Study on sealing effect of pre-drainage gas borehole in coal seam based on air-gas mixed flow coupling model

The low gas extraction concentration is one of the main factors that affect the prevention and control effect of coal and gas outburst. Based on this, the factors of air leakage around the roadway and borehole are analyzed. The effect of extraction time, seal length and air leakage on the gas concentration is discussed by the fluid-solid coupling model, which is proved by the borehole environmental parameter instrument.The research findings show :(1) Influenced by roadway excavation and drilling, a large number of cracks are created inside the coal around the roadway and the boreholes, which are the main air leakage channels for the air to flow from roadway into borehole; (2) Inside the borehole, the closer it is to the bottom of the borehole, the fewer the cracks in the coal seam, the higher the gas concentration. The critical point of air leakage in which the gas concentration is 100 % moves continuously to the bottom of the borehole with extraction time, until it is stable. The effective sealing length must exceed the position of critical point of air leakage; (3) The gas concentration is related to the sealing length and the amount of air leakage. Increasing the sealing length, and reducing the air leakage can greatly improve the extraction efficiency. Based on the above results, combined bag-type segmented grouting technique, which is to inject different sealing materials into different regions divided by the stress, is proposed to block the air leakage channel by comparing the crack characteristics of the coal surface in the abnormal stress zone and the normal stress zone by SEM technology. According to the field test results, the application of the new sealing technology blocks the crack channel, improves gas concentration and reduces the risk of coal and gas outburst. The results have important guiding significance for improving gas utilization rate, reducing greenhouse gas emission and enhancing safety in production. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.