Multiple Reservoir Creation and Evaluation in the Ogachi and Hijiori HDR Projects, Japan

It was considered to be necessary to create a multiple reservoirs and multiple wells system for extracting heat effectively from relatively shallow Hot Dry Rock (HDR) geothermal reservoirs in Japan. In the Ogachi HDR project, two reservoirs were created at different depths from one 1,000 m deep injection well and two production wells were drilled to penetrate the reservoirs. In the Hijiori HDR project, two reservoirs were also created from two 1,800 m deep and 2,200 m deep injection wells and two production wells were drilled to penetrate the reservoirs. These project results showed that the production wells succeeded to penetrate the reservoirs based on the micro-earthquake hypocenter distributions but it was difficult to control balance of water flow in the reservoirs. In the Ogachi project most of water flowed and short cut occurred in the lower reservoir. In the Hijiori the lower reservoir was cool down faster than the upper reservoir and this phenomenon made scaling problem in the production wells. The author reviewed the characteristics and performances of the multiple reservoirs and multiple wells systems.