To study the effects of the three deformation instability modes of gas drainage borehole on gas drainage, the deformation instability mechanism of soft coal seams is analyzed, three deformation instability modes are proposed for soft coal seams, namely, complete holes, collapse holes, and plug holes, and a solid‐fluid coupling model incorporating dynamic change of borehole suction pressure is established. The results of the study show the following. (1) When there is no borehole deformation (i.e., complete borehole), the suction pressure loss of drainage system in the borehole is very small, whose effect on gas drainage can be neglected. (2) In case of borehole collapse, the suction pressure loss is big at the collapse segment, and the total suction pressure loss of the drainage system in the borehole is bigger than that in the complete hole. However, it is smaller than the suction pressure of the drainage system and exerts limited effect on gas drainage. As the borehole collapse deteriorates, the effective drainage section of the borehole becomes smaller, while the suction pressure loss in the borehole increases continuously; thus, the gas drainage effect continuously worsens. (3) In case of plug hole, a continuous medium forms between the plug segment coal body and the surrounding coal seam, the plug segment drainage pressure turns into coal‐bed gas pressure, and effective drainage length of the borehole shortens, seriously affecting the gas drainage effect. The study carries important theoretical guiding significance for improving gas drainage effect and effectively preventing gas disasters.

The development of the coal pore system is extremely significant for gas disaster prevention in coal mines and coalbed methane exploitation. To accurately, quantifiable, and non-destructively characterize the coal pore system, the relationship between total porosity, effective porosity, pore size distribution, and resistivity of coals with different degrees of metamorphic was studied by resistance test and low field NMR techniques. The dimensionless parameter ρ_P that characterize the coal pore system was defined by the variety of resistivity in dry and saturated conditions. The achieved results are presented as follows: (1) The greater the ρ_P (the influence of the coal pore system on the conductivity), the larger the total porosity; (2) The proportion of effective pores increased linearly with the addition of the ρ_P; (3) The proportion of micropores and small pores decreased linearly as the ρ_P increased, revealed that these pores had poor connectivity and complicated structure, and had a significant negative effect on the conductivity of saturated coal. (4) The proportion of mesopores and macropores gained linearly as the ρ_P increased, indicated these pores had well connectivity, and had a positive influence on the conductivity. The results showed that the variety of coal resistivity from dry to the saturated condition was mainly controlled by the pore system. Therefore, a new method to characterize the coal pore system was proposed, which was exceedingly effectively, conveniently, and non-destructively.

The difference in the development of coal pore system in bedding direction is of great significance to the exploration and development of coalbed methane and the prevention and control of gas disasters. In order to improve the understanding of the fluid storage and transportation rules in the development of horizontal wells for coalbed methane, the typical low-rank coals in western China are taken as the research object. Based on the NMR experiments, the effective pores, pore size distribution and pore connectivity of low-rank coals in parallel and vertical bedding directions are analyzed. Meanwhile, the reason for the difference in pore structure of parallel and vertical bedding coal is also explained. The results obtained are as follows: (1) All types of pores in low-rank coal are developed. The proportion of T₂ spectrum area of large pores in parallel and vertical bedding coal samples is 55% and 43%, respectively. After centrifugation, the T₂ spectral area of large pore peaks in parallel bedding coal samples reduced more than 85%, and the vertical bedding was reduced by 80%, indicating that the large pores in the coal sample were developed and the pores communicated with each other and had good connectivity, and most of the water escaped from the medium and large pores. (2) The total porosity, effective porosity, and movable fluid saturation of the parallel bedding are all larger than the vertical bedding, and the parallel bedding T_{2 C} is less than the vertical bedding at about 1 ms, indicating that the permeability of the parallel bedding is better than the vertical bedding. (3) The hole was squeezed more seriously in the vertical bedding direction, and the micro-cracks extended along the bedding direction. However, the poor continuity in the vertical bedding direction is the main reason for the difference in coal structure between the parallel bedding and vertical bedding directions.