ABSTRACT
We conducted HF (hydraulic fracturing) experiment using SC-CO2 (Supercritical Carbon Dioxide) in a granitic hot rock mass 7.40 m below a tunnel floor. We succeeded to make a bi-wing crack, which is indicated by planar AE (Acoustic Emission) distribution observed just after BD (Breakdown). From 75 s after the BD, the active region of the AE events expanded along another vertical plane, likely indicating that the CO2 intruded into a pre-existing crack. Many AE events along those two planes showed implosion mechanism, which has never been observed in our HF experiments using water, whereas such type of seismic events were often observed in volcanic regions and are interpreted as a result of crack closing due to degassing from magma. Although CO2 likely intrude easily into pre-existing cracks due to their low viscosity and likely enhances AE activities on cracks, those implosion type events unlikely increase the generation risk of large earthquakes no matter how many such events are induced because they were not associated with fractures of rock including frictional slip.
