Field trials of nitrogen injection enhanced gas drainage in hard-to-drain coal seam by using underground in-seam (UIS) boreholes

•Field trials of nitrogen injection enhanced gas recovery were carried out in underground coal mine through in-seam boreholes for the first time.•Significant gas flows were observed after nitrogen injections and a strong post-effect was monitored.•Residual gas content (Q3) can be effectively reduced through nitrogen injection and the total recovery ratio of coal seam gas is enhanced.