Performance Evaluation of CO2 Pre-Pad Energized Fracturing in Shale Oil Reservoir

Carbon dioxide (CO2) exhibits strong compressibility and expansion behavior, which enables efficient pressure transmission in tight reservoirs. This property promotes reservoir pressure, improves fracturing fluid flowback, slows production decline, and finally enhances oil recovery. CO2 pre-pad energized fracturing has been widely adopted in shale oil development. However, its theoretical basis and controlling mechanisms are still not well understood. In this study, the energization mechanism of CO2 pre-pad energized fracturing in shale oil reservoirs was investigated. The energization performance was quantitatively evaluated by the pressure coefficient and the energy replenishment range under different geological conditions and operational parameters. The results indicate that pre-injected CO2 increases pressure and reduces effective stress. Part of the CO2 can be permanently stored in the reservoir. The CO2 injection volume is identified as the dominant controlling factor. Increasing the CO2 injection volume expands the energy replenishment range to a maximum of 12,470 m2. Lower injection rates favor CO2 penetration and diffusion, leading to a wider energy replenishment range. An energy replenishment range exceeding 7000 m2 can be achieved when the CO2 injection rate is less than 4 m3/min. Lower reservoir permeability restricts the energization region, whereas longer soaking time allows pressure to diffuse more fully and enhances CO2–oil interaction. The results provide a basis for further optimization and future studies for the CO2 pre-pad energized fracturing in shale oil reservoirs.