Change Laws of Pore-Fracture Structure of Coal under High-Temperature Steam Shock

Injecting steam into coal seam is an important means to accelerate gas desorption and improve gas extraction efficiency. However, the change law of pore-fracture structures of coal after high-temperature steam shock (thermal shock) is still unclear. Through this study, pore-fracture structures of coal samples before and after thermal shock were compared and analyzed based on the experimental methods of surface pore and fracture extraction, scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) spectroscopy. The results show that after thermal shock, the surface porosity, max equivalent fracture width, fracture lengths, fracture number, and probability entropy of coal samples increased significantly, and the increment of bituminous coal was greater than that of anthracite. This indicates that thermal shock can promote the development of coal pores, which is significantly better for bituminous coal than anthracite. A SEM analysis reveals that fractures tend to appear at the interface between minerals and coal matrix. The NMR analysis demonstrates that the absolute increment of micropores is the largest, followed by that of mesopores, and that of macropores is the smallest. The increase of porosity in coal shows pore enlargement and penetration, which enhance the connectivity between the pores, thus providing a smoother channel for methane migration. Heterogeneous distribution of mineral components with different thermal expansion coefficients as well as the temperature gradient is the fundamental mechanism behind thermal stress-induced porosity development. The research results provide theoretical support for enhanced gas extraction technology by high-temperature steam injection into coal seams.