含衰减地层微地震震源机制反演及其反演分辨率

微地震震源机制的反演对于非常规油气开发具有至关重要的作用.微地震信号主频高、能量小,容易受地层吸收衰减作用的影响使其波形发生畸变,本文提出了一种考虑地层吸收衰减作用的微震源机制反演方法,并利用费雷谢偏导矩阵的SVD分解(特征值分解)方法,分析研究了地层的吸收衰减因子的变化对于微地震震源机制反演分辨率的影响,根据理论计算给出了不同地震数据对各种震源机制反演的适用条件.理论计算证明,采用直达P波和S波数据联合反演震源的T值,单独利用直达P波反演震源的k值,可以有效降低地层吸收衰减作用对反演结果的影响.

Inversion for microseismic focal mechanisms in attenuated strata and its resolution

Microseismic focal mechanism inversion is significant for the unconventional oil and gas development. Microseismic signal is characterized by high frequency and small energy, so it is easy to be influenced by formation attenuation effect, and to cause serious waveform distortion and unreliable inversion results. After analyzing the previous microseismic moment tensor inversion methods and some of their deficiency, this paper proposes a new method for microseismic focal mechanism inversion considering the stratum attenuation effect. On the other hand, the Q factor of actual strata is hard to estimate precisely and the microseismic events are very complex. So we design a simple observation model with three vertical wells and four basic moment tensors which are added 30% random perturbation to simulate the actual field situation. In the process of numerical computation, we assume the sediments' Q factor has different levels of error. Based on this assumption, we can calculate the inversion error of the new moment tensor inversion method and analyze the inversion resolution by using the SVD of Frechet derivative matrix due to Q factor error. According to the theoretical calculation, the application conditions of the various focal mechanism inversions can be summarized for different types of microseismic data. Furthermore, the theoretical calculation proves that only using direct P waves can obtain a better inversion result than using direct P and S waves simultaneously. After analyzing the inversion error and the inversion resolution, we conclude that the joint inversion by using direct P and S wave data can estimate a better T value and a better k value can be inversed by using direct P wave separately. Moreover the direct P wave has a less sensitivity about the sediment Q factor. Based on this conclusion we can effectively reduce the negative influence on the focal mechanism inversion result by utilizing different types of wave data to determine the different parts of microseismic focal mechanisms.