Study on the influence mechanism of air leakage on gas extraction in extraction boreholes

Abstract

Borehole leakage is an important cause of gas extraction failure. In order to study the influence mechanism of borehole leakage on gas extraction, the borehole air leakage mode was studied, and a mathematical model was established to describe the borehole air leakage of the gas extraction. In addition, this paper also analyzed the change rules of borehole air leakage and gas extraction effect at different borehole sealing depth, negative extraction pressure, degree of fracture development and borehole sealing quality. Research shows that: (1) The hole sealing depth has a certain influence on the borehole air leakage and the gas extraction effect. As the hole sealing depth increased, the borehole air leakage gradually decreased. Moreover, the net amount of gas extraction gradually decreased, but the gas extraction concentration increased to some extent, and the increasing range gradually decreased. The reasonable sealing depth of this coal seam was 9∼10 m. (2) The negative pressure of borehole extraction has little influence on borehole air leakage and gas extraction. With the increase of the negative extraction pressure, the gas extraction net amount and borehole air leakage both increased, but the gas extraction concentration decreased. In the later stage of the gas extraction, the negative pressure can be properly reduced to improve the gas extraction effect. (3) The borehole sealing quality has a great influence on the borehole air leakage and gas extraction effect. With the improvement of the sealing quality, the coal-rock permeability gradually decreased. Moreover, the borehole air leakage significantly reduced, and the gas extraction concentration increased significantly. Therefore, attention should be paid to the selection and improvement of sealing materials and sealing techniques in the actual production so that the airtightness and sealing quality of the hole sealing section can be improved.