Oil Recovery Processes

This invention relates to the recovery of natural fluids from subterranean reservoirs. More particularly, this invention relates to a process for recovering hydrocarbons from hydrocarbon-bearing formations. More specifically, this invention relates to post primary hydrocarbon recovery processes in which the formation is first treated with a sacrificial agent followed by injection of microbial agents.

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 hydrocarbons. However, it is recognized that these primary recovery techniques may recover only a minor portion of the petroleum present in the formation particularly when applied to reservoirs of viscous crudes. Even the use of improved recovery practices involving steam, surfactantflooding, waterflooding and polymerflooding can result in up to 50-60 percent of the original hydrocarbons being left in place.

Thus, many large reserves of petroleum fluids from which only small recoveries have been realized by present commercial recovery methods remain in place.

It is known that bacteria can be introduced into subterranean formations for the purpose of beneficially influencing the factors which control the quantity of valuable fluids which can be recovered from the formations or which control the facility with which such fluids are recoverable. Microbial cells have a strong surface charge and, therefore, are readily adsorbed by the rock surface and clay particles in porous media. Consequently, microbial cells are very difficult to transport through porous media because the microorganisms are readily adsorbed on the reservoir rock.

The positive but indirect manner by which in-situ bacterial cell metabolism enhances oil recovery depends on the successful positioning of such cells deep in the oil bearing formation. Once in position, cell metabolism can give rise, e.g., to gas production such as CO.sub.2 or surfactant production such as carboxylic acids which can be beneficial to enhanced oil recovery operations. The positioning of bacterial cells in subterranean rock strata is hindered by the natural tendency of such cells to be strongly adsorbed on the rock surface. This adsorption prevents the facile injection of the cells and severe plugging of the rock matrix results at a short distance from the injection site. The present invention proposes the use of known sacrificial agents to facilitate the injection and thus the successful positioning of bacterial cells deep in the rock strata, i.e., at a great distance from the site of bacterial cell injection. It is contemplated that the sacrificial agent, usually as an aqueous solution, can be injected prior to the injection of the bacterial cells or simultaneously with the bacterial cells.

The general use of sacrificial agents in the prior art to diminish the adsorption of subsequently or simultaneously injected surfactants and/or polymers in enhanced oil recovery operations is disclosed, e.g., in patents such as U.S. Pat. Nos. 3,532,166; 3,469,630; 3,478,823; 4,157,115; 4,217,958; 3,804,173; 4,005,749; 4,207,946; 3,797,574 and 3,778,399. These patents encompass a variety of sacrificial agents such as alkali metal carbonates and polyphosphates; low molecular weight water soluble polymeric viscosifiers; lignosulfonates and polybasic carboxylic acids. The use of these agents for the facilitation of bacterial cell injection into subterranean rock strata does not appear to be disclosed in the prior art.

Since it is recognized that significant amounts of surfactant and/or polymer are "lost" by rock adsorption in enhanced oil recovery operations, it is contemplated that such previously surfactantflooded and/or polymerflooded reservoirs are amenable to bacterial cell injection because a significant proportion of the rock adsorption sites would be occupied by surfactant and/or polymeric species. This adsorption gradient would be expected to facilitate the in depth penetration of the subsequently injected bacterial cells.

Accordingly, an object of this invention is to provide an improved process for the recovery of oil from subterranean oil reservoirs.

A further object of this invention is to provide an improved process for the post-primary or enhanced recovery of oil from subterranean formations.

A further object of this invention is to provide an improved process for the utilization of microbial cells in recovery of hydrocarbons from subterranean formations.

Other objects, aspects, as well as the several advantages of the invention will be apparent to those skilled in the art upon reading the specification and the appended claims.

In accordance with the invention a subterranean oil reservoir is treated near an injection well with a sacrificial agent to prevent adsorption of subsequently injected microbial or bacterial cells on rock near the injection well.

More specifically the invention relates to a process for producing petroleum from subterranean formations having injection and production wells in communication therewith comprising injecting into the formation via an injection well sacrificial agents, such as inorganic salts, polymeric viscosifiers, lignosulfonates, polybasic carboxylic acids, and the like, and subsequently or simultaneously injecting microbial or bacterial cells in an amount sufficient and under conditions to produce metabolic products such as carbon dioxide, carboxylic acids, and the like, which are potentially useful to increase post-primary, oil recovery by enhancing oil displacement from the subterranean formation to the surface of the earth.

In accordance with the invention, it has heen found that sacrificial agents are strongly adsorbed on reservoir rock surfaces to establish an adsorption gradient, thereby facilitating the subsequent transport of less-highly adsorptive microbial cells to a site deep in the formation, i.e., at the "distal end" of the previously mentioned adsorption gradient.

The present invention is carried out utilizing spaced injection and production wells in communication with a subterranean oil reservoir. The injection and production wells can be located and spaced from one another in any desired arrangement. In carrying out the invention, at least one sacrificial agent is introduced into the reservoir prior to injection of microbial or bacterial cells in an amount sufficient to significantly reduce the availability of adsorption sites for the subsequently introduced microbial cells.

A sacrificial agent is injected according to the invention through an injection means comprising one or more injection wells into a subterranean petroleum containing formation to occupy or cover all potential adsorption sites of the rock within the subterranean formation thereby reducing the extent of adsorption of the microbial cells or agents injected therebehind. The sacrificial agent performs best when it exhibits high adsorption on active sites of rock surfaces and thus diminishes microbial cell adsorption. Active adsorption sites on the formation rock refer to those areas of rock surfaces capable of adsorbing a material from a solution on contact.

It is within the scope of the invention to use any sacrificial agent which exhibits high adsorption on active sites of rock surfaces. Suitable sacrificial agents include inorganic salts, such as alkali metal carbonates and polyphosphates, e.g., sodium carbonate and sodium polyphosphate, lignosulfonates, e.g., sodium lignosulfonate, polymeric viscosifiers, such as polyacrylamides, as well as a number of other known materials such as disclosed in U.S. patents referred to hereinbefore.

The amount of sacrificial agent employed can vary somewhat depending upon the desired extent of penetration into the formation desired and the depth desired for penetration of the subsequently introduced microbial cells.

In actual operation, the amount of sacrificial agent injected into the reservoir can be an amount up to and including that required to occupy all of the active sites of the formation matrix. If less than the maximum amount is used, there will be a corresponding increase in the adsorption of microbial cells from the injected solution containing same onto the formation matrix although the amount of increase will not be as in the case where the formation is completely free of the sacrificial agent. The upper concentration limit is the amount of sacrificial agent needed to occupy the active sites on a formation matrix. The detriment resulting from using excess sacrificial agent would be an increase in cost of operating the oil recovery program. The amount of sacrificial agents, such as inorganic salts, carboxylic acids, polymeric viscosifiers, lignosulfonates, and the like, needed in the process of this invention depends on the particular formation, the area or pattern to be swept, and other formation characteristics. Those skilled in the art can determine the exact quantity needed to afford the desired amount of protection. Generally, it has been found that the sacrificial agent in the solution slug injected into the formation will be effective in a concentration of from about 0.01 to about 10 percent by weight of the total solution while an effective volume of these materials will range from about 0.01 to about 1 pore volume of the aqueous solution containing the sacrificial agent.

As indicated hereinbefore, microorganisms are injected into the formation following treatment of the subterranean formation with a sacrificial agent. A medium containing the microorganisms can be introduced into a subterranean formation immediately following injection of sacrificial agent or after the formation has been contacted with an aqueous solution following injection of the sacrificial agent. Thus, in actual practice, the formation can be treated sequentially by the steps of:

(1) contacting of the formation with a solution containing sacrificial agent,

(2) contacting the thus treated formation with an aqueous solution,

(3) introducing a solution containing a microorganism, preferably an aqueous, and more preferably an isotonic saline suspension,

(4) introducing additional aqueous medium, e.g., an isotonic saline solution,

(5) introducing the same or a different microorganism in an aqueous suspension, and

(6) Further introducing an aqueous medium, preferably an isotonic saline solution.

The subterranean formation can be treated with the microorganisms by any procedure found most convenient and the invention is not limited to any particular method of introduction. The microorganism cultures can be introduced into formations with brine or water which is being injected into a subterranean formation for the purpose of a water drive.

The type of nutrient medium employed with the microorganisms will vary with the type or types of microorganisms being handled as is well known in the microbiological art. A mineral salt type medium is usually used.

The amount of microorganism introduced into the formation will depend upon the results desired. Anyone skilled in the art can determine the amount needed to provide the enhanced recovery desired.

The term "microbial cells" as used herein is intended to include bacteria, related microorganisms such as actinomycetes and fungi, and enzymes.

After introduction of the microbial cells or agents deep into the formation, the formation can be shut in for a sufficient period of time to allow the microbial cells to produce metabolic products, such as carbon dioxide, carboxylic acid and the like. After sufficient time is past to allow formation of the desired products, the formation can be subjected to the introduction of a drive fluid or other post-primary means for displacing the oil from the formation to the surface through a production well.

The potential of this invention ior positioning microorganisms deep within a porous media such as formation rock in oil reservoirs is demonstrated by the following laboratory runs which are presented for the purpose of illustration and are not intended to limit the scope of the invention as defined in the claims.

The following examples substantiate the operability of the present invention.

Berea sandstone cores which had been oilflooded and then water flooded to residual oil saturation were used in all the experimental work. The Berea cores were treated with the sacrificial agent prior to the injection of the bacterial cells. This was a screening procedure and no attempt was made to optimize the effect of the sacrificial agent in regard to concentration or pH effects.

The usual injection sequence was (a) sacrifical agent solution, (b) saline solution, (c) bacterial cell suspension, and (d) saline solution. Bacterial counts were made by plate count on aliquots of the effluent solution. At the conclusion of each experiment, bacterial cells were still flowing through the core so the improvement in cell passage is only relative and not absolute. In addition, no quantitative determination of the death of cells within the core could be made. Only viable cells were counted in the core effluent which may have been a small fraction of the total number of cells (both viable and nonviable) which passed through the core.

In the following examples, the bacterial cells were suspended in a salt solution containing a small amount of peptone. This type of composition exhibited a stabilized cell count for several hours. Since there was no technique to determine the fate of the cells in the core, the cell count could have increased or decreased relative to the total number of cells injected. The sacrificial agent probably had no effect on the cell count so the change of cell number in the control cores (no sacrificial agent) should have paralleled the cell count change in the experimental cores.

This example used a commercially available partially hydrolyzed polyacrylamide (mol. wt. ca. one million) as the sacrifical agent (aqueous solution 500 ppm polymer). All experimental saline solutions were 0.85% sodium chloride. The waterflooded residual oil saturated Berea sandstone control core was treated sequentially with:

(a) one pore volume of saline solution

(b) one pore volume of Serratia marcescens saline suspension

(c) five pore volumes of saline solution

(d) one pore volume of Escherichia coli saline suspension

The salt solutions passing through the control core after cell introduction were plated for cell count: S. marcescens was determined by the red pigmentation of the colonies; E. coli was plated on EMB agar and only the typical E. coli colonies were counted. Flow rate through the core was equivalent to one foot per day.

In the control core, 2.48.times.10.sup.8 cells of S. marcescens were introduced and 4.69.times.10.sup.4 cells passed through. With E. coli a sample of 3.5.times.10.sup.8 cells were introduced and no cells were detectable in the core effluent.

ln the invention core, the Berea sandstore core was treated sequentially with:

(a) one pore volume of polyacrylamide (500 ppm) solution

(b) one pore volume of saline solution

(c) one pore volume of Serratia marcescens saline suspension

(d) five pore volumes of saline solution

(e) one pore volume of Escherichia coli saline suspension

(f) five pore volumes of saline solution

The injected samples of Serratia marcescens contained 2.48.times.10.sup.8 cells: the untreated Berea sandstone core permitted the passage of 4.69.times.10.sup.4 cells whereas the treated core permitted the passage of 2.94.times.10.sup.7 cells. This represents about a 62686% improvement in the ability of the polyacrylamide treated core to pass cells relative to the untreated core's capacity to allow the passage of bacterial cells.

The injected samples of Escherichia coli contained 3.5.times.10.sup.8 cells: the untreated Berea sandstone core did not permit the passage of a detectable quantity of cells whereas the polyacrylamide treated core permitted the passage of 2.15.times.10.sup.6 cells. This represents an infinite improvement in the capacity of the treated core to pass cells relative to the capacity of the untreated core to allow the passage of bacterial cells.

In an additional run with the polyacrylamide sacrificial agent similar results were ohtained. In this run the injection sequence was changed to: one pore volume of aqueous polyacrylamide (500 ppm), one pore volume of saline solution, one pore volume of a saline suspension of Serratia marcescens cells and ten pore volumes of saline solution. The injected samples of S. marcescens contained 4.5.times.10.sup.9 cells: the untreated Berea sandstone core permitted the passage of 4.2.times.10.sup.4 cells whereas the treated core permitted the passage of 5.45.times.10.sup.7 cells. This represents a 129762% improvement in the capacity of the polyacrylamide treated core to pass cells relative to the capacity of the untreated core to pass bacterial cells.

In this example, a saline solution which was 0.1 weight percent in sodium carbonate and 0.2 weight percent in sodium polyphosphate was used as the sacrificial agent. The injection sequence in this experiment was one pore volume of the carbonate/polyphosphate solution, one pore volume of saline solution, one pore volume of a saline suspension of Serratia marcescens bacterial cells and five pore volumes of saline solution. The injected samples of S. marcescens contained 4.73.times.10.sup.9 cells: the untreated Berea sandstone core permitted the passage of 1.28.times.10.sup.4 cells whereas the treated core permitted the passage of 1.22.times.10.sup.5 cells. This represents about a 953% improvement in regard to the treated core's ability to pass bacterial cells compared to the capacity of the untreated core.

The example procedure in this run was the same as that used in Example II. An aqueous solution of a xanthan biopolysaccharide was used as the sacrificial agent and cells of Serratia marcescens were injected into the Berea cores. The injected samples of S. marcescens contained 3.43.times.10.sup.9 cells: the untreated Berea sandstone core permitted the passage of 8.6.times.10.sup.4 cells whereas the treated core permitted the passage of 7.9.times.10.sup.5 cells. This represents about a 918% improvement in the capacity of the xanthan treated core to pass cells relative to the untreated core's capacity to pass bacterial cells.

The example procedure in this example was essentially the same as that of Example II except for the use of a ten pore volume slug of saline solution in the injection sequence rather than a five pore volume slug. Aqueous 1 weight percent ammonium lignosulfonate was sacrificial agent. The injected samples of Serratia marcescens contained 7.75.times.10.sup.8 cells: the untreated Berea sandstone core permitted the passage of 3.75.times.10.sup.4 cells whereas the treated core permitted the passage of 3.2.times.10.sup.5 cells. This represents about an 853% improvement in regard to the core's capacity to pass bacterial cells after the ammonium lignosulfonate treatment.

The example procedure in this example was the same as that used in Example II. An aqueous solution of guar gum was used as the sacrificial agent. Injected samples of Serratia marcescens contained 1.89.times.10.sup.9 cells: the untreated core permitted the passage of 8.79.times.10.sup.4 cells whereas the guar gum treated core permitted the passage of 3.25.times.10.sup.5 cells. This represents about a 370% improvement in the capacity of the treated core to pass cells relative to the untreated core's capacity to allow passage of bacterial cells.