Characterization of 4.2-km-Deep Fractured Granodiorite Cores from Pohang Geothermal Reservoir, Korea

The rock core test is the most direct and reliable way to measure fundamental physical, hydraulic, and mechanical properties for underground reservoir characterization. A 10-cm diameter and 3.6-m-long granodiorite core was retrieved from a 4.2-km-deep geothermal well at the Pohang Enhanced Geothermal System (EGS) site in Korea. Numerous natural fractures were detected in the core and induced core disks were observed. We optimized the sample preparation with this limited core for various tests based on the scanned X-ray CT images that visualized all the fractures in the core. We measured the basic mechanical—the deformation and strength properties of intact rock—and thermal properties of intact samples. In particular, fracture deformation and strength properties were directly measured by conducting direct shear tests on pre-existing natural fractures. The seismic velocity and normal fracture stiffness were compared with the wireline logging and injection test measurements, respectively. The discrepancies between the in situ data and laboratory experiment are due to the stress dependency of properties. The measurements presented in this paper provide essential inputs for the EGS reservoir modelling and a dataset of properties for the fractured granite reservoir at a great depth from which few samples have been retrieved thus far.