AU633262B2

AU633262B2 - Well drilling fluid and method

Info

Publication number: AU633262B2
Application number: AU39535/89A
Authority: AU
Inventors: James V. Fisk Jr.
Original assignee: Baroid Technology Inc
Current assignee: Baroid Technology Inc
Priority date: 1988-08-15
Filing date: 1989-08-11
Publication date: 1993-01-28
Anticipated expiration: 2009-08-11
Also published as: IT8967701A0; BR8904091A; NO893150L; GB2221940A; NO893150D0; NL8902056A; CA1332502C; SG128992G; GB2221940B; IT1232916B; DE3926970A1; DK398889D0; DK398889A; AU3953589A; GB8917892D0

Description

I ~pU-U~ -~~la 633262 S F Ref: 103630 FORM COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE: Class Int Class Complete Specification Lodged: Accepted: Published: Priority: Related Art:

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Name and Address of Applicant: Address for Service: Baroid Technology, Inc.

3000 Sam Houston Parkway East Houston Texas 77032 UNITED STATES OF AMERICA Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia i::i r a- 1 i

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.rt Complete Specification for the invention entitled: 4 Well Drilling Fluid and Method The following statement is a full description of this invention, including the best method of performing it known to me/us 58A5/5 Jean McLaren Assistant Secretary The Conumissioner of Patents, 1- Abstract of' the Disclosure A drilling fluid comprising an aqueous medium, a watersoluble cationic polymer, a low molecular weight hydroxyethyl cellulose and a generally water-insoluble weighting agent.

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t Background of the Invention 1. Field of the Invention The present invention relates to drilling fluids and, more particularly, to drilling fluids exhibiting good fluid loss control and stabilization of shale formations.

2. Description of the Background It is known that shale is predominantly formed of clays which swell and disperse when contacted with a water-based drilling fluid. This swelling and dispersion can result in a phenomena known as "heaving" in which the borehole walls can collapse. To reduce this tendency of the clays in the shale to swell and disperse, it is common practice to add cationic salts to render the clays generally hydrophobic.

However, in doing this, anionic polymers which are commonly :i5 used as viscosifiers in drilling fluids are also rendered hydrophobic, thereby losing their ability to viscosify the drilling fluid or mud and reduce fluid loss from the formation.

It is known to use mixtures of cationic polymers and '20 high molecular weight hydroxyethyl cellulose (HEC) when drilling shale formations in order to stabilize the shale by flocculating the clay and impart viscosification. However, prior art cationic drilling mud systems containing HEC suffer from the disadvantages of higher than desired fluid loss and the inability to effectively suspend weighting agents, such as barite, at a viscosity which permits pumping of the fluid.

It would therefore be desirable to have a cationic polymer-based drilling fluid which would stabilize the shale, exhibit low fluid loss, viscosify and effectively suspend weighting af'ents, such as barite.

t *1t -,c~--LIIILlllllle~l~ IIPLIIILL3111C- -3- Summary of the Invention It is therefore an object of the present invention to provide an improved drilling fluid.

Another object of the present invention is to provide a cationic polymer-based drilling fluid exhibiting low fluid loss.

Still another object of the present invention is to provide cationic polymer-based drilling fluids which will effectively suspend weighting agents, exhibit low fluid loss and stabilise shale formations.

Yet another object of the present invention is to provide a method of drilling a borehole using a drilling fluid containing a cationic polymer and a low molecular weight hydroxyethyl cellulose polymer.

The above and other objects of the present invention will become apparent from the description given herein and the appended claims.

According to a first embodiment of the present invention there is provided a drilling S 15 fluid comprising an aqueous medium, from 1.43 to 8.58gl- 1 of said aqueous medium of a i water-soluble cationic polymer which will stabilise a shale containing water-swellable clay, from 1.43gl 1 to 8.58gl- 1 of said aqueous medium of hydroxyethyl cellulose having Sf o 00ooo i a molecular weight of from 3.000 to.4= and from 2.86 to 858gl- 1 of said aqueous medium of a generally water-insoluble weighting agent.

In the present specification and claims the term "generally water-insoluble weighting agent" refers to a weighting agent which remains as a circulating solid such as barite as opposed to a weighting agent such as zinc bromide or other such heavy metal halides which are water soluble.

Preferably, the cationic polymer is selected from the group consisting of (a) branched emulsion polymers of diallyldimethylammonium chloride having a molecular "i weight of at least 5,000, dialkylamino-alkyl acrylic ester polymers, (c) dialkylaminoalkyl methacrylic ester polymers, dialkylaminoalkyl acrylic acidacrylamide copolymers, dialkylaminoalkyl methacrylic acid-acrylamide copolymers, N-(Dialkyl-aminoalkyl) acrylamide polymers, N-(Dialkylaminoalkyl) methacrylamide polymers, poly(2-vinylimidazioline), poly(alkyleneamines), (i) poly(hydroxalkylene polyamines) and mixtures thereof.

In cases where the aqueous medium contains a salt of a multivalent cation, the drilling fluid will also contain a non-water-swellable clay which can be added to the drilling fluid prior to commencement of drilling or picked up by the drilling fluid, in situ, during the drilling operation.

10330cl I- I -4- According to a second embodiment there is provided a method of drilling an earth borehole comprising circulating in said borehole during the drilling operation a drilling fluid comprising an aqueous medium, from 1.43 to 8.58 gl- 1 of said aqueous medium of a water-soluble cationic polymer which will stabilise a shale containing water-swellable clay, from 1.43 to 8.58 gl-1 of said aqueous medium of hydroxyethyl cellulose having a 4+ Oj 00) molecular weight of from about 3,000 to 4trfrom 2.56 to 85" gl- of said aqueous medium of a generally water-insoluble weighting agent and from about 1 to about pounds per barrel of said aqueous medium of a non-water-swellable clay.

In the method of the present invention, the drilling fluid described above is circulated in the borehole during the drilling operation.

4 t a 1 a €a t t 4 4 103630c1 Description of the Preferred Embodiments The aqueous medium used in the drilling fluid compositions of the present invention can be fresh water, brines of monovalent cations, such as sodium chloride solutions, potassium chloride solutions, brines of multivalent cations, such as calcium chloride solutions, sea water, etc. The nature of the aqueous medium, as seen hereafter, determines the composition of the drilling fluid.

The cationic polymers which are useful in the compositions and method of the present invention are those cationic polymers which will stabilize, i.e. prevent erosion o .4 or dispersion, shale containing water-swellable clays so as to -prevent heaving during the drilling operation, which are Sa substantially water-soluble, or dispersible in the aqueous medium, and which act to effectively suspend weighting S. agents, such as barite. The cationic polymers will be present in amounts ranging from about 0.5 to about 3 pounds per barrel (ppb) of the aqueous medium. Non-limiting examples of suitable cationic polymers include branched emulsion polymers of diallyldimethylammonium chloride having a molecular weight of at least 5,000, dialkylaminoalkyl acrylic ester polymers, dialkylaminoalkyl methacrylic a. ester polymers, dialkylaminoalkyl acrylic acidacrylamide copolymers, dialkylaminoalkyl methacrylic acid-acrylamide copolymers, N--(Dialkylaminoalkyl) acrylamide polymers, N-(Dialkylaminoalkyl) methacrylamide polymers, poly(2-vinylimidazioline), (i) a poly(alkyleneamines), poly(hydroxalkylene polyamines) and mixtures thereof.

The diallyldimethylammonium chloride polymers useful in the compositions and method of the present invention can be homopolymers or copolymers of other monomers such as j acrylamides. Preferably, the polymers are branched emulsion-type polymers which can employ branching agents 35 such as triallylmethylammonium chloride, tetraallylammonium chloride as well as bis-diallylammonium salts such as tetraallylpeperizinium chloride and N,N,N',N'-tetraallyl- N,N'-dimethylhexamethylenediammonium chloride. The emulsion -1 ra r -6polymers can be prepared by emulsion or suspension polymerization techniques such as those described in U.S.

Patent No. 3,968,037, incorporated herein by reference, and may contain from about 95 to about 99.99 mole percent diallyldimethylammonium chloride and from about 0.01 to about 5 mole percent of one of the aforementioned branching agents. The branched emulsion polymers can have molecular weights ranging from 5,000 and upward, such branched emulsion polymers wherein the molecular weight is from about 4 0,000 to about 5,000,000 being preferred. Especially preferred are homopolymers of dimethyldiallylammonium o' chloride having a molecular weight of from about 1,000,000 oo eO to about 5,000,000.

o 0o Another cationic polymer especially suitable for use in ova a ."00 5 the compositions and method of the present invention are the 00 0 N-(Dialkylaminoalkyl) acrylamide polymers, such as, for S° example, polymers prepared via the Mannich Reaction wherein a polyacrylamide is reacted with formaldehyde and an amine to produce an aminomethylated polyacrylamide. Especially preferred are such polyacrylamides having molecular weights ranging from about 40,000 to about 4,000,000.

Another preferred type of cationic polymer for use in o^ the compositions and method of the present invention are the dialkylaminoalkyl derivatives of a water-soluble copolymer formed from an ethylenically unsaturated amide monomer and a comonomer selected from the group consisting of acrylic acid, alkyl-substituted acrylic acids and mixtures thereof, Ssuch as, for example, the copolymer of acrylamide and methacrylic acid. Such polymers, which can have molecular weights ranging from 40,000 to 4,000,000, can be produced, for example, by the method described in U.S. Patent No.

3,923,756, incorporated herein by reference for all purposes. A particularly preferred class of copolymers are the dialkylaminoalkyl acrylamide-methacrylic acid copolymers wherein the copolymer has a molecular weight ranging from about 40,000 to about 4,000,000. Especially preferred, nonlimiting examples of such copolymers include the dimethylaminocthyl sulfate and chlorides of copolymers of -7acrylamide and methacrylic acid.

It has been found that the molecular weight of the particular cationic polymer has very little effect on the polymer's ability to stabilize the shale or suspend the weighting agent. Thus, as noted above, cationic polymers of widely varying molecular weights can be employed.

The compositions of the present invention also employ hydroxyethyl cellulose as a viscosifier and fluid loss control additive. The HEC, which will generally be present in the compositions in amounts ranging from about 0.5 to about 3 pounds per barrel, preferably 0.5 to 2.5 pounds per barrel, of the aqueous medium, must have a molecular weight ranging from about 3,000 to about 40,000 depending upon the degree of viscosification desired. HEC's having higher molecular weights cannot be used to form weighted muds as per the compositions of the present invention.

In cases where the drilling fluid of the present invention contains a salt of a multivalent cation, e.g., calcium chloride, sea water or the like, it is necessary, in o order to achieve acceptable fluid loss control, to include a non-water-swellable clay. Such clays can be dispensed with if the aqueous medium is fresh water or contains only the salt of a monovalent cation, such as sodium chloride. The non-water-swellable clay can be added to the drilling fluid at the commencement of the drilling if the formation through which the drilling progresses does not contain a non-watero swellable clay. Alternately, the clay can be picked up, in Ssitu, by the drilling fluid during the actual drilling operation since many formations contain such non-waterswellable clays which form part of the drill cuttings. The non-swellable clay will normally be present in the drilling j fluid in amounts ranging from about 1 to about 15 pounds per barrel of the aqueous medium. Suitable, non-limiting examples of such non-water-swellable clays include kaolin, j 35 attapulgite, sepiolite, etc.

The compositions of the present invention also include a water-insoluble weighting agent such as barite, although Sother weighting agents such as galena, hematite and other -8-

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4,, 0 ao *4 4 *0o 0d mineral materials may be employed. The weighting agent will generally be present in the compositions in amounts of from about 1 to about 300 pounds per barrel of the aqueous medium.

5 The compositons of the present invention may contain other materials or additives, such as additional viscosifiers or fluid loss control additives, salts, etc. to tailor the mud to desired needs.

In the method of the present invention, the drilling fluid, if the formation contains a non-water-swellable clay, is circulated in the well bore, the non-swellable clay being incorporated in situ into the drilling fluid.

Alternatively, the drilling fluid having added non-swellable o clay is circulated in the borehole during the drilling .15 operation, this being the method utilized when the formation through which the borehole is being drilled is devoid of non-water-swellable clay.

To more fully illustrate the invention, the following non-limiting examples are presented.

Example 1 0 A series of drilling muds were prepared of varying Scompositions and tested. In all cases, unless otherwise indicated in this and all the other examples, the muds were prepared by mixing 20 minutes on a Multimixer following by hot rolling for 16 hours at 150 0 F prior to testing. The compositions of the drilling fluids and test results are shown in Table 1 below.

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o *000 *04 Formulation Tap water, bbl [Cl, lb LV 2141 283 El, lb

HEC-OP

2 300, lb HEC-Qp 3 4400 lb

ZEOGEL

1 lb BAROID ,lb l Kaolin, lb BARABRINE DEFOAN 6 lb Plastic Viscosity, c Yield Point, lb/l00 3q ft lO-si c gel, lb/lO0h aq ft gel, lb/l00 sq ft Fluid Temperature,*7

PH

API Fluid Loss, ml Mud wt, lb/gal Mud No. 1 .75 21 1 1.5 10 25 200 Mud' No. 2 2.75 21 1.5 10 25 200 MAud .75 21 1 3.0 10 25 200 Mud No. 21 .75 21 1 3.0 10 25 Mud N~o. 1 4 a .75 21 1 25 200 Tabl I Mud Mud No. 4Jb No. 5 .75 .75 10 200 21 1 3.0 200 Mud No. -6 .75 21 1 3.0 200 10 Mud No. 7 .70 21 1 1.0 300 20 Mud No. 8 .70 21 1 1.5 300 Mud no. 9 2.70 21 3.0 300 Mud No. 215 21 4100 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 .2 15 11 11 415 741 8.21 16 13.0 18 19 741 8.21 12.5 13.0 23 12 6 210 741 8.41 6.0 13.0 To Thick To Mix On Multi Mixer 13.0 147 13 3 7 741 8.0 8.2 16 21 5 7 741 7.8 50+ 37 6 2 3 721 8.41 13.0 422 10 2 3 721 8.41 5.0 13.0 17 5 21 7 721 7 .4 11 121.5 20 10 21 7 741 7.1 7.2 121.5 56 19 3 741 7.1 121.5 83 37 21 7 741 21.8 16.0 'Aqueous solution of 7.7 solar (cationic activity) copolymers having a molecular weight of 21,000,000.

of dimethylaminoethyl sulfate salt of acryl amide-se tbacryl ic acid 2 HEC marketed by Union Carbide having a molecular weight of 3,000.

3 HEC marketed by Union Carbide having a molecular weight of 20,000.

2 Non-water-swellable clay marketed by HL Baroid, Inc.

5 Barite weighting agent marketed by ML Baroid, Inc.

6 Mud defoamer marketed by WL Baroid, Inc.

7Heteropolyzaccharide marketed by Kelco Rotary.

As can be seen from the data in Table 1, the drilling fluids of the present invention exhibit excellent rheological properties and fluid loss control. With reference to Mud No. 4, it can be seen that when too much HEC having a higher molecular weight, 40,000, is present, the drilling fluid becomes unacceptably thick.

Indeed, it has been found that using HEC's having molecular weights of several million makes it virtually impossible to formulate an acceptable drilling fluid containing a waterinsoluble weighting agent. Although Mud No. 4a shows acceptable rheological properties, it has been found that it shows very poor shale stability indicating the necessity of o the presence of the cationic polymer to achieve shale stability. Note that when the HEC is not present (Mud No.

15 4b), there is virtually no fluid loss control. The data in Table 1 also demonstrates that the weighted muds can contain up to 400 pounds per barrel of the weighting agent and still be an acceptable drilling fluid. However, it is to be noted (See Mud No. 10) that the yield point of such heavily 20 weighted mud is higher than would normally be desired.

0 0 000 0 49Q or q 00 00 *U 0 0 60 0 o 00 4 00 a 4s n*, 0 S0 0 U 60 0 00r 59 Example 2 Several drilling fluid formulations were prepared and tested. The compositions of the muds and the test results are shown in Table 2 below.

.90009 0 4 0 0OL -11- Table 2 Formulation Mud No. 1 Mud No. 2 Tap water, bbl 0.7 0.7 Sea Salt, Ib 11 11 LV 214 283 E, lb 1 1 b HEC-QP 300, lb 3.0 BAROID, lb 300 300 XC Polymer, Ib ZEOGEL, Ib BARABRINE DEFOAM, lb .2 .2 Premix (1 bbl Tap Water 100 lb Kaolin), bbl Plastic Viscosity, cP 60 54 Yield Point, lb/100 sq ft 22 21 gel, 3b/100 sq ft 3 4 t1>-Min gel, lb/100 sq ft 4 7 SFluid Temperature, oF 74 74 pH 7.5 7.4 API Fluid Loss, ml 50± Mud wt, Ib/gal 14.5 14.5 SThis example shows that when salts of multivalent i cations, such as are contained in sea salt are present, a non-water-swellable clay must be present in order for the drilling fluid to exhibit acceptable fluid loss properties.

As can be seen, when no ZEOGEL is present (Mud No. there is virtually no fluid loss control. Tue results in Table 2 are to be contrasted with the results in Table 1 wherein the aqueous medium contain only the salt of a monovalent cation (KC1) and wherein acceptable fluid loss control could be achieved even in the absence of any non-water-swellable clay (see Mud No. 9 in Table 1).

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i ,l i -j i ri -12- Example 3 A series of drilling fluids were prepared by mixing the ingredients for 20 minutes on a multimixer. The drilling fluids were then hot rolle 'or 16 hours at 150 0 F and tested. The compositions of the muds and the test results are shcmn in Table 3 below.

Table.

Mud Mud Mud Mud Mud Mud Mud Mud Mud Mud Formulation No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 No. 9 Tap water, bbl .70 .70 .70 .70 .70 .70 .70 .70 .70 KC1, lb 21 21 21 21 NaC, lb 30 30 Seasalt, lb 11 11 11 11 E-9051 lb 1 1 1 1 1 SE-904 2 lb 1 1 1 1 HEC-QP 300, lb 3 3 3 3 3 3 ZEOGEL, lb 15 15 15 15 15 15 15 15 15 DAROID, lb 300 300 300 300 300 300 300 300 300 300 XC Polymer, lb .25 .25 .25 .25 .25 .25 .25 .25 .25 Mud Temperature, OF 75 75 75 75 75 75 75 75 75 Plastic Viscosity, cp 42 50 53 45 49 50 15 11 14 9 Yield Point, lb/100 sq ft 29 17 43 20 22 40 35 54 36 41 gel, lb/100 sq ft 13 5 9 14 7 27 20 24 24 22 10-min gel, lb/100 sq ft 57 30 71 46 37 67 22 27 22 pH 8.2 8.1 7.7 8.2 8.0 7.7 7.6 7.5 7.6 API Fluid Loss, ml 7.5 6.8 10.5 10.5 10.0 13.5 50+ 50+ 50+ Mud wt lb/gal 14.5 14.5 14.5 14.5 14.5 14.5 14.4.14.5 14.5 14.5 1 Trademark of an aqueous solution of 20 molar percent (cationic activity) of a homopolymer of dimethyldallylammonium chloride having a molecular weight of 40,000 and marketed by Calgon Corp.

2 Trademark of an aqueous solution of 15 molar percent (cationic activity) of a homopolymer of dimethyldiallylammonium chloride having a molecular weight of 2,000,000 and marketed by Calgon Corp.

ir ft t ft f 'ft.I ftc ft ft t. f As can be seen from the data in Table 3, drilling fluids made in accordance with the present invention have excellent rheological properties and exhibit good fluid loss control. Note that when there is no low molecular weight HEC (Mud Nos. 7-10), there is essentially no fluid loss control.

Example 4 To demonstrate that the drilling fluids of the present invention are effective in stabilizing shale, a series of drilling fluids were prepared and compared with a conventional prior anionic polymer-based drilling mud commonly used where shale stabilization is important. In testing the ability of the drilling fluids to stabilize the .15 shale, Pierre shale samples weighing 15 grams and having a diameter between 0.08 and 0.19 inches were hot rolled at 150 0 F over a six day period. The shale sample was removed from each drilling fluid and re-weighed periodically to determine the amount of erosion that was occurring, The mud 20 compositions and test data are shown in Table 4 below.

ft tf 0 ft 'ft ft f ft f 64 0 i Table 4 Formulation No. 1 No. 2 No. 3 No. 4 No. Tap water, bbl 1 1 1 1 1 SKC1, lb 25 25 25 25 KOH, 1b 0.5 0.5 0.5 0.5 SHEC-QP 300, lb 3.0 E-905, lb 1 Drispac i b 1.5

PAC-L

2 lb 1.0

EZ-MUD

3 Ib 1 1 THERM-CHEK, b -3 Kaolin, lb 10 10 5 10 erosion values at various times 6 hr 0 0 0 1 0 24 hr 9.0 9.2 15.5 28.5 15.2 48 hr 27.4 29.2 53.5 53.6 38.9 72 hr 47.0 47.8 73.5 70.1 55.2 144 hr 77.4 39.3 88.6 78.5 1 Trade name of carboxymethyl cellulose marketed by NL Baroid, Inc.

2 Trademark of a low molecular weight carboxymethyl cellulose marketed by NL Baroid, Inc.

3 Trade name of a partially hydrolyzed polyacrylamide (anionic polymer) marketed by NL Baroid, Inc.

4Trademark of a sulfonated acrylamide copolymer marketed as a fluid loss additive by NL Baroid, Inc.

Mud No. 2 is a commonly used prior art anionic polymerbased drilling fluid used in drilling shale formations.

While as can be seen, Mud No. 2 shows acceptable shale stabilization, it cannot be effectively weighted with weighting agents such as barite or other generally nona~nc=mr~~

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:1 20 I 1 i- -16water-soluble weighting agents. On the other hand, Mud No.

1 made in accordance with the present invention is equally as effective at shale stabilization and as shown in previous data can be easily weighted with barite. As can also be seen, drilling fluids which contain neither the cationic polymer nor HEC are not effective at stabilizing the shale (see Mud No. 3 and Mud No. It can also be seen (Mud No.

that unless both the cationic polymer and the low molecular weight HEC are present, the drilling fluids are not as effective in stabilizing the shale, even in the presence of an anionic polymer commonly used for shale stabilization.

It can thus be seen that the drilling fluids of the present invention are effective at shale stabilization, exhibit low fluid loss and can be easily weighted with commonly used, generally water-insoluble weighting agents such as barite.

The foregoing disclosure and description of the invention is illustrative and explanatory thereof, and various changes in the method steps may be made within the scope of the appended claims without departing from the spirit of the invention.

Claims

17- The claims defining the invention are as follows: 1. A drilling fluid comprising an aqueous medium, from 1.43 to 8.58gl 1 of said aqueous medium of a water-soluble cationic polymer which will stabilise a shale containing water-swellable clay, from 1.43gl 1 to 8.58g1- 1 of said aqueous medium of 14.0) 0o0 hydroxyethyl cellulose having a molecular weight of from 3,000 to X0Qi and from 2.86 to 858gl 1 of said aqueous medium of a generally water-insoluble weighting agent. 2. The drilling fluid of claim 1 wherein said cationic polymer is selected from the group consisting of branched emulsion polymers of diallyldimethylammonium chloride having a molecular weight of at least 5,000, dialkylaminoalkyl acrylic ester polymers, dialkylamino-alkyl methacrylic ester polymers, dialkylaminoalkyl acrylic acid- acrylamide copolymers, dialkylaminoalkyl methacrylic acid-acrylamide copolymers, N-(Dialkylaminoalkyl) acrylamide polymers, N-(Dialkylaminoalkyl) S.;methacrylamide polymers, poly(2-vinylimidazioline), poly(alky!eneamines), (j) poly(hydrox-alkylene polyamines) and mixtures thereof. 3. The drilling fluid of claim 2 wherein said branched emulsion polymer has a molecular weight of from about 40,000 to about 5,000,000. 4. The drilling fluid of claim 2 or claim 3 wherein a member selected from the group consisting of triallylmethylammonium chloride, tetraallylammonium chloride, tetraallylpiperazinium chloride and N,N,N',N',-tetraallyl-N,N'-dimethylhexamethylene- diammonium chloride is utilized as a branching agent in forming said branched emulsion polymers, said branched polymers containing from about 95 to about 99.99 mole percent diallylmethylammonium chloride and from about 0.01 to about 5 mole percent of said branching agent. 5. The drilling fluid of any one of claims 1 to 4 wherein said water-soluble cationic copolymer comprises a dimethylaminoalkyl acrylamide-methacrylic acid copolymer. 6. The drilling fluid of claim 5 wherein the molecular weight of said dimethylaminoalkyl acrylamide-methacrylic acid copolymer is from about 40,000 to about 4,000,000. 7. The drilling fluid of any one of claims 1 to 6 wherein said weighting agent comprises barite. 8. The drilling fluid of any one of claims 1 to 7 wherein said aqueous medium contains a salt of a monovalent cation. 9. The drilling fluid of any one of claims 1 to 8 wherein said aqueous medium contains a salt of a multivalent cation and said drilling fluid contains a non-water- swellable clay. r- T~- I -18- A method of drilling an earth borehole comprising circulating in said borehole during the drilling operation a drilling fluid comprising an aqueous medium, from 1.43 to 8.58gl 1 of said aqueous medium of a water-soluble cationic polymer which will stabilise a shale containing water-swellable clay, from 1.43 to 8.58gl 1 of said aqueous medium of hydroxyethyl cellulose having a molecular weight of from about 3,000 to) 4 rom 2.86 to 858gl 1 of said aqueous medium of a generally water-insoluble weighting agent and from about 1 to about 15 pounds per barrel of said aqueous medium of a non-water- swellable clay. 11. The method of claim 10 wherein said cationic polymer is selected from the group consisting of branched emulsion polymers of diallyldimethylammonium chloride having a molecular weight of at least 5,000, dialkylaminoalkyl acrylic ester polymers, dialkylaminoalkyl methacrylic ester polymers, dialkylaminoalkyl acrylic acid- acrylamide copolymers, dialkylaminoalkyl methacrylic acid-acrylamide copolymers, N-(Dialkylaminoalkyl) acrylamide polymers, N-(Dialkylaminoalkyl) I 15 methacrylamide polymers, poly(2-vinylimidazioline), poly(alkyleneamines), (j) 'poly(hydroxalkylene polyamines) and mixtures thereof. 12. The method of claim 11 wherein said branched emulsion polymer has a molecular weight of from about 40,000 to about 5,000,000. 13. The method of claim 11 or claim 12 wherein a member selected from the 20 group consisting of triallylmethylammonium chloride, tetraallylammonium chloride, tetraallylpiperazinium chloride and N,N,N',N',-tetraallyl-N,N'-dimethylhexamethylene- dimonium chloride is utilized as a branching agent in forming said branched emulsion S'polymers, said branched polymers containing from about 95 to about 99.99 mole percent Sdiallylmethylammonium chloride and from about 0.01 to about 5 mole percent of said I 25 branching agent. The method of any one of claims 10 to 13 wherein said water-soluble cationic polymer comprises a dimethylaminoalkyl acrylamide-methacrylic acid copolymer. The method of claim 14 wherein the molecular weight of said dimethylaminoalkyl acrylamide-methacrylic acid copolymer is from about 40,000 to about 4,000,000. X 116. The method of any one of claims 10 to 15 wherein said weighting agent comprises barite. 17. The method of any one of claims 10 to 16 wherein said aqueous medium contains a salt of a monovalent cation.

18. The method of any one of claims 10 to 17 wherein said aqueous medium Scontains a salt of a multivalent cation. 103630c I~ I--

19- 19. A drilling fluid substantially as hereinbefore described with reference to any one of the Examples but excluding the Comparative Examples. A method of drilling an earth borehole comprising circulating in said borehole during the drilling operation the drilling fluid according to any one of claims 1 to 9 or 19. EIGHTH day of OCTOBER 1992 Baroid Technology, Inc. Patent Attorneys for the Applicant SPRUSON FERGUSON I i 103630cl