Measurement of Tectonic Stress by Hydraulic Fracturing in Japan

Abstract

Using the hydraulic fracturing technique, we have made a series of in-situ stress measurements in wells drilled in the Sengan geothermal area. The wells are all 250 m deep and 158 mm in diameter. The stratigraphy of the well is tight dacite and rhyolite welded tuff of the late Miocene-Pleistocene Periods. From the results of the well logging, ultrasonic borehole televiewer, and the observation of the core samples, we have selected five intervals for each well. Vertical stress was calculated as overburden pressure. Horizontal principal stress was calculated from the pressure records. The impression packer and ultrasonic borehole televiewer have been used to detect the orientation of the fractures. In many cases, impression packer results show clearly two vertical fractures (approximately 180 degrees apart). Calculated stress results show that the maximum principal stress is the maximum horizontal stress, the intermediate principal stress is the minimum horizontal stress, and the minimum principal stress is the vertical stress. Direction of the maximum horizontal stress is NW to SE. Absolute value of the stress shows that the state of stress is very compressive. Absolute value and the direction of the stress by our hydraulic fracturing stress measurements are in harmony with the tectonic stress. This shows that Tamagawa welded tuff is hard and tight enough to maintain tectonic stress and act as a cap rock of the underlying geothermal reservoir. Since we expect that a less compressive stress field is dominant in large scale geothermal areas, hydraulic fracturing stress measurements in deeper wells are necessary to understand the Sengan geothermal area.