Dynamic evaluation of heterogeneity in pore-fracture system of different rank coals under different confining pressure based on low-field NMR

Abstract

ABSTRACT In order to quantitatively analyze the dynamic changes in pore-fracture system coals under confining pressure loading, six coal samples were tested by low-field nuclear magnetic resonance (NMR). The experiment result show that with the increase of coal rank, the total pore space (0.01 ms < T 2 < 10000 ms) increases firstly and then decreases, the proportion of adsorption space (0.01 ms < T 2 < 2.5 ms) increases gradually, and the seepage space (T 2 > 20 ms) shows the opposite trend. The fractal dimension of adsorption space (D 1, 0.5 ms < T 2 < 2.5 ms) and seepage space (D 2) of different rank coal under different confining pressure were discussed in this paper. The D 1 of low- and medium-rank coals are higher than high-rank samples. With the increase of coal rank, the D 2 is gradually increased, and the dynamic change of D 2 is closely related to the composition and pore development of coal. As the stress increases, the change in D 1 is more complicated, and D 2 shows the “two-stage” characteristic, when the stress is lower than 8 MPa, the heterogeneity becomes complicated, when the stress is higher than 8 MPa, the heterogeneity tends to be stable. Additionally, from the definition of pore compression coefficient, the pore compression coefficient theory expression is derived. It is concluded that the pore compression decreases with the increase of confining pressure. Compared with the medium- and high-rank coals, there is a negative correlation between the pore compression coefficient of low-rank coal and D 2. This study helps to better understand the pore and heterogeneity changes under coal rank and stress and provides a quantitative evaluation method for different pores under stress changes, which has a predictive effect on permeability changes.