HEC and the BOHRAMYL C~ combine to synergistically increase ~he viscosity and decrease the fluid loss of the aqueous brine. The fluid loss results in the absence of the 5 BARACARB bridging agent are particularly outstanding. The aqueous brines, after hot rolling, were completely clear as all of the polymers were dissolved.

3~ ( Example 3 To demonstrate the effect of HEC and BOHRAMYL CX on heavy brines containing less than 20% by weight ZnBr2, the following procedure was carried out: a 15.3 ppg CaBr2/ZnBr2 solution containing 15.7% ZnBr~ and 43.9~ CaBr2 was pr~pared by mixing together a 19.2 ppg solution containing 57% by weight ZnBr2, and 20~ by weight CaBr2 and a 14.2 ppg solution containing 53 by weight CaBr2 in a volume ratio of 0.78/~.22,~respectively.
. To three separate portions of this brine soluti~n were mixed, .10 on a Multimixer for 15 minutes, the following:;
r 1. 3 pp~ BOHRAMYL CR
2. 1 ppb HEC
3. 3 ppb BOHRAMYL CR and 1 ppb HEC.
The Fann V-G meter viscosities were then obtained, and after rolling the solutions for 3 hours and overnight at 150F. The data obtained are given in Table 5.

Table 5 BOHR~MYL CR, ppb 3 3 NATROSOL 250 HHR, ppb 0 _ . _ Fann V-G Rheology After_l_ Mlnutes Mixing Apparent Viscosity 7.5 8 *
Plastic Viscosity 7.5 - 7.5- *
- Yield Point ` 0 1 ~ ~ *
- 10-Sec. Gel Str. 0 0 *
After Rolling 3 Hours at 150F
Apparent Viscosity 13.5 20 *
Plastic Viscosity 13.5 19 *
Yield Point - 0 2 *
13-Sec. Gel Str. 1 1 *
After Rolling Overnight at 150F
Apparent Viscosity 14 23.5 **
Plastic Viscosity 14 22 **
Yield Point 0 3 **
10-Sec. Gel Str. 0.5 1 **

* No HEC hydration and dispersion ** Large hydrated lu~ps on top of solution '7~

The data clearly indicate the synergistic res~lts obtained on adding both the BOHRAMYL CR and HEC to the brine.
Indeed, it was noted that the HEC would not hydrate and disperse in the brine when added without the BOHRAMYL CX.

Example ~
-In this example two samples of non-cross-linked hydroxy-ethyl starch were evaluated and compared with BOHRAMYL CR. The HEC sample used was Sample A of Example 2. The samples were evaluated as per the procedure ~iven in Example 2. The two non-cross-linked hydroxyethyl starches had hydroxyethyl degrees of substitution of 0.29 and 0.83. The data, given in Table 6, clearly indicate that the hydroxyethyl ~tarch must be cross-linked in order to inter,act with the ~EC to syner-gistically decrease the fluid loss of the brine.

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Table 6 -HEC, ppb. 1.0 0 1.0 0 1.0 0 1-0 HESl, 0.29 DS, ppb 0 2.0 2.0 0 0 0 0 HES, 0.83 DS, ppg 0 0 0 2.0 2.0 0 0 BOHRAMYL CR, ppb O O O O 0 2.0 2.0 Initial Properties After 15 Minutes on a Multimixer AppareQt Viscosity 22 8 22 9 23 8 26 Plastic Viscosity 19 8 19 9 20 ~ 8 21 Yield Point 7 0 7 0 6 0 11 10-Sec. Gel Str. 1 1 1 1 1 0 Prop~rties After Hydrating Overnig~It at 74F
Apparent Viscosity42 12 43 12 45 10 50 Plastic Viscosity29 12 33 12 31 10 33 Yield Point 27 0 21 0 29 0 34 10-Sec. Gel Str. 4 1 3 1 4 0 4 Fluid Loss 42 19 22 21 90 15 15 Properties After Rolling Overnight at 150F
Apparent Viscosity55 19 73 11 57 13 77 Plastic Viscosity35 19 47 11 37 13 37 Yield Point 41 0 53 0 40 0 81 10-Sec. Gel Str. 6 1 8 0 5 0 10 Fluid Loss 60~ 11 80 35 158 10 3 Non-cross-linked hydroxyethyl starch - .

Example 5 This example demonstrates the synergistic effect on viscosity and fluid loss in fresh water and in lower density brine solutions (11.6 ppg CaC12). The samples used were prepared as follows:
11.6 ppb Brine The indicated amounts of BOHRAMYL CR and ~EC were added to the brine and mixed for 15 minutes on a Multi~ixer. After - obtaining the API viscosities, the samples were rolled at ;10 150F for 16 hours, cooled to 74~, and the API viscosities and API fluid 105s obtained.
Tap Water These samples were prepared and evaluated as in the case of the 11.6 ppg brine with the exception that 0.35 ppb of magnesium oxide were added to each sample to raise the pH and decrease the hydration time of the HEC.
The data, shown in Table 7, clearly indicate synergistic increase in both viscosity and fluid loss on the ~resh water and the 11.6 ppg brine.

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.~ 3 3 3 ' Example 6 This example compares the effect of cross-linked hydroxyethyl starch and non-cross-linked hydroxyethyl starch in combination with HEC in a 10% NaCl solution. The indicated alnounts of HEC and the hydroxyethyl starch were added to separate 350 ml portions of a 10% NaCl solution. Rheological c1ata obtained aEter 25 minutes of mixing and after rolling at 150F, cooling at 74F, and mixing for an additional S minutes - are shown in Table 8. As the data in Table 8~-clearly show, cross-linked hydroxyethyl starch in admixture with HEC syner-gistically interacts to decrease the fluid loss and increase the viscosity. However, samples of hydroxyethyl starch which are not cross-linked only synergistically interact with the HEC to increase the viscosity.

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The invention may be embodied in other specific forms without departing from the spirit or essential character-istics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restric-tive, the scope of the invention being indicated by theappended claims xathex than by the foregoing descr;ption, and all changes which come within the mean~ng and range of equivalence of the claims are therefore~ intended to be ~ embraced therein.

Claims (22)

I claim:

1. A method of decreasing the fluid loss of aqueous well servicing fluids comprising dispersing in said fluid an effective amount of a cross-linked hydroxyethyl starch and an effective amount of a hydroxyethyl cellulose, the relative amounts of said hydroxyethyl starch and said hydroxyethyl cellulose being such as to synergistically decrease the fluid loss of said aqueous fluid.

2. The method of Claim 1 wherein said aqueous fluid comprises an aqueous solution of at least one water soluble salt of a multi-valent metal ion.

3. The method of Claim 1 wherein said aqueous medium has a density greater than about 11.7 pounds per gallon.

4. The method of Claim 2 wherein said water soluble salt is selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide, and mixtures thereof.

5. The method of Claim 2 wherein the density of said aqueous brine is from about 12.0 pounds per gallon to about 19.2 pounds per gallon.

6. The method of Claim 2 wherein said aqueous brine contains from about 0.5% to about 20% by weight zinc bromide and has a density in the range of from about 14.2 pounds per gallon to about 16.25 pounds per gallon.

7. The method of Claim 1 wherein the weight ratio of said hydroxyethyl starch to said hydroxyethyl cellulose is from about 10 to 90 to about 90 to 10.

8. The method of Claim 7 wherein said weight ratio is from about 33 to 67 to about 75 to 25.

9. The method of Claim 1 wherein said hydroxyethyl starch and said hydroxyethyl cellulose are activated prior to being dispersed in said aqueous fluid.

10. A composition for increasing the viscosity and decreasing the fluid loss of aqueous well servicing fluids comprising a mixture of a cross-linked hydroxyethyl starch and a hydroxyethyl cellulose, the relative amounts of said hydroxyethyl starch and said hydroxyethyl cellulose being such as to synergistically decrease the fluid loss from said aqueous fluid.

11. The composition of Claim 10 wherein the weight ratio of said hydroxyethyl starch to said hydroxyethyl cellulose is from about 10 to 90 to about 90 to 10.

12. The composition of Claim 11 wherein said ratio is from about 33 to 67 to about 75 to 25.

13. The composition of Claim 10 wherein said hydroxy-ethyl starch and said hydroxyethyl cellulose are activated.

14. A well servicing fluid comprising:
an aqueous medium; and an effective amount of a cross-linked hydroxyethyl starch and an effective amount of a hydroxyethyl cellulose, the relative amounts of said hydroxyethyl starch and said hydroxyethyl cellulose being such as to synergistically decrease the fluid loss of said aqueous medium.

15. The composition of Claim 14 wherein said aqueous medium comprises a solution of at least one water soluble salt of a multi-valent metal ion.

16. The composition of Claim 14 wherein said aqueous medium has a density greater than about 11.7 pounds per gallon.

17. The composition of Claim 15 wherein said water soluble salt is selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide, and mixtures thereof.

18. The composition of Claim 15 wherein the density of said aqueous medium is from about 12.0 pounds per gallon to about 19.2 pounds per gallon.

19. The composition of Claim 14 wherein said aqueous medium contains from about 0 5 to about 20% by weight zinc bromide and has a density in the range of from about 14.2 pounds per barrel to about 16.25 pounds per barrel.

20. The composition of Claim 14 wherein said hydroxy-ethyl starch and said hydroxyethyl cellulose are activated.

21. The composition of Claim 14 wherein the weight ratio of said hydroxyethyl starch to said hydroxyethyl cellulose is from about 10 to 90 to about 90 to 10.

22. The composition of Claim 21 wherein said ratio is from about 33 to 67 to about 75 to 25.