Critical Aspects In Surfactant Flooding Procedure At Mixed-wet Conditions

Abstract

Abstract The theory and procedures for surfactant flooding were developed for water-wet sandstone, but many sandstone reservoirs are today characterized as mixed-wet. The objective for the study was to carry out critical investigation of experimental methods used to evaluate the potential for surfactant flooding in mixed-wet sandstone reservoirs. Water flooding and surfactant flooding experiments were performed in mixed-wet Berea outcrop sandstone, and in reservoir sandstone rock. During water flooding long tail-end production was observed for both the mixed-wet Berea and reservoir rock. The relative permeability (kr)-curves estimated by history matching were similar for these rocks. Since kr of oil (kro) is low at higher water saturations, the residual oil saturation (Sor) will not be reached with realistic injection volumes. The remaining oil saturation (ROS) was both in the water floods and surfactant floods strongly dependent on the volumes injected. The focus should therefore be on the alteration of kr-curves by the surfactant rather than reduction of Sor. Solubilization of oil into the surfactant phase is challenging in relative permeability experiments at mixed-wet conditions. When injected surfactant solution was not in equilibrium with the oil, it was not possible to correct the oil production for the solubilization because the surfactant phase was not in equilibrium with the remaining oil in the rock. The estimated kr-curves should therefore have been at higher water-saturations. The uncertainty related to solubilization of oil can be reduced by flooding with surfactant systems with equilibrium between the phases. In mixed-wet rock having negative Pc at higher water saturation (Sw), capillary forces will act to reduce the oil production rate. The effect increases in shorter core plugs and at lower injection rates and it also seems to be more important if displaced oil phase has low viscosity. It is recommended to always evaluate capillary end effects when dealing with mixed-wet rock material to secure that additional oil production due to reduced end effects is not included in the surfactant flooding potential. Simulation of surfactant flooding in mixed-wet formations require a fair representation of the capillary number (Nc) dependent kr curvature. It is demonstrated that this is possible using a simple surfactant model available in commercial simulators and a practical cut-off value for Sor at low Nc in the model. Recommendations are given on how to secure the quality of surfactant flooding experiments and the interpretation of these experiments to allow robust data for scaling of the laboratory results to field.