Investigation of thermal-hydro-mechanical coupled fracture propagation considering rock damage

Thermal-hydro-mechanical (THM) coupled fracture propagation is common in underground engineering. Rock damage, as an inherent property of rock, significantly affects fracture propagation, but how it influences the THM coupled fracturing remains stubbornly unclear. A pore-scale THM coupling model is developed to study this problem, which combines the lattice Boltzmann method (LBM), the discrete element method (DEM), and rock damage development theory together for the first time. This model can more accurately calculate the exchanged THM information at the fluid-solid boundary and fluid conductivity dependent on fracture and rock damage. Based on the developed model, the synergistic effect of injected temperature difference (fluid temperature below rock temperature) and rock damage (characterized by the parameter critical fracture energy, abbreviated as CFE) on fracture propagation of shale are investigated particularly. It is found that: (1) the generation of branched cracks is closely related to the temperature response frontier, and the fracture process zone of single bond failure increases in higher CFE. (2) through the analysis of micro failure events, hydraulic fracturing is more pronounced in the low CFE, while thermal fracturing displays the opposite trend. The fluid conductivity of fractured rock increases with a higher injected temperature difference due to the more penetrated cracks and wider fracture aperture. However, this enhancement weakens when rock damage is significant. (3) in the multiple-layered rock with various CFEs, branched cracks propagating to adjacent layers are more difficult to form when the injection hole stays in the layer with significant rock damage than without rock damage.

Automated summary

Rock damage has a significant influence on the thermal-hydro-mechanical (THM) coupled fracture propagation. A pore-scale THM coupling model is developed to study the synergistic effect of injected temperature difference and rock damage on shale fracture propagation. The model calculates the exchanged THM information and fluid conductivity accurately based on fracture and rock damage. It is found that the generation of branched cracks is closely related to temperature response frontier and the single bond failure increases with higher critical fracture energy (CFE). Hydraulic fracturing is more pronounced in low CFE while thermal fracturing shows the opposite trend. The fluid conductivity of fractured rock increases with higher injected temperature difference, but it weakens when rock damage is significant. Branched cracks propagating to adjacent layers are more difficult to form when the layer has significant rock damage.