Dual-damage constitutive model to define thermal damage in rock

The evolution of the mechanical properties of rock as a function of thermal damage is relevant to various engineering applications, such as nuclear waste repositories, underground coal gasification, dry-fracture shale system and geothermal energy extraction. The thermal properties of heterogeneous rock control heat transmission with differential thermal expansion potentially resulting in significant changes in the mechanical and transport behavior of reservoir rocks. We define a novel dual-damage thermal-mechanical model accommodating the interaction of thermal conductivity, thermally-induced deformation, rock mechanical deformation and damage to define the evolution of the thermal and mechanical properties of rock during thermal treatment. Importantly, the dual-damage constitutive model is capable of using elastic modulus and strength to solve for asynchronous changes in peak strain and peak strength, respectively. The proposed model is validated against analytical solutions and laboratory data. The results indicate that thermally-induced damage increases rock porosity and permeability while simultaneously decreasing elastic modulus and strength. Thermal treatment causes a realignment in the rock microstructure and results in a change in the ultimate failure pattern. Thermally-induced damage causes irreversible increases in rock porosity and permeability even as temperature is restored. Furthermore, it is confirmed that thermally-induced damage in rock is dominated by the type of tensile damage during the thermal expansion. The proposed dual-damage constitutive model better explains the non-proportional relationship between peak strength and peak strain observed in many experiments.