Simulation of Coupled THMC Processes in Fractured Rocks Using a Dual-Media Approach

Abstract Simulation of coupled thermo-hydro-mechanical-chemical (THMC) processes in fractured rocks is relevant to many areas of applied geoscience. Groundwater flow and reactive transport (hydro-chemical processes) in hard rocks are primarily controlled by networks of rock fractures, and therefore best modelled in a discrete fracture network, or DFN. By contrast, heat and stress (thermo-mechanical processes) are mediated via the rock mass, and thus best represented as a continuum. Here, we present an approach using dual, coincident meshes, each simulating the relevant processes in their ‘natural’ media. A sequential coupling between thermo-mechanical and hydro(-chemical) processes is achieved by computing the effective stress on individual fracture planes, and updating their mechanical apertures and hydraulic properties at selected times during model simulations. The simulated processes are modelled in PFLOTRAN, using a linear elastic constitutive model for rock deformation. Several updates to the PFLOTRAN code were required to enable this, as verified against analytical test cases and benchmarked against alternative finite element codes. We present an example application using data from a spent nuclear fuel repository in Finland, which simulates the evolution of thermal stress due to radiogenic heating from spent fuel canisters. The results demonstrate how continuum-based thermo-mechanical processes exert an important influence on near-field flows around the repository that can only be accurately captured in a DFN, thus demonstrating the advantages of a dual-media approach.