Measuring changes in fracture properties from temporal variations in anisotropic attenuation of microseismic waveforms
Measuring changes in fracture properties from temporal variations in anisotropic attenuation of microseismic waveforms
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DOI：
10.1111/1365-2478.12551
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发表时间：
2017-07
影响因子：
2.6
通讯作者：
P. Usher;J. Kendall;C. Kelly;A. Rietbrock
中科院分区：
文献类型：
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作者：
P. Usher;J. Kendall;C. Kelly;A. Rietbrock
我们从水力压裂增产过程中获得的微地震数据集中研究了裂缝引起的衰减各向异性。该数据集包含888个震级为−3.0到0.0的事件。我们使用对数-谱-振幅-比方法来估计在注入流体的半小时内t *的变化，并从先前的S波分裂分析中推断出压裂的增加。使用Pearson相关分析来评估衰减随时间的变化是否具有统计学意义。在这段时间内，P波没有显示出t *的系统变化。相反，垂直于裂缝的极化S波随时间明显增加，而平行于裂缝的极化S波则呈现微弱的负趋势。我们还比较了两个S波之间的t *，发现Δt *随着时间的推移而增加。采用裂缝诱导衰减各向异性的孔隙弹性岩石物理模型来解释结果。该模型表明，观察到的t*变化与断裂密度增加有关，最高可达0.04。这比之前基于S波速度各向异性估计的0.025±0.002要高得多，但在衰减测量中有相当多的散射。这可能是由于衰减测量对非定向裂缝、裂缝形状和流体性质的敏感性增加。然而，该初步研究表明，衰减测量对裂缝密度和长径比等裂缝特性非常敏感。
We investigate fracture‐induced attenuation anisotropy in a cluster of events from a microseismic dataset acquired during hydraulic fracture stimulation. The dataset contains 888 events of magnitude −3.0 to 0.0. We use a log‐spectral‐amplitude‐ratio method to estimate change in t∗ over a half‐hour time period where fluid is being injected and an increase in fracturing from S‐wave splitting analysis has been previously inferred. A Pearson's correlation analysis is used to assess whether or not changes in attenuation with time are statistically significant. P‐waves show no systematic change in t∗ during this time. In contrast, S‐waves polarised perpendicular to the fractures show a clear and statistically significant increase with time, whereas S‐waves polarised parallel to the fractures show a weak negative trend. We also compare t∗ between the two S‐waves, finding an increase in Δt∗ with time. A poroelastic rock physics model of fracture‐induced attenuation anisotropy is used to interpret the results. This model suggests that the observed changes in t* are related to an increase in fracture density of up to 0.04. This is much higher than previous estimates of 0.025 ± 0.002 based on S‐wave velocity anisotropy, but there is considerably more scatter in the attenuation measurements. This could be due to the added sensitivity of attenuation measurement to non‐aligned fractures, fracture shape, and fluid properties. Nevertheless, this pilot study shows that attenuation measurements are sensitive to fracture properties such as fracture density and aspect ratio.