Hijiori HDR Reservoir Evaluation by Micro-Earthquake Observation

Abstract

Hot Dry Rock geothermal energy development experiments were conducted from 1986 to 2002 at Hijiori in Yamagata prefecture, Japan by the New Energy and Industrial Technology Development Organization (NEDO). In this program two injection wells and two production wells were drilled into granodiorite to extract heat from two artificially created reservoirs at depths of around 1,800 m and 2,200 m where temperatures were measured at 230 degree C and 270 degree C, respectively. The upper reservoir was created from 1,788 m to 1,802 m in the injection well, SKG-2, by injecting a total of 1,080 tons of water. The lower reservoir was produced from 2,151 m to 2,205 m in another injection well, HDR-1, by injecting a total of 2,115 tons of water. These reservoirs were created by injecting water at a high flow rate for a short period of 11 and 12 hours for improving permeability of natural fractures of the rock immediately the injection wells. It was not intended to extend fractures long distance. If the created fractures extend long beyond the injection and production wells area, water recovery during water circulation will be small. The location and size of the two reservoirs were evaluated from micro-earthquake hypocenter distributions. The hypocenters were distributed within a few hundred meters of the injection wells during reservoir creation. This result means that the hydraulic fracturing improved permeability of the rock immediately around the injection wells. However the hypocenters were distributed in a wide area during the water circulation tests. The recovery ratio of produced steam and hot water amount to injected water amount was small of 34.6 % to 54%. This result means that the fractures extended longer and some of the injected water leaked away from the injection and production wells through the fractures. It is needed to plug some fractures to prevent water leaking during water circulation tests.