|Abstract: ||Nowadays, coal mining is extending to deeper and deeper levels, facing ever increasing coal seam gas contents, much higher gas emissions and outburst risks. Capturing coal seam gas before it migrates into atmosphere has been seen as an effective approach to simultaneously improve mining safety, reduce greenhouse gas emissions, and produce clean energy.
Thick seams account for a considerable share of global coal reserve. The application of longwall top coal caving (LTCC) method to extract thick seams generally yields a much higher productivity and is more efficient in comparison to a mechanised single-slice longwall panel. However, the greater productivity achieved by LTCC may further exacerbate the gas emission problems often faced in longwall mining. Geomechanical response of the strata and associated gas emission patterns around thick seam layouts are significantly different from coal mining under thinner multi-seam mining conditions, which is not well understood.
This thesis focuses on establishing an understanding of the stresses, pressure regimes, and gas emission patterns around advancing LTCC faces. During the PhD research, gas pressure and gas concentration were measured in a large number of boreholes in and around an advancing LTCC face at a coal mine. These data are complemented with ventilation and seismic monitoring programmes at the same LTCC district. An integrated analysis of the monitoring data has been carried out and conceptual models for gas emission and drainage for LTCC faces have been developed. These were later used as the basis for numerical modelling research.
A two-way sequential coupling of a geomechanical simulator with a reservoir simulator has been achieved, whereby mining induced stresses and pressures are linked by two coupling parameters: permeability and pore pressure. By applying this approach, gas emission during coal extraction at a LTCC panel in the study coal mine has been successfully modelled and history matched with field data. Recognising that coal and gas outbursts are the most serious and violent gas emissions in both thick and thin seam mining, the application of the coupled modelling approach has been further extended to model two common types of outbursts experienced in an outburst-prone coalfield.
Gas drainage before mining is a standard gas emission control technique, however, its application is largely limited to high permeability coal seams and roof/floor source seams undermined/overmined by single level longwall mining. The feasibility of utilising mining induced permeability enhancement zones to drain gas at thick and tight seams mined by multi-level LTCC method was studied via field trials and numerical models. Building upon the gas emission model developed earlier, a parametric study was carried out to assess different borehole layouts in order to optimise gas drainage designs.
It is believed that the findings of this research and gas drainage methods developed for thick seam mining will create a safer underground environment for miners at high productivity LTCC panels.|