A coupled model of air leakage in gas drainage and an active support sealing method for improving drainage performance

Abstract

Air leakage is the principal reason for low methane concentrations during coal seam gas drainage. To investigate the air leakage mechanism, a coupled model of coal deformation and air-CH4 flow in coal seams was developed, considering gas adsorption and the elastoplasticity of coal. The model was first validated by comparing model results with field data. Subsequently, the model was implemented to analyze air flow distribution in the seam, and discuss the impacts of time, active support pressure and drainage pressure on the air leakage. Modeling results reveal that (1) air flow is distributed widely in the seam near the roadway side, and in the area away from the roadway, the air flow is concentrated around the gas drainage borehole; (2) air leakage is notably affected by time, and increases rapidly initially, then tends to gradually stabilize; and (3) the air leakage decreases with an increase in the active support pressure acting on the drainage borehole wall, or when the drainage pressure (absolute pressure) is improved. Therefore, an active support sealing method was proposed with a double-expansive (DE) material as the sealing material. The expansion pressure of the DE material provides an active support onto the hole-wall, inhibiting coal deformation to control air leakage. Furthermore, field test results prove that the air leakage can be reduced effectively by the active support sealing method, and that the average methane concentration is 21.5% higher than that of the boreholes sealed by the traditional sealing method.