Distributed Acoustic Sensing as a Distributed Hydraulic Sensor in Fractured Bedrock
Distributed Acoustic Sensing as a Distributed Hydraulic Sensor in Fractured Bedrock
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DOI：
10.1029/2020wr028140
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发表时间：
2020-09-01
影响因子：
5.4
通讯作者：
Ciervo, C. C.
中科院分区：
文献类型：
--
作者：
Becker, M. W.;Coleman, T., I;Ciervo, C. C.
分布式声学传感(DAS)最初的目的是测量光缆上频率为1赫兹或更高的振荡应变。最近，在更低频率下的测量为在钻孔中使用DAS作为动态应变传感器提供了可能性。机械连接到地质地层上的光缆会因孔洞和裂缝中的水力应力而产生应变。DAS探测仪可以测量井筒中的动态应变，它可以通过地层的机械柔度特性与流体压力有关。由于DAS进行分布式测量，它既能够定位水力活动特征，又能够量化地层中的流体压力。我们介绍了将一根光缆机械耦合到两个结晶岩石钻孔上的现场实验。在另一口井采用交替注水和抽水的方式对地层施加水力压力。DAS仪器在已知水力活动的断裂带位置测量了振荡应变。测量了小于1 nm的岩石位移。实验室实验证实，位移测量正确。这些结果表明，埋藏在地质地层中的光缆可以用于绘制三维裂隙网络中的水力连接。这种方法的一个很大优点是，应变是水力应力的间接测量，可以在不事先知道与井眼相交的流动裂缝的情况下进行测量。该技术在水资源、地热能、二氧化碳封存、裂隙基岩地下水修复等方面有明显的应用。
Distributed acoustic sensing (DAS) was originally intended to measure oscillatory strain at frequencies of 1 Hz or more on a fiber optic cable. Recently, measurements at much lower frequencies have opened the possibility of using DAS as a dynamic strain sensor in boreholes. A fiber optic cable mechanically coupled to a geologic formation will strain in response to hydraulic stresses in pores and fractures. A DAS interrogator can measure dynamic strain in the borehole, which can be related to fluid pressure through the mechanical compliance properties of the formation. Because DAS makes distributed measurements, it is capable of both locating hydraulically active features and quantifying the fluid pressure in the formation. We present field experiments in which a fiber optic cable was mechanically coupled to two crystalline rock boreholes. The formation was stressed hydraulically at another well using alternating injection and pumping. The DAS instrument measured oscillating strain at the location of a fracture zone known to be hydraulically active. Rock displacements of less than 1 nm were measured. Laboratory experiments confirm that displacement is measured correctly. These results suggest that fiber optic cable embedded in geologic formations may be used to map hydraulic connections in three-dimensional fracture networks. A great advantage of this approach is that strain, an indirect measure of hydraulic stress, can be measured without beforehand knowledge of flowing fractures that intersect boreholes. The technology has obvious applications in water resources, geothermal energy, CO(2)sequestration, and remediation of groundwater in fractured bedrock.