Estimation of Fracture System and Stress State of Ogachi HDR Reservoir by Using AE Multiplet Analysis

Reservoir structure and stress field at the Ogachi Hot Dry Rock field, Japan, have been examined using hydraulically induced AE (Acoustic Emission / Microseismic) events. Two large scale hydraulic fracturing tests were conducted by CRIEPI (Central Research Institute of Electric Power Industry, Japan) in 199J and 1992. The fracturing tests were carried out at two different depth intervals, 990m ∼ 1000m and 711 m∼719 m. AE events were observed by using near-surface networks deployed around the fracturing well. The source locations of induced AE events for the two experiments distribute towards the NNE and ENE directions, respectively. The AE multiplets, which are the group of AE events with very similar waveforms in spite of different origin times, are analyzed to evaluate the fractures in the reservoir and to estimate the stress field. AE multiplets are considered to be the AE events that occurred on the same fracture planes with the same source mechanism. The precise source locations of the multiplets are estimated using the cross spectrum analysis. The orientations of fracture planes derived from the source distribution of each group of similar AE events suggest that the seismicaliy activated fractures have orientation in the NE-SW direction for both the 1991 and 1992 experiments, even though the entire source distributions are different in each case. The stress field is also estimated by using the orientation of fractures and the slip directions of the fractures, which were evaluated using P-wave polarities. The stress field is calculated assuming that the derived fracture planes caused shear slip during the. pressurization of the fractures according to the Coulomb's criteria. Under this assumption, it is suggested that the maximum principal stress direction is nearly WSW-ENE, and that the ratio of minimum and intermediate stresses is small and the ratio of maximum and minimum stresses is large.