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Comparison between Distributed Acoustic Sensing and Geophones: Downhole Microseismic Monitoring of the FORGE Geothermal Experiment
Seismological Research Letters ( IF 2.6 ) Pub Date : 2020-08-19 , DOI: 10.1785/0220200149
Ariel Lellouch 1 , Nathaniel J. Lindsey 1 , William L. Ellsworth 1 , Biondo L. Biondi 1
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Cite this article as Lellouch, A., N. J. Lindsey, W. L. Ellsworth, and B. L. Biondi (2020). Comparison between Distributed Acoustic Sensing and Geophones: Downhole Microseismic Monitoring of the FORGE Geothermal Experiment, Seismol. Res. Lett. XX, 1–13, doi: 10.1785/0220200149. We compare the performance of a downhole distributed acoustic sensing (DAS) fiberoptic array with that of conventional geophones. The downhole collocated arrays are part of the Frontier Observatory for Research in Geothermal Energy (FORGE) geothermal experiment, in which stimulation of the rock volume in an enhanced geothermal system (EGS) causes microseismic events. The DAS acquisition system yields data sampled at every 1 m at 2000 samples per second for the entire length of the well, spanning to a depth of 985 m from the surface. Whereas single DAS channels are substantially noisier than geophones at the same location, their large number and spatial coherency allow for the application of effective array processing techniques. We follow a complete workflow for the fiber-optic array: velocity model building, event detection, event location, and magnitude estimation. Estimated velocity models agree well with sonic logging in a nearby well and map a granitic contact accurately. Detection performance is somewhat worse than geophones and yields magnitude completeness of −1:4 compared to −1:7 for geophones. Using a single vertical fiber array, we cannot retrieve the azimuth of the events relative to the well. However, we can very accurately estimate their depth and horizontal distance from the array. Magnitude estimation with DAS approaches geophone results to within a standard deviation ofM 0:115 and negligible mean difference. The DAS processing results outperform a regional and local surface array, consolidated with a shallow borehole sensor. Although downhole geophones in the FORGE experimental layout performed better, DAS holds several critical practical benefits that were not demonstrated. Thanks to its heat resistance, it can be deployed much closer to the reservoir; fibers can be deployed along cased active wells, eliminating the need for a dedicated monitoring well; the permanently installed fiber can be used for years or decades. Therefore, we argue that DAS holds vast potential for long-term monitoring of EGS projects.

中文翻译:

分布式声学传感与地震检波器的比较:FORGE 地热实验的井下微地震监测

将本文引用为 Lellouch, A.、NJ Lindsey、WL Ellsworth 和 BL Biondi(2020 年)。分布式声学传感和地震检波器之间的比较:FORGE 地热实验的井下微地震监测,Seismol。水库 莱特。XX, 1–13, doi: 10.1785/0220200149。我们将井下分布式声学传感 (DAS) 光纤阵列的性能与传统地震检波器的性能进行了比较。井下并列阵列是地热能研究前沿天文台 (FORGE) 地热实验的一部分,在该实验中,增强地热系统 (EGS) 中岩石体积的刺激会导致微震事件。DAS 采集系统在井的整个长度上以每秒 2000 个样本的速度每 1 m 采集一次数据,覆盖距离地表 985 m 的深度。虽然单个 DAS 通道在同一位置比地震检波器噪声大得多,但它们的数量和空间相干性允许应用有效的阵列处理技术。我们遵循光纤阵列的完整工作流程:速度模型构建、事件检测、事件定位和幅度估计。估计的速度模型与附近井中的声波测井非常吻合,并准确地绘制了花岗岩接触图。检测性能比地震检波器差一些,与地震检波器的 -1:7 相比,产生的幅度完整性为 -1:4。使用单个垂直光纤阵列,我们无法检索事件相对于井的方位角。但是,我们可以非常准确地估计它们与阵列的深度和水平距离。使用 DAS 进行的震级估计使地震检波器结果接近 M 0:115 的标准偏差和可忽略的平均差异。DAS 处理结果优于区域和局部地表阵列,并与浅钻孔传感器相结合。尽管 FORGE 实验布局中的井下检波器表现更好,但 DAS 具有几个未证明的关键实际优势。由于其耐热性,它可以部署在更靠近水库的地方;光纤可以沿套管活动井部署,无需专门的监测井;永久安装的光纤可以使用数年或数十年。因此,我们认为 DAS 在长期监测 EGS 项目方面具有巨大潜力。DAS 处理结果优于区域和局部地表阵列,并与浅钻孔传感器相结合。尽管 FORGE 实验布局中的井下检波器表现更好,但 DAS 具有几个未证明的关键实际优势。由于其耐热性,它可以部署在更靠近水库的地方;光纤可以沿套管活动井部署,无需专门的监测井;永久安装的光纤可以使用数年或数十年。因此,我们认为 DAS 在长期监测 EGS 项目方面具有巨大潜力。DAS 处理结果优于区域和局部地表阵列,并与浅钻孔传感器相结合。尽管 FORGE 实验布局中的井下检波器表现更好,但 DAS 具有几个未证明的关键实际优势。由于其耐热性,它可以部署在更靠近水库的地方;光纤可以沿套管活动井部署,无需专门的监测井;永久安装的光纤可以使用数年或数十年。因此,我们认为 DAS 在长期监测 EGS 项目方面具有巨大潜力。它可以部署在离水库更近的地方;光纤可以沿套管活动井部署,无需专门的监测井;永久安装的光纤可以使用数年或数十年。因此,我们认为 DAS 在长期监测 EGS 项目方面具有巨大潜力。它可以部署在离水库更近的地方;光纤可以沿套管活动井部署,无需专门的监测井;永久安装的光纤可以使用数年或数十年。因此,我们认为 DAS 在长期监测 EGS 项目方面具有巨大潜力。
更新日期:2020-08-19
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