CA1168850A - Suspension of hydrophilic polymer with an inorganic salt - Google Patents

Abstract

Abstract of the Disclosure A method of preparing a well servicing fluid containing a hydrophilic polymer in which the hydrophilic polymer and water are admixed to form a uniform polymer/water suspension followed by the dissolution of a dry inorganic salt having a positive heat of solution, the amount of salt added being such as to raise the temperature of the polymer/water suspension to above about 160°F.

Description

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Bac~g~ound of the Invention The present invention relates to the preparation of polymer containing brines useful in various applications wherein an increase in viscosity, filtrate control or other functional property is derived from the polymer composition contained therein.
Polymer containing brines are useful as well servicing fluids such as driling fluids, workover fluids, completion fluids, packer fluids, well treating fluids, subterranean 1~ formation treatiny fluids, spacer fluids and hole abandon-ment fluids and in other applications wherein thickened aqueous mediums are required. It is known to use hydrophilic polymers such as hydroxyethyl cellulose (HEC), for example, as thickening agents for aqueous mediums such as those used in well servicing fluids. However, such polymers are not readily hydrated, solvated or dispersed in aqueous solutions containing one or more water soluble salts of multivalent cations such as the heavy oil field brines having a density greater than about 11.6 pounds per gallon (ppg) preferred for the preparation of well servicing fluids. Elevated tempera-tures and/or mixing under high shear for extended periods of time are required for effective thickening of such brines with hydrophilic polymeric materials in order to obtain a homogeneous mixture. In many cases, as, for example, in workover operations, the equipment available for preparing the well servicing fluids does not really lend itself to such conditions. Accordingly, it is usually necessary, if it is desired to use such thickened brines, to prepare them off the well site or to circulate the fluid in the hot borehole.

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Summary oE the Invention It is, therefore, an object of the present invention to provide a method fcr the prep~ration of thickened polymer containing brines, especially heavy brines having a density yreater than 11.6 ppg under conclitions of lo~ shear mixing without the application of heat.
Other objects and advantages of the inven-tion ~
become apparent Erom the following description thereof, together with the appended claims.
~ccording to the invention, a suspension of a hydro-philic polymer and water is formed by generally uniformly dispersing the polymer in the water~ An inorganic salt having a positive heat of solution is then added to the suspension, in the absence of external heating, the amount of salt added being sufficient to raise the temperature of the dispersion to above about 160F as a result of the heat of solution of the salt. This suspension, depending on the density desired, can be used directly as a well servicing fluid.
In another embodiment of the invention, there is added to the liquid suspension of the polymer, a sufficient quantity of a heavy aqueous brine to provide a quantity of a well servicing fluid of the desired density.
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Descri~tion of the Preferred Embocliments The hydrophilic polymers useful in the practice of the invention are articulate organic polymers which are gener-ally water soluble or water dispersible and which upon solution or dispersion in an aqueous medium increase the viscosity o the system but which do not readily hydrate, solubilize or disperse upon addition to heavy brines having a density greater than 11.6 ppg and containing soluble salts of multivalent cations. Such polymers are selected from the group consisting of cellulose derivatives, water dispersible starch derivatives, polysaccharide gums, and mixtures thereof. Exemplary cellu:Lose derivatives are the carboxyalkyl cellulose ethers, such as carboxymethyl cellu-lose and carboxyethyl cellulose; hydroxyalky1 cellulose15 ethers suchas hydroxyethyl cellulose and hydroxypropyl cellulose; and mixed cellulose ethers such as: carboxy-alkyl hydroxyalkyl cellulose, i.e. carboxymethyl hydroxy-ethyl cellulose; alkyl hydroxyalkyl cellulose; i.e. methyl hydroxyethyl cellulose, methyl hydroxypropyl cellulose;
20 ~ alkyl carboxyalkyl cellulose, i.e. ethyl carboxymethyl cellulose. (See U.S. Patent No. 4,110,230). Exemplary starch derivatives are the carboxyalkyl starch ethers such as carboxymethyl starch and carboxyethyl starch; hydroxy-alkyl starch ether$, such as hydroxyethyl starch and25 hydroxypropyl starch; and mixed starch ethers such as:
carboxyalkyl hydroxyalkyl starch, i.e. carboxymethyl hydroxyethyl starch; alkyl hydroxyalkyl starch, i.e. methyl hydroxyethyl starch; alkyl carboxyalkyl starch, i.e. ethyl-carboxymethyl starch. Exemplary polysaccharide gums include: the biopolymers such as Xanthomonas (xanthan) gum; galactomannan gums, such as guar gum, locust bean gum, tara gum; glucomannan gums; and derivatives thereof, particularly the hydroxyalkyl derivatives (See U.S.
Patents Nos. 4,021,355 and 4,105,461). Other polymers which can be used include pre-gelatinized starch powder and stabilized partially dextrinized polysaccharide powder, toxic non-ionic.

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Particularly preferred are the HEC polymers which are generally high yield, water so1uble, non-ionic materials produced by treating cellulose ~7ith so~ium hydroxide followed by reaction with ethylene oxide. Each anh~dro-glucose unit in the cellulose molecule has three reactivehydroxy groups. The average number of moles of the ethylene oxide that b~comes attached to each anhydroglucose unit in cellulose is cal]ed moles of substituent combined. In general, the greater the degree of substitution, the greater the water solubility. In general, it is preferred to use H~C
polymers having as high a mole substitution as possible.
Usually, upon the addition of one of the dry particulate hydrophilic polymers described above to aqueous mediums such as brines, the polymer particles undergo surface hydration preventing the interior of the particle from readily hydrating, solvating or otherwise dispersing in the aqueous medium. Accordingly, high shear, long mixing times and/or elevated temperatures rnust be applied in order to obtain a homogeneous system. Using the method of the present invention, the hydrophilic polymers readily hydrate, dissolve or disperse in the aqueous brine at relatively low shear and a~bient temperatures.
In the initial step of the method, the hydrophilic polymer and water as, for example, fresh water, distilled water, etc., are admixed under conditions so as to provide a uniform dispersion of the polymer in the water. The term "uniform dispersion" as used herein refers to a condition in which the polymer and water form a generally homogeneous system whether it be a solution or a mixture in which discrete polymer particles are generally uniformly distributed through the suspension of polymer and water. ~he polymer and water can be admixed by conventional mixing techniques and no special conditions of temperature, mixing times or other such parameters are required. It is only sufficient that the polymer and water be admixed sufficiently to provide the uniform dispersion of the polymer suspension in the water.
In the next step of the method, an inorganic salt(s) is 8 ~ 0 added, in dry form, to the suspension of the polymer and water, the sall: beiny of a type which has a positive heat of solution and brine generates heat upon dissolving in water.
The amount of the inorganic salt added to the polymer suspension will be such as to provide a temperature of above about 160F as a result of the heat of solution of the salt and without the addition of external heating. Dissolving of the salt in the polymer suspension can be conducted with usual mixing techniques.
The inorganic salt or salts which can be employed in the second step of the method are any water soluble salts which generate heat upon dissolving in water and which preferably form brines which are useful in hydrocarbon recovery operations. Preferred salts are those selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide, and mixtures thereof. As noted, preferably the amount of salt added should be such as to raise the temperature of the polymer/water suspension to above about 160F. However, it is preferred that the amount of salt added be such as to raise the temperature to at least 180F
and most preferably to at least 200F. It ~ill be apparent that different salts have different positive heats of solu-tion and, therefore, the amount of salt or salts added wil be dependent upon the particular salt(s~ which are selected.
The polymer/water suspensions prepared as above can themselves be used as well servicing fluids if the amount of inorganic salts added are sufficient to achieve the desired density. Thus, for example, in a typical case, the amount of polymer, water and inorganic salt admixed may be sufficient to form a thickened brine of the desired density. More frequently, however, there is added to the polymer/water suspension containing the dissolved salt an aqueous brine solution of a given density, the aqueous brine being added in an amount so as to provide a well servicing fluid having a pre-determined density. In this latter embodiment of the method of the present invention, the polymer, water and inorganic salt are mixed as above to hydrate the polymer and -1 1688~0 form the polymer/water suspension. Following this, the aqueous brine is admixed with the polymer/water suspension containing the inorganic salt and the well servicing fluid thus prepared. The aqueous brines which can be admixed with the polymer/water suspensions generally contain soluble salts such as, for example, a soluble salt of an alkali metal, an alkaline earth metal, a Group Ib metal, a Group IIb meta]
as well as water soluble salts of ammonia and other cations.
Generally speaking, such aq~leous brines contain soluble salts of multivalent cations, e.g. Zn and Ca. Thus, aqueous brines comprised of a salt selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide, and mixtures thereof are especially preferred. The aqueous brines will generally have densities ranging from about 11.6 ppg to about 19.2 ppg.
The amount of the hydrophilic polymer used in the method of the present invention will be such as to provide a final concentration of from about 0.1 to about 10 pounds per barrel (ppb) regardless of whether the ultimate well servicing fluid comprises (a) the polymer/water suspension prepared by mixing the polymer, water and the inorganic salt, or (b) the polymer, water, inorganic salt and an amount of an aqueous brine.
While the mechanism of the method of the present inven-tion is not completely understood, it has been found thatbrine solutions prodllced thereby have improved rheological and filtration properties as opposed to brine solutions prepared simply by dispersing the hydrophilic polymer, in dry form, in a brine and then heating the mixture to solvate the polymer. ~he application of artificial heat to the mixture of a hydrophilic polymer and a brine while giving some enhanced results, does not achieve the remarkable results obtained by the method wherein the polymer is first dispersed in water and this suspension then brought to elevated temperature via the mechanism of the natural heat of solution of the inorganic saltts) dissolving in the polymer/water suspension. To more fully illustrate the present invention, 1 ~68~50 the following non-limiting examples are presented.
Unless otherwise indicated~ all physical proper~y measurements were made in accordance with testing procedures set forth in STANDARD PROCEDURE FOR TESTING
DRII.LING FLUID, API RP 13B, Seventh Edition, April, 1978. In the following examples, the following hydrophilic polymers were employed:
Hi Vis CELLE ~ (carboxymethyl cellulose marketed by N~ Industries, Inc.).
BARAZA ~ (xanthan gum marketed by NL Industries, Inc.).
NATRASOL~ 250 HHR (hydroxyethyl cellulose marketed by Hercules Chemical Company).
DRISPA ~ Ipolyanionic cellulose powder marketed by Drilling Specialities).
BOHRAMYL~ (cross-linked hydroxyethyl starch marketed by Auebe, N~V.).
IMPERMEX~ (pregelatinized starch powder marketed by NL Industries, Inc.)-DEXTRI ~ (stabilized partially dextrinized polysccharide powder, toxic nonionic marketed by NL Industries, Inc.).
.

1 ~688~0 g Example 1 Several hydrophilic polymers were used to prepare thickened aqueous brines as described below. Approximately

2 g of the polymer was mixed in 204.4 g of water by means of a Multimixer for about 10 minutes. Thereafter, there was added to the prehydrated polymer 11~.0 g of CaC12 pellets (94-97~) and 280.5 g of CaBr2 (91%) followed by 16.8 ml of a 19.2 ppy CaBr2/ZnBr2 brine to bring the density of the resulting brine to 15.2 ppg. The heat of solution of the added salts brought each sam~le to boiling (212F). The resulting thickened mixtures were allowed to stand overnight at ambient temperature and the rheological and filtration properties of each mixture were then determined. Rheological properties were measured using a Fann ~lodel 35A Viscometer and a Brook-field RVT Viscometer. The filtration properties weremeasured on an API filtration press. The properties reported are plastic viscosity (PV) cp, Yield Point (YP) lb/100 ft2, apparent viscosity (AV) cp, 10-second gel strength (GEL 10 s) lb/lOO ft2, and API filtrate (API-FIL) ml. All filtration testing was performed after 10 lb/bbl CaCo3 was added as a bridging agent. Xesults of the measurements made are presented in Table 1 below. Table 2 gives the same informa-tion for identical samples after being rolled for 16 hours at 150F.

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PV YP AV 10 s FIL

CELLEX ~IV - 0 150~ 24.5 1.5 BARAZAN 92 30 109 2.5 6.5 DRISPAC - - 150+41.0 0.5 BOHR~YL 80 4 82 2.0 3.0 I~lPERMEX 72 6 74 2.0 100 DEXTRID 73 6 76 1.5 53 Table 2 GEL API
- PV YP AV 10 s FIL
CELLEX ~V - - 150+ 11.0 0.5 BARAZ~N - - 150~ 6.0 4.5 DRISPAC - - 150+ 9.0 0.5 BOHRAMYL 76 4 78 2.0 1.0 IMPE~5EX 62 4 64 2.0 77 DEXTRID 68 6 71 2.0 89 '~
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8~50 Example 2 Control thickened aqueous brine solutions were prepared by adding 2 g of each of the dry polymers eTnployed in Example 1 to a pre-made 15.2 ppg brine prepared by mixing 21~.2 g o H2O, 119.5 g of CaC12 , 2~4 g -of CaBr2 and 16.2 ml of a 19.2 ppg CaBr2/ZnBr2 brine. Data on the rheological and filtration properties of the control samples after standing overnight are presented in Table 3 below while similar data on identical samples after being rolled for 16 hours at 150F
are presented in Table 4 below. These data compared to those in Tables 1 and 2 demonstrate that the brines prepared by the method of the invention (Example 1) wherein the polymer is first hydrated and the dry salts added thereto exhibit superior viscosity and give lo~er filtrates in every case before hot rolling and in substantially all cases after hot rolling.

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Table 3 GEL API
_ YP AV 10 s FIL_ CELLEX HV 54 0 54 2.0 180 BARAZAN 46 0 46 1.5 201.
DRISPAC 55 1 55 1.5 174 BOHRAMYL 49 1 49 1.0 190 IMPEP~IEX 51 1 52 2.0 172 DEXTRID 51 1 51 1.5 170 Table 4 GEL API
PV YP AV 10 s FIL
CELLEX HV 49 0 50 2.0 250 BARAZAN 67 - 67 1.5 DRISPAC 50 3 52 1.5 262 BOHRAMYL 73 - 71 1~5 6 IMPE~IEX 67 6 70 2.0 42 DEXTRID 65 - 64 1.5 6 ~ . ` ": ' , .

~ 1~88~0 Example 3 Using the method described in Example 1, polymer con-taining aqueous brines of several different densities wereprepared. Five grarns of Hi Vis Cellex were prehydrated in water by mixiny for 10 minutes. Dry CaC12 pellets~7ere added to the prehydrated polymer with mixiny to obtain a }li Vis Cellex concentrate of 11~6 ppy density. One hundred forty m]
of the concentrate was added to 210 ml of an aqueous brine of 11.6ppg density to achieve a 2 ppb polymer concentration. In the same manner, heavy aqueous brines of 14.2 and 17.0 were prepared. The composition of each of the a~ueous brines is listed in Table 5 below.

Table 5 11.6 14.2 17.0 Water, ml 299.6 234.2 1]2.2 CaC12 g 197.7 137.9 62.6 CaBr2, g ~ 224.3 154.1 19.2 ppg brine, ml - - 166.6 :: `

1 lB~850 Control samples of thickened aqueous brines were prepared by mixing 2 g of the dry Hi Vis Cellex with premade brines having densities of 11.6, 14.2 and 17.0 ppg, respec-tively. Rheological and filtration measurements of the samples prepared by the method of Example 1 and the control samples wcre made as described in the foregoing examples.
Results are presented in Table 6. In Table 7, the data obtained on all the samples after hot rolling at 150F for 16 hours are presented. It is obvious from these data that the apparent viscosities of the brines prepared by the method of the invention have values twice as large or more than the controls. The superior filtration properties of the samples prepared by the method of the invention are readily evident as well from a comparison of these data.

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1 16~0 Exam~le 4 Brines having a density of 15.4 ppg and containing 1 ppb carboxymethyl cellulose, either Hi Vis CELLEX (high Vis-cosity Grade) or DRISPAC, were prepared by adding the polymer to 176 ml (equivalent to 0.5036 bbl) water and mixing to dissolve the polymer. Thereafter, there were added, while mixing, 114 g CaC12 (95%) and 180 g CaBr2 (91~) (equivalent to 114 and 180 ppb, respectively). The heat of solution of these salts increased the temperature to 212F. The API
rheology and fluid loss was determined on these viscous solutions after cooling to room temperature. The data obtained are given in Table 8.

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Thereafter, the indicated amount of CaC12 (95~ active) was added while mixing. The heat of solution of the CaC]2 increased the temperature about 180F and the solution became - more viscous. The indicated amount of a 14.2 ppg CaBr2 solution was slowly added followed by the indicated amount oE
a 19.2 ppg ZnBr2/CaBr2 solution. After cooling to room temperature in one hour, the API rheology and fluid loss were obtained. The solutions were then rolled at 150F for 16 hours, cooled to room temperature, and the API rheology and fluid loss determined. The data are given in Table 9.

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Example 6 Three Hi Vis Cellex polymer concentrates were prepared by dispersing 2 grams of the polymer in 3.89 ml of water. One concentrate (Control Sample) was diluted with 155.5 ml of H2O
followed by the addition of 114.0 grams of CaC12, 280.5 gramc of CaBr2 and 16.8 ml of 19.2 ppg CaBr2/ZnBr2 brine. The temperature of this sample reached 212F.
A second concentrate (Sample A) was added to a brine solution at room temperature. I'he brine was prepared with 155.5 ml of H2O, 114.0 grams of CaC12, 280.5 grams of CaBr2, and 16.8 ml of 19.2 ppg CaBr2/ZnBr2 brine. The mixing in of the concentrate caused heat evolution, bringing the sample temperature to 114F. It was noted that insoluble clumps of carboxymethyl cellulose formed immediately. When cooled overnight, a large amount of suspended strings were formed which eventually floated to the surface. It did not appear that any of the polymer concentrate went into the brine solution.
A third concentrate (Sample B~ was prepared as in the case of Sample A and once again heat was evolved when the concentrate was mixed in, the temperature reaching 111F.
Following this, Sample B was rolled at 212F for 3 hours in an aging cell. Sample temperature was measured at 160F after aging. Although some suspended strings of polymer formed and floated to the top, most of the polymeric concentrate appears to have been dispersed.
The above three samples were then rolled for 64 hours at 150F. ~ollowing cooling, they were run on a Brookfield Viscometer at 50 rpm. The Control Sample maintained a reading of 1590 cp and exhibited a smooth consistency, as before rolling.
Mosty of the insoluble clumps and all of the suspended strings dissolved in Sample A. The Brookfield reading was, nevertheless, only 180 cp.
Sample B, which appeared homogeneous, showed a Brook-field reading Oc 250 cp.
From the above results, it can be seen that simply pre--1 ~ 6~50 hydrating the polymer in water alone (Sample A) is not thc mechanism of the salt activated method of the present inven-t' tion. In other words, it is also necessary, following dispersion of the polymer in water, that the dry, positive heat of solution salt(s) be added to the polymer concen-trates. While the application of art:ificial heat (rolling at 212F) brings some response (250 cp Brookfield reading on Sample B, as compared to ]80 cp Brookfield reading on Sample A), the effect is still no where the 1590 cp Brookfield reading used on the Control Sample prepared by the salt activated method.

~ 16~850 ~- 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 the appended claims rather than by the foregoing description, and all changes which co~e within the meaning and range of equivalence of the claims are therefore intended to be embraced therein.

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Claims (17)

We claim:

1. A method of preparing a liquid suspension of a hydrophilic polymer in an aqueous salt solution comprising mixing a hydrophilic polymer and water under conditions which provide a generally uniform dispersion of said polymer in said water, dissolving in said dispersion an inorganic salt having a positive heat of solution, said salt being added in an amount so as to achieve a temperature in said dispersion above about 160°F as a result of the heat of solution of said salt.

2. The method of Claim 1 wherein said hydrophilic polymer is selected from the group consisting of carboxyalkyl cellulose ethers, hydroxyalkyl cellulose ethers, mixed cellulose ethers, carboxyalkyl starch ethers, hydroxyalkyl starch ethers, mixed starch ethers, polysaccharide gums, pregelatinized starch powder, stabilized partially dextrin-ized polysaccharide powder, toxic nonionic, and mixtures thereof.

3. The method of Claim 1 wherein said inorganic salt is selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide and mixtures thereof.

4. The method of Claim 1 wherein the temperature generated by the heat of solution of said inorganic salt is above about 180°F.

5. The method of Claim 1 wherein the temperature generated by the heat of solution of said inorganic salt is above about 200°F.

6. The method of Claim 1 wherein said hydrophilic polymer is present in said suspension in an amount of from about 0.1 to about 10 pounds per barrel.

7. The method of Claim 1 wherein said hydrophilic polymer comprises hydroxyethyl cellulose.

8. A method of preparing an aqueous brine useful as a well servicing fluid comprising mixing a hydrophilic polymer and water under conditions which provide a generally uniform dispersion of said polymer in said water, dissolving in said dispersion an inorganic salt having a positive heat of solution, said salt being added in an amount so as to achieve a temperature in said dispersion above about 160°F as a result of the heat of solution of said salt, and adding to said dispersion an aqueous brine of a given density in an amount sufficient to produce a well servicing fluid of a pre-determined density.

9. The method of Claim 8 wherein said hydrophilic polymer is selected from the group consisting of carboxyalkyl cellulose ethers, hydroxyalkyl cellulose ethers, mixed cellulose ethers, carboxyalkyl starch ethers, hydroxyalkyl starch ethers, mixed starch ethers, polysaccharide gums, pregelatinized starch powder, stabilized, partially dex-trinized polysaccharide powder, toxic, nonionic, and mixtures thereof.

10. The method of Claim 8 wherein said inorganic salt is selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide and mixtures thereof.

11. The method of Claim 8 wherein the temperature generated by the heat of solution of said inorganic salt is above about 180°F.

12. The method of Claim 11 wherein the temperature generated by the heat of solution of said inorganic salt is above about 200°F.

13. The method of Claim 8 wherein said aqueous brine contains at least one salt selected from the class consist-ing of water soluble salts of alkali metals, alkaline earth metals, Group Ib metals, Group IIb metals, and mixtures thereof.

14. The method of Claim 13 wherein said aqueous brine comprises a water soluble salt selected from the group consisting of calcium chloride, calcium bromide, zinc chloride, zinc bromide, and mixtures thereof.

15. The method of Claim 8 wherein said hydrophilic polymer comprises hydroxyethyl cellulose.

16. The method of Claim 8 wherein said polymer is present in an amount of from about 0.1 to about 10 pounds per barrel.

17. The method of Claim 8 wherein said well servicing fluid has a density of from about 11.6 to about 19.2 pounds per gallon.