Investigation of longwall goaf gas flows and borehole drainage performance by CFD simulation

• A 3-D permeability model and gas release model were used to model longwall goaf gas flow.
• Flow patterns under different conditions were obtained.
• Methane flows first horizontally and then flow down along the annular higher permeability zone.
• Boreholes located in the annular area draw higher methane flow rate.
• Perforated section of the boreholes located in the range of fractured zone draw more methane.

To reduce methane emissions into the workings of a longwall panel, the mechanisms of gas emissions and migration within the longwall goaf must first be understood. Additionally, high performance drainage boreholes must be designed. Many variables affect the goaf gas flow and borehole drainage efficiency, namely the gas release characteristics of gas sources, the heights of caved and fractured zones of the goaf, the location of the drainage borehole and the position of the effective drainage section (the portion of the perforated case and the uncased open hole). This paper illustrates how these variables influence the goaf gas flow patterns and borehole performances through CFD simulations. A permeability model and a gas release curve, which reflect observations from experiments, geomechanical modellings, mine site monitoring and borehole drainage data, were constructed for the mining-disturbed strata and incorporated into the CFD model. The CFD model was calibrated with the actual drainage data from the surface boreholes at a coal mine. Simulation results show that the gas drainage velocities around the perimeter of the panel goaf are higher than in the central goaf, which is consistent with the overlying annular fracture zone model established from CSIRO previous research. Boreholes located within the annular area drew methane at a higher flow rate than those located in the central area, leading to a significant reduction of methane emissions to the ventilation system. Goaf geometry with higher caved and fractured zones has lower pressure and methane concentration but higher gas flow velocity. The location of the effective drainage section of the borehole has significant influence on drainage performance. Boreholes with the bottom end located in the lower region of the fractured zone, (20 m above the roof), can draw more methane than those located in the upper region of the fractured zone (70 m above the roof) and those located in the caved zone (2 m above the roof).