Alternate Slugs Of Gas And Dilute Surfactant For Mobility Control During Chemical Flooding

Abstract

Abstract Studied was a novel technique for mobility control during chemical flooding, namely, the injection of alternate slugs of inert gas and surfactant solution. This method of reducing mobility offers a possible alternative to polymer thickened water. Performance was investigated in consolidated sandstone and carbonate. Sandstone included cores, and large slabs of rock oriented so as to maximize gravity segregation of gas. Reported data suggest that, for mobility control in laboratory rocks, performance of alternate slugs of gas and dilute surfactant compares favorably with water soluble polymers, without the many disadvantages of the latter. During experiments in large sandstone slabs, excessive foam drainage, and gravity segregation of gas, did not occur. Introduction Surfactant enhanced waterflooding (micellar flooding) is an oil recovery method for reservoirs depleted by waterflood. The preferred method of conducting micellar floods is to:inject a small (less than 1 Vp) slug of relatively concentrated surfactant, andfollow with a less expensive drive liquid. The surfactant slug lowers oil-water interfacial tension, promotes oil-water miscibility, and reduces So. If the surfactant slug is effective, a reconnected oil bank is generated, and driven, by the slug, which is, in turn, displaced by the drive. Since surfactant slugs are often as small as 0.1 Vp, or perhaps less, formation of viscous fingers can lead to bypassing, dilution, and process breakdown. For process efficiency, the process breakdown. For process efficiency, the mobility of the slug and drive must be less than the mobility of the displaced liquids. Mobility control in the surfactant slug is accomplished either by injecting the surfactant as an inherently viscous microemulsion or soluble oil, or by the addition of water soluble polymer. Drive is generally polymer thickened water. Dilute polymer solutions are used as drives because of economy, and the lack of suitable alternatives. The two classes of polymers used most frequently are polyacrylamides and Xanthan gums; neither is ideal. At elevated temperatures, both can degrade with a consequent loss of solution viscosity. Both can form gels with multivalent cations, and can flocculate finely divided solids. The result is injection well plugging. In addition, polyacrylamide is degraded by mechanical shear at polyacrylamide is degraded by mechanical shear at shear rates low enough to preclude injection through chokes, and, in many instances, through perforations. As marketed, Xanthan gums contain bacterial residue which must be removed by filteration. They are also a favorite diet of bacteria, and bacterial growth must be inhibited. It is not known for certain whether either class of polymer can be transported totally intact through a reservoir. In this paper, we present a different method for achieving mobility control during chemical flooding, namely, the injection of alternate slugs of gas and surfactant solution. The object is to generate, in porous media, a low mobility dispersion of gas in liquid. For brevity, we shall refer to the dispersion of gas in liquid as foam. Foam has been suggested as a means for providing mobility control for a variety of processes, providing mobility control for a variety of processes, e.g. flooding with CO2, steam, solvent, or even water. It offers a unique advantage, however, in the case of chemical flooding. The surfactant adsorption requirement of the reservoir rock is satisfied by the micellar slug.