Estimate of In-situ Permeability and Fluid Flow Paths from Hypocenter Migration of Microearthquakes during the Build-up Tests of Geothermal Wells in the Takinoue Geothermal Area

Abstract

It is of crucial importance to map and evaluate properties of fractures (for example permeability) in geothermal areas, because it has been recognized that fractures act as dominant fluid flow paths. However, there is little information on the permeability of fractures and fluid flow paths. In this paper, we report an estimate of in-situ permeability from hypocenter migration of microearthquakes that were observed during the build-up tests in the Takinoue geothermal area. If we assume that microearthquakes are triggered when pore pressure reaches a certain threshold level, we may expect hypocenters to migrate as pore pressure changes during the build-up tests. Then we might be able to estimate permeability from the migration rate of hypocenters, and fluid flow paths from distribution of hypocenters. From simple calculation, we found that it is possible to estimate permeability if we fit the space-time relation of hypocenters by t∼x² curve, even if pressure history is not known, as in the case of the buildup tests in the Takinoue geothermal area.