水圧破砕により形成される円板形フラクチャの寸法と形状

Abstract

The Los Alamos National Laboratory in the United States, with participation from Japan and West Germany, proved the technical feasibility of hot dry rock energy production. Two inclined wells were drilled into hot rock and reached a vertical depth of about 4, 200 m and a temperature of 320°C at Fenton Hill test site. A hydraulic fracturing test, Exp. 2061 was conducted from June 29 through July 2, 1985 to create a larger and hotter reservoir. Water was injected in the interval between 3, 830 m and 4, 017 m at an average rate of 1.6 m3/min with a wellhead pressure of 48 MPa. Cumulative injection during 63 hours of pumping was 5, 228 m3. No hydraulic connection between injection and production wells was obtained in this experiment.The radius and aperture of the fracture system were simulated as a single fracture using the finite element code FEHM (Finite Element Heat and Mass Transfer Code). It is found that for the first 12 hours of pumping, the aperture decreased rapidly towards the tip of fracture and the shape of the fracture remained similar at different times. After 12 hours, the aperture remained nearly constant from the center of the fracture to some extent and then decreased rapidly toward the tip. The distance for which the aperture remained constant increased with the pumping time. However, the extension rate of the fracture decreased with time. The fracture extended even after shut-in when the aperture at the fracture center decreased with time.