To investigate the effects of carbon dioxide phase change on dynamic multi-fracturing process during supercritical carbon dioxide (SC-CO2) fracturing, polymethyl methacrylate (PMMA) specimens of 20 × 20 × 10cm3 are used in experiments with SC-CO2 and water as fracturing fluids, respectively. The process of fracture propagation is recorded using high speed camera to determine fracture propagation velocity and morphology. Simple models related to expansion work and heat energy caused by SC- CO2 phase change are to explain the experimental observations and results. The experimental results show that, due to strength weakening from CO2 adsorbed into the nano-scale fracture surface, the magnitude of fracture initiation pressure of SC-CO2 fracturing is lower than that of water-based fracturing. It is of interest that fracture propagation velocity of SC-CO2 fracturing is two order greater in magnitude than that of water-based fracturing, which shows that the energy release rate maybe sufficient for fracture branching. The expansion and thermal stresses induced by CO2 phase change, when the fracture instability occurs, further extend the fractures or even create multi-fractures. Eventually, the intermittent dynamic growth of multi-fractures occurs under the effect of CO2 phase change in SC-CO2 fracturing.

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