A Prediction Method for the Initiation Pressure of Pre-CO 2 Fracturing Considering the Modification of Rock Mechanical Parameters after CO 2 Treatment

Abstract The utilization of CO₂ fracturing fluid presents a notable reduction in the challenges associated with reservoir opening, particularly in dense and unconventional reservoirs. The widespread adoption of this technique underscores its efficacy. The establishment of a more realistic CO₂ fracturing model serves to elucidate the intricate mechanisms underlying CO₂ fracturing transformation. Additionally, it furnishes a foundational framework for devising comprehensive fracturing construction plans. However, current research has neglected to consider the influence of CO₂ on rock properties during CO₂ fracturing, resulting in an inability to precisely replicate the alterations in the reservoir post-CO₂ injection into the formation. This disparity from the actual conditions poses a substantial limitation to the application and advancement of CO₂ fracturing technology. This work integrates the variations in physical parameters of rocks after complete contact and reaction with CO₂ into the numerical model of crack propagation. This comprehensive approach fully acknowledges the impact of pre-CO₂ exposure on the mechanical parameters of reservoir rocks. Consequently, it authentically restores the reservoir state following CO₂ injection, ensuring a more accurate representation of the post-fracturing conditions. The research findings reveal that post-CO₂ treatment, the elastic modulus of reservoir shale experiences a reduction of 12.5%, Poisson's ratio decreases by 11.8%, tensile strength decreases by 7.9%, permeability increased by 180%. Additionally, the pre-injection of CO₂ into the reservoir induces a notable increase in pore pressure in the near wellbore zone. In comparison with conventional numerical simulation methods, the approach outlined in this paper yields a reduction in the error associated with predicting fracturing pressure by 9.8%. The model and methodology presented herein serve as a practical tool for accurately forecasting the initiation pressure of CO₂ fracturing.