[S-SS08] Fault Rheology and Earthquake Physics

[SSS08-25] Microseismic Cloud Growth Process Mainly Controlled by In-Situ Stress During Hydraulic Stimulation

*Yusuke Mukuhira¹, Meihua Yang^5,1, Kyosuke Okamoto⁴, Takuya ISHIBASHI⁴, Yusuke Kumano², Hirokazu Moriya³, Hiroshi Asanuma⁴, Kangnan Yan^5,1, Justin Rubinstein⁶, Yinhui Zuo⁵, Markus O Häring⁷ (1.Institute of Fluid Science, Tohoku University, 2.JAPEX Co., Ltd., 3.School of engineering, Tohoku University, 4.Fukushima Renewable Energy Institute, National Institute of Advanced Industrial Science and Technology, 5.State Key Laboratory of Oil and Gas Geology and Exploitation, Chengdu University of Technology, 6.United States Geological Survey, 7.Häring GeoProject)

Keywords:induced seismicity, Permeability, pore pressure, In-situ stress, Shape of reservoir

Forecasting the shape of the area deformed by hydraulic stimulation is essential for the design of an energy extraction system. Here we use microseismicity as a proxy for the area of deformation, given the limitation that aseismic deformation is not easily observed. Microseismic clouds caused by hydraulic stimulation are typically observed to be elongated in the direction of maximum principal stress. However, there are exceptions to this empirical relationship, and spatio-temporal evolution of microseismicity is not fully understood. We investigate the microseismic cloud growth process at the Basel, Switzerland geothermal site, focusing on the correlation with in-situ stress. We apply principal component analysis to examine the temporal evolution of the distribution of microseismicity. The smallest axis of the microseismic cloud is stable and nearly identical to the minimum horizontal stress. The largest axis of the seismicity dips during the stimulation, but after injection it is near- vertical. This observation suggests the microseismic cloud growth behavior was different before and after the stimulation, likely being correlated with the dynamic and static permeability tensors. The microseismic cloud grew radially within the plane perpendicular to the minimum horizontal stress. The radial growth is consistent with nearly identical maximum horizontal and vertical stress. We hypothesize that the microseismic cloud did not grow in the direction of least principal stress because the permeability is lower in this direction, while the microseismic cloud did grow parallel to the orientations of maximum and intermediate stresses. Based on these observations, we conclude that microseismic cloud growth is mainly controlled by in-situ stress given a wide distribution of fault orientations, as can be found in Basel. This study suggests that in-situ stress measurements taken before stimulation can be used to help forecast geothermal reservoir shapes.