Understanding methane emissions in underground coal mines is critical for a safe and productive mine. In addition to reasonable estimation of initial coalbed reservoir parameters, it is also crucial that changes in effective stress due to mining and pore pressure reduction are taken into account due to their effects on porosity and permeability. Primary parameters for estimation of emissions or modeling of the mining environment for this purpose are porosity and permeability which can change dramatically as a result of stress redistribution associated with mining and gas desorption from a large coal volume. These parameters affect the emission rates and ventilation requirements, as well as water inflow into the working environment. Stopping leakage, on the other hand, is a secondary stress dependent factor in estimation of emissions, as convergence of the roof and floor strata, compromising the integrity of the stopping, may result in leakage, making prediction of ventilation requirements difficult.
This paper aims to examine the effects of porosity and permeability changes of the coal seam on methane emissions in an underground continuous miner section. The models were developed and executed in a dynamic fashion to simulate an advancing section. Through this process, the changes of effective stress in coal, particularly their change paths, on porosity and permeability were incorporated into the models and methane emissions, concentrations, air requirements, water inflow and possible leakage from the stoppings were investigated using a conventional coalbed methane reservoir model.
Coalbed methane (CBM) is both a valuable domestic energy source in the United States and a safety concern in underground coal mines. In 2007, US underground coal mine operators reported 73 methane ignitions in underground coal mines. In unfortunate situations, these ignitions may lead to an explosion and to a major mining disaster. Thus, the ability to model methane emissions in underground coal mines as realistically as possible allows for the optimization of coalbed methane degasification systems ahead of mining, appropriate design of the mining geometry, and appropriate design of the mine and section ventilation systems, providing for enhanced safety and productivity.
There are various parameters involved in coalbed methane reservoir modeling that span from depth of seam, coal thickness, porosity, permeability, pressure, methane content and isotherms. Particularly, the influence of in-situ and mining induced stresses on coal porosity and permeability during mining cannot be underestimated for effective degasification and ventilation of coal mines.
Additionally, mining-induced and the resultant effective stress on a coal seam plays an important role in the integrity of ventilation controls, such as stoppings, in underground mines. Roof and floor convergence will usually result in some structural damage to stoppings, allowing leakage of air from intake to return and reducing the quantity of air at the working face.
By accounting for the influence of in-situ and mining-induced stress on the reservoir parameters, methane emissions in an active continuous mining section can be forecasted more accurately.
