Impact of shale anisotropy on the microseismic wavefield of passive sources
WU Ruyue1,2,3, LI Han1,2, CHANG Xu1,2, YAN Hongyong1,2, WANG Yibo1,2
1. Key Laboratory of Petroleum Resource Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029; 2. Institution of Earth Science, Chinese Academy of Sciences, Beijing 100029; 3. University of Chinese Academy of Sciences, Beijing 100049
Abstract:Shale reservoir is featured by strong anisotropy. Ignoring the impact of shale anisotropy on the microseismic wavefield of passive sources (induced by fracturing stimulation) could affect the credibility of reservoir characterization. We proposes a method for quantitatively evaluating the deviation in travel time and amplitude of seismic wavefields,and study the influence of shale anisotropy on the microseismic wavefield of passive sources based on synthetic numerical test. Firstly,we use moment tensor to define explosive and shear microseismic sources,and caculate the microseismic wavefield of passive sources based on the elastic wave equation for three-dimensional anisotropic media and the staggered-grid finite-difference algorithm. Secondly,by comparing theoretical wavefields induced by different sources in different anisotropy media,we calculate the deviation in travel time and amplitude of P wave and S wave relative to isotropic models,and quantitatively and qualitatively analyze the impact of shale anisotropy on the microseismic wavefields under different source mechanisms. Numerical test show that for a surface microseismic monitoring system and a horizontally layered VTI medium model,shale anisotropy has significant influence on the travel time and amplitude. The travel time can deviate by 30%,and the deviation of amplitude can be up to 60%. The deviation of the travel time increases with increasing offset. In addition,under different source mechanisms,the impact of shale anisotropy on the microseismic wavefield are similar to each other.
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