COAL MINE DRAINAGE AND OUTBURSTS.
Outbursts are expulsions of coal and gas from the working face of a mine. They can be lethal due to the violence of the explosion and by asphyxiation. Outbursts are a gas driven failure, as described in the ACARP paper, "Outbursts and Research Needs".
Because of this, outburst prevention can be achieved by gas drainage provided that the gas can be drained. Where it cannot be achieved by conventional drilling, alternative techniques such as hydrofracture or large hole de-stressing drilling need to be adopted. Gas pressure or content is not however the sole factor determining whether a coal will outburst. Gouge material in faults is far more susceptible to outbursting and whether outbursts occur from solid coal is substantially influenced by the toughness of the coal.
Gas storage in coal is by a process of sorption into the coal structure. By this means the coal holds much more gas at a given pressure than if gas were simply held in void space. Gas movement through the coal is by diffusion, darcy flow in the cleats and sometimes in major fractures. The darcy flow is influenced by two phase effects between gas and water. These effects are described more fully in the papers, "The Physical Process of Gas Storage and Movement in Coal Seams" and "Determining the Gas Production Characteristics of Coal Seams". If the cleats or fractures do not exist or if they have been sealed by mineral deposits then gas movement may simply be limited to diffusion. From Sigra's experience we have measured permeability ranges from zero to 3000 millidarcies in coal.
Sigra has designed and can build gas flowmeters to monitor flows from boreholes. These flowmeters and the electronics used with them are described in the section on the two wire data acquisition system. The bulk of this ACARP report refers to an intrinsically safe data acquisition system that can gather information from up to 255 modules, each capable of having up to four analogue devices attached. The two wire cable can be up to 2 km long and was originally intended to span a longwall block.
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Field and Laboratory Services
Sigra provides a range of field and laboratory services to assess how prone a seam is to outbursting and the drainability of gas. The underground techniques are in principal documented in the paper "Reservoir Engineering in Coal Seams: Part 2 Observations of Gas Movement in Coal Seams", though more updated equipment is available for testing now. This provides test capability in longer holes and improved automation.
Testing from surface is described in the section on Coal Seam Methane - Field and Laboratory Services.
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Consulting
Sigra provides a consulting service to assess outburst risk and to provide solutions to control outbursts. Sigra advises on gas drainage regimes from the viewpoint of reservoir mechanics,
mine gas drainage installations, directional drilling and measurement while drilling technology.
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Equipment
In addition to the equipment that Sigra has for its own internal use there are a number of items that are available for sale for use in the area of outburst assessment and gas drainage.
Sigra manufactures packers and packer systems for testing coal seams from underground or in surface boreholes. These may be used in pressure or incremental flow testing. They also have uses in hydrofracture stimulation of coal seams.
Sigra supplies automated desorption test equipment as shown in photograph P6.
Sigra can also supply automatic safety shutoff valves to fit on standpipes before the main gas drainage range. In the event of a pressure fluctuation brought about by main pipeline damage these are designed to close off the flow from individual boreholes.
Sigra also can supply drilling tools to detect the rock type being drilled. These are particularly useful in the detection of outburst prone structures. Information on these can be found in directional drilling and measurement while drilling equipment.