Study on technology of gas injection pressurization and strong production of underpressured gas during drainage exhaustion period

Enhanced gas drainage technology is an effective measure for local gas control, in order to improve the gas drainage effect at the working face during the exhaustion period, the reason of the decrease of gas drainage flow during drainage exhaustion period was analyzed based on fluid mechanics. Periodic gas injection, pressurization and strong production of underpressure gas technology was proposed in this paper. Its mechanism, theoretical models and field trials are studied. The research results show that with the gradual reduction of gas pressure in the coal seam, the driving force for gas flow is insufficient and the drainage enters the exhaustion period. The gas injection pressurization and strong mining technology has the advantages including “promoting flow”, “increasing permeability” and “gas replacement”. A binary gas convection mathematical model for gas injection and forced production is derived. Based on the model, the effect during gas injection and methane recovery is analyzed quantitatively through numerical simulation. Through on-site trials in the Sydney Basin in Australia, it is found that gas flushing can rapidly increase the mixed volume and scalar of the extracted gas.During the gas injection, the gas content is reduced by 5.28 m3/t, a decrease of 73.90%, and the reduction of gas content during conventional drainage was 2.93 m3/t, a decrease of 39.1%. The results show that, compared with conventional drainage, gas injection can improve the coal seam gas drainage effect more effectively, and realize the rapid and strong extraction of the underpressured coal seam, and reduce the risk of coal and gas outburst possibility and methane emission rate.