页岩水力压裂实验声发射事件震源机制研究

As one kind of unconventional oil and gas resource, shale gas has been widely concerned by all walks of life. However, the current microseismic monitoring technology is limited, and the microseismic focal mechanism during the hydraulic fracturing of shale reservoir cannot be accurately retrieved and evaluated. So, the shale samples with different sedimentary bedding directions are selected as the research object. Based on the hydraulic fracturing experiment, CT scanning technique and geophysical inversion method are used to study the time-changing characteristic of P/S velocity and attenuation of the shale during the hydraulic fracturing process. And the source mechanism of acoustic emission events due to rock burst is also analyzed. By processing the high signal-to-noise acoustic emission data and comparing the CT scanning image of fractures network before and after fracturing, the source mechanism of rock failure in the shale samples is inversed and verified. The quantitative relationship between the shale sedimentation and the source mechanism is established and the influence of sedimentary bedding direction on fracture development is revealed. Then, the time-dependent model of P/S-wave attenuation value of shale during the hydraulic fracturing process is used to quantitatively analyze the inversion results of different types of focal mechanism, which is influenced by the variation of attenuation value. The study of the focal mechanism for the acoustic emission by shale hydraulic fracturing experiment will provide reliable experimental data to support the hydraulic fracturing scheme, and lay a theoretical foundation for improving the inversion accuracy of the microseismic source mechanism in the field.

页岩气作为一种非常规油气资源已经得到了各界的广泛关注。然而，当前微地震监测技术水平有限，页岩储层水力压裂过程中的微地震震源机制不能被准确反演与评价。本项目选取不同沉积层理方向的页岩样品为研究对象，以水力压裂实验为基础，应用CT扫描技术和地球物理反演方法，对水力压裂过程中页岩纵横波速度与衰减时变特征、岩石破裂声发射事件震源机制进行分析与研究。通过处理高信噪比声发射数据，对比压裂前后岩样裂缝网络CT扫描图像，反演并验证页岩样品的水力压裂震源机制，建立页岩沉积层理方向与震源机制类型间的量化关系，揭示沉积层理面对裂缝发育的影响。然后，利用页岩水力压裂过程中纵横波衰减值的时变模型，定量分析纵横波衰减值变化对不同类型震源机制反演的影响。页岩水力压裂实验声发射事件震源机制研究，将为优化水力压裂施工方案提供可靠的实验数据支持，为提高野外微地震震源机制反演精度奠定理论基础。

水力压裂储层改造技术是页岩气开发的关键技术手段。大量高压流体注入页岩储层会显著改变岩石的各项物性特征。本研究中，我们设计了不同的水力压裂岩石物理实验，对比了龙马溪组页岩超声纵横波速度值、衰减值和岩石力学参数的各向异性特征。然后利用诱发的声发射数据分析了页岩破裂的震源机制和其反演结果误差。分析实验数据可知：（1）在各向同性压缩阶段，流体作用可以降低页岩的杨氏模量，削弱Thomsen参数对于加载应力的敏感性，从而增强杨氏模量与泊松比各向异性参数的计算误差；（2）在差应力加载中，流体作用减弱，持续增大的轴压使得页岩垂直层理方向的抗压强度得到更快的增强；（3）在水力压裂的破坏阶段，页岩样品的纵横波衰减值对裂缝发育具有更高的敏感性，但在裂缝处于水饱和状态后，纵波衰减对流体注入失去了敏感性。（4）在应力卸载阶段，储存在样品裂缝网络中的高压水诱发了数百个声发射事件；通过对比声发射事件定位和震源机制与裂缝CT扫描图像，我们验证了反演结果的准确度，由此推断声发射事件主要来源于裂缝的扩张/压缩，以及页岩沿裂缝平面的剪切滑移；（5）定量分析地层衰减误差对震源机制反演结果的影响表明声发射事件的剪切分量比体积分量对介质衰减变化更敏感，尤其是当衰减值被高估时。