Abstract The present study is aimed at developing a numerical model to reproduce coupled hydro-mechanical processes associated with fault reactivation by fluid injection in low permeability rock, as part of the DECOVALEX-2019 project Task B. We proposed a modeling approach for simulating the processes using the TOUGH-FLAC simulator, and modeled a fault reactivation experiment conducted at Mont Terri Rock Laboratory in Switzerland. The first step of the study involved benchmark calculations considering a simplified fault plane and geometry. Fluid flow along a fault was modeled using elements of aperture-sized thickness on the basis of Darcy's law and the cubic law in TOUGH2, whereas the mechanical behavior of a single fault was represented by zero-thickness interface elements in FLAC3D upon which a slip and/or separation is allowed. A methodology to connect a TOUGH2 volume element to a FLAC3D interface element was developed for handling the hydro-mechanical interactions on the fault during fluid injection. Two different fault models for describing the evolutions of hydraulic aperture by elastic fracture opening and failure-induced aperture increase were considered in the benchmark calculations. In the coupling process, the changes in geometrical features and hydrological properties induced by mechanical deformation were continuously updated. The transient responses of the fault and host rock to stepwise pressurization were examined during the simulation. The hydro-mechanical behavior, including the injection flow rate, pressure distribution around the borehole, stress conditions, and displacements in normal and shear directions were monitored in the surrounding rock and along the fault. The results of benchmark calculations suggest that the developed model reasonably represents the hydro-mechanical behavior of a fault and the surrounding rock. This modeling approach was applied to the fault reactivation experiment of the Mont Terri Rock Laboratory. In this interpretive modeling, a parametric study was conducted to examine the effects of input parameters regarding in situ stress and fault properties on the hydro-mechanical responses of the fault to water injection. Then, an optimal parameter set to reproduce the field experiment results was chosen by trial-and-error. The injection flow rate and pressure response during fault reactivation closely matched those obtained at the site, which indicates the capability of the model to appropriately capture the progressive pathway evolution during fault reactivation tests at the site. The anchor displacements were overestimated by the model, but a fair agreement was obtained in terms of the order of magnitude and the variation tendency.

中文翻译：

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使用耦合流体流动和机械界面模型对流体注入引起的断层再激活进行建模

摘要 作为 DECOVALEX-2019 项目任务 B 的一部分，本研究旨在开发一种数值模型来重现与低渗透性岩石中流体注入断层再激活相关的耦合水力机械过程。我们提出了一种模拟这些过程的建模方法使用 TOUGH-FLAC 模拟器，并模拟了在瑞士 Mont Terri 岩石实验室进行的断层再激活实验。研究的第一步涉及考虑简化断层平面和几何形状的基准计算。沿断层的流体流动基于 TOUGH2 中的达西定律和三次定律使用孔径大小的元素建模，而单个断层的力学行为由 FLAC3D 中的零厚度界面元素表示，在此元素上，滑动和/ 或允许分离。开发了一种将 TOUGH2 体积元素连接到 FLAC3D 接口元素的方法，用于处理流体注入过程中断层上的流体力学相互作用。在基准计算中考虑了两种不同的断层模型，用于通过弹性裂缝张开和破坏引起的孔径增加来描述水力孔径的演变。在耦合过程中，机械变形引起的几何特征和水文性质的变化不断更新。在模拟过程中检查了断层和主岩对逐步加压的瞬态响应。流体力学行为，包括注入流速、钻孔周围的压力分布、应力条件、监测围岩和断层沿线的法向和剪切位移。基准计算的结果表明，所开发的模型合理地代表了断层和围岩的水力学行为。这种建模方法被应用于蒙特特里岩石实验室的断层再激活实验。在这种解释性建模中，进行了参数研究，以检查有关地应力和断层特性的输入参数对断层对注水的流体力学响应的影响。然后，通过反复试验选择一个最佳参数集来重现现场实验结果。断层再激活过程中的注入流量和压力响应与现场获得的非常匹配，这表明模型在现场故障再激活测试期间适当地捕捉渐进路径演变的能力。模型高估了锚的位移，但在数量级和变化趋势方面获得了公平的一致性。