The Response of Microstructure and Heterogeneity to Different Degassing Temperatures in Low-Rank Coal Reservoir

Abstract

Low-pressure nitrogen adsorption is a widely utilized experimental technique for evaluating the pore structure of coal with diameters between 0.35 and 300 nm. This work analyzes the effect of degassing temperature on nitrogen adsorption findings by analyzing low-rank coal from the Xishanyao Formation, employing low-pressure N₂ adsorption and nuclear magnetic resonance to evaluate pore structure parameters at different degassing temperatures. The findings reveal that the nanoscale pore geometry is predominantly defined by “ink bottle”-shaped pores, with micropores and small pores being the most common. Furthermore, the T₂ relaxation times display three unique peak spectra, underscoring the significant formation of micropores in the low-rank coal. Moreover, the structural fractal dimension (D₂₂) values for mesopores, spanning from 2.86 to 2.93, highlight the structural heterogeneity of the coal. NMR examination of the pore size and fluid states delineates five unique types of fractal dimensions: D_B, D_M, D_T, D_A, and D_S. The D_A for adsorption pores is notably below 2, signifying a lack of fractal properties. In contrast, D_B and D_T, denoting closed and total pores, respectively, demonstrate restricted fractal behavior within the closed pores. Nonetheless, high goodness-of-fit values for D_M and D_S in open and gas seepage pores suggest the existence of notable fractal characteristics. A negative association was observed between D_M, D_S, and the degassing temperature. The correlation study indicates that diminished fractal dimensions correlate with a more uniform pore distribution and enhanced pore connection, potentially improving coalbed methane production.