Underground » Ventilation, Gas Drainage and Monitoring
During underground longwall coal mining, roof rocks collapse to fill the mined area as mining progresses. This creates fractures on the strata above the goaf area, which provide flow pathways for the gas and water in the overlying strata to migrate into the mining area. Gas may also migrate to the mining area from the underlying strata due to differences in pore pressure.
Since the quantities of gas from overlying and underlying strata to the mining area may become significant, it is important to estimate this flow using simulation models. For such models to be accurate, they must represent gas storage behaviour in other strata, such as organic rich mudstone/shale (via adsorption and compressed free gas), sandstone/siltstone (via compressed free gas) and coal (via adsorption). Flow behaviour in different types of strata rocks, goaf and induced fractures must also be included.
In this work, SIMED II was used to model different gas storage and flow models in different strata rocks and goaf, as well as gas flow in fractures. SIMED II models gas and water flow from the overlying strata through the goaf, while simultaneously calculating gas drainage from mining faces, boreholes and goaf boreholes. The sensitivities of strata rock parameters on gas and water flow to the working area through a series of simulation cases was also investigated. The parameters studied included permeability, strata porosity and adsorption behaviour, strata pressure and water saturation.
It was found that effective permeability of the fractured zone has the highest impact on gas and water from the overlying strata to the working area. This was followed by the effective permeability of the goaf, while the original permeability of the strata has less impact. Adsorption behaviour in the coal also affects gas content and leads to large differences in gas migration to the working area.
The porosity of the overlying tight sandstone/siltstone determines the amount of free gas, along with pressure and water saturation. Although a rise in porosity will increase the amount of free gas in the sandstone strata, it also leads to more water in the sandstone strata. This affects the relative permeability behaviour of the sandstone, causing complex impacts on gas and water flow.
Gas pressure has little impact on the gas flow behaviour. However, gas pressure in the overlying strata, especially in the overlying coal seam, has a significant effect on gas flow to the working area through the goaf. Water saturation affects the amount of free gas in the overlying sandstone, and more importantly, the gas and water relative permeability. Water saturation therefore affects gas and water flow into the working area.
In summary, the main sources of gas to the working area through the goaf are free gas from the sandstone strata and adsorbed gas from the overlying coal seam. The effective permeabilities of the overlying strata fractured zone and the goaf are the most important factors affecting gas and water flow. Therefore, if the degree of fracturing is high in the overlying strata and goaf, the gas flow rate to the working area will be high, making gas management more challenging. The researchers suggest using goaf boreholes and/or pre-drainage of overlying coal seam (where practicable) to mitigate this problem.