N2 injection to enhance coal seam gas drainage (N2-ECGD): Insights from underground field trial investigation

Pre-gas drainage plays a significant role of preventing gas disasters in underground coal mines. However, it is difficult to reduce the gas content to below the threshold values, especially in low permeability coal seams in China. Previous field trials on the enhanced coalbed methane recovery (ECBM) by nitrogen injection mostly focused on the ground. Until now, few attempts on N-2 injection to enhance coal seam gas drainage (N-2-ECGD) and no sufficient evidences can indicate the good effect. In this study, exploratory tests of N-2-ECGD were carried out in underground low-gas coal seams (with gas content less than 5 m(3)/t). An experiment to displace the CH4 in coal by N-2 injection under constant pressure conditions was first carried out. The results show that continuous N-2 injection can promote the desorption of CH4 in the coal pores, which cannot be achieved under normal pressure. N-2-ECGD field trials were conducted in the working face of a coal mine in Hejin City, Shanxi Province, China. The effective pressure of N-2 injection (EPI) was investigated. Intermittent and continuous N-2 injection trials were carried out to study the response characteristics of the gas flow rate, CH4 concentration, and flow rate. The results show an EPI of 0.45 MPa and gas injection radius of 5 m. The gas flow rate increased significantly after N-2 injection. Although the CH4 concentration decreased, the CH4 flow rate remained high. On the basis of the change characteristics of gas drainage parameters, the physical process of N-2-ECGD was analyzed. The research results obtained in this study can serve as a reference for the application of N-2-ECGD in low permeability coal seams. The study also provides a new technical approach for local gas prevention and control in underground coal mines. (C) 2021 Elsevier Ltd. All rights reserved.