Coupled thermo-hydro-mechanical (THM) processes play a key role in a variety of geomechanical applications, including, enhanced oil recovery from unconventional reservoirs, deep geological disposal of heat-emitting nuclear waste, and enhanced geothermal systems (EGSs). The research work presented herein introduces a novel fully-coupled, THM formulation for the finite-discrete element method (FDEM) specifically developed to exploit the computing parallelism of graphics processing units (GPUs), ultimately enabling a substantial reduction of computing time. The new logic is thoroughly verified using published closedform solutions for a variety of boundary value problems involving several heat transport mechanisms such as conduction in solid rock, advection, and convective heat transfer. Application of the THM logic to practical geomechanical problems is highlighted by a case study focusing on heat extraction in an EGS. Numerical results provide novel insights into coupled subsurface THM processes, including the parameters controlling the reservoir temperature evolution, as well as the stresses and displacements induced by fluid injection.
Research interest in the effect of coupling thermal (T), hydraulic (H) and mechanical stresses/deformations (M) in geological systems has been growing steadily since the 1980s. As described by Tsang, 1991, the term "coupled processes" is commonly used in geomechanics to indicate that the rock mass response to natural, or artificial, perturbations cannot be correctly described by considering each process in isolation, or in simple succession, as the continuous inter-play between the three processes must be analyzed together. An overview of possible couplings between THM processes can be visually represented by the interaction matrix of Fig. 2.
Advances in the theoretical description and numerical simulation of coupled THM processes have been mostly driven by two major geoengineering applications, namely, deep geological storage of nuclear waste and enhanced (or engineered) geothermal systems (EGS). In the case of the deep geological storage of nuclear waste, a number of coupled THM phenomena (e.g,, thermomechanical stresses) are expected to develop around the repository in response to the release of radiogenic heat into the host formation (Tsang et al., 2000). Therefore, the role of such couplings needs to be addressed as part of the design process and performance and safety assessment of any underground repository. Since the early 1990's key developments of mathematical models and testing procedures have been realized through the DECOVALEX project (see Birkholzer et al., 2019, for an overview). In the case of the development of geothermal energy from EGS, energy is produced from dry, low permeability reservoirs by circulating a fluid through an engineered fracture network (Olasolo et al., 2016). As such, EGS are characterized by coupled convective heat transport, heat exchange in fractured media, fluid migration, and rock mass deformations. Prediction of these phenomena is crucial for energy recovery quantification, lifetime performance assessment, as well as induced seismicity risk analysis (e.g., Taron and Elsworth, 2009; Fu et al., 2016; Sun et al., 2017). Other engineering applications controlled by THM processes include enhanced oil recovery (e.g., Elyasi et al., 2016), borehole integrity and breakouts (e.g., Andersson et al., 2009; Tao and Ghassemi, 2010), hydraulic fracturing (e.g., Tomac and Gutierrez, 2017), CO2 storage and sequestration (e.g., Cappa and Rutqvist, 2011; Zhang et al., 2015), underground gas storage and compressed air energy storage (e.g., Zhuang et al., 2014), and gas production from hydrate bearing formations (e.g., Rutqvist et al., 2012).