Effects of Pore Structure Variations Among Different Lithotypes on Methane Diffusion: An Experimental Study Based on Progressive Pulverization

Diffusion is a key process linking methane desorption and seepage in coalbed methane reservoirs and is strongly affected by lithotype. Coal samples of bright and dull coal from the No. 2 coal seam in the Fengfeng mining area, Hebei, China, were investigated using stepwise pulverization, low‐temperature nitrogen adsorption, and high‐pressure methane desorption experiments to evaluate pulverization‐induced pore accessibility and diffusion kinetics. The results show that bright coal is dominated by slit‐shaped and parallel plate pores. After pulverization to 200 mesh or finer (<75 μm), the volume of newly accessible micropores increases exponentially, indicating strong pulverization‐induced activation of previously inaccessible micropores. Dull coal mainly contains ink‐bottle‐shaped pores, with poorly developed micropores, and its increase in newly accessible pore volume is controlled mainly by transitional pores. Methane diffusion is jointly regulated by lithotype and temperature. In bright coal, diffusion is closely associated with the development and accessibility of transitional pores, whereas dull coal is limited by insufficient methane supply from micropore desorption at low temperature. Temperatures above 75°C, enhanced molecular thermal motion promotes desorption and diffusion. Changes in the initial effective diffusion coefficient further confirm stronger closed‐pore activation in bright coal, while dull coal shifts from diffusion inhibition to enhancement at high temperature.