PDF(Pubmed)
Abstract：
Distributed acoustic sensing (DAS) is a promising technology for seismic data acquisition, particularly in downhole applications. However, downhole DAS measurements can be affected by the deployment method of the fibre-optic cable. These effects were explored in a field trial in two wells (one vertical and one deviated) drilled at the Otway International Test Centre. The trial in the vertical well shows that (1) fibre-optic cables cemented behind the casing provide data of the highest quality due to the best coupling to the formation, and (2) tubing-conveyed cable shows only slightly weaker coupling, but the data quality can be severely degraded by source-generated noise. A cable loosely suspended in the deviated well provided data quality comparable to that of the cemented DAS cable. To better understand the nature of the observed effects, the field experiments were supplemented by numerical modelling with a 1.5D full wave reflectivity algorithm (3D wave propagation in a 1D model), where cement, casing and wellbore were represented by infinite vertical layers. The results show that (1) a cement layer has only a slight effect (<5%) on the DAS amplitude; (2) the vertical strain in a liquid-filled borehole is comparable to that in the formation; and (3) the strain amplitude in the cable is of the same order of magnitude both in the formation and in the fluid. The strain in the cable is zero both when the cable\'s Poisson\'s ratio is zero and when the borehole fluid is air. The results confirm the feasibility of borehole DAS measurements with fibre-optic cables suspended in a borehole liquid (but not gas!).
摘要：
分布式声传感(DAS)是一种很有前途的地震数据采集技术,特别是在井下应用。然而,井下DAS测量可能受到光纤电缆的部署方法的影响。在Otway国际测试中心钻探的两个井（一个垂直和一个偏离）的现场试验中探索了这些影响。在垂直井中的试验表明，（1）由于与地层的最佳耦合，胶接在套管后面的光纤电缆提供了最高质量的数据，和（2）管道输送电缆显示只有稍弱的耦合，但是源产生的噪声会严重降低数据质量。松散地悬挂在斜井中的电缆可提供与胶结DAS电缆相当的数据质量。为了更好地了解观察到的效果的性质，现场实验由数值建模与1.5D全波反射率算法（三维波传播在一维模型）补充，水泥，套管和井筒由无限垂直层表示。结果表明：（1）水泥层对DAS振幅的影响很小（<5％）；（2）充满液体的钻孔中的垂直应变与地层中的垂直应变相当；（3）电缆中的应变振幅在地层和流体中均具有相同的数量级。当电缆的泊松比为零时以及当井眼流体为空气时,电缆中的应变均为零。结果证实了使用悬挂在井眼液体（而不是气体！）中的光纤电缆进行井眼DAS测量的可行性。
