ABSTRACT Downhole monitoring with fibre‐optic Distributed Acoustic Sensing (DAS) systems offers unprecedented spatial resolution. At the same time, costs are reduced since repeated wireline surveys can be replaced by the permanent installation of comparatively cheap fibre cables. However, the single component nature of fibre data requires novel approaches when designing a monitoring project such as cross‐well seismics. At the example of the shallow CO2 injection test site in Svelvik, Norway, we model the evolution of velocity changes during CO2 injection based on rock physics theory. Different cross‐well seismic design scenarios are then considered to evaluate the best design and the limits of this method to detect containment breach. We present a series of evaluation tools to compare the effect of different well spacings for cross‐well seismic tomography. In addition to travel‐times, we also consider characteristic amplitude changes along the fibre unique to DAS strain measurements, which might add a constraint to the inversion. We also compare the effect of using helical fibres instead of classical straight fibres. We thus present a toolbox to evaluate and compare different monitoring design options for fibre optic downhole installations for cross‐well monitoring.

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使用 DAS 进行井间地震的设计考虑：CO 2 封存案例研究

摘要 光纤分布式声学传感 (DAS) 系统的井下监测提供了前所未有的空间分辨率。同时，由于可以通过永久安装相对便宜的光缆来代替重复的电缆勘测，因此成本降低了。然而，在设计监测项目（如井间地震）时，纤维数据的单组分性质需要采用新的方法。以挪威 Svelvik 的浅层 CO2 注入试验场为例，我们基于岩石物理理论模拟了 CO2 注入过程中速度变化的演变。然后考虑不同的井间抗震设计方案来评估最佳设计和该方法检测安全壳破裂的限制。我们提出了一系列评估工具来比较不同井距对井间地震层析成像的影响。除了旅行时间，我们还考虑了 DAS 应变测量特有的沿光纤的特征幅度变化，这可能会增加反演的约束。我们还比较了使用螺旋纤维代替经典直纤维的效果。因此，我们提供了一个工具箱来评估和比较用于井间监测的光纤井下安装的不同监测设计选项。