Nitrogen injection enhanced coal seam gas recovery in low permeable seams

Gas pre-drainage is carried out in underground gassy coal seams to decrease the gas content and create a safe working environment for underground coal mining. However, in some low permeable coal seams, particularly those of high carbon dioxide content, the performance of gas pre-drainage is far from satisfactory, and thus leading to delays in production schedules. This problem has been encountered in several coal mines located in the southern Sydney Basin, including Metropolitan Colliery, Appin Colliery and coal mines in Queensland. In order to improve the gas drainage performance and reduce the gas drainage leading time, a new technique inspired by enhanced coalbed methane production (ECBM) is put forward by injecting nitrogen (N2) into the targeted coal seams via gas boreholes to accelerate gas drainage. The objective of this study is to investigate the mechanism of N2 injection enhanced gas drainage in underground coal mines