Fracture Investigation of the Granitic Basement in the HDR Ogachi Project, Japan

Abstract

Natural fractures developed in the granitic basement at the Ogachi Hot Dry Rock (HDR) experiment site were investigated in order to evaluate the reservoir characteristics. The granitic basement is located inside a Neogene caldera, in a zone where the basement is uplifted and also in a NNWtrending mylonite zone.

Natural fractures are highly developed in the Ogachi HDR reservoirs, especially in the upper one (~700 -900 m in depth). This supposedly is due to hydrothermal brecciation associated with the caldera formation. Mylonitization seems to have had little effect on the natural fracture formation. Chlorite, epidote, anhydrite and calcite veins were observed. Anhydrite veins are dominant in the lower reservoir (~900 -1100 m in depth). This explains the result that a high content of calcium and sulfate ions was detected in the returning water from the lower reservoir and indicates that the reaction between anhydrite and injected water was extensive.

Fracture orientations in both the injection and production wells were revealed by BHTV surveys. Overall, fractures with high dips are dominant throughout the reservoir. No dominant orientations were observed in the upper reservoir, whereas in the lower reservoir, fractures with a N-S trend and moderately-high W dips are dominant in the injection well and those with a NE-SW trend and high SE dips are dominant in the production well. It was assumed from AE analysis that artificially induced hydraulic fractures propagated to the east and to the NNE in the upper and lower reservoirs, respectively. From the standpoint of fracture analysis, the propagation of hydraulic fractures is largely restricted to the natural fracture system. This is because the eastward propagation in the upper reservoir coincides with relatively thick veins and andesite dikes oriented along an E-W trend. The NNE propagation of the lower reservoir coincides with the dominant natural fracture orientations in the lower reservoir.