A systematic model- and experimental approach to hydro-mechanical and thermo-mechanical fracture processes in crystalline rocks

The paper presents the key findings of Task G SAFENET of the DECOVALEX 2023 project “Safety Assessment of Fluid Flow, Shear, Thermal and Reaction Processes within Crystalline Rock Fracture NETworks”. It utilizes a systematic and experimental approach to numerically simulate mechanical (M), hydro-mechanical (HM), and thermo-mechanical (TM) fracture processes in brittle rocks. The Task team introduced, applied, and compared a wide range of numerical methods, including both continuum and discontinuum methods, for simulating related fracture processes. Task G is based on three key experiments: the Freiberg, GREAT cell, and KICT experiments, which analyze M, HM, and TM processes respectively. Classic HM and THM benchmark exercises serve as a common basis by using analytical solutions for a plane line discontinuity in a poro-elastic medium (Sneddon and Lowengrub, 1969) and a point heat source in a thermo-poro-elastic medium (Booker and Savvidou, 1985), (Chaudhry et al., 2019). These solutions also serve as a reference for rough fractures and simple fracture networks. A systematic set of new benchmark cases has been derived based on the GREAT cell experiments. An analysis of the constant normal load (CNL) experiment has been conducted using micro- and macroscopic approaches, based on the Freiberg experiment. The GREAT cell experiments provided a database for evaluating the mechanical and hydro-mechanical responses of various rock samples (resin, greywacke, gneis) in triaxial tests with a rotational stress field. Fracture permeability was determined as a function of normal stresses in the rotational stress field. The KICT experiments were used to investigate thermally induced shear slip and dilation processes. The SAFENET Task contributed to the Open Science concept in DECOVALEX by providing a freely accessible Jupyter notebooks for selected benchmark exercises.