Sulfoxide Cosurfactants in Enhanced Oil Recovery Processes

This invention relates to surfactant systems. More specifically, this invention relates to the use of new surfactant systems for oil recovery.

Water flooding and surfactant flooding are processes well known in the art to recover vast quantities of oil which remain in the formation after primary oil recovery. In the case of surfactant flooding, it is not uncommon to employ a cosurfactant wherein the latter aids in forming microemulsions with the oil to be recovered.

The oil recovery effectiveness of surfactant systems though, can be diminished by the presence of a highly saline environment (i.e., greater than 2 weight percent total dissolved solids) in the waters associated with the oil to be recovered. This is because high salinity solutions can cause precipitation of surfactants and hence destroy their effectiveness in the oil recovery process. A highly saline environment can also diminish the effectiveness of mobility buffers by reducing their viscosity. Therefore, it is highly desirable to develop surfactant systems which are effective in highly saline environments.

It is therefore an object of this invention to provide a surfactant system comprising a surfactant and cosurfactant which are effective for tertiary oil recovery even in highly saline environments. A further object of this invention is to provide a surfactant flooding process which enhances tertiary oil recovery.

These and other objects, advantages, details, features and embodiments of this invention will become apparent to those skilled in the art from the following detailed description of the invention and the appended claims.

In accordance with this invention it has been found that sulfoxides can be efficiently used as cosurfactants in surfactant flooding operations for oil recovery.

Thus, in accordance with a first embodiment of this invention, there is provided a new surfactant system comprising a hydrocarbon sulfonate surfactant and a sulfoxide as a cosurfactant.

The preferred surfactant system of this invention is an aqueous system, optionally containing a protective agent. Generally, water is used containing a certain quantity of sodium chloride for practical reasons. Typical and preferred compositions of a surfactant system of this invention are shown in the following table:

The surfactant used in the surfactant system of this invention is a hydrocarbon sulfonate. It can be made from relatively pure hydrocarbons and hydrocarbon mixtures. The preferred surfactant is a petroleum sulfonate. The petroleum sulfonates are commercially available products. The presently preferred surfactants used in oil recovery are petroleum sulfonates having an average equivalent weight in the range of 325 to 600. Best results with presently known reservoirs are believed to be obtainable with petroleum sulfonates having average equivalent weights in the range of about 325 to 500.

In accordance with this invention, sulfoxides are used as cosurfactants. These sulfoxides having a maximum of 14 carbon atoms can be represented by the following structural formulas: ##STR1## wherein R is an alkyl, aryl, alkaryl, or aralkyl group and Q is a linear or branched alkylene group, the sulfoxide having a minimum total of 2 carbon atoms and a maximum total of 14 carbon atoms.

Representative examples of sulfoxides of the type (I) suitable for use in the present invention include dimethyl sulfoxide, diphenyl sulfoxide, methyl n-octyl sulfoxide, diisobutyl sulfoxide, ethyl n-dodecyl sulfoxide, isopropyl n-decyl sulfoxide, di-n-propyl sulfoxide, n-butyl p-tolyl sulfoxide, di-p-tolyl sulfoxide, benzyl methyl sulfoxide, and mixtures thereof.

Representative examples of the sulfoxides of the type described in formula (II) suitable for use in the present invention include tetramethylene sulfoxide, pentamethylene sulfoxide, trimethylene sulfoxide, 2,6-dimethyl pentamethylene sulfoxide, 4-phenyl pentamethylene sulfoxide, 2-n-propyl 5-p-tolyl tetramethylene sulfoxide, and mixtures thereof.

Dimethyl sulfoxide is the preferred sulfoxide for use in the present invention because of its commercial availability and its demonstrated effectiveness as a cosurfactant as shown in the example.

Whatever sulfoxide is used must, however, demonstrate some solubility or miscibility with water. Generally, the sulfoxide should have a water solubility of 0.1 to 20, preferably 1-10 weight percent.

An oil recovery process using the surfactant system of this invention as defined above constitutes another embodiment of this invention. This process involves generally the conventional steps of post primary oil recovery and distinguishes over the known procedures primarily in the use of sulfoxides defined as cosurfactants.

It is optional to carry out a preflush step preceding the post-primary oil recovery operation. Such preflush operations are known in the art. Generally, a brine compatible with the surfactant system is injected via at least one injection well into the subterranean formation. Such a brine typically contains 2000-50,000 ppm salts, predominantly sodium chloride. Preferably a brine solution as utilized in the production of the surfactant system is also used in this preflush step.

The quantity of the preflush employed will generally be in a range of about 0.01 to 2, preferably 0.25 to 1 pore volume, based on the total pore volume of the formation or reservoir subjected to the recovery.

After the optional preflush step the surfactant fluid of this invention is injected into the reservoir via at least one injection well. The surfactant system is injected in an amount usually in the range of about 0.001 to 1.0, preferably 0.01 to 0.25 pore volume based on the pore volume of the total treated and produced formation.

The preferred operation makes use of the aqueous saline surfactant system in the form of a single phase. Usually the surfactant system contains water, surfactant and the sulfoxide compound cosurfactant as the principal ingredients. The single phase surfactant system is introduced into the formation via one or more injection wells and the generation of a microemulsion takes place in-situ as the injected surfactant system contacts the oil in place. It is contemplated that surfactant systems characterized by the presence of more than one phase are preferably subjected to continuous mixing during the injection operation.

The present invention can be utilized for a variety of subterranean reservoirs. The invention is, however, particularly preferred in reservoirs containing hard brine connate water. Such hard brines are characterized by a high content of Mg++ and Ca++ ions in the reservoir water. Typical hard brines contain more than 100 ppm of Ca++ and/or Mg++.

Protective agents are an especially preferred ingredient in the surfactant system of this invention when used for oil recovery from reservoirs with hard brines. They aid in solubilizing the surfactant in a high salinity environment. Examples for such protecting agents are polyethoxylated fatty alcohols and polyethoxylated alkylphenols. In addition, the sodium salts of sulfated polyethoxylated fatty alcohols and polyethoxylated alkylphenols are known in the art to function as protective agents.

Following the surfactant slug it is presently preferred, although again not necessary, to inject a mobility buffer solution into the reservoir. This buffer helps prevent fingering and enhances the efficiency of the oil recovery. Mobility buffer solutions are aqueous solutions of thickening agents. Examples of useful mobility buffers are aqueous and nonaqueous fluids containing mobility reducing agents such as high molecular weight partially hydrolyzed polyacrylamides, biopolysaccharides, cellulose ethers and the like. The mobility buffer contains 50 to 20,000, preferably 200 to 5,000 ppm of the mobility reducing agent in the fluid.

The injection of the mobility buffer fluid can be at a constant composition or the mobility buffer can be graded, i.e., the injection starts out at a relatively high concentration at the leading edge and the concentration tapers off toward the trailing edge. As an example, the mobility buffer can start with a concentration of 2500 ppm of polyacrylamide in the water and end with 250 ppm of polyacrylamide in water. These mobility buffer fluids are well known in the art.

The invention will be still more fully understood from the following detailed example which is intended for illustrative purposes only and not for an undue limitation of the scope of this invention.

The following experimental procedure was used to condition Berea sandstone cores for the surfactant flood runs.

Berea sandstone cores measuring approximately 3 feet in length and 3 inches in diameter were dried under vacuum for 24 hours at 250.degree. F. Polycarbonate disc end plates with centrally located 1/8" threaded openings were secured to each end of the core with epoxy adhesive before applying an epoxy coating to the outside surface of the core. The epoxy coating material was formulated by mixing 550 g of a commercially available epoxy resin, 50 g of a suitable activator and 140 g diatomaceous earth. This mixture was stirred until smooth before applying to the surface of the core. The cores were rotated continuously as the epoxy mixture was applied with a 2" paint brush. Four gauze strips measuring 2".times.12" were applied to the core in the following manner: a first gauze strip was applied to the core and covered with epoxy as the core was rotated; the remaining three strips were then individually incorporated in a similar manner. The core coating was cured over a period of about 4 hours at ambient temperature as the core was rotated. One-eight inch male connector fittings were placed on each end of the core and pipe plug caps were put on the core.

The core was weighed to determine the dry weight before being saturated with brine of the desired concentration. A vacuum of about 1 mm was pulled on the core before saturating the core with approximately 500 mL of brine. After saturation, approximately 100 to 200 mL of brine were pumped through the core before determining the original permeability to water. A 1 mL portion of effluent brine was collected from the saturated core and thereafter during a period of one minute, the volume of additional effluent collected and the pressure in psi were recorded with these values. The original permeability to water, e.g., on the order of 3.2 mL/min at 43 psi could be recorded. The pore volume of the core was calculated by the relationship: ##EQU1##

The brine-saturated core was oil flooded in the conventional manner until oil break-through became detectable by the presence of alternate globules of oil and water in the effluent line. The oil flood was carried out to completion by the 24 hour recycling of oil through the core to remove all the displaceable water. The total water displaced, i.e., water displaced at the point of oil break-through and water displaced by the 24 hour recycle procedure was recorded as water displaced by oil flood. If desired, oil permeability was determined in a manner analogous to that used above for establishing original permeability to water. Prior to waterflood, the effluent line was air blown to remove oil.

The oil-flooded core was waterflooded in the conventional manner until water break-through became detectable by the presence of alternate globules of oil and water in the effluent line. The waterflood was carried to completion by the 24 hour recycling of water through the core to remove all of the displaceable oil. The total oil displaced, i.e., oil displaced at the point of water break-through and oil displaced by the 24 hour recycle procedure was recorded as oil displaced by waterflood. If desired, water permeability after waterflood can be determined in a manner analogous to that used above for original permeability to water. The residual oil volume remaining in the core was calculated by subtracting the oil volume displaced by the waterflood from the water volume displaced by the oilflood. At this point, the core simulated an oil reservoir which had been exhaustivelywaterflooded. Cores were routinely conditioned in this manner prior to carrying out surfactant flood tests.

This example demonstrates the oil recovery effectiveness of an aqueous saline surfactant system containing petroleum sulfonate and dimethylsulfoxide. Waterflood residual oil from a waterwet Berea sandstone core was recovered by surfactant flooding in the conventional manner, i.e., sequential injection of the inventive surfactant system and a thickened aqueous mobility buffer slug of polyacrylamide graded back logarithmically with Arkansas-Burbank water. (Arkansas-Burbank water is relatively fresh river water taken from the Arkansas river having on the average 500 ppm of total dissolved solids.) The surfactant slug had the following composition:

8.71 g Witco TRS 10-410 (62 wt. % active petroleum sulfonate)

4.50 g Dimethyl sulfoxide (DMSO)

3.45 g Sodium Chloride

133.34 g Arkansas-Burbank Water (A-B Water)

Thus, each component was present in the following weight percentages:

The pore volume of the 3".times.3" cylindrical Berea sandstone core was 821 mL. A 63 mL (7.6% PV) slug of the above surfactant system was injected followed by 410.5 mL (0.5 PV) of Betz Hi Vis polyacrylamide (ca. 1700 ppm) in Arkansas-Burbank water graded back logarithmically with Arkansas-Burbank water.

In preparing the core, 585 mL of water (i.e., 2.3 wt. % NaCl in Arkansas-Burbank water) was displaced from the water-saturated core by oilflood indicating the introduction of approximately 584 mL of oil into the core. Subsequent waterflood resulted in about 287 mL of oil being displaced from the core leaving about 297 mL of waterflood residual oil in the Berea sandstone core prior to the surfactant flood.

In the course of the surfactant flood, a total effluent of 2.41 pore volumes (ca. 1985 mL) was collected which contained 258.8 mL of tertiary oil representing 87.1% of the waterflood residual oil.

The data demonstrate the efficiency of the petroleum sulfonate/dimethyl sulfoxide system in recovering waterflood residual oil from a waterwet sandstone core.

Reasonable variations and modifications which will become apparent to those skilled in the art can be made in this invention without departing from the spirit and scope thereof.