Micro-mechanical behaviors of granite under cyclic heating/cooling conditions investigated by GBM3D

During the exploitation of hot dry rock, the surface of the convective heat transfer channel undergoes repeated cycles of heating and cooling. Investigating the microstructure of granite after such cyclic thermal treatments is crucial for ensuring the stability and efficiency of enhanced geothermal systems (EGS). In this study, a 3-Dimensional Grain-Based Model (GBM3D) within the Particle Flow Code (PFC) framework was developed to analyze the evolution of micro-cracks in granite under cyclic heating and cooling, considering the spatial distribution of different minerals. The results demonstrate that this method effectively captures the changes in the macroscopic properties of granite under thermal cycling. The failure modes become more pronounced due to the increased number of pathways created by thermally induced micro-cracks. After the first heating cycle, the number of micro-cracks exhibits a quadratic increase with rising temperatures. In contrast, subsequent heating cycles, crack initiation and propagation are significantly suppressed, as the area and number of tensile contact forces decrease with each cycle. During the cooling process, thermal shock has a more significant impact on the specimen compared to heating. Pre-existing micro-cracks play a less significant role during the cooling phase compared to the heating phase, and their number continues to increase in subsequent cooling cycles. Thermal treatment primarily induces inter-granular cracks, and the number of micro-cracks increases nonlinearly with temperature during cyclic heating and loading. These thermally induced micro-cracks predominantly form at the boundaries of specimen because of temperature gradients, with inter-mineral cracks occurring more frequently than intra-mineral cracks.