Investigation of the CO2 Pre-Fracturing Mechanism for Enhancing Fracture Propagation and Stimulated Reservoir Volume in Ultra-Deep Oil Reservoirs

CO2 pre-fracturing is an innovative technique for enhancing oil and gas production in unconventional reservoirs. Despite its potential, the mechanisms of CO2 pre-fracturing influencing fracture propagation, particularly in ultra-deep reservoirs, remain inadequately understood. This study investigates the CO2 pre-fracturing process in ultra-deep sandstone reservoirs of the central Junggar Basin. A 3D geomechanical model was established using RFPA3D-HF based on rock mechanical parameters from laboratory experiments. The study examines the effect of in situ horizontal stress differences, CO2 pre-injection volume, and slickwater injection rate on fracture complexity index (FCI) and stimulated reservoir volume (SRV). The results reveal that in situ horizontal stress differences are the primary factor influencing fracture propagation. In ultra-deep reservoirs, high horizontal stress difference hinders fracture deflection and bifurcation during slickwater fracturing. CO2 pre-fracturing, through the pre-injection of CO2, reduces formation breakdown pressure and increases reservoir pore pressure due to its low viscosity and high permeability, effectively mitigating the effect of high horizontal stress differences and significantly enhancing fracturing effectiveness. Furthermore, appropriately increasing the CO2 pre-injection volume and slickwater injection rate can increase fracture complexity, resulting in a larger SRV. Notably, adjusting the CO2 pre-injection volume is more effective than adjusting slickwater injection rate in enhancing oil production. This study provides scientific evidence for selecting construction parameters and optimizing oil recovery through CO2 pre-fracturing technology in deep unconventional oil reservoirs and offers new insights into CO2 utilization and storage.