Microscopic Response and Damage Degree of Coal under Stepwise Loading of Low-Temperature Liquid Nitrogen Based on Nuclear Magnetic Resonance

Low-permeability coal seams are widely found in China; therefore, an efficient and environmentally friendly penetration enhancement technology is needed to improve the gas extraction efficiency. Liquid nitrogen fracturing, as a new low-temperature phase change permeability enhancement method, has shown significant advantages in applications, but the fracture closure effect induced by geostress in the permeability enhancement of coal seams severely limits its long-term effectiveness. The pore microstructure is an important factor affecting the permeability, and it is of great significance to simulate the change of pore structure of frozen coal under vertical ground stress to improve the optimization of engineering parameters of liquid nitrogen fracturing technology. To address this issue, this study uses liquid nitrogen fracturing as a means of increasing permeability to explore the microstructural response of anthracite coal under stepwise compression. Therefore, with the help of precise and nondestructive nuclear magnetic resonance technology, this study investigates the pore structure evolution law of anthracite coal under stepwise loading with different liquid nitrogen action times and analyzes the mechanical properties, water-containing space, fractal dimensions, and damage degree of the coal. The tests showed that the coal pore space was essentially a dynamic change of compaction and dilatancy during the compression process. At low stress levels (<7.5 MPa), a decrease in coal sample porosity occurs. Specifically, there is an increase in small pores and a decrease in large pores in coal samples. This effect is exacerbated by liquid nitrogen freezing. At high stress levels (>7.5 MPa), the coal samples undergo macroscopic damage, and the porosity grows significantly due to the sharp increase in macropores. During the stepwise loading process, the small and large pore throats of coal undergo transformation to medium-sized pore throats. The seepage space of coal samples decreases sharply during stepwise loading but increases sharply when the stress increases to a certain level. The fractal dimension of coal samples is negatively correlated with liquid nitrogen fracturing time and axial pressure. The general law of fracture evolution of liquid nitrogen frozen coal samples in stepwise loading is freezing fracture → fracture closure → fracture development → specimen destruction. These findings can provide a reference for the study of microstructural response and quantitative analysis of macroscopic damage in liquid nitrogen frozen coal during compression.