Summary Rayleigh frequency-shift-based distributed strain sensing (RFS-based DSS) is a fiber-optic-based diagnostic technique, which can measure the strain change along the fiber. The spatial resolution of RFS-based DSS can be as low as 0.2 m, and the measuring sensitivity is less than 1 με. Jin et al. (2021) presented a set of DSS data from the Hydraulic Fracture Test Site 2 project to demonstrate its potential to characterize near-wellbore fracture properties and to evaluate perforation efficiency during production and shut-in periods. Extensional strain changes are observed at locations around perforations during a shut-in period. At each perforation cluster, the observed responses of strain changes are significantly different. However, the driving mechanisms for the various observations are not clear, which hinders accurate interpretations of DSS data for near-wellbore fracture characterization. In this study, we applied a coupled flow and geomechanics model to simulate the observed DSS signals under various fractured reservoir conditions. The objective is to improve understanding of the DSS measurements and characterize near-wellbore fracture geometry. We used our in-house coupled flow and geomechanics simulator, which is developed by a combined finite-volume and finite-element method, to simulate strain responses within and near a fracture during shut-in and reopen periods. Local grid refinement was adopted around fractures and the wellbore, so that the simulated strain data can accurately represent the DSS measurements. The plane-strain condition is assumed. Numerical models with various fracture geometries and properties were constructed with representative parameters and in-situ conditions of the Permian Basin. The simulated well was shut-in for 4 days after producing 240 days, and reopened again for 1 day, following the actual field operation as shown in Jin et al. (2021). The characters of the strain changes along the fiber were analyzed and related to near-wellbore fracture properties. A novel diagnostic plot of relative strain change vs. wellbore pressure was presented to infer near-wellbore fracture characteristics. The impacts of permeability and size of the near-wellbore-stimulated region, fracture length, and near-perforation damage zone on strain responses were investigated through sensitivity analysis. The strain responses simulated by our model capture the observed signatures of field DSS measurements. During the shut-in period, clear positive strain changes are observed around the perforation locations, forming a “hump” signature. The shape of the “hump” region and peak value of each “hump” are dependent on the size and permeability of the near-wellbore fractured zone. Once the well is reopened, the strain changes decrease as the pressure drops. However, in one cycle of shut-in and reopen, the strain-pressure diagnostic plot shows path dependency. The discrepancy between the shut-in and reopen periods is highly influenced by the properties of near-wellbore fractured zones. The differences in the strain-pressure diagnostic plots can help to identify the conductive fractures. This study provides better understandings of the DSS measurements and their relations to the near-wellbore fracture properties, which is of practical importance for near-wellbore fracture characterization and completion/stimulation optimization.

中文翻译：

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模拟高分辨率分布式应变传感数据对近井筒裂缝区特征的新认识

总结 基于瑞利频移的分布式应变传感（RFS-based DSS）是一种基于光纤的诊断技术，可以测量沿光纤的应变变化。基于RFS的DSS空间分辨率可低至0.2 m，测量灵敏度小于1 με。金等人。（2021 年）展示了来自 2 号水力裂缝试验场项目的一组 DSS 数据，以证明其在表征近井筒裂缝特性和评估生产和关井期间的射孔效率方面的潜力。在关井期间，在射孔周围的位置观察到拉伸应变变化。在每个穿孔簇中，观察到的应变变化响应显着不同。然而，各种观察的驱动机制尚不清楚，这阻碍了对 DSS 数据的准确解释，以用于近井筒裂缝表征。在这项研究中，我们应用耦合流动和地质力学模型来模拟在各种裂缝储层条件下观察到的 DSS 信号。目的是提高对 DSS 测量的理解和表征近井筒裂缝几何形状。我们使用我们的内部耦合流动和地质力学模拟器，它是通过组合有限体积和有限元方法开发的，来模拟关闭和重新打开期间裂缝内和附近的应变响应。在裂缝和井筒周围采用局部网格细化，使模拟的应变数据能够准确地代表DSS测量值。假定平面应变条件。使用二叠纪盆地的代表性参数和现场条件构建了具有各种裂缝几何形状和特性的数值模型。模拟井在生产 240 天后关闭 4 天，再次开井 1 天，按照 Jin 等人的实际现场操作。（2021 年）。分析了沿纤维的应变变化特征，并与近井筒裂缝特性相关。提出了一种新的相对应变变化与井筒压力的诊断图，以推断近井筒裂缝特征。通过敏感性分析研究了渗透率、近井改造区尺寸、裂缝长度、近射孔损伤区等对应变响应的影响。我们的模型模拟的应变响应捕获了现场 DSS 测量的观察特征。在关井期间，在射孔位置周围观察到明显的正应变变化，形成“驼峰”特征。“驼峰”区域的形状和每个“驼峰”的峰值取决于近井筒裂缝带的大小和渗透率。一旦油井重新打开，应变变化随着压力下降而减小。然而，在关闭和重新打开的一个周期中，应变-压力诊断图显示了路径依赖性。关井期和重开期之间的差异在很大程度上受井筒附近裂缝带特性的影响。应变-压力诊断图的差异有助于识别导电裂缝。