Removal of gas from the coal seam and surrounding strata has proven to be an effective method in reducing or eliminating outbursts.
The benefits of gas drainage are:
- Gas given off from the virgin coal is reduced so that gas percentage in intake airways and at working faces are kept within statutory limits and coal production rates are not governed by gas emission rates. This is achieved by degasification of coal prior to mining, i.e., pre-drainage
- Gas given off from the adjacent strata, mainly the lower coal seams after extracting an area of coal, is controlled so gas percentages in return airways are maintained below statutory limits. This is achieved by drainage from adjacent strata after mining, i.e., post drainage
- A reduction in the intensity of outbursts of coal and gas
Gas drainage in coal mines can be either by
(a) mine ventilation system, and
(b) drilling technique.
The drainage by mine ventilation is normally used in mines with relatively low coal gas content. However, in coal deposits with high gas content beyond threshold limits, gas drainage can be carried out by drilling boreholes. Drainage by bore hole drilling can be carried out either from the surface or from underground. In either case the drainage of gas can be carried out either prior to mining- called “pre-drainage” or after mining of the coal seam- called “post drainage”
Pre drainage of coal seam and surrounding deposits can be carried out by drilling holes from the surface and even prior to the commencement of the mining operations. Holes drilled can be vertical, angled (Ray 2004) or deviated as per need. In seam drilling can also be carried out from the surface by deviated drilling. See figure opposite.
Surface drainage generally involves drilling boreholes on a grid pattern from the surface to the coal seam. The spacing between the holes varies between 250-500m. The location of the boreholes is influenced by surface topography and eased of access to the drill site. Above all it can also be influence by the angle of drilling. Currently the grid spacing of the surface holes are widened significantly because of borehole deviation and branching. Hydrofracing and deviated drilling are also carried out from the surface. Drilling from the surface can also be carried out for post drainage. The effectiveness of post drainage is influenced by the competency of the overburden stratification. The longwall extracted areas can be efficiently sealed off after completion of the extraction process. The exposed cavities and/or goaf areas form ‘free gas’ reservoirs that can be extracted by post-drainage techniques at a predetermined controlled rate. To avoid dilution of high percentage methane behind the seals, the quantity of captured gas should be in equilibrium with gas desorption rate from the strata gas sources. Part of the gas usually leaks directly to the ventilation system, however, the majority of high percentage methane could be recovered by goaf gas drainage systems. The extracted gas can be diffused into the ventilated mine workings or transported by a methane drainage network to the surface for utilisation or controlled exhaustion to the open atmosphere. The diameter of surface holes is 150 to 300 mm, which depends on working seam depth, gas and mining conditions as well as the source of gas transportation (suction or free flow – buoyancy effect).
Underground drainage of gas can be carried out as pre drainage and post drainage and these include:
Local drainage ahead of the development headings:
Figures opposite show a typical method of drilling holes ahead of the roadway. These holes are either drilled to both sample gas in advance of the developing coal face and extract any residual gas ahead and adjacent to the proposed roadway.
Inseam gas drainage across the panel and along the panel:
Figure opposite shows typical drilling patterns used in inseam drainage of a panel. The extent of the drill hole density, drill pattern and drill fan orientation with respect to the panel direction is dependent upon the coal permeability, coal gas make, gas composition, ground principal stress direction, coal cleat orientation and lead time until mining. Holes, typically 96 mm in diameter, are normally drilled in advance of the working face, from gate roads or main headings to dewater and degas the coal over time intervals of 3 -18 months, depending on virgin gas content, permeability and hole spacing. Holes are typically 250-400m in length but can reach over 1500m if required. The coal gas is drained of the coal either by its natural gas pressure or with the aid of vacuum pumps.
Cross measure drilling, below or above the working seam:
This type of drilling practice is primarily focused on controlling and managing gas, which has the potential to migrate into the mine workings. Holes are drilled into the floor and/ or roof of the coal seam being worked and in advance of the longwall coal being extracted. These holes are usually inactive or dormant prior to longwall extraction and only start to liberate significant gas flow as the floor and / or roof fractures with the extraction of the coal. Figure opposite shows the cross measure holes (in Green) drilled to the floor of the mined coal seam, intersecting the gas horizons and coal seams beneath the mined seam.
Drilling for gas sampling and drainage in advance of development heading