CA1054359A - Clay-free thixotropic wellbore fluid - Google Patents

Abstract

ABSTRACT OF THE DICLOSURE

An aqueous clay-free thixotropic wellbore fluid for use in subterranean formations in the earth comprises water, at least 1% by weight of a brine forming soluble salt or mixtures of salts, MgO and a heteropolysaccharide prepared by the action of bacteria of the genus Xanthomonas on carbo-hydrates; the fluid is injected into a formation to contact the formation with the fluid, and the fluid is returned to the surface for regeneration and reinjection into the formation.

Description

~os~s~
Thi~ invention relate~ to wellbore fluid~, including drilling fluids, completion fluid3, woxkover fluids, packer fluids, that is, all of tho~e fluid~ which are employ~d over the course ~-o~ the li~e of a well.
Thi~ application is a divi~ion of C~nadian Patent Application Serial ~umber 217,114, filed December 30, 1974.
Generally wellbore fluids will be either clay-based or -brines which are clay-free~ These two classes are exclusive, that i3, clay-based drilling fluids are not brine~. A wellbore fluid can perform any one or re of a number of functions~ For example, the drilling fluid will generally provide a cooling medium for the rotary bit and a means to carry off the drilled particles~ Since great volumes of drilling fluid are required for these two purposes, the fluids have been based on water.
Water alone, ho~ever, does not have the capacity to carry the drilled particles from the borehole to the ~urface~
In the drilling fluid class, clay based fluids have for ;~
years preempted the field, because of the traditional and widely held theory in the field that th~ visco~ity suitable for creating a particle carrying capacity in the drilling fluid could be achieved only with a drilling fluid having thixotropic -properties, that i3, the vi~co~ity must be supplied hy a mate-- rial that will have sufficient gel strength to prevent the drilled particle~ from 3eparating from the drilling fluid when ., .
agitation of the drilling fluid has cea3ed, for example, in a holding tank at the ~urface.
In order to obtain the requi~ite thixotropy or gel strength, hydratable clay or colloidal clay bodie~
~uch a~ bentonite or fuller'~ earth have been employed. A~
a re~ult the drilling fluids are u~ually referred to as "muds". The u3e of clay-based drilling mud~ has provided , . .

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the means of meeti ~ two basic requirements o~ drilling ~luids, i.e., cooling and particle removal. However, the elay-based drilling muds have created problems for which solutions a~e needed. For e~ample, since the clays must be hydrated in order to function, it is not possible to employ hydration inhibitors, such as calcium chloride, or if employed, their presence must be at a level which will not interfere with the clay hydration. In eertain types o~
shales generally ~ound in the Gulf Coast area of Te~as and Louisiana, there is a tend~ncy for the shale to disinte-grate by swelling or eracking upon contaet with the water i~ hydration is not limited. Thus the uninhibited clay-based drilling fluids will be prbne to-shale disintegration.

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Tha drilled particles and any heaving shale material ~ ~ -will be hydrated and taken up by the conventional elay-based drilling ~luids. The continued addition o~ e~traneous ~i hydrated solid particles to the drilling fluid will increase the viscosity and necessltated costly and eonstant thin-ning and reformulation-of the drilling mud to maintain its original properties.
Another serious disadvantage o~ the elay-based ~luids is their susceptibility to the detrimental e~feet ... .
o$ brines which are o~ten found in drilled ~ormations, particularly Gul~ Coast formations. Such brines can have a hydration inhibiting ef~eet, detrimental to the hydration requirement f or tha elays.
Other disadvantages of elay-based drilling fluids are their (1) tendency to prevent th0 eseape o~ gas bubbles, when the viscosity o~ the mud rises too high by the inei-dental addition of hydratable material, which ean resultin blow-outs; (2~ the need ~or eonstant human co~trol and

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:~5~35g supervision of the clay-based fluids because of the expectable, yet unpredictable, variations in properties' and (3) the forma-tion of a thick cake on the internal surfaces of the wellbore.
The brlne~ have the advantage of containing hydration inhibiting materials such as potassium chloride, calcium chlor-ide or the like. Quite apparently any solid particulate mater-ial would be easily separated from khe brine solution since it is not hydrated. Thus, the properties of the brine are not changed by solid particulate matter from the wellbore. Simi-larly, since there is no opportunity for gas bubbles to become ;
entrapped, blowouts are less likely in a clay-free brine-type wellbore fluid.
~ on-argillaceous (clay-free) wellbore fluids based on non-thixotropic viscosifiers have been developed, which over-come the problems noted above with the clay-based fluids, such , . ..
as a brine containing a viscosifying amount of magnesia stabi-lized hydroxyethyl cellulose described in detail in U~S. patent

3,852,201 of Jack M. Jackson, issued December 3, 1974.

Thus, although these two pxincipal water-based, com~ ~;-j; .
peting and incompatible systems are commercially available and used, there i~ yet a hiatus, in a manner of speaking, between the capacities and desirable properties of these two systems.
Thus, even though the clay-free systems described avoid the ~ -problems of the clay-based systems, they are not suitable for -1 systems where weighting materials such aq calcium carbonate ', are nece~sary or desixed, especially if the weighting material is used in substantial quantities, A material which has come into expanding use in well-bore fluids is heteropolysaccharide produced by the action of bac-teria of the genus Xanthomonas on carbohydrate~, such as des-cribed in U.S. Patent~ 3,198,268; 3,208,526; 3,2Sl,417;

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~L()5~5g 3,243,000; 3,305,016, and 3,319,715. This material has be~n employed for a number of functions in wellbore fluids, e.g fluid lo~s additive, foaming agent, and viscosifier. Generally these heteropolysaccharides are employed with clays, however, they need not be, and in U.S. Patent 3,319,715 they are di~closed to be use-ful in brine completion fluids.
It is a feature of the present invention that the hetero-polysaccharides produced by the action of the genus Xanthomona~
bacteria is employed a~ a thixotropic viscosifier in a clay-free wellbore fluid in conjunction with a specified class o~ water los~
~` control additives having improved down hole properties, and which can contain weighting materials. These and other featuras and - advantage~ will be apparent from the following discussion and description of the invention and the preferred embodiment~.
It ha~ been found that an improved clay-freé wellbore fluid having thixotropic gel proper ies for u~e in subterranean formations in the earth is compri~ed of water, a viscosifying amount of a het~ropoly~accharide produced by the action of bacteria of the genu~ Xanthomona~ on carbohydrates, a stabilizing amount of MgO, and a brine forming soluble ~alt.
In particular the wellbore fluid comprises at least 1%
by weight of the brine forming ~oluble salt or mixtures of salts.
The fluid may al30 include an organic ~taxch derivative in accordance with the invention claimed in the parent Canadian application S.N. 217,114.
There i~ also disclosed a dry mix additive package for use in aqueous non-argillaceous brine wellbore fluids comprising an intimate mixture. The materials in the dry mix to provide 0.72 to 7.14 grams of heteropoly~accharide prepared by the action of bacterial of the genus Xanthomona~ on carbohydrates; 0~50 to 15 gram3 of MgO and 0.15 to 30 grams of organic starch derivative per liter of wellbore fluid or more preferably 0.72 to 3.57 grams, 1.0 to 7.0 grams, and 0.9 to 12 grams per liter, rPspectively.

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~05~35~
A further a~pect of ~he present invention i~ the method of employing the previou~ly de3cxibed wellbore fluid.
Thu~ there is provided the m~thod of treating a sub-texranean formation comprising injecting into ~aid formation clay free wellbore fluid comprising at least 1% by weight of a brine formlng ~oluble salt or mixtures of salts, a viscosifying amount of a heteropolysaccharide prepared by the action of bacteria of the genu~ Xanthomonas on carbohydrates and a stabilizing amount of MgO, contacting said foxmation with said .-10 fluid and returning said fluid ts the surface for regeneration and reinjection into said formation.
, The heteropoly~accharides are those produced as generally de~cribed in the aforementioned patents, The hetero~
poly~accharides are commercially available. Methods for the preparation are also described in the following references: an ?
~` article by J. G. Leach, VO G. Lilly, H. SO Wilson and M. R. Purvis, JrO ~ entitled "The ~Tature and Function of the Exudate Produced by Xanthomonas phaseoli," which appeared in Phytopathology, :, volume 47, pageq 113 through 120 ~1957); an article by V. G. Lilly, H. A. Wil~on and J, G. Leach entitled "Bacterial Polysaccharides Laboratory-Scale Production of Poly~accharides by Species : . .
of Xanthomonas," which was published in Applied Microbiology, : :;
volume 6, pages 105 through 108 (1958); a paper by R. F. Anderson, S. P. Rogovin, M. C. Cadmu~ and R. W. Jackson, "Polysaccharide , . .
Production by Xanthomonas ~Ç~e~Ea~" presented at the 136th ~Tational Meeting of the American Chemical Society in Atlantic City, New Jer~ey, on September 14-18, 1959, and a paper by A.R~ Jeanes, J.E, Pittsley, J.H. Sloneker and F,R. Senti, "Composition and . . .; .
Properties o~ a Heteropolysaccharide Produced from Glucose by Xanthomonas campestris ~RRL B-1459," which was delivered at the 136th I~ational Meeting of the American Chemical Society in Atlantic City, ~Tew ~ersey, on September 14-18, 1959.

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A typical heteropoly~accharide material is produced by the action of Xanthomonas campestris ~RRL B-1459 upon car~ohydrates. The purified product can be characterized as a soft, bulky powder having a slight tint, which swells rapidly in the presence of small quantities of water and dissolves in larger quantities. Generally from 0~72 to 7,14 grams o~ the hetero .' ' ~

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~ 5~3S~7 polysaccharide are dissolved per liter o~ wellbore fluid and more pre~erably 0.7~ to 3.57 ~rams per liter. Amounts less than O.72 grams per liter (.25 pounds per barrel) are ine~ec-tive to provide the necessary thi~otropic properties in the fluid. Amounts above 7.14 grams per-li-ter (2.5 pounds/barrel~
render the wellbore fluid too VlSCOUS ~or handling at the surface, A very important adjunct $or use in wellbore ~luids containing the'heteropolysaccharides is M~O (magnesia) which ~,', 10 serves as a stabilizer. Generally only a stabilizin~ amount '~
o~ MgO will be employed, e.g., about .05 percent hy weight based on drilling ~luids, which is about 0.50 grams per liter of wellbore fluid ~.188 pounds/barrel). The ~gO is only very slightly so]uble in the brine under the conditions ,~
presented here, about 10 ppm of magnesium i'on concéntration.
- Hence, the MgO is employed in quantities substantially greater than its solubility~' The use o~ larger'amounts of MgO is not harmful,,but generally no more than about 15 grams par liter ' of wellbore ~luid is employ~d. MgO could o~ course be employed ' 20 ' in larger quantiti~s as indicated below as a weighti~ material ,, ' should the cost be justifiable.,,Pre~erably for stabilization the ~gO will be employe,d in the range, of 1.0 to 7.0 grams per , liter o~ wellbore $1uid.
' The term "organlc deriva~iva o~ starch" or "organic '` starch derivative" means amylaceous substances which have been ,' modified, ~or é~ampl0, by etheri~ication ~r e~terification.
, Starch has been employed in both clay-free brine and ;;

~', clay-based wellbore fluids to aid in water loss control and u,nder certain limited conditions it has been e~fective. How- '~

ever, in clay-~ree brine wellbore fluids serious drawbar,ks have .~ .
; ' .

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been obserYed with starches. At temperatures around 300 F.
~luid loss control is a'brogated; that is, the starch no longer provides any ~luid loss control.
Another area where starches have proved unsatis$actory is,in clay-free brine completion ~lulds? workover fluids and the like, where acid (~enerally HCl). is employed. The problem arises because the starches are not su-fficiently acid soluble.
This problem is paxticularly serious in injection wells where the insoluble starch can create pockets or block strata which .
" 10 the acid will not leach out, thus resulting in irregular in-,jection into the-~orma'tion when the well is employed.~or that . . , purpose.

A particular problem encountered in using starch in clay-free brine.wellbore ~luids is the instability of ~he ,starohes. in the prqsence of calcium chlori~e brines, .Gener-.ally, the..starches begin to'break dovrn after about twenty four ~' , hours in the pre.sence of calcium chloride.
, . . . . . .
. .'Starch may undergo re,trogradation which is a spontan-~' eous,tendency to associate and partially crystalli~e... The ;, 20 associa~ed particles may precipitate and there appears to be ' a reverting to.original cold water.insolubility.

... . . .
Thus although starches.,llave been employed in clay based ~luids, they have generally not been successfully em-ployed with the brine wellbore fluids.. It is not surprising :; . .
` to note that the ~rk has grouped all starches tog~thsr and '~ have considered $he starch derivative~ as no better or sub-stantially equivalent'to unmodi~ied starches. Thus in U. S. ~::
i Pate~t 3,032,498 a cyanoethylated starch ~as describsd a's a ~,, water 105s reduction additive, which ls not in itsel~ sur-,' 30 prising; what is un~ortunate is ths exclusion o~ brlne-type ~' _7_ '. ' ' '' ' .

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fluids and the furt~ler compounding o~ t.his error by explicit statements tha-t there must be a thin impervious layer formed on *he walls of the borehole by.the clay-based mud. The amy-laceous substances from which ~he present modified starches are prepared may be derived from any source, including corn, wheat, potato, tapioca, wa~y maize, sago, rice, grain sorghum and arrowroot. It has been found that, where~s ordlnary un-modified starch has the disadvantages.previously shown, the derivative starches of the prese~t inventio~ are far superior and not so dlsadvantaged for use.in wellbore fluids. The mechanism for this unèxpected superiority o~ the present mod-ied starches.is not presently known with certainty. However, a possible e~planation for the surprising performance of the derivative starch is that the derivative group makes the starch molecule more bulky and less prone to crystalline s~ructure7 i.e., the stereospecific arr~ngement o~ the starch molecule may have been disarranged so that the derivative .
starch is atactic.

The derivative modi~iad s~arches of the parent in-. 20 vention may be added to the wellbore 1uid in either the gel-. atinized or ungelatinized iorm~ ~Pregelatinization is not :~ necessary. The present modified starches provide fluid loss control and all of the improvements noted herein, when employed .- in either gela~.inized or ungelatinized form.

The wellbore fluids concerned in the pre~ent i~vention ~ are those typically know~ as "brines". As the term brine is : employed here it means at least 1.% by weight o~ soluble salt ;
of potassium, sodium or calcium in water. In addition, the brine may contain other soluble salts of, ~or e~ample, ~inc~
-~ 30 chromium, iron, copper and the like. Generally, the chlorides ~:
are employed because o~ availability, but other salts such as . the bromides, sul~ates and the like can be used. The soluble .

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~OS~35~ ' ; salt~ of the brine not only furnish weighting material by adju~ting the density of the -qolution, but al~o typically furni~h the cations for inhibiting the fluid against hydration o solid materials.
The modified ~tarches, when pre~ent, are preferably , employed in an amount which will provide the maximum fluid loss -~ 'reduction and beyond which additional derivative starch has little additional effec~. This amount will vary not only as a result of the other components of the brine but also as a function of the 3ubterranean formation in which it is employed.
As a general ob~ervation, it has been found that the fluid loss reduction is obtained with starch derivative present in at least an amount in the range of about .15 to 30 gram~
per liter of the wellbore fluid. More preferably, at least an amount of starch derivative in the range of 0.9 ~o 12 grams per liter of wellbore fluid would be employed.
It is a unique property of the non-clay based wellbore fluids that rather large excesses of starch can be tolerated without any significant effect on the properties of -20 the wellbore fluid. This is not the case with clay-based .
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drilling fluids, where the fluid is physically crowded hy the clay particles and he addition of starch for water loss con-trol, for example, can appreciably increase the viscosity of ~`
the fluid.
; Numerous derivatives of starch have been described in the art~ Their synthesis and properties are outlined in detail in hundreds of papers and patents. ~n excellent and relatively recent compilation of much of this information is presented in "Starch and Its Derivatives", 4th Ed., J. A. Radley, Chapman and Hall Ltd.; London 1968. The particular method of prepara- ;~
tion is not of interest here and forms no part of this inven-tion insoar as the derivative product.
Included among the suitable organic derivatives of starch ~; :
` are etherified starch, esterified starch and partially oxidized -starch.
Some particular etherified starches would include alkylated ethers, prepared for example by treating the starch with an alkyl sulfate and alkali to convert the free hydroxy groups to ` alkoxyl producing, e. g., a methyl or ethyl ether derivative.
Other types of ethers such as hydroxyethylated starch, prepared by mixing starch with dry powdered sodium hydroxide, aging, followed by treatment with ethylerle oxide are included. Simi larly ca~boxyalkyl ethers such ascarboxymethyl ether of starch prepared by the action of chloroacetic acid on 5tarch in the presence o~ alkali; sulfur containing ethers such as those taught in British Patent 895,406 and the phosphorus analogues are suitable. The so-called "cationic" nitrogenous starch ~' ethers such as the derivative from the reaction of starch with the re~ction product of epihalohydrin and a tertiary amine or ~0 ~he amine salts in the presence of strongly all~aline catalysts are also suitable for the present invention. Other nitrogenous starch et~ers include t~le cyanoal~-yl ethera produced by the ,, . " , ,.
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1~5~359 reaction of starch and acrylonitrile. A further listing of suitable nitrogenous starch ethers is described, for example, in U. S. Patents 2,813,093; 2,842,541; 2,894,94~; 2,917,506 and 2,970,140.
A broadly applicable method of ether preparation for a large nu~er of suitable ethers was disclosed by Graver et al in U. 5. Patents 2,671,779i 2,671,7~0 and 2,671,731, which briefly involved the reaction of an alkalinated starchate with an organic halogen compound.
A particularly preferred class of starch derivati~es are starch ethers of the general formula `~
starch - O - C - R2 ! R3 .. , . , ~
i where Rl is oH,CH2O~, or H; R2 is hydrocarbyl or H;
- R3 is hydrocarbyl, H, COOH, CH2R4OH, NR5R6; R4 is hydrocarby~
R5 and R6 are H or hydrocarbyl. Generally each hydrocarbyl ~-group has from 1 to 8 carbon atoms and is alkyl~ cycloalkyl, aryl, alkaryl or aralkyl~ Most preerably, the hydrocaxbyl groups are alkyl of l to 6 carbon atoms. Each hydrocarbyl is - independently selected.
-The starch esters may be generally prepared by treating the starch with an organic acid 7 acid anhydride or acid chloxide in presence of an alkaline catalyst such as a tertiary amine or an alkall hydroxide~ Specifically water soluble starch formate, starch acetate, starch benzoate and the like have been prepared as well as mixed starch esters such as acetate-butyrate and acetate-formate.

The partial oxidation of starches, for example, wi~h nitric acid introduces carboxyl and carbonyl groups into the starch to produce suitable organic starch derivatives for use 30 in this invention.

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' , S9~35~
: Many of the organic s-tarch derivatives deseribed above are commercially available and have been used in the past as sizing agents for paper and cloth or ~or other pur-poses. I-t should be appreciated that the present starch derivatives are as.varied as the starch starting materials and suitable derivative starches for this in~ention may have number average molecular wei~hts o~ ~rom 20 9 000 to several hundred thousands, e.g., 400,000-600,000.
It is apparent that since there are.multiple.sites .. 10 available for ~sterif.icati.on and/or etherification on the - starch moleculs that there.may be ~rom one to several ester . ~ .
. or ether functions on a single starch molscule. Thus the ~ :

.~. chemically modified starches. may contain up to the theore- ~

. tical value o~ substituent groups or components thereo~, based ~ --on the glucose units available or may contain only a.frac-tional portion of functional groups based on available sites.

`: Similarly the starches.may be c~oss linked by the use of di-.. or trifunctional esteriiication or etheri~icatio~ agents.

.~ Within the limits previously given, all such normal and ob-vicus variants o~ the chemically modi~ied starch are wlthln `~ tha scope o~ the present i~vention... It is also within the - present invention.to employ mi~tures o~ starch ~erivat~ives, i.e.,.di~erent ethers or mixtures o~ ethers and esters a~d .~ :.

'. partially oxidized starches.
::
.. In addition to ~oluble brine ~alts and heteropoly-.; saccharides an~ MgO, the optional modified starche~, the ::
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present wellbore fluid~ can contain other conventional well- ~
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bore additive~, such as oil for producing water-in-oil or oil-.. in-water emu1sion~, vi~cosifiers such as hydxoxyethyl cellulose, gums, and the like, lignosulfonate salt~ such as calcium or ;:~ chromium ... . . .

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~5~359 lignosulfonates, emulsifiers, weigh-tirlg agcnts, calcium c~r-bonate, magnesia and o-ther agents. It is understood that not all of these possible constituents will be present in any one.wellbore fluid ~ut th~ir selec.tion and use will be governed by other constituents and the use for which the wellbore ~luid is intended.
In a particular embodlment a powder~d weighting agent is incorporated in the wellbore ~luid. The term "pow-dered weighting agent" is used harein and in the claims to describe all such materials known to the prior art o~ well ; working ~luids as weighting.agents, such as barium sulfate (barite or barytes) BaS04, the various lead o~ides, chiefly litharge ~PbO) and red lead (Pb304~, the iron ores or iron oxides, chie~ly magnetite (Fe304) and hematite (~e203), and ;. .
~ powdered.iron and lead or ot.her pow~lered heavy metals and their solid or other water lnsoluble, stable compounds, such : as calcium carbonate .. The various compone~ts may be added to a wellbore -~luid as indivi~ual components.or may be preblended as a dry ready mi~ additive package or packages in such propor- :
tions that the relative amounts of each component will be within the ranges recited above in the fluid.
. , ~ ..
SPECIFIC EMBODIMENTS
E~amples 1-4 In this s~ries of run~ the improvemen~ in fluid 1058 characteri~tics of the brine solution~ with the heteropoly-saccharide, MgO and the optional modified starch additive com~
: ponent is d~monstrated~ A dry blend of the components was ,~
prepared and added to the brine stirred ~or five minutes at room temperature and immediakely hot rolled at 175F. ~or 18 ~ hour~, ccoled, stirred 5 minutes and ~ubjected to API filter ..

- los~ te3t (100 p.s.i. differential ' ' .

for 30 minutes) . The brine was a CaC12 brine weighing 11. 3 lbs. per gallon. The composition and test results are given below in Table I.

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In these examples two types o$ modified starches are compared in a fluid which is loaded with a weighting material. A decline in the water loss e~iciency ~or one typs of modified starch was observed; howsver, this was ;~
overcome by the addition vf calcium lignosulfonate A dry blend o~ the components was prepared and added to a CaCi2 brine weighing 11.3 pounds per gallon.
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This mixture is aged ~or one hour at room temperature. Then the weighting material is added, stirred ~or 5 minutes and `~
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- tested. The compositions and test results are glven in ,i ' -.
TABLE II. ~

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- ~L0S~35'-3 ; EXAMPLES 9-12 These examples demonstrate that the effect of "green" cement as might be found in wellbore fluids in some situations. It can be seen that tha cement caused no substantial detrime-nt to the properties of the fluid.
~; The fluid preparation procedure described above ~or Exam- :
ples 5-8 was employed. ~he brine, howe~er, was ~ NaCl ~ -brine weighing 9.8 pounds per gallon. :
~ The compositions and test results are set out in Table III.

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Claims (2)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An aqueous clay-free thixotropic wellbore fluid for use in subterranean formations in the earth comprising water, at least 1% by weight of a brine forming soluble salt or mixtures of salts, MgO and a heteropolysaccharide prepared by the action of bacteria of the genus Xanthomonas on carbohydrates.

2. The method of treating a subterranean formation comprising injecting into said formation clay-free wellbore fluid comprising at least 1% by weight of a brine forming soluble salt or mixtures of salts, a viscosifying amount of a heteropolysaccharide prepared by the action of bacteria of the genus Xanthomonas on carbohydrates and a stabilizing amount of MgO, contacting said formation with said fluid and returning said fluid to the surface for regeneration and reinjection into said formation.