Influence of geotechnical factors on gas flow experienced in a UK longwall coal mine panel

Methane drainage has become an integral part of modern coal mining operations when gas emissions cannot be practically dealt with using conventional ventilation methods alone. Boreholes are often drilled above and below the caving zone and connected to a drainage range located along the return gate. This paper describes the construction and analysis of the results obtained from the two- and three- dimensional geomechanical and gas flow models experienced around an active deep UK longwall coal production panel. The models constructed using the commercial FLAC codes were undertaken to provide information to the ventilation engineers at the mine on the likely gas sources and gas flow paths into the face line areas and gate roads. This information allows for the correct design of the orientation, length and support of the boreholes to maximise gas capture. The paper describes the method adopted to derive the relevant rock mass parameters and the laboratory tests conducted to obtain the stress-dependent permeability of coal measure rock strata. A functional relationship is proposed whereby the intrinsic bulk permeability of a sheared coal measure rock may be predicted from the confining stress. A detailed discussion of the geomechanical modelling methodology and the derivation of the strata permeabilities and gas flow modelling adopted is presented. The output of the models is described and used to interpret the major potential gas sources and pathway into the workings.