抄録
In-situ heat extraction tests were carried out at Toyoha mine. An artificial fracture was made by hydraulic fracturing and used as a heat exchange plane. An initial direction of the artificial fracture was controlled by disk-shaped slot which had been created by abrasive water jet. After hydraulic fracturing test, circulation tests were conducted twice. Two circulation tests named 93b and 94a consist of one injection borehole and four, seven production boreholes, respectively. Although seven production horeholes had been drilled, water was produced mainly from only one production borehole. In experiment 93b and 94a, water recovery were 44 and 35%, respectively. After circulation tests, the rock mass around boreholes was blasted and removed to observe the fractures. From the observation, the artificial fracture initiated from the tip of the disk-shaped slot was estimated to be connected to two natural fractures. On the basis of the test results and observation result, a fracture model including artificial fracture and natural fractures in a rock mass was constructed. The constructed fracture model was simplified to satisfy the connectivity between boreholes and fractures and a numerical simulation using FEHM (Finite Element Heat and Mass Transfer Code) was carried out. After trial and error, permeabilities of fractures were decided to match the observed temperature drawdown during the circulation test 93b. A close agreement between observed and calculated values was obtained. Using the parameters which gave a best fit to the circulation test 93b, numerical simulation for the circulation test 94a was also conducted. There was an apparent difference between observed and calculated temperature drawdown. The reason for this discrepancy might be that the code could not simulate convectional phenomena within the borehole. There was another possibility that the permeability of a fracture had been increased after experiments 93b. By using a higher fracture permeability, a temperature drawdown for the circulation test 94a was re-calculated and a good agreement between calculated and measured values was obtained. This re-calculation supports the assumption that the fracture permeability had increased after experi
