Gas flow analysis for the impact of gob gas ventholes on coalbed methane drainage from a longwall gob

In this paper, the numerical simulation of gas flow in gob gas reservoirs and the in-situ assessment of GGVs drainage are carried out to investigate the mechanism of the control of gob gas flow with GGVs drainage. Firstly, based on fluid mechanics in porous media the mathematical model of gob gas transport is developed. Then, the gas pressure distribution, gas flow patterns and methane concentration distribution in the gob under the conditions of no drainage measures, drainage with buried pipe in the upper-corner and GGVs in different locations are discussed by conducting numerical simulation, respectively. In addition, the influence of distance between GGV and return airway on the gob methane flow and the strong control scope of GGVs are analyzed in detail. It is found that: 1) GGVs has the “flow-interception effect” on the gas in the deep zone of the gob and the “flow-reversal effect” on the gas in the gob close to the longwall face, which accounts for that GGVs drainage advantage over underground drainage. 2) GGVs can be applied to drain the gas in the deep zone of the gob massively and to control the gob gas migration effectively, so they are a top choice for the extraction of gob gas and the control of gob gas emission. Finally, both GGVs drainage and underground drainage are adopted to extract gob gas in east-102 working face in Weijiadi coal mine. The comparison of the extracted and emitted volume of gob gas with two different technologies shows that, GGV can drain the gob gas with greater flux and can better control the gob gas emission, which validated the prediction of numerical stimulation.