CO₂ injection-induced fracture and fault activation, potentially leading to seismic events, is a significant concern requiring careful risk assessment. The injected CO₂ readily undergoes phase changes (gas, liquid, supercritical) depending on pressure-temperature conditions, yet the mechanical impact of these phase transitions on fault stability remains poorly understood. This study experimentally investigates the effect of CO₂ phase change on injection-induced fault slip behavior in granite. Results demonstrate that the gas-liquid phase transition during injection could result in the dynamic weakening of the fault friction, and then may induce the stick-slip instability with the continuous injection. In stark contrast, the gas-supercritical phase transition triggers a quick increase in pore pressure, acting as the primary driver for substantial stick-slip. It is noted that, the gas-supercritical phase transition will further lead to a net strengthen of the fault's shear strength, and then causes the fault become stable after several stick-slip. These findings reveal that the phase behavior of CO₂ fundamentally alters fault mechanics, promoting instability and hardening compared to conventional single-phase injection fluids.
 

Fault slip,injection,friction,CO2 phase change