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.