Modeling air leakage around gas extraction boreholes in mining-disturbed coal seams

Abstract

Air leakage is one of the most important factors that affect the underground gas extraction in mining-disturbed coal seams. It was demonstrated that the gas concentration in most in-seam boreholes decreased to a very low level in a short time. Such a low concentration may lead to secondary disasters such as coal spontaneous combustion, gas explosion and gas combustion. Aiming at this problem, a fully coupled mechanical- composite gas flow (MCF) model was developed to reveal the mechanism of air leakage around the borehole. The reliability of the model was verified by matching the calculating results with gas extraction data monitored in the field. Then, with the MCF model, factors affecting the gas extraction effect were systematically investigated. It was found that the mining-disturbed zone led to an increase of both the gas and air flow rate, but a decrease of the gas concentration in the borehole. Both the gas and air flow rate reduced with an increase of the sealing length of the borehole, while the gas concentration showed an increase trend. The optimal sealing length of the in-seam borehole in Pingmei 8th Mine was identified as 10-14 m. In addition, the attributes of the coal seam itself also have obvious influence on the quality of the gas extracted. The results indicate that both the borehole and the mining-induced fractures around the borehole should be appropriately treated to improve the quality of gas extracted. Therefore, we propose a new sealing method which integrates cement grouting and gel injection. In this method, after the sealing of the gas extraction boreholes, the sodium silicate and ammonium bicarbonate aqueous solutions were mixed and injected into the coal seam. The produced silicone gel could block the mining-induced fractures and the crescent-shaped gap between the sealing material and borehole wall, thus preventing the air leakage. The research results could help to optimize borehole sealing and improve the quality of gas extracted, thus prevent disasters such as coal spontaneous combustion and gas explosion, so as to guarantee the safety of mining and reduce the greenhouse gas emission. 

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Modeling air leakage around gas extraction boreholes in mining-disturbed coal seams