CA1063336A - Drilling of subterranean formations with drilling fluids containing radiation induced fluid loss agents - Google Patents

Abstract

ABSTRACT OF THE INVENTION
Improved drilling of a well with aqueous drilling fluids is obtained by incorporating within the water phase a polymer obtained as a product of radiation-induced polymerization of acrylamide, methacrylamide, acrylic acid, methacrylic acid, alkali metal salts thereof, or mixtures thereof.
Polymerization is preferably carried out in 10-60% aqueous monomer solution with gamma radiation. The aqueous monomer solution, before radiation, preferably contains 25-99%
acrylamide and 75-1% sodium acrylate. Drilling fluids containing about 0.01 to about 10% by weight of polymer in the water phase exhibit improved fluid loss control properties.
These polymers also impart reduced friction loss properties to the drilling fluid.

Description

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BACKGROUND OF THE INVENTION
; Field of the Invention.
This invention relates to a process of improved ~;
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drilling of holes in subterranean reservoirs with .

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~63336 1 aqueous drilling Eluids having dispersed therein ~

2 polymer to impar-t ~luid loss control properties, etc.

3 Description of the Prior Art.

4 The art has generally experienced water loss with water-based drilling fluid while drilling through sub-6 terranean formations. Excessive water loss can cause 7 numerous problems, e.g. filter cake build-up within the 8 bore hole to produce hole cons-trictions; in some cases, ;~
9 cake build-up can cause sticking o~ the drill pipe within the bore hold or cause sloughing and cavin~ of shale 11 formations, or cause difficulty with electrical log 12 interpretation, or cause completion problems, especially 13 where the water enters the pay formation. To prevent 14 water loss, the prior art has incorporated within the drilling fluid bentonite, pregelatinized starch, car-16 boxymeth.yl cellulose, polyacrylates, gums, qmulsi~ied 17 oil, partially hydrolyzed, high molecular weight poly-18 acrylamides and other types of chemical agents.
19 Patents representative of the art include:
20~ Morgan, in U.S. 2,775,557, improves water loss 2~ properties~in drilling muds by incorporating a copolymer 22 of acrylic acid and acrylamide within the mud. The co- i 23 polymer is prepared in the presence o~ an oxidation 24 catalyst and can have molecular wei~hts up to about 611,000. Concentrat.~ons of the copolymers can range .
26 from about 0.1 to 8 lbs pex barrel of drilling fluid.
27 Copolymers which contain 25-40~ o~ acrylate groups are 28 more effective than copolymers of lower carboxylic acid 29 content.

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1 Oldham e~ al, in U.S. 2,718,497, use polymers of 2 unsa-turated aliphatic monoc~rbox~lic acid ~e . g . acrylic 3 acid) and the hydrol~zed polymer of the~r acid-formin~ ;
4 derivatives to improve water loss control properties of drilling fluids. The molecular weights can vary up 6 to about 2 million. These polymers increase the vis~
7 cosity of the drilling mud but also decrease the gel '-8 s~rength. ~ -9 Lummus et al, in U.S. 3,323,603, used 0.005-0.15- --10 lb/barrel of drilling fluid to bene~it ben-tonite in the 11 drilling fluid and to flocculate other cla~s.
12 Siegele et al, in U.iS.' 3,434,970, use copolymers 13 ~ of acrylic acid and acrylamide having molecular weights 14 of at least 200,000 to improve drilling mud properties.
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~' ~ 15 Such drilling muds have comparativel~ high viscosities 16 but permit the'drill cuttings to separate xelatively ; ' 17 '' easily on standing. The polymer is produced by hydrol~sis 18 of polyacrylonitrile, or the polymer can be produced by 19 hydrolysis of polyacrylamide or by copolymerization'of , acrylic acid and acrylamide. Polymers up to 5 million 21 molecular weight are useful,~but the preferred molecular 22 weight range is 200,000 to about 2 million.
23 Applicants have'discovered a radiatio~ induced 2~ polymer that imparts improved 1uid loss'co~trol propex-' ties to water-based drilling muds. ' 27 SUMMAR~ OF THE INVENTION
28 Applicants ha~e discovered that improved drillin~
29 of subterranean formations, especially in fluid loss 710019-B -3- ' .
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1 control properties J may be obtained by incorporating wi-thin 2 the water phase of a drilling fluid a polymer (preferably 3 water-soluble) obtained by radiation polymeri~ation o:E acryl-4 amide, methacrylamide, acrylic acid, methacrylic acid, alkali metal salts thereof or mixtures thereof. The 6 agueous solution to be polymerized contains about 10-60 7 by weight of monomer. A preferred mixture o~ monomers is 8 25-99~ acrylamide and 75-1% by weight, based on the 9 ~ total weight of monomer, of sodium acrylate. Radiation :
intensity is 250-1,000,000 rads/hr., and the dosage is 11 500-300,~00 rads. The radiation proa~ct may be diluted 12 with water and used directly, or the polymer can be .:
13 separated from the reaction product, dried and there-14 after resolubilized. Concen~ration ranges o~ about 0.01 to about 10% by weight, based on the.aqueous me~ium~ are 16 useful with this invention. Reduced friction loss prop-17 erties are also realized with these polymers in the : 18 drilling fluid. . .
, 19 ' '' ' ' , ', PREFERRED EMBODIMENTS OF TH~ INVENTION ..
21 . The monomer is preferably a combination of ak least 22 one compound selected from the group consisting oE acrylamide 23 and methacrylamide and at least one compound selected from 24 the group consisting of acr~lic acid, methacrylic acid, alkali metal acrylate, and alkali metal me-thacrylate.
26 However, the monomer may be a single compound selected 27 from the above group of monomers. Minor amounts of addi-28 tional ethylenically unsaturated copolymerizable monomers 29 710019-B -4- . . -.

' ' 11C~63336 1 may also be present. Preferably, the monomer is a mixture 2 o~ acrylamide and sodium acrylate. It is preferred that 3 the monomer contain abou~ 1-75% and preferably 1S-55%
4 and more pre~er~bly 25-50Po of acrylic acid or alkalî
m2tal salt thereoE, e.g. sodium acrylate.
6 Irradi~tion of the monomer is preferably carried out 7 in an aqueous solutio~ containing about lO~i to about 60%~:
8 and more preferably about 15~i to about 45% by weight 9 of dissolved monomer. At the lower monomer concentra- ~
tions, the~produc~ is generally a pourable polymer solu- : .
11 tion and at concentrations of above about 15~ by weight, 12 the product is generally a nonpourable gel.~ A water~in-13 soluble product may result at concentrations above about 14 60% monomer; thus, such high concentrations are undesirable.
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of course, the particular limits of monomer concentration 16 will depend, among other things, on the xadiation condi-17. tions used, monomers used, and on the desired product for a 18 particular use. The intrinsic viscosity of the polymer 19 product increases as the monomer concen~ration increases, up to the point where the amount of cross-linking becomes 21 appreciable, provided all other variabl~s are kept con- :
, 22 ~ ~s~itant.
. 23 The aqueous monomer solution preerably contains 24 not more than about S ppm oE transition metal ion~, such as nLckel, iron, and cobal~, and no more than about 0.5 .. . . . . .
26 ppm of cuprous and cupric ions .
27 Irradiation o~ the aqueous monomer solution may 28 be accomplished with high energy ionizing rad.iation.
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~0tj3336 1 Radiation wavelengths less than 3,500 and preferably 2 less than 2,000 Angs-troms are useful. The radiation 3 employed may be par-ticula-te or electromagnetic in nature.
Examples include accelerated electrons, protons, neutrons, etc., as well as X-rays and gamma-rays, the latter being 6 pre~erred.
- 7 Radiation intensity is preferably about 1,000 to 8 abou~ 300,000 rads/hr. and more preferably abou-t 5,000 9 to about 200,000 rads/hr. Intensity directly influences the molecular weight of the copolymer. That is, under 11 otherwise identical conditions, low intensities generally~
12 give higher molecular weights.
13 The radiation dose is pre~erably at least about , 14 1,000 xads and more pre~erably a~ least about 1,500 rads.
The maximum dose level is preferably not more than 100,000 16 rads and more preferably not more than 50,000 rads~
17 The radiation dose used directly influences the 18 intrinsic viscosity and degree of monomer-to-polymer con-19 version. At a given radiation intensity and monomer con-centration, an increase in radiation ~ose generall~
~21 ~tends to result in a décrease in the intrinsic vis-22 cosity o~ the polymer produced and an increase in the .
23 degree o~ monomer-to-polymer conversion. The xadiation 24 dose may also in~luence the water-solubilit~ o~ the polymer ~as it has been ~ound that a radiation dose may render 26 the resulting polymer water-insoluble. At the pre~erred 27 do~e ratesr conversions up to lOO~and preferably 80-100%
28 of the monomer to polymer may be obtained without undue 29 insolubil~zation.
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1C~63336 1The pH of -the aqueous monomer solution is generally 2 not critical except th~t very low pH values ~.ay cause 3 insoluble products to form. Preferably the pH is wi-thin 4the range of 3-13 and more preferably a~out 8 -to about 11.
Although hJ.~her and lower pH values may be used, it should 6 be recognizea that hydrolysis may occur at pH values much ~7 below about 3 and much above about 11.
8While the process described above may be used to - :~
9 prepare polymers having an intrinsic viscosity from about 6 to about 30 dl/g (deciliters per gram) in 2 normal 1~ sodium chloride a-t 25.5C, the process is~modified some-12 what to prepare polymers having an intrinsic viscos.ity 13 . below about 6.dl/g or above about 30 dl/g in 2 normal .
~;: - 14 sodium chloride at 25.5C. Polymers having an intrinsic :
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viscosity below about 6 dl/g are prepared by carrying ou~
16 the poIymerization reaction described above in the presence : 17 of a chain transfer agent. The chain trans~er agent ; 18tends to restrict the growth o~ active polymer chains and ~ 19thereby results in the formation of polymers ha~ing lower .
~ 20~ molecular weight ~lower intrinsic viscosit~). The chain ~
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21 transfer agents which may be used herein may be any chain ; 22 transer agent which tends to restrict the. gxow~h o~ the , ~ .
23 polymer chains and thereby aid the formation o~ lower 24 ~olecular welght (lower intrin~ic viscosity) polyme~s,.which i5 soluble in khe reaction medium, and which does not inter~ere . ~ . ~I . . . .
~: ~ 26 with the polymerization by reacting with the monomer.
27 Illustrative examples of ahain transfer agents whi.ch may 28 be used include lower alkyl alcohols, such as methanol, : 29 ethanol, and isopropanol; halogenated compounds, such , : : 30 710019~B . . -7- . .
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, , . -~63~36 1 as trlchloroacetic a~id; thiosorbitols c~ntainincJ two thio ~roups and four secondary hydrox~l groups; and mer-3 captans. The amount of chain transfer agent used depends 4 upon the in-trinsic viscosity desired, the monomer concen-tration, the radiation conditions used, and the chain 6 transfer constant of the chain transfer agent used. The use 7 of a chain transfer agent is not necessary in order to ~ -8 prepare polymers having intrinsic viscosities from about 6 9 to about 30 dl/g; but lf desired, such polymers may be prepared in the presence o~ chain transfer agents~
11 In order to prepare polymers having~an intrinsic 12 viscosi~y above about 30 dl/g, the polymerization reaction .
13 is terminated when less than about 75~ and preferably when~
14 less than about 60~ ~y weigh~ o the monomer has been converted to polymer. It has been found that the 16 ~ ~ intrinsic viscosity of the resulting pol~mer tends to 17 decrease as the percent conversion o monomer to pol~mer 18 increases. For reason5 of economy, it is not practïcal 19 ; to tolerate conversions lower than about 20%.
~ More specifically, to prepare polymers having 21 intrinsic viscosities fro~ 30 to about 60 dl/g, it is 22 preerred that the radiation intensity be below about 23 100,000 and more preerably below about 50,000 rads/hr 24 and the monomer conce~tration o~ the aqueous solution to be irradiated be abo~ 20 to about 60% and prefèrably 26 25 ~to about 50% and ~hat the conversion of the monomer 27 to the pol~mer be about 15 to about 75%, and more prefer-28 ably less than 60 : . , .
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~L~63336 - 1Where i~ is desirecl tha~ the polyme~s have ~he hicJh~
- 2es-t molecular wei~Jh~ alid the lowest ~!ugyins cons~ant, ~he ,~
3 reaction conditions should be such that the dosaye and 4 conversion be low, the monomer concentration be relatively hi~h, and the radiation intensity be rel~tively low--these 6 preferred parameters being within the range of reaction 7 conditions taught herein.
8Also, higher intrinsic viscosities, shorter xeaction 9 times, better con~ersion and a more linear polymer can be obtained by incorporating at least one watex-soluble salt 11 in the aqueous monomer solutlon to be irradiated. Salt 12 concentrations of about 3~ to saturation in the aqueous , . :.':
13 monomer solution are useful Examples of salts, etc. are 14~undin cope,n~ngCanadian Patent applicati~ Serial No. 220,741, . .
15 ,"Polymerization o Unsaturated Monomers with Radiation in ;,;
16 ,the Presence o~ Salt", filed February 25, 197;5. "
17Where it i5 desired to obtain a less water-soluble ~8 polymer, the polymer can ba branched or partially cross , .
19 linked. Such can be accomplished by overirradiation, e.~.
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by continuing the radiatlon after all the monomer has been ,, 21 converted to polymer~ or continuing the radiation a~ter 22 insoluble polymer begins to appear. Also, partial, ,, 23cross-linking can be e~ected using bthylenically unsatur- ~, 2~ , ated water-soluble copolymerizable monomers containing more ~5 than one ethylenically unsaturated bond. Examples of 26 such monomers include methylene bisacrylamide, polyacrylates 27like sorbitol polyacrylate and polyallyl ethers ,or sorbitols ,, 2,8 like hexallyl sorbitol and lik~ monomers. Where these . .
29 710019-B -9- , , ,", :~ ,: . . .
' ~' 1 monomers ar~ used in the radia~iorl polymerization process, 2 they are preferably present in concentration$ o~ about 3 0.01 to about 10~ and prefera~ly about O.OS to about 5 4 and more preferably 0.1 to about 3% by weigh-t.
The variables of radiation intensiky, total radiation 6 dose, and monomer concentration discussed above are inter-7 dependent variables. While useful polymers may be prepared 8 at all monomer concentrations, radiation intensities, 9 and radiation dosages within the ranges given heretofore, all combinations of concentration, dose, and intensity lI within these ranges may not be used to prepare polymers 12 useful in the process of this invention. For example, 13 while a polymer useful in the process of this invention may 14 be prepared at a monomer concentration o 60% by weiyht, ~ . .
provided the radiation dose used is sufficiently low to 16 result in the formation of wàter-soluble polymers, the 17 use of a monomer concentration oE 60~ by weight, an 18 intensity of 25~ rads per hour, and a dose oE 300,000 19 rads, results in the formation o water-insoluble pol~mers.
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~ 20 In view of this interdependency of intensity, dose, and .
~ 21 ~monomer concentration, it may be necessary to per~orm a ~, :
~ 22 limited amount of experimentation in order to prepare a :
23 polymer having the desired intrinsic v:iscosity. However, 24 this e~perimentation may be kept to a minimu~ in view oE the disclosure in ~able l below oE the preparation of 26 a variety o polymers o dierenk viscosities and in 27 view of the discussion above on the effect o intensity, 28 dose, monomer concenkration, degree o conversion, and .~ ' ' ' ' ' ' ~ ' , ` ' : ' - ' ' ' ' ' 1 chain transfer agent on the in-trinsic viscosity of the 2 poly.~er. ~ccordingly, the reaction conditions which may 3 be used to prepare a water-soluble polymer having an 4 intrlnsic viscosity different from the intrinsic vis-cosities of the polymers described in Table 1 may be : -6 readily determined by minor modification of the reaction 7 conditions given in Table 1 for the preparation of the 8 polymer having the intrinsic.viscosity nearest to the 9 intrinsic viscosity of the polymer which is desired ~o .
10 be prepared. Such modification may be made in view of the 11 discussions above on the effect of intensity, dose, monomer ` 12 concentration, percent conversion of monomer to polymer, 13 and chain transfer agent on the intrinsic viscosity of the ,~
14 polymer. Yor example, a polymer having an intrinsic viscosity of about 16 dl/y may be prepared;by using the same 16 reaction conditions employed in Example F, Table 1, 17 except that the radiation intensity is increased, the .
18 total radiation dose is increased, the monomer concentra-19 tion is lowered, the percent monomer conversion is increased, and/or the reaction is carried out in the presence of a 21 chain transfer agent. . It is generally preferred, however, 22 that the said decrease in intrinsic viscosity be obtained 23 by increasing the rad.iation intensity, lowering the monomer .
24 conaentration, and/or u~ing a chain transf~r agent.
~he product of irradiation is an aqueous so~ution 26 o~ the water-soluhle polymer which may be in the ~orm of 27 a pourable liqu:id or a nonpourable, rubbexy gel, depending 7 28 upon the polymer concentration and intrins.ic viscosity of 29 710019-B . -11 ` ~ :

,, 1 the polymer deslred. The viscosity of the polymer 2 solution tends to increase as the polymer concentration 3 and inkrinsic viscosity oE the polymer increase. The 4 polymer solutions produced by the radia-tion may be admixed with water and used directly or the polymer solution may 6 be concentrated by conventional means or it may be 7 ~ recovered in particulate form, i.e. dry form~ For example-, 8 a nonpourable gel may be ~inely subdivided and -the water 9 removed by conventional drying techniques. Or, the water may be extracted from the subdivided gel with a water-11 miscible, volatile organic Iiquid, e.g. wi-th methanol, 12 which has no affinity for the copolymer.
13 The polymer, of course, is desirably compatible with 14 other components in the drilling fluids. The polymer may contain cations which are preferably monovalent cations .
16 and preferably sodium.
17 ~ The polymers obtained from this'radiation polymeriza-18 tion have relatively low Huggins constants. This constant`
19 is related to the lineari-ty of the polymer where molecular weights are constant, i.e. for two copolymer5 having similar 21 molecular weights, but different Huggins constant, the~
22 lower Huggins constan;t indicates a more linear polymer.
23 'Polymers having Huggins constants below ~1 and preferably 24 below 0.7 and more preferabl~ below 0.5 are most often' used with this invent'ion. A more detailed definition of 26 Huggins constant and a method for determinging Huggins 27 constant of a polymer is found in "Textbook of Polymer Chem-28 istry," Billmeyer, Interscience Publishers, ~.Y., 1957, 29 pp. 128-139.

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~;063336 1 Ink~insic viscosity of the polymer can vi~ry from les~
2 than abou-t 1 to about 60 dl/g ox ~ore and pre~ierably about 5 3 to about 35 dl/g. The permeability of the reservoir rock 4 may in~luence the desired intrinsic viscosity, i.e. g~n-erally, a lo~ler permeability reservoir rock can use lower 6 intrinsic viscosities. For example, permeabilities less 7 than about 50 md can use intrinsic viscosities of about 10, 8 whereas permeabilities of about 200 md or more wlll generjally 9 demand intrinsic viscosities greater than abou-t 20. The intrinsic viscosiky numbers referred to are measured in 11~ 2 normal sodium chloride at 25.5C. Of course, polymers 12 with higher intrinsic viscosities can "plug" or "bridge"
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large pore holes in the reservoir rock; thus, the efficiency 14 of the polymer to prevent fluid loss is improved with higher intrinsic viscosities. The degree of branching on the 16 polymer can also affect the efficiency of the polymer to 17 pluy the reservoir rock.
18 rrhe polymer may be solubllized and diluted to the 19 desired concentration with water. The use of water contain-ing large amounts of polyvalent metallic cations which may ~ .
21 ~ be present in the aqueous polymer solution is dependent 22 upon the specific polyvalent metallic cation present, the 23 temperature and pH of the solution, and the in-trinsic 24 vi5cosity and anionic content of the pol~mer. Xn general, the polymer becomes less tolerant of polyvalent cations 26 as the intrinsic viscosity, anionic content, and concentra-27 tion of the polymer increase. The use of water containing 28 substantial amounts of copper ions and/or iron ions is , :

pr~erably avoided due to the advers~ ~Ef ~ct such ions 2 may have on the water solubili-ty o~ the polymer. Where 3 maximum viscosity is desired for a given polymer concentra-4 tion, the water pre~erably co~t~ins iess than about 500 ppm of TDS (total dissolved solids). Also, where rnaximum vis-6 cosities are desired, the wa-ter preferably contains less 7 than about 50 ppm of divalent cations, such as calcium -8 and/or magnesium, i.e. the water is classified as "softl'.
9 Shearing o~ the polymer upon dissolu-tion and use should ~ be minimized if maximum viscosity is desired. To ob~ain .
11 maximum viscosity with the gel form o~ the polymer, the 12 gel is first extruded and then cut into ~ine pieces, e~g.

13 the size of BBs, and thereafter agitated in aqueous 14 solution at low shear rates. Pumps characterized by low . . , .~ .
lS shear rates as well as agitators run at low speeds 16 are especially useful. Water-soluble alkaLine salts, that 17 is, salts which give pH above 7 in water, such as alkal~

18 metal carbonates, may be added to the aqueous solution 19 to facilitate soLubillzation of the polymer. A pre~erred alkaline salt is sodium carbonate. The amount of alkaline 2L salts added to the water must be carefully controlled if 22 one desires to avoid hydrolysis of the pol~mer. Other 23 additives known to the a~t are also useful.
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24 The pol~mers o~ this invention exhibit non-Newtonian behavior and high viscosities at low shear rates. Such 26 properties impart good suspension characteristics to drill-27 ing fluids to keep well bore cuttings in suspension, drill-28 ing fluid component in suspension, etc. but also 29 exhibit relatively low viscosities in high-shear regions, e.g. near the drill bit. ~ -. ' ~ 1063336 1 The polymers o~ this invention can be selected to 2 exhibit ~ery high viscosities at low shear rates--this 3 proper-ty improves suspension characteris-tics, reduces 4 ~luid loss at the rock surface as well as inside the rock matrix, etc.
6 Also, the poly~ers can be selected to have improved 7 shear degradation characteristics. That is, polymers 8 generally tend to degrade as they pass in and out o high 9 shear rate regions, e.g. the drill bit region. By selecting a polymer with some degree of branching or some degree of 11 cross-linking, the polymer will be less sensitive to 12 shear degradatlon. Thus, for given molecular weight poly-13 mers, a polymer with a higher Huggins constant will be 14 less sensitive to shear degradation.
The water phase containing drilling flùids can con-16 tain numerous additives known to the drilling mud art.
- 17 These additives are desirably not reactive with the polymer 18 of this invention, i.e. the additives should not adversely 19 influenae the water-loss characteristics of the polymer.
Examples of additives are highly bentonitic clays commonly 21 known in the art;~these alays can be high yield and/or , ~ . .
22 ~ high density clays. Or, a low-yield clay can be used;
23 such clays are characteristic of the Ventura Field in 24 Cali~ornia, U.S.A. ~lso, the drilling ~luid can contain 2S hydrocarbons normall~ used in watex and h~drooarbon emul-~; . . .
26 sion drilling fluids and water-based drilling fluids. The ~7 make up of -the drilling 1uid as well as preferred com-23 ponents for a particular formation are known in the art 29 and are intended to be included within the scope of this invention.

, - :1063336 1 The concentration of the polymer in the drilling fluid 2 can be about 0.01 to about 5~ and more preferably is about 3 0.05 to about 2 weight percent. Preferably, the cIrillin~
4 fluid contains a ben~onitic clay. Supplemen-tal water-loss control agents can be added if desired. Conditioning 6 agents such as nigroslne, linae, starches, lignin deriva-~
7 tives, and alkali metal polyphosphates ~e.g~ tetra-sodium 8 pyrophosphate, sodium tetraphosphate, and sodium hexameta-- ~ ........................... - .
9 phosphate), etc.; wall building agents such as quebracho, nigrosine, impectate pulp, etc.; weighting agents such as 11 iron oxides, barytes, etc.; are particularly useful with 12 ~ this invention.
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13 Additional polymer and optionally higher or lower 14 - intrinsic viscosity polymers~ in relation ~o the original~ -pol~mer, may be added during the drilling operation to 16 impro~e the fLuid loss control properties. Also, other 17 additives desired to impart needed properties ~o the 18 drilling fluid can be added as the drilling operation pro-19 gresses.

~ ~ Drilling fluids applicable with this invention include 21 those containing a water phase. Examples o~ such fluids 22 include~water-in-oil emul9ions and microemulsions, oil-in-23 water emulsions and microemulsions, conventional water-:;::
24 base drilling fluids,; fluids containing bot.h water and hydrocarbon, ~luids ~ontaining little or no h~drocarbon, etc.

26 The following e~amples are presented to teach specific 27 working embodiments of the invention; such are not meant 28 to limit the interpretation of the invention. Unless 29 otherwise specl~ied, all percents are based on volume.

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1 Pre aration OL- the Copo ymers 2 Polymers used Eor testin~J are prepared with Cobalt 3 60 gamma radiation; radiation intensities and dosages 4 are outlined in Table 1. The process for preparing Polymer S A is explained; preparation o~ the other polymers is 6 similar except for variations indicated in Table 1.
7 To 24,000 gms of deionized water there are added 8 692 gms of sodium hydroxide. A~ter cooIinq -the solution 9 to 30C, 1,250 gms of acryl1c acid are added. Thereafter,

5,000 gms of acrylamide are added while mixing and the 11 pH is adjusted to 9.4. The resulting solution contains 12 75% by weight acrylamide (AAd) and 25~ b~ weight sodium -.
13 acrylate tNaA~) and has a total monomer concentration of ' - ' . ~ . , I4 21.4% by weight. The solu-tion is purged with N2 ~or 20 minutes and therafter sealed~ The sample is irradiated 16 with Cobalt 60 gamma radiation at an intensity o~ 18,000 17 rads/hr (R/hr) to a total dose of 8,800 rad5 (R). The 18 resulting product is a gel-like mass.
19 A portion o the gel is weighed, and thereafter extr~cted with methanol to precipitate the polymer. The polymer is 21 dried in a vacuum oven at 36C and 0.02 psia ~or 24 hours 22 and then to constant weight at 110C. Weight oE -the dried 23 ~ prod~ct divided by the theoretical weight gi~es a monQmer 24 convexsion o~ 93~. .
A portion o~ the gel is solubilized in water by 26 first extruding it th`rough a "meat" grinder; th~ "spaghetti'^-27 like extrusion is cu~ into "BB" size particles and then 28 dissolved in water b~ agitating at a low rpm to prevent 29 substantial shearing of the pol~mer.
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1 The remainin~ gel is processed to dry powder Eorm 2 as ollows: first ~.he gel is extruded and the yel strands 3 are stirred into excess of metha~ol to remove the wat~r.
4 The thus dewatered and hardened polymer strands are then ground to less than 20-mesh size and -the ground poly~er

6 is slurried .in methanol, filtered, and finally dried at

7 60C in a vacuum o~en.

8 The intrinsic viscosity is measured at 25.5~C in ,

9 a 2 normal NaCl aqueous solution. The Huggins constant .
is measured by the method described in l'Textbook o~ Pol~-11 mer Chemistry," Billmeyer, Interscience Publishers, New 12 York, 1957, pp. 128-139.
13 The monomer used in Example "G" is dissolved in water 14 containing 9.1~ by weight o~ methanol.
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o ~ x EX~IPLE I
2 To show that the copolymers of this invention give 3 unexpectecl results over polymers and copolymers o~`the 4 prior art, this example is presented.
Fluid loss control properties are simulated in the 6 laboratory by flooding water-saturated reservoir core 7 samples with aqueous pol~mer solutions at a gi~en ~rontal 8 velocity. Permeability reduction, determined by a water 'f 9 flush at the same frontal velocity, lS the measure of fluid loss control property. That is, high permeability 11 reduction is desirable to minimize fluid loss. The front 12 core section permeability reduction simulates a well bore 13 environment in which fluids "bleed" out into the reservoir ' 14 matri~ while the borehole is being drilled.
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Sandstone cores 1" in diameter by 3" long are 100ded 16 with the following polymer solutions at a frontal velocit~
of 10 ft/day. The polymers are dissolved in water con-18 taining Table 2-indicated TDS. Water containing 500~ppm ~: . : : . : .
19 TDS is used to measure core permeabilities and flushed permeabilities.
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Run~; 1 throu~h 6, as compared tc) Runs 7-9, e~chibit higher 2 front and rear sec-tion permea~ility reductions than com-3 mercially available polymers. These data indicate the 4 polymers will impart improved fluid loss co~trol properties as compared to prior art pol~mers. Polymer E in Table 1 6 at concentrations and water conditions identical to 7 copolymers 1 and 2, has ~ Broo~ield viscosity at 6 ~pm 8 o~ 52 cp.
9 EXAMPLE I I .
. . Examples of drilling fluid composi-tlons con-taining 11 the polymers of this invention include:
12 Sample A: -13 52.5 parts of bentonite are dry blended with 2.5 14 parts of a copolymer ha~ing an intrinsic viscosity of 18 dl/g in two normal sodium ch~oride at 25C and contains 16 50 mole percent oE polymerized sodium acrylate and ~0 17 mole percent of polymerized acrylamide. The mixture is 18 then added to 17,500 parts of deionized water and stirred .
19 for 30 minutes to form a drilling fluid having excelle~ ~
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~ 20 rheological properties.
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21 Sample B

22 20 parts of a homopolymer of sodium acrylate havin~

23 an intrinsic viscosity of 25 dl/g in -two normal sodium 2~ hydroxide at 25C is mixed with a ~olution o~ 140 parts o~ sodium carbonate and about 1740 parts of water. To 26 this mixture is added about 100 parts o~ a bentonitic 27 clay and the mi~ture is stirred until the clay is thor-28 oughly wetted. At this point, the fluid contains an 29 estimated 5% of clay, 7~ of sodium carbonate, and l~ by weight of sodium polyacrylate. The drilling fluid is .

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)63336 1 then ~:ei~hted with barium sulfate until a weicJh-t of about 80 2 pounds per cubic ~oot is obt~ined. This dxilliny fluid 3 possesses a suitable Viscoslty and water loss ch~rac-te~-4 istics to permit its use in general drilling operations~
5 Sa~ple C ~ ' 6 A drilling mud which is particularly effective for 7 forming a filter cake on the wall of the well and which 8 reduces the water loss through the filter cake without 9 substantially lncreasing the viscosity o~ the drilllng fluid is prepared by mixing 60 parts of water, 20 parts .
;~ ' 11 of kaolin, 2 parts of bentonite, 17 parts of barium sul-12 fate, and one pound of polymer per barrel of drilling ~ .
~ ~ 13 fluid. The pol~mer is a copolymer of 55% acrylamide ana - . .
14 45% sodium acrylate and has an intrinsic viscosity ~' 15 in two normal sodium chloride at 25C of 30 dl/g.
; 16, SampleS D-L
17 'These samples illustrate additional drilling fluids 18 of the present invention. These drilling fluids are - ~ 19 water,based drilling fluids comprislng bentonite clay, ; 20 ~ polymer containing the indicated amount o sodium acrylate 21; ~and/or acrylamid~e~and water. The amount of bentonite clay 22 and polymer are given in pounds per barrel a~ drilling .
23 fluid. The compositions of the polymers emplo~ed are 2~ given in the Table. ,~he intxinsic viscosit~ ~iven in Samples E and G throu,gh L are measured in 2 N NaCl. The ;.~ , .
26 ' intrinsic viscosity given in Samples D and F ar~ measured 27 in 2 N NaOH.

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1 s~mpl~ ~l 2 A water-external emulsion drillin~ fluid is prepared 3 by emulsifying 6 parts by weight of crude oil and 94 4 parts of water using 0.3 par-ts by weight of a mixture of 70% of po]yoxyethylene sorbitan mono-tall oil ester and 6 30~ o~ the isopropyl amine salt of dodecylbenzene sul~onic 7 acid. The resulting emulsion is then blended with ben~on- -8 ite and a polymer in proportions such as to obtain 12 pounds 9 of bentonite per barrel of drilling fluid and ~.75 parts of polymer per barrel of drilling fluid. The polymer is a 11 homopolymer of a sodium polyacrylate and has an intrinsic 12 viscosity of 20 dl/g in two normal sodium hydroxide at 25C.

14 A porous rock is efficiently drilled usin~ the drilling mud of Sample C. No fluid loss is experienced during the 16 drilling.
17 It is not intended tha-t the invention be limited by 18 the above examples. Rather, compositions and components 19 Of water-based drilling fluids known in the art and obvious to those skilled in the art are intended to be incorporated , 21 within the scope of the invention as defined herein. For 22 example, it is ob~io~s to those skilled in the art what 23 components and concentrations th~reo~ can be incorporated 24 into the drilling fluid along with the polymer5 o~ this invention; and, in ce;rtain cases, prefe~red components will-26 obviously be preferréd with particular reservoirs.

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Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. CLAIM 1 In a process of drilling a well into a subterranean formation wherein a water containing drilling fluid is circulated in the well while drilling into the formation, the improvement comprising incorporating into the drilling fluid a fluid loss control agent obtained as a product of radiation polymerization of at least one monomer selected from the group consisting of acrylamide, methacrylamide, acrylic acid, methacrylic acid, alkali metal acrylate, and alkali metal methacrylate in concentrations of about 10% to about 60% by weight dissolved monomer in an aqueous medium, the radiation intensity being about 250 to about 1,000,000 rads/hr, and the radiation dose being about 500 rads to about 300,000 rads.
2. CLAIM 2 The process of claim 1 wherein the aqueous monomer solution contains about 25 to about 99% by weight of one monomer selected from the group consisting of acrylamide and methacryl-amide and about 1 to about 75% by weight of at least one monomer selected from the group consisting of acrylic acid, alkali metal acrylate and alkali metal methacrylate.
3. CLAIM 3 The process of claim 1 wherein the monomer is acryl-amide.
4. CLAIM 4 The process of claim 1 wherein the monomer is sodium acrylate.
5. CLAIM 5 The process of claim 1 wherein the polymer is a co-polymer obtained by polymerizing about 25 to about 99% by weight of acrylamide and about 1 to about 75% by weight of sodium acrylate.
6. CLAIM 6 The process of claim 1 wherein the radiation dose is within the range of about 1,000 to about 100,000 rads.
7. CLAIM 7 The process of claim 1 wherein the aqueous medium is at a pH of about 3 to about 13.
8. CLAIM 8 The process of claim 1 wherein the pH of the aqueous medium is about 8 to about 11.
9. CLAIM 9 In a process of drilling a well into a subterranean formation wherein drilling fluid containing a water phase is circulated in the well while drilling into the formation, the improvement comprising incorporating into the water phase a fluid loss control agent obtained as a product of radiation polymerization of about 25 to about 99% by weight of at least one monomer selected from the group consisting of acrylamide and methacrylamide and about 1 to about 75% by weight of at least one monomer selected from the group consisting of acrylic acid, methacrylic acid, alkali metal acrylate, and alkali metal methacrylate in concentrations of about 10% to about 60% by weight dissolved monomer in an aqueous medium, the radiation intensity being about 250 rads to about 1,000,000 rads/hr, and the radiation dose being about 500 rads to about 300,000 rads.
10. CLAIM 10 The process of Claim 9 wherein one of the monomers is acrylic acid.
    
    The process of Claim 9 wherein one of the monomers is methacrylic acid.
    
    The process of Claim 9 wherein one of the monomers is sodium acrylate.
    
    The process of Claim 9 wherein one of the monomers is sodium methacrylate.
    
    The process of Claim 9 wherein one of the monomers is acrylamide.
    
    The process of Claim 9 wherein the polymer is a copolymer obtained by copolymerizing acrylamide and sodium acrylate.
    
    The process of Claim 9 wherein the radiation intensity is within the range of about 1,000 to about 300,000 rads/hr.
    
    The process of Claim 9 wherein the radiation dose is within the range of about 1,000 to about 100,000 rads.
    
    The process of Claim 9 wherein the aqueous medium is at a pH within the range of about 3 to about 13.
    
    The process of Claim 9 wherein the pH of the aqueous medium is within the range of about 8 to about 11.
    
    The process of Claim 9 wherein the fluid contains a bentonitic clay.
    
    In a process of drilling a well into a subterranean formation wherein a drilling fluid containing a water phase is circulated in the well while drilling into the formation, the improvement comprising utilizing as the drilling fluid a com-position comprising an aqueous bentonitic clay dispersion and a water soluble fluid loss control agent obtained as a product of radiation polymerization of an aqueous solution comprised of about 10% to about 60% by weight of a mixture of about 25% to about 99%
    acrylamide and about 75% to about 1% sodium acrylate, the radiation intensity being within the range of about 5,000 to about 200,000 rads/hr and the radiation dose being within the range of about 1,500 rads to about 50,000 rads.
    
    The process of claim 21 wherein the copolymer obtained from the radiation polymerization is in the form of a gel.
    
    The process of claim 21 wherein the aqueous solution contains about 15% to about 55% by weight of sodium acrylate.
    
    The process of claim 21 wherein the radiation is gamma radiation.
    
    The process of Claim 9 wherein the drilling fluid contains about 0.01% to about 10% of the fluid loss control agent.
    
    The process of Claim 9 wherein the polymer is partially cross-linked.
    
    The process of Claim 21 wherein the polymer is partially cross-linked.