CA1156444A - Aromatic compound with an aqueous hydrophilic polymer fluid - Google Patents

Abstract

AROMATIC COMPOUND WITH AQUEOUS
HYDROPHILIC POLYMER FLUID
D#76,672-F
ABSTRACT OF THE DISCLOSURE
Disclosed is an improved viscous, aqueous, hydrophilic polymer-containing fluid suitable for injection into porous media such as subterranean petroleum-containing earth formations and an oil recovery method in which the aqueous fluid is injected into the subterranean petroleum-containing formation. The fluid contains an effective amount of an aromatic treating substance, preferrably benzene, toluene, xylene, and low alkyl-substituted benzene or toluene. The improvement resulting from incorporation of this additive in the polymer fluid includes greatly increased resistance to microbial degradation of the polymer, improved screen factor, and improves the injectivity of the fluid.

I

Description

_IELD OF I~ INVENTION
This invention conc~rns an improved viscous fluid and an enhanced oil recovery process using the 1uid. More particularly, this improvement concerns a viscous, aqueous, hydrophilic polymer-containing fluld, the fluid con~aining an aromatic additive which protects the pol~mer from bacterial attack and improves the injectability of the fluid and reduces plugging sometimes experienced when similar fluids are injected into subterranean formations.

BACKGROIJND OF THE INVENTION
Persons skilled in the art of recovering oil or petroleum fxom subterranean formations ordinarily employ so-called primary recovery techniques first, so long as oil may be reco~ered under acceptable economic conditions thereby.
Once primary production is no longer economically feasible, some form of supplemental or enhanced recovery is applied to the subterranean formation. One of the earliest used and most popular forms of enhanced recovery is water injection, in which either fresh water or brine is injected into the ~0 subterranean fonmation to displace or push the residual oil through the formation toward a spaced-apart production well, from which it is recovered to the surface of the earth. Since the viscosity of the oil present in the subterranean formation is usually higher than the viscosity of water or other aqueous fluids injected into the formation, there is a strong tendency for the more mobile (less viscous) aqueous fluid to bypass a substantial portion of the oil. This is sometimes raferred to in the literature as viscous fingering.
The result is that only a portion of the residual oil is displaced by the aqueous fluid.

This problem h~s been recogniæed by persons skilled in the art of oil recovery, an~ various literature references describe method~ for increasing the ability of the flooding medium to displace residual oil. Ik is well known in the art of oil recovery and described in the literature pertaining thereto ~hat incorpora~ion of sufficient ~nount of certain polymeric materials in the aqueous flooding medium to increase the viscosity thereof to a value more nearly equal to or greater than the viscosity of the oil, reduces or eliminates the tendency for the injected aqueous fluid to bypass or finger through the residual oil in the formation.
Many substances have been disclosed in the literature for incorporation in the flooding medium for the purpose of increasing the viscosity of the injected fluid. U.S. Patent

2,827,964 and U.S. Patent 3,039,529 describe the use of high molecular weight, partially hydrolyzed polyacrylamides as thickening agents for a~ueous fluids employed in oil recovery operations. U.S. 3,581,824 describes the use of heteropoly-saccharid~s produced by fermentation of carbohydrates by bacteria of the genus Xanthomonas for the same purpo~e.
It is important to recognize the difference between the effect achieved by injecting a viscous, hydrophilic polymer-containing fluid in~o a forma~ion as contrasted to injecting an agueous fluid containing a surface active agent i.e., a surfactant. The surfactant-containing fluid decreases the interfacial tension between the residual oil and the flooding medium in the flow channels through which the fluid passes, and will therefore reduce the residual oil in the portion of the formation contacted by the injected surfactant fluid. A fluid containing any of ~he hydrophilic 4 ~ ~

pol~mers normally used for viscous floodlng oil recovery methods daes not reduce the interacial tension between residual oil and the injected a~ueous medium, and SG does not reduce the oil saturation in ~he flow channels through which it passes. The purpose for using a hydrophilic polymer-containing fluid is to increase the number o~ flow channnels contacted by the injected fluid, or to improve the volumetric sweep efficiency of the oil recovery method. It is common practice to employ both a surfactan~ solu~ion and a viscous, hydrophilic polymer-containing fluid in an optimum state-of-the-art chemical flooding process, although either may be used alone without the other.
A~ueous fluids containing sufienct hydrophilic polymer to increase the viscosity thereof to a value equal to or greater than the oil viscosity for the purpose of increasing the volumetric sweep efficiency, are commonly referred to in the art as mobility control or mobility buffer fluids. The ability of the variou~ classes of polymers employed in mobility c~ntrol fluid to produce the desired increase in the viscosity of the injected fluid depends on various actors including the salinity of the aqueous fluid present in the formation, the physical and chemical characteristics o~ the formation, and the nature of the residual oil.
It is recognized by persons skilled in the art of enhanced oil recovery processes employing mobility control fluids, that numerous problems are encountered in the use of these fluids. Injectivity problems are sometimes encountered due to improper hydration of the polymer, bacterial growth and other contaminants.

Another important property of an aqueous mobili~y control fluid relating to the flow resistance of the pol~ner fluid through a porous medium such as a pe~neable, subterranean oil-containing earth formation is recogni~ed and a "screen factor" has been defined, which relates to the ability of the fluid to flow under such conditions. The scrPen factor is a measure of the viscoelastic behavior of ~he polymer fluid.
Another serious problem which has been recognized as occuring in the use of all of the hydrophilic polymers descxibed in the literature for use in mobility control fluids, is bacterial degradation of the polymer contained in the fluid, which causes loss in fluid viscosity. It is not unusual for fluids injected into subterranean earth formations for oil recovery purposes to remain in the formation for many months or even years, and so the fluid properties will be adversely affected even though the rate of bacterial decomposition of the polymer is relatively slow.
Many methods have been described in the literature for reducing the problem associated with bacterial decomposition of hydrophilic polymers, but most which have been described heretofore are either of limited effectiveness or are prohibitively expensive.
DESCRIPTION OF PRIOR ART
U. S. 3,410,342 describes the use of organic materials including benzene, toluene, or xylene to stabilize the miscibility of the components of a surfactant fluid.
U. S. 3,800,877 describes the use of aldehydes such as formaldehyde as an oxygen scavenger and bactericide for a polymer fluid.

SUMMARY OF INVENTI~N
The present invention concerns a method of treatiny an a~ueous, hydrophilic polymer-containing fluid with an affective amount of an aromatic material.
The aromatic treating materials afford a very high degree of protection against bacterial decomposition of the hydrophilic polymer, thereby preventing decrease in fluid screen factor and viscosity. '~hP affective aromatic materials include benzene, toluene, ~ylene, and C1 - C5 alkyl-substituted benzene and toluene.

The present invention cencerns an improved aqueous fluid containing a viscosifying amount of a hydrophilic polymer, which fluid exhibits more stable injec~ivity characteristics, and the screen factor and viscosity remain constant over longer periods of time since the fluid is more resistent to attack by bacteria present in oil field brines or from surface contamination than presently-used fluids.
The fluid is especially suitable for use in an oil recovery method in which the aqueous mobility control fluid is injected into the formation for the purpose of increasing the volumetric efficiency of the displacement process. The fluid may be used as substantially the only fluid injected in~o the formation, or it may be used in combination with, preferably immediately aftex injection of, an aqueous fluid containing a surface active agent or surfactant, which reduces the residual oil saturation in the portion of the formation contacted by the surfactant fluid. The fluid injected into the formation may contain at least one surfactant and at least one polymer.

_5_ 4 ~ ~
One preerred class of hydrophilic polymers suitable for use in carrying out the present invention, include ionic polysaccharides such as those available co~nercially which are produced by fermentation of carbohydrates by bacteria of the genus Xanthomonas. Examples of such heteropolysaccharides are those produced by action of Xanthomonas Campestris, Xanthomonas Begonia, Xanthomonas Phaseoli, Xanthomonas Hederae, Xanthomonas Incanae, Xanthomonas Carotae, and Xanthomonas Translucens. Of these, the preferred species is ionic polysaccharide B-1459, which is prepared by culkurring the bacterium Xanthomonas Campestris in NRRL B~1459, U. S. Department of Agriculture, on a well aerated medium containing commercial glucose, organic nitrogen sources, dipotassium hydrogen phosphate and various trace elements. Fermentation is carried to completion in four days or less at a pH of about 7 and a temperature of 28C. Polysaccharide B-1459 is available under the tradename Xanflood~ 9702 from Kelco Company.
Production of this and related heteropolysaccharides is well described in Smiley, K.L. "Microbia Polysaccharide--A
Review", Food Technolo~y 20,9:112-116 (1966) and in Moraine, R.A., Rogovin, S.P., and Smiley, K. L. "Kinetics of Polysaccharide B-14S9 Synthesis", J. ermentation Technolo~y, 44, page 311-132 (1966). Other fermented polymers used for oil recovery such as that produced by the fungus species sclerotium may be used in this invention.
- Another preferred class of hydrophilic polymer which may be employed beneficially in the fluiding process of this invention includes the commercially available, water soluble high molecular weight, unhydrolyzed or partially hydrolyze~ po]yacrylamides having molecular weights in khe range of above 0.2 x 106, preferrably from 0.5 x 106 to 40 x 106, and more preferrably from 3 x 106 to 10 x 106. Co-polymers of acrylamide and acrylic acid within the same molecular weight range, may also be used. If the polymer employed is a partially hydrolyzed polyacrylamide, up to about 70% and preferably from 12 to 45% of the carboxylamid groups are hydrolyzed to carboxyl groups. A number of partially hydrolyzed polyacrylamides and or co-polymers of acrylamide and acrylic acid are available commercially and commonly employed for mobility control buffer fluid formulation. These include, for example, materials marketed by the Dow Chemical Company under the trade name "Pusher 700"
and "Cyanatrol" available from American Cyan~mid.
Nakurally occurring pol~n~rs may also be employed as the hydrophilic polymer in ~his process. Included in this class of effective materials are Guar gum, Locus Bean Gum, natrual starches and derivatives thereo~, cellulose and its derivatives including hydroxy ethyl cellulose.
Any of the above described materials may be employed as the only hydrophilic polymer present in the mobility control fluid utilized in the oil recovery process aspect of this invention. It is well recognized that under certain conditions, improved results are obtained when a combination of two or more of the above-described hydrophilic polymers are utilized in an aqueous fluid for oil recovery purposes, and it is contemplated that this combination of polymers is within the scope of the present invention.
I~ preparing the aqueous polymer-containing fluid according to the process of this invention, one or more of the above described hydrophilic polymers are dissolved in water in any suitable fashion in order to provide an aqueous li~uid having the desired viscosity. In oil recovery processes, it is sometimes desirable to prepare the aqueous fluid in a moderate salinity brine whose salinity is about egual to the salinity of the water remaining in the formatio~
at the time ~he fluid is to be injected thereinto. Since the salinity of the fluid affects the viscosity obtained from any particular concentration of hydrophilic polymer, great care must be taken to ensure that the resulting fluid viscosity is sufficient to provide the desired bene~icial mobility ratio between the injected fluid and the residual oil present in the formation. One ef~ectlve method for preparing fluids for injection into high salinity forma~ions, including processes employing use of surfactant fluid injection, is to prepare the mobility fluid using relatively fresh water, or in water whose salinity is at least significantly less than the salinity o the brine present in the formation at the time the fluids are injected thereinto.
The concentration of polymer mixed with water or brine to form the viscous a~ueous fluid can vary over a fairly wide range, from about 50 part~ per million to about 5 weight percent, although the pre~erred range is ordinarily from about 500 parts per million to about 3000 parts per million. The controlling parameters are the resultant viscosity of the solution, rather than any particular con-centration, since the viscosity produced by addition of the polymer varies with numerous factors. For oil recovery purposes, the controlling factor should be, that the mobility of the mobility buffer fluid is less than the mobility of the residual oil present in the forma~ion under formation conditions. Ordinarily this requires that the viscosity of the polymer fluid be e~ual to or greater than the viscosity of the residual oil, although other factors are well recognized in the literature pertaining to polymer flooding oil recovery mothods, and it is sometimes possible to formulate an aqueous polymer fluid having the desired mobility (less than ~he mobility of the oil present in the formation) even though the viscosity of the polymer fluid i5 somewhat less than the viscosity of the petroleum. The viscosities of polymer fluids commonly employed for oil recovery purposes can range anywhere from several centipoise to several hundred centipoise.
The total volume of polymer solution prepared and injected into a formation in practicing this invention is in the range of from about .05 to 1.0 pore volumes and preferrably from 0.2 to 0.5 pore volumes based on the pore volumes of the oil containing formation to be swept by the oil recovery fluid. Of course, injection of larger amounts of polymer fluid will not decrease the amount of oil recovered, but the increased cost will make the economics of the process quite unsatisfactory. It is common practice to inject one or more slugs of polymer-containing fluid into the formation and to displace that through the formation by injecting field brine or other less expensive drive fluid. It is also recognized that the concentration oE pol~mer may be decreased in a continuous or step wise fashion from the initial value to 0, thereby obtaining continuously efficient displacement of the polymer fluid by the subsequently-injected drive fluid.

The additive incorpora~ed in the pol~mer containingre~istance to bacterial degradation is an aromatic cornpound having the following formula:
R~ ~ 2 ~``J
I

wherein R1, R2, and R3 are each hydrogen or C1 to C5 and preferably Cl - C3 alkyl with the total number of carbon atoms in Rl, R2 and R3 being from 0 to 5 and preferably from 0 to 3. Examples of preferred operable species are:
benzene toluene xylene ethyl benzene propyl benæene propyl toluene butyl benzene butyl toluene The concentration of any one or more of the above described additives should be in the range from 0.001 to 0.2 and preferably from 0.005 to 0.15 percent by volume. The above described additive may be incorporated in the water prior to adding the polymer thereto, or it may be added simultaneously with the polymer, or it may b~ added to ~he fluid ater the polymer has been dissolved and or dispersed in the water. It is understood that the above-stated concentration range exceeds the solubility of some of the aromatic compounds described above, particularly the alkyl-substituted benzenes or toluene. I have found that the amount 4 4 ~
of aromatic compound added to the polymex fluid may exceed the solubility without adverse effects of fluid properties.
The excess aromatic material is dispersed or emulsified in the a~ueous phase. The presence of excess, undissolved bactericide is sometimes an advantage, since loss of aromatic compound ~rom the polymer fluid may occur in the formation, and in such case, the excess aromatic bactericide then dissolves in the fluid, thereby maintaining the concentration of dissolved bactericide sufficiently high ta maintain the bactericidal action.
As mentioned previously, the polymer fluid prepared according to this invention may be injected into the formation via one or more injection wells and displaced away from the wells by injecting field brine or suitable drive fluid, without injecting any other fluids. This process will improve the volume of formation swept by the injected fluid, but will not ordinarily reduce the oil saturation in the pore spaces and flow channels of the formation contacted by the 1uid. This process will, however, by virtue of contacting greater volumes of formation, recover more oil than could be recovered under ordinary circumstances using water injection alone. Still greater oil recovery is possible i the viscous fluid is employed in combination with a fluid which reduces the oil saturation in the portions of the formakion through which the 1uid passes, such as an a~ueous fluid containing one or more effective surfactants, or a miscible fluid such as a hydrocarbon, or an emulsion or micellar dispersion comprising both an aqueous surfactant-containing phase and a hydrocarbon phase, all of which are well described in the literature pertaining to enhanced oil recovery methods. The polymer and suractant may also be incorporated in one 1uid.

4 ~ 4 The invention will be further described by the following examples, which are illustrative o specific modes of practicing the invention but are not intended to be in any way limitative of the scope of the invention which is defined by the appended claims.
FIELD EXAMPLE
For the purpose of illustrating a typical perferred method of applying the process of th~ invention to a subterranean oil containing foxmation, the following field example is described.
A subterranean petroleum-con~aining formation is located at a depth of 4700 feet, and the average thickness of the formation is 38 feet. The porosity is 42% and the permeability is 125 millidarcies. The oil contained in the formation is 20 API gravity crude. This formation has been produced ~y primary production processes until the oil production rate has declined and the water- oil ratio has increased to a point at which further oil production is economically unfeasible.
The salinity of the water present in the formation is approximately 1200 parts per million total dissolved solids including 120 parts per million divalent ions, principally calcium. It is determined that a suitable mobility control fluid for ensuring a favorable mobility ratio between an injected fluid and the residual oil in the formation can be prepared using field water whose salinity is 800 parts pex million total dissolved solids, and having dissolved or dispersed therein approximately 1000 parts per million of a commercially available partially hydrolyzed polyacrylamide. To this is added 1000 parts pex million 4 ~

toluene to prevent bacterial decomposition of the fluid, to ensure a desireable screen factor, and to ensure that no injectivity problems will be encountered during the time the fluid is being injected into the formation.
Although the -tokal field comprises a numbex of ~ive spo~ patterns, only one will be considered for this field example. The wells are located on the corners of a square, each side being approximately 120 feet in length, and with an injection well centered at about the center of each square grid. It is known tha~ the total volumetric efficiency of an oil recovery process using a polymer fluid in this type pattern is about 80%. Accordingly, the pore volume of formation to be contacted by injected fluid in each grid unit 1 s :
120 ~ 120 x 38 x 0.42 ~ 0.8 = 183,859 square feet One pore volume is eguivalent to 1,375,000 gallons of fluid.
The slug size of mobility buffer fluid e~ployed in this test is appro~imately .05 pore volumes or 5 pore volumes pexcent. Accordingly, the volume of the mobility buffer ~luid is ~8,772 gallons. This quan~ity of relatively fresh water (salinity equal 800 parts per million total dissolved solids) is utilized for preparing the fluid. The amoun~ of polymer required to produce an average concentration of 1000 parts per million in this quantity of fluid is 573 pounds.
The same weight of toluene is added to the fluid at the same time the polymer is added, and the fluid is mixed sufficiently to produce a homogeneous fluid.
In this particular application, no surfactant or other oil recovery agent is employed, and the polymer fluid is injected into the formation and followed by injecting ;4~

field hrine of approximately lS00 parts per million total dissolved solids. Brine injection is continued until the ~luid being recovered from the production well is in excess of 99% by volume water, indicating that substantially all of ~he oil that can be recovered economically by this proce~s has been recovered.

EXPE:RIl!~ENTAL SECTION
_ The following laboratory tests were performed to demonstrate the benefits a~hieved by treatment of a~ aqueous polymer-containing fluid with khe aromatic treating compounds according to the process of this invention.
In the first series of experiments, the ability of xylene to inhibit microbial growth in Xanflood polymer was studied. A solution comprising 10,000 gm/m3 (1.0% by weight) 15 Xanflood~, a biopolymer, was prepared in deionized water.
Xylene was added to two samples of the concentrat~d polymer solution, and the concentrate was stored for a pexiod of 5 days at room temperature. The concentrated polymer solution was then diluted wi~h 800 parts per million total solid brine to obtain a polymer concentration o l,000 gm/m3 (1,000 parts per million). The fluid viscosity o~ the diluted ~amples was measured, for the purpose of determining the rate of viscosity loss o~ the polymer fluid which indicates the rate at which the polymer in the concentrate has been decomposed by bacterial action. Three concentrate sclmples were studied, all containing lO,000 gm/m3 polymer, with xylene concentrations of 0, 1,000 and 3,000 parts per million. The following data were obtained:

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TABLE I
INHIBITION OF MICROBIAL GROW'~l IN A POLYMER WITfl ~fYLENE

Viscosity, cp Visco~i~y, ~p vi5c08ity, ~p Con~rol 1,000 gm/m 3tO00 gm/0 Days Aged(NO Additive~ Xy1e~e Xylene O 36.6 37.4 36.2 21.6 36.4 36.8 It can be seen from the foregoing that in only five days, the pol~mer solution without xylene e~perienced a drop in viscosity from 36.6 to ~1.6, a loss of 41% of its initial viscosity. The sample containing 1,000 gm/m3 xylene experienced only negligible, approximately 3.6% lose in viscosity in the same five day interval. The sample containing 3,000 ~m/m3 xylene lost essentially no viscosity, indicating inhibition of microbial attack was complete. It is concluded from this series of tests that xylen~ is a very effective material for inhibiting the degradation of Xanflood~ polymer by microbial action as is evidenced by degradation in viscosity.
Another series of tests were conducted to determine the ef~ectiveness of ~oluene for inhibiting loss of viscosity due to microbial attack on a con~nercially available, partially hydrolyzed polyacrylamide. The polymer investigated was Cyanatrol WF 940S~ a hydrolyzed polyacrylamide available from American Cyanamid Corporation.
The fluid was prepared by dissolving the partially hydrolyzed polyacxylamide in a mixture of produced water and field water, which mixture had a salinity of 3400 parts per million ~otal dissolved solids. The fluid contained 1,000 gm/m3 Cynatrol~. One sample was prepared without a stabilizing additive for use as a control, and another sample contained one cubic cen-timeter toluene per 1,000 cubic ~15-;4~

centimeters of fluid ( equivalent to 1000 parts per million toluene). The viscosity and screen ~ac~or of the fluids were determined ini-tially and again ater the flu:ids had been aged for three weeks at ambient laboratory temperature. The data are contained in Table II below.
TABLE II
THE EFFECTIVENESS OF TOLUENE
AS A BACTERACIDE FOR PARTIALLY
HYDROLYZED ACRYLAMIDE

c~ a~ 6 rDm Inltial After 3 WksInitial After 3 Wks CONTROL 3 32 11.5 17.0 8.9 1000 gm/m 33 32 17.0 16.g Toluene It can be seen from the above data that toluene was essentially completely effective during a three week aging period for stabilizing both viscosity and the screen factor, whereas without additive, the viscosity of an otherwise identical fluid dropped from 32 to 11.5 centipoise, a drop of 64% and the screen factor dropped from 17 to 8.9, a drop of about 48%.
Another series of tests were conducted to verify the effectiveness of toluene as a bactericide for use in combination with partially hydrolyzed polyacrylamide polymer under conditions approximating that which would be experienced in the field. Solutions conkaining 1000 gm/m3 Cynatrol~ and 1000 gm/m3 toluene were prepared in field water whose salinity was approximately 3400 parts per million total dissolved solids. The viscosity and screen factor of the toluene-protected polymer solution was determined initially, as well as after aging 14 days and 30 days at 49C (120F).
The data observed in this series of tests are presented below.

TABL~ III
Days ~gedViscosity Screen at 49CmPa s (cP) at 10rpm Factor o 30.3 14.6 14 30.1 12.8 33.1 1~.9 It can be seen from ~he foregoing data that toluene was essentially completely efective for preserving viscosity and screen factor values of the fluid over the 30 day period at the elevated temperatures, since no loss in viscosity was experienced and in fact a slight increase was obserYed, although this difference is within the ~imits of experimental error in determining viscosity. The screen factor declined only very little from 0 to 14 days, and experienced no loss from 14 to 30 days, indicatiny excellent stability of the screen factox as well as viscosity.
Still another experiment was conducted to evaluate the effectiveness of toluene as an inhibitor to prevent the loss of viscosity, screen factor, and possibly other physical properties as a result of biological degradation of a partially hydrolyzed polyacrylamide polymer. In these tests, several samples of solution were prepared containing 1000 parts per million Cyanatrol~, a commercially available polyacrylamide sold by American Cyanamid. One sample was not treated with a bactericide, to serve as a control to the other experiments. The second sample was treated with 150 gm/m3 of Dowicide B~, a commercially available bactericide sold by Dow Chemical Company for use as a bacterial inhibitor for polyacrylamide. The third was treated with 1000 gm/m3 toluene. The observed data are shown below.

~ 1~5~

TABLE IV
EFFECTIV~NESS OF TOLU~N~ AS A
BACTERICI~E FOR POLYACRYLAMIDE
TIME CONTROL DOWICIDE B~ TOLU~,Na Weeks Viscosity Screen Viscosity Screen Vi~c08ity Screen - Factor Factor Factor 0 30.3 1~.6 37.0 1S.2 2~.5 14.4 1 17.5 ~
4 11.7 3.0 28.1 14.3 30.4 14.0 6 3.3 1.8 2~.7 13.3 27.7 13.4 It can be seen from the above the severe 105s in both viscosity and screen factor of the control sample containing no toluene or other ~actericide indicates th~
severity of the problem of microbial degradation. Dowicide B
provided good skability although the viscosi~y of the sample treated with Dowicide B~ dropped from 37.0 to 28.8, a lose of 22%. A slight drop in screen factor was also observed. The sample ~reated wi~h toluene experienced a drop in viscosity from 28.5 to 27.7, less ~han a 3% decrease. The screen factor similarly declined from 14.4 to 13.4, a decline of less than 7%.
The foregoing indicates that toluene is an extremely effective inhibitor for preventing loss of viscosity and screen factor, as well as deteriation of other physical properties in the polymer solution as result of bacterial attack. While the concentration level for treatment of Dowicide B~ was considerably less than the treatment level of toluene employed, the cost of treating a solution under field conditions with 150 gm/m3 Dowicide B~
would be approximately $135.00 per 1000 barrels of polymer fluid, versus only $23.00 for treating the same volume of polymer fluid with 1000 parts per million toluene.
. . .
Accordingly, it can be seen that the process of this inven~ion provides substantially improved cost effectiveness for treating polymer solutions to prevent loss of physical properties due to bacterial att~ck.
Another ~eries o~ tests were performed to determine the effect of variations in concentration o~ toluene on its effectiveness for stabilizing Xanflood~ polymer against loss of filterability and viscosity as a result of microbial attack. All of the solutions contained 1000 parts per million polymer prepared in dionized water, plus the indicated amount of toluene. The samples were aged at room temperature for the periods indicated a~d the presence of microbial growth was detected gualitatively based on visual observation, odor, etc. The data are contained in Table V
below.
TABLE V
TOLUENE CONCENTRATION EFFECT
Concentration of Toluene Days Aged gm/m3 3 10 24 0 (control) + ~ +
+ + +
+ + +
100 + ~ +
500 ~/_ +

2000 _ _ _ 3000 _ _ _ = microbial growth observed - = no visible sign of microbial growth It can be seen from the data contained in Table V
above that toluene is inefective under these conditions o~
polymer concentration and salinity below about 500 parts per million. Above 500 parts per million, toluene was ~uite effective for preventing microbial growth over the time period of these tests. The minimum concentration of toluene needed for bactericidal action depends on the particular polymer and brine, and protection is observed under other conditions at concentrations far below 500 partg per million~
It is encouraging that over treatment causes no adverse effects, although ordinarily the pre~erxed method of applying the invention is to use only as much toluene as is necessary to achieve the desired protection against microbial decomposition under the conditions and or the time ~or which the polymer containing fluid will be present in the orma~ion.
Another series of test~ were performed to compare performace of various level6 of toluene treatment in fresh water and brine (111,000 ppm total dissolved solids.) In these tests, the toluene was added to a 1% polymer concentrate, and aged 24 days. Diluted samples were then prepared and the physical properties measured. The data are given in Table VI below.

TABLE VI
COMPARISON OF EFFECTIV~NESS
OF TOLVENE AS BACT~RIClDe IN
~R~SH WAT~R AND BRIN~ POI,~MRR SOLUTION
-Fresh Wat~r Brine __, Toluene Days Filt.1 Visc.2 Apr.3 Filt.1 Visc.2 Apr. 3 - ~ Content Aged 0 0 8631.3 b 69 35 b 0 24 -- 1.4 a -- 1.6 a 101000 24 13330.9 c 124 36 c 2000 2~ -- -- 116 35.6 c 3000 24 -- -- -- 90 36.9 d 6000 24 -- - - 64 35 e 1 Filt. - volume (cc) filtered in 300 sec through 0.8 micron filter wi~h 20 psi pressure 2 Visc. - viscosity measured at 7.3 sec 1 (6 rpm) at am~ient temperature

3 Apr. - appearance-a = precipltate b = slightly cloudy c = very clear d = clear e = ~loudy It can be seen from the foregoing that viscosity and filterability, 1000 parts per million toluene is adequate concentration to prevent loss of physical properties due to microbial a~tack in fresh water. In the pol~mer fluid prepared in brine, excellent st~bilization of viscosity characteristics occurred in all four treating levels. The cloudy appearance of the brine 1uids treated with 6000 parts per million toluene suggests that this treatment level is excessive for these conditions.
.

CONCLUSION
The use of from 10 to 2000 and preferably from 50 to 1500 parts per million of an aromatic treating agent, preferably benzene, toluene, xylene, or short alkyl chain substituted benzene or toluene efectively reduces lose of

4 ~. 4 viscosity and screen fac-tor of a hydrophilic pol~mer containing solution due to microbial action under relatively long term aging conditions.
While this invention has been described in terms of S a number of illustrative embodiments, this is done for the purpose of complete disclosure and is not intended to be in any way limitative or restrictive of the scope of the inven~ion. Many variations will become apparent to persons skilled in the art of oil recovery, without departing form the true spirit and scope of this invention. It is m~
intention that my invention be limited and restric~ed only by those limitations and restrictions appearing in the claims appended immediately hereinafter below.

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Claims (14)

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

1. A method of recovering oil from a subterranean, permeable, oil-containing formation penetrated by at least one injection well, and by at least one production well, both wells being in fluid communication with the formation, comprising injecting an aqueous fluid comprising water containing a viscosifying amount of a hydrophilic polymer, wherein the improvement comprises:
incorporating in the fluid from 10 to 2000 parts per million of a bactericide comprising an aromatic treating compound having the following formula:

wherein R1, R2 and R3 are each hydrogen or C1 - C5 alkyl with the total number of carbon atoms in R1, R2 and R3 being from 0 to 5.

2. A method as recited in Claim 1 wherein the concentration of aromatic bactericide is from 50 to 1500 parts per million.

3. A method as recited in Claim 1 wherein R1, R2 and R3 are each C1 - C3 alkyl.

4. A method as recited in Claim 1 wherein the aromatic treating compound is selected from the group consisting of benzene, ethyl benzene, propyl benzene, butyl benzene, toluene, ethyl toluene, butyl toluene, propyl toluene, xylene and mixtures thereof.

5. A method as recited in Claim 4 wherein the treating compound is toluene, xylene or a mixture thereof.

6. A method as recited in Claim 4 wherein the fluid also contains a surfactant.

7. A method as recited in Claim 1 wherein the hydrophilic polymer is partially hydrolyzed polyacrylamide, a co-polymer of acrylamide and acrylic acid, a polysaccharide, a naturally occurring polymer, or a mixture thereof.

8. A fluid composition suitable for injecting into porous earth formations comprising a viscosifying amount of a hydrophilic polymer and from 10 to 2000 parts per million of bactericidal aromatic treating agent having the following formula:

wherein the R1, R2 and R3 are each hydrogen or C1 - C5 alkyl with the total number of carbon atoms in R1, R2 and R3 being from 0 to 5.

9. A fluid composition as recited in Claim 8 wherein the concentration of the aromatic treating agent is from 50 to 1500 parts per million.

10. A fluid composition as recited in Claim 8 wherein the aromatic treating agent is benzene.

11. A fluid composition as recited in Claim 8 wherein the aromatic treating agent is toluene.

12. A fluid composition as recited in Claim 8 wherein an aromatic treating agent is xylene.

13. A fluid composition as recited in Claim 8 wherein the hydrophilic polymer is polyacrylamide, partially hydrolyzed polyacrylamide, a co-polymer of acrylamide and acrylic acid, a biopolymer, a natural gum, or a mixture thereof.

14. A fluid composition as recited in Claim 8 wherein said fluid additionally contains a surfactant.