Experimental investigation of N2 injection to enhance gas drainage in CO2-rich low permeable seam

Seam gas pre-drainage, is widely used as an effective method to control gas and coal outburst in underground coal mines. However, in CO2 abundant low permeable seams, this technology seems to be less efficient due to the CO2 sorption characteristics and the lower safe mining threshold limit for CO2 applied in many outburst risk management plans. This paper presents the experimental investigations of the applicability of nitrogen (N2) injection to improve gas drainage in CO2 rich seams. Core specimens were obtained from Bulli coal seam, Sydney Basin and N2 flood tests were conducted under different permeability conditions (0.3 mD and 0.06 mD). A triaxial permeability test rig equipped with a back pressure regulator was used to conduct the test. The variation of gas composition, gas outlet flow rate, N2 flushing efficiency, and permeability variation were analyzed. In addition, a comparative study between N2 flushing coal seam CO2 and N2-ECBM was conducted and it was observed that CO2 was much more difficult to drain out and with longer time compared with CH4. Based on the test results, a two-stage flushing mechanism was obtained. In the first stage, the original free phase coal seam gas accounted for the large percentage and this stage last for a shorter time. In the second stage, desorption time governed the flushing efficiency and desorbed gas was the primary gas. It took much longer time than the first stage. The final recovery rate in 0.3 mD and 0.06 mD scenarios were 90.5% and 87.7%, and the N2 consumption/CO2 production ratios were 15 and 11.2, respectively. Through these laboratory experiments, we concluded that N2 injection can significantly decrease coal seam CO2 content level and increase gas drainage efficiency.