Experimental study of supercritical CO2 fracturing on initiation pressure and fracture propagation in shale under different triaxial stress conditions

Supercritical carbon dioxide (SC-CO2) fracturing technique used in the development of shale gas has attracted increasing attention in the past decades. Using a self-developed physical simulation system equipped with acoustic emission (AE) and computed tomography (CT) system, the fracture initiation pressure (FIP) and fracture propagation mechanism of shale in the process of SC-CO2 fracturing was invested. The results show that the FIP for the shale without pre-existing fractures obtained from experiment is consistent with the theoretical values under different triaxial stress conditions, indicating that the theoretical calculation for FIP is feasible. The FIP increased gradually with increasing in situ stress for the shale without pre-existing fractures. The pre-existing fractures will affect the FIP and the fracture propagation direction in the shale. AE signals are obviously observed in the fracture initiation and the fracture propagation stages. In addition, the hit, energy, and amplitude are increased stepwise with increasing fracturing time. In the triaxial stress state, the initiation position and the propagation direction of cracks are random in the process of the SC-CO2 fracturing of shale and are along the direction of weak structural planes or pre-existing fractures. The SC-CO2 fracturing technique integrated with AE monitoring and CT scanning can be used to analyse the crack initiation position, the direction and sequence of crack propagation, and the number of the fracturing-generated cracks in shale. This research lays a foundation for SC-CO2 fracturing of shale in the Longmaxi formation in the Sichuan Basin.