Effect of Clays on the Spontaneous Imbibition Behavior of Surfactant Solutions in Shale Oil Reservoirs

Surfactant-enhanced spontaneous imbibition is a proven method of enhancing oil recovery from shale reservoirs. However, a significant knowledge gap concerning the impact of clay minerals on surfactant-enhanced imbibition in shale reservoirs remains. Therefore, this study first analyzed the mineral composition and pore structure of the shale reservoirs. Subsequently, three surfactants were selected for spontaneous imbibition experiments. The influence of various clay minerals on the surfactant imbibition was assessed by considering various properties of the imbibition system. The results showed that the shale pores primarily consisted of macropores (44.03%) and mesopores (43.94%). Surfactants enhance the imbibition recovery by altering the wettability of various pore types. Nuclear Magnetic Resonance results show that mesopores and macropores are the main contributions to imbibition recovery. Under the influence of clay minerals, the effect of surfactant on improving imbibition recovery is zwitterionic type (26.6%) > anionic type (20.57%) > nonionic type (11.56%) > formation water (7.03%). And the imbibition curve shows four stages different from the typical tight reservoir. At the macroscopic scale, clay minerals induce fractures along the shale laminae, thereby expanding the imbibition area and creating additional oil drainage channels. At the microscopic scale, osmotic pressure acts as an additional driving force to enhance oil recovery from micropores and mesopores. The imbibition process of surfactant solutions in shale reservoirs follows this sequence: "imbibition - energy increase (swelling and osmotic pressure) - wettability synergy - imbibition displacement." Maximizing the synergistic effects of swelling clay minerals and surfactants is crucial for enhancing oil recovery in continental shale reservoirs.