Influence of spontaneous imbibition on post-fracturing well soaking in shale oil reservoirs

To address the current challenges such as the determination of well soaking after fracturing in shale oil reservoirs and lack of theoretical guidance on the design of soaking duration, this study established an oil-water two-phase flow model in consideration of capillary imbibition. By employing finite element-finite volume methods, joint simulation of well soaking and production was performed, which took into account whether the imbibition of the shale matrix will produce micro-fractures and their influence on imbibition and production. Furthermore, the influences of well soaking duration and capillary force on the water absorption of shale matrix during well soaking process and the shale oil production after well opening were investigated. The simulation results revealed that if micro-fractures are generated in the shale matrix due to capillary imbibition of fracturing fluid, imbibition significantly enhances shale oil production. With increasing well soaking duration and shale hydrophilicity, more micro-fractures will be generated in the shale matrix after fracturing, and the water-oil displacement effect becomes stronger, leading to increased shale oil production. In cases where capillary imbibition does not introduce micro-fractures in the shale matrix, the imbibition effect will reduce shale oil production. With soaking duration and shale hydrophilicity increase, water saturation in the matrix rises, causing a decrease in the relative permeability of the oil phase, resulting in reduced shale oil production. This study clarifies that not all shale oil reservoirs are suitable for well soaking after hydraulic fracturing, which provide a crucial guidance for the design of hydraulic fracturing in shale oil reservoirs.