CA1169759A - Oil recovery with dilute surfactant systems - Google Patents

Abstract

Abstract of the Disclosure The combination of a dilute surfactant solution with an alkaline compound is effective in recovering oil from petroleum deposits after primary and secondary recovery operations have been completed. The combination is injected into the petroleum bearing formation and as it travels through the structure oil is washed free of the rock and is recovered from a production well. A preferred alkaline compound is sodium silicate. The use of sodium silicate appears to maxi-mize and concentrate the recovery of the surfactant so that it can be recycled.

Description

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Back round of the Invention g _ _ _ This invention generally relates to an improved method ~or recovering oil from petroleum deposits af ter primary and secondary recovery operations have been carried 5 out. In particular, the method involves injection of a dilute solution containing an alkaline material and a sur-factant into the oil-bearing formation and recovering the oil released from the structure.
It is well known that significant amounts of oil remain in petroleum-bearing formations after primary and secondary recovery operations become unproductive Numerous methods have been proposed to recover this oil, which is largely trapped in small spaces within the formation. U.S.
Patents Nos. 3,174,542; 3,185,214; 3,208,517; 3,804,170;
4,011,908 and 4,037,659, among many others, disclose various methods for recovering additional oil from watered out deposits. Patents ~los. 4,011,908 and 4,037,659 appear to be prior art that is relevant to the method of our invention.
The first patent discloses a method which includes the steps of injecting an alkali metal silicate solution into the petroleum-bearing formation followed by the sequential in-jection of a "micellar solution" and a drive fluid. The second patented method reverses the injection sequence for the alkali metal silicate and the "micellar solution" prior to use of the drive fluid. In these processes the silicate is of the highly alkaline type such as sodium orthosilicate or sodium metasilicate. The micellar solution can include surfactant/water systems in macromolecular form, or water and at least one surfactant in amounts that exceed the criti-cal micelle content. These, as well as other enhanced oilrecovery systems, involve relatively high concentrations of surfactant, up to 20%, and depend upon the formation of an oil bank to obtain efficient oil recovery. This dependency is undesirable, since once the integrity of this oil bank is compromised, effective oil flow ceases.

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It is an objective of this invention ko provide an improved method for enhanced oil reco~ery using dilute sur-factant solutions with an alkaline material, thereby enabling the recovery of oil without formation of an ~oil bank. n It is a further objective of this invention to provide a method of enhanced oil recovery that allows recover~ of the sur-factant for reu~e by using an alkali metal silicate with the dilute surfactant.

Brief Description of the Drawings 10Figures 1 and 2 show the interfacial kension (IFT) of a Kansas crude oil as a function of various process variables. Figure 3 shows the IFT of an Illinois crude as a function of the concentration of surfactant with and with-out silicate. Figure 4 summarizes the results from several linear core floods by showing the ~ansas crude recovered from a sandstone core as a function of the amount of liquid in-jected while varying certain process conditions. Figure 5 also summarizes the results of several linear core floods by showing the amount of Illinois crude recovered as a function of the liquid injected while varying the alkaline compound used. Figure 6 shows the surfactant recovered as a function of the liquid injected into a sandstone core with alkaline material in said liquid. Figures 7, 8, 9 and 10 summarize the same information when the injection liquid contains sodium silicate, sodium tripolyphosphate, sodium carbonate, or sodium hydroxide, respectively. The curves in Figure 11 compare the residual oil recovery of linear and radial core floods with and without silicate. Figures 12 and 13 show the oil remaining in slabs after radial floods with silicate and no silicate,respectively.
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Solutions containing low concentrations of surfactant ; when combined with an alkaline substance ~uch as sodium carbonate, sodium silicate, ~odium hydroxide or sodium ;` .
, tripolyphosphate tSTPP) are very effective in recovering additional oil from "watered out" petroleum formation~.
- Injection of up to 4 pore volumes (PV) of solu~ions contain-ing not more than about 0.~% ~urfactan~ and 105% of said alkaline material continually washes oil from the rock matrix ~o that oil is recovered at the production well during most of the injection process. When ~odium silicate is supplied as the alkaline compDnent, the surfactant is recovered more ; co~pletely and in a concerted fashion so tbat the surfactant can be reused with the addition of only small guantities of make-up surfactant. Our process does not appear t~ depend upon forming an "oil bank" that moves through the formation as a slug. Our combination treatment appears to provide de-creased surfactant adsorption, reduced crude oil interfacial tension and enhanced bilization of the oil. These effects promote displacement of the oil and recovery earlier in the injection sequence along with more complete production of the remaining oil. Our treatment also increases "sweep efficiency" when compared to the prior art.

The Invention The inorganic alkaline chemical used in combination with dilute solutions of surfactants may be any such chemical that is compatible with the particular surfactant being used - and is capable of reducing the IFT between the crude oil and the aqueous injection fluids to below 0.1 dynes/cm. Examples of useful materials include the alkali metal silicates, car-bonates, polyphosphates and bicarbonates. Alkali metal ` hydroxides provide some benefits. In general, we use sodium silicate, sodium carbonate, sodium tripolyphosphate and sodi~m hydroxide. We prefer the silicates, carbonates and ; tripolyphosphates. We most prefer sodium silicate. These are well known articles of commerce usually prepared by dis-solving the glass that results from the fusion of a mixture of sodium carbonate and sand. Such solutions can also be prepared by dissolving silica ~n sodium hydroxide. The sili-cate solutions that are useful in the process of our invention contain about 1.5 to 4.0 moles of SiO2 for each mole of Na20.

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Surfactants that are compatible with alkaline solu-; ti~ns and promote the desired mobility ~f the oil are satisfactory. Numerous nonionic and anionic surfactants satisfy these criteria and can be used. Anionics, particul-S arly alkyl aryl sulfona~es in the form of ~heir sodium salt are preferred. The most preferred surfactants are the so-;; called natural petroleum sulfonates. These materials are produced by sulfonating a mixture of petroleum hydrocarbonsl and their properties are controlled by using different mole-cular weight fractions of the petroleum. Different surfac-tants may be required for the conditions in various petroleumfields.
The process of ~ur invention can be carried out by injecting the alkaline surfactant solution into the petroleum 1~ bearing formation using the so-called "ive spot," nline drivel' patterns or any other pattern desired. Sufficient surfactant must be present to promote the desired mobility of the oil through the formation. The concentration can be 0.05 to 0.8% by weight. We prefer to use 0.1 to 0.5% by weight. The alkaline compound must be present in sufficient quantities to decrease the interfacial tension between the crude oil and the aqueous environment. The concentration can be 0.1 to 2.5% by weight ~ile we prefer 0.2 to 1.5~.
The preferred range is especially applicable to sodium silicate.
A preflush of salt or other equivalent inorganic chemical solution may be injected into the formation to pro-; vide a favorable ionic environment or to make the rock less reactive. Salt is also an important constituent of the dilute surfactant-alkaline mAterial solution causing the surfactant to partition between the oil and water so that most of it is at the oil/water interface, which reduces the IFT. Salt is also important in maintaining the integrity of the rock and clay surfaces, thereby preventing plugging. Salt concentra-tions of about 0.5 to 5.0~ appear to be useful; we prefer ~`\ ( 7S~

salt concentrations of 0.5 to 2.0~. The composition of our -~ co~bination is:
Component Useful Range (%) Preferred Range (%) Surfactant 0.05 to 0.8 0~1 to O.5 Alkaline Material 0.1 to ~.5 0.2 to 1.5 NaCl 0.5 to 5.0 0.5 to 2.0 Water to 100 to 100 About 1/4 to 4 or more PV of this solution are injected - through a drive well. Significant amounts of crude oil are recovered at production wells soon after injection of the combined solution is initiated. After injection of the surfactant-containing solution is completed, the usual drive fluids are ~tilized to continue the process.
The use of the combination of surfactant-alkaline-compound-salt, especially when sodium silicate is used, pro-vides quicker and more complete recovery of the remaining oil in a watered-out formation. For instance, a typical dilute surfactant linear flood with no alkaline additive recovers 30 to 50% of the residual oil. The addition of the alkaline material according to the teachings of our in-vention promotes the recovery of 40 to 70% of the residual oil. "Sweep efficiencies" for floods carried out according to the teachings of our invention are extremely good. In addition, surfactant recovery is promoted when odium sili-cate and sodium tripolyphosphate are used. These resultsand others are summarized in the drawings, and further illus-trated and discussed more specifically in the examples.

Examples The following examples and the figures which summar-ize the results of said examples illustrate certain embodi-ments of our invention. These examples do not establish the scope or limits of our invention; said scope i5 fully defined in the disclosure and the claims. Two light mid-continent crudes, one from Kansas and the other from Illinois, were 7~

used in the tests discussed in the examples. The properties of these crude oils and their associated brines are sumnarized in the following table.
Crude Kansas Illinois Gravity 40.6 API 36.3 API
Y Viscosity 48 cp 37 cp Reservoir Temperature75F (23.9~C~80F (26.7C) Brine Composition:
Na 9,280 ppm 39,350 ppm Ca+~ 1,500 ppm 1,190 ppm Mg+ 500 ppm 435 ppm Cl 18,440 ppm 64,030 ppm Petrostep~ petroleum sulfonate surfactants were used in these examples. Petrostep~ is a trademark of Stepan Corporation. These surfactants have molecular weights rang-ing from 420 to 465 and are about 60~ active ingredients.
- Sodium silicate solutions having 3.3 moles of SiO2 per mole of Na2O were used.

Example 1 (Fi~ures 1-3) It is generally accepted that interfacial tension (IFT) values of less than 0.1 dynes/cm æe required to effect-ively displace most crude oils from capillary pores and channels in the reservoir rock. I~T values were measured by the spinning drop technique at 2, 5, 10 and 15 minutes.
The average of the four values is reported. These results are summarized in Figures 1 to 3.
Figure 1 shows the log of IFT of the ~ansas crude as a function of the concentration of ~laCl in the solution which contains 0.1% surfactant. Curve I represents the re-sults with no alkaline material present, while Curve IIrepresents the results with 0.37% sodium silicate solids ~ present. These results indicate that increasing the salt `` content of the surfactant solution without silicate has a very limited eifect ~n the IFT. The addition of silicate . ~, 75~

initially lowers the IF~ and promstes lcw~r IFT ValURB as the concentration of salt is increased.
Figure 2 shows the log of IFT of the Kansas crude as a function of the concentration of sodium silicate solution at 37.5% 501ids with 0.1% surfactant present. CurYe V re-presents the results with 1.0% NaCl added while Curve Vl represents the results with 2.0~ llaCl added. These curves show that increasing the concentration of silicate reduces the IFT and reinforce the indication that the presence of silicate enhances the effect of the salt present.
Figure 3 shows the log of IFT of the Illinois crude as a function of the concentration of the surfactant with 1.5~ NaCl present. Curves VII and VIII have no silicate and 0.37% silicate present, respectively. These curves show that silicate enhances the effect of the surfactant and that with silicate present a minimum IFT i5 found at about 0.5%
surfactant, The results illustrated in Figures 1 to 3 definitely show that the presence of an alkaline material such as sodium silicate enhances the IFT reduction of surfactant and salt as well as producing a lower IFT as its concentration increases.
Examples 2 and 3 . .
` ~xamples 2 and 3 illustrate linear core floods wherein `~ 25 treatment of 2 n x 2 ~ x 24 1~ Berea sandstone cores simulated recovery of residual oil from a petroleum formation. The cores were sealed with fiberglass and epoxy resin, then evacuated and saturated with a br.ine solution containing 9,300 ppm Na , 1500 ppm Ca , 500 ppm Mg and 18,500 ppm Cl .
The pore volume ~PV) of the core ~as determined from the vol-ume of brine injected. Injection was continued until the composition recovered was the same as that injected. The cores were then saturated with either of the crude oils by pumping oil into one end of the core at about 10 to 15 psi.

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Typically 55 to 65% of the pore space ~ecomes ~illed wikh oil. Then the core was flooded wnth brine solution to simulate a simple waterflood. The flood was continued until almost no oil was produced. This recovery usually ~mounted to about 35 to 40% of the oil in place so that 35 tD 40~ of the pore spaces still contained oil.
The core floods were carried out as follows:
1. Injection of a preflush of 0.25 PV of salt solution;

2. Injection of the dilute surfactant-alkaline compound solution (0.5, 1.0 PV and continuous flushes were used);

3. Injection of a postflush that was the same as the pre-flush; and

4. Injection of brine as a drive fluid.
The oil was recovered by separating it from the aqueous fluids and the amount determined as a volume percent of the residual oil present in the core at the start of the dilute surfactant alkaline compound flood.
Example 2 (Figure 4) A series of four core floods was carried out with cores that contained ~ansas crude. These floods are repre-sented by curves X to XIII on Figure 4 wherein the percent of recovered residual oil is shown as a function of the total PV of all liquids injected into the core. The preflush contained 0.1% NaCl, while the surfactant flood contained 0.1% surfactant, 1.0% NaCl and either no silicate or .37%
sodium silicate solids. Curve X represents a continuous flood that contained silicate, and it indicates the oil is produced when less than about 0.5 PV of the surfactant-silicate solution was injected and that most of the oil is produced in about 2.5 YV of total injection fluids. Curve XI
also represents a flood that contains silicate, but in this case only 0.5 PV of the surfactant-silicate solution was injected, followed ~y a postflush and brine. This curve is ' .

the same as Curve X until the injection of the sur~actant-silicate ceases, at which time the production of oil virtually stops. 17e believe these results support our hypothesis that oil is continually washed from the pores by our surfactant-silicate solution instead of forming a so-called "oil bank" which moves through the formation to the pro-duction well. Curves XII and XIII represent floods with no silicate that are continuous, and 0.5 PV of surfactant solution, respectively. Oil is produced later and at a slower rate for the floods that contain no silicate. These results clearly indicate the benefit derived from adding silicate to the surfactant solution since the oil is pro-duced earlier and at an accelerated rate.
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, Example 3 (Figure 5) A series of five continuous core floods with Illinois crude were carxied out with various alkaline compounds while the other conditions ~ere kept constant. The constant con-ditions were: a preflush of 0.25 PV of 1.0% NaCl with 0.25%
surfactant and 1% NaCl in flood solution. Curves XV, XVI and XVIII represent floods that contained 0.37% sodium tripoly-phosphate, sodium silicate solids and ~la2CO3, respectively.
Curve XVII represents a flood that contained 00185% silicate - and 0.185% Na2CO3. ~he flood shown by Curve XIV contained no alkaline component. The results summarized in these curves show that alkali addition promotes quicker and more complete recovery of oil from the sandstone cores. We do not believe, at this time, that the small differences in recovery patterns and/or amount are significant. The impor-tant difference between the various sources of alkali is illustrated in Example 4.

Example 4 (Figures 6-l0) Five core floods were run to determine the degree to which the surfactant is adsorbed by the reservoir rock.
These floods were carried out as previously described with 7~

a 0.25% level of Burfactantl except that no oil was in place in the core. Figures 6-10 summarize the results from these floods. The curves in these figures show the concentration of surfactant in the produced ef~luent as a function of the injection liquids. The legend at the top of the drawings - identifies the chemical i~jected as follows:
1. Brine 2. 0.1% NaCl--preflush or postflush 3. 0~25% surfactant, 1% NaC1, 0.37% alkali or no alkali.
Curve XX, Figure 6, represents a flood with ~o alkaline material added and shows that very little of the surfactant is recovered. Curve XXI, Figure 7, represents a flood with sodium silicate and shows that a significant amount of the ` surfactant is recovered in a concerted period of time.
This slug of surfactant with some make-up material can be reinjected into the formation for further oil recovery operations. Curve XXII, Figure 8, represents a flood with sodium tripolyphosphate. In this case surfactant is re-covered, but not as much as when silicate is used. This recovered surfactant can also be reinjected. Curve XXIII, Figure 9, represents a flood with sodium carbonate. Sur-factant is recovered, but not in a concerted manner. This material could be reinjected, but with more difficulty and requiring more make-up surfactant than when silicate and polyphosphate are used. Curve XXIV, Figure 10, represents a flood with NaOX. The use of NaOH does not appear to promote recovery of surfactant.
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Example 5 (Fi~ures 11, 12 and 13) Two linear core floods were carried out as previously described. Two radial core floods were carried out using the same conditions so that "sweep efficiencies" could be - determined and compared. The "~weep efficiency" is deter-mined by the comparison of the radial flood to the linear flood. The radial flood more closely approximates the . 10 `

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conditions in a petroleum formation, and generally much less oil is recovered than in a linear flood.
The radial core floods were carried out in 12" x 12" x 1.5" slabs of Berea sandstone ~o simulate the two-dimensional flood pattern in one quarter of a conven-tional "~ive-spot" drilling pattern with one injection well and four producing wells. The injection point was in one corner with the recovery point in the opposite cornPr. The slabs were sealed with epoxy resin and saturated with brine and oil as previously described. The slabs were water-flooded until very high water-to oil ratios, equivalent to those of the linear floods, were achieved. All of the floods (linear and radial) were carried out as follows:
1. Preflush of 1/4 PV of 0.1% NaCl, 3 PV;
152. Surfactant Flood of 3 P~ of 0.25% surfactant, 1% NaCl with 0.37% silicate or no silicate;
3. Postflush of 1/4 PV of 0.1% NaCl.
Curve XXXI of Figure 11 represents the linear flood con-taining silicate while Curve XXXII represents the radial flood containing silicate. Curves XXXV and XXXVI represent the linear and radial floods containing no silicate, respectively. These curves show that the floods containing silicate were more effective than those that contained no silicate. Additionally, the curves show a high n sweep efficiency" for the floods containing silicate, i.e., very little difference between the curves, while a much lower "sweep efficiency" is indicated for the floods with no silicate. The "sweep effisiencies" calculated from these results were about 95~ for the floods containing silicate and about 60% for the floods containing no silicate.
The slabs used to carry out the radial core floods represented by Curves XXXII and XXXVI were sawed into nine ~` pieces as indicated by the dashed lines of Figures 12a, 12b, 13a and 13b. Each of these sections was examined for . . 11 :

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crude oil still in place~ The diagonally hatched areas represent the oil remaining after the flood. Figures 12a and 13a show the slab, the injection and recovery points and how he slab was sawed apart, as well as ~he cross-

5 section of the slab along the lines indicates. T meanstop and B means bottom in these representations. Figures 12b and 13b are projections of the top and bottom of the slab as indicated with the oil remaining as indicated by the hatched area. Figures 12a and 12b show the oil re-maining after a flood containing silicate, and Figures13a and 13b the oil remaining after a flood with no silicate.
Figure 12 shows that most of the oil was removed from the slab. Figure 13 shows that the flood fingered through and much of the oil remained in the slab. These results further indicate the difference between the process of our inven-tion and the prior art methods.

Claims (6)

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

1. A method of recovering crude oil from a petroleum deposit comprising: injecting into said deposit a dilute aqueous solution consisting of 0.05 to 0/8% by weight of an alkaline material and 0.5 to 5% by weight of NaC1, said alkaline material being capable of reducing the interfacial tension between the oil and the dilute solution to 0.1 dynes/cm or less and compatible with said surfactant; and recovering the crude oil pro-duced from the deposit.

2. A method of recovering crude oil from a petroleum deposit penetrated by at least one injection well and one producing well comprising: injecting into said deposit a dilute aqueous solution consisting of 0.05 to 0.8% by weight of a nonionic or anionic sur-factant, 0.1 to 2.5% by weight of an alkali metal sili-cate and 0.5 to 5% by weight of NaC1, and recovering crude oil and components of said injected solution that are not adsorbed or consumed in the deposit from said production well.

3. The method of Claim 2, wherein the dilute solution consists of 0.1 to 0.5% by weight of surfactant, 0.2 to 1.5% of silicate, and 0.5 to 2.0% by weight of NaC1.

4. The method of either of Claims 2 or 3, wherein the surfactant is an alkylaryl sulfonate.

5. The method of Claims 2 or 3, wherein the surfactant is a petroleum sulfonate.

6. The method of either of Claims 2 or 3, wherein the silicate is sodium silicate and contains 1.5 to 4.0 moles of SiO2 per mole of Na2O.