There have been numerous technical and social studies related to the Pohang EGS project and associated Mw 5.5 seismic event offering unprecedented lessons to be learned. This presentation intends to present a series of recent studies associated with hydraulic stimulations at the Pohang EGS project. Deep rock cores retrieved from the 4.2 km deep geothermal reservoir directly confirmed prevalent fractures existing in the reservoir and provided invaluable information of mechanical and thermal properties in situ. Among other key parameters, we emphasize the need to better characterize the stress dependent dilation angle and nonlinear fracture normal behavior. Combination of numerous observations of drilling, induced seismicity, hydraulic stimulation and borehole logging provided a more reliable comprehensive stress model of the site. Two contrasting stimulation mechanisms were identified in a very clear manner in two boreholes in Pohang – one hydraulic shearing dominant and the other hydraulic jacking dominant. The contrasting hydromechanical responses observed in the same reservoir at the two nearby wells emphasize the importance of proper design and operation of drilling and completion with close consideration of stimulation strategy. Coupled hydro-mechanical numerical modeling for hydraulic stimulations improves the understanding on the coupled behavior caused by hydraulic stimulations. The key hydro-mechanical processes of shear slip and dilation and hydraulic jacking observed in the fractured reservoir can be successfully reproduced in the numerical modeling. Coupled hydromechanical numerical modeling of five stimulations showed that injection-induced hydraulic jacking and fracture shearing induce immediate stress transfer that plays a significant role in understanding seismic response at the fault associated with the Mw 5.5 event. Coulomb failure stress (CFS) immediately increases after the initiation of the water injection prior to the migration of fluid. During all stimulation periods, the stress change has a dominant impact on the CFS change at the Mw 5.5 fault. Geothermal energy may be duly called ‘hydrogeothermal energy’ in order to do justice on the underlying principle and emphasize the critical role of securing sufficient permeability and water.

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