Modeling Coupled Thermo-Hydro-Mechanical-Chemical Processes in Subsurface Geological Media
Authors: Huang Y. , Lei Z. , Lipnikov K. , Moulton J. D. , Sweeney M. R. , Hyman J. D. , Knight E. , Stauffer P. H. 
Published: September 2023

ABSTRACT
The complex coupling interaction phenomena among rock mechanics, fluid flow, heat transfer and geochemical reactions has become a critical topic in complex subsurface systems including the production of unconventional oil and gas. In this paper, we introduce a fully coupled Thermo-Hydro-Mechanical-Chemical (THMC) framework that is being developed at Los Alamos National Laboratory (LANL). The framework integrates four LANL-developed codes: HOSS, Amanzi, dfnWorks and InyanCC. HOSS simulates deformation of the rock matrix as well as the opening, closing and shear sliding in the discrete fractures (mechanics), while Amanzi solves subsurface multiphase flow and reactive transport, dfnWorks generates meshes with complex discrete fracture networks, and InyanCC links the mechanics and flow solvers while controlling the whole simulation processes. The advantages of this coupling framework are: 1) it is based on hybrid continuum-discontinuum approaches which overcomes the limitations seen with pure continuum assumptions; 2) both mechanics and subsurface flow solvers are fully parallelized for distributed memory systems which allows the users to simulate large scale problems on HPC clusters. Different selected benchmarking problems are simulated using this THMC framework. The results show good agreement with the analytical solutions, which verifies the accuracy of the framework.

INTRODUCTION
Current applications of geotechnical engineering and geo-energy in the subsurface rely significantly on complex coupling process among rock mechanics, fluid flow, heat transfer and geochemical reactions, including geothermal production, unconventional oil and gas production and underground nuclear explosions. Hence, Modeling Thermo-Hydro-Mechanical-Chemical (THMC) processes is essential in understanding the coupled processes in subsurface geological media.
Fully addressing the computational challenge of coupled THMC process simulation has been exacerbated by the inability to simulate coupled processes in both the rock matrix and discrete fractures. However, modern subsurface simulators taking advantage of the high-performance computation have been proposed to overcome these challenging problems. Cheng, 2016; Rutqvist et al., 2001; and Wang, 2000 proposed different approaches for modeling the evolution of pressure, stress, and temperature fields in porous media, including equations for pressure diffusion, mechanical equilibrium, and energy transport. Rutqvist and Stephansson (2003) introduced a coupled THM model based on sub-grid scale fracture networks. Min and Jing (2003) reported numerical simulations of hydro-mechanical coupling in fracture networks. These models capture the contribution of discrete fracture deformation to permeability anisotropy through the effective properties such as permeability and porosity. However, these methods have limitation in modeling time-evolving large scale THM system when the characteristic length of network structures is much larger than the grid block scale.