A 2D FDEM-based THM coupling scheme for modeling deformation and fracturing of the rock mass under THM effects

Abstract

Deformation and fracturing of the rock mass are heavily affected by complex thermal-hydraulic-mechanical (THM) processes in the subsurface environment. This study develops a THM coupling formulation based on the 2D combined finite-discrete element method (FDEM) to investigate the rock deformation and fracturing behavior driven by coupled THM phenomena in the rock mass. The coupled scheme consists of three subsolvers, i.e., a hydraulic solver, thermal solver, and mechanical solver. In the developed hydraulic solver, a flow network searching algorithm is proposed based on the pore network models and the special mesh discretization in the FDEM. The laminar viscous flow in the flow network is solved using the cubic law approximation, and the fluid pressure field is determined by a linear compressibility model. The developed thermal solver accounts for the complex thermal transport processes decomposed into conduction in the solid and the fluid, advection by fluid flow, and fluid-solid thermal convective transfer. The coupled THM formulation is implemented by iterating the three subsolvers using an explicit, partitioned scheme. This approach is then progressively verified by a series of benchmark problems. Finally, the deformation and branching of a single fracture embedded in the rock mass due to cold fluid injection are simulated, in which the gradual aseismic fracture slip/opening, as well as the branch fracture initiation and propagation, and their potential impact on seismic events are discussed. The results show the potential of the developed numerical tool in modeling the coupled THM process-driven deformation and fracturing of the rock mass. Improvements in the implementation are needed, such as improvements in modeling the thermo-poroelastic effects and temperature- and pressure-dependent physical properties.