Secondary Recovery Process

1. Field of the Invention

This invention relates to a process for recovering hydrocarbons from a subterranean hydrocarbon-bearing formation penetrated by an injection well and a production well wherein a drive fluid such as water thickened with a copolymer of (a) vinyl sulfonic acid alkoxylated with ethylene oxide or a mixture of ethylene oxide and propylene oxide and (b) acrylamide is utilized to displace hydrocarbons in the formation toward a production well.

2. Prior Art

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 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 up to 70-80 percent of the original hydrocarbons in place.

Thus, many large reserves of petroleum fluids from which only small recoveries have been realized by present commercial 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 result in recovery of 30-50 percent of the original hydrocarbons left in place. However, generally the application of water flooding to many crudes 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 of this process is enhanced through the beneficial effects of the drastic viscosity reduction that accompanies an increase in temperature. 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 formation, that is, the sorptive capacity of the sand for the crude is great. In addition, interfacial tension between the immiscible phases results in entrapping 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. Another disadvantage is the tendency of the aqueous drive fluid to remove additional gas by diffusion from the in-place oil thus further reducing the already lowered formation oil volume and increasing the viscosity of the oil.

There is a definite need in the art for a water flooding process in which the disadvantages discussed above are largely eliminated or avoided.

This invention relates to a process for recovering hydrocarbons from a subterranean hydrocarbon-bearing formation penetrated by an injection well and a production well which comprises:

(A) injecting into the formation via an injection well a drive fluid comprising water having dissolved therein a small amount of a copolymer comprising repeating units of vinyl sulfonic acid alkoxylated with ethylene oxide, or a mixture of ethylene oxide and propylene oxide wherein the weight percent of ethylene oxide in the said mixture is from about 60 to about 95 and (b) acrylamide.

(B) forcing the said fluid through the formation and

(C) recovering hydrocarbons through the production well.

An additional embodiment of this invention relates to the driving fluid compositions utilized in step (A) which may be saturated with carbon dioxide and/or natural gas.

Prior to practicing the process of this invention it is sometimes desirable to open up a communication path through the formation by a hydraulic fracturing operation. Hydraulic fracturing is a well-known technique for establishing a communication path between an injection well and a production well. Fracturing is usually accomplished by forcing a liquid such as water, oil or any other suitable hydrocarbon fraction into the formation at pressure of from about 300 to about 3000 psig which are sufficient to rupture the formation and to open up channels therein. By use of this method it is possible to position the fracture at any desired vertical location with respect to the bottom of the oil-filled zone. It is not essential that the fracture planes be horizontally oriented, although it is, of course, preferably that they be. After the fracture has been established, and without diminishing the fracture pressure, a propping agent may be injected into the fraction in order to prevent healing of the fracture which would destroy its usefulness for fluid flow communication purposes. Gravel, metal shot, glass beads, sand, etc. and mixtures thereof are generally employed as propping agents. When sand is utilized as the propping agent particles having a Tyler mesh size of from about 8 to about 40 are preferred (i.e., from about 0.016 to about 0.093 inches.)

The water-soluble copolymers utilized in the drive fluids of this invention comprise repeating units of (a) alkoxylated vinyl sulfonic acid and (b) acrylamide. In the said copolymers the weight percent of the alkoxylated vinyl sulfonic acid units ranges from about 5 to about 35 with the balance being acrylamide units. Generally the number average molecular weight of the alkoxylated vinyl sulfonic acid-acrylamide copolymers will range from about 10,000 to about 2,000,000 or more. The copolymers of alkoxylated vinyl sulfonic acid and acrylamide can be prepared by the usual vinyl compound polymerization methods at temperatures of about 30.degree. to about 100.degree. C. employing a suitable polymerization catalyst such as azo-bis-(isobutyronitrile), ammonium persulfate, etc. The preparation of vinyl sulfonic acid-acrylamide polymers is described in detail in Norton et al U.S. Pat. No. 3,779,917 and in DePaolo U.S. Pat. No. 3,405,003 which are incorporated herein in their entirety by reference.

The alkoxylated vinyl sulfonic acid employed in preparing the copolymers of (a) alkoxylated vinyl sulfonic acid and (b) acrylamide comprises vinyl sulfonic acid alkoxylated with from about 2 to about 100 percent by weight of ethylene oxide or with a mixture of ethylene oxide and propylene oxide and wherein the weight percent of ethylene oxide in the said mixture is about 60 to about 95.

The alkoxylation of vinyl sulfonic acid can be conveniently conducted using methods well known in the art. For example, an aqueous solution of the vinyl sulfonic acid comprising about 10 to about 30 weight percent or more of the vinyl sulfonic acid in water along with about 0.5 weight percent or more of powdered potassium hydroxide or sodium hydroxide is charged to an autoclave and the autoclave and contents heated to a temperature of about 125.degree. to about 200.degree. C. after which the required weight of ethylene oxide or a mixture of ethylene oxide and propylene oxide is pressured with nitrogen into the autoclave over a period of 1 to about 3 hours or more following which the autoclave is allowed to cool to room temperature and then vented. The reaction product remaining is then stripped to remove unreacted materials, etc. to yield the water-soluble, alkoxylated vinyl sulfonic acid.

A number of other methods are set out in the art for conducting such alkoxylation reactions including those described in U.S. Pat. Nos. 2,213,477, 2,233,381; 2,131,142; 3,879,475; 2,174,761; 2,425,845 and 3,062,747.

The following example which illustrates the preparation of the alkoxylated vinyl sulfonic acid is to be considered not limitative.

A total of 425 cc of water, 6.5 g. of powdered potassium hydroxide and 48 g. of a vinyl sulfonic acid are added to an autoclave which is then heated to a temperature of 120.degree. C. Ethylene oxide in the amount of 36 g. is added to the autoclave under nitrogen pressure over a 1.15 hour period during which time the temperature of the autoclave is maintained at 120.degree. C. Next, the autoclave and contents are allowed to cool to room temperature after which the autoclave is vented. The reaction mixture is then stripped of volatiles using a nitrogen purge. The resulting water-soluble product is vinyl sulfonic acid alkoxylated with about 42 weight percent of ethylene oxide.

The following example illustrates the preparation of a copolymer of (a) alkoxylated vinyl sulfonic acid and (b) acrylamide and is to be considered not limitative.

A reaction flask equipped with a moderate speed stirrer is charged with 80 parts by weight of acrylamide, 220 parts by weight of ethanol, 0.62 parts by weight of azobis(isobutyronitrile) and 25 parts by weight vinyl sulfonic acid alkoxylated with 54 weight percent of ethylene oxide prepared as described in EXAMPLE I. The reaction mixture is then flushed with carbon dioxide to remove oxygen following which the polymerization is conducted for 20 hours at 65.degree. C. The reaction mixtures was then stripped of volatiles using a nitrogen purge yielding the alkoxylated vinyl sulfonic acid-acrylamide copolymer which exhibits a molecular weight of about 130,000.

In the secondary recovery process of this invention, generally the aqueous drive fluid will contain from about 0.01 to about 5.0 weight percent or more of the alkoxylated vinyl sulfonic acid-acrylamide copolymer. Optionally, the aqueous drive fluid may be saturated with carbon dioxide and/or natural gas at the injection pressure which generally will be from about 300 to about 3000 psig or more.

If desired, the aqueous drive fluid having dissolved therein the above-described polymeric thickening agent may be made alkaline by addition of an alkaline agent. The advantageous results achieved with the aqueous alkaline medium used in the process of this invention are believed to be derived from the wettability improving characteristics of the alkaline agent.

Useful alkaline agents include compounds selected from the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides, and the basic salts of the alkali metal or alkaline earth metals which are capable of hydrolyzing in an aqueous medium to give an alkaline solution. The concentration of the alkaline agent employed in the drive fluid is generally from about 0.005 to about 0.3 weight percent. Also, alkaline materials such as sodium hypochlorite are highly effective as alkaline agents. Examples of these especially useful alkaline agents include sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide, sodium hypochlorite, potassium hypochlorite, sodium carbonate and potassium carbonate.

A wide variety of surfactants such as alkylaryl sulfonates, alkyl polyethoxylated sulfates, etc. may also be included as a part of the aqueous drive fluid composition. Generally about 0.001 to about 1.0 or more weight percent of the surfactant will be included in the drive fluid.

This invention is best understood by reference to the following example which is offered only as an illustrative embodiment of this invention and is not intended to be limitative.

In a field in which the primary production has already been exhausted, an injection well is completed in the hydrocarbon-bearing formation and perforations are formed between the interval of 5720-5735 feet. A production well is drilled approximately 425 feet distance from the injection well, and perforations are similarly made in the same hydrocarbon-bearing formation at 5725-5740 feet.

The hydrocarbon-bearing formation in both the injection well and the production well is hydraulically fractured using conventional techniques, and a gravel-sand mixture is injected into the fracture to hold it open and prevent healing of the fracture.

In the next step water at a temperature of 70.degree. F. saturated with carbon dioxide at about 1050 psig to which there has been added about 0.21 weight percent sodium hydroxide and containing dissolved therein about 0.32 weight percent of a copolymer of (a) an acrylamide-vinyl sulfonic acid alkoxylated with 39 weight percent of ethylene oxide and (b) acrylamide having a number average molecular weight of about 172,000 is injected via the injection well into the formation at a pressure of about 1050 psig and at the rate of 1 barrel per minute. In the alkoxylated sulfonic acid-acrylamide copolymer the weight percent of acrylamide units is about 78 with the balance being alkoxylated vinyl sulfonic acid units. Injection of the driving fluid is continued at the rate of 1 barrel per minute and at the end of 73 days the rate of production of oil is substantially greater than with water injection alone.

The sodium, potassium and ammonium salts of the above-mentioned copolymers are also useful as thickeners for the drive fluids employed in the process of this invention.