Enhancing Water Flooding Oil Recovery by Regulating Rock Surface Under-Oil–Water Wettability

Traditionally, the efficiency of water flooding for oil recovery has been attributed to whether the formation is “water-wet” (hydrophilic) or “oil-wet” (oleophilic). However, water flooding is fundamentally a process in which the aqueous phase displaces the oil phase from the rock surface, involving the replacement of the “oil-rock” interface with a “water-rock” interface. This process is closely related to the under-oil–water wettability of the rock surface. In this study, alkoxysilane modification was used to regulate rock surfaces, achieving three wetting states: under-oil hydrophilic (under-oil–water contact angle of 34.9°), under-oil neutral (under-oil–water contact angle of 92.4°), and under-oil hydrophobic (under-oil–water contact angle of 129.2°). The simulated water flooding and spontaneous imbibition experiments demonstrate that enhancing the under-oil hydrophilicity of rock significantly improves both water flooding oil recovery and imbibition recovery. Water flooding results show that, compared to untreated natural cores, the oil recovery of under-oil hydrophilic cores increased by 10.5%, with a corresponding reduction in displacement pressure of 1.4 MPa. In contrast, under-oil hydrophobic cores exhibited a 13.7% decrease in recovery and a 1.3 MPa increase in pressure. Spontaneous imbibition tests revealed that under-oil hydrophilic cores achieved an imbibition recovery of 34.02%, whereas under-oil hydrophobic cores showed almost no effective oil production via imbibition. It also confirms that it is not the overall water or oil wettability (in air) of the rock surface but the under-oil–water wettability that determines the efficiency of water flooding. These findings can provide insightful guidance for searching for more efficient water flooding methods.