Changes in coal pore structure and permeability during N2 injection

Abstract

Previous studies on N2-ECBM (N2-enhanced coalbed methane) have focused on experimental investigation or numerical simulation of coalbed methane recovery processes using pure N2 or binary gas (CO2/N2) injection. Experimental studies on the changes in pore structure and permeability during N2 injection have been limited. In this study, N2 injection experiments, mercury intrusion porosimetry and permeability measurements were conducted to investigate the changes in pore structure and permeability caused by N2 injection of semi-anthracite coal from the Lu'an mining area in the Qinshui basin, Shanxi Province, China. The results show that the total pore volume markedly increases during N2 injection, with increases in transition pores, mesopores and macropores of 8.0%, 50.0% and 138.3%, respectively. Nitrogen injection improves the pore size distribution: the incremental pore volume variances of transition pores, mesopores and macropores after treatment are 2.1%, 47.8% and 141.0%, respectively. Porosity and permeability markedly rise during N2 injection, by 22.6% and 29.9%, respectively. These results demonstrate that N2 injection mainly affects macropores, followed by mesopores and transition pores, and reformation of the micropores is limited. Nitrogen injection alters the pore structure of coal, which leads to an increase in the pore volume and improvement of the pore size distribution and connectivity: these changes facilitate the diffusion and transfusion of coalbed methane. The permeability of the coal sample was improved as a result of N2 injection, indicating that N2 injection could be used to enhance the permeability of CBM reservoirs. These findings will lead to a better understanding of the interactions between pores and N2 during N2 injection and hence can be applied to improve CBM recovery for non-productive or low-productivity CBM wells.