A 2D Fully Coupled, Thermo-hydro-mechanical Modeling for Fault Activation Using the Combined Finite–Discrete Element Method (FDEM)

Within the framework of the combined finite–discrete element method (FDEM), we propose a coupled thermo-hydro-mechanical (THM) model to investigate the underlying mechanism of fault reactivation induced by cold-fluid injection during geothermal heat extraction. The coupled model consists of three parts, i.e., a hydraulic solver, a thermal solver, and a mechanical solver. For the hydraulic solver, we use the finite volume method to implement the pore seepage in the rock matrix, and fracture seepage following the cubic law only occurs in the broken cohesive elements or pre-existing fractures. The thermal solver involves heat conduction both in solid and fluid, heat transfer between discrete bodies, heat exchange between solid and fluid, and fluid heat convection. The mechanical solver is responsible for capturing the solid deformation, solid fracturing and contact interaction. We conduct a suite of numerical benchmarks to verify the accuracy and robustness of the proposed THM model. As an application, we perform a cold-fluid injection simulation to investigate the effect of initial rock temperature, injected pressure and thermal expansion coefficient on the fault activation during geothermal heat extraction. Our simulations demonstrate that the increase of initial rock temperature and thermal expansion coefficient would produce more obvious deformation in terms of fault slip displacement and aperture, thus also leading to a more active and wider distribution of acoustic emission (AE) events. The proposed THM model may provide a new perspective to uncover the fault activation mechanism during unconventional energy extraction.

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