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.