Underground sequestration of carbon dioxide (CO2) is a promising method for mitigating its greenhouse gas effect. This method will become much more feasible by compensating for its cost, if it can be combined with economically viable processes such as shale gas recovery and geothermal energy extraction from hot dry rock. When CO2 is injected deeper than 1000 m, the temperature and pressure usually satisfy the condition to make CO2 to be supercritical state, which has much lower viscosity than that of water. To examine behavior of the supercritical state CO2 (SC-CO2) in rock, we conducted hydraulic fracturing experiments using SC-CO2, water, and viscous oil in granite specimens under a tri-axial deviatoric stress condition. Source distributions of acoustic emission events showed that hydraulic fracturing with SC-CO2 can induce more intense cracks branching and spreading wider than those with water. The result suggests that SC-CO2 fracturing forms more effective pathways for shale gas recovery and hot dry rock geothermal extraction while enabling economically feasible underground sequestration of CO2.
