Mechanism of Non-blasting Supercritical CO₂ Fracturing in Rocks Under Reverse Fault Conditions: Dynamic and Quantitative Monitoring Using Coda Waves

Supercritical carbon dioxide (SC-CO₂) fracturing has emerged as a promising non-blasting rock fracturing method within the carbon capture, utilization, and storage (CCUS) portfolio. Understanding the evolution of global fracture network, which is a complex and dynamic process, is essential for enhancing fracturing effectiveness. However, traditional research methods have been inadequate for dynamically and quantitatively tracking and characterizing the development of these networks. In this study, SC-CO₂ fracturing experiments and active source acoustic emission monitoring tests were conducted under three different confining pressures (CPs) caused by the reverse conditions at different burial depths. The characteristic parameters of global fracture network evolution were analyzed using coda wave signals, including frequency, transmissivity, relative velocity (dv/v), and decorrelation coefficient (DC). The relationships between these parameters and the combined effects of CPs and fracturing pressure were established, and the mechanisms of global fracture network evolution were discussed. The results indicate that the microscopic mechanism of SC-CO₂ fracturing involves fissure tip extension driven by expansional energy from SC-CO₂ phase transition, which is influenced by the compression-shear effect induced by CPs. Additionally, the compression-shear effect governs the developmental rate and direction of the global fracture network, determining the final failure pattern. The combined influence of CPs and fracturing pressure on all investigated parameters was linked to an increase in coda wave travel time, resulting in a decay trend in frequency, transmissivity, and dv/v, while the DC exhibited an increasing trend. This study introduces a novel method for monitoring the SC-CO₂ fracturing process, enhances the quantitative characterization of global fracture network evolution, and promotes the application of SC-CO₂ fracturing in situ.