Permeability characterization of the Soultz and Ogachi large-scale reservoir using induced microseismicity

Hydraulic fracturing is a common procedure to increase the permeability of a reservoir. It consists in injecting high-pressure fluid into pilot boreholes. These hydraulic tests induce locally seismic emission (microseismicity) from which large-scale permeability estimates can be derived assuming a diffusion-like process of the pore pressure into the surrounding stimulated rocks. Such a procedure is applied on six data sets collected in the vicinity of two geothermal sites at Soultz (France) and Ogachi (Japan). The results show that the method is adequate to estimate large-scale permeability tensors at different depths in the reservoir. Such an approach provides permeability of the medium before fracturing compatible with in situ measurements. Using a line source formulation of the diffusion equation rather than a classical point source approach, improvements are proposed for accounting in situation where the injection is performed on a well section. This technique applied to successive fluid-injection tests indicates an increase in permeability by an order of magnitude. The underestimates observed in some cases are attributed to the difference of scale at which the permeability is estimated (some 1 km(3) corresponding to the seismic active volume of rock compared to a few meters around the well for the pumping or pressure oscillation tests). One advantage of the proposed method is that it provides permeability tensor estimates at the reservoir scale.