GB2044321A - Drilling Fluids and Methods of Drilling - Google Patents

Abstract

During drilling of a subterranean well a drilling fluid is used that contains, as a filtration control agent, a copolymer of (1) a (meth)acrylamido alkyl sulphonic acid or alkali metal salt thereof; and (2) a (meth)acrylamide or N- alkyl(meth)acrylamide. The copolymer may be cross-linked with a quaternary ammonium salt cross-linking agent.

Description

SPECIFICATION Drilling Fluids and Methods of Drilling Drilling an oil or gas well is generally conducted by a rotary system. This system depends upon the rotation of a string of drill pipe to the bottom of which is attached a multi-pronged drilling bit. The bit cuts into the earth causing the cuttings to accumulate as drilling continues. As a result, a drilling fluid must be used to carry these cuttings to the surface for removal, thus allowing the bit to continue functioning and the bottom hole to be kept clean and free of cuttings at all times. Drilling systems other than the rotary system are sometimes used but these also require a drilling fluid to remove the bore hole cuttings and to perform functions related to drilling fluids.

Oil-producing formations are generally porous layers having varying degrees of permeability to the flow of fluids such as oil, water, or gas. Consequently, the rate of oil production is largely determined by the rate of flow through these permeable formations which, in turn, is dependent upon the porosity or permeability of the sand or stone present. In drilling through such a porous layer, it is desirable to employ a drilling fluid having such characteristics that excessive amounts of liquids or solids are prevented from penetrating through the porous formation. The ability of the drilling fluid to prevent excessive formation fluid penetration is called filtration control.

Materials that have been used in the past to control filtration rates of aqueous drilling fluids by plugging, producing cakes, or similar methods, have included materials such as pregelatinized starch, sodium carboxylmethylcellulose, sodium polyacrylates and iignites. Each of these materials have certain limitations. For example, lignite becomes ineffective in high salt concentrations.

Acrylic and methacrylic derivatives, such as those which are copolymerised with hydrocarbon substituted styrenes, such as alpha methyl styrene, para methyl styrene, 2-4 dimethyl styrene, and the like have been utilised in drilling fluids. For example, U.S. Patent No. 2,718,497, teaches the use of relatively high molecular weight polymers of these materials to control water loss characteristics of aqueous muds and clay dispersions. Additionally, U.S. Patent No. 2,650,905 teaches the use of water soluble sulphonated polystyrene derivatives for filtration control in water-based drilling fluids.

Acrylic acid derivatives have been proposed as thickeners for numerous commerical purposes, including utilisation in drilling fluids. U.S. Patent No. 4,059,552 teaches the use of acrylamide-sodium acrylate or acrylic acid-substituted acrylates. U.S. Patent 4,037,035 describes use as thickener an acrylamide-sodium acrylate constituent with an acrylamide-acrylic acid alkanolamine. Similarly, copolymers of acrylamide and sodium acrylate and acrylate derivatives thereof formed by irradiation polymerisation are utilised as thickeners, as disclosed in U.S. Patent No. 3,926,576. U.S. Patent No.

3,897,404 teaches utilisation as thickeners for printing paste of substituted acrylamide-acrylic acidacrylate derivatives.

Hydrophilic gels derived from 2-hydroxyethyl methacrylate have been found to be useful in a number of medical applications as material for gel filtration, such as copolymers of acrylamide, acrylic ester-2-hydroxyethyl methacrylate, as disclosed in U.S. Patent No. 3,948,841.

Acrylamide-sodium acrylate-2-hydroxyethyl acrylate cross-linked agents of a comparatively high molecular weight are utilised as soil stabilisers as disclosed in U.S. Patent No. 3,651,002.

Acrylic acid derivatives such as copolymers of acrylamide and sodium acrylate derivatives have been frequently and commerically utilised as flocculants for drilling fluids, and are disclosed in U.S.

Patents Nos. 3,558,545, and No. 3,472,325. Similarly, a copolymer derived from acrylic acid and acrylamide is disclosed in U.S. Patent No. 3,323,603 as a flocculant for aqueous drilling fluids.

Acrylamide/2-acrylamido-2-methylpropane sulphonate and polymers of N-sulphohydrocarbonsubstituted acrylamides have been utilised as viscosifiers in water-flooding operations for secondary and tertiary petroleum recovery, such as in U.S. Patent No. 3,679,000, "A Comparative Evaluation of Polymers for Oil Recovery-Rheological Properties", by Miklos T. Szabo, Society of Petroleum Engineers, A.I.M.E., Paper Number SPE 6601-A; and "Recent Advances in lon-Containing Polymers", by M. F. Hoover and G. B. Butler (J. Polymer SCL: Symposium No. 45, 1-37, 1974).

An aqueous drilling fluid according to the invention comprises a clayey substance and, as a filtration control agent, a copolymer of (1) a (meth)acrylamido alkyl sulphonic acid or alkali metal salt thereof; and (2) a (meth)acrylamide or N-alkyl(meth)acrylamide.

in the invention a well is drilled through a subterranean formation and, during drilling, a drilling fluid containing a filtration control agent to decrease loss of fluid is circulated into the well, and the drilling fluid contains, as the filtration control agent, a copolymer of (1) a (meth)acrylamido alkyl sulphonic acid or alkali metal salt thereof; and (2) a (meth)acrylamide or N-alkyl(meth)acrylamide.

Although it is not fully understood it is believed that the copolymer together with the other constituents of the drilling fluid will produce a filter cake along the bore hole so as to maintain effective filtration control during circulation of the drilling fluid within the well.

By the invention it is possible to obtain satisfactory filter cake formation or other means of filtration control sn when exposed to high temperatures and salt environments.

Any acrylamido alkyl sulphonic acid or alkali metal salt thereof may be utilised as component (1) of the copolymer. For example, one may utilise 1,3-acrylamido propyl sulphonic acid, or, preferably the sodium salt thereof. Preferably, sodium 2-acrylamido-2-methyl propane sulphonate is utilised.

Any (meth)acrylamide or N-alkyl(meth)acrylamide which is capable of polymerisation with the acrylamido alkyl sulphonic acid or alkali metal salt thereof may be used as component (2) of the copolymer. Preferably, acrylamide will be selected as the monomer, because of its comparatively low cost, availability, and ease of reaction with other reagents.

It is generally preferred that the amount of component (1) should be from 8 to 70 mole % and the amount of component (2) should be from 30 to about 91 mole %. The copolymers may be substantially free of cross-linking agent, for instance being formed solely of the two defined components. One preferred copolymer contains about 20 mole % of component (1), preferably 2-acrylamido-2methylpropane sulphonate, and about 80 mole % of component (2), generally acrylamide. However other preferred polymers contain 30 to 70 mole % of component (1) and 70 to 30 mole % of component (2).

It is sometimes desirable for the copolymer to be formed of monomers including also a quaternary ammonium salt.

It is sometimes desirable for the copolymer to be in a cross-linked form. This appears to provide more effective filtration control in salt environments, such as sea water.

The cross-linking agent may be a quaternary ammonium salt, such as 3-methacryloyloxy-2hydroxy propyl trimethyl ammonium chloride, 3-acrylamido-3-methylbutyl trimethyl ammonium chloride, diallyl dimethyl ammonium chloride, N-methyl 2-vinyl pyridinium methyl sulphate, N-methyl 4-vinyl pyridinium methyl sulphate, N-propyl acrylamido, trimethyl ammonium chloride, or, preferably, a (meth)acryloyloxy alkyl tri methyl ammonium salt, especially 2-methacryloyloxy-ethyi trimethyl ammonium methosulphate. An amount of (meth)acryloyloxy alkyl trimethyl or other ammonium salt from between about 0.2 mole % to about 4.4 mole % is suitable which may be about 0.2 mole % methacryloyloxy-ethyl trimethyl ammonium methosulphate being preferred.

In these cross-linked polymers the amount of component (1) is preferably about 9 to 25 mole % and the amount of component (2) is preferably about 73 to 90 mole % with preferred products having 9 to 10 mole % of component (1) and substantially 90 mole % of component (2). Preferred polymers contain about 9.8 mole % of component (1), about 90 mole % of component (2) and about 0.2 mole % quaternary ammonium salt.

The copolymers used in the invention preferably consist solely of components (1) and (2), and optionally the described ammonium salt or other cross-linking agent, but of course small amounts of other comonomers may be incorporated provided they do not significantly detract from the desired properties of the copolymers. The maximum amount of such additional monomers is normally 5 mole % or at least 10 mole % but preferably is zero mole %.

The copolymers may be made by polymerisation of the chosen monomers using, for example, emulsion, suspension or bulk polymerisation techniques although solution polymerisation is preferred.

The polymerisation may be promoted by typical initiators used in aqueous systems such as peroxides, persulphates or persulphatesulphites. It has been found that the copolymer can be made with small quantities of initiator when the reagents are warmed to about 600 C. The amount of initiator under such condition does not affect the properties of the copolymer as a filtration control agent.

The copolymer is preferably present in the drilling fluid in an amount of at least 0.25 ppb (pounds per 42 gallon barrel) and preferably in amounts of 0.25 to 5 ppb. Generally it is added to the aqueous base drilling fluid at the drilling or rig location. The precise amount required for optimum properties will depend upon the particular type of aqueous drilling fluid utilised, such as brine or sea water, the weight of the given drilling fluid, the clayey substance or substances in the fluid and the presence and amount of other chemical additives, such as lignosulphonate defiocculants. Simple and commerically available testing techniques may be easily utilised at the well site to determine the amount of filtration control additive which must be added to the circulatable drilling fluid to provide effective filtration control in the subterranean well.Because of the loss of material in the well, such as through adsorption onto the surface of the drilled solids, it may be necessary to make incremental additions of the copolymer to the drilling fluid from time to time to maintain the required concentration. The clayey substances used in the invention will be suspended in the fluid and may be any of the materials typically used in drilling fluids and may be present in typical amounts.

Effectiveness of the copolymer to control the fluid loss from drilling fluids may be determined by utilisation of a simple filtration test. A standardised procedure for determining the filtration rate is described in the "A.P.I. Recommended Practice RP 13 B Standard ProcedureforTesting Drilling Fluids", 2nd Edition (April 1969).

The preparation and use of the copolymer to control fluid loss in an aqueous drilling fluid is further described in the examples which foliow: Example 1 The present Example demonstrates a preparation of the copolymer of the present invention wherein acrylamide, sodium 2-acrylamido-2-methyl propane are utilised as the initial monomeric reagents with and without a cross-linking agent. A solution was prepared by mixing 30.2 grams of water and 12.3 grams of 50% caustic soda, which was cooled by an ice-water bath, followed by the incremental addition of 31.8 grams of 2-acrylamido-2-methyl propane sulphonic acid. The acid was added at a rate to maintain the temperature of the solution below about 300C. The pH of the solution was adjusted and maintained between about 6.8 and about 7.0. A 64.3 gram charge of acrylamide was thereafter added.The solution was warmed to about 300 C. to effect dissolution of the reagents.

The solution was transferred to a 250 ml. addition funnel.

A 1-litre jacketed reaction flask equipped with a mechanical stirrer, thermometer, reflux condenser and 250 ml. addition funnel containing the mixture of the monomers was charged with 263 grams of deionised water. The water was heated to 600C. Approximately 1/10th of the monomer solution was added followed by 1 ml of an aqueous solution containing 1 8 mg of potassium persulphate. An exothermic reaction was noted after approximately 10 minutes after addition of the initiator. The remaining monomer mixture was added dropwise during a none-hour period, and one ml.

aliquots of aqueous solution containing 1 8 mg. of potassium persulphate was added every 6 minutes.

The reaction was maintained at a nearly constant temperature by circulating water through the jacket from a constant temperature water bath at about 600 C. The temperature of the reaction mixture increased to about 650C. during the next three hours.

The very viscous gel-like reaction mixture was added slowly to and rapidly stirred with 2 litres of acetone. The product was precipitated into small filterable particles when the mixing and addition technique was optimised. The product was filtered on a Buchner funnel, washed with small portions of a 1 -to-1 mixture of water and acetone. The product was dried at 1 050C to a constant weight, requiring about 24 hours. The yield of the product was nearly quantitative. The product was analysed for sulphur, which was calculated at 4.87, and was found to be 4.9.

A cross-linked copolymer was prepared, as above, by reacting 1.8 grams of 80% methacryloyloxy ethyl trimethyl ammonium methosulphate in water, with the monomeric reagents prior to warming of the reagents to 300 C. The procedure and reaction was continued as described for the preparation the un-cross-linked copolymer.

The monomeric composition of the samples given in Table I below are representative of the copolymer (Samples G through L) and the cross-linked copolymer (Samples A through F), of this invention.

Table I 2-Methacryl oyloxy-ethyl Sodium 2-acryl- trimethyl am amide-2-methyl monium metho- Meth Acrylamide propane sulphon- sulphate (mole acrylamide Sample (Mole %) ate (Mole %) %J (Mole /0) A 73.2 24.4 2.4 B 78.3 19.5 2.2 C 75.0 24.8 0.2 D 90.0 9.8 0.2 E 75.0 24.5 0.5 F 90.0 9.5 0.5 G 70.0 30.0 H 68.3 31.7 50.0 50.0 J 50.8 49.2 K 30.0 70.0 L 50.0 50.0 Example II In the present Example, and the Examples which follow, the copolymer of the present invention was tested in simulated drilling fluid environments to determine filtration control characteristics.The indicated amount of the copolymer was slowly sifted into barrel equivalents of a base mud while shearing at moderate speed on an electric mixing apparatus after which shearing was continued for a period of 30 minutes. The pH of the sample was adjusted to about 9.5 with incremental additions of sodium hydroxide, when necessary. Thereafter, each sample was hot rolled in an oven at 1 500F for 1 6 hours. Thereafter, each sample was permitted to cool to room temperature and the pH was readjusted to about 9.5, when necessary. Flow properties and A.P.I. filtrate tests were performed. Thereafter, some of the samples were again hot-rolled between 3000F and 3500F for the indicated time and thereafter cooled to room temperature, the pH again being readJusted to about 9.5, when necessary.

Thereafter, flow properties and A.P.I. filtrate were again determined.

In the present Example, a mud system was prepared to simulate a sea water mud. The sample mud had the following composition: 20 ibs./bbl. bentonite; 10.7 Ibs/bbl of commerically available sea water salt; and 1 T Ibs./bbl chrome lignosulphonate. The pH of the mud composition was adjusted to about 9.5 with sodium hydroxide. A typical filtrate analysis of this base mud is as follows: 1 5,000 mg/l chloride ion; 900 mg/i total hardness, measured as calcium ion.

In the present Examples, a mud sample was treated with the indicated copolymers described in Example I and evaluated, along with the base mud containing no additive, for purposes of determining effective filtration control, as well as for measuring the rheology of the fluid. The results of the tests are given in Table II, below.

Table II Concen- Identi- 10 tration fication Fann Readings Initial Minute API lb/hbl. Sample 600 300 200 100 6 3 Gel Gel pH filtrate 0.5 A 18 11 9 6 2.5 2 2 8.5 9.5 27.5 1.0 A 30.5 18.5 14.5 9.5 4 3.5 4 9.0 9.6 20.0 2.0 A 39 25.5 21 15 8 7.5 7.0 20 9.6 12.2 0.5 B 19 12 9.5 6.5 3 2.5 2.5 8 9.7 30.0 1.0 B 24 14 11 7 3 2.5 3 7 9.6 20.4 2.0 B 40 25.5 20.5 14 7 6.5 6.5 17 9.7 11.8 0.5 C 15.5 9 7 4.5 2 1.5 1.5 5 9.6 35 1.0 C 22 13 10 7 3 2.5 2.5 7 9.6 26.6 2.0 C 35 20 15 10 3.5 3 3 11 9.6 13.2 0.5 D 16 9 7 4.5 1 1 2 5 9.7 30.2 1.0 D 17.5 10.5 8 5 1 1 2.5 6 9.5 25.5 2.0 D 31 18.5 14 9 2.5 2 4 7 9.6 13.0 0.5 E 20.5 12 9 6.5 3 2.5 3 8 9.7 22.8 1.0 E 30 16.5 12.5 7.5 2.5 2 2 9 9.7 14.0 2.0 E 57.5 35 28 18 7.5 6.5 6 22 9.6 7.2 0.5 F 16.5 9 7 4 1 1 2 6 9.5 23.0 1.0 F 20.5 12 9 6 1 1 2 8 9.6 8.2 2.0 F 35 21 16 10 3 3 5 14 10.0 8.6 Base 15 8 6.5 4 1.5 1.5 1.5 5.5 9.7 54.0 Example Ill In order to determine the effect of temperature upon the ability of the filtration control agent of the present invention to effectively control filtration in a sea water mud sample, Sample "D", as identified in Table I of Example I, was added to the simulated sea water mud identified in Example II at the 1 and 2 Ib/bbl. concentration levels. Each sample was first hot rolled at 1 500F for 16 hours after which rheological properties and A.P.I. filtrate were measured. Thereafter, each sample was again hot rolled at a temperature of 3250F for 10 hours. Thereafter, the samples were permitted to cool, the pH was adjusted, and rheological properties and A.P.I. filtrate readings were taken.The results of this test indicated that the filtration control agent of this invention continues to provide filtration control even after exposure to temperatures above 3000F. The results of this test are set forth in Table Ill below.

Table Ill In!- 10 API Hot Fann Readings tial Minute Fll- Sample Rolled 600 300 200 100 6 3 Gel Gel pH trate D 1500F16hrs. 30 16.5 12.5 7.5 2.5 2 2 9 9.7 14.0 (1 Ib/bbl) 3250F 10 hrs. 21 13 10 7 2 1.5 1 2 9.5 23.8 D 15O0F16hrs. 57.5 35 28 18 7.5 6.5 6 22 9.6 7.2 (2 Ib/bbl) 3250F10hrs. 34 21 17 11 2 1.5 1.5 2.5 9.7 10.0 Base Mud 1500F16hrs. 15 8 6.5 4 1.5 1.5 1.5 5.5 9.7 54 3250F10hrs. 12 7 5 3 1.5 1 1 5 9.5 66.5 Example IV The present Example demonstrates the ability of the filtration control agent of the present invention to provide effective filtration control in fresh water and saturated salt water mud environments. One fresh water mud system (Mud "lV-A") comprised deionised water containing: 1 7.5 Ib/bbl bentonite; 1 Ib/bbl gypsum; 2lb/bbl sodium chloride; and 1.5 Ib/bbl chrome ignosuiphonate. The other fresh water mud (Mud "lV-B") was comprised of 22 Ib/bbl bentonite and 10.7 Ib/bbl sodium chloride. The pH was adjusted to about 9.5. A typical filtrate analysis of the first mud indicated: a chloride ion content of 3,800 mg/l; 300 mg/l calcium ion; and a total hardness of 350 mg/l. The saturated salt water mud system (Mud "lV-C") was a saturated aqueous sodium chloride solution containing 1 5 Ib/bbl of attapulgite.The rheological properties and A.P.I. filtrates of these muds Nere measured as in earlier tests. Table IV below reflects the results of these tests.

Table IV A Fresh Water Mud IV-A Concen- 10 tration Sample Fann Readings Initial Minute API lb/bbl No. 600 300 200 100 6 3 Gel Gel pH Filtrate Hot Rolled at 1 50 OF. 16 hrs.

0.5 A 13.5 7.5 5.5 3 1 1 1 1.5 9.7 19.0 1.0 A 16 8.5 6 3.5 1 1 1 1.5 9.7 16.0 1.5 A 18.5 10.5 8.5 4.5 1.5 1 1 1.5 9.9 11.8 0.5 B 11 6 4 2.5 1 0.5 0.5 1 9.8 18.6 1.0 B 15.5 8.5 6.0 3.5 1 1 1 1 9.6 14.4 1.5 B 20 11 8.0 5 1.5 1 1 1.5 10.1 11.4 0.5 C 14 8 5.5 3.5 1 1 1 1 9.9 17.8 1.0 C 18 10 7 4 1 1 1 1 9.9 13.6 1.5 C 31 17.5 13 8.5 3 2.5 2.5 7 9.5 14.0 0.5 D 13.5 7.5 5 3 1 1 1 1 9.9 27.2 1.0 D 18 10.5 8 5.5 2 1.5 1.5 4 9.6 18.2 1.5 D 22 12 8.5 5 1 1 1 1.5 10.1 11.6 0.5 E 23 13 9.5 6 1 1 1.5 3 9.4 12.4 1.0 E 20 12 8.5 5 1 1 1.5 3 9.6 9.0 1.5 E 28 17 13 8 2 1 2 5 9.9 6.8 0.5 F 13 7 5.5 3.5 1 1 1 3 9.9 12.8 1.0 F 15.5 9 6 3.5 0.5 0.5 1 2 9.8 9.4 1.5 F 20 11.5 8.5 5 1 1 1 4 9.9 8.2 Base Mud 10 5 4 2 1 0.5 1 1 9.5 28.4 Table IV B Saturated Salt Water Mud IV-C Concen- Fann 35 Rheology 10 tration Sample 600 300 200 100 6 3 Initial Minute API ./6/66/ No. Gel Gel pH Filtrate Hot Rolled 1 50 OF. 16 hrs.

2.0 A 30 19 15 11 5 4 5 7 9.8 36.8 4.0 70 49 40 29 9 6 8 16 9.5 8.4 2.0 B 38 22 17 13 5 4 5 7 9.5 44.8 4.0 69 48 29 28 7 5 6 9 9.5 7.6 2.0 C 49 38 33 26 5 5 7 8 9.6 25.0 4.0 52 33 25 15 2 1.5 1.5 2.5 9.5 7.1 2.0 D 45 33 27 21 7 4 6 10 9.6 28.0 4.0 70 48 39 26 6 5 5 11 9.6 7.8 2.0 E 56 40 34 26 10 7 8 10 9.7 32.0 4.0 69 44 34 22 4 3 3.5 6 9.5 6.8 2.0 F 43.5 28 23 12.5 5.5 4 7 7 10.2 36 4.0 98 71.5 60 45 10 7.5 10 22 10.4 6.2 1.0 G 24 16 13 10 4 3 4 4 8.6 137 3.0 74 51 40 38 9 7.5 7 9 8.8 7.0 1.0 H 22.5 15 12 7 3.5 3 4 4 9.2 110.6 3.0 56 33 24.5 10.5 3.5 3 3 5 9.1 6.0 3.0 1 56 38 31 22.5 8 6.5 6 7 9.3 10.4 1.0 J 26 19 16 13 5.5 4 5 5 9.0 99 Table IV B (control Saturated Salt Water Mud IV-C Concen- Fann 35 Rheology 10 tration Sample Initial Minute API Ib/bbl No. 600 300 200 100 6 3 Gel Gel pH Filtrate Hot Rolled 1 50 OF. 16 hrs.

3.0 34 22 18 13.5 7 6.5 8 10 9.4 51 1.0 K 20.5 14 12 9 4.5 4 6 6 8.9 99 3.0 40 30 25 19 8.5 7.5 8 9 9.3 43.2 Base Mud 32 25 22 19 12 11 11 18 9.8 163 Table IV C Fresh Water Mud IV-B Concen- Fann 35 Rheology 10 tration Sample Initial Minute API Ib/bbl No. 600 300 200 100 6 3 Gel Gel pH Filtrate 0.5 G 59 49 44 33.5 29 28 26 29 8.1 19.4 1.0 78 59 52 44 30 29 30 45 8.2 13.0 0.5 H 54 44 39 34 23 23 23 28 8.4 26.6 1.0 61 48 43 37 25 25 25 47 8.3 16.6 0.5 1 54 45 40.5 36 24 24 23 29 8.1 23.4 1.0 65 52 46 40 27 27 28 28 8.2 18.3 0.5 J 51 43 39 34 26 25 23 25 8.3 25.0 1.0 55 45 41 35 23 23 24 25 8.2 22.4 0.5 K 49 40 37 33 23 23 22 24 8.2 25.2 1.0 59 47 42 35 23 23 22 33 8.3 22.0 0.5 L 50 41 38 33 22 22 22 26 8.3 23.0 1.0 49 39 36 31 21 20 20 30 8.3 17.9 Base Mud 46 39 36 32 22 22 20 15 8.3 32.5

Claims (22)

Claims

1. An aqueous drilling fluid comprising a clayey substance and, as a filtration control agent, a copolymer of (1) a (meth)acrylamido alkyl sulphonic acid or alkali metal salt thereof; and (2) a (meth)acrylamide or N-alkyl(meth)acrylamide.

2. A fluid according to claim 1 in which the amount of component (1) in the copolymer is 8 to 70 mole %.

3. A fluid according to claim 1 or claim 2 in which the amount of component (2) in the copolymer is 30 to 91 mole %.

4. A fluid according to any preceding claim in which the copolymer is substantially free of crosslinking agent.

5. A fluid according to claim 4 in which the amount of component (1) in the copolymer is about 20 mole % and the amount of component (2) is about 80 mole %.

6. A fluid according to claim 4 in which the amount of component (1) in the copolymer is about 30 to 70 mole % and the amount of component (2) is about 70 to 30 mole %.

7. A fluid according to any of claims 1 to 3 in which the copolymer is cross-linked.

8. A fluid according to claim 7 in which the amount of component (1) in the copolymer is about 9 to 25 mole % and the amount of component (2) is about 73 to 90 mole %.

9. A fluid according to claim 7 or claim 8 in which the amount of cross-linking agent is 0.2 to 4.4 mole %.

10. A fluid according to any of claims 7 to 9 in which the amount of component (1) is 9 to 10 mole % and the amount of component (2) is substantially 90 mole %.

11. A fluid according to claim 7 in which the copolymer is formed of 9.8 mole % component (1), 90 mole % component (2) and 0.2 mole % cross-linking agent.

12. A fluid according to any of claims 7 to 11 in which the cross-linking agent is a quaternary ammonium salt.

13. A fluid according to claim 12 in which the quaternary ammonium salt is a (meth)acryloyloxy alkyl trimethyl ammonium sait.

14. A fluid according to claim 12 in which the quaternary ammonium salt is 2-methacryloyloxyethyl trimethyl ammonium methosulphate.

15. A fluid according to any preceding claim in which component (1) is sodium 2-acrylamido-2methyl propane sulphonate.

1 6. A fluid according to any preceding claim in which component (2) is acrylamide.

1 7. A fluid according to any preceding claim in which the copolymer is a copolymer of substantially 20 mole % sodium 2-acrylamido-2-methyl propane sulphonate and substantially 80 mole % acrylamide.

1 8. A fluid according to any of claims 1 to 1 6 in which the copolymer is a copolymer of substantially 9.8 mole % sodium 2-acrylamido-2-methyl propane sulphonate, substantially 90 mole % acrylamide and substantially 0.2 mole % 2-methacryloyloxy-ethyl trimethyl ammonium methosulphate.

1 9. A fluid according to any preceding claim containing at least 0.25 Ibs copolymer per 42 gallon barrel.

20. A fluid according to claim 1 substantially as herein described with reference to any of the Examples.

21. A method in which a well is drilled in a subterranean formation and in which during drilling an aqueous drilling fluid is circulated into the well and in which the drilling fluid contains, as filtration control agent, at least 0.25 Ibs per barrel of copolymer as defined in any of claims 1 to 1 8.

22. A copolymer as defined in any of claims 1 to 18 for use in an aqueous drilling fluid as a filtration control agent.