| Abstract |
The Central Research Institute of the Electric Power Industry (CRIEPI) has conducted some experiments to create artificial geothermal reservoir at the Ogachi test site in southern Akita prefecture, Japan, from 1989 to 2002 as an HDR project, but now it must be called an EGS project. An injection well, OGC-1 was drilled to a depth of 1,000 meters (m) in which a temperature was measured 228 degree C at a depth of 990 m. Two vertical separated reservoirs at depths of 990-1,000 m (lower) and 711-719 m (upper) were created by hydraulic stimulation in OGC-1 with the casing cutter and sand plug (CCSP) method. Values of permeability of the original rock at the lower and the upper reservoirs were measured 3×10-15 m2 and 1×10-16 m2, respectively. The created reservoir size and location was evaluated by micro-earthquake or acoustic emission (AE) events locations induced by the stimulation. A production well, OGC-2, was drilled to penetrate both the two reservoirs. Some water circulation tests were conducted between OGC-1 and OGC-2 through the reservoirs. Because of a low water recovery rate from OGC-2 in the early circulation tests, some hydraulic stimulation operations were conducted in OGC-1 and OGC-2. After the stimulation operations, the permeability of the rock increased to 1×10-13 m2 in the lower reservoir and 1×10-14 m2 in the upper, respectively. The water recovery rate increased from 3 % in the 1993 circulation test to 25 % in the 1995 circulation test. Numerical simulation results showed that the water recovery rate may increase to 44.5 %, if another production well is added to the water circulation system. If more production wells are added and/ or a down-hole pump is applied to the system, the water recovery rate may be no problem in water circulation operations. The results of tracer tests during the circulation experiments in 1994, 1995 and 1997 showed that characteristics of the water flow paths in the reservoirs changed by stimulation and water circulation operations. The modal volume of the reservoirs and values of widths at 1/2 height decreased after these operations in 1994 and 1995. These results showed that artificial fractures were progressed by the hydraulic stimulation operations and permeability of the rock increased. But the modal volume change showed that the water flow between OGC-1 and OGC-2 might concentrate in a small part of the reservoirs, which means that a short circuit might occur by the stimulation and water circulation operations. The tracer test results after sand plug set and removal used for the 1977 individual flow test showed that value of widths at 1/2 height increased. This result means that the sand particle used for sand plug remained fractures in the reservoir and contributed to prevent from short circuit of the flow path. |