GB2128659A - Drilling fluids - Google Patents

Abstract

Drilling fluids for high temperature subterranean drilling operations are disclosed containing, as an improved water loss control additive, a copolymer of molecular weight to the range 100.000 to 500.000 and derived by free radical polymerisation of a monomer charge containing a combination of hydrophilic monomers, particularly acrylic and/or methacrylic acid, and hydrophobic monomers. particularly acrylate or methacrylate esters or vinyl esters of saturated C1-C3 monocarboxylic acids. in a mole ratio of from 22:1 to 1:1.

Description

SPECIFICATION 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, present loss of water and drilling fluids into the formation through which the bore hole is being drilled, and to control the entry of liquids into the bore hole from various formations being penetrated during 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 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 3600F (1 82 C), 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 with previous polymers noted above.

It is an object of the present invention to provide improved drilling fluid additives.

Another object of the present invention is to provide an improved method of producing drilling fluid additives.

A further object of the present invention is to provide an improved method of drilling a borehole.

Still another object of the present invention is to provide an improved drilling fluid for use in the drilling of a borehole.

Yet another object of the present invention is to provide an improved drilling fluid which exhibits high temperature stability over extended periods of time.

Another object of the present invention is to provide an improved drilling fluid which provides enhanced water loss control characteristics.

A further object of the present invention is to provide an improved drilling fluid additive characterized by enhanced water loss control characteristics coupled with a minimum increase in viscosity when employed in a drilling fluid.

The foregoing and other objects, advantages and aspects of the present invention will become readily apparent from the following detailed description of the invention and the appended claims.

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 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 methacrylic acid-methyl acrylate copolymer (MAA-MA), a methacrylic acidmethyl methacrylate copolymer (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-trifluoroethane is commercially available under the registered trademark Freon-l 13at. 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 1 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 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 copolymers 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, 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-MMA mole ratios range from about 3.2:1 to 2.1:1, corresponding to a AA-MMA 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 1:1, and from about 22:1 to about 2:1. For the acrylic acid-methyl methacrylate-vinyl acetate copolymer, the weight ratios of AA-MMA-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 8.3:1 :3.5, and from about 4.8:1:0.58 to about 9.7:1:2.3.

Generally, in preparing the polymers of the instant invention, the total monomer level with 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 250C to about 1000C, preferably from about 500C to about 700 C, and most preferably at about 500C for the best results. The polymers prepared at about 500C exhibit the best control of water loss in the tests used.

Each polymerization was conducted for a sufficient length of time to obtain substantially 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 mL of reaction medium containing from about 0.1 to about 1.5 weight percent, and preferably fromabout 0.1 to about 0.8 weight percent t-butyl peroxypivalate and a reaction temperature ranging from about 500C to about 700C, 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 trademark Lupersol-1 1 s 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 temperature, for example at 500C or 700C, 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 NaCI. The second mud, designated saturated saline mud B, 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 1 to about 3 pounds per barrel of drilling fluid or mud. As used herein, the term "barrel" is defined at 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 corripensate'fdr 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, e.g., about 250 C. The samples were then stirred an additional two minutes with the mixer just before determining their initial plastic viscosities and yield points at about 250C with a model 35 Fann V-G meter, a direct indicating viscosimeter, in accordance with API RP 138, 2nod ed., April 1 969, "Standard Procedure for Testing Drilling Fluids", American Petroleum Institute, Division of Production, Dallas, Texas.Initial water loss at about 250C 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 800C and cooled to room temperature.

Selected polymers were also evaluated under more severe conditions. For these tests, base mud A was treated with 12 Ib/bbl of bentonite, 2 Ib/bbl of a commercially available thinner sold under the trademark Desco and available from Chromalloy Corp., Conroe, Texas, and 3 Ib/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 Ib/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 3600F (1 820C). Following this second series of tests an additional water loss test was performed at about 3250F (1 630C) 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 monomer weight. In each run the total monomer weight was about 20.0 g: 0.20 g of t-butyl peroxypivalate (BPP) solution containing 0.15 g 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 500C until conversion was substantially complete. The polymers were isolated, washed with n-hexane, Freon-113s or the like, and vacuum dried.The yields ranged from about 95 to about 1 00 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 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 sedond series, 200 mL of Freon-1 1 3, equivalent to about 313 g, was employed the reaction medium. In the third series, 200 mL 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 Ib/bbl and of 3 Ib/bbl 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 250C) for plastic viscosity (PV) in centipoises (cp), yield point (YP) in Lob/1 00 ft2, and water loss (WL) in mix30 minutes, in the manner previously described. The results are presented in Tables 1 A, 1 B and 1 C.

TABLE 1A AA-MMA Copolymers Prepared In Freon-1 13e at 500C Saline Mud A Saturated Saline Mud B 1 Lb/Bbl 3 Lbs/Bbi 1 Lb/Bbl 3 Lbs/Bbl Polymer Wt.%MMA PY(a)/YP(b) WL(c) PV(a)/YP(b) WL(c) PW(a)/YP(b) WL(c) PV(a)/YP(b) WL(c) 0 4/5 68.0 6/6 20.5 7/15 55.1 14/6 13.0 5 4/6 69.0 6/6 21.0 8/12 44.0 13/7 13.0 10 3/6 58.0 7/4 23.0 7/11 38.0 12/10 14.0 20 5/3 40.0 7/4 23.0 7/4 24.0 19/9 10.9 30 4/2 26.5 7/4 23.0 7/5 19.0 9/5 8.5 Mud A Only 5/10 129 - - - - - - Note: A dash signifies no determination was made.

(a) cp.

(b) lbs./100ft2 (c) mL/30 min.

TABLE 1B AA-MMA Copolymers Prepared In Freon-113# at 500C Saline Mud A Saturated Saline Mud B 1 Lb/Bbl 3 Lbs/Bbl 1 Lb/Bbl 3 Lbs/Bbl Polymer Wt.%MMA PY(a)/YP(b) WL(c) PV(a)/YP(b) WL(c) PW(a)/YP(b) WL(c) PV(a)/YP(b) WL(c) 0 2/5 60.0 4/4 14.0 6/12 70.0 7/5 18.0 5 3/2 53.0 10/9 14.5 5/8 57.0 10/3 12.5 10 8/4 50.0 6/3 16.0 6/6 46.0 13/7 10.4 20 4/1 48.0 6/3 24.5 7/6 31.0 16/9 12.5 30 3/2 34.0 6/3 20.5 8/4 21.0 13/8 6.0 40 3/1 17.5 6/1 10.0 10/6 29.0 9/3 4.3 (a) cp.

(b) lbs./100 ft2 (c) mL/30 min.

TABLE 1C AA-MMA Copolymers Prepared In Tert-Butyl Alcohol at 500C Saline Mud A Saturated Saline Mud B 1 Lb/Bbl 3 Lbs/Bbl 1 Lb/Bbl 3 Lbs/Bbl Polymer Wt. % MMA PY(a/YP(b WL(c) PW(a)/YP(b) WL(C) PV(a)/YP(b) WL(c PV(a)/YP(bI WLIC? 0 - - 4/4 53.0 - - 6/4 54.0 5 - - 5/5 66.0 - - 9/3 38.0 10 - - 4/5 65.0 - - 7/3 39.5 20 3/5 79.0 5/5 63.0 - - 4/2 17.0 30 3/4 61.0 6/4 62.0 - - 6.1 16.1 Note: A dash signifies no determination was made.

(a)cp.

(b) lbs/100ft2 (c) mL/30 min.

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 data also show that the polymers made in n-hexane and Freon-1 1 3e are more effective than the polymers made in t-butyl alcohol.

EXAMPLE II 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-113# in another series, as water loss control agents in the base muds was ascertained. The polymer samples were prepared as in Example I 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, in turn, leads to an increase in polymer molecular weight. Thus, these polymers are of higher molecular weight than those shown in Example I.

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

TABLE 2 Higher Molecular Weight AA-MMA Copolymers Prepared at 500C.

Saturated Saline Mud B Results n-Hexane Medium Freon 113# Medium 1 Lb/Bbl 3 Lbs/Bbl 1 Lb/Bbl 3 Lbs/Bbl Polymer Wt. % MMA py(a)/yp(b) WL(c) RW(a?/YP(b) WL(c) PV(ai/YP(b? WL(c) PV(a),,,YP(b? WL(Ci 0 8/11 47.0 9/11 0.0 7/4 40.0 9/1 5.0 5 5/3 29.0 7/2 6.0 6/4 37.0 9/3 7.0 10 5/5 28.0 10/1 6.0 7/1 33.0 10/3 6.5 20 6/4 20.5 8/2 5.5 6/1 18.0 10/2 6.5 30 7/1 15.0 12/0 6.0 8/1 18.0 11/4 5.5 40 8/1 16.0 15/5 4.5 6/1 25.0 14/6 4.5 50 - - - - 7/3 44.0 19/7 5.5 * foam (a) cp.

(b) lbs./1 00 ft.2 (c) mL/30 min.

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 AA:MMA copolymer which is not usually considered desirable in a drilling mud.

EXAMPLE Ill 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 in was Freon 113s. Several initiator levels were also employed in each series. A polymerization time of about 21 hours was employed in preparing the 60:40 AA:MMA copolymers in n-hexane and in Freon-113s. All other polymers were given about a 25 hours polymerization time. Each polymer was prepared at about 500 C. 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 from about 6 to about 8.The mud was also treated with 12 Ibs/bbi bentonite clay and 2 Ib/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 3600F (1 820C) in brass bombs, cooled to room temperature, and the shear 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 3250F (1 630C) and at a differential pressure of about 500 psi was also performed. The results are reported in Tables 3A, 3B and 3C.

TABLE3A AA-MMA Copolymers, High Temperature Runs Variable lnitiator (BBP) Levels, n-Hexane Medium Properties after 16 Hours at 360 F (182 C) Wt.% Wt.% lnitial Water Loss 1Lb/Bbl. 2Lbs/Bbl. HTWL (a) MMA BPP 1Lb/Bbl SS(b) PV(c)/YP(d) WL(e) SS(b) PV(c)/YP(d) WL(e) 1Lb/Bbl 2LbS/Bbl 40 075 8.0 5.9 150 10/23 10.0 150 12/23 5.4 46 27 40 0.38 6.0 4.4 230 9/25 10.0 210 16/22 5.4 48 30 40 0.19 5.2 4.3 230 12/24 10.7 160 8/29 5.0 42 28 30 0.75 5.2 4.3 200 10/25 13.0 150 11/30 5.8 38 30 30 0.38 5.2 4.2 180 11/28 11.2 150 14/23 5.7 39 30 30 0.19 5.2 3.7 150 11/24 10.4 135 14/28 5.9 41 28 Base mud only, PV(c)/YP(d)=5/20. WL=11.0 initially; after 16 hours aging at 360 F(182 C), PV/YP=7/10, WL=35.0(e), SS=135(b); HTWL(a)=100.

(a) HTWL is high temperature water loss, mL/30 minutes; (B) SS is shear strength, lbs/100 square feet; (c) cp.; (d)lbs/100ft2; (e) ml/30 min.

TABLE 3B AA-MMA Copolymers, High Temperature Tests Variable lnitiator (BBP) Level, Cyciohexane Medium Properties after 16 Hours at 360 F (183 C) Wt.% Wt.% lnitial Water Loss 1Lb/Bbl. 2Lbs/Bbl. HTWL(a) MMA BPP 1Lb/Bbl 2Lbs/BbL SS(b) PV(c)/YP(d) WL(e) SS(b) PV(c)/YP(d) WL(e) 1Lb/Bbl 2LbS/Bbl 40 0.75 6.3 5.5 160 12/21 10.6 170 12/23 6.0 36 31 40 0.38 8.6 5.4 150 10/22 11.0 150 15/20 5.6 36 29 40 0.19 6.7 6.7 160 11/24 10.5 - 10/22 5.4 37 32 40 0.075 5.5 5.0 210 10/26 12.0 - 10/23 6.0 35 30 (a) HTLW is high temperature water loss, mL/30 min.; (b)SS is shear strength, lbs./100ft2; (c) cp.

(d) lbs./100ft2; (e) mL/30 min.

Note: A dash signifies no determination was made TABLE 3C AA-MMA Copolymers, High Temperature Tests variable lnitiator (BBP) Level, Freon-113# Medium Properties 16 Hours at 360 F (182 C) Wt.% Wt.% lnitial Water Loss 1Lb/Bbl. 2Lbs/Bb1. HTWL(a) MMA BPP 1Lb/Bbl SS(b) PV(c)/YP(d) WL(e) SS(b) PV(C)/YP(d) WL(e) 1Lb/Bbl 2Lbs/Bbl 40 0.75 - 4.3 - - - 170 12/18 5.4 - 26 40 0.19 - 4.0 - - - 200 17/23 5.8 - 36 30 0.75 - 5.2 - - - 170 15/24 5.6 - 29 30 0.19 - 4.0 - - - 170 11/30 5.4 - 31 (a) HTWL is high temperature water loss, mL/30 min.; (b) SS is shear strength, lbs./100ft2; (c) cp.; (d) lbs/100ft2; (e) mL/30 min.

A comparison of the data presented in Tables 3A, 3B and 3C shows that the polymers prepared in n-hexane, cyclohexane or Freon-l 13e are all about equivalent in performance as water loss control agents, particularly when employed at a concentration of 2 Ib/bbl in the mud under test. At this concentration, after aging for about 16 hours at about 3600F (1 820C) the results indicate that the polymers may slightly lose some water loss control. The results in Example III indicate that the 60:40 or 70:30 AA:MMA 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 IV 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 I and Table 1 B, prepared at about 500C in Freon-113e, were also tested in the bentonite clay/Desco thinner-treated mud described in Example III, this mud being additionally treated with 1 or 2 Ib 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 3600F (1 820C). High temperature water loss values were also determined. Unless specified otherwise, each treated mud was also treated with 3 Ib of polymer per barrel of mud. The results are presented in Table 4.

TABLE 4 AA-MMA Copolymers, High Temperature Tests in Gypsum Mud Polymer Gypsum Initial Properties Aged 16 Hours at 3600F (1 820C) Wt.% MMA Lbs/Bbl PV(a)/YP(b) WL(c) PV(a),,'YP(b) WL(c) 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 2/26 17.0 6/22 43.0 - 5 0 9/4 6.5 5/2 28.0 58 5 1 8/4 11.0 4/7 20.0 280 10 0 9/5 8.0 4/5 31.0 74 10 1 7/8 15.5 4/7 16.0 86 20 0 9/4 10.0 4/3 43.0 80 20 1 8/7 20.0 4/6 31.0 74 30 0 30 1 10/5 15.0 4/2 16.0 54 0 2 9/10 18.5 6/9 34.0 - 40* 0 10/15 10.9 5/4 32.2 70 40 1 13/1 8.0 4/0 12.0 124 40 2 11/7 6.8 4/5 15.0 - * Test made with 2.37 Ibs. polymer per barrel (a) cp.

(b) Ibis/100 ftt; (c) mL/30 min.; (d) HTWL is high temperature water loss, mL/30 min.

Note: A dash significant, no determination was made.

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 AA:MMA copolymer achieved better results in a gypsum-contaminated mud than the did the other copolymers 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 I. Thus, about 20 g total monomers, about 200 mL of Freon-113e and about 0.75 weight percent BPP 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.

TABLES AA-MMA-VA Terpolymers As Water Loss Control Agents Saline Mud A Saturated Saline Mud B Wt. Percent 1 Lb/Bbl 3 Lbs/Bbl 1 Lb/Bbl 3 Lbs/Bbl AA MMA VA WL(A) WL(a) WL(a) WL(a) 60 0 40 22 5.5 74 5.7 60 10 30 22 6.5 41 5.5 60 20 20 21 7.5 22 7.5 60 30 10 19 7.0 17 4.7 60 40 0 17 10.0 21 4.4 70 0 30 24 6.8 71 8.3 70 10 20 27 6.5 30 6.8 70 20 10 35 8.0 22 10.0 70 30 0 33 10.5 19 7.0 (a) my/30 min.

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:MMA:VA terpolymer will perform effectively in drilling fluids as a water loss control agent.

EXAMPLE Vl Copolymers containing 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 3250F (1 63 OC) and at a differential pressure of about 500 psi to determine their high temperature water loss (HOWL) 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/bbl of the previously mentioned Desco thinner and 5 Ib/bbl of Tannathin lignite. The 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 MMA-MA and AA-MA Copolymers as High Temperature Water Loss (HTWL) Control Agents Gypsum Copolymer HTWL, (c) Copolymer (a) Concentration, (b) at 3250F (1 63 OC) (d) 0 1 28 (d) 0 2 30 (d) 1 2 51(f) (d) 1 3 26 (d) 2 2 136 (d) 2 3 135 (e) 0 1 44 (e) 0 2 28 (e) 1 3 120 (e) 1 2 (e) 2 3 260 (e) 2 2 258(f) (a) Ib gypsum per 42 gal bbl of mud (b) Ib 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?A 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 methacrylate content of these polymers is in 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 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 2/1 for a good balance of polymerization rates, copolymer latex viscosity and adequate heat transfer. As in the previously described slurry polymerization process, various free radical sources can be employed including both water soluble or oil soluble 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 ractions 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 monomers are copolymerized in the presence of a water-insolubie organic liquid (usually a hydrocarbon), 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 monoleate. 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 571. Free radical sources described above can also be utilized in the water-in-oil polymerization 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 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 chromatography (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 polymeric 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 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 I.V. measurements wherein appropriate calibration measurements have already been made or can be made. For example, the quantitative esterification 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 Polymer 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 highly effective water loss control agents in drilling fluids of various types, e.g. KCI-containing inhibited muds, workoverfluids containing fibrous additives, etc. The methacrylic acid (MAA) copolymers, ie.

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 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 1 750F (790C) up to about 7000F (371 OC).

EXAMPLE VII A series of methacrylic acid (MAA)/methyl methacrylate (MMA) copolymers were prepared by an oil-in-water polymerization process at 500 C. Sulfonated castor oil (2 phm of 50% by wt. active ingredient) was employed as the surfactant and 0.3 phm potassium persulfate (K2S2O8) as the initiator.

The polymers were neutralized in situ with 5M NaOH and recovered by evaporation 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 3250F (1 630C) under 500 psi differential pressure in the saline mud of Example Vl with and without contaminants gypsum or addiitonal salt (Table 8). Results for certain acrylic acid (AA) copolymers are also shown.

TABLE 7 RTWL Values'a' Saturated Saline Saline Mud A Mud B Monomer Polymer Ratio 1 Lb/ 3 Lb 1 Lb/ 3 Lb/ No. MAA/MMA Bbl Bbl Bbl Bbl 1 95/5 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 5 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 10 50/50 49 9.5 - 86 AA/MA(c) 11 75/25 18 9 21 5.6 AA/MMA(d) 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 TABLE 8 HTWL Values @ 2 Lb/Bbl Polymer(a) Polymer Ratio Mud With No gypsum NaCI No.MAA/MMA Contaminant @ 1 Lb/Bbl @ 10 Lb/Bbl 1 95/5 23 28 2 90/10 24 32 32 3 85/15 23 38 32 4 80/20 26 32 38 5 75/25 24 28 24 6 70/30 28 36 30 7 65/35 26 40 46 8 60/40 - - 9 55/45 - - 10 50/50 - Base Mud - no polymer 150 AA/MA 11 75/25 28 120 160 AA/MMA 12 75/25 24 170(b) - 13 70/30 28 94(b) 140 (a) High Temperature Water Loss, 500 psi, 3250F (b) Average values of two tests The results in Tables 7 and 8 show that the MAA/MMA 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, 1 2 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.

EXAMPLE Vlil Other properties were measured of the drilling mud composition used in Table 8 of Example VII and containing Polymers 1-7 (B 2 Ib/bbl of mud) of Example VII. 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 &commat; 360 F(182 C) Polymer No. PV.YP(a) Gels(b) pH WL(e) 55(d) PV/YP Gels pH WL 1 23/13 16/31 10.3 3.8 180 14/15 11/40 7.7 5.2 2 27/13 16/32 10.3 3.8 190 16/8 14/45 7.6 4.6 3 26/11 15/31 10.4 3.8 190 16/15 10/39 7.7 4.2 4 25/17 16/32 10.5 3.6 180 25/8 13/40 7.5 4.3 5 24/21 16/36 10.3 - 200 15/15 8/35 7.7 4.5 6 27/27 22/52 10.3 2.7 190 10/17 8/- 7.6 - 7 24/38 36/64 10.3 2.8 200 15/22 14/- 7.6 4.5 Base Mud 13/3 6/13 10.7 - < 60 9/10 11/- 7.8 Part B - 1 Lb/8bl Gypsum Added 1 23/13 10/33 10.3 5.3 160 14/5 19/48 7.6 6.9 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 5 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 7 22/33 22/65 10.4 4.7 190 21/43 38/60 7.6 6.3 Part C - 10 Lb/Bbl 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 4 18/22 16/36 9.8 5.0 160 13/16 13/37 7.2 8.3 5 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 (a) PV = Plastic Viscosity, cp;YP=Yield Point, lb/100 ft2 (b) Gels = Gel Strength C' 10 sec (16/100 ft2)/Gel Strength &commat; 10 min (16/100 ft) (c) WL = API Water Loss, mV30 min (d) SS = Shear Strength, lb/100 ft2 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 polyanoinic cellulose identified as Drispace 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 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 Vl and also in Example VII (Table 8). The results of these runs are presented in Table 10 below.

TABLE 10 Muds Aged 16 Hours &commat; 2500F (1210C) Conc. Ib/bbl HTWL &commat; 3250F Run No. Polymer NaCI Gypsum 500 psi, m230 min 1 (A)2 0 1 28 2 (A)2 0 1.5 46 3 (A)1 15 0 37 4 (A)1 20 0 40 5 (A)i.5 20 0 32 6 (A)2 20 0 32 7 (A)2 50 0 28 8 (A)2 60 0 66 9 (A)2 70 0 124 10 (B)2 0 1 38 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 15 (C)i 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 20 (C)2 60 0 36 21 (C)2 70 0 40 Muds Aged 64 Hours &commat;; 176 F(80 C) 22 (A)2 0 1 26 23 (A)2 0 2 150 24 (A)2 20 0 28 25 (C)2 0 1 34 26 (C)2 0 2 37 27 (C)2 20 0 50 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 lb/bbl mud. However, MAA/MMA copolymer (A) became 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 Ib/bbl, polymer (C) was more effective than (A).

EXAMPLE X A large lot (about 10,000 Ib) 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 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 Flosale from Drilling Specialties Co. Bartlesville, Oklahoma, was determined in a series of tests on aqueous solutions containing both. For comparison, a commercially available polyanoinic 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 TABLE 11 Viscosity(a), cp Run Flosal, Polymer, No.Ib/bbl &commat; 1 lb/bbl tZ 600/300 RPM Gels(c) PV/YP(d) WL(b) 1 1 A 28/20 0/0 8/12 200 2 1 Drispac 26/18 0/0 8/10 65 3 2 A 32/22 1/2 10/12 168 4 2 Drispac 30/20 0/0 10/10 64 5 4 A 37/30 1/2 7/23 116 6 4 Drispac 36/26 3/5 10/16 31.5 Aged Overnight &commat; 800C 7 1 A 27/17 0/0 8/9 140 8 1 Drispac 25/15 0/0 10/5 39 9 2 A 29/19 1/2 10/9 108 10 2 Drispac 29/20 0/1 9/11 30 11 4 A 38/28 4/6 10/18 96 12 4 Drispac 36/26 3/5 10/16 27 (a) Determined with Fann viscometer (b) APl water loss determined at room temperature, 100 psi in mL/30 minutes (c) Gels=Gel Strength &commat; 10 sec (lb/100 ft2)/Gel Strength &commat; 10 min (lb/100ft2) 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 Ib/bbl and polymer (Polymer A or Drispac) at 2 Ib/bbl. 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 &commat; 3000F Run Additive No. Polymer 2 ib/bbl PV/YPC Gelsd WLe PV/YPC Gelsd WLB 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 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 &commat;4000F 5 A Bentonite 17/32 6/8 28 9/11 3/3 30 6 A CaCO3 16/28 5/7 22 18/28 5/7 20 7 A Bentonitea 25/18 8/11 28 9/16 3/7 26.8 andCaCO3 &commat; RT (75 C) 8 A Cementb 12/16 2/3 6 11/14 3/5 10.7 a) Both present (B 2 Ib/bbl fluid.

b) Present &commat; 1.5 Ib/bbl fluid.

c) PV = Plastic Viscosity, cp; YP = Yield Point, Ib/1 00 ft2 d) Gels = Gel Strength, &commat; 1 0 sec lb/i 00 ft2)/Gel Strength &commat; 10 min (Ib/1 00 ft2) e) API water loss determined at room temperature, 100 psi in mL/30 minutes 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 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 wells.

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 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.

From the foregoing it will be seen that the polymers of the instant invention readily achieve the objects and advantages set forth above. Although the invention has been described in terms of specified embodiments which are set forth in detail, it should be understood that this is by way of illustration only and that the invention is not necessarily limited thereto, since alternative embodiments and operating techniques will become apparent to those skilled in the art in view of the instant disclosure. Accordingly, modifications are contemplated which can be made without departing from the spirit and scope of the described invention limited only by the claims appended hereto.

Claims (12)

1. An aqueous drilling mud for use in subterranean drilling operations containing, as a water loss control additive, a copolymer having a molecular weight in the range 100,000 to 500,000 and obtained by the free radical polymerisation of a monomer charge comprising one or more hydrophilic monomers and one or more hydrophobic monomers in a molar ratio of from 22:1 to 1:1.

2. A drilling mud according to claim 2, wherein the hydrophilic component of said copolymer is derived from acrylic and/or methacrylic acid.

3. A drilling mud according to claim 1 or 2, wherein the hydrophobic component of said copolymer is derived from one or more of the following: an acrylate or methacrylate ester and vinyl esters of saturated monocarboxylic acids containing from 1-3 carbon atoms in the molecule.

4. A drilling mud according to claim 3, wherein the hydrophobic component of the copolymer is derived from methyl methacrylate.

5. A drilling mud according to any one of claims 1-4, wherein the copolymer is obtained from a monomer charge containing said hydrophilic and hydrophobic monomers in a mole ratio of from 7:1 to 1:1.

6. A drilling mud according to claim 5, wherein said ratio is from 3.5:1 to 2:1.

7. A drilling mud according to claim 1, wherein said copolymer is a copolymer of acrylic acid and methyl methacrylate in a mole ratio of from 3.2:1 to 2.1 :1, the carboxylic acid groups in said copolymer being in neutralised salt form.

8. A drilling mud according to claim 1, wherein said copolymer is a copolymer of methacrylic acid and methyl methacrylate in a mole ratio of from 22:1 to 2:1, the carboxylic acid groups in said copolymer being in neutralised salt form.

9. A drilling mud according to claim 1, wherein the copolymer is a terpolymer of acrylic acid, methyl methacrylate and vinyl acetate in a mole ratio of from 2.8:1:0.33 to 8.3:1 :3.5, the carboxylic acid groups in the terpolymer being in neutralised salt form.

10. A drilling mud according to claim 9, wherein said ratio is from 4.8:1:0.58 to 9.7:1:2.3.

11. A drilling mud according to any one of claims 1 to 10, wherein the copolymer is present in an amount of from 2.9 to 8.6 g/l.

12. A method of drilling a subterranean bore hole wherein there is used as the drilling fluid a drilling mud as claimed in any one of claims 111.

1 3. A method according to claim 12, wherein the temperatures encountered by the drilling fluid when injected into the bore hole are in the range 790C to 371 C.