GB2173507A - Process for the preparation of polymer additives for use in aqueous drilling fluids - Google Patents

Abstract

Polymer compositions suitable for use as water loss control additives in aqueous drilling fluids are prepared by copolymerizing a hydrophilic vinyl monomer and at least one hydrophobic vinyl monomer, said polymerization being carried out in an inert, low free radical chain transfer, organic liquid medium at a polymerization temperature and for a period of time sufficient to produce a copolymer comprising said hydrophilic monomer and said hydrophobic monomer and having a weight average molecular weight in the range from 100,000 to 500,000.

Description

1 GB2173507A 1

SPECIFICATION

Process for the preparation of polymer additives for use in aqueous drilling fluids This invention relates to a process for the preparation of polymer additives for use in aqueous 5 drilling fluids.

Drilling fluids are used in the drilling of oil and gas wells to cool and lubricate the rotating drill bit and drill string shaft, convey rock cuttings to the earth surface for removal, prevent loss of water and drilling fluids into the formation through which the borehole is being drilled, and to control the entry of liquids into the borehole from the various formations being penetrated during 10 the drilling. To accomplish these ends, a drilling fluid, generally referred to as drilling mud, comprises several components. For example, weighting solids such as barites are often added to such drilling muds to develop the desired density in the mud, while bentonite or various clays are often added to increase the drilling mud viscosity to improve the capability of the drilling muds to convey rock cuttings upwardly through the borehole from the drill bit and remove the 15 cuttings from the borehole.

It is also known to add various polymers to drilling fluids or drilling muds to function as viscosifiers, dispersants and water loss additives in the drilling fluids-. The use of polymers for such purposes meets with many problems. Such polymers are often unstable at high borehole temperatures, e.g. temperatures around about 360'F (182oC), and tend to lose their desired physical characteristics, such as plastic viscosity, yield point and resistance to water loss. The previous use of polymers for water loss control in drilling fluids has often resulted in an undesirable increase in viscosity of the drilling fluids.

The present invention provides polymers suitable for use as water loss additives for drilling fluids and methods for the preparation thereof which overcome the disadvantages encountered 25 with previous polymers noted above.

According to the invention, such polymers are prepared by copolymerizing a hydrophilic vinyl monomer and at least one hydrophobic vinyl monomer, said polymerization being carried out in an inert, low chain transfer, organic liquid medium at a polymerization temperature and for a period of time sufficient to produce a copolymer comprising said hydrophilic monomer and said 30 hydrophobic monomer and having a weight average molecular weight in the range from 100,000 to 500,000.

The polymers of this invention which are employed with drilling fluids as water loss additives after at least partial neutralization are, preferably, those polymers derived from copolymerizing a hydrophilic vinyl monomer selected from among acrylic acid (AA), methacrylic acid (MAA), other 35 related monomers and mixtures of any two or more thereof, with at least one hydrophobic vinyl monomer selected from among acrylic acid esters, such as, for example, methyl acrylate and ethyl acrylate, methacrylic acid esters, such as, for example, methyl methacrylate and ethyl methacrylate, vinyl esters of saturated monocarboxylic acids having 1 to 3 carbon atoms, such as, for example, vinyl formate and vinyl acetate, other related monomers, and mixtures or combinations of any two or more thereof. Presently preferred polymers of this invention are those selected from among an acrylic acid-methyl methacrylate copolymer (AA-MMA), a metha crylic acid-methyl acrylate copolymer (MAA-MA), a methacrylic acid-methyl methacrylate co polymer (MAA-MMA), an acrylic acid-vinyl acetate copolymer (AA-VA), and an acrylic acid methyl methacrylate-vinyl acetate terpolymer (AA-MMA-VA).

The polymers are prepared in any suitable inert, low free radical chain transfer, organic liquid medium. Non-polar diluents or solvents such as n-hexane, cyclohexane, chlorofluorocarbons such as 1,1,2-trichloro-1,2,2-trifluoroethane, and mixtures of any two or more thereof, are preferred since the polymer formed is insoluble in the medium and can be readily separated therefrom, if desired, by suitable means such as filtration. The diluent or solvent 1,1, 2-trichloro-1,2,2-trifluoro50 ethane is commercially available under the Registered Trademark Freon-1 13. The separated product can be washed, dried, slurried in water and treated with an appropriate amount of sodium hydroxide to obtain the sodium salt as an aqueous solution. An appropriate amount of sodium hydroxide for this purpose would be, for example, an amount sufficient to raise and maintain the pH of the slurry to a value in the range from about 5.3 to about 12, preferably from about 5.5 to about 11, and more preferably from about 6 to about 8.

A free radical initiator is normally employed in the polymerization process. Such initiators are well known in the art and include azo compounds such as azobisisobutyronitrile and organic peroxy compounds such as t-butyl peroxypivalate. The amount of initiator used, based on the weight of monomers employed, depends on the monomers chosen as well as the chosen 60 reaction medium and the polymer molecular weight desired. Sufficient initiator is employed such that the weight average molecular weight of the resulting polymers will range from about 100,000 to 500,000. For example, when preparing the acrylic acid-methyl methacrylate co polymers in n-hexane with t-butyl peroxypivalate as the initiator, the initiator level can range from about 0.05 to about 1.0 weight percent, preferably from about 0. 1 to about 0.8 weight percent, 65 2 GB2173507A 2 based on the weight of monomers charged.

The mole ratio of hydrophilic monomer or monomers to hydrophobic monomer or monomers can be any mole ratio which provides a copolymer having the desired characteristics, but the mole ratio generally can range from about 22:1 to about 1.1, preferably from about 7:1 to about 1: 1, and more preferably from about 3.5:1 to about 2: 1. For example, for the most preferred acrylic acid-methyl methacrylate copolymers, the AA-1VIMA mole ratios range from about 3.2A to 2.1A, corresponding to a AA-1VIMA weight ratio range from about 70:30 to about 60:40.

For the methacrylic acid-methyl methacrylate copolymers, the weight ratios of MAA-MMA can range from about 95:5 to about 46:54, and from about 95:5 to about 65:35 for best results.

The equivalent mole ratios are from about 22:1 to about 1A, and from about 22:1 to about 10 2A. For the acrylic acid-methyl methacrylate-vinyl acetate copolymer, the weight ratios of AA-1VIMA-VA can range from about 60-30-10 to about 60-10-30 and from about 70-20-10 to about 70-10-20 for best results. The equivalent mole ratios are in the range from about 2.8:1:0.33 to about 8.3A:3.5, and from about 4.8A:0.58 to about 9.7:1:2.3.

Generally, in preparing the polymers of the instant invention, the total monomer level with 15 respect to the reaction medium can range from about 5 to about 30 weight percent, more preferably from about 10 to about 20 weight percent based on the weight of the reaction medium. Expressed differently, the weight ratio of monomers to the reaction medium is generally in the range from about 5:95 to about 30:70, and preferably in the range from about 10:90 to about 20:80.

Polymerization temperatures are generally in the range conventionally practiced and can range, for example, from about 25'C to about 100'C, preferably from about 50'C to about 70'C, and most preferably at about 50'C for the best results. The polymers prepared at about 50'C exhibit the best control of water loss in the tests used.

Each polymerization was conducted for a sufficient length of time to obtain substantially 25 quantitative conversion. Generally, a polymerization time in the range from about 10 minutes to about 30 hours is satisfactory. More specifically, when about 20 g of total monomers are employed in about 200 mi- of reaction medium containing from about 0.1 to about 1.5 weight percent, and preferably from about 0.1 to about 0.8 weight percent t- butyl peroxypivalate and a reaction temperature ranging from about 50'C to about 70'C, a polymerization time of from about 15 to about 30 hours is used for convenience; however, the conversion is substantially complete in less than four hours. In commercial operations, it may be desirable to reduce the polymerization time to about one hour or less, or even to about 10 minutes.

In a typical bench scale preparation, the appropriate molar ratio of acrylic acid and methyl methacrylate (20.0 g total) was charged to a 10 fl. oz. (296 mL) crown top beverage bottle containing 200 mL of the reaction medium and the desired quantity of t- butyl peroxypivalate initiator solution, usually about 0.75 weight percent based on total weight of monomers. The initiator, available commercially under the Registered Trademark Lupersol- 1 1 from Pennwalt Corp., was contained as a 75 weight percent solution in mineral spirits. The monomers were of technical grade purity and used as received.

Each crown top bottle was degassed for about 20 minutes with argon after charging the components. capped, and immediately placed in a water bath maintained at a desired tempera ture, for example at 50'C or 70'C, and rotated the desired time. All polymerization runs produced polymer slurries. Each polymer was isolated by filtration, dried, and weighed to confirm that the degree of conversion was substantially quantitative, e.g. , from about 95 to about 100 percent of theoretical.

After determination of yields, the polymers were slurried in distilled water to which was added a predetermined quantity of sodium hydroxide sufficient to raise the polymer solution pH and maintain it at a value in the range of from about 6 to about 8 and produce about a 10 weight percent solution of the neutralized polymer or dry polymer salt in the water.

The effectiveness of the polymers as water loss control agents and rheological control agents were determined at several temperatures in a moderately saline drilling mud and in a saline saturated drilling mud.

The first mud, designated saline mud A, comprised 3.5 weight percent attapulgite clay in water containing 5 weight percent NaC]. The second mud, designated saturated saline mud B, 55 comprised 3.5 weight percent attapulgite clay in water saturated with NaCI.

A suitable amount of the polymer salt solution was added to the mud samples to obtain the desired concentrations. While the concentration of the polymer salt solution contained in a drilling fluid or mud can be any concentration which will achieve the desired results from the drilling fluid or mud, it is presently preferred to employ a concentration in the range from about 60 1 to about 3 pounds per barrel of drilling fluid or mud. As used herein, the term---barrel-is defined as having a capacity of 42 U.S. gallons. After the addition of the polymer sample to the mud slurry, withholding part of the water used in making the mud slurry to compensate for the water introduced by the polymer solution when used, the mud samples were mixed for twenty minutes with a Hamilton-Beach Multimixer and then aged at least 2 hours at room temperature, 65 3 GB2173507A 3 e.g., about 2WC. The samples were then stirred an additional two minutes with the mixer just before determining their initial plastic viscosities and yield points at about 25C with a model 35 Fann V-G meter, a direct indicating viscosimeter, in accordance with API RP 1313, 2nd ed., April 1969, -Standard Procedure for Testing Drilling Fluids-, American Petroleum Institute, Division of Production, Dallas, Texas. Initial water loss at about 2WC was also determined in accordance with this reference. Additionally, the pH of each sample was determined. These tests were then performed again after the samples were aged overnight at about 80C and cooled to room temperature.

Selected polymers were also evaluated under more severe conditions. For these tests, base mud A was treated with 12 1b/bbi of bentonite, 2 1b/bbl of a commercially available thinner sold 10 under the trademark Desco and available from Chromalloy Corp., Conroe, Texas, and 3 1b/bbl of polymer and NaOH. A series of the treated muds was also additionally treated with gypsum to ascertain its effect on the polymers. For example, 1 1b/bbl gypsum when added to the mud samples provides about 400 ppm calcium ion in the mud filtrates as determined by versenate titration.

These treated mud samples were generally tested initially at room temperature and again at room temperature after aging overnight at about 360'F (182'C). Following this second series of tests an additional water loss test was performed at about 325'F (1 63'C) with a differential pressure of about 500 psi. These tests were made with about 600 psi applied to the water loss cell and about 100 psi applied to the filtrate collector.

EXAMPLE 1

Three series of acrylic acid-containing polymers were prepared in which the acrylic acid (AA) content varied from about 60 weight percent to about 100 weight percent, and the methyl methacrylate (MMA) content varied from zero to about 40 weight percent, based on the total 25 monomer weight. In each run the total monomer weight was about 20.0 g; 0. 20 9 of t-butyl peroxypivalate (13PP) solution containing 0.15 9 BPP in concentration mineral spirits, which is equivalent to about 0.75 weight percent BPP based on the total monomer weight, was used; and the reaction was conducted for a period of time in the range from about 16 to about 25 hours in 200 mL of the specified liquid organic compound at about WC until conversion was 30 substantially complete. The polymers were isolated, washed with n-hexane, Freon-1 13 or the like, the vacuum dried. The yields ranged from about 95 to about 100 percent of theoretical.

Each polymer was slurried in about 135 g of distilled water and treated with sufficient 6 M NaOH solution to obtain an aqueous solution having a constant pH value in the range of about 6 to about 8 and containing about 10 weight percent solids based on the total weight of the 35 aqueous solution.

In one series, the reaction medium was n-hexane, with 200 mL of n-hexane being equivalent to about 132 g. In the second series, 200 mL of Freon-1 13, equivalent to about 313 9, was employed as the reaction medium. In the third series, 200 mi- of t-butyl alcohol, equivalent to about 156 g, was employed as the reaction medium.

Each polymer was evaluated at a loading (dry polymer salt) of 1 lb/bbi and of 3 lb/bbi in saline mud A (5% salt water mud) and in saturated saline mud B (saturated salt water mud), said mud being described previously.

The initial properties of each treated mud sample were evaluated at room temperature (about 2WC) for plastic viscosity (PV) in centipoises (cp), yield point (YP) in lb/100 ft2, and water loss 45 (WL) in mL/30 minutes, in the manner previously described. The results are presented in Tables 1 A, 1 B and 1 C.

Polymer 5 Wt. % MNA 0 10 4.11 TABLE IA

AA-HMA Copolymers P ared In Freon-113f f!T 4 at WC Saline Mud A 1 LbTB-bi 3 Lbs/B51 py(a) /YP (b) WL "c P0a)/yp(b) wr(c) 4/5 68.0 616 4/6 69.0 616 3/6 58.0 714 5/3 40.0 714 30 4/2 26.5 7/4 Mud A Only 5110 129 - Note: A dash signifies no determination was made (a) cp.

(b) lbs.1100 ft 2 is (c) mL/30min.

20.5 21.0 23.0 23.0 23 0 Saturated Saline Mud B 1 Lb/Bbl PW (a) /'JL K WL(C) 7/15 8/12 7111 7/4 7/5 55.1 44.0 38.0 24.0 19.0 3 Lbs/Bbl (a) (b) (CY PV /Yp WL.

14/6 13/7 12/10 1919 9/5 13.0 13.0 14.0 10.9 8.5 (n Polymer 5 Wt - % HMA 1 W/M Py(a),yp(b) WL(C-Y TABLE 1B

AA-MA Copolymers Prepared In Freon-1138 at 500C Saline flud A 3 US7Bbl PV (a) lyp (b) WLUC7 Saturated Saline Mud B 1 Lb/Bbl 3Lbs/Bbl (a) (b).jnc (a) (b) PW lyp WL PV /YP 0 215 60.0 4/4 14.0 6112 70.0 7/5 18.0 3/2 53.0 10/9 14.5 5/8 57.0 10/3 12.5 8/4 50.0 613 16.0 6/6 46.0 13/7 10.4 411 48.0 613 24.5 7/6 31.0 16/9 12.5 30 3/2 34.0 6/3 20.5 8/4 21.0 1318 6.0 3/1 17.5 611 10.0 10/6 29.0 9/3 4.3 (a) cp. (b) lbs./100 ft 2 (c) mL/30min.

(n 0) Polymer 5 Wt. % HMA TABLE IC

AA-MMA Copolymers Prepared In Tert-Butyl Alcohol at 50% Saline Mud A 1 Lbl 1 3 LE7IBIET- py(a) lyp (b) WL(C) pW(a),yp(b) WL(c) Saturated Saline Mud B 1 Lb/Bbl 3 Lbs/Bbl pV(a)/yp(b) WL(cl pV(a) /YP (b) WL(C-Y 0 4/4 53.0 - 6/4 54.0 515 66.0 - - 9/3 38.0 - - 4/5 65.0 - - 7/3 39.5 3/5 79.0 5/5 63.0 - - 4/2 17.0 30 314 61.0 6/4 62.0 - - 6/1 16.1 Note: A dash signifies no determination was made. (a) cp. (b) lbs/100 ft2 (c) mL/30min.

m 7 GB2173507A 7 Inspection of the data presented in Tables 1 A, 1 B and 1 C shows that the performance of the polymers at moderate temperatures generally improve as the methyl methacrylate content of the copolymers approaches 30 or 40 weight percent, which is the preferred level range based on these results. At these levels of methyl methacrylate content, the plastic viscosity and yield point of the muds are generally not significantly increased by the presence of the polymers. The 5 data also show that the polymers made in n-hexane and Freon-1 13 are more effective than the polymers made in t-butyl alcohol.

EXAMPLE 11

The effect of higher molecular weight acrylic acid-methyl methacrylate copolymers (as the sodium salts), prepared in n-hexane in one series and in Freon-1 13 in another series, as water loss control agents in the base muds was ascertained. The polymer samples were prepared as in Example 1 except that the BPP level was decreased from about 0.75 weight percent to about 0.19 weight percent. A decrease in initiator concentration causes a decrease in the number of free radical fragments available in the monomer solution to initiate polymerization. This decrease, 15 in turn, leads to an increase in polymer molecular weight. Thus, these polymers are of higher molecular weight than those shown in Example 1.

Each polymer salt was evaluated as before in saturated saline mud B. The results are given in Table 2.

00 TABLE 2 Higher Molecular Weight AA-MMA Copolymers Prepared at 500C. Saturated Saline Mud B Results n-Hexane Medium Freon 113 Medium Polymer -1 Lb/Bbl 3 Lbs/Bbl 1 Lb/Bbl - 3 Lbs/Bbl Wt. % HMA py(a)lyp(b) WL(c) pW(a)/yp(b) WL(c) PV(a) /YP (b) WL(C) Pv(a), yp(b) WL(c) 0 8111 47.0 9111 0.0 714 40.0 911 5.0 5/3 29.0 712 6.o 614 37.0 9/3 7.0 5/5 28.0 1011 6.o 7/1 33.0 10/3 6.5 20 614 20.5 812 5.5 6/1 18.0 10/2 6.5 7/1 15.0 12/0 6.o 8/1 18.0 11/4 5.5 8/1 16.0 1515 4.5 6/1 25.0 14/6 4.s so - - - - 7/3 44.0 19/7 5.5 f oam (a) cp. 2 (b) Ibs./100 ft.

(c) aL/30 min.

G) m N I.i W 01 0 I.i CO 9 GB2173507A 9 Comparison of the results shown in Table 2 with those shown in Tables 1 A and 1 B shows that somewhat better water loss control is achieved with the higher molecular weight polymers at the moderate temperatures employed than with the equivalent lower molecular weight polymers. It is noted that foaming occurred with the 50:50 AAWIVIA copolymer which is not usually 5 considered desirable in a drilling mud.

EXAMPLE 111

This example was directed to the determination of high temperature water loss properties. Three series of copolymers were prepared in the manner previously described. In one series the reaction medium was n-hexane, in the second series it was cyclohexane, and in the third series 10 it was Freon-1 13. Several initiator levels were also employed in each series. A polymerization time of about 21 hours was employed in preparing the 60:40 AAWIVIA copolymers in n-hexane and in Freon-1 13. All other polymers were given about a 25 hours polymerization time. Each polymer was prepared at about WC. As before, each polymer was evaluated in treated saline mud A as'the sodium salt in a 10 weight percent aqueous solution at a pH value in the range 15 from about 6 to about 8. The mud was also treated with 12 Ibs/W bentonite clay and 2 1b/bbl Desco thinner to reduce the adverse effects of high temperature aging on the clays, and sufficient NaOH to give and maintain a pH value in the range from about 10 to about 11.

After determining the initial properties of the mud samples at room temperature, the samples were aged at about 360OF (182'C) in brass bombs, cooled to room temperature, and the shear 20 strengths of the muds were then measured. The muds were then removed, stirred for about 20 minutes with the previously described multimixer, and the tests of plastic viscosity, yield point and room temperature water loss were repeated. An additional water loss test at about 325'F (163'C) and at a differential pressure of about 500 psi was also performed. The results are reported in Tables 3A, 313 and 3C.

Wt. % wt. % HMA BPP 40 40 30 30 30 0.75 0.38 0.19 0.75 0.38 0.19 Initial Water Loss 1 Lb/Bbl 2 Lbs/Bbl 8.0 6.o 5.2 5.2 5.2 5.2 5.9 4.4 4.3 4.3 4.2 3.7 TABLE 3A AA-HRA Copolymers, High Temperature Runs Variable Initiator (UP) Levels, n-Hexane Medium Properties after 16 Hours at 3601F (182%) 1 Lb/Bbl. SSM PV(c)/YP(d) WL(e) 10/23 10.0 230 9/25 10.0 230 12/24 10.7 10/25 13.0 11/28 11.2 11/24 10.4 2 Lbs/Bbl.

SS(b) Mc)/YP(d) WL(e) 12/23 210 16122 8129 11/30 14/23 14/28 5.4 5.4 5.0 5.8 5.7 5.9 Base mud only, PV(c)/YP(d)=5/20, WL=11.0 initially; after 16 hours aging at 360OF(I82OC), PVIYP=7/10, WL=35.0(e), SS=135(b); HM(a)=100.

(a) HTWL is high temperature water loss, mL/30 minutes; (b) SS is shear strength, Ibs/100 square feet; (c) %;l0Oft.2 (d) lb / (e) mL/30 min.

H T W L (a) ILb/Bbl 2Lbs/Bbl 46 48 42 38 39 41 27 30 28 30 30 28 TABLE 3B

AA-MRA Copolymers, High Temperature Tests wt. % wt.% MMA BPP 40 40 40 0.75 0.38 0.19 0.075 Initial Water Loss 1 Lb/Bbl 2 Lbs/Bbl 6.3 5.5 8.6 5.4 6.7 6.7 5.5 5.0 1 Lb/Bbl.

Variable Initiator (BPP) Level, Cyclohexane Medium Properties after 16 Hours at 360'F (182%) 2 Lbs/Bbl. SSM PV(c)/YP(d) WL(e) SSM PV(c)/YP(d) WL(e) 12/21 10.6 170 12123 6.o 10/22 11.0 150 15120 5.6 11/24 10.5 - 10/22 5.4 210 10/26 12.0 10/23 6.o (a) HTLW is high temperature water 12ss, mL/30 min.

(b) SS is shear strength, lbs.llooft (c) cp. 2 (d) lbs./looft (e) mL/30 min.

Note: A dash signifies no determination was made H T W L (a) lLb/Bbl Ms/Bbl 36 36 37 35 31 29 32 30 N) TABLE 3C AA-MMA Copolymers, High Temperature Tests Variable Initiator (BPP) Level, Freon-113@ Medium Wt. % Wt. % MNA 40 30 30 BPP 0.75 0.19 0.75 0.19 Initial Water Loss 1 Lb/Bbl.

1 Lb/Bbl 2 Lbs/Bbl 4.3 4.0 5.2 4,o Properties after 16 Hours at 360'F (182C) SSM PV(c)/YP(d) WL(e) (a) IM is high temperature water los, mL/30 min.; (b) SS is shear strength, lbs.110Oft. (c) (d) (e) cp.; 2 Ibs/100 ft mL/30 min.

- 170 170 170 2 Lbs/Bbl.

SS(b) PV(c)/W(d) WL(e) 12/18 5.4 17/23 5.8 15124 5.6 11/30 s.4 H T W L (a) ILb/Bbl 2Lbs/Bbl - 26 - 36 - 29 - 31 G) ca N) I.i W (n 0 lli N 13 GB 2 173 507A 13 A comparison of the data presented in Tables 3A, 313 and 3C shows that the polymers prepared in n-hexane, cyclohexane or Freon-1 13 are all about equivalent in performance as water loss control agents, particularly when employed at a concentration of 2 lb/bbi in the mud under test. At this concentration, after aging for about 16 hours at about 360OF (1820C) the 5 results indicate that the polymers may slightly lose some water loss control. The results in Example Ill indicate that the 60:40 or 70:30 AA:1VIMA copolymers are about equivalent in performance. The initiator level used in preparing the polymers did not appear to be a controlling factor in this series. The high temperature water loss properties shown by the polymers are good.

EXAMPLE 1V

This example is directed to the determination of high temperature water loss properties of the polymers of the instant invention in gypsum muds. The polymers described in Example 1 and Table 1 B, prepared at about WC in Freon- 113, were also tested in the bentonite clay/Desco thinner-treated mud described in Example ill, this mud being additionally treated with 1 or 2 lb 15 of gypsum per barrel. As described above, plastic viscosity, yield point and water loss were determined initially and again after aging for about 16 hours at about 360'F (182,C). High temperature water loss values were also determined. Unless specified otherwise, each treated mud was also treated with 3 lb of polymer per barrel of mud. The results are presented in Table 4.

P.

TABLE 4

AA-HMA Copolymers, High Temperature Tests in Gypsum Mud 1 Polymer Gyps Initial Properties Aged 16 Hours at 36CF(I82'C) Wt.% MNA Lbs/Bbl PV(a)/YP(b) WL(c) PV(a)/YP(b) WL(O HTWL(d) 0 0 9/4 7.0 4/5 21.5 60 0 1 7/11 11.5 6/7 16.0 70 0 2 2126 17.0 6/22 43.0 - 0 9/4 6.5 5/2 28.0 58 1 814 11.0 417 20.0 280 0 9/5 8.0 415 31.0 74 1 718 15.5 4/7 16.0 86 0 9/4 10.0 4/3 43.0 80 1 8/7 20.0 4/6 31.0 74 0 - - - - - 1 1015 15.0 4/2 16.0 54 0 2 9110 18.5 6/9.34.0 - 0 10/15 10.9 5/4 32.2 70 1 13/1 8.0 410 12.0 124 2 11/7 6.8 4/5 15.0 - Test made with 2.37 lbs. polymer per barrel (a) cp;/looft, (b) lb (c) mL/30 min.; (d) HTWL is high temperature water loss, mL/30 min.

Note: A dash significant, no determination was made.

G) m N) Ili W (n 0 1.4 -91 GB2173507A The gypsum-contaminated muds present a severe test for the polymers being evaluated and the severity of this test is reflected in the relatively high high temperature water loss (HTWL) value shown in Table 4. Based on these values, the results suggest that the 70:30 AkIVIMA copolymer achieved better results in a gypsum-contaminated mud than the did the other co5 polymers tested.

EXAMPLE V

This example is directed to the evaluation of acrylic acid-methyl methacrylate-vinyl acetate (AA-MMA-VA) terpolymers as water loss agents. The polymers were prepared in the same manner described in Example 1. Thus, about 20 9 total monomers, about 200 mL of Freon-1 13 10 and about 0.75 weight percent BW based on total monomers weight were charged to the crown top beverage bottle, followed by degassing and rotation for about 24.5 hours to effect polymerization. Conversions averaging from about 90 to about 100 percent were achieved. Each sample was isolated and converted to the sodium salt as previously described. Each polymer was evaluated for water loss control in saline mud A and saturated saline mud B as described before at room temperature. The results are presented in Table 5.

0) TABLE 5

AA-HMA-VA Terpolymers As Water Loss Control Agents Saline Mud A Saturated Saline Mud B Wt. Percent 1 Lb/Bbl 3 Lbs/Bbl 1 Lb/BBI 3 Lbs/Bbl AA MMA VA WL(A) WL(a) WL(a) WL(a) 0 40 22 5.5 74 53' 10 30 22 6.5 41 5.5 20 20 21 7.5 22 7.5 30 10 19 7.0 17 4.7 40 0 17 10.0 21 4.4 0 30 24 6.8 71 8.3 10 20 27 6.5 30 6.8 20 10 35 8.0 22 10.0 30 0 33 10.5 19 7.0 (a) mL130 min.

G) m N -j W M 0.1 m 17 GB2173507A 17 The data presented in Table 5 demonstrate that the terpolymers can function as water loss control agents at moderate temperatures. The results suggest that a 60:30:10 AA:1VIMA:VA terpolymer will perform effectively in drilling fluids as a water loss control agent.

EXAMPLE V]

Copolymers contaning 75 weight percent methacrylic acid (MAA) and 25 weight percent methyl acrylate (MA), based on the total weight of the monomers, and containing 75 weight percent acrylic acid (AA) and 25 weight percent methyl acrylate (MA), based on the total weight of the monomers, were separately prepared in n-hexane in the manner previously described. As before, the copolymers, in their sodium salt form, were each evaluated at about 325,F (163'C) 10 and at a differential pressure of about 500 psi to determine their high temperature water loss (HTWL) properties in saline mud at various copolymer-mud concentrations in the absence or presence of gypsum in the mud at various gypsum-mud concentrations. The gypsum-free saline mud was prepared in water containing about 4 weight percent NaCI, and was additionally treated with 2 ib/bbi of the previously mentioned Desco thinner and 5 lb/bbi of Tannathin lignite. The 15 solids content of the finished mud was calculated to be about 2.7 weight percent bentonite clay, about 2.19 weight percent illite clay and about 30 weight percent barite, based on the total weight of the finished mud. The initial pH of the mud samples ranged from about 10-0 to about 10.9. The results are given in Table 6.

TABLE 6 MAA-MA and AA-MA Copolymers as High Temperature Water Loss (HTWL) Control Agents Gypsum Copolymer HTWL, (c) 25 Copolymer (a) Concentration, (b) at 325'F (163'C) (d) 0 1 28 (d) 0 2 30 (d) 1 2 51 (f) 30 (d) 1 3 26(d) 2 2 136 (d) 2 3 135 (e) 0 1 44 (e) 0 2 28 35 (e) 1 3 120 (e) 1 2 (e) 2 3 260 (e) 2 2 258(f) 40 (a) lb gypsum per 42 gal bbl of mud (b) lb copolymer per 42 gal bbl of mud (c) high temperature water loss, mL/30 min (d) MAA-MA copolymer, 75 wt.% MAA, 25 wt.% MA (e) AA-MA copolymer, 75 wt.% AA, 25 wt.% MA (f) Average value for two separately prepared mud samples Note: A dash signifies no determination was made.

The results show that the MAA-MA copolymer is substantially more effective as a high temperature water loss agent than is the AA-MA copolymer in gypsum- contaminated muds under the conditions employed.

As discussed earlier, the polymers of this invention are preferably employed as the sodium salts since the salts are water soluble. However, it is possible to use the acid forms of the polymers alone although this is less preferred because the polymers in this form do not dissolve well in aqueous solutions. In fact, when the methyl methaerylate content of these polymers is in 55 the range from about 30 weight percent or higher, the acid forms of the polymers are insoluble in water. In spite of this factor, the acid forms of the polymers are capable of acting as water loss control agents.

Although not previously emphasized, another feature of the polymers of the present invention is that they contribute little or not at all to the viscosity of the mud in which they are placed. This feature is of extreme importance when the polymers are to be added to muds having a high solids content.

In addition to the slurry polymerization method described above, suitable copolymers of hydrophilic vinyl monomers with hydrophobic vinyl monomers can be prepared in a variety of other methods. However, all of the methods employed are characterized as free radical polymerizations 65 18 GB2173507A 18 wherein the steps of polymer initiation, propagation, and termination take place substantially as free radical reactions One suitable method of preparation of copolymers in accordance with the present invention is a polymerization process of the oil-in-water type wherein the monomers are copolymerized in the presence of a suitable surfactant, a free radical source, and water. A wide variety of surfactants and free radical sources can be employed. Surfactants effective at pH values below 7 are generally preferred. In this system the water/monomer weight ratio can vary over a wide range but will usually be about 211 for a good balance of polymerization rates, copolymer latex viscosity and adequate heat transfer. As in the previously described slurry polymerization pro- cess, various free radical sources can be employed including both water soluble or oil soluble 10 chemical compounds which thermally decompose to free radicals, e.g. potassium persulfate or benzoyl peroxide or 2,2'-azo-bis-isobutyronitrile. In addition, irradiation with ultraviolet light or gamma rays from a radioisotope such as 60Co can also serve as free radical sources for this polymerization system as well as the previously described slurry polymerization process. When using ultraviolet irradiation, sensitizing compounds can be employed to promote radical forming 15 reactions as is known in the art.

The copolymer latex obtained as a product from the oil-in-water type polymerization process can be added directly to a drilling fluid already containing sufficient alkali metal base compound to neutralize the polymer in situ. Alternatively, the polymer latex and alkali metal base can be added simultaneously to a drilling fluid for neutralization in situ. As a further alternative, the polymer latex can be added to a drilling fluid, first followed by subsequent addition of sufficient alkali metal base for in situ neutralization. The copolymer latex can be reacted directly with sufficient alkali metal base, e.g. aqueous NaOH 50% by wt., to convert the latex to an aqueous neutralized polymer solution. The polymer solution can be added directly to a drilling fluid to obtain the drilling fluid composition desired. Alternatively, the neutralized polymer solution can be heated under reduced pressure or otherwise dried by a suitable drying technique, such as, for example, microwave radiation, to remove water to obtain a substantially dry particulate product which can be easily admixed with a drilling fluid.

Another suitable method of preparing copolymers in accordance with the present invention can be characterized as a polymerization process of the water-in-oil type. In this system the mono- 30 mers are copolymerized in the presence of a water-insoluble organic liquid (usually a hydrocar bon), water, a suitable surfactant and a free radical source. Surfactants suitable for use in such systems are well known in the art, e.g. sorbitan monooleate. The weight ratio of "oil" to water in this system can range widely but will usually be in the range from about 0.1/1 to about 5/1.

Free radical sources described above can also be utilized in the water-inoil polymerization 35 method.

The various methods noted above with regard to the oil-in-water polymerization process, can also be used for providing the neutralized polymer produced by the water- in-oil polymerization process to a drilling fluid.

Polymer containing carboxyl groups can present difficulties in certain conventional methods of 40 molecular weight determination due to intra- or intermolecular interaction involving the carboxyl groups. A convenient procedure for circumventing these difficulties is to quantitatively esterify the carboxyl groups in such polymers without altering the polymer chain length. The esterified polymer can then be handled in molecular weight analyses such as gel permeation chromato- graphy (GPC) or intrinsic viscosity (I.V.) wherein calibration measurements have already been made or can be made for use in the calculations.

Several reagent combinations and conditions are disclosed in the article by H. L. Cohen, J.

Polymer Sci.: Polymer Chem. Ed., Vol. 14, 7-22 (1976) for quantitative esterification of poly meric carboxylic acids. A particularly useful procedure is the prolonged (24-48 hr) heating of the polymer in dimethylformamide (DMF) diluent with excess trimethyl orthoacetate. The polymer 50 may be initially insoluble but swells and then dissolves in the DMF reaction diluent as the reaction proceeds. DMF is also believed to have a promoting effect on the esterification reaction.

As noted earlier the esterified polymer can be readily handled in procedures for molecular weight determination such as GPC or IN. measurements wherein appropriate calibration mea surements have already been made or can be made. For example, the quantitative esterification 55 of a methyacrylic acid/methyl methacrylate (MAA/MMA) copolymer converts the copolymer to a poly(methyl methacrylate). This polymer has been extensively studied in a wide variety of solvents to establish intrinsic viscosity-molecular weight relationships which have been published.

Furthermore, poly(MMA) standards for GPC analysis are commercially available, e.g. from Poly mer Laboratories, Stow, Ohio. Hence, the quantitatively esterified MAA/MMA copolymer can be used to determine the molecular weight of the -original- MAA/MMA copolymer.

A more direct method of determining the molecular weight of the carboxylic copolymers of this application is by light scattering techniques which are well known. However, this method is also known to be experimentally difficult at times.

The carboxylic copolymers of this invention, in their neutralized (alkali metal salt) form, are 65 19 GB2173507A 19 highly effective water loss control agents in drilling fluids of various types, e.g. KCl-containing inhibited muds, workover fluids containing fibrous additives, etc. The metahacrylic acid (MAA) copolymers, i.e. MAA/MMA copolymers, have been found to retain their effectiveness as water loss control agents in the presence of significant levels of commonly encountered contaminants such as NaCI (salinity), calcium ion (gypsum), and cement. Furthermore, the polymers of this application have been found to retain their effectiveness as water loss control agents at elevated temperatures. This is a very important property since there is greatly increased interest in drilling geothermal wells in addition to the high temperatures being encountered in many deep gas and oil wells. It is known that cellulosic polymers which otherwise are very useful as drilling fluid additives may be degraded when subjected to high temperature conditions even for a relatively 10 short period of time. In contrast, the polymers of this invention are resistant to thermal degradation in drilling fluids over relatively long periods of time, e.g. up to 3-5 days and even longer. Thus, the copolymers of this invention are particularly useful at well temperatures of from about 175'F (79'C) up to about 700'F (371'C).

EXAMPLE V11

A series of methaerylic acid (MAA)/methyl methaerylate (MMA) copolymers were prepared by an oil-in-water polymerization process at 50'C. Sulfonated castor oil (2 phm of 50% by wt.

active ingredient) was employed as the surfactant and 0.3 phm potassium persulfate (K,S,0,') as the initiator. The polymers were neutralized in situ with 5M NaOH and recovered by evaporation 20 of water.

The copolymers were evaluated as water loss control additives in saline muds A and B at room temperature (Table 7) and at about 325'F (163'C) under 500 psi differential pressure in the saline mud of Example VI with and without contaminants gypsum or additional salt (Table 8).

Results for certain acrylic acid (AA) copolymers are also shown.

TABLE 7

RTWL Values(W Saline Mud A Saturated Saline Mud B Monomer Polymer Ratio 1 Lb/ 3 Lb/ 1 Lb/ 3 Lb/ No. MAA/MMA Bbl Bbi Bbl Bbi 35 1 9515 29 7.4 33 7 2 90/10 24 8.8 30 5.5 3 85/15 28 7.8 26 5.5 4 80/20 20 7 28.5 5.5 40 75/25 17 7 32 5 6 70/30 16 6.8 43 6.5 7 65/35 19 8 69 12 8 60/40 20 7 -(b) 28 9 55/45 30 8.7 56 45 50/50 49 9.5 86 AAIMAW 11 75/25 18 9 21 5.6 50 AAIMMA11) 12 75/25 30 9.5 54 8.2 13 70/30 22 8.5 23 5.8 (a) Room Temperature Water Loss, API, mL/30 minutes (b) A dash indicates not determined (c) Acrylic acid/methyl acrylate copolymer (d) Acrylic acid/methyl methacrylate copolymers 1 2 3 4 6 7 8 9 Base Mud-no polymer 150 TABLE 8

Polymer Ratio No. MAA/MMA 95/5 90/10 85/15 80/20 75/25 70/30 65/35 60/40 55/45 50/50 HTWL Values @ 2 Lb/BIJI Polymer(a) Mud With No Contaminant 23 24 23 26 24 28 AAIMA 75/25 28 AAIMMA 12 75/25 24 13 70/30 28 gypsum NaCI @ 1 Lb/Bbl @ 10 Lb/Bbl 28 - 32 32 38 32 32 38 28 36 24 30 46 160 170(b) 941b) 140 (a) High Temperature Water Loss, 500 psi, 325'17 (b) Average values of two tests The results in Tables 7 and 8 show that the MAA/1VIMA copolymers over a wide range, e.g. 95/5 to 65/35, of monomer ratios show satisfactory RTWL values and uncontaminated mud HTWL values but that the values are not greatly different from those shown by acrylic acid/methyl acrylate and acrylic acid/methyl methacrylate copolymers 11, 12 and 13 in these muds. However, in muds contaminated with gypsum or NaCI at high temperature the MAA/MMA copolymers are outstandingly better at water loss control than the AA/MA or AA/MMA copolymers.

GB2173507A 20 EXAMPLE VIII

Other properties were measured of the drilling mud composition used in Table 8 of Example 40 V11 and containing Polymers 17 (@ 2 lb/bbi of mud) of Example V11. These tests were also made with and without added contaminants gypsum or NaCI. The results of these tests are shown in Table 9 below.

TABLE 9

Part A-No contaminant added Initial Aged 16 Hours @ 36017 (1 82'C) GB2173507A 21 Polymer No. PV/YPO) GelsM pH WL(O SS(d) PV/YP Gels pH WL 1 23/13 16/31 10.3 3.8 180 14/15 11/40 7.7 5.2 10 2 27/13 16/32 10.3 3.8 190 16/8 14/45 7.6 4.6 3 26111 15/31 10.4 3.8 190 16/15 10139 7.7 4.2 4 25117 16/32 10.5 3.6 180 25/8 13/40 7.5 4.3 24/21 16/36 10.3 - 200 15/15 8135 7.7 4.5 6 27/27 22/52 10.3 2.7 190 10/17 8/- 7.6 - 15 7 24138 36/64 10.3 2.8 200 15/22 14/- 7.6 4.5 Base Mud 13/3 6113 10.7 - <60 9/10 11/- 7.8 Part B-1 Lb/Bbi Gypsum Added 1 23/13 10/33 10.3 5.3 160 14/5 19/48 7.6 6.9 20 2 23/12 8/22 10.4 4.8 140 15/15 23/47 7.7 5.8 3 24/9 8/22 10.4 5.4 160 13/17 18/48 7.5 6.6 4 26/11 9/22 10.4 4.6 140 31/0 21/49 7.6 7.0 23/20 11/29 10.4 4.3 160 23/7 17/46 7.5 7.7 6 23/26 18/45 10.4 4.8 170 20/32 26/58 7.5 7.0 25 7 22/33 22/65 10.4 4.7 190 21/43 38/60 7.6 6.3 Part C-10 Lb/13bl NaCI Added 2 28/8 15/30 9.8 5.0 110 13/16 11/36 7.2 8.2 3 23/12 14/29 9.8 4.5 95 11/16 12/35 7.1 8.0 30 4 18/22 16/36 9.8 5.0 160 13/16 13/37 7.2 8.3 19/30 20/42 9.9 4.2 120 14/13 9/31 7.2 8.5 6 25/32 28/48 9.9 4.7 120 12/20 13/37 7.0 8.6 7 21/33 22/55 9.8 5.2 120 13/21 21/47 7.0 6.8 35 (a) PV=Plastic Viscosity, cp; YP=Yield Point, 1b/100 ft2 (b) Gels=Gel Strength @) (c) W1_=API Water Loss, mL/30 min (d) SS=Shear Strength, lb/100 ft2 sec (16/100 ft2) /Gel Strength @ 10 min (16/100 ft) The results in Table 9 show that the MAA/MMA copolymers tested did not drastically increase values for mud viscosity, gel strength, yield point and shear strength. Hence, muds containing said polymers should be readily handled in pumping or other like operations.

EXAMPLE IX

Other tests were made which compared the water loss control effectiveness of a (75/25) MAA/MMA copolymer (A) against a (70/30) AA/MMA copolymer (B) and a commercially available polymer (C) based on a polyanionic cellulose identified as DrispaC, Superlo obtained from Drilling Specialties Co. Bartlesville, Oklahoma. These runs determined high temperature water loss (HTWL) values on muds containing differing concentrations of polymer, gypsum or 50 NaCI after being aged under different conditions. The mud (base) used for these runs was the same as that employed as the saline mud of Example VI and also in Example V11 (Table 8). The results of these runs are presented in Table 10 below.

22 GB2173507A 22 TABLE 10

Muds Aged 16 Hours @ 250'T (121'C) Conc. lb/bbl Run No. Polymer NaCI Gypsum HTWL @ 325'F 500 psi, mL/30 min 1 (A) 2 0 1 28 10 2 (A) 2 0 1.5 46 3 (A) 1 15 0 37 4 (A) 1 20 0 40 (A) 1.5 20 0 32 6 (A) 2 20 0 32 15 7 (A) 2 50 0 28 8 (A) 2 60 0 66 9 (A) 2 70 0 124 10 (B) 2 0 1 38 20 11 (B) 1 20 0 126 12 (B) 1. 5 20 0 92 13 (B) 2 20 0 36 14 (C) 2 0 1 36 25 (C) 1 15 0 48 16 (C) 1 20 0 72 17 (C) 1.5 20 0 48 18 (C) 2 20 0 30 19 (C) 2 50 0 38 30 (C) 2 60 0 36 21 (C) 2 70 0 40 Muds Aged 64 Hours @ 176'F (80Q 22 (A) 2 0 1 26 35 23 (A) 2 0 2 150 24 (A) 2 20 0 28 (C) 2 0 1 34 26 (C) 2 0 2 37 27 (C) 2 20 0 50 40 The above results indicate that the MAA/MMA copolymer (A) was more effective than the AA/MMA copolymer (B) and commercial polymer (C) in a salinity range of from 4% to almost saturation and at a gypsum level of 1 1b/bbl mud. However, MAA/MMA copolymer (A) became 45 less tolerant of salt as aging temperature increased. As salinity neared saturation polymer (C) also became more effective in water loss control (HTWL) than the MAA/MMA copolymer (A). Also, at a gypsum level of 2 lb/bbi, polymer (C) was more effective than (A).

EXAMPLE X

A large lot (about 10,000 1b) of an 80/20 MAA/MMA copolymer was prepared in several reactor batches by a polymerization process of the oil-in-water type. The polymer latex was converted to a polymer solution of about 10% solids in water by neutralization of the polymer with NaOH.

An aqueous polymer solution for testing in drilling muds was obtained by combining samples 55 from several reactor batches to provide a composite sample identified hereafter as Polymer A.

The compatability of Polymer A with a commonly employed additive in drilling fluids, flaked fibers of chrysotile asbestos identified as Flosal from Drilling Specialties Co. Bartlesville, Okla homa, was determined in a series of tests on aqueous solutions containing both. For compari- son, a commercially available polyanionic cellulosic polymer of high molecular weight (Drispac also from Drilling Specialties Co.) was also tested at the same time. The results of the tests are given in Table 11.

23 GB2173507A 23 TABLE 11

Viscosity(a), cp Run Flosal, Polymer, 5 No. lb/bbi @ 1 lb/bbi @ 600/300 RPM Gels(cl PV/Yp(d) WL(b) 1 1 A 28/20 010 8/12 200 2 1 Drispac 26118 0/0 8/10 65 3 2 A 32/22 1/2 10/12 168 10 4 2 Drispac 30/20 0/0 10/10 64 4 A 37/30 112 7/23 116 6 4 Drispac 36/26 315 10/16 31.5 Aged Overnight @ WC 15 7 1 A 27/17 0/0 8/9 140 8 1 Drispac 25/15 0/0 10/5 39 9 2 A 29119 1/2 1019 108 2 Drispac 29/20 0/1 9/11 30 11 4 A 38/28 4/6 10/18 96 20 12 4 Drispac 36/26 3/5 10/16 27 (a) Determined with Fann viscometer (b) API water loss determined at room temperature, 100 psi in mL/30 minutes (c) Gels=Gel Strength @ 10 sec (Ib/100ft2)/Gel Strength @ 10 min (ib/ 1 00ft2) (d) PV=Plastic Viscosity, cp; YP=Yield Point, lb/100 ft2 The results in Table 11 show that Polymer A is compatible with Flosal in fluids containing both. However, water loss control with the Flosal- Polymer A combination was not as good as with the Flosal-Drispac polymer combination. Other tests were carried out on fluids containing other materials in addition to the Flosal at 4 lb/bbi and polymer (Polymer A or Drispac) at 2 lb/bbi. These tests also were conducted after aging at elevated temperatures in addition to the room temperature runs. Results from these tests presented in Table 12.

TABLE 12

Aged Overnight Initial @ 300'F 40 Run Additive No. Polymer 2 lb/bbi PV/YP, GelSd WU, PV/Y1P, GelSd WL, 1 A 0 14/22 1/2 64 10/5 1/5 139 2 Drispac 0 20/38 6/10 20 7/12 3/4 35 45 3 A Bentonite 16/30 4/6 20 21/27 3/6 14 4 Drispac Bentonite 25/65 11/11 13.2 9/11 5/9 19.8 @ 400'F 50 A Bentonite 17/32 6/8 28 9/11 3/3 30 6 A CaCO, 16/28 5/7 22 18/28 5/7 20 7 A Bentonitea 25/18 8/11 28 9/16 3/7 26.8 and CaC03 55 @ RT (75'F) (a) (b) (c) (d) 65' (e) 8 A Cementb 12/16 2/3 6 11/14 3/5 10.7 Both present @ 2 lb/bbi fluid. Present @ 1.5 lb/bbi fluid. PV=Plastic Viscosity, cp; YP=Yield Point, lb/100 ft2 Gels=Gel Strength, @ 10 sec lb/100 ft2)/Gel Strength @ 10 min (I1J/100 ft2) API water loss determined at room temperature, 100 psi in mL/30 minutes 24 GB2173507A 24 The results in Table 12 show that addition of bentonite or calcium carbonate can greatly improve the water loss control property of fluids containing Polymer A such that the performance is equal to or better than Drispac after aging at elevated temperatures. It is further shown that cement contamination of a fluid containing Flosal and Polymer A does not cause a drastic 5 increase in viscosity of the fluid.

The tests reported in Tables 11 and 12 indicate that Polymer A should be well suited for use in well workover fluids containing Flosal fiber additive especially in wells encountering high temperatures such as in geothermal walls.

In operation, the drilling fluids or muds described above are preferably circulated downwardly through a tubular drill string and out through the drill bit on the lower end of the drill string in 10 the bottom of a borehole. The drilling fluids are further circulated upwardly through the annulus between the drill string and the wall of the borehole whereby cuttings from the bottom of the borehole are conveyed to the earth's surface and the polymeric water loss additive of the drilling fluid contacts the penetrated formations to control water loss from the borehole into the thus contacted formations. After each passage of the drilling fluids through the drill string and borehole annulus the drilling fluid is preferably passed through a settling tank or trough where sand and drill cuttings are separated, with or without screening. The drilling fluid is then again pumped into the drill string by a mud pump to continue the circulation as described.

In conclusion attention is directed to our copending application No. 8326883 (Publication No.

Claims (1)

1. 2,128,659 A), from which the present application is divided, and in which

   there is claimed a 20 method of drilling a borehole in a subterranean formation and in which there is used an aqueous drilling fluid containing the copolymers of this invention as a water loss control additive.

   CLAIMS 25 1. A process for the preparation of polymer compositions suitable for use as water loss control additives in aqueous drilling fluids, which comprises copolymerizing a hydrophilic vinyl monomer and at least one hydrophobic vinyl monomer, said polymerization being carried out in an inert, low chain transfer, organic liquid medium at a polymerization temperature and for a period of time sufficient to produce a copolymer comprising said hydrophilic monomer and said 30 hydrophobic mononer and having a weight average molecular weight in the range from 100,000 30 to 500,000. 2. A process according to claim 1, wherein said hydrophilic vinyl monomer is or comprises acrylic acid, methacrylic acid or a mixture of the two. 3. A process according to claim 1 or 2, wherein the hydrophobic vinyl monomer is or comprises an acrylic acid ester, a methacrylic acid ester, or a vinyl ester of a saturated monocarboxylic acid having from 1 to 3 carbon atoms, or a mixture of two or more thereof.

   4. A process according to any one of claims 1-3, wherein the polymerization is conducted in the presence of a free radical initiator.

   5. A process according to claim 4, wherein said free radical initiator is an azo compound or an organic peroxy compound, or a mixture of two or more thereof.

   6. A process according to any one of claims 1-5, wherein said inert, low chain transfer, organic liquid medium comprises n-hexane, cyclohexane, a chlorofluorocarbon, or a mixture of two or more thereof.

   7. A process according to any one of claims 1-6, wherein polymerization is effected at a temperature in the range 25C to 1OWC.

   8. A process according to claim 7, wherein said temperature is in the range from WC to 700C.

   9. A process according to any one of claims 1-8, wherein the polymerization is in the range minutes to 30 hours.

   10. A process according to any one of claims 1-9, wherein the mole ratio of hydrophilic 50 monomer to hydrophobic monomer is in the range 22:1 to 1A.

   11. A process according to claim 10, wherein said ratio is in the range 7:1 to 1A.

   12. A process according to claim 10, wherein said ratio is in the range 3. 5:1 to 2A.

   13. A process according to any one of claims 1-12, wherein the total monomer concentra- tion is from 5 to 30 per cent by weight based on the reaction medium.

   14. A process according to claim 13, wherein said concentration is from 10 to 20 per cent.

   15. A process according to any one of claims 1-14, wherein the hydrophilic monomer is acrylic acid (AA) and the hydrophobic monomer is methyl methacrylate (MMA) and the AA:MMA mole ratio is in the range 3.2:1 to 2.1A.

   16. A process according to any one of claims 1-14, wherein the hydrophilic monomer is 60 methacrylic acid (MAA) and the hydrophobic monomer is methyl methacrylate (MMA) and the MAA:1VIMA mole ratio is in the range 22:1 to 1: 1.

   17. A process according to claim 16, wherein said MAA:MMA ratio is in the range 22:1 to 2A.

   18. A process according to any one of claims 1-14, wherein the hydrophilic monomer is 65 GB2173507A 25 acrylic acid (AA) and the hydrophobic monomer comprises both methyl methacrylate (MMA) and vinyl acetate (VA) and the AAWMA:VA mole ratio is in the range 2.8:1:0.33 to 8.3A:3.5.

   19. A process according to claim 18, wherein said AAMMA:VA ratio is in the range 4.8A:0.58 to 9.7:1:2.3.

   20. A process according to any one of claims 1-19, wherein, following polymerization, the 5 product copolymer is separated, washed and dried, and then slurried in water and treated with sodium hydroxide thereby to provide an aqueous solution of the copolymer in the form of its sodium salt.

   21. A process according to claim 20, wherein sufficient sodium hydroxide is used to raise the pH of the slurry to a value in the range 5.3 to 12.

   22. A process according to claim 2 1, wherein said pH is in the range 6 to 8.

   Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935, 1986, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.