The occurrence of nano- and micro-scale pores and their controls on the selective migration of gases in the coals of different ranks

The nano- and micro-scale pores distribution of coals are important in determining the gas occurrence mechanism and gas migration behavior in the coal beds. Taking from coals of different ranks in China, the nano- and micro-scale pores distribution characteristics and their controls on the selective migration of gases was studied. It shows that the pore volume of the micro-scale pores presents a decreased trend and evolves towards the nano-scale micropores with the increased coal rank. For coals of different ranks, there is no adsorption energy in the nano-scale mesopores and all the micro-scale pores for CH4, N2, and CO2, where these pores play the roles of gas migration. Instead, the nano-scale micropores provide the sites for gas adsorption. Because of the differences in the adsorption energy for different sizes of pores in the nano-scale micropores section, these pores show selective sorption characteristics and variable gas migration behaviors. In the process of pressure drawdown, the gases in the coals have a dynamic diffusion mode. Of which, the transitional diffusion in the coals evolves from the micro-scale micropores towards the micro-scale transitional pores, mesopores and macropores. Meanwhile, the Fick diffusion in these corresponding pores cannot occurs, which will result in a decreased gas migration ability in the coals. For the CO2 geo-sequestration, the preponderant micro-scale micropores in the high rank coal make more transitional diffusion but less Fick diffusion occurring in the coal, which will result in the injected CO2 difficult to reach the nano-scale micropores for gas adsorption and storage.