Nuclear magnetic resonance study of the influence of the liquid nitrogen freeze-thaw process on the pore structure of anthracite coal

In recent years, liquid nitrogen (LN2) fracturing technology has been applied in coalbed methane (CBM) development. However, the impact of the liquid nitrogen freeze-thaw process on the pore structure of anthracite coal has not yet been systematically investigated. In this study, nuclear magnetic resonance (NMR) analysis and scanning electron microscopy (SEM) imaging of coal samples after treatment with liquid nitrogen for different freezing times and freeze-thaw cycles were performed to study the pore structure damage of anthracite subjected to the liquid nitrogen freeze-thaw process as well as the variation in the porosity, fracture evolution, and permeability. The results show that the LN2 freeze-thaw process can enlarge the pore size, enhance the pore connectivity, and form a fracture network on the surface of coal samples, which increases the total porosity, residual porosity, effective porosity, and permeability. This study provides a theoretical value for LN2 fracturing in the development of CBM.