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( Hyung-mok Kim ),( Arshad S. A. Shahid ) 대한지질공학회 2019 대한지질공학회 학술발표회논문집 Vol.2019 No.2
Fault zones are most of the time sealed because of the gauge-minerals and the frictional forces between the fault planes. When these fault zones are injected with fluid for stimulation, these initially stable faults can reactivate. For instance, the Pohang EGS field was injected with 12,800 ㎥ of water for stimulation of the fault zone in January 2016. Intense seismic activity was detected by the geophone arrays near the Pohang EGS site, which is now believed to be triggered by the water injection into the rock for EGS. On the 15th November 2017 an earthquake of 5.4 (Mw) was observed accompanied with foreshocks and aftershocks. The proper modelling of induced seismic activity is needed to describe the reasons behind the technically unexpected earthquake magnitude, and to allow future prediction of induced seismicity as well. A detailed coupled geomechanics and fluid-flow-modelling approach is required to describe both, fluid flow behaviour during injection and fault deformation causing the seismicity. Shahid (2015) developed a coupled modelling approach for fracture reactivation in shale-gas reservoirs and predicting micro-seismicity during hydraulic fracturing. In this study, the same modelling approach was extended for simulating induced seismicity of EGS stimulation job. We demonstrate that presence of initially unknown secondary fault influence how the main fault zone failed during stimulation. Parametric studies also suggested that uncertainty of mechanical and hydraulic parameters on secondary fault result in much different seismicity profile. This study gives insight into the reasons for the unexpected seismicity observed for many EGS cases.