Abstract
Pore is the main adsorption and desorption space of coalbed methane (CBM). Pore size configuration and connectivity affect the adsorption/desorption hysteresis effect. Using tectonically deformed coal (TDC) and original structure coal of medium- and high-rank coal as the research objects, through the N2/CO2 adsorption experiment to analyze the pore size distribution and connectivity of different scales. We investigate the control mechanism of heterogeneous evolution in the key pore scales against adsorption/desorption hysteresis characteristics during coal metamorphism and deformation by combining the CH4 isothermal adsorption/desorption experiment under 30°C equilibrium moisture. The findings indicate that the super micropores (<2 nm) are mainly combination ink bottle-shaped pores and have worse connectivity as the degree of metamorphism and deformation increases. The super micropores occupy the vast majority of pore volume and specific surface area; its pore size distribution curve change presents an “M” bimodal type and is mainly concentrated in two pore segments of 0.45–0.70 nm and 0.70–0.90 nm. The effect of ductile deformation exerts a significantly greater effect on super micropores than brittle deformation. The exhibited adsorption–desorption characteristics are the result of the combined effect of the unique pore structure of the TDCs and different moisture contents. The presence of a large number of super micropores is the most important factor influencing the degree of gas desorption hysteresis. The “ink-bottle effect” is the primary cause of gas desorption hysteresis. For CBM development, some novel methods to increase desorption and diffusion rate at the super micropores scale should be considered.
Highlights
Coalbed methane (CBM) is a significant clean energy resource, as an unconventional natural gas and mainly adsorbed in coal
The author discusses the control mechanism of heterogeneous evolution in pore structure during coal deformation against CH4 adsorption/desorption hysteresis characteristics, which is important for understanding the nature of gas adsorption and desorption, and provides a theoretical basis for coalbed methane (CBM) exploration and development in medium- and high-rank tectonically deformed coal (TDC) and original structure coal (OSC) distribution areas
Experimental Instruments and Methods The LTNA experiment used the QuadraWin SI Specific surface area (SSA) and pore size analyzer produced by Quantachrome, United States, with a pore size test range of 0.35–500 nm
Summary
Coalbed methane (CBM) is a significant clean energy resource, as an unconventional natural gas and mainly adsorbed in coal. Studies have shown that ductile deformation coal has a higher pore fractal dimension than brittle deformation coal, showing more complex pore structure and increased heterogeneity, which results in increased adsorption capacity, enhanced micropore capillary condensation effect, and prominent desorption hysteresis loop (Zhang et al, 2014a; Wang and Li, 2016; Li et al, 2017). The author discusses the control mechanism of heterogeneous evolution in pore structure during coal deformation against CH4 adsorption/desorption hysteresis characteristics, which is important for understanding the nature of gas adsorption and desorption, and provides a theoretical basis for CBM exploration and development in medium- and high-rank TDCs and OSC distribution areas.
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