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
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
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.