Pore Water Microscale Response of Different Water-Saturated Coal Samples under a Freeze–Thaw Degradation Effect

Pore water content directly affects the effect of liquid nitrogen (LN2) penetration, and the study of the freeze–thaw response of the pore water of different saturated coal samples in the process of LN2 penetration is crucial for promoting the development of LN2 fracturing technology. In this work, a LN2 freeze–thaw study was carried out with three types of coal samples, namely, dry, naturally water-saturated, and fully water-saturated samples, and the pore water microscale response of the coal samples under freeze–thaw degradation was analyzed by a low-field NMR technique. The results show that (1) the swelling effect caused by the pore free water phase transition is the key to the low-temperature penetration enhancement of coal. (2) As the coal samples were subjected to different times of LN2, the pore water fugacity was affected by the pore evolution; in 0–50 min, the pore water content of the BBT coal sample was positively correlated with the freezing time, while the opposite was true for the BT coal sample; in 50–150 min, both samples showed a tendency to increase first and then decrease. The pore water content before and after freeze–thaw increased by 26.90% in a GT coal sample, 237.20% in a BBT coal sample, and 118.86% in a BT coal sample. (3) The pore water of GT coal sample is mainly in the closed pores, which is reduced during freeze–thaw. The pore water of BBT and BT coal samples are mainly distributed in open pores, the effective pore water of BBT coal sample is the highest at the end of freezing and thawing, the effective pore water of BT coal sample is the most at the beginning of freezing and thawing, and the maximum effective pore water content of the two samples is close to each other. Compared with BT coal sample, BBT coal sample have a better fracturing effect and better pore water storage. (4) As the specimen is degraded by freezing and thawing, internal pore channels are developed, the frozen specimen forms a combination with coal as the main body and that with ice and water as the auxiliary body, the heat conduction of the specimen is weakened, and the conduction efficiency is reduced. By increasing the duration of LN2 freezing, the influence of pore water on coal fracturing can be enhanced and the efficiency of LN2 action can be improved.