Evolution rules of coal mesopore structure under the action of hydraulic coupling

In order to study the influence of the cycle of water injection pressure and cycle of ground stress on the mesopore structure of coal during the coal seam water injection process, this paper uses low-field nuclear magnetic resonance and high-pressure hydraulic coupling in situ test system to carry out cycle of confining pressure test and cycle of water pressure test. The changes of pore size distribution, porosity, and fractal dimension of coal in the test were analyzed. The results show that confining pressure loading causes pores to be gradually compacted, resulting in a reduction in pore volume, while unloading of confining pressure causes pores to gradually expand and the proportion of macropores to increase. The pore structure of coal developed significantly after cycle of confining pressure, indicating that cycle of confining pressure can promote the development of coal pore structure. With loading of water pressure, the pore volume of each scale in the coal gradually increases with the increase of water pressure, and the proportion of macropores is significantly higher than before loading stage. During the water pressure unloading stage, the pore volume of the coal gradually decreases with the unloading of water pressure, and the proportion of micropores gradually increases. The fractal dimension results show that the impact of confining pressure cycle on the pore structure of coal is significantly greater than that of water pressure cycle. Loading confining pressure will cause pore compaction and pore structure damage, and the pores will gradually expand as the confining pressure is unloaded.