Numerical analysis of phase transition effects on CO2 leakage along a fault during geologic carbon storage

Carbon dioxide (CO 2 ) leakage through fault is an important concern for the long-term safety and effectiveness of carbon capture, utilization and storage projects. Variations in pressure and temperature gradients, caused by complex reservoir conditions and burial depth, inevitably induce CO 2 phase transitions. This study proposes a non-isothermal hydro-thermal model to simulate CO 2 phase transitions in fractured porous media. The interphase heat transfer is achieved in this model. CO 2 leakage along fault is simulated to explore the pressure and temperature distribution and phase transition mechanism. Results show that CO 2 leakage progression can be divided into two stages dominated by pressure and temperature, respectively. Phase transition induces pressure drops and temperature declines during leakage. In the pressure-dominated stage, as the pressure difference between reservoir and the fault decreases, the leakage flux decreases sharply. The liquid CO 2 converts back to gaseous phase in the temperature-dominant stage, and this gasification process has a negative effect on the leakage. The sensitivity analysis indicates that CO 2 leakage temperature determines the type of phase transition but has a limited impact on leakage mass flux. The reservoir overpressure and fault aperture affect the extent and duration of the pressure-dominant phase transition, respectively. Moreover, the increases in overpressure and fault aperture further lead to higher leakage mass flux. Therefore, it is recommended that monitoring wells be preferentially deployed in areas characterized by high overpressure and large fault aperture, enabling early warning and targeted risk control in CCUS projects. • A TH coupled phase transition model during CO 2 leakage along fault is proposed. • Phase transition process is divided into two stages dominated by pressure and temperature. • CO 2 gasification promotes leakage when temperature dominates. • Effects of CO 2 leakage temperature, overpressure and fault aperture are discussed.