Thermo-hydro-mechanical (THM) coupling in fractured/porous geomaterials

Abstract

Thermo-hydro-mechanical (THM) coupling prevails in various rock engineering projects, such as nuclear waste disposal, geothermal exploitation and shale gas recovery. THM coupling is a very complex system and it features a wide spectrum of knowledge. Especially for THM coupling simulation, a diversity of approaches are theoretically possible. Previous THM coupling studies abound, whereas the methodology behind them does not vary too much. In the methodology sense, most of the previous studies only explored a fraction of THM coupling, which is insufficient to illuminate a full landscape of THM coupling.

This thesis aims to systematically explore the THM coupling in fractured/porous geomaterials. Two overarching objectives are intended: 1) create a comprehensive and in-depth theoretical framework to guide THM coupling simulations and 2) try to provide engineering implications. TM coupling is first attempted. An elusive phenomenon, thermal strengthening in rock material, is thoroughly investigated by laboratory experiments. Fracture stimulation is then targeted. An open-source code DDFS3D using the boundary element method is developed to model arbitrary 3D fracture propagation under I, II and III mixed-mode loadings. Afterwards, a 3D THM coupling simulator Hybrid-THM is developed integrating DDFS3D, finite volume method and finite element method to simulate THM coupling in the fractured geomaterial. Two stages, HM coupling and TH coupling, are implemented in Hybrid-THM, therefore Hybrid-THM is a pseudo- THM coupling simulator. Finally, an authentic fully THM coupling simulator HENGYI is developed in a pure FEM framework to simulate THM coupling in the porous geomaterial. In particular, the 3D Streamline Upwind Petrov Galerkin (SUPG) method is developed and implemented in HENGYI to suppress the numerical oscillation incurred by advection-dominated heat transfer.

In terms of the theoretical framework to guide THM coupling simulations, two strategies for developing a THM coupling simulator are explored: integrate multiple numerical methods (Hybrid-THM) and focus on a pure FEM framework (HENGYI). The staggered and monolithic solution strategies are implemented in both Hybrid-THM and HENGYI. Performance comparison in terms of accuracy, efficiency and convergency is carried out to better understand the applicability of different solution schemes. 3D SUPG is developed to deal with advection-dominated situations. The conclusions drawn are highly distilled and thus transferable to other cases of multiphysics simulations.

In terms of providing engineering implications, the temperature range of thermal strengthening investigated in this thesis is between 25 and 200 ℃, which prevails in most underground rock engineering projects. In addition, three simulation examples are provided, including Enhanced Geothermal Systems, nuclear waste disposal and hydrothermal exploitation. The reservoir performance during 30, 1000, and 30 years is analyzed in great detail for the three examples, respectively. The conclusions drawn are of high value to understand these projects and then be used to guide engineering practice. Furthermore, three in-house codes, DDFS3D, Hybrid-THM and HENGYI are developed, and among them DDFS3D is open-source. These codes have been verified to be competent to perform field-scale simulations, thus they are of great potential to be used to aid engineering design.