Low-Field NMR Experimental Study on the Effect of Confining Pressure on the Porous Structure and Connectivity of High-Rank Coal

To study the influence of different confining pressures on the pore structure and connectivity of high-rank coal, the high-rank raw coal of the Shanxi Xinjing Mine No. 9 coal seam was studied. A low-field nuclear magnetic resonance (LNMR) test system and a vacuum pressurized water saturation system were used to analyze the samples. The T-2 spectra of samples, saturated with water under different confining pressures and containing residual water after centrifugation, were tested. The coal sample pore size distributions, permeabilities, free fluid values, bound fluid values, and other parameters were obtained, and a calculation model of the coal pore connectivity ratio was established. The results were as follows. When the saturated pressures were 5, 10, 15, 20, 25, and 30 MPa, the pore diameters of the coal samples were mainly concentrated in the ranges of 0.00023-0.069 and 1.29-24.09 mu m. Among them, micropores (<10 nm) and small pores (10 < 100 nm) account for the main part, mesopores (100 < 1000 nm) were underdeveloped, and relatively few macropores (>1000 nm) and fissures developed. As the confining pressure increased, the coal porosity and connectivity showed a trend of decreasing, then increasing, and finally remaining basically unchanged. The total pore connectivity rates of the coal samples were 37.0-62.6%. The interconnection rates of the micropores, small holes, mesopores, and macropores are 2.90-34.55, 89.09-99.03, 97.09-100, and 100%, respectively. The total pore connectivity followed an exponential functional relationship with permeability, and the critical confining pressure of high-rank coal was 25 MPa. These results provide a scientific basis for the high-pressure water injection of high-rank coal seams.