Geological discontinues in the coalbed, such as faults, sandstone channels, permeability facies, lithotype changes, and large scale partings in the coal seam, can cause intermittent production problems or produce unexpected amounts of water or gas from degasification boreholes. These discontinuities not only can impact methane emissions into the mine workings, especially if they hinder proper and effective degasification of the coalbed but may act as conduits for methane flow from gassy strata into advancing mine workings. The effects of these discontinuities are still under debate by mining and gas production experts. This study presents a numerical investigation using reservoir simulations of the effects of partings and permeability facies and lithotype changes on the production performances of vertical and horizontal degasification boreholes, as well as the emissions during longwall operations.
In this work, the grid block was designed in a multi-layer 3-D structure which enabled spatial descriptions of the geometries and properties of the discontinuities within the coal seam. The studied coal seam discontinuities and their properties were distributed arbitrarily within the model. Production was simulated by vertical and horizontal boreholes. Degasification boreholes operated two years prior to start of the mining.
The results of this numerical study show that in coal seams with major heterogeneities and discontinuties, the geometry and location of the boreholes are important for improved gas production. These considerations are also important for controlling methane and water inflow into the working environment during longwall mining.
Occasionally, unanticipated and unusually high emissions, despite normal ventilation controls in coal mines, may result in an explosive mixture at the working face that can be easily ignited during mining. Abnormal, unanticipated mine gas emissions in quantities sufficient to create hazardous conditions have often been attributed to various geologic features. During burial and diagenesis of the organic matter that ultimately forms mineable coal beds, dispersed organic matter many occur also in adjacent strata that is not being directly exposed by the mining process. This material can produce methane in quantities far exceeding the storage capacity of the coal and surrounding rock (Juntgen and Klein, 1975). The flow of this gas at high rates into the mine workings may be either facilitated or temporarily impeded by the presence of geologic structures or anomalies during mining of the nearby coal seams.
One of the most effective approaches to alleviate the gas emissions from mined coal seams before mining starts is to drill stimulated vertical or horizontal boreholes to drain excessive gas from the coalbed (Noack, 1998; Diamond, 1994; Thakur, 1997). This approach has been proven to be very effective in various field applications to degasify fairly continuous and uniform coal seams (Perry et al., 1978; Prosser et al., 1981; Diamond, 1994; Aul and Ray, 1991; Zuber, 1998; Kelafant et al., 1988; Ertekin et al., 1988; Young et al., 1993; Cameron et al., 2007; Diamond et al., 1989; Young et al., 1991).