水力压裂诱发的剪张型微地震震源机制矩张量反演方法

Hydraulic-fracturing shear-tensile microseismic focal mechanism moment tensor inversion

受井中接收器数量的限制以及井中监测张角小且方位单一等因素的影响,井中微地震监测信号的震源定位和震源机理分析结果有时不够准确。为此,研究了利用地表接收的P波初动振幅信息反演压裂裂缝震源机制解的方法原理,首先利用点源在均匀弹性介质中P波和S波的质点运动方程计算格林函数,然后通过理论模型计算振幅信息,最后测试、分析矩张量反演方法的应用效果,得到以下认识:①采用全矩张量反演时各向同性(ISO)分量相对补偿线性矢量偶极(CLVD)分量对误差不敏感,这是由于在矩张量分解时,ISO分量是矩张量特征值的线性函数,而CLVD分量是非线性函数;②观测系统对非双力偶(非DC)分量的反演结果非常重要,合适的观测系统可增加地震矩张量DC和非DC分量分析的可信度;③辐射花样可观测破裂面的几何分布和破裂机制类型,不同震源类型的辐射花样间存在差异,因此有限的震源球覆盖导致反演多解性。

Due to limited receivers,small monitoring angles,and a single measure direction in a well,focal positioning and focal mechanism analyses based on microseismic monitoring signals in wells are not accurate enough.So a hydraulic-fracturing focal mechanism calculation with P-wave first-motion amplitude inversion is proposed.The moment tensor inversion uses P-wave and S-wave particle motion equations in uniform elastic medium to calculate Green’s functions.Based on tests of the proposed method on model data,the following understanding is obtained:(1)Isotropic(ISO)components are not sensitive to errors compared with compensated linear vector dipole(CLVD)components when using the full moment tensor inversion.This is because ISO components are linear functions of moment tensor eigenvalues when the moment tensor is decomposed,but CLVD components are nonlinear functions;(2)The acquisition geometry is very important for inversion results of non-double-coupled(non-DC)components.An appropriate acquisition geometry increases the credibility of seismic moment tensor DC and non-DC component analysis;(3)The geometric distribution and the type of rupture mechanism can be observed from radiation patterns.Radiation patterns of different focal types have some differences.The limited focal sphere coverage will bring multiple solutions to the inversion.