Characteristics of microseismic data recorded by distributed acoustic sensing systems in anisotropic media
Characteristics of microseismic data recorded by distributed acoustic sensing systems in anisotropic media
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DOI：
10.1190/geo2019-0776.1
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发表时间：
2020-07-01
期刊：
影响因子：
3.3
通讯作者：
Clarke, A.
中科院分区：
文献类型：
--
作者：
Baird, A. F.;Stork, A. L.;Clarke, A.
光纤分布式声学传感（DAS）电缆现在用于监测非常规气藏水力压裂增产期间的微震活动。与地震检波器阵列不同，DAS系统对沿光纤方向沿着的单轴应变或应变率敏感，因此提供1C记录，这使得难以识别入射波的方向性和偏振。使用合成的例子，我们已经显示了一些基本特征的微震记录DAS系统的目的，在各向异性（垂直横向各向同性[VTI]）页岩的水平井中的水力压裂监测。我们确定SH到达占主导地位的记录的信号，因为他们的极化是沿着水平电缆在近偏移对齐，虽然SV将通常占主导地位的事件直接上方或下方的阵列。沿着电缆的相干剪切波（S波）到达的幅度呈现出具有双峰的特征模式，其宽度与事件距电缆的距离有关。此外，我们发现，DAS系统上记录的S波分裂可用于推断入射波的倾斜度，克服了当前事件位置的限制，这些事件位置限制了事件位于水平面上。低振幅SV到达表明事件深度类似于DAS电缆。相反，陡峭的到达产生更高振幅的SV波，S波分裂随着沿电缆的偏移沿着增加。最后，我们确定如何极性反转观察到的P和SH阶段可以用来提供强有力的约束源机制。
Fiber-optic distributed acoustic sensing (DAS) cables are now used to monitor microseismicity during hydraulic-fracture stimulations of unconventional gas reservoirs. Unlike geophone arrays, DAS systems are sensitive to uniaxial strain or strain rate along the fiber direction and thus provide a 1C recording, which makes identifying the directionality and polarization of incoming waves difficult. Using synthetic examples, we have shown some fundamental characteristics of microseismic recordings on DAS systems for purposes of hydraulic fracture monitoring in a horizontal well in anisotropic (vertical transverse isotropy [VTI]) shales. We determine that SH arrivals dominate the recorded signals because their polarization is aligned along the horizontal cable at the near offset, although SV will typically dominate for events directly above or below the array. The amplitude of coherent shear-wave (S-wave) arrivals along the cable exhibits a characteristic pattern with bimodal peaks, the width of which relates to the distance of the event from the cable. Furthermore, we find that S-wave splitting recorded on DAS systems can be used to infer the inclination of the incoming waves, overcoming a current limitation of event locations that have constrained events to lie in a horizontal plane. Low-amplitude SV arrivals suggest an event depth similar to that of the DAS cable. Conversely, steep arrivals produce higher amplitude SV-waves, with S-wave splitting increasing with offset along the cable. Finally, we determine how polarity reversals observed in the P and SH phases can be used to provide strong constraints on the source mechanisms.