A CONTINUUM MODEL FOR SIMULATING MINE WATER INFLOW AND GAS EMISSION

This paper describes a three-dimensional numerical model, called COSFLOW. It uses a Cosserat continuum approach for the efficient description of mechanical stress changes and deformation in weak layered rock, typical of coal measures. This mechanical model is coupled with a two-phase dual porosity fluid flow model to describe flow of water and gas through porous rock, desorption of gas from the matrix and subsequent flow of water and gas through the fracture network. The coupling includes simulation of permeability and porosity changes with rock deformation. Further the rock mass consisting of many interconnected fractures is idealized as an equivalent porous continuum using an equivalent anisotropic hydraulic conductivity matrix defined in terms of mean fracture spacing and mean aperture. This formulation is amenable to easy evaluation of modifications to the hydraulic conductivities as a function of stress induced changes in fracture aperture. The numerical code is used to simulate water inflow and gas emission in two Australian coal mines. The models at the two mine sites require significant geotechnical and hydrogeological data for adequate calibration. Many parameters in the model are not directly measurable and must be inferred by back-analysis of existing deformation, stress and hydrological data obtained during previous mining. The calibrated model is then used to make predictions for future mining panels. Water inflows at Mine A were predicted to increase significantly as mining progressed and this was supported by later measurements. This increase was attributed to wider longwall panels and increased roof rock permeability as more panels are mined. At Mine B, the model used measurements of gas production from predrainage boreholes for calibration and provided accurate predictions of average gas emissions into the longwall panel and post-drainage boreholes, although transient fluctuations were seen in the measurements. These were probably caused by local variations in geology or gas content or other factors not incorporated in the model.