黏弹性VTI介质微地震正演模拟与震源机制全波形反演

Microseimic forward modeling and full-waveform inversion of focal mechanisms for viscoelastic vertically transverse isotropic media

微地震矩张量能够描述储层岩石破坏的细节,有助于了解水力压裂储层的地质力学特征,震源机制矩张量反演对于非常规油气开发具有重要的作用.水力压裂通常在页岩油气储层中进行,震源区介质表现出速度和衰减各向异性,需要研究衰减各向异性介质中微地震信号的传播规律.基于观测到的微地震记录,通过波形反演能够获得矩张量,忽视震源区介质的衰减和速度各向异性会导致震源机制反演结果的误差.本文提出了一种考虑地层各向异性吸收衰减作用的震源机制全波形反演方法,对于黏弹性VTI介质,采用GPU并行的伪谱法计算合成地震记录,通过全波形反演方法匹配合成波形和记录波形,借助优化算法获得震源机制矩张量反演结果.本文通过实验测试验证了该方法的有效性,分析了震源区各向异性速度和衰减参数、噪声以及观测系统等对于微地震震源机制各分量及成分反演结果的影响,研究表明考虑模型中的黏弹性各向异性对于获得可靠的震源机制反演结果非常重要.

The microseismic moment tensor can describe the details of rock failure in reservoirs,which helps reveal geomechanical characteristics of hydraulic fracturing reservoirs.Moment tensor inversion of focal mechanism is important to development of unconventional oil and gas.Hydraulic fracturing is usually conducted in shale oil and gas reservoirs.The media near the source typically have anisotropic velocity and anisotropic attenuation.So it is necessary to study microseismic signal propagation in anisotropic attenuative media.Based on microseismic records,the moment tensor can be calculated through waveform inversion.While ignoring the velocity anisotropy and attenuation of the source area will lead to errors in the focal mechanism inversion.To solve this problem,we propose a full-waveform inversion method for focal mechanism considering anisotropic attenuation.The vectorized Graphics Processing Unit(GPU)parallel pseudospectral method is used to simulate synthetic seismic records in viscoelastic Vertically Transverse Isotropic(VTI)media.Through the full-waveform inversion to match the synthetic waveforms and received waveforms,the focal mechanism moment tensor can be obtained by the optimization algorithm.Numerical simulation tests verify the effectiveness of this method.We further analyze the influences of anisotropic velocity,anisotropic attenuation,noise and observation system on the microseismic focal mechanism inversion results.Research suggests that considering the attenuation anisotropy is important for reliable inversion of focal mechanism.