Thermal damage in crystalline rocks: the role of heterogeneity

Accurately evaluating the impact of microstructural heterogeneity on thermal damage and failure mechanisms in crystalline rocks is crucial for geothermal energy development and the establishment of nuclear waste repositories. However, in thermal damage analyses of crystalline rocks using the discrete element method, most studies fail to account for the reduction in mineral mechanical and thermal properties caused by temperature increases. This study uses a Grain-Based model to simulate the microscopic mineral structure of crystalline rocks, focusing on analyzing the effect of heterogeneity. Thermal damage resulting from the uneven expansion of minerals is simulated by assigning specific thermal properties to each mineral. The temperature-induced degradation of mechanical and thermal properties is incorporated by introducing a temperature-dependent relationship for these characteristics in crystalline rocks. Along with moment tensor inversion theory, the microseismic behavior of crystalline rocks is explored to enhance understanding of rock failure mechanisms. The results indicate the following: (1) Thermal stress depends on the mineral’s size, shape and arrangement. The sharp corners and edges of irregularly shaped minerals are more likely to become stress concentration points under the influence of temperature. (2) Peak strength gradually decreases with increasing heterogeneity and larger grain sizes. A negative correlation between quartz content and peak strength becomes more evident when the temperature exceeds 450 °C. (3) The combined effects of high temperature and heterogeneity lead to a more significant increase in the b value. Smaller grains increase the complexity of crack paths, leading to more frequent branching at grain boundaries and resulting in higher b values. (4) The uneven distribution of grain sizes is the primary factor influencing the mechanical properties of crystalline rocks, while grain size is a secondary factor. At the same temperature, samples with more uniform grain distributions and smaller grain sizes exhibit higher thermal stability.