Efficient and accurate modeling of the coupled thermal-hydraulic-mechanical-chemical (THMC) processes in various rock formations is indispensable for designing energy geo-structures such as underground repositories for high-level nuclear wastes. This work focuses on developing and verifying an implicit finite element solver for generic coupled THMC problems in geological settings. Starting from the mass, momentum, and energy balance laws, a specialized set of governing equations and a thermoporoelastic constitutive model is derived. This system is then solved by an implicit finite element (FE) scheme. Specifically, the residuals and the Jacobians are scripted in a user-defined element (UEL) subroutine which is then combined with the general-purpose FE software Abaqus Standard to solve initial-boundary value problems. Considering the complexity of the system, the UEL development follows a stepwise manner by first solving the coupled hydraulic-mechanical (HM) and thermal-hydraulic-mechanical (THM) equations before moving on to the full THMC problem. Each implementation step consists of at least one verification test by comparing computed results with closed-form analytical solutions to ensure that the various coupling effects are correctly realized. To demonstrate the robustness of the algorithm and to validate the UEL, a three-dimensional case study is performed with reference to the in-situ heating test of ATLAS at Belgium in 1980s. A hypothetical radionuclide leakage event is then simulated by activating the chemical-concentration degree of freedom and prescribing a constant high concentration at the heater's surface. The model predicts a limited contaminated regime after six years considering both diffusion and advection effects on species transport.

中文翻译：

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饱和地质介质中热-水力-机械-化学（THMC）耦合过程的用户定义单元（UEL）的实现和验证

对各种岩层中的热-水力-机械-化学（THMC）耦合过程进行高效、准确的建模对于设计能源地质结构（例如高放核废料的地下储存库）是必不可少的。这项工作的重点是开发和验证地质环境中通用耦合 THMC 问题的隐式有限元求解器。从质量、动量和能量平衡定律出发，导出了一组专门的控制方程和热多孔弹性本构模型。然后通过隐式有限元 (FE) 方案对该系统进行求解。具体来说，残差和雅可比行列式在用户定义元素 (UEL) 子程序中编写，然后与通用有限元软件 Abaqus Standard 相结合来解决初始边界值问题。考虑到系统的复杂性，UEL 开发遵循逐步的方式，首先求解耦合液压机械 (HM) 和热液压机械 (THM) 方程，然后再继续解决完整的 THMC 问题。每个实施步骤都包含至少一个验证测试，通过将计算结果与封闭式解析解进行比较，以确保正确实现各种耦合效应。为了证明算法的鲁棒性并验证UEL，参考20世纪80年代比利时ATLAS的原位加热测试进行了三维案例研究。然后通过激活化学浓度自由度并在加热器表面规定恒定的高浓度来模拟假设的放射性核素泄漏事件。