Interaction between Fracturing Fluids and Shale from the Ziliujing Formation of the Jurassic in the Sichuan Basin and the Mechanism of Imbibition-Driven Oil Displacement

Hydraulic fracturing is a necessary method for reservoir stimulation in shale oil and gas development. During the fracturing process, a large amount of fracturing fluid enters the reservoir and remains underground, leading to complex interactions between the fracturing fluid and shale. These interactions modify the reservoir’s pore structure and physical properties, while imbibition further influences fluid distribution and oil-gas displacement efficiency in the reservoir, thereby affecting the development effectiveness of shale oil and gas. To investigate the dynamic interaction mechanisms between fracturing fluids and shale reservoirs and the laws of spontaneous imbibition oil displacement, this study focused on the shale reservoirs of the Jurassic Ziliujing formation in the Fuxing area of the Sichuan Basin. Using experimental methods such as porosity and permeability testing, low-temperature nitrogen adsorption, high-pressure mercury intrusion, micro-CT scanning, and FIB-scanning electron microscopy imaging, the pore structure characteristics of shale samples are systematically characterized. The interactions between different fracturing fluids and shale and their effects on spontaneous imbibition oil displacement efficiency are thoroughly explored. Results indicate that the primary storage space in shale consists of nanopores, with complex and highly heterogeneous pore systems. The fracturing fluids significantly enhanced the porosity and permeability of shale, with fracturing fluids containing integrated emulsion drag reducers showing more pronounced effects in promoting fracture formation and improving permeability. Micro-CT and SEM results indicate that fracturing fluids promote fracture propagation and mineral detachment; however, prolonged interactions can lead to fracture closure and reduced connectivity. NMR T₂ spectrum analysis shows that the oil displacement efficiency of fracturing fluids with integrated emulsion drag reducers (17.55%) is higher than that of those with suspended drag reducers (15.09%), suggesting that optimizing fracturing fluid formulations can effectively enhance the oil and gas recovery of shale reservoirs. This study provides theoretical support for shale reservoir stimulation, fracturing fluid optimization, and improving shale oil and gas recovery efficiency.