Thermo-hydro mechanical coupling in a discrete modelling: Large-scale 3D application to thermal hydrofracturing

This work aims to model an in-situ heating test (CRQ) conducted by the French National Radioactive Waste Management Agency (Andra) within the Meuse/Haute-Marne Underground Research Laboratory (MHM URL), using a discrete approach. The modelling of the CRQ test is part of numerical simulations performed through the international research project DECOVALEX. The goal of the CRQ test is to study the conditions under which thermal hydrofracturing can occur in the Callovo-Oxfordian claystone (COx) formation and to identify its influence on pore pressure evolution. The present discrete model introduces thermo-hydro-mechanical (THM) coupling into the Itasca discrete code 3DEC, which represents an assembly of elastic deformable blocks with interfaces modelled as joints. The THM formulation is implemented in the 3DEC code using an iterative approach. At each step, this iterative numerical solving starts by the thermal simulation. Then, the hydro-mechanical calculation is carried out by a series of hydraulic and mechanical computations until equilibrium is reached. This iterative process repeats at each timestep until the final calculation time is achieved. To model the fracturing process in the COx, a failure criterion based on Mohr Coulomb with tensile cut-off is used for the joints. The THM coupling implementation is first validated against a poro-elastic closed-form solution by considering a heat source within an infinite saturated porous medium. Afterwards, the CRQ experiment is considered with particular attention to the phenomenon of thermal fracturing, as the main advantage of the discrete model lies in its explicitly representation of fractures. This study also demonstrates the ability of a discrete model in dealing with a large model that includes multiple processes (THM coupling and rock failure).