页岩各向异性对被动源微地震波场的影响

Impact of shale anisotropy on the microseismic wavefield of passive sources

页岩储层具有较强的各向异性特征,忽略页岩各向异性对被动源微地震波(水力压裂产生)传播的影响,可能会降低储层表征结果的可信度。为此,提出一种定量评估地震波场旅行时和振幅偏差的方法,并基于理论数值算例,研究页岩各向异性对被动源微地震波场的影响。首先,利用矩张量模型描述爆炸型和剪切型微地震震源,并基于三维各向异性介质弹性波动方程和交错网格有限差分算法,正演模拟被动源的微地震波场;然后,对比不同震源、不同程度各向异性介质的理论波场,计算其P波和S波相对于各向同性介质波场的旅行时及振幅偏差,定性分析和定量评估不同震源机制下页岩各向异性对微地震波场特征的具体影响。数值算例的结果表明:对于所采用的地面微地震监测系统和平层VTI介质模型,页岩各向异性对微地震波场旅行时和振幅的影响均较为显著,旅行时偏差可达子波周期的30%,振幅相对偏差则高达60%,且旅行时偏差随炮检距增加而增大。此外,不同震源机制下,微地震波场特征受页岩各向异性影响的规律相似。

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.