Crack Expansion and Fracturing Mode of Hydraulic Refracturing from Acoustic Emission Monitoring in a Small-Scale Field Experiment

We conducted a hydraulic fracturing (HF) experiment at a 500-m-level gallery in Mizunami Underground Research Laboratory in central Japan. We drilled a hole downward from the gallery floor and initially injected water at a flow rate of 10 mL/min in a section of 36 mm in diameter and 160 mm in length that was selected to avoid a pre-existing joint. The first breakdown (BD) occurred at 9.20 MPa, whereupon we increased the flow rate to 30 mL/min and induced a second BD in the form of “refracturing” at 9.79 MPa, larger than the first BD pressure. Acoustic emissions (AEs) monitored with 16 sensors in four boreholes located 1 m away from the HF hole exhibited two-dimensional distributions, which likely delineate a crack induced by the fracturing. Expansions of the regions in which AEs occurred were observed only immediately after the first and second BDs. Many AE events in other periods were distributed within the regions where AE events had already occurred. The initial motion polarities of P-waves indicate that tensile-dominant AE events occurred when the regions expanded and they were distributed primarily on the frontiers of the regions where AE events had already occurred. The experimental results suggest that increasing the injection flow rate is effective for generating new cracks in the refracturing, with the new crack expansions being induced by tensile fracturing.