GB2032929A

GB2032929A - Copolymers for use as drilling fluid additives, and methods for drilling

Info

Publication number: GB2032929A
Application number: GB7933542A
Authority: GB
Original assignee: Milchem Inc
Current assignee: Milchem Inc
Priority date: 1978-09-27
Filing date: 1979-09-27
Publication date: 1980-05-14
Also published as: CA1144298A; GB2032929B; FR2441049A1; MX150927A

Abstract

Copolymers of an alkali metal salt of acrylic acid, a hydroxy alkyl acrylate and acrylamide are described. They are of particular use as drilling fluid additives, for instance as filtration control agents so as to decrease loss of fluid during drilling of a well.

Description

SPECIFICATION Copolymers for use as drilling fluid additives, and methods of drilling Oil or gas wells are generally drilled by rotary drilling systems. These depend upon the rotation of a column of drill pipe to the bottom of which is attached à multi-pronged drilling bit. The bit cuts into the earth causing the cuttings to accumulate as drilling continues. A drilling fluid is circulated through the well 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 employed during drilling operations. Nevertheless, these systems still require a drilling fluid to remove the bore hole cuttings and to otherwise 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 mud having such characteristics that excessive amounts of liquids or solids are prevented from penetrating the porous formation. The ability of the drilling mud 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 pre-gelatinized starch, sodium carboxylmethylcellulose, sodium polyacrylates, and Signites.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 co-polymerized with hydrocarbon substituted styrenes, such as alpha methyl styrene, para methyl styrene, 2-4 dimethyl styrene, and the like have been utilized 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 commercial purposes, including utilization 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 No. 4,037,035 describes an acrylamidesodium acrylate constituent with an acrylamide-acrylic acid alkanoiamine. Similarly, copolymers of acrylamide and sodium acrylate and acrylate derivatives thereof formed by irradiation polymerisation are utilized as thickeners, as disclosed in U.S. Patent No. 3,926,576. U.S. Patent No. 3,897,404 teaches utilization as thickeners for printing paste of substituted acr,ylamide-acrylic acid-acrylate 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 utilized as soil stabilisers as disclosed in U.S. Patent No. 3,651,002.

Acrylic acid detivatives such as copolymers of acrylamide and sodium acrylate derivatives have been frequently and commercially utilized 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.

A copolymer according to the invention that is of particular value for use as a drilling fluid additive, is a copolymer of a monovalent alkaline metal salt of acrylic acid, a hydroxy alkyl acrylate and acrylamide.

Suitable monovalent aikaline metal salts are the sodium, potassium and cesium salts, sodium acrylate being preferred. Di and trivalent salts are less acceptable because of their tendency to precipitate in aqueous systems. Salts of analogues of acrylic acid, for instance methacrylic acid salts, are also less satisfactory because they are less soluble in aqueous systems. Because of the high solubility of the chosen acrylic acid salts in aqueous environments their copolymerisation activity is enhanced, thus making them particularly suitable for use in the preparation of the chosen copolymers.

The hydroxyalkyl acrylate of the copolymer appears to provide hydrogen bonding between chains incorporating a carboxyl group and to provide a bulkier polymer end product. Additionally, the utilization of an hydroxy alkyl acrylate also enhances the viscosity of the terpolymer and renders it less sensitive to adverse effects of heavy salt concentrations, such as those found in aqueous brine solutions encountered as a base constituent of an aqueous drilling fluid. Also, by incorporating the hydroxy alkyl acrylate material into the terpolymer, good hydrogen-bonded cross-linking can be achieved between the chains which will aid in resisting salt sensitivity of the resultant terpolymer material.

Numerous hydroxy alkyl acrylates may be utilized, such as 2-hydroxy alkyl acrylates and 3-hydroxy alkyl acrylates, for example 2-hydroxyethylacrylate, 2-hydroxymethylacrylate, 2-hydroxybutylacrylate or, preferably, 2-hydroxypropylacrylate.

The third monomer included in the preparation of the copolymer is acrylamide which has been found to have a certain electrolyte reaction when incorporated into the terpolymer utilized in the present invention. Although not fully understood, it is believed that the acrylamide monomer when polymerised with the other constituents of the terpolymer will act as a dispersant of carboxyl groups on the resultant polymeric chain. Although methacrylamide may be substituted for the acrylamide monomer of the present invention, it has been found that it is less tolerant of the heavier concentrations of salt solutions and environments and is less water soluble than the acrylamide. Therefore, the acrylamide monomer has been found to be more acceptable for use in general in the present invention.

The copolymers are preferably terpolymers consisting solely of these three monomers 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 most 10 mole %, but preferably the amount is zero mole %. All monomers are preferably only difunctional.

The amount of acrylic acid salt incorporated into the preparation of the copolymer is preferably from 5 to 62 mole % of monomers used, most preferably about 23 mole %, especially when the salt is sodium acrylate.

The amount of hydroxy alkyl acrylate is preferably 2 or 2.1 mole % up to 40 mole %, most preferably about 4.0 mole %, especially when the monomer is 2-hydroxypropylacrylate.

The amount of acrylamide is preferably from 31 to 91 mole % and is most preferably substantially 70 mole %. For instance it may be 70 + 10 mole %. Preferably it is about 73 mole %.

Copolymers of the invention are useful as drilling fluid additives and in particular have use as filtration control agents. It seems that the copolymer participates in the production of a filter cake along the bore hole and that this cake assists in maintaining effective filtration control during circulation of the drilling fluid within the well.

Filter cakes may be formed that are substantially unaffected by comparatively high bore hole temperatures and pressure. The filtration control agents of the invention can be formulated so that they do not materially affect the viscosity of the drilling fluid system being used. They are easily prepared anc of relatively low cost. They are relatively insensitive to salt environments and concentrations within the aqueous system. They may be used in highly weighted drilling fluids.

The drilling fluids will generally comprise water, a clayey substance and the added copolymer.

The invention includes also methods of drilling a well in a subterranean formation in which, during drilling, a drilling fluid is circulated in the well and a filter cake is formed on the wall of the well to reduce loss of fluid, and in these methods the drilling fluid comprises a copolymer as defined above.

The copolymer may be prepared using a variety of known techniques. For example, emulsion, suspension or bulk polymerisation techniques may be utilized.

As an effective filtration control agent the terpolymer may be added to any aqueous base drilling fluid at the drilling or rig location in an amount from between about 0.25 ppb to about 5 ppb (pounds per 42 gallon barrel). Preferably the components and molecular weight of the copolymer are such that ii is soluble in the fluid. The amount needed will vary, of course, depending upon the particular type of aqueous drilling fluid utilized, such as brine, sea water, or the like, the weight of the given drilling fluid, the clayey substances appearing therein, and the presence and amount of other chemical additives, such as lignosulphonate deflocculants, and the like.Simple and commercially available testing techniques may be easily utilized 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 and the like, it may be necessary to incrementally add additions of the terpolymer to the drilling fluid from time to time to maintain the required concentration.

A measure of the ability of the drilling fluid to form a thin impervious compressible filtercake may be determined by utilization of a simple filtration test in which the filter cake is formed and pressed against the membrane or filter which is permeable to water. A standardised procedure for determining the filtration rate is described in "API Recommended Practice RP 13 B Standard Procedure for Testing Drilling Fluids," 2nd Edition (April 1969).

The preparation and use of the terpolymer to control filtrate in an aqueous drilling fluid is further described in the examples which follow: EXAMPLE 1 A terpolymer of sodium acrylate, 2-hydroxypropyl acrylate and acrylamide was prepared utilizing an emulsion polymerisation process. The mole ratios of acrylic acid to acrylamide, mole percent of 2hydroxypropyl acrylate and molecular size were varied by controlling the reaction parameters. Six representative samples of the terpolymer were prepared with the compositions of the various terpolymers being estimated based upon their total nitrogen content. In the reaction, ammonium persulphate was utilized as the polymerisation initiator or catalyst. Reagent grade acetone and reagent grade anhydrous methanol were utilized to form the solvent. The emulsifier was sodium lauryl sulphate, and 2N aqueous sodium hydroxide was utilized to neutralise the reactants. The dried products were thereafter milled and passed through a number 30 mesh screen. The reaction was conducted over a period of three hours at a reaction temperature of about 600C.

The six polymers prepared by this technique and the varying amounts of monomers and reagents are given in the table below, together with the resultant product yield for the terpolymer. TABLE 1 Terpolymer Preparation

Polymer Acrylic 2-(hydroxy propyl) Sodium Lauryl Yield No. Units Acid Acrylate Acrylamide (NH4)2S2O8 Sulphate Water g % 1 g* 89.0 18.0 43.0 1.00 7.00 608 157 88 M* 1.61 0.18 0.79 5.2x10-2 30x10-2 2 g 46.0 10.0 88.0 1.00 7.00 808 149 97 M 0.67 0.08 1.29 4.9x10-2 24x10-3 3 g 46.0 10.0 88.0 0.60 10.00 608 132 86 M 0.84 0.10 1.63 3.4x10-3 43x10-3 4 g 89.0 18.0 43.0 0.60 10.00 1061 105 59 M 1.17 0.13 0.57 2.5x10-3 3.1x10-3 5 g 72.00 14.0 69.0 1.20 4.00 968 107 61 M 1.03 0.11 1.00 5.4x10-3 14x10-3 8 g 75.0 28.5 72.0 0.40 4.00 1180 157 78 M 0.88 0.19 0.86 1.5x10-3 11x10-3 *g - grams *M - mole/kg EXAMPLE II The present example was conducted to determine the effect of exposure of various types and concentrations of inorganic salts to selected terpolymers made as in Example I, above. Polymer samples No. 2 and No. 3 of Example I, which contained a mole ratio of 2-to-1 of acrylamide-to-acrylic acid, were soluble in various aqueous-saturated salt solutions, while those polymer samples having a smaller mole ratio were precipitated by excess amounts of calcium ion.

The effect of the selected salt concentrations on the given polymer samples was determined by using swelling tests. The procedure for the swelling tests incorporated utilization of a 1 5 millilitre centrifuge tube which was charged with 0.50 grams of the selected dry terpolymer which was powdermilled to pass through a No. 30 mesh screen. The tube also was charged with water or the selected aqueous solutions containing varying amounts of one of the salt samples. The total volume in the tube for each sampled material was 13 millilitres. The tubes were stoppered, shaken vigorously to disperse the terpolymer and heated in a steam bath for about 10 minutes.Thereafter, the tubes were cooled to room temperature, shaken to disperse the terpolymer, and centrifuged at 1 800 rpm for about 20 minutes using an International Clinical Centrifuge which was equipped with a head having an 8.5 cm radius of gyration. The volume of the swollen terpolymer was measured as the difference between the total volume of 13 millilitres and the volume of the supernatent. The volume of the unswollen polymer was measured utilizing isopropyl alcohol as the non-solvent.

The criteria for measuring the degree of swelling for the samples was based upon the application of q as a criterian for measurement of the chemical potential of an electrolyte in a given solvent containing a given amount of electrolyte which is based upon theoretical models developed by Flory, in "Principles of Polymer Chemistry", (Cornell University Press, 1953) and by Hildebrand, in "Regular and Related Solutions" (Van Hostrand Reinhold Company, 1970) which explains the unique solubility of a polymer in various solvents.

The results given in Table 2, below, indicate that the polymer samples No. 2 and 3 of Example I were soluble in aqueous solutions which are saturated with various salts, while poiymer Nos. 1, 4, 5 and 6 were somewhat precipitated by excess amounts of calcium ion. These tests indicate that these polymers have potential application as filtration control agents exposed to high salt environments.

TABLE 2 DEGREES OF SWELLING, q, VERSUS IONIC STRENGTH OF AQUEOUS SALT SOLUTIONS

Salt conc. lonic Terpolymer number mole/kg Strength 1 2 3 4 5 6 Salt M mhom/kg() Degree of Swelling q H2O 0 0 gel gel gel gel gel gel CaSO4 0.01 0.04 - gel gel 20 gel (2) gel CA(OH)2 0.02 0.06 gel gel gel gel (2) gel (2) gel KCI 4.02 4.02 7 gel gel (2) 10 8 11 MgSO4 1.21 4.84 9 gel - 8 gel (2) 11 NaCI 6.00 6.00 7 gel -gel (2) 8 12 12 CaCI2 2.04 6.12 2 gel gel (2) 2 8 7 (t) S = 1/2 S (MV2), V is valence of ion.

(2) Gel formed with some swollen polymer particles suspended in solution.

EXAMPLE lil The filtration control effectiveness of the terpolymer samples prepared as in Example I, together with a sample of a commercially available filtration control agent generically described as a polyanionic cellulose (hereinafter referred to as "P.A.C.") were tested in a 22 ppb Wyoming Bentonite 3% sodium chloride suspension. The filtration control test was the API test as referred to above. The terpolymer and P.A.C. samples were sifted into barrel equivalents of muds while shearing at moderate speed on an electric mixer after which shearing was continued for thirty minutes. Thereafter, the samples were hot rolled at 1 500F for about 1 6 hours and thereafter cooled to room temperature, before flow properties and API filtrate properties were determined.

The results of this test clearly indicated that the terpolymer samples were effective in reducing the filtrate, when compared to a sample of each drilling fluid containing no filtration control additive.

The results of this test are set forth in the tables below: TABLE 3A FLOW AND FILTRATION CHARACTERISTICS OF 22 ppb WYOMING BENTONITE 3% NaCl SUSPENSION TREATED WITH EXAMPLE I TERPOLYMERS Fann 35 Rheology Room Temperature 600 300 200 100 6 3 IG 10G pH API Filtrate 0.5 ppb Sample 1 52 40 34.5 28 18 17 12 42 9.0 16.0 1.0 ppb Sample 1 52 38 33 25 10 9 3 18 8.5 9.5 0.5 ppb Sample 2 44 25 19 13 3 2.5 3 11 8.2 13.0 1.0 ppb Sample 2 71.5 42 32 21 4 4 4 7 8.3 8.7 0.25 ppb Sample 3 46.5 38 35 31 28 28 8.1 19.9 0.5 ppb Sample 3 70 52 46 39 26 19 18 49 8.4 10.6 1.0 ppb Sample 3 76 63 56 46 3629 22 69 8.3 7.2 0.5 ppb Sample 4 52 36 32 26 16 16 10 47 8.3 15.3 1.0 ppb Sample 4 50 36 32 26 13 13 8 44 8.4 9.2 0.25 ppb P.A.C. 55 47 43 37 24 23 21 26 8.2 18.8 0.05 ppb P.A.C. 75 63 57 48 29 28 28 47 8.1 11.0 Blank 46 39 36 32 22 22 20 15 8.3 32.5 TABLE 3B FLOW AND FILTRATION CHARACTERISTICS OF ppb ATTAPULGITE CLAY SATURETED NaCl SUSPENSION TREATED WITH TERPOLYMER SAMPLES OF EXAMPLE I Fann 35 Rheology Room Temperature 600 300 200 100 6 3 IG 10G pH API Filtrate, ml P.A.C.

1 ppb 78 60 51 42 20 12 13 12 8.3 130.2 2 ppb 120 87 72 52 16 13 11 17 8.7 15.1 3 ppb 137 99 82 57 12 9 8 17 9.0 8.0 Sample 3 1 ppb 75 46 36.5 27 12 7 10 11 8.5 96.8 2 ppb 95 66 51.5 33.5 11 9 7 12 8.5 11.8 3 ppb 102 72 57 38 11 9 7 12 8.5 5.0 Sample 2 1 ppb 54 40 34 27 13 11 10 10 8.3 116.4 2 ppb 73 47 37 26 8.5 7 7 11 8.3 11.9 3 ppb 87.5 56 43.5 29 7 5.5 5 8 8.2 6.8 From the rheological data given in each of Tables 3A and 36, above, it may also be observed that the addition of the terpolymer filtration control agent of the present invention did not adversely affect the rheological properties of the drilling fluid, and each sample had satisfactory rheology properties at the given readings.

EXAMPLE IV Nineteen terpolymer samples containing various mole percents of sodium acrylate, 2hydroxypropyl acrylate and acrylamide were prepared for evaluation as filtration control agents. The procedure utilized to prepare the samples was as in Example I, above. The monomers and about 70 to 75 grams of water were mixed and the acrylic acid was neutralised to about pH 7 with 5 M sodium hydroxide. The temperature of the solution was kept below 300C with the aid of an ice water bath. The monomers in solution and one-half of the initiator-ammonium persulphate were added over a one-hour period to the mixture of water, one-half of the initiator and about 1 0% of the solution of monomers. The temperature of the reaction was maintained at about 600C for a total reaction time of three hours.The terpolymer-water mixture was broken into small samples, added to 1 litre of isopropyl alcohol, and allowed to stand for several hours. The solvent was decanted and the residue was dried at 90"C for about 24 hours, followed by drying in a vacuum oven at 900C for another 24 hours or to a constant weight. The samples were milled to pass through a 30 mesh screen. In all cases, the product yield was quantitative.

The various terpolymers composed of varying mole percents of sodium acrylate, 2-hydroxypropyl acrylate and acrylamide were prepared using the quantities of monomers, initiator and water given in the table below. The product yield was quantitative.

The reactants utilized in the preparation of the terpolymer samples are set forth in the table below: TABLE 4 MONOMERS AND INITIATOR CONCENTRATIONS OF TERPOLYMER PREPARATIONS Sample No. (1) (2) (3) (4) 7 N* 25.6 6.3 68.1 M* 0.357 0.092 1.000 3.51 8 N 16.1 19.8 64.1 M 0.249 0.308 0.995 3.49 9 N 16.0 3.9 80.1 M 0.333 0.082 1.667 3.51 10 N 24.0 4.0 72.0 M 0.750 0.124 2.251 0.88 11 N 22.2 3.6 74.1 M 0.500 0.082 1.667 1.75 12 N 24.0 4.0 72.0 M 0.750 0.124 2.251 1.75 13 N 8.0 4.0 88.0 M 0.250 0.124 2.750 1.75 14 N 8.0 27.8 64.2 M 0.116 0.404 0.931 3.27 15 N 30 4.0 66.0 M 0.627 0.083 1.378 5.86 19 N 0 0 100.0 M 0 0 1.352 3.51 20 N 10.0 15.0 75.0 M 0.200 0.299 1.498 1.75 21 N 10.0 25.0 65.0 M 0.231 0.575 1.494 1.75 22 N 5.0 14.9 80.1 M 0.093 0.275 1.482 1.73 23 N 6.0 2.9 91.0 M 0.142 0.069 2.132 1.75 24 N 15.0 40.0 45.0 M 0.267 0.713 0.802 1.75 25 N 12.9 2.6 84.5 M 0.306 0.062 2.001 1.75 *N mote %, M mole/kg 3 acrylamide 1 sodium acrylate 4 ammonium persulphate 2 2-hvdroxypropyl acrylate EXAMPLE V The present Example demonstrates the effectiveness of the present terpolymer composition to provide effective filtration control at increased temperatures. For this test, selected samples made as in Example I and Example IV were utilized in a mud system comprised of 172 pounds of Wyoming Bentonite to which was added 1 ppb of gypsum, 2 ppb sodium chloride and 2 ppb of chrome lignosulphonate. The initial mud system was adjusted to a pH of about 9.5 with sodium hydroxide. The selected samples of the terpolymer were added to aliquots of the base mud in concentrations of 2 ppb and 1 ppb.The samples were hot rolled at 1 500F for 16 hours, and were permitted to cool to room temperature prior to rheological measurements being made and API filtrate being established for each sample. Thereafter, the sample containing 1 ppb polymer was hot rolled in a 3000F oven for a period of 3 hours and permitted to cool to room temperature before rheological data and API filtrate volumes were measured. Samples of the base mud treated with P.A.C. together with a base mud treated with another commercially available filtration control agent identified as a sodium polyacrylate, (hereinafter referred to as "S.P.A.") also were prepared and tested, as above, for comparative purposes.This test clearly indicated that the terpolymer of the present invention is an effective filtration control agent even after exposure to increased temperatures as high as about 300OF. Additionally, this test also indicates that the terpolymer of the present invention will not affect rheological properties of the drilling fluid. The results of this test are further illustrated in the following table: TABLE 5 FLOW PROPERTIES AND API FILTRATE OF TERPOLYMER TREATED MUD BEFORE AND AFTER HOT-ROLLING AT 300"F Fann Rheology, Rm Temp.

Ib/bbl Sample Hot-Rolled 600 300 200 100 6 1.0 13 150 F 62 35 26 16 3 1.0 300OF 48 30 24 16 3 1.0 150 F 69 40 30 20 5 1.0 300"F 53 33 25 15 1.0 150OF 36 20 15 9 1.5 1.0 300 F 71 46 38 25 4 1.0 8 150 F 66 39 29 18.5 3.5 1.0 300"F 64 37 27.5 17 3 0.5 20 150"F 36 21 16.5 9.5 2 1.0 150"F 44 25 19 10 2 1.0 300"F 52 30 22 13.5 2.5 0.5 22 150"F 24 13.5 10 6 1.5 1.0 150 F 42 24 17 10 2 1.0 300"F 57 32.5 23 13.5 2 0.5 23 150"F 19 11 8 5 1 1.0 150 F 25 14 16.5 6.5 1.5 1.0 300"F 39 23.5 18 11 2.5 0.5 24 150"F 43 25 19 12 2.5 1.0 150"F 53 31 23 14 2.5 1.0 300"F 72 43 32 19 3.5 0.5 25 150"F 19.5 11 8 5 1 1.0 150 F 26 15 11 6.5 1.5 1.0 300"F 42 25 18.5 11.5 2.5 1.0 7 150"F 28 15.5 11.5 7 1 1.0 300"F 26.5 14.5 10.5 6 1 1.0 19 150"F 37 33.5 18 11.5 2.5 1.0 300 F 13.5 7.5 6 3.5 1 1.0 150 F 300 295 200 20 13 1.0 300"F 29 16 12 7 1.5 0.25 P.A.C. 150OF 25 14.5 11 6.5 1.5 0.25 150"F 20 12 8.5 5 1.5 0.5 150 F 35 21 15.5 10 2 0.5 150"F 26.5 15 11 7 1.5 1.0 150 F 49 30.5 24 15.5 4 1.0 300"F 47 29 22 14 3.5 1.0 150"F 43.5 26.5 20 13 3 1.0 300 e F 31 19 14 9 2 TABLE 5 continued Fann Rheology, Rm Temp.API IbIbbI Sample Hot-Rolled 3 1G 10G pH Filtrate ml 1.0 13 150 F 2.5 3 10 9.2 6.1 1.0 300F 2 2 3 9.1 6.1 1.0 150"F 4 4 23 9.0 4.8 1.0 300"F 2 4 5 9.4 6.2 1.0 150 F 1 2 3 - 4.8 1.0 300"F 3 3 60 9.6 5.9 1.0 8 150 F 2.5 3 7 8.9 6.2 1.0 300"F 2 3 3 9.5 8.6 0.5 20 150"F 1.5 1.5 2 9.2 8.2 1.O 150 F 1.5 1.5 2 9.0 6.4 1.0 300"F 2 2 2 8.3 8.1 0.5 22 150 F 1 1 2 9.0 10.4 1.0 150"F 1.5 1 3 9.0 7.2 1.0 300"F 1.5 2 2 8.4 10 0.5 23 150 F 1 1 1.5 9.2 7.4 1.0 150 F 1 1 2 9.2 5.8 1.0 300 F 2 2 3 8.6 6.5 0.5 24 150"F 2 2 6 8.9 8.2 1.0 150 F 2 2 6 8.7 8.8 1.0 300 F 2.5 2.5 3 8.1 10.0 0.5 25 150"F 1 1 11 9.1 9.0 1.0 105"F 1 1.5 2 9.3 6.4 1.0 300"F 2 2 2 8.4 6.5 1.0 7 150 F 1 1 2 8.9 8.6 1.0 300 F 1 1 1 8.1 12.6 1.0 19 150 F 2 3 8 9.2 37.8 1.0 300'F 1 1 1 8.2 38.0 1.0 150 F 10 13 25 - 14.6 1.0 300 F 1 1 2 8.9 9.0 0.25 P.A.C. 150 F 1 2 3 8.6 10.8 0.25 150 F 1 4 2 9.2 13.6 0.5 150"F 2 2 7 8.9 8.4 0.5 150"F 1 3 2 8.8 10.8 1.0 150 F 3.5 4 22 8.7 5.4 1.0 300"F 3 4 13 9.4 9.0 1.0 150 F 2 4 12 9.2 6.6 1.0 300 F 1.5 3 7 9.3 9.7 TABLE 5 continued Fann Rheology, Rm. Temp.

lb/bbl Sample Hot-Rolled 600 300 200 100 6 0.25 3 $150 F 68 45 35 23 14 0.5 $150 F 59 37 28 17 2.5 1.0 150'F 59 37 29 19 3 1.0 300 F 61 35 25 15 2 0.25 14 150 F 44 29 23 15 3 0.5 150'F 73 43 33 20 5 1.0 150'F 93 58 45 29 6 1.0 300'F 82 52.5 41 27 4.5 0.25 S.P.A. $150 F 93 59 48 33.5 20 0.5 $150 F 76 43 32 18 3 1.0 150 F 85 48 35 20 3 1.0 300 F 169 119 95 64 13 1.0 1509F 49 28 20 12 2 1.0 300 F 69.5 41.5 31.5 18.5 2.5 0.5 11 150 F 61 38 28.5 17 3 1.0 150 F 80 60 45 30 7 1.0 300 F 78.5 45 35 21 3.5 1.0 150'F 77 49 38 26 4 1.0 300"F 72 42 30 18 0.5 12 150 F 80 51 39 25 6 1.0 150 F 88 62 49 40 18 1.0 300 F 80 50 38 23 4 0.5 15 150"F 49 31 23 14 2 1.0 150 F 52 31 23 13.5 2 1.0 300,'F 59 34 25 14.5 2 0.5 10 150 F 68 42 33 21 7 1.0 150"F 97 65 53 39 20 1.0 150"F 87 58 47 34 11 1.0 300"F 115 66 47 26 4 1.0 150 F 86 54 43 37 5 1.0 300 F 115 67 53 30 0.25 4 150"F 174 146 135 F 111 70 0.5 1500F 88 49 35 20 3 1.0 150 F 64 38 28 17 2 1.0 300 F 125 86 69 47 10 0.25 1 150 F 92 67 55 44 33 0.5 150 F 57 34 25 15 2.5 1.0 150 F 44 24 18.5 10 1.5 1.0 300 F 139 96 77 53 11 0.5 9 150 F 26 15 11.5 7 2.5 1.0 150"F 35 20 15 9 2 1.0 $300 F 61 35 26 16 3 1.0 150"F 39 23 17 10 2 1.0 300 F 52 32 24.5 15 3 Base Mud 150 F 18 9 6.5 3.5 1 $300 F 20 10.6 7 4 1 $150 F 21 11 7.5 4 1.5 300 F 26.5 14 9.5 5 1 TABLE 5 continued Fann Rheology. Rm. Temp. API Ib/bbl Sample Hot-Rolled 3 1G 10G pH Filtrate ml.

0.25 3 150"F 14 5 63 - 12.8 0.5 150 F 2 2 8 8.7 7.6 1.0 150 F 2 2 8 8.7 5.2 1.0 300 F 1.5 2 2 9.3 13.0 0.25 14 150 F 2 2 55 8.8 11.0 0.5 150"F 4 4 38 8.4 9.6 1.0 150 F 5 6 49 8.6 6.8 1.0 300"F 3 4 5 9.4 14.6 0.25 S.P.A. 150 F 19.5 19.5 68 9.1 22.0 0.5 150 F 2 3 3 9.0 15.2 1.0 150"F 2 2 4 9.2 9.0 1.0 300 F 8.5 9 - 9.4 20.6 1.0 1500F 1.5 1.5 2 9.6 9.0 1.0 300"F 1.5 1.5 2 8.8 16.4 0.5 11 150"F 2 2 21 9.2 6.8 1.0 150"F 6 5 47 9.2 5.4 1.0 300"F 3.5 4 8 9.0 6.6 1.0 150 F 3 3 55 9.2 4.8 1.0 300 F 2 3 5 9.3 7.9 0.5 12 150 F 5 3 47 9.0 7.0 1.0 $150 F 14 12 95 9.3 5.0 1.0 300"F 3 4 6 9.0 6.7i 0.5 15 150qF 1.5 2 13 9.2 7.2 1.0 150"F 1.5 2 3 9.1 6.6 1.0 300'F 2 3 3 9.3 9.3 0.5 10 150 F 6 5 40 9.2 8.8 1.0 150 F 18 19 51 9.3 5.0 1.0 150 F 9 7 103 9.4 4.8 1.0 $300 F 3 3 4 8.9 8.2 1.0 $150 F 4 4 66 8.9 5.0 1.0 300 F 3 4 4 9.0 8.6 0.25 4 150"F 61 54 55 9.2 19.8 0.5 150 F 2 2 4 9.0 14.6 1.0 150 F 1.5 2 3 9.1 7.2 1.0 300 F 7 9 17 - 19.3 0.25 1 150 F 32.5 34 70 9.1 21.6 0.5 150 F 2 4 6 9.2 14.8 1.0 150"F 1 2 2 9.3 7.2 1.0 300"F 8 8.5 13 9.6 18.8 0.5 9 150 F 2 3 4 9.0 7.4 1.0 150 F 1 2 3 9.1 5.2 1.0 300 F 2.5 3 3 9.0 6.1 1.0 150 F 1.5 1.5 2.5 9.2 5.4 1.0 300"F 2 3 9 9.2 5.6 Base Mud 15G'F 1 1 1 9.3 16.4 300"F 0.5 1 1 9.0 20.6 150"F 1 1 1 9.7 20.8 300"F 0.5 1 1 9.7 21.4 EXAMPLE VI The present Example demonstrates the ability of the terpolymer of the present invention to provide effective filtration control in an aqueous drilling fluid after being subjected to 3000F over an extended period of time. For this test the base mud as in Example V, was utilized.To the base mud was added a 1 ppb treatment of the samples identified in the Table below. The sample mud with the filtration control additive added thereto was first hot rolled for a period of 1 6 hours at a temperature of 1 50OF. There after, each sample was permitted to cool to room temperature prior to measurements of rheological properties and API filtrate volumes. Thereafter, the initial hot rolled sample was broken into four separate test specimens, and each specimen was hot rolled in an oven at a temperature of 3000F. The first test specimen was hot rolled for a time period of 3 hours, the second for 5 hours, the third for 7 hours and the 4th for 1 6 hours. After the respective time period of hot rolling for each sample, the sample was permitted again to cool to room temperature prior to rheological measurement made and API filtrate being established.The same procedure was utilized to measure the rheology and API filtrate of a blank sample of the base mud.

The results of this test clearly indicated that the terpolymer of the present invention provided effective filtration control of the drilling fluid sample, even after exposure to a temperature of 3000F, over extruded periods of time, up to 16 hours. Additionally, the present test indicates that the filtration control agent is more effective after exposure to a temperature of 3000F than comparative samples of commercially available filtration control agents as described above.

The results of this test are further reflected in the Table below: TABLE 6 EFFECT OF TIME AT 300 F ON THE FLOW AND FILTRATION CHARACTERISTICS OF TERPOLYMER TREATED MUD Fann 35 Rheology, Rm. Temp. API 600 300 200 100 6 3 1G 10G pH Filtrate 1 lb/bbl Sample 9 *HR 150 F, 16 Hrs. 39 23 17 10 2 1.5 1.5 2.5 9.2 5.4 HR 300 F, 3 Hrs. 52 32 24.5 15 3 2 3 9 9.2 7.1 5 Hrs. 80 51 39 25 5.5 4 4 7 9.2 7.1 7 Hrs. 80 51 38 24 5 3.5 4 5 9.3 9.0 16 Hrs. 86 54 41 26 5 4 4 7 9.3 9.2 1 lb/bbl Sample 13 HR 150 F, 16 Hrs. 36 20 15 9 1.5 1 2 3 - 4.8 HR 300 F, 3 Hrs. 71 46 38 25 4 3 3 60 9.6 5.9 5 Hrs. 102 65 49 31 7 5 6 15 9.3 7.0 7 Hrs. 54 33 24.5 10.5 3.5 2.5 2.5 3 8.7 6.6 16 Hrs. 48 29 22 14 2.5 2 2.5 3 9.0 8.0 1 lb/bbl P.A.C.

HR 150 F, 16 Hrs. 55 33.5 25.5 16.5 16.5 4 4 16 9.5 5.8 3 Hrs. 51 33 26 17 17 5.5 7 22 9.4 7.8 5 Hrs. 40 26 20 14 14 4 5 17 9.2 10.5 7 Hrs. 39 24 18 11.5 11.5 2.5 3 11 9.8 10.4 16 Hrs. 34.5 19.5 14.5 8.5 8.5 1 2 4 9.2 13.6 Base Mud HR 150 F, 16 Hrs. 21 11 7.5 4 4 1 1 1 9.7 20.8 HR 300 F, 3 Hrs. 26.5 14 9.5 5 5 0.5 1 1 9.7 21.4 5 Hrs. 22 12 8.5 5 5 1 1 2 8.0 22.8 7 Hrs. 18 9.5 7 4 4 1 1 1 8.0 21.6 16 hrs. 32 17 12 6.5 6.5 0.5 1 2 9.3 23.2 1 lb/bbl S.P.A.

HR 150 F, 16 Hrs. 49 28 29 12 12 1.5 1.5 2 9.6 9.0 HR 300 F, 3 Hrs. 69.5 41.5 31.5 18.5 18.5 1.5 1.5 2 8.8 16.4 5 Hrs. 85 53 40 24.5 24.5 2.5 2.5 - - 20.2 7 Hrs. 79 48.5 36.5 22 22 2 2 6 9.4 19.2 16 Hrs. 95.5 58.5 43.5 26.5 26.5 3 3 6 8.4 22.8 * hot rolled

Claims

1. A copolymer of a monovalent alkali metal salt of acrylic acid, a hydroxy alkyl acrylate and acrylamide.

2. A copolymer according to claim 1 for use as a drilling fluid additive.

3. A copolymer according to claim 1 or claim 2 for use as a filtration control agent.

4. A copolymer according to any of claims 1 to 3 in which the amount of the salt of acrylic acid is 5 to 62 mole /O, the amount of hydroxy alkyl acrylate is 2 to 40 mole % and the amount of acrylamide is 31 to91 mole%.

5. A copolymer according to claim 4 in which the amount of acrylic acid salt is 10 to 30 mole %, the amount of hydroxy alkyl acrylate is substantially 4 mole % and the amount of acrylamide is substantially 70 mole. %.

6. A copolymer according to claim 5 formed of about 23 mole % acrylic acid salt, about 4 mole % hydroxy alkyl acrylate and about 73 mole % acrylamide.

7. A copolymer according to any preceding claim in which the acrylic acid salt is sodium acrylate.

8. A copolymer according to any preceding claim in which the hydroxy alkyl acrylate is 2hydroxypropyl acrylate.

9. A copolymer according to claim 1 substantially as herein described.

10. An aqueous drilling fluid containing water and a clayey substance and at least 0.25 pounds per barrel of a copolymer according to any preceding claim.

1 A method in which a well is drilled in a subterranean formation and in which, during drilling, a drilling fluid is circulated in the well and a filter cake is formed on the walls of the well to decrease loss of fluid, and in which the drilling fluid comprises a copolymer according to any of claims 1 to 9.