Multi-scale pore structure characteristics of coal under alternating hydraulic intrusion pressure.

With the increase of coal mining depth, coal seam has the characteristics of complex pore structure and low porosity under the influence of ground stress, which limits the effect of coal seam water injection. Alternating hydraulic intrusion is an effective method to improve the permeability of low permeability coal seam. By applying low water pressure circulation, it can force the pore structure of coal to be damaged, which is helpful to change its pore structure. However, there is limited research on the evolution of the multi-sacale pore structure in coal under alternating hydraulic pressure using low-field nuclear magnetic resonance (LF-NMR) technology. In this study, we investigate the evolution of coal pore structure under alternating hydraulic pressure using LF-NMR. The pore size distribution (PSD), porosity, fractal dimension, and sensitivity of pores to alternating hydraulic pressure were analyzed. The results indicate that the pore volume of coal exhibits an increasing trend initially and then decreases with the increase of cycle times. An appropriate number of alternating cycles contribute to the development of the coal pore structure. Fractal dimension analysis reveals that the pore fractal dimension decreases first and then increases during the test, indicating a significant effect of alternating hydraulic pressure on the development of pores. Analysis of the sensitivity of pores to alternating hydraulic pressure shows a decreasing trend overall with an increasing cycle time, although the rate of decrease gradually slows non-significant with continued cycling. Mechanism analysis suggests that during the alternating hydraulic intrusion process, the development of new pores and the compaction of pores occur simultaneously under the hydraulic pressure. With the continuous loading of alternating hydraulic pressure, the development rate of new pores gradually becomes lower than the compaction rate of pores, which leads to the phenomenon that the pore structure of coal expands first and then compacts. This study provides theoretical support for the application of alternating water injection in coal reservoir engineering. [ABSTRACT FROM AUTHOR]