Pore-scale dynamic behavior and displacement mechanisms of surfactant flooding for heavy oil recovery

Abstract

Surfactant flooding is widely used in enhanced heavy oil recovery. It is important to understand the mechanisms involved in different stages of surfactant flooding at the pore scale. First, the effects of surfactant flooding at different surfactant concentrations were evaluated, and the optimal concentration was determined by analyzing the interfacial tension (IFT), emulsification states, turbiscan stability index (TSI), and viscosity reduction rate. Then, the microscopic displacement mechanism at different stages of the surfactant flooding of heavy oil was visualized by a microscopic visualization model. Furthermore, the residual oil was classified. The optimal surfactant concentration was found to be 0.5 wt%. At this concentration, the IFT was only 0.092 mN/m, the average droplet size of O/W emulsion is 7.1 μm, and the viscosity reduction rate was as high as 99.37%. The surfactant flooding process occurred in three stages. The main mechanisms in the initial stage are cutting emulsification and stable displacement. Temporary blocking in the small throat resulted in a change in the diversion ratio, Further, the cutting and tension-fracture effects of the small throat on the heavy oil droplets and the stretching effect on the adhered oil film were found to be the main mechanisms of middle stage. The main mechanisms of the last stage were the pulling, peeling, and swirling–carrying effects of surfactant flooding on the heavy oil in the stagnant zone, which were controlled by the large throats. Moreover, liquid-flow carrying, and emulsification-cutting effects in the pore system controlled by the small throats. The primary residual oil did not flow migrate throughout the development process. Further, secondary residual oil was generated by the plugging of heavy oil droplets at very small throats, resulting in a diversion ratio of zero. The results are highly significat for optimizing surfactant flooding for enhanced oil recovery technology and the development of high-efficiency surfactants.