Adsorption/Desorption of Sulfonate by Reservoir Rock Minerals in Solutions of Varying Sulfonate Concentrations

In micellar flooding, reservoir rocks are exposed to surfactant solutions of varying concentrations as the surfactant slug advances through the reservoir. Therefore, the attachment and detachment of sulfonates with rocks that are already exposed to surfactant solutions of higher or lower concentrations is of major interest. In this study, the abstraction behavior of purified Na-dodecylbenzenesulfonate on Na-kaolinite by stepwise increase in surfactant concentration is determined. Deabstraction* occurring after reductions in surfactant concentrations at various stages also is determined. Most importantly, the results of incremental abstraction, individual abstraction, and deabstraction showed the system to exhibit hysteresis or memory effects. Also, abstractions obtained at various pH values and during stepwise changes in pH exhibited marked differences. The deabstraction isotherms showed the presence of maximum in certain cases, indicating the occurrence of maximum on the abstraction isotherms to be a real phenomenon. Possible reasons for the hysteresis are phenomenon. Possible reasons for the hysteresis are considered, and the practical implications of these memory effects on micellar flooding and depletion experiments using cores are discussed. Introduction Loss of surfactants owing to their interactions with reservoir rocks and fluid is possibly the most important factor that can determine the efficiency of a micellar flooding process. While there has been considerable work with process. While there has been considerable work with dilute surfactant solutions, mechanisms by which surfactants interact with rocks in their critical micelle concentration (CMC) range have not been studied in detail. Nevertheless, some limited data that have been reported in the literature do suggest that the adsorption characteristics of systems made up of concentrated surfactant solutions (above the CMC) are markedly different from those of systems involving dilute solutions. Adsorption isotherms above CMC have been reported to exhibit shapes that have not been encountered elsewhere. Our past work on abstraction of dodecylbenzenesulfonate on Na-kaolinite clearly showed the complex nature of the process, which depends on a number of system variables such as the nature and concentration of inorganic electrolytes, surfactant concentration, pH, and temperature. Under certain conditions, the systems exhibited a maximum in the region of CMC and, in some cases, a minimum at higher concentrations. Most interestingly, the presence of the maximum in the abstraction isotherm depended strongly on the type of inorganic electrolyte in the system. From a practical point of view, it would indeed be useful to be able to control the abstraction of sulfonates by rock minerals by controlling the inorganic electrolytes in the system. However, laboratory batch-type adsorption tests cannot be used directly for micellar flooding systems for a number of reasons. One important consideration in this regard is that the reservoir rocks are exposed to surfactant solutions of varying concentration as the surfactant slug advances through the reservoir. To examine the role of this effect, the abstraction behavior of sulfonates by kaolinite during incremental increase and decrease in surfactant concentration has been determined in this study. Comparison of the abstraction isotherms obtained by conventional batch-type tests (B-isotherms) with those obtained by stepwise changes in surfactant concentration (S-isotherms) and the deabstraction of isotherms of sulfonate upon dilution of the system should help in developing an understanding of the surfactant abstraction behavior as well as the phenomenon of abstraction maximum. Materials and Methods Kaolinite Kaolinite used was a well-crystallized Georgia sample with a B.E.T. surface area of 9.8 m2/g [105 sq ft/g]. Homoionic Na-kaolinite prepared according to a procedure described earlier was used for all the procedure described earlier was used for all the adsorption tests discussed here. Surfactants and Chemicals Sodium dodecylbenzenesulfonate (DDBS) purchased from Lachat Chemical Inc. (specified to be 95 % active but analyzed to be 85 %) was purified in the following manner. purified in the following manner. SPEJ P. 343