Grain-Based Finite-Discrete Element Modeling of Thermal Cracking in Stanstead Granite

ABSTRACT: Granitic rocks constitute many of geothermal energy plays and host candidate for future underground nuclear waste storage facilities. However, producing hot fluids or injecting cold fluids into geothermal reservoirs or storing hot decaying radioactive material will generate temperature changes and induce local thermo-elastic stresses within the surrounding rock. Such stress changes have the potential to initiate cracks within the host rock affecting its mechanical and hydraulic properties. In this paper, we investigate the process of thermal cracking of unconfined Stanstead granite samples, thermally treated up to 400 °C, combining optical microscopy imaging of thin sections and grain-based finite-discrete element method (GB-FDEM) modeling. The heterogeneous nature of this granite was represented by its three most abundant minerals (i.e., quartz, feldspar, and biotite) and thermal properties variation was captured using temperature-dependent thermal properties. Petrographic microscopy image analysis reveals significant increase in the microcrack density with increasing thermal treatment temperatures, in which the number of grain boundary microcracks dominates such increase over the number of intragranular microcracks – consistent with typical observations in other granitic rocks. GB-FDEM simulations showed that tensile cracking is the prevailing failure mode with few mixed and shear cracks initiating towards 400 °C. Furthermore, most of the initiated cracks are intergranular cracks and few intragranular cracks appeared at 400 °C in quartz and feldspar.