The influence of SC-CO2 on the shales’ fracture behavior described by using ultra-fast time resolution method and the damage fracture constitutive model: A case study of the Cretaceous Qingshankou formation in Gulong Depression, Songliao Basin, NE China

Supercritical CO₂ (SC-CO₂), as an innovative non-aqueous fracturing fluid, has gained extensive application in arid regions. However, its influence on the fracture propagation behavior of shale in very short timescales remains poorly understood. Therefore, a series of three-point bending tests were conducted on shale samples with 0°-90° bedding inclinations. To describe the influence of water-heat-SC-CO₂ treatment, a damage fracture constitutive model based on a normal distribution was developed. The theoretical fitting curves of this constitutive model agree well with the experimental results. The whole crack propagation processes were captured using the ultra-fast time resolution method, achieving a time resolution of fast to 15 ps. Identical three-point bending tests were repeated after subjecting the shale samples to water-heat-SC-CO₂ treatment for 48 h. The experimental results reveal that after the water-heat-SC-CO₂ treatment, there were significant changes in the key mechanical parameters of shale samples. Moreover, the influence of water-heat-SC-CO₂ was observed to increase progressively with the bedding inclinations. And by combining the image captured by the ultra-fast time resolution method with this constitutive model, the fracture behavior change of the shale sample after water-heat-SC-CO₂ treatment predominantly is affected by the weak planes of shale, with negligible influence on the matrix.