We injected carbon dioxide (CO2) into a hole drilled around 8 m long in a granitic hot rock from the floor of the tunnel. We drilled four AE (acoustic emission) monitoring holes parallel 1 m far away from the injection hole and monitored AE events induced with hydraulic fracturing (HF). When the breakdown (BD) pressure was recorded, the pressure and the temperature satisfied the supercritical condition of CO2. The AE source distribution showed that two vertical cracks were initiated from the injection hole with BD. After 75 seconds from the occurrence of BD with no pressure increase, the AE sources started to distribute along the direction almost normal to that of the initial crack from the position around 0.7 m far away from the injection hole. It is most likely that the cracks initiated in intact rock with BD by HF and one of them bended and extended along pre-existing crack. These results suggest that CO2 migrates easily and enhances AE occurrence in a pre-existing joint.

1. Introduction

The carbon capture and storage (CCS) in underground is a promising and feasible method for mitigating the greenhouse effect by decreasing the amount of CO2 emissions. If we can utilize CO2 for energy production and compensate for the cost of CCS, which is called carbon capture, utilization and storage (CCUS), CCS will be much more eagerly developed and adopted. For example of CCUS, Xie et al. [1] shows enhance the recovery of oil (CO2-EOR), coalbed methane (CO2-ECBM), geothermal systems (CO2-EGS), natural gas (CO2-EGR), shale gas (CO2-ESG) and others.

For EGS, EOR, ESG and ECBM, CO2 is usually injected into rocks at a depth of more than 1000 m and sometimes more than 3000 m, and the temperature and pressure at these depths make CO2 supercritical state. The viscosity of supercritical CO2 (SC-CO2) is one or two order of magnitude smaller than that of normal liquid water. To examine effect of fracturing fluid viscosity on a crack features induced by hydraulic fracturing (HF), Ishida et al. [2, 3] made HF experiments using SC-CO2, liquid CO2 (L-CO2), water and viscous oil in 170×170×170 mm cubic granite blocks with a center hole of 20 mm diameter. They found in the laboratory experiments that fracturing with low-viscosity fluid such as SC-CO2 tends to induce three-dimensionally sinuous cracks with many secondary branches, which seem to be desirable pathways for EGS, ESG and ECBM. However, the effect of fracturing fluid viscosity on HF in real rock mass including pre-existing cracks has not been still clarified. Thus, we made small field HF experiment using CO2 in a hot rock mass under a tunnel floor which satisfies the temperature to form SC-CO2, and monitored acoustic emission (AE) induced by HF to clarify crack extension.

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