Determining the pore structure and fluid characteristics of different ranks of coal using nuclear magnetic resonance

Abstract

The microstructure of coal reservoirs and their fluid-endowment characteristics are important indicators for evaluating coalbed methane during mining. This study aimed to investigate the influence mechanism of the nanopore structure of different coal ranks on the fluid distribution characteristics. Low-field nuclear magnetic resonance (NMR), centrifugal force, and heat treatment tests were conducted. The results indicated that as the degree of coal metamorphism increases, micro-transition pores increase significantly, mesopores–macropores decrease, and the nonhomogeneity of the pores increases. The optimum centrifugal forces for low-, middle-, and high-rank coals were 1.68, 1.08, and 1.08 MPa, respectively. The corresponding mean T2C1 (first T2 cutoff) values were measured as 8.01, 1.61, and 5.09 ms, respectively, while the mean T2C2 (second T2 cutoff) values were 0.23, 0.26, and 1.25 ms, respectively. Pore sizes less than 11.6 nm indicate unrecoverable fluid, those higher than 88 nm indicate free fluid, and the rest (11.6–88 nm) indicate capillary bound fluid. As the degree of coal metamorphism increases, free fluid saturation decreases, capillary bound fluid saturation increases and then decreases, and unrecoverable fluid saturation increases.