Novel insights into dynamic imbibition mechanisms of surfactant solution enhancing continental shale oil recovery

Shale oil is an unconventional energy resource, and its efficient development is a key focus of current energy research. China’s continental shale resources are abundant, with high clay content and poor physical properties. The development of shale oil reservoirs after horizontal well fracturing remains suboptimal, requiring further recovery enhancement. Surfactant imbibition oil recovery technology is recognized as one of the effective methods for enhancing oil recovery in unconventional reservoirs. This study proposes a novel experimental apparatus and method for dynamic imbibition, and also develops a three-dimensional (3D) micro pore-fracture dynamic imbibition model. Furthermore, surfactant selection criteria based on wettability and interfacial tension are established. The results show that Cetyltrimethylammonium bromide (CTAB) forms ion pairs on the surface of shale pores, rapidly removing the oil film covering the shale surface. This induces a wettability reversal, reducing the water-phase contact angle from 132° to 55.1°. The use of the CTAB surfactant solution resulted in an 11.5 percentage points improvement in final imbibition efficiency. After CTAB surfactant solution detaches oil droplets from the shale surface, more of the water phase comes into contact with the clay minerals and micro-damage structures in the shale. Hydration induces new micro-damage in the shale, which accumulate and evolve into micro-fractures, creating flow channels for shale oil transport. Another key mechanism for enhancing the dynamic imbibition efficiency of continental shale oil is to modify the shale surface wettability while strategically adjusting interfacial tension. The selection criteria for surfactants used in shale oil dynamic imbibition should require not only wettability alteration capability but also an appropriate interfacial tension ranging from 2 to 10 mN/m. This balances the promoting effect of capillary forces with the suppressing effect of the Jamin effect, thereby facilitating the movement of oil droplets within deformed pores. Significant variations in average pressure within the pores enhance micro dynamic imbibition efficiency in shale. In the practical development of shale oil reservoirs, providing effective short-cycle energy replenishment and altering the average pressure within shale pores are crucial for guiding the enhancement of recovery in continental shale oil reservoirs.