Vesicular Dispersion Delivery Systems and Surfactant Waterflooding

Abstract

Abstract Surfactant waterflooding processes that rely on ultralow interfacial tensions suffer from surfactant retention by reservoir rock and from the need to avoid injectivity problems. New findings reported here open the possibility that by delivering the surfactant in vesicle form, more successful low-concentration, alcohol-free surfactant waterflooding processes can be designed. Basic studies of low concentration (less than 2 wt %) aqueous dispersions of lamellar liquid crystals of a model surfactant, Texas No. 1, have established the role of dispersed liquid crystallites in the achievement of ultralow tensions between oil and water. Recent work, including fast-freeze, cold-stage transmission electron microscopy (TEM), reveals that sonication both in the absence and the presence of sodium chloride converts particulate dispersions of Texas No. 1 into dispersions of vesicles, which are spheroidal bilayers or multilayers, less than 0.1 mum in diameter filled with aqueous phase. Vesicles ordinarily revert only very slowly to the bulk liquid crystalline state. We find, however, that their stability depends on their preparation and salinity history, and that contact with oil can accelerate greatly the reversion of a vesiculated dispersion and enable it to produce low tensions between oil and water. Tests with Berea cores show that surfactant retention and attendant pressure buildup can be reduced greatly by sonicating Texas No. 1 dispersions to convert liquid crystallites to vesicles. In simple core-flooding experiments both the unsonicated liquid crystalline dispersions and the sonicated vesicle dispersions are able to produce substantial amounts of residual oil. We point out implications and directions for further investigation. Introduction Methods of enhancing, petroleum recovery, especially tertiary recovery, following the primary and secondary stages, are under intense research and development. Among these are at least two classes of surfactant-based recovery methods-surfactant waterflooding and so-called micellar or microemulsion flooding. Gilliland and Conley suggest that of the various enhanced-recovery methods, surfactant waterflooding has the potential for the widest application in the U.S. Residual oil is trapped as blobs in porous rock by capillary forces. The number of mechanisms is limited both for reducing entrapment and for mobilizing that residual oil remaining entrapped, there by improving the microscopic displacement efficiency of a petroleum recovery process. Taber and Melrose and Brandner established that tertiary oil recovery by an immiscible flooding process is possible by increasing the capillary number, which measures the ratio of Darcy flow forces of mobilization to capillary forces of entrapment. In practice this can be achieved by lowering the oil-water interfacial tension to about 10 mN/m or less. That this is feasible in the surfactant waterflooding range-i.e. at surfactant concentration less than those characterizing the microemulsion flooding range-and in the absence of cosurfactants or cosolvents that typify microemulsions is well established. Gale and Sandvik suggested four criteria for selecting a surfactant for a tertiary oil-recovery process:low oil-water interfacial tension,low adsorption.compatibility with reservoir fluids, andlow cost. For a given oil and type of surfactant, it has been shown that the interfacial tensions are extremely sensitive to surfactant molecular weight. SPEJ P. 37^