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

Authors

Jia Lin, University of WollongongFollow
Ting X. Ren, University of WollongongFollow
Gongda Wang, China Coal Research Institute, University of WollongongFollow
Patrick Booth, University of WollongongFollow
Jan Nemcik, University of WollongongFollow

RIS ID

118000

Publication Details

Lin, J., Ren, T., Wang, G., Booth, P. & Nemcik, J. (2018). Experimental investigation of N2 injection to enhance gas drainage in CO2-rich low permeable seam. Fuel, 215 665-674.

Abstract

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 CO 2 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.