CA1069043A - Oil recovery process using a tapered surfactant concentration slug - Google Patents

Abstract

OIL RECOVERY PROCESS USING A
TAPERED SURFACTANT CONCENTRATION SLUG
(D#74,012-FB) ABSTRACT OF THE DISCLOSURE
A process for recovering oil from a subterranean reservoir in which a surfactant slug injected into the forma-tion via an injection well is passed through the formation and oil is recovered via a production well penetrating the formation. In this process a tapered surfactant concentration slug is employed in which the surfactant concentration is increased, either incrementally or gradually, during the slug injection time. The slug can be a simple aqueous solution of the surfactant, a hydrocarbon-in-water emulsion containing the surfactant or an aqueous solu-tion of a polymer containing the surfactant.

Description

~ ~ ^ D.74,012-FB
,~ ~ :

9~3 .
This invention is concerned with a method of recovering oil from a subterranean reservoir. More partl-cularly, this invention relates to an improved oil recovery process in which a slug comprising water and a surfactant is injected into the formation via an injection well, is forced through the formation, and oil is recovered via a production well penetrating the same formation. During this reeovery process the concentration of the surfactant is increased during the slug injection time. The slug can be a simple aqueous solution of the surfactant, a hydrocarbon-in-water emulsion containing the surfactant, or an aqueous solution of a polymer containing the surfactant.

The production of petroleum products is usually accomplished by drilling into a hydrocarbon-bearing formation --- and utilizing one of the well-known recovery methods for the recovery of the hydrocarbons. It is recognized, however, that these primary recovery techniques may recover only a minor portion of the petroleum products present in the formation, particularly when applied to reservoirs of viscous crudes. Even the use of improved recovery practices involving heating, miscible flooding, water flooding and steam processing may still leave 50% or more of the original hydro-carbons in place.

Thus, many large reserves of petr!oleum fluids from which ~i~ only small recoveries have been realized by present commer cial recovery methods, are yet to reach a potential recovery approaching their estimated oil-in-place.

Water flooding is one of the more widely practiced secondary recovery methods. A successful water flood may

- 2 -.

;` ` ~.(~69~3 result in ultimate recovery of up to about 50 percent of the original hydrocarbons in place. Howe~er, generally the application of water flooding to many oil reservoirs results in much lower recoveries.

The newer development in recovery methods for heavy crudes is the use of steam injection which has been applied in several modifications, including the "push-pull" technique and through-put methods, and has resulted in significant recoveries in some areas. Crude recovery by this process is enhanced through the beneficial effects of the drastic viscosity reduction that accompanies an increase in tempera-ture. This reduction in viscosity facilitates the production of hydrocarbons since it improves their mobility, i.e., it increases their ability to flow.

However, the application of these secondary recovery techniques to depleted formations may leave major quantities of oil-in-place, since the crude is tightly bound to the sand particles of the ~ormation, that is, the sorptive capacity of the sand for the crude is great. In addition, interfacial tension between the immiscible phases results in entrapping the crude in the pores, thereby reducing recovery.
Another disadvantage is the tendency of the aqueous drive fluid to finger, since its viscosity is considerably less than that of the crude, thereby reducing the efficiency of the processes.

Consequently, process modifications have been developed which may incorporate additives to lessen the above cited disadvantages and thereby improve the efficiency of these processes. For example, surface-active agents and miscible liquids are utilized to decrease the interfacial tension between the water and the reservoir crude, and thickeners have been developed to adjust viscosity so as to inhibit fingering.

~ ~6~3 The present invention provides a method of recovering oil from a subterranean oil-bearing formation having in comm-unication therewith at least one injection well and at least one production well comprising:
; (a) injecting a slug comprising water and a surfactant into the formation via the iniection well;
(b~ injecting water into the formation via the injection well; and (c) recovering oil from the formation via the ~aid ~ production well wherein the concentration of the surfactant in the slug of step (a) is increased as the slug is injected into the said formation.

The increase in the surfactant concentration in the slug may be accomplished in a variety of ways such as incrementally in from about 2 to 5 steps or more over the entire slug injection time or alternatively, the concentration of the surfactant in the surfactant slug may be increased continuously and in a linear manner if desired during the timP in which the slug is being in~ected into the formation via the injection well. Optionally, after.step (a) and before step (b), a slug of a polymer solution, such as a solution in water of polyacrylamide, partially hydrolyzed polyacryl-amide, a polysaccharide, a polyvinyl aromatic sulfonate or a polyethylene oxide, may be injected into the formation~

The accompanying Drawing is a graph showing the increased . ,J"' recovery of oil achieved when limestone cores are flooded with surfactant solutions in which the concentration of the surfactant is increased during the injection time.

Generally in the surfactant flooding process of this invention a preflush slug of water e~ual to about 0.05 to 0.30 reservoir pore volume, preferably about 0.10 to 0.20 69~343 reservoir pore volume, is injected into the ~ormation via an injection well before injection of ~he surfactant-containing slug. The water employed in the preflush step, which generally is of low salinity (i.e~ less than 50,000 parts per million, e.g. 20,000 to 50,000 parts per million), should be compatible with water containing various additives injected in subse~uent steps in the process, and in addition, the water may contain, if desired, a sacrificial agent, preferably sodium tripolyphosphate, to reduce surfactant losses from the surfactant slug to the reservoir matrix.
In the next step of this surfactant flooding process a slug comprising water and a surfactant, equal in amount to about 0.10 to 0.60 pore volume, is injected lnto the formation via the injection well. As previously pointed out, one of the novel features of this invention is that during the injection of the surfactant slug the concentration of the surfactant therein is increased in any convenient manner, such as incrementally or continuously and, for example, in a linear manner. For example, a slug having an initial concentration of about 0.2 percent by weight ~i.e.
0.7. lbs/bbl) of the surfactant may be followed by an equal slug size having 0.4 percent by weight surfactant (i.e.
1.4 lb/bbl), following which a third slug of equal size is injected containing a surfactant concentration of 0.80 percent by weight (i.e. 2.8 lbs/bbl). Typically, the size of the three slugs may each be equal to about 0.10 pore volume for a total pore volume of 0.30. A final slug equal, for example, ~ ~, in size to 0.20 pore volume may then bP injected at a maximum surfactant concentration of about 1 percent by weight (3.5 lbs/bbl). The surfactant concentration employed in the surfactant slugs may be varied over a wide range and generally will be from about 0.05 to 2.0 or more weight percent.

. . .

69~ 3 The total surfactant slug size employed in this process will range from about O.lo to 0.75 pore volume or more. The total slug size and the maximum surfactant concentration are usually determined from laboratory and field flood tests.
When a water soluble polymer is included in the surfactant slug, its amount will range from about 100 to abou~ 1000 ppm, and preferably will be from about 200 to about S00 ppm.

When the surfactant slug is a hydrocarbon-in-water emulsion, the concentration of the hydrocarbon in the emulsion may be varied from about 1 to about 12% ~y weight, or may be even more, and praferably it will be from bout 2 to about 12~ by weight, based on the weight of the emulsion slug.

Hydrocarbons useful in preparing the hydrocarbon-in-water emulsions used according to this embodiment of the invantion include crude petroleum oil, such as crude oil derived from the same field in which the process of the invention is being operated; and refined or semi-refined petroleum products such as gasoline, naphtha, stove oil or diesel oil, etc. Especially useful hydrocarbons include pentane, butane~
propane, mixtures of Cl-C5 paraffins, xylene and toluene.
Mixtures of these same hydrocarbons may be employed in the process of this invention, if desired. Alcohols such as secondary butyl alcohol may also be employed in small amounts, along with any of the above-mentioned hydrocarbons or mixtures thereof in forming the emulsion slugs utilized in this embodiment of the invention.

Emulsion slugs suitable for use in the practice of this embodiment of the invention can be prepared in any convenient manner. In one method the hydrocarbon and surfactant together with the secondary butyl alcohol, if used, are combined with mixing, after which the required amount of water or brine is added with continued mixing. Preferably the water employed will have a salinity of less than about 100,000 ppm of total dissolved solids (TD5).

, )6~3 In the final step of this surfactant process, a water drive is employed. ~he ~osition of the water injected via the injection well during the last phase should be compatible with all slug compositions injected during the process. Injection of the drive water is continued to completion of the chemical flood process.

If desired, after step (a) and injection of the drive water in step (b) a polymer slug may be injected into the formation for mobility control. Generally, the polymer slug will comprise an aqueous solution of a polymer, e~ual in volume to about 0.05 to about O~SO pore volume, or even more, containing a low concentration of polymer, varying from about 200 to 1000 parts per millian, preferably from about 200 to about 800 parts per million. If desired, the polymer slug may be tapered in a linear or any other convqnient manner by decreasing the polymer concentration from its maximum value to 0. This mobility control procedure is a desirable measure which results in minimization of fingering (or channeling) of the drive water through the oil bank and prevents early breakthrough of the flood.

One clas~ of hydrophilic polymer water thickening materials suitable for use in this invention is the poly-saccharide or biopolymer compounds, many of which are readily a~ailable commercially. For example, in U.S. Patent No. 3,208,518 there is disclosed a water flooding process wherein the viscosity of the flooding medium is increased by ..; A: .
the use of high molecular weight polymers specifically an ionic polysaccharide produced by the fermentation of carbo-hydrates by bacteria of the genus Xanthomonas, under controlled pH conditions.

Numerous other polymers are suitable for use in this -~ 6~043 invention, such as partially-hydrolyzed polyacrylamide, : which may be graphically illustrated by the following formula:

~ I r 2 ~ ~ ~ CH2 ~ p~l ~ CH2 ~ C
; . C=O m C=O r ~P ¦
oY NH2 -- J J z wherein Y represents hydrogen, ammonium, an alkali metal or an alkaline earth metal, R represents hydrogen or methyl, X represents chlorine, alkoxy, acyloxy or cyano, m is from 12 to 67, n is from 33 to 88, ~ is from O to 10, and the sum of _, n and p equals 100, and z is at least about 60.
This class of polymers is known for the purpose of increas-ing the viscosity of the injected fluid and the efficiency with which the injected fluid displaces petroleum, speclfi-cally being disclosed in U.S. Patent No. 3,039,529 (1962).
Polyacrylamide itself is also a highly satisfactory polymer for use in the polymer slug of this invention.
Other excellent polymers for use in the aqueous polymer slug are polyvinyl aromatic sulfonates having relatively high molecular weights and yet being soluble in water. Polymers of this class have the following general formula: .
H I :
f 3 .'-,J.
-~ wherein Ar is divalent monocyclic aromatlc hydrocarbon unit, derived from benzene or a benzene deri~ative, R is hydrogen : or a metal ion, and M is hydrogen, sodium or potassium. The use of such polymers for mobility ratio improvement in oil recovery operations is more fully detailed in U.S. Patent No. 3,282,337.

.
Another class o~ hydrophilic polymers which can, if desired, be used for mobility control in the polymer slug comprises the flocculant grade water soluble polyethylene oxides such as described in U.S. Patent No. 3,021,231.

Any of the polymers mentioned above may also be used in the slug in step (a) in that embodiment of the invention where the slug in step (a) is an aqueous solution of a polymer containing the surfactant.

In selecting a surfactant or surfactants for use in the surfactant water slug employed in this invention, any of the surfactants commonly employed in petroleum recovery systems may be utilized.

Surfactants are classified on the basis of the hydro-philic or water soluble groups in the molecule, and are generally recognized as falling in one of the following groups:
1. Anionic, meaning that the surfactant molecule has one or more anionic, hydrophilic or water soluble groupsO
The most typical of these groups are carboxyIate, sulfonate, sulfate and phosphate. The anionic sur~actants constitute by far the most important group, and are the sur~actants of choice in oil recovery operations unless there are compelling reasons to resort to the use of other types of surfactant.
2. Cationic surfactants, wherein the hydrophilic or water soluble groups are primary, secondary or tertiary ~ ,~, ' .' ' amine or quaternary ammonium groups.

3. Nonionic surfactan~s in which the hydrophilic character o~ the molecule is provided by hydroxy groups or polyoxyethylene chains.

4. An amphoteric group is sometimes recognized comprising molecules having hoth anionic and cationic groups.

_ 9 _ -- L~ gL3 The hydrophobic or oil soluble component of a surfactant molecule is almost always a hydrocarbon or halogen ... .
substituted hydrocarbon group.

The molecular weight of surfactants generally ranges from around 200 to about 20,000, or even higher, and is preferably from about 300 to about 600. Commercially available surfactants are seldom composed essentially of a single molecular species; rather they are more often polydispersed compounds (i.e. the molecules all have the same functional groups but vary in chain length or some other structural detail).

Petroleum sulfonates which are at present among the more popular classes of surfactants being considered for supplemental oil recovery techniques may be employed in the process of this inve~tion. A preferable petroleum sulfonate is described in U.S. Patent No. 3,30~,713, disclosing a petroleum sulfonate prepared from a petroleum fraction whose boiling range is from 700F to 1100F which corresponds to a molecular weight range of from about 350 to about 600. The sodium salt of the sulfonation product of this petroleum fraction is an excellent material for use in the subject invention.

One surfactant composition which has been shown to be especially good~is a dual surfactant system comprising a sodium salt of tridecylbenzene sulfonate (Conoco 650) and, as a solubilizer surfactant, a sodium sulfated/sulfonated polyethoxylated nonyl phenol. These particular materials, which are anionic surfactants, have average molecular weights of 362 and 518 respectively. In this dual surfactant system, 3 parts by weight of sodium tridecylbenzene sulfonate and 1 part by weight of sulfated/sulfonated ethoxylated nonyl phenol are employed in aqueous solution containing about 1%
by weight of total dissolved solids. Such surfactant 6~143 compositions are highly effective in mobilizing and recovering oil from limestone formations. Other dual surfactant systems, e~g. the sodium or ammonium salt of alkyl aryl sulfonic acid in conjunction with nonionic surfactants, such as polyethoxylated alkyl phenols, are also effective in the process of this invention. Another anionic surfactant useful in this process as a solubilizer in the surfactant so7ution is sodium tridecyl polyethoxylated sulfate.
Petroleum sulfonates which likewise are also useful in the process of this invention require increased concentrations.

The results obtained in an experiment in which oil recovery by surfactant flooding was determined as a function of surfactant concentration in field water (salinity - 10,000 ppm) are shown in the accompanying Drawing. A Cordova Cream limestone core was initially water and oil saturated and then allowed to imbibe water and produce oil at 200 psig. After 75 hours of imbibition the oil saturation was reduced from 0.65 pore volume to 0.43~ The experiment was continued by water flooding the core to a residual oil saturation of 0.39 ~3jm3. A surfactant flood followed the water flood using a surfactant concentration of approximately 2 kg/m3 (0~2%
by weight). After 2.8 pore volume of injection, the residual oil saturatlon decreased to 0.33 m3/m3. At this time, the core was flooded again, with the same surfactant having a concentration of approximately 4 kg/m3 ~0.4% by weight).
The additional 2.32 pore volume injected during the second . ,.
surfactant flood reduced the residual oil saturation to 0.23 m3/m3. This test shows that oil recovery is directly related to the total surfactant injected through the core.

This relationship is shown in the upper curve of the Drawing, where the oil recovery is expressed as a function of cumu-lative surfactant, i.e. pore volume injected times the surfac-tant concentration. The lower curve is the result of a ~ ;90~3 . . ` .
surfactant flood using a similar "Cordova Cream" limestone core and a constant surfactant concentration of approximately 8 kg/m3 (0.8% by weight). All floods were made with the same field liquids and surfactant type. The results shown in the Drawing indicate that surfactant injection with initially a lower concentration tapered gradually to a greater concentration, give more efficient oil recovery than a surfactant slug injecti~n having a constant concentFation.

If desired, the drive fluids, which is preferably water, may contain dissolved therein from about 0.01 to about l.00~ by weight or more of a solubilizing agent, and in addition the drive fluid employed in this invention may include an alkaline agant in an amount sufficient to form a 0.001 to about Ool molar solution. Useful alkaline agents include sodium hydroxide, potassium hydroxide and sodium hypochiorite.

A wide variety bf solubilizing agents may be utilized in the drive fluid of this invention. An especially use~ul group of solubilizing ag~nts are water-solublel oxyalkylated, alkyl alkanolamine compounds of the ~ormula:
R - I - R'O(CH2CH20)mA
R~
wherein R is alkyl having from 1 to about 16 carbon atoms;
R' is alkylene having from 2 to 4 carbon atoms, e.g. ethylene, propylene, butylene or isobutylene and m is from 4 to about 60 or more; A is hydrogen, sodium, potassium or ammonium and R" is hydrogen, alkyl having from l to about 16 carbon atoms or a group of the formula -R'O(CH2CH20)mH.
Typical alkyl groups include methyl, ethyl, isopropyl, n-butyl, isohexyl, n-heptyl, n-decyl and n-dodecyl.

Examples of such compounds include CH3 - NH - (C~2CH20)12 . .

1~9~3 CH3 - N(CH2)CH2 - CH20 (CH2CH20)10 4 2E15 ~ NH ~ CH2 ~ CH(CH2)CH2 (CH2CH2o)l8H

C H - N (CH3) CH2 CH2 - CH - CH2 (CH2CH20~20 CH3 - Cl~ (CH2) N (CH3) CH (CH2) CH20(CH2CH20)13H
'`- ICH 3

5 11 1 . CH - CH20 (CH2CH~0)2~H
fH- CH20 (CH2CH20)28 :, 3 ( 3) C2H5 NH-cH - CH2- CH2o(cH2cH2o)34K

fH~
; 7 ~5 I CH CH2 ~CH2cH2)20H

..
f2H5 C H - CH - C4Hg - ~ - (C~2C 2 16 ( 2 2 )16H

The above-described, oxyalkylated, alkyl alkanol amines can be conveniently prepared by methods well known ` in the art rom commercially available alkyl alkanol amines.
I For example, these oxyalkylated compounds can be made by the methods set out in U.S. Patents No. 3,062,747; 2,174,761 or 2,435,755. In general, the procedure consists of reacting the alkyl alkanolamine with ethylene oxide in the presence of an oxyalkylation.catalyst, such as sodium hydroxide, at about 150C in a stirred autoclave.

`Another group of solubilizer compounds which are highly useful in the aqueous drive fluid employed in the ..
. process of this invention are the sulfated and sulfonated , ~69~3 derivatives oE the previously described oxyalkylated alkyl alkanol amines as well as the alkali metal and ammonium salts of such sulfated or sulfonated derivatives.

Sulfated, oxyalkylated, alkyl alkanolamines useful as solubilizers in the drive fluid of this invention include compounds of the formula:

R - N - R'0 (CH2CH20)mS03 '' '~' ' ' ~ , , .

wherein R, R', m and A have the meanings given above and R''' is hydrogen, alkyl having from 1 to about 16 carbon atoms or a group of the formula R'O(CH2CH20)mS03A. The sulfated, oxyalkylated alkyl alkanolamines can be prepared by methods well known in the art, such as by treatment of the oxyalkylated, alkyl alkanolamine with sulfuric acid followed by neutralization to form the metal or ammonium salt, if desired.

.
Sulfonated derivati~es of these same oxyalkylated alkyl alkanolamines can likewise be prepared by reacting the above-described sulfated products with sodium sulfite at temperatures of about 150C to about 200C, or even more, for about 5 to 10 hours. The sulfonated compounds and their metal and a~monium salts suitable for use as solubilizing agents in the polymer slug or drive fluid of this invention have the formula:

R - N - R'0 (CH2CH20)mCH2CH2 3 "-.:~x, .~: ~
wherein R, R', m and A have the meanings given above andR"" is hydrogen, alkyl having from 1 to about 16 carbon atoms, or a group of the formula -R'O(CH~CH20)mCH2CH2S03A.

:, 3~69~3 .

Examples of such sulfonated solubilizing agents include:
f 3 CH - CH2 - NH - CH (CH3) CH2)(CH2C~20)8C 2 2 3 ':
CH - N (CH3) CH (CH2) CH2 - CH20(C~2CH20)12 ~ 2 3 CH - CH - N - (CH2CH20)17C~2CH2S 3 C2H5 2~5 fH3 1 3 CH3 - ~ - NH - CH ~ CH20 CH2C~2S03H
H

.
13 H27 ~ (cH2cH2o)locH2cH2so3H
C13EI~7 C16H33 ~ NH ~ CH2 ~ CH (CH3) CH2 (CH2cH2) 12CH2CH2S03NH4 CH
C H -- N -- CH CH20 (CH2cH2o) 30 2 2 3 CH3 - CH - CH20(CH~CH20) 30CH2CH2S03H

The following example illustrates one embodiment of this invention employing an aqueous solution of surfactant.

petroleum-containing formation located at a depth of - 6200 feet in a limestone formation is exploited by means of conventional water flooding operations using an inverted five spot pattern until the water-oil ratio rises above about 30. The formation thickness is 20 feet and the porosity is 25~. In this inverted five spot pattern, the centre well is employed as an injection well while the four remaining wells serve as production wells. The dimensions of the .
- 15 ~

~169~3 square grid on which the inverted five spot pattern is based is 500 feet and it is known that only 75~ of the reservoir volume will be swept by the injected fluid using the standard five spot pattern. The pore volume of the pattern swept by the injected fluid will be 500 x 500 x 20 x 0.25 x 0.75 =
937,500 cubic feet. The salinity of the water contained in the formation is about 60~000 parts per million. A total of O.l pore volume t93,Z50 cubic feet) of a preflush solution having a salinity of about lO,000 parts per million and having dissolved therein 0.01% by weight of sodium tripoly-phosphate is injected into the formation via the injection well. This is followed by the injection into the formation of the surfactant slug which consists of four parts. In the first part a surfactant slug equal to about O.l pore volume - or 93,750 cubic feet comprising water containing 0.2~ by weight of a dual surfactant system (0.7 lb/bbl) consisting of the sodium salt o tridecylbenzene sulfonate and sodium sulfated/sulfonated ethoxylated nonyl phenol having 4 oxy-ethylene units and consisting of 80% by weight of the sulfate and 20~ by weight of the sulfonate, is injected into the formation. Next, a second surfactant slug, equal to about O.l pore volume, comprising water containing 0.4 weight percent (i.e. l.4 lb/bbl) of the same surfactants in the same proportions is injected into the formation followed by injection of a third slug of equal size comprising water containing 0.80 weight percent (i.e. 2.8 lb/bbl) of the same surfactants. Finally, a fourth surfactant slug of about i 0.2 pore voLume comprising water and l weight percent (i.e.

3.5 lb/bbl) of the same surfactants is injected into the formation. About 3 parts by weight of the sodium salt of tridecylbenzene sulfonate was utilized for each part by weight of the sodium sulfated/sulfonated ethoxylated nonyl phenol included in each of the surfactant slugs. Next 0.4 - ~69~t4L3 pore volume (375,000 cubic feet~ of water containing 800 parts per million of the polysaccharide Kelzan MF is injected into the formation. This is followed by the injection of drive water having a salinLty of about 10,000 parts per million and containing 0.02 weight percent of a sulfonate solubilizer of the foxmula:

C2H NH - C~ (C~3) C~2(CH2CH ) H2CH SO NH
2 10~ 2 3 4 to displace the oil, surfactant solution and thickened water through ~he formation. Oil ls produced through the associated production wells in the five spot pattern, and the amount of oil produced is substantially in excess of that produced by water flooding alone.

The increased recovery achieved in the process of this invention is also shown by the results set out in Tables 1 and 2 which follow. All flood tests included in these tables were conducted on cores which had first been water flooded until no additional oil was produced, after which a single surfactant slug of specific surfactant concentration (i.e~ a non-tapered slug) or a surfactant slug which comprised from 2 to 4 parts equal or different pore volume size, in which the concentration of the surfactant was increased in each successive part (i.e. a tapered slug) was injected into the core. Following the surfactant slugs, field brine with 1000 parts per million of a polysaccharide (commercially available under the trade name Kelzan MF from the Xelco Company, San Diego, California) was forced through the core until no additional oil was produced. The total amount of oil recovered by injection of the surfactant slug and the polymer thickened water was measured to determine the amount of tertiary oil recovered in this tertiary recovery process. The relative efficiency

6~43 ;of the recovery process was determined by the following formula:
Tertiary Recovery to Surfactant Ratio = Er .- (Vp X ~Cs~
where Er = tertiary oil recovery in percent of oil in place (i.e. after water flooding), Vp = injected pore volume, ml/ml, %Cs = surfactant concentration in percent~

; The following example illustrates the embodiment of this invention in which an emulsion containing a surfactant is used.

A petroleum-containing formation located at a depth of 5800 feet in a limestone formation is exploited by means of conventional water flooding operations using an inverted five spot pattern until the water-oil ratio rises above ;about 30. The formation thickness is 30 feet and the porosity is 25~. The inverted five spot pattern is the same as in Example 1. The pore volume o the pattern swept by the injected fluid will be 500 x 500 x 30 x 0.25 x 0.75 = 1,406,250 cubic feet. The salinity o the water contained in the formation is 90,000 parts per million. A total of 0.1 pore volume ~140,600 cubic feet) of a preflush solution having a salinity of about 35,000 parts per mlllion and having dissolved therein 0.06~ by weight of sodium trlpolyphosphate is injected into the formation via the injection well.
~x~This is followed by the injection into the formation of the emulsion slug which consists of four parts. In the first part an emulsion slug equal to about 0.1 pore volume or 140,600 cubic feet comprising water containing about 4~ by weight of pentane and about 0.2% by weight of a dual surfactant system (0.7 lb/bbl) consisting of the sodium salt of tridecylbenzene sulfonate and sodium sulfated ethoxylated nanyl phenol having the formula:

36~43 C9H19 ~ ~ 2 2 )5S3Na was injected into the formation. Next, a second emulsion slug equal to about 0.1 pore volume, comprising water containing about 4% by weight of pentane and about 0.4~ by weight (i.e. 1.4 lb/bhl) of the same surfactants was injected into the formation, followed by injection of a third slug of equal size, comprising water containing about 4~ by weight of pentane and about 0.80% by weight (i.e. 2.8 lb/bbl) of the same surfactantsO Finally, a fourth emulsion slug of about 0.2 pore volume compxising water and about 4% by weight of pentane and 1% by weight of the same surfactants was injected into the formation. About 3 parts by weight of the tridecylbenzene sulfonate was utilized for each part by weight of the sodium sulfated ethoxylated nonyl phenol included in each of the emulsion slugs. Next 0.4 pore volume, (562,500 cubic feet) of water containing 400 parts per million of polysaccharide polymer and containing about 0.02 weight percent of the sulfonate solubilizer used in Example 1 is injected into the formation. This is followed by the injection of drive water having a salinity of about 35,000 parts per million to displace the banked emulsion slug oil, and thickened water through the formation. Oll is produced through the associated production wells in the five spot pattern, and the amount of oil produced is substantially in excess of that produced by water flooding alone.
EXA~LE 3 The following example illustrates that embodiment of this invention using a slug containing a surfactant and a polymer.

A petroleum-containing formation located at a depth of 3800 feet in a limestone formation is exploited by means , i9C! a~3 .
of conventional water flooding operations using an inverted five spot pattern until the water-oil ratio rises above about 30. The formation thickness is 40 feet and the porosity is 25 percent. The inverted five spot pattern is the same as in Examples 1 and 2. The pore volume of the pattern swept by the injected fluid will be 500 x S00 x 40 x 0.25 x 0.75 - 1,875,000 cubic feet. The salinity of the water contained in the formation is 55,000 parts per million. A
total of 0.1 pore volume (187,500 cubic feet) of a preflush water solution having a salinity of about 28,000 parts per million and having dissolved therein 0.05% by weight of sodium tripolyphosphate is injected into the formation via the injection well. This is followed by the injection into the formation of a slug of the aqueous polymer solution which consists of four parts. In the first part a slug equal to about 0.1 pore volume or 187,500 cubic feet comprising water containing about 0.05 percent by weight of the polysaccharide Kelzan MF and about 0.2 percent by weight of a dual surfact-ant system (0.7 lb/bbl) consisting of the sodium salt o~
tridecylben2e~e sulfonate and sodium sulfated/sulfonated ethoxylated nonyl phenol was injected into the formation.
Next, a second aqueous polymer slug equal to about 0.1 pore volume comprising water, containing about 0.05% by weight of Kelzan MF and about 004~ by weight (i.e. 1.4 lb/bbl) of the same surfactants, was injected into the formation followed by injection of a third aqueous polymer slug of equal size, comprising water containing about 0.05~ by weight of Kelzan MF and about 0.80% by weight (i.e. 2.8 lb/b~l) of the same surfactants. Finally, a fourth aqueous polymer slug of about 0.2 pore volume comprising water and about 0.05% by weight of Kelzan MF and 1% by weight of the same surfactants was injected into the formation. About 3 parts by weight of the tridecylbenzene sulfonate was utilized for ea~h part by weight of the sodium sulfaked/sulfonated ~._/ \

6904~
ethoxylated nonyl phenol having 4 ethylene oxide units included in each of the polymer slugs. Next 0.4 pore volume (750,000 cubic feet), of water containing 0.07 percent by weight of polysaccharide Kelzan MF is injected into the formation.

This is followed by the injection of water having a salinity of about 28,000 parts per million and containing about 0.05 weight percent of the sulfonate solubilizer used in Examples 1 and 2 to displace the banked oil, aqueous polymer solution and thickened water through ~he formation.
Oil is produced through the associated productlon wells in the five spot pattern, and the amount of oil produced is substantially in excess of tha~ produced by water flooding alone.

:

.

0~3 TAB LE

Tertiary Flood Results in 4 to 8 in. Limestone Cores (2 in. dia. ) .
Permeability (k). = 11 to 40 md.
Porosity (0) = 17 to 21 percent Surfactant slug - ethoxylated dodecylphenol sulfonate in 85 ,OOO ppm salinity water .

FLOOD NO. SURFACTANT OIL RECOV- r T.YPE ERY BY TER- Vp X %Cs TIARY FLOOD,~ Tapered Non-Tapered Slug Slug 1 *4E0 33.6 0.77 2 " 53.6 1.07 3 " 38.0 0.56 4 . " 42.7 0.85 : `
" 28.1 0.55 6 . " 52.~ 0.55 .7 " 57.9 1.52 ; - 8 " 56.9 1.49 9 " 39.6 0.79 " 55.4 1.11 11 " 38.3 1.19 AVE~AGE RATIOS 1.32 0.81 * The single surfactant utilized was an ethoxylated dodecylphenol sulonate having 4 ethylene oxide units.

Tertiary Flood Results in 7" Berea (sandstone) Cores k = 110 to 302 md 0 = 20 to 22%
Surfactant slug - ethoxylated dodecylphenol sulfonate in 85,000 ppm salinity water.

:,. .

FLOOD NO. SURFACTANT OIL RECOV- Er ! TYPE ERY BY TER
TIARY FLOOD~(%) Tapered Non-T~e' Slug Slug : .
12 *4E0 57.3 0,88 13 " 43.4 1.11 14 " 63.1 1.87 -~

*The single surfactant was the same as in Table 1.
.
The results in both Tables 1 and 2 indicate that substantially more oil i5 recovered when the tapered : surfactant concentration slug is utilized.

: .

.~ , .

:: :

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 oil-bearing formation having in communication therewith at least one injection well and at least one production well comprising:
(a) injecting a slug comprising water and a surfactant into the formation via the injection well;
(b) injecting water into the formation via the injec-tion well; and (c) recovering oil from the formation via the said production well characterized in that the concentration of the surfac-tant in the slug of step (a) is increased as the slug is injected into the said formation.

2. A method according to Claim 1, characterized in that the slug of step (a) is an aqueous solution of the surfactant.

3. A method according to Claim 1, characterized in that the slug of step (a) is a hydrocarbon-in-water emul-sion containing the surfactant.

4. A method according to Claim 3, characterized in that the emulsion contains 1 to 12% by weight of hydrocar-bon.

5. A method according to Claim 1, characterized in that the slug of step (a) is an aqueous solution of a polymer containing the surfactant.

6. A method according to Claim 5, characterized in that the aqueous solution contains from 100 to 1000 ppm of the polymer.

7. A method according to Claim 2, 3 or 5, charac-terized in that the surfactant is an anionic surfactant.

8. A method according to Claim 2, 3 or 5, charac-terized in that the concentration of the surfactant in the slug is incrementally increased as the slug is injected into the said formation.

9. A method according to Claim 2, 3 or 5, charac-terized in that the concentration of the surfactant in the slug is continuously increased as the slug is injected into the formation.

10. A method according to Claim 2, 3 or 5, charac-terized in that a preflush slug of water is injected into the formation via the inejction well before step (a).

11. A method according to Claim 2, 3 or 5, charac-terized in that a slug of an aqueous polymer solution is introduced into the formation.

12. A method according to Claim 2, 3 or 5, charac-terized in that the water injected in step (b) is saline water.

13. A method according to Claim 2, 3 or 5, charac-terized in that the water injected in step (b) contains a small amount of a solubilizing agent selected from (a) a compound of the formula:
wherein R is alkyl having from 1 to about 16 carbon atoms;
R' is alkylene having from 2 to 4 carbon atoms, m is from 4 to about 60, A is hydrogen, sodium, potassium or ammonium and R" is hydrogen, alkyl having from 1 to about 16 carbon atoms or a group of the formula -R'O(CH2CH2O)mA;
(b) a sulfated compound of the formula:
wherein R, R', m and A have the meanings given above, R"' is hydrogen, alkyl having from 1 to about 16 carbon atoms or a group of the formula R'O(CH2CH2O)mSO3A, and (c) a sulfonated compound of the formula:
wherein R, R', m and A have the meanings as given above; and R"" is hydrogen, alkyl of from 1 to about 16 carbon atoms, or a group of the formula -R'O(CH2CH2O)mCH2CH2SO3A.

14. A method according to Claim 1, characterized in that the water injected in step (b) also contains an alka-line agent in an amount sufficient to form a 0.001 to 0.1 molar solution.