Obervation and Analysis of AE Events Accompanying Hydraulic Injection at the Hijiori Hot Dry Rock Site

Abstract

AE events accompanying hydraulic injection experiments at the Hijiori hot dry rock site was monitored by a network of ten borehole stations deployed at an average distance of 2 km from an injection well. AE events induced during the 1988 and 1992 hydraulic fracturing experiments with a high injection pressure were located near the injection point in the early stage of the experiments and clearly migrated towards the east and distributed along a vertical plane. The strike of epicenter distribution of AE vents is nearly parallel to the direction of the maximum principal stress. AE events induced during the 1989 and 1995 circulation tests with a low injection pressure were diffused. The permeability was estimated from the hypocenter migration as 10^-16m², which is intermediate between the permeability of core samples of granodiorite taken from a production well and the permeability of fractured rocks obtained by an injection test between the injection well and the production well. This indicates that the diffusion of AE events accompanying the circulation test is due to the permeation of water into joints which slip when the effective stress is reduced by the increased pore fluid pressure accompanying the hydraulic injection. The stress state at Hijiori was estimated based on the inversion from focal mechanisms of AE events. The best fit stress model obtained by inverting 58 focal mechanisms of AE events simultaneously indicates that the maximum principal stress σ₁ is vertical, while the minimum principal stress σ₃ is horizontal and trends north-south. The stress estimates obtained by the focal mechanism inversion essentially agree with other stress estimates previously obtained. It is therefore concluded that the focal mechanism inversion method provides a useful tool for estimating the stress state. Source parameters of AE events associated with the 1995 experiment were analyzed. Seismic moments and source radii estimated from the spectra of AE waveforms range from 8 to 585×10⁷ N⋅m and 8 to 36 m, respectively. The S-wave Q is estimated to be 191. Comparison of the present source parameters with those reported for a similar magnitude range in a hard-rock formation indicates that our estimates of seismic moment and corner frequency are comparable.