Effects of pore structure on methane adsorption behavior of ductile tectonically deformed coals: An inspiration to coalbed methane exploitation in structurally complex area

The distinctive pore structure of tectonically deformed coals (TDCs) has a considerable influence on their methane adsorption behavior, resulting in higher coalbed methane (CBM) pressure and higher CBM content, and thus increasing the difficulty of CBM development in TDCs distribution area (especially ductile TDCs). In this paper, the supermicropore-microfracture structures of primary structure coals (PSCs) and ductile TDCs from the Suxian mining area were characterized by using mercury intrusion porosimetry (MIP) and gas adsorption methods (N-2 and CO2). Methane adsorption test was conducted to study the methane adsorption behavior. Then, the control mechanism of pore structure on methane adsorption behavior of ductile TDCs was investigated. Results show that the supermicropores (<2 nm) surface area generally increases and then decreases. The micropores (2-10 nm) and small transitional pores (10-40 nm) volume of ductile TDCs is slightly larger than that of PSCs, while the volume of big transitional pores (40-100 nm) experiences a steady increase. The mesopores (100-1000 nm) and macropores (1000-10000 nm) volume displays a similar pattern of "steep increase-slightly decrease-stable increase" with the enhancement of deformation. The microfractures (>10000 nm) volume first increases and then decreases from the PSCs to mylonitized coals. Langmuir volume (V-L,V- daf) displays no obvious change rules with the increasing ductile deformation, but the Langmuir pressure (P-L,P- daf) of ductile TDCs is smaller than that of PSCs and gradually decreases to its minimum in the mylonitized coals. The Pearson correlation coefficients show that the volume of big transitional pores (40-100 nm), mesopores and macropores both shows obvious negative correlations with the P-L,P- daf, which indicates that the development of 40-10000 nm pores in ductile TDCs causes a lower P-L,P- daf, resulting in higher CBM pressures and contents in the ductile TDCs distribution area under the in-situ pressure condition.