[S-SS08] 地震発生の物理・断層のレオロジー

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

*椋平 祐輔¹、Yang Meihua^5,1、岡本 京祐⁴、石橋 琢也⁴、熊野 裕介²、森谷 祐一³、浅沼 宏⁴、Yan Kangnan^5,1、Rubinstein Justin⁶、Zuo Yinhui⁵、Häring Markus⁷ (1.東北大学　流体科学研究所、2.石油資源開発株式会社、3.東北大学　工学部、4.産総研　福島再生可能エネルギー研究所、5.成都理工大学、6.United States Geological Survey、7.Häring GeoProject)

キーワード：誘発地震、透水性、間隙水圧、地殻応力、貯留層形状

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.