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gas drainage/outbursts |
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drainage methods |
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Drainage Methods
Gas drainage for the purpose of eliminating the outburst hazard consists of drilling holes within the seam being mined to allow gas to bleed off from the coal into the hole. In its virgin state, the gas is "adsorbed" onto the coal, a physical bonding largely governed by pressure. As the pressure is reduced (as will occur when the coal is excavated, including around a borehole drilled into the coal) the gas will "desorb" from the coal, the amount released increasing as the pressure becomes lower. Once desorbed, the gas can flow through microfractures or other structures in the coal and so be released into the excavation, or borehole. If the gas can be captured from the end of the borehole and led into a pipe range, it can be removed without polluting the mine ventilating air.
Purely drilling boreholes and allowing the gas to flow of its own accord will be effective in reducing the likelihood of outbursts and would normally be sufficient to reduce gas levels to below thresholds without the use of pipework. However most mines would find the gas in the ventilation system to be an additional hazard, difficult to handle, and therefore connect the boreholes to a pipe range to remove the gas to an area where it will not cause a problem to the mine environment, usually to the surface (ignoring at this stage any atmospheric pollution problems this may cause on the surface).
Most mines that carry out gas drainage for whatever purpose utilize a surface "gas drainage plant" which consists of vacuum pumps (with ancillary equipment) attached to an underground gas pipe reticulation system usually via boreholes. The vacuum has a minimal effect on the effectiveness of gas flow from the coal – the gas is frequently released at a pressure of the order of 1500 to 2000 kPa in the coal and few gas plants would provide a vacuum of more than 20 to 40 kPa at the boreholes, an additional very small amount in comparison. The main purpose of the vacuum is to take the gas away and deliver it where required. The vacuum may have some minor benefit in removing a small amount of extra gas after the natural gas pressure has reduced to a very small amount and most of the gas has gone. In order for gas to flow through the coal, any water in the coal has to be removed first. This flows into the boreholes and the vacuum does serve a purpose in removing the water from the boreholes, allowing gas flow to occur. The removal and handling of water from the boreholes and pipe system can be a major process in itself and has to be included in the design of a gas drainage system.
The number and size of drainage boreholes necessary to be effective will vary from seam to seam and mine to mine. The borehole density required for effective drainage will depend on the permeability of the coal which controls the rate of flow through the seam once the gas has been desorbed (the effect of water on porosity is the reason it has to be removed before gas flow is effective) and the time available before mining takes place in the location being drained.
Most mines now use directional drilling (i.e. types of drill rigs where the direction of the borehole being drilled can be surveyed and controlled) to allow long holes to be drilled with accurate targeting of the hole for its full length. Typical holes are around 90 mm diameter and vary in length from 100m or even less up to 1km (possibly even longer but few underground drill rigs can successfully drill far beyond this distance). Typically holes are drilled in a fan pattern from relatively few locations (relocating drill rigs is a major operation), often curved from the starting point to then form a series of parallel holes over much of their length.
Ideally the location of a future set of roadways would be drained by holes drilled from a roadway parallel to the line of the future ones, this being the simplest drainage pattern and allowing drilling to be separated from the development process. Frequently however, such drilling sites are not available and drilling has to be carried out from the standing end of a set of roadways to cover the next section ahead. This can create problems with continuity as development has to stop and wait for drainage to occur.
At times when drainage has been unsuccessful, either because the gas will not flow effectively or there has been insufficient time for successful drainage, it may be possible to complete drainage to threshold levels by drilling a very dense hole pattern ahead of the face, usually for relatively short lengths of advance.
The increasing use of coal bed methane production, dealt with elsewhere, will have a major effect on gas drainage as the need for any further drilling and draining from the mine workings may have been removed or at least reduced.
To check that an area has been successfully drained, further drilling is required to obtain coal samples in order to measure the remaining gas content and ensure threshold levels have been attained. Most commonly this entails relatively short holes drilled ahead of a development panel, often using purely rotary drilling (not directional drilling) and surveying the holes on completion. There are several variations to this process at various mines.
Because of the high risk and consequences involved in seams liable to outbursts, the drainage and testing process is strictly controlled with procedures set out in Management Plans as one of the major hazards to be controlled.