Experimental and numerical simulation study on the CO2 pre-injection fracturing optimization parameters in continental shale oil

The CO2 pre-injection fracturing technique is widely used in the development of shale oil. To address the unclear mechanism of fracture creation in CO2 pre-injection fracturing, a study was conducted on the multi-fracture propagation model for lacustrine shale oil using CO2 pre-injection fracturing. This study revealed the changes in rock mechanical parameters during CO2 pre-injection fracturing, clarified the mechanisms of shale failure and fracture propagation, and optimized the parameters for CO2+water-based fluid fracturing to guide field operations. The research findings indicate that water-based fracturing fluid has a greater impact on the tensile strength and fracture toughness of shale samples compared to CO2+water-based fracturing fluid. CO2 can mitigate the adverse effects of water-based fracturing fluid on the rock's tensile strength and fracture toughness. Introducing CO2 before slickwater can partially restore the brittleness of shale, which is conducive to increasing the complexity of fracture networks. Reducing the viscosity of the water-based fracturing fluid and increasing the volume of CO2 injection are effective measures to enhance the modified volume. Increasing the stage spacing can improve the modified volume but may reduce fracture complexity. As the angle between natural fractures and the maximum horizontal principal stress decreases, the complexity of fractures created by CO2 pre-pad fracturing initially increases and then decreases, while the modified volume initially decreases and then increases.