Control of Geothermal Reservoirs Consisting of Multiple Cracks

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The main objective of the present project is to clarify the mechanical response of geothermal engineered reservoirs, such as HDR/HWR reservoirs, within a rock mass with many planes of weakness. To this end, we have constructed models for the case that the reservoir consists of innumerable joints and for the case that the reservoir consists of predominant reservoir cracks. We have clarified the basic characteristics of such reservoirs. Then, we have applied them successfully to the results of massive hydraulic fracturing/stimulation conducted at the Fenton Hill HDR field and at the Yunomori HWR field. It is preferable that the basic characteristics of the models can be estimated by using the geometrical and mechanical characteristics of fracture surfaces of fractures within core samples retrieved from the depth of boreholes. Therefore, we developed new methods for estimating the hydraulic properties of fractures from the fractal characteristics of fracture surfaces based on the laborato … More ry experiments under tri-axial stress conditions. Furthermore, we conducted mini-hydraulic fracturings in a driftway at the Kamioka mine to make a man-made fracture and to characterize the fractal characteristics of the surfaces created by hydraulic fracturing to find that the fractal characteristics are different from those of surfaces of fractures made by explosion and of slicken sides which are the surfaces subjected to shear movements. We have also developed new techniques on crosshole seismic measurement, borehole radar measurement and precise AE measurement for estimating the structure of multicrack geothermal reservoirs and the in-situ stress state of surrounding rocks. We have analyzed data sets observed at geothermal fields and have estimated reservoir characteristics by using these new techniques. It was found that it is highly probable that man-made reservoir cracks are accompanied by many-small secondary cracks generally. The hydraulic and heat transfer properties have been investigated for reservoir cracks accompanied by the secondary cracks by conducting laboratory experiments, where the rock with secondary cracks is simulated by a packed cube bed. Less