CA1075596A - Oil recovery process employing a hydrocarbon slug and a thickened aqueous surfactant slug - Google Patents

Abstract

SLUG AND A THICKENED AQUEOUS SURFACTANT SLUG

Abstract of the Disclosure The recovery of oil by surfactant waterflooding involving the injection of a hydrocarbon slug followed by a thickened aqueous surfactant slug. The hydrocarbon slug is injected into the reservoir in an amount of at least 0.02 pore volume and has a viscosity less than that of the reservoir oil. The thickened surfactant slug comprises an aqueous solution of a surfactant system which includes an organic sulfonate surfactant and which functions to increase the viscosity of the aqueous liquid to a value at least as great as the viscosity of the reservoir oil while decreasing the oil-water interfacial tension. The surfactant slug may include one or more organic sulfonates, either alone or in mixture with a water-soluble aliphatic alcohol.

Description

~-75596 9452 Back round of the In~ention This invention relates to the reco~ery of oil from subterranean ~il reservoirs and more particularly to improved waterflooding operations involving the injection ` 5 of a hydrocarbon solvent followed by the injection of a thickened aqueous surfactant slug.
In the recovery of oil from oil-bearing reservoirs, it is usually pos~ible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces presen~ iff the reservoir. Thus a variety of supplemental recovery techniques has been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used ~upplemental recovery technique is waterflooding which involves the injection of water into an oil-bearing reservoirO As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
; It has long been recognized that factors uch a6 the interfacial tension between the lnjec~ed water and the - reservoir oil, the relative mobilitie~ of the reservoir oil and in~ected water, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amou~t of oil recovered by waterflooding.
Thus it has keen proposed to add surfactants to the injected water in order to lower the oil-water interfacial tension and/or to alter the wettability characteristics of the - ' ~6 9452 reservoir rock. Also, it has been proposed to add thickening agents- to all or part of the injected water in order to increase the viscosity thereof, thus decreasing the mobility ratio between the injected water and oil and ~mproving the sweep efficiency of the waterflood.
Processes which involve the injection of aqueous surfactant solutions in order to reduce the oil-water interfacial tension are commonly referred to as low tensi~n waterflood ng techniques. To date one Qf the more promising low tension waterflooding techniques involves the injection of aqueous solutions of petroleum sulfonates within a designated equivalent weigkt range and under c~ntrolled conditions of salinity. For example, in a paper by W. R. Foster entitled "A Low-Tension Waterflooding Process", Journal of Petroleum Teehnology, ~ol. 25, Feb. 1973, pp. 205-210, ~here is disclosed a procedure which involves the~sequential injection of a protective slug, a surfactant slug~ and a mobility control 81ug.
The surfactant slug comprises an aqueous solution -20 of petroleum sulfonates having an average molecular weight within the range of 350-500 in concen~rations ranging fr~m about 1.0-3.0 ~eight percent. The 6urfactant slug contains sodium chloride in a concentration, typically about 1.0 to

2.0 weight percent, which will promote the desired low inter~acial tension between the injected water and the reservoir oil. m e subsequently injected thickened water slug contains a viscosifier such aæ a water-soluble biopolymer . ' - . ~

.
9451 in a graded concentration in order to provide an initial viscosity greater than the viscosity of the re~ervoir oil and a terminal viscosi~y near that of water. This mobility control slug has a lower sodium chloride concentra~ion ~han S the surfactant slug. This somewhat lower salini~y functions to increase the desorption of the previously adsorbed surfactant to move the surfactant through the reservoir by a chromatographic-desorption process.
Various modifications of, or alternatives to, surfactant waterflooding involve the injection of a surfactant and a hydrocarbon 81ug or the injection of surfactants in both a hydrocarbonaceous solution and an aqueous solut~on. For example, U.S. Patent No. 3,468,377 to Dunlap et al. discloses the injection of an aqueous solution of petroleum sulfonates having a median molecular weight within the range of about 375 to about 430. The aqueous surfactant solution may be - preceded by a hydrocarbonaceous solution of surfac~ants in a ~olume of about one-tenth of to about equal to the volume - of the aqueous solution with the total volume of the hydrocarbonaceous and aqueous solution being from about 0.01 to about 0.2 pore volume. U.S. Patent No. 3,491,834 to Ahearn et al. discloses the in~ection of a nonpolar (hydrocarbon) slug containing a preferentially oil-soluble sulfonate surfactant followed by a polar (aqueous) slug containing a somewhat lower molecular weight sulfonate which is preferentially water-soluble. The size of the nonpolar slug is said to be between 0.5 percent and ' ~ `": ~

9452 20 percent of the reserv~ir pore volume and preferably between 2 percent and 10 percent. The polar slug varies from 0.5 percent to 100 percent of ~he pore volume, prefierably from 25 percent to 75 percent, and may contain S a thickening agent.
U.S. Patent No. 3,865,187 to Carlin et al.
discloses an oil recovery process which involves an ; emulsification mechanism resulting from the injection of a hydrocarbon solvent containing a mono-unsaturated secondary alcohol followed by an aqueous solution containing a sulfate salt of a fatty alcsho~. Each of the respective slugs varies in size from about 5 percent to about 50 percen~ reservoir pore volume with the alcohol present in a concentration within the range of 0.1-10 percent by weight and the alcohol L5 sulfate being present in amo~nts from abo~t 0.1 to about ` 2.0 percent by weight. The aqueous slug may be ollowed by water containing a thickening agent in an amount from about 0.01 to 0.5 weight percent. Another process disclosed in U.S. Patent ~o. 2,669,306 to Teter et al. involves the injection of a liquefied normally gaseous hydrocarbon such as propane, followed by the injection of drive water. The patentees disclose that recovery of hydrocarbons may be impro~ed by * e addition of surface-acti~e agents selected so as to avoid emulsification difficultles.
~anadia~ Patent Application No. 284,548 discloses an i~prcved waterflooding process involv~ng the sequential ' "

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9452 injection of a relati~ely low viscosity hy~rocarbon slu~, a surfactant slug, and a mobility control slug. The hydrocarbon is in~ected in an amount withi~ the range of 0.01-0.04 pore volume followed by the surfactant slug in an amount within the range of 0.0~-0.3 pore ~olume and which contains a surfactant in-an amount within the range of 0.5-4.0 weight percent. The surfactant slug is followed by an aqueous mobility control slug, at ~east a portion of which has a viscosity at least as great as the viscosity of the hydrocar~on slug.
. Canadia~ Patent Applic~tion No. 278312 (corresponding rO ~.S.
Patent No. 4~042030) discloses an improved waterflooding process --in which at least a portion of the injected water is thickened by employing an alkylaryl sulfonate surfactant having an average molecular weight within the range of 350-500 in combination with a water-soluble C4-C6 aliphatic alcohol ha~ing a hydrocarbon chain link of at least 3 carbon atoms. The thickened aqueous liquid exhibits a monovalent salt sali~ity and a surfactant concentration within the ranges of 0.5-3.0 ~eight percent and~0.5-4.0 weight percent, respecti~ely, and more specifically9 within the range of 0.8-2.5 and 0.5-3.0 weight percent, respectively. m e ; aliphatic alcohol is present in concentration such that the ratio o~ the sulfonate surfactant to the sum of the amount of sulfonate surfactant and alcohol is within the range of 0.3-0.8.

, . , ' 452 . Another waterflooding process involving the ~- injec~ion of a thickened ~urfactant ~olution is discl~sed . in our co-p ~ lng Ca~dlz~ P~e~t Appl~cat~n ~0. 29~,930 f~led ~ current~y ~th ehe pre~ t appl~cation, ~ to~ ~o~ep~ Geor~e Sa~ , Jcrr~
S Wai~e, and Ralph F. Burdyn, entitled WATERFL~OD~NG
EMPLOYING MIXIURES OF SULFONATE SURF~CIANTS~
This application discloses the in3ection of a vi~cous susfactant ~lug . oontaining a petroleum sulfonate having a relatively .
. 10 broad lecular weight distribution and a synthetic sulfonate .
having a relatively narrow m~lecular weight distr~but~on -' which interact synergistically to thicken ~he aqueous liquid . for mobility control purposes. This multicomponent surfactant : ~ system may optionally contain a water-soluble C3-C6 aliphatic . 15 alcohol.
` SummarY of the Invention - In accordance with ~he present invention, there i8 provided a new and impro~ed waterflooding process which involves the ~eque~tial injection of a hydrocarbon slug and .j ., .
- 20 a thickened surfactant slug. In carrying out the ~nvention, a hydrocarbon having a ~iscos~ty less than that of the .. .
resexvo~r oil iB injected into the seservoir vi- a ~uitable injection system in an Emcunt of at least 0.02 pore volume.
. i Subse~uent to the i~ection of the hydrocarbon ~lug, a 2~ thickened aqueous surfactant slug is inj ected. me surfactant slug contains a surfactant system comprising an organic sulfonate surfactant which functions to incxease the viscosity ~ .

1~75S96 of the aqueous liquid to a value at least as great as the viscosity of the reservoir oil while decreasing the interfacial tension between the aqueous liquid and the reservoir oil. Subsequent to the injection of the thickened aqueous surfactant slug, an aqueous flooding medium is injected into the reservoir in order to displace reservoir oil to a spaced production system from which the oil is recovered Description of Specific Embodiments The aforementioned Canadian Patent No. 1,053,11~7 discloses that the injection of a low viscosity hydrocarbon slug in conjunction with the injection of a surfactant slug and subsequent mobility control slug decreases the surfactant utilization necessary to achieve satisfactory oil recovery and lessens the maximum water viscosity needed for effective mobility control. ~n accordance with the present invention, roughly equivalent oil recovery without excessive surfactant utilization can be achieved through the injection of a hydrocarbon solvent followed by a thickened surfactant slug but without the need for a separate mobility control slug.
The initially injected hydrocarbon slug may take the form of any suitable hydrocarbon which exhibits a viscosity under reservoir conditions which is less than -the viscosity of the reservoir oil. Suitable hydrocarbons are disclosed in the aforementioned Waite et al. Canadian appli- :
cation and include low to intermediate molecular weight alkanes such as propane, hexane, and decane as well as relatively low viscosity crude oils and hydrocarbon mixtures such as found in liquefied petroleum gas (LPG). The hydrocarbon slug is injected in an amount of at least 0.02 pore volume.
It usually will be desirable to limit the hydrocarbon slug to a maximum size of 0.2 pore volume for reasons of economy.
The thickened surfactant slug may contain any suitable surfactant system which includes an organic sulfonate and which functions to increase the viscosity of the injected aqueous liquid to a value equal to or greater than the viscosity of the reservoir oil. While the relationship between oil-water interfacial tension and oil displacement is not well defined, a pronounced reduction in oil-water interfacial tension, normally to a value of less than 0.1 dyne per centimeter, is usually required for significant oil recovery. Preferably the surfactant system is one which will reduce the oil-water interfacial tension to a value of .005 dyne per centimeter or less although , interfacial tensions of up to .04 dyne per centimeter can produce 100 percent oil recovery.
One type of surfactant system which may be employed in carrying out the present invention comprises a mixture of an organic sulfonate surfactant and a water-soluble aliphatic alcohol which interact synergistically to produce a thickening effect. Preferred surfactant systems of this type are disclosed in Canadian Patent No. 1,053,147.
The sulfonate surfactant is an alkylaryl sulfonate having an average molecular weight within the range of 350-500.

107559~;
The alkylaryl sulfonates may be synthetic sulfonates such as those derived from sulfonation of products such as keryl benzenes or they may be petroleum sulfonates derived from sulfonation of petroleum oils or petroleum oil fractions.
The aliphatic alcohols employed in conjunction with the alkylaryl su~fonate contain from 4 to 6 carbon atoms and have a hydrocarbon chain link (the chain link of the hydrocarbon portion of the alcohol molecule exclusive of the carbinol group) of at least 3 carbon atoms. The sulfonate and alcohol components are employed in relative amounts such that, the ratio a of the amount of the sulfonate surfaclt'an~ `to'`'thé-'~sum--~ofithe~'sù'lfonate'surfactant a~d ~t~'e7 alc'oho'l'~c$om~onen-'t is within the range of 0.3-0.8.
The sulfonate surfactant is employed in a concentration within the range of 0.5 4.0 weight percent in an aqueous liquid having a monovalent salt salinity within the range of 0.5-3.0 weight percent as explained in greater detail in the aforementioned Canadian Patent No.
1,053,147. Examples of alkylaryl sulfonates which may be employed include the petroleum sulfonates "TRS 10-80"*
available from the Witco Chemical Company and "Stepan 107"**
available from the Stepan Chemical Company. Synthetic alky1aryl sulfonates useful in carrying out this embodiment of the invention include the monoethanolamine alkylaryl sulfonate available from the Exxon Chemical Company under the trademark "FA 400" and the sodium alkyl orthoxylene sulfonate available under the trademark ~Trademark **Trademark : :

"Synacto 426" from Esso Chemie France, an affiliate of the Exxon Chemical Company. Exemplary of the alcohols which may be employed in combination with the alkylaryl sulfonates includes isobutyl alcohol, n-butyl alcohol, and the amyl and hexyl alcohols such as n-pentanol and n-hexanol.
Normally the greatest thickening effect is achieved through the use of the but~13,alcohols, particularly n_butyl alcohol.
As disclosed in Canadian Patent No. 1,053,147, the thickening action of the surfactant-alcohol system is time and temperature dependent in the sense that ~t occurs upon aging of the system in aqueous solution and further in the fact that the thickening process may be accelerated by the application of heat. The thickening effect is also salinity dependent in the sense that the viscosi~y of the surfactant-alcohol system may be increased or decreased ; by varying the salinity within the range of 0.5-3.0 weight percent. Changes in viscosity may also be effective by varying the ratio 0 within the range of 0.3-0.8 For a more detailed description of these thickened surfactant slugs, reference is made to U.S. Patent No. 4,042,030, issued May 24, 1976.
Another surfactant system which is preferred for use in carrying out the present in~ention comprises a '~ mixture of a petroleum sulfonate and a synthetic alkyl or alkylaryl sulfonate as disclosed in the aforementioned Canadian application Serial No. 298,930. The petroleum sulfonate has a --11_ 10~5596 relatively broad molecular weight distribution and the synthetic alkyl or alkylaryl sulfonate has a moaecular weight distribution which is narrower than that of the petroleum sulfonate. These sulfonates are employed in concentration such that the ratio of the petroleum sulfonate to the synthetic sulfonate is within the range of 1: a-l 1 . Preferably the petroleum sulfonates and the synthetic alkyl or alkylaryl sulfonates have average molecular weights within the range of 350-500. Suitable synthetic sulfonates include the alkylaryl sulfonate Synacto 426, previously identified, and a sodium polybutene sulfonate available from Amoco Chemicals Corporation under the trade mark "Amoco Sulfonate 151". "Amoco Sulfonate 151"
has an average hydrocarbon chain length of about C21-C23 and an average molecular weight of about 400 to 420.
Suitable petroleum sulfonates include "TRS 10-80", pre~ ously identified, and "Petrostep 420"-:~ available from the Stepan Chemical Company. "Petrostep 420" is a sodium petroleum sulfonate derived by sulfona-tion of a gas oil fraction.
It has an average molecular weight of about 420 and a molecular weight distribution ranging from about 200 or less to about 600 or more. The petroleum sulfonate-synthetic sulfonate system may optionally contain a water-soluble aliphatic alcohol containing from 3 to 6 carbon atoms as disclosed in Canadian application Serial No. 298,930.
The alcohol is not an essential component since the synergistic thickening effect is achieved through *Trademark , IO~SS96 the use of the petroleum sulfonate and synthetic sulfonate in the relative concentrations described previously. However, the alcohol is useful in attaining maximum oil displacement particularly where the surfactant ratio approaches the upper end of the range 1:3_1:1 or where the surfactant solution is ont at optimum salinity. It is preferred to employ the alcohol in an amount to provide a surfactant-alcohol ratio 0 within the range of 0.3-0.8 as described in Canadian Patent No. 1,053,147.
The petroleum sulfonate and synthetic alkylaryl sulfonates are employed in any suitable concentrations provided that the ratio of the petroleum sulfonate and - the synthetic sulfonate is maintained within the range previously described. Normally, the total surfactant concentration will be within the range of 1.0_5.0 weight percent and preferably within the range of 1-2 weight ; percent. In order to obtain good oil displacement without excessive surfactant utilization the petroleum sulfonate~
synthetlc sulfonate surfactant system~ 7 S employed in an aqueous liquid exhibiting a monovalent salt salinity within the range of 1.5-4.0 weight percent. Where the salinity is ~~ near the upper end of this range, it is desirable to use a i?
~ relatively concentrated surfactant solution of about 3 to ' 5 weight percent in order to attain adequate thickening of the water for good mobility control. Preferably the salinity of the surfactant solution is within the range of 1.5-3.0 weight percent, particularly where somewhat :
i' :

-13_ lower surfactant concentrations of 1.0-2.0 weight percent are employed. For a further description of the use of petroleum sulfonate-synthetic sulfonate mixtures in formulating the thickened surfactant slug, reference is made to the aforementioned Canadian application Serial No.
298,930 of Savins et al.

Surfactant systems other than the preferred systems described above can be employed in formulating the thickened surfactant slug. For example, the petroleum sulfonate, "Petrostep 420", and the synthetic alkylaryl sulfonate, "Synacto 426', exhibit a viscosifying effect when used alone, even in the absence of an alcohol, under certain carefully controlled conditions of salinity and surfactant concentration. Tables I and II present a summary,of the viscosity and interfacial tension characteristics for "Petrostep 420" and "Synacto 426", respectively, at different surfactant concentrations and salinities. In each of Tables I and II, the first column indicates the salinity ; 20 of the surfactant solution and the remaining columns indicate the viscosities, , in centipoises and the interfacial tensions, , in millidynes per centimeter as measured for solutions containing surfactant concentrations of 1.0, 1.5, 2.0, and 4.0 weight percent. The legend "dd"
in Table II indicates that the interfacial tension measurements were not obtained because the oil drop dispersed or disappeared and the legend "T" in Table I indicates that the surfactant solution was too turbld to see the oil drop and thus obtain an interfacial tension measurement. The viscosity measurements set forth in Tables I and II were obtained at 1~.7~sec~

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C~ 1 u~ou)Ou~o l~1 " ou~ou~o Z ~l ~ Z 21 9452 Fro~ an examination of the data presented in Tables I and II, it can be seen that the use of the petroleum sulfonate or the synthetic alkylaryl sulfonate can produce significant thickening within narrow regions of surfactant concentration and salini~y. For exæmp~e, the Petrostep 420 in a solution of 1.5 weight percent sodium chloride showed viscosities of 8.8, 4.4, and lO centipoises at concentration~
of 1, 1.5, znd 2.0 weight percent, respectively. Howe~er, at a surfactant concentration of 4.0 weight percent a ~ignificant increase in viscosity occurred in the 1.5 weight percent sodium chloride solution. Even higher viscosities were obser~ed at salinities of 2.0 and 2.5 weigh~ percent.
Similar viscosity increases within narrowly defined ranges of salinity and surfactant concentration were observed for the synthetic alkylaryl sulfonate, Synacto 426. For example, a significant viscosifying effect was observed for a surfactant concentration of 1.5 weight percent in a 2.5 weight percent sodium chloride solution and at a surfactant concentration of 2.0 weight percent in a 1.5 weight percent sodium chloride solution. For the more concentrated 4 percent surfactant solution, signif;cant viscosity yields were observed within the range of 1.0-2.0 weight percent. In many cases the single component surfactant systems produced interfacial ~ensions which were somewhat marginal in terms of optimum oil displacement. However, it is probable that the systems where the interfacial tension measurement could no~ be obtained because the oil drop dispersed or disappeared actually produced interfacial tensions which were low enough for maximum oil displacement.
The results achieved by the present invention in terms of oil displacement and surfactant requirements are illustrated by laboratory oil displacement tes~s-c~rried out on a crude oil for which the displacement efficiency by conventional surfactant flooding was relatively poor. This crude oil is the same as that employed in the tests summarized in Table I of the previously identified Canadian application No. 284,548 and thus the results of the displacement tests described hereiln may be compared with those described ; in Canadian application No. 284,548. The crude oil was employed in two forms, one a "stock tank oil" having very little dissolved gases therein and the other a "separator oil!' recovered from an oil gas separator and thus containin~
light hydrocarbons. The stock tank oil exhibited a pour point of approximately 22 C. and viscosities at 25 C., 38 C., and 54 C., and 60C. of about 80, 12, 7, and 6 centipoises, respectively. The viscosity of the separator oil was not measured but its viscosity characteristics appeared to be similar to those of the stock tank oil. The surfactants employed in these displacement tests were the petroleum sulfonates "TRS 10-80" and "Petrostep 420", and the synthetic alkylaryl sulfonate "Synacto 426". Alcohols used were isobutyl alcohol, n-butyl alcohol, and hexanol. The hydrocarbon banks employed in the displacement experiments were formulated from propane or hexane. The test parameters and results for these oil displacement experiments are set forth in Table III. ~ -; 11D75596 . 9452 E~ 0~ o o~
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~i ~ C~ ~I C~ l ~ O N ~ o o 0 J~ r~ i N C~i 9452 The displacement experiments were performed in plastic or glass tubes having inside di2meters of about 1/4 to 5/16 inch. The tubes were 3 feet long except in the case of Run 12 where the tube had a length of 6 feet.
In each tube run, the tube was packed with unconsolidated Berea sand and then saturated with water containing 2.0 weight percent sodium chloride. The crude oil was then flooded into the tube until ~he effluent from the tube contained no ~ater in order to arrive at an initial oil saturation. A waterflood ~hen was simulated by injecting water until no more oil was produced from ~he tube in order to arrive at a waterflood residual oil saturation.
Run No. 2 was not preceded by a separate simulated waterflood, as explained hereinafter.
After injection of the initial water, displacement experiments were carried out employing a thickenéd surfactant ; 81ug with and without the injection of a prior hydrocar~on 61ug. In Runs 1-4, the displacement experiments were carried out at a temperature of 60 C. and in the remaining runs at a temperature of 35 C. In Run 1, the thickened surfactant slug comprised 3 weight percent TRS 10-80 and

3 weight percent isob~tyl alcohol. In Runs 2, 3, and 4, the s~rfactant slug contained 4.0 weight percent TRS 10-80 and 2.0 weight percent isobutyl alcohol~ In the remaining runs, Runs 5~12, the surfactant system employed was composed of Petrostep 420 and Synacto 426, each in a concentration of 0.75 weight percent to provide a total surfactant -9452 concentration of 1.5 weight percent and n-butyl alcohol in an amount of 0.75 weight percent.
In Table III, ~he second and third columns set forth the salinity and pore volume amount, respectively, of the water injected immediately ahead of the hydrocarbon slug or, where no hydrocarbon slug was employed, ahead of the surfactant slug. In Runs 1, 3, and 4, the - pre-surfactant waterflood involved the injection of about 2-l/2 pore volumes of 1.2 weig~t percent sodium chloride `
solution (not shown in Table III) which was followed by an aqueous solution of 0.8 weight percent sodium chloride, 0.6 weight percent sodium carbonate and 0.2 weight percent sodium tripolyphosphate in the pore volume amounts indicated in the third column. Run 2 was carried out at the completion of Run 1 in the same tube. Thus, at the completion of Run 1, 0,12 pore volume of an aqueous solution of 0.8 weight percent sodium chloride, 0.6 weight percent sodium carbonate, and 0.2 weight percent tripolyphosphate was injected followed by injection of the thickened sur~actant slug. With respect to Runs 5-12, the pore volume amount and salinity of the pre-surfaetant waterflood are as shown in Table III. In Run 5, the simulated waterflood contained 1.6 weight percent sodium chloride, and O.l weight percent esch of sodium carbonate and sodium tripolyphosphate. In the remaining uns, Runs 6-12, the simNlated waterflood contained 2.0 weight percent sodium chloride and 0.1 weight percent each of sodium carbonate and sodium tripolyphosphate The .

.. .

. .

; 9452 fourth column sets forth the pore volume amount of the hydrocarbon slug employed. In Run S, the hydrocarbon slug was composed of propane and in the remaining runs, Runs 6-12, - the hydrocarbon slug was hexane. Columns 5, 69 and 7 characterize the thickened surfactant slug in terms of pore volume amount, æalinity, and viscosity, respectively. In Run 4, the aqueou~ surfactant solutions containing the TRS 10-80 and isobutanol were injected in two parts. The first exhibi~ed a viscosity of 170 centipoises and was injected in an amount of 0.18 pore volume. This was followed by 0.32 pore volume of thickened surfactant solution having a viscosity of 320 centipoi&es. In Runs 1-4, the surfactant slug con~ained ; 0.6 and~0.2 weight percent of sodium carbonate and sodium tripolyphosphate, respectively, plus 0.8 weight percent sodium chloride to provide a tot~l salini~y of 1.6 weight percent. In Runs 5-12, the surfactant 31ug contained 0.2 weight percent each of sodium carbonate and sodium tripolyphosphate plus sufficient sodium chloride to provide the total salinity indicated in column 6.
Run 3 involved the continuous injection ~f a thickened surfactant slug in a total pore volu~e amount of 1.38. The remaining runs involved the injection of a limited pore volume ~mount of thickened surfac~ant which was then followed by a drive water injected in such amounts as necessary to carry the run to a concluding point at which ' - no fur~her oil was recovered. The sodium chloride concentration of the driving fluid is set forth in column 8 ,. . .. .

~ .

9452 With the exception of Runs 5 and li, the driving fluid contained no other additives. In Run 11, the first 0.1 pore volume of the drive water contained 0 7~ weight percent n-butyl alcohol and, in Run 5, 0.75 weight percent n-butyl alcohol was present ~hroughout the drive water.
Column 9 of Table III presents the percent oil recovery, R, at the conclusion of the displacement run and column 10 sets forth the remaining residual oil saturation~
The percent oil recovery is calculared as the percentage of waterflood residual oil recovered and thus is repreæentative of tertiary oil recovery. Column 11 in Table III sets forth the amount, ST, of surfactant employed in terms ~f milligrams of surfactant per gram of sand. The last column pre~ents the amount of surfactant employed in milligrams per græm as normalized to reflect 100 percent oil recovary. This normalized amount, Sr, is derived at by dividing the amount of surfactant, ST, in milligrams of surfactant per gram of sand by the decimal equivalent of the percent oil recovery.
- From an examin~tion of the data set forth in Table III it ca~ be seen that the injection of the thickened ~surfactant slug in fractional pore volume amounts resulted in rela~ively low oil recoverieæ. Run No. 1 which involved the injection of 0.13 pore volume of surfactant slug achieved an oil recovery of only 28 percent. Run 4 which involved the injection of a total amount of thicklened surfactant slug of 0.5 pore volume resulted in a recovery of 60 percen~. Only in the case of ~un 3 which involved ` ~075596 .

9452 the continuous injection of a thickened surfactant solution was 100 percent oil recovery achieved.
The displacement Runs 5-12 simulating the practice of the present invention generally produced significantly higher oil recoveries and required much lower surfactant utilization and lower viscosi~y for effective mobility control. Further, only a relatively small pore volume amount of the surfactant slug is required to accomplish both microscopic and macroscopic oil displacement. In RNn 5, 100 percent oil recovery was achieved employing only 0.08 pore volume of the thickened surfactant slug. However, the slug was displaced by water containing ~utanol throughout and this probably acted by desorption of previously adsorbed surfactant to produce a somewhat larger pore volume amount of thickened aqueous solution wit~in the sand pack. I~
; will be noted in this regard that Run 6 which involved the in~ec~ion of 0.09 pore volume of surfac~ant slug resulted in a tertiary oil recovery of 43 percent. While recovery here was probably limited by ~he amount of surfactant available, as well as by the sizè of the thickened surfactant slug, the data indicates that the ~hickened surfactant slug should be employed ~n an amount of at least 0.1 pore volume unless relatively high surfactant concentrations on the order of 4 or 5 percent are employed. Preferably, the thickened sur~actant slug is employed in an amount of at least 0.2 pore volume and not more than 0.5 pore volume. As noted previously~ the thickened surfactant slug has a viscosity at least as great as that of the reservoir oil. In mo~t cases the surfactant slug viscosity will be within the range of 1 to 4 times the viscosity of the reservoir oil.
The driving fluid injected immediately after the surfactant slug preferably exhibits a relatively low salinity of one-half or less than that of the surfactant slug in order to enhance the chromatographic movement of the surfactant components through~the formation. This low salinity drive water may be injected in an amount within the range of 0.5 to 1.5 pore volume and may contain an aliphatic alcohol of the type employed in formulating the thickened surfactant slug as described previously. The low salinity water-is followed by any water which is lcoally available and not incompatible w~rth the formation. This driving fluid is injected in such amounts as is necessary to carry the process to conclusion.
The present invention may be carried out utilizing injection and production systems as defined by any- suitab~e arrangement- of wells. One well arrangement commonly used in waterflooding operations and suitable for use in carrying out the present invention is an integrated five-spot pattern of the type illustrated in the aforementioned Canadian ' Patent No. 1,053,147. Other well arrangements may be used in carrying out the invention examples of which are set forth in sa~d Canadian Patent No. 1,053,147.
By the term "pore volume" as used herein is meant the pore volume .~

_24_ ~075596 of that portion of the formation underlying the well patt~rn employed as described in greater detail in Canadian Patent No. 1,053,147.
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Claims (8)

9452 Claims We claim:

1. In the recovery of oil from a subterranean oil-containing reservoir penetrated by spaced injection and production system, the method comprising:
(a) injecting into said reservoir via said injection system at least 0.02 pore volume of a hydrocarbon slug having a viscosity lower than the viscosity of the reservoir oil, (b) thereafter injecting into said reservoir via said injection system a thickened aqueous surfactant slug comprising an aqueous solution of a surfactant system which includes an organic sulfonate surfactant and which functions to increase the viscosity of said aqueous solution to a value at least as great as the viscosity of the reservoir oil while decreasing the interfacial tension between said aqueous solution and said reservoir oil, (c) thereafter injecting into said reservoir via said injection system an aqueous flooding medium to displace oil to said production system, and (d) recovering oil from said production system.

9452 2. The method of claim 1 wherein said surfactant system comprises a mixture of an organic sulfonate surfactant and a water-soluble aliphatic alcohol.

3. The method of claim 2 wherein said organic sulfonate surfactant is an alkylaryl sulfonate having an average molecular weight within the range of 350-500 and said alcohol is a C4-C6 aliphatic alcohol having a hydrocarbon chain length of at least 3 carbon atoms, said surfactant and said alcohol being present in said aqueous solution in concentrations to provide a ratio 0 of said surfactant to the sum of said surfactant ant said alcohol within the range of 0.3-0.8.

4. The method of claim 1 wherein said surfactant system contains a petroleum sulfonate having a relatively broad molecular weight distribution and a synthetic alkyl or alkylaryl sulfonate having a molecular weight distribution narrower than that of said petroleum sulfonate with said surfactants being present in said aqueous solution in concentrations to provide a ratio of said petroleum sulfonate to said synthetic sulfonate within the range of 1:3-1:1.

5. The method of claim 4 wherein said surfactant system includes a C3-C6 aliphatic alcohol.

9452 6. The method of claim 1 wherein said hydrocarbon slug is injected in an amount within the range of 0.02-0.2 pore volume.

7. The method of claim 1 wherein said thickened aqueous surfactant slug is injected in an amount of at least 0.1 pore volume.

8. The method of claim 1 wherein said thickened aqueous surfactant slug is injected in an amount within the range of 0.2-0.5 pore volume.