Effect of cyclic thermal stimulation on the pore structure and fluid space of coal and inspiration for coalbed methane production

Based on the modified programmed temperature-raising system, cyclic thermal stimulation with different temperature gradients was carried out for lignite, bituminous and anthracite coal, and each group of coal samples after thermal stimulation cycle was subjected to nuclear magnetic resonance (NMR) testing. The NMR T1-T2 spectra and the fractal dimensions of coals in different ranks during thermal simulation quantitatively described the dynamic evolution of the pore fluid storage space of coals. The variation of T2 distribution and pore throat were used to reflect the pore development process of coals. The results shows that the pores and pore throat of lignite developed the best among the three coal ranks and the structure of the pore is complete after the cyclic thermal simulation; the T1-T2 signal peaks increased from 17.05 at 30 °C to 163.00 at 180 °C; the seepage of lignite increased from 6.54 % to 12.43 %; total pore fractal dimension DT has a decreasing trend, indicating reduced inhomogeneity of pores. This indicates that the cyclic thermal stimulation treatment can promote the growth of seepage pores to a greater extent, improve the seepage space, and enhance the connectivity of the pores in the coal samples.