Experimental Study on Permeability Enhancement of Anthracite by High Pressure Nitrogen Injection and Soaking

Nitrogen treatment is a widely used coalbed methane (CBM) recovery technique for low permeability CBM reservoirs. We conducted a series of experiments to investigate the nitrogen injection and soaking (NI-n-S) induced coal structure alteration and, thus, the permeability variations. Two sets of experimental variations were investigated, which are the treatment pressure and the treatment duration. The experiments were carried out with nitrogen pressures of 6, 9, 12, and 15 MPa for 2, 12, 24, and 36 h, respectively. The core-scale permeability was employed as an indication of the gas flow enhancement capability and effectiveness. Both X-ray CT and field emission scanning electron microscopy (FESEM) were used for the imaging analyses for cross-comparison before and after NI-n-S treatments. The permeability of the coal samples after NI-n-S has shown a significant increase by 13.6–524.1%, with an average of 98.5%. The permeability increase ratio positively correlates with nitrogen treatment pressure; high-pressure nitrogen produced six types of damage modes on microfractures. The main mode is improving the microfracture opening width, and it increased by 4.4–166.7% (mean 45.1%) of the coal specimen with NI-n-S.