Hydraulic Fracturing In Situ Stress Estimations in a Potential Geothermal Site,Seokmo Island,South Korea |
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Authors: | Chandong Chang Yeonguk Jo Yangkyun Oh Tae Jong Lee Kwang-Yeom Kim |
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Affiliation: | 1. Department of Geology, Chungnam National University, Daejeon, 305-764, South Korea 4. Mineral Resources Division, Daewoo International Corporation, Seoul, South Korea 2. Geothermal Resources Department, Korea Institute of Geoscience and Mineral Resources, Daejeon, 305-350, South Korea 3. Geotechnical Engineering Research Department, Korea Institute of Construction Technology, Goyang, 411-712, South Korea
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Abstract: | We conducted hydraulic fracturing (HF) in situ stress measurements in Seokmo Island, South Korea, to understand the stress state necessary to characterize a potential geothermal reservoir. The minimum horizontal principal stress was determined from shut-in pressures. In order to calculate the maximum horizontal principal stress (S Hmax) using the classical Hubbert–Willis equation, we carried out hollow cylinder tensile strength tests and Brazilian tests in recovered cores at depths of HF tests. Both tests show a strong pressure rate dependency in tensile strengths, from which we derived a general empirical equation that can be used to convert laboratory determined tensile strength to that suitable for in situ. The determined stress regime (reverse-faulting) and S Hmax direction (ENE–WSW) at depths below ~300 m agrees with the first order tectonic stress. However the stress direction above ~300 m (NE–SW) appears to be interfered by topography effect due to a nearby ridge. The state of stress in Seokmo Island is in frictional equilibrium constrained by optimally oriented natural fractures and faults. However, a severe fluctuation in determined S Hmax values suggests that natural fractures with different frictional coefficients seem to control stress condition quite locally, such that S Hmax is relatively low at depths where natural fractures with low frictional coefficients are abundant, while S Hmax is relatively high at depths where natural fractures with low frictional coefficients are scarce. |
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