Overview
This work addresses the verification of a thermo-hydrologic-mechanical (THM) capability, implemented within the PFLOTRAN framework. The focus is on a benchmark-based assessment of this THM implementation. The methodology incorporates thermal-hydrologic (TH) equations for mass and energy balance solved on control-volume blocks or Voronoi cells, coupled with a quasi-static momentum balance solved on an element-based dual mesh. The coupling mechanism employed is a strictly sequential, non-iterative fixed-stress split strategy.
Research Context
The research is situated within the broader context of modeling coupled thermal-hydrologic-mechanical processes in geologic porous media. Such processes are relevant for various applications, including enhanced geothermal systems and other subsurface energy storage initiatives. Establishing a robust modeling foundation for these complex interactions is a key objective.
Approach
The THM implementation in PFLOTRAN utilizes a sequential, non-iterative fixed-stress split strategy. Within this strategy, the TH system is solved implicitly to determine pressure and temperature. Subsequently, a mechanics update is performed to resolve the displacement unknowns. For discontinuities, such as fractures, a specific treatment based on mapping between mechanical and flow degrees of freedom is proposed.
Verification of this formulation involved setting up several problems against established poroelastic and thermo-poroelastic benchmarks. The outcomes of the PFLOTRAN implementation were then compared against analytical or semi-analytical benchmark responses. This comparative analysis was conducted for key variables including pressure diffusion, the temperature field, and mechanical deformation.
Findings
- The thermo-hydrologic-mechanical capability implemented within the PFLOTRAN framework demonstrated agreement with analytical or semi-analytical benchmark responses.
- This agreement was observed across several verification problems for pressure diffusion, the temperature field, and mechanical deformation in poroelastic and thermo-poroelastic benchmarks.
- The proposed treatment for discontinuities, which involves mapping between mechanical and flow degrees of freedom, was validated through comparison with an analytical solution.
Why This Matters
This work establishes a verified basis for thermo-poroelastic coupling within PFLOTRAN. It provides a solid modeling foundation applicable to a range of applications involving coupled thermal-hydrologic-mechanical (THM) processes in geologic porous media. This includes areas such as enhanced geothermal systems and other subsurface energy storage applications.