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.

The waterless fracturing method with liquid nitrogen (LN₂) as the fracturing fluid has been proposed and successfully applied in coalbed methane (CBM) production in recent years. The temperature of the coal reservoir sharply decreases, causing damage to the pore structure of the coal reservoir due to the ultralow temperature of LN₂ during fracturing. Thus, in this paper, infrared thermal imaging (ITI) and nuclear magnetic resonance (NMR) were used to measure the temperature distribution and pore evolution law of anthracite coal. The results demonstrate that the temperature of the coal sample after being frozen by LN₂ was far less than 0°C, which causes the internal water of the coal sample to freeze and turn into ice and produce a frost‐heave force. In addition, the temperature of the coal samples was not fixed but fluctuated, which led to the formation of a temperature gradient and induced thermal stress. The T₂ spectra variation showed that LN₂ freeze‐thaw cycles can promote the development of pores in coal samples and enhance the connectivity of pores. Some of the micropores gradually connect and expand to form a large number of mesopores and macropores under the influence of frost‐heave force and thermal stress. The total porosity, residual porosity, and effective porosity increase with the number of LN₂ freeze‐thaw cycles. The NMR imaging directly reflects the change characteristics of the internal pore structure before and after LN₂ freeze‐thaw, which provide a new way to reveal the pore evolution law of anthracite. These results show that LN₂ freeze‐thaw cycles can damage the pore structure of anthracite coal, and a large number of mesopores and macropores are formed to provide channels for CBM migration, which improves the efficiency of LN₂ fracturing.

To investigate the effect of freeze–thaw on damage and pore structure characteristics, some coal samples were freeze-thawed by freeze–thaw test machines. The evolution law of the pore structure of coal samples under freeze–thaw cycles, the porosity, and the bound and free fluid volume of coal samples under freeze–thaw were studied by low nuclear magnetic resonance, the complexity of the pore structure of coal samples under different freeze–thaw cycles was analyzed by fractal dimension, and the influence mechanism of freeze–thaw on the pore structure of coal samples was discussed. The results indicate that: (1) freeze–thaw could damage the pore and fracture structure and reduce the wave velocity of coal samples, and the wave velocity of coal samples after 120 freeze–thaw cycles decreased 66.5% compared with that of before the freeze–thaw cycle. (2) Freeze–thaw can effectively promote the development of pore and fracture. With the increase in freeze–thaw cycles, the proportion of micropores of coal samples decreases, while the proportion of mesopores and macropores gradually increases. (3) The freeze–thaw cycle can promote the increase in the coal sample porosity, and the increment of total, residual, and effective porosity is 3.47%, 1.94%, and 1.53%, respectively, after 120 freeze–thaw. (4) The fractal dimension of total, adsorption, and seepage pore of coal samples all decrease with the increase in freeze–thaw cycles, indicating that liquid nitrogen can reduce the complexity of the pore structure and weaken the heterogeneity of the pore structure.

Liquid nitrogen (LN₂) fracturing technology as a kind of waterless fracturing technology has received extensive attention in recent years. In order to investigate the evolution law of fissure structure and seepage characteristics of coal samples under LN₂ freezing and freeze–thaw, the evolution of fissure of coal samples before and after LN₂ freeze–thaw was monitored by micro-computed tomography, the change of permeability of coal samples under different LN₂ freezing time and freeze–thaw cycles was measured, and the damage mechanism of LN₂ freezing and freeze–thaw to coal was discussed. The experimental results show that (1) LN₂ freeze–thaw can cause the shrinkage of the coal matrix, resulting in damage to the fissure structure of the coal sample, which promotes the initiation, expansion, and extension of fissure of coal sample to form new fissure and, finally, forms a fracture network. (2) The permeability and the increment of permeability of coal samples increase with increase of LN₂ freezing time and the number of freeze–thaw cycles under different gas pressure and confining pressure condition. (3) Under the same freezing time, the permeability growth rate of coal samples under LN₂ freeze–thaw condition is significantly greater than that of coal samples under LN₂ freezing condition. (4) The frost-heave force and thermal stress are the main factors leading to the damage, promoting fissure formation and increasing permeability of coal samples LN₂ freezing and freeze–thaw. This study provides a theoretical basis for the understanding of fracturing technology with LN₂.

To better apply nuclear magnetic resonance (NMR) to evaluate the petrophysical characterization of high-rank coal, six anthracite samples from the Baijiao coal reservoir were measured by NMR. The porosity, T ₂ cutoff value, permeability and pore type were analysed using the transverse relaxation time ( T ₂ ) spectrum before and after centrifugation. The results show that the T ₂ spectrum of water-saturated anthracite can be divided into a discontinuous and continuous trimodal distribution. According to the connectivity among pores, three T ₂ spectrum peaks were identified at the relaxation times of 0.01–1.7 ms, 1.7–65 ms and greater than 65 ms, which correspond to the micropores (less than 100 nm), mesopores (100–1000 nm) and macropores (greater than 1000 nm), respectively. Based on the T ₂ cutoff value, we divided the T ₂ spectrum into two parts: bound fluid and free fluid. By comparing two classic permeability models, we proposed a permeability model to calculate the permeability of anthracite. This result demonstrates that NMR has great significance to the exploration of coal reservoirs and to the understanding of the development of coalbed methane.