Structural and evolutionary characteristics of pores-microfractures and their influence on coalbed methane exploitation in high-rank brittle tectonically deformed coals of the Yangquan mining area, northeastern Qinshui basin, China

The Yangquan mining area is an important anthracite production base in China, but various types of tectonically deformed coals (TDCs) are well developed in its mines and gas outburst accidents take place frequently. By using methods such as optical microscope observation, mercury intrusion porosimetry (MIP) and low pressure nitrogen/carbon dioxide gas adsorption (LP-N-2/CO(2)GA), we tested the structural and evolutionary characteristics of pores-microfractures and their influence on coalbed methane exploitation in high-rank (R-o,R-max = 2.00-2.72%) primary structure coals and brittle TDCs (i.e., cataclastic-, mortor-, and granulitic coals) from the Yangquan mining area. The results show that the tectonic deformation can change the pore-microfracture structure of coals and, thus, their reservoir physical properties. In general, with the enhancement of tectonic deformation, the density of microfractures in coal, which become more and more mussy, gradually increases, the total pore volume (TPV) of coal (pores with a diameter of 5.37-27000 nm) and the pore volume of each pore size range increase, with the greatest increase occurring in pores of 100-1000 nm, and the connectivity between pores is gradually reduced as well. From primary structure coals to granulitic coals, the TPV by the Barret-JoynerHalenda (BJH) model increases with a greater increase in mesopores, and the specific surface area (SSA) by the Brunauer-Emmett-Teller (BET) model shows a trend of increase-decrease-increase, while the average pore diameter changes follow the pattern of decrease-increase-decrease in 0.7-206 nm. However, tectonic deformation has little effect on micropores with a diameter of less than 1 nm in brittle TDCs. The difference in pore-microfracture structures of different brittle TDCs indicates that the cataclastic coals distribution area is favorable for coalbed methane (CBM) exploitation, while the granulitic coals distribution area is a gas outburst-prone area.