Stress estimated using microseismic clusters and its relationship to the fracture system of the Hijiori hot dry rock reservoir

A method for estimating the in situ stress state by analyzing microseismic clusters induced by hydraulic injections is proposed. The method is based on focal mechanism analysis, supplemented with microseismic doublet analysis. The latter is used to discriminate between the fault plane and the auxiliary planes of the focal mechanism solution. The method was applied to estimate the stress field within the Hijiori hot dry rock geothermal reservoir and yielded results which were consistent with other estimates. The mechanics of permeability creation within the reservoir during major hydraulic injections was then examined by evaluating the interaction of the inferred stress field with the natural fracture system. The characteristics of the latter were determined by integrating information obtained from core samples, acoustic borehole televiewer images and pressure, temperature and spinner logs. The results indicated that the observed microseismicity and orientation-dependent permeability characteristics of the fracture population are well explained by shear failure inferred from Coulombʹs friction theory. It is also shown that the growth direction of the reservoir is strongly controlled by the distribution of favorably oriented pre-existing fractures and their interaction with the stress field.