Page 315 - Advanced Mine Ventilation

292 Advanced Mine Ventilation

Advance horizontal drilling is done for pressure relief and gas drainage, but only
limited success has been achieved because of low permeability [7].

17.3.4 Ukraine
Coalfields of Ukraine are liable to gas outburst. They use the following criteria:
1. Depth.
2. Coal rank.
3. Gas content.
Similar criteria are also used in Germany (North Rhine Westphalia) and Russia.
Pre-mining methane drainage has been done with only limited success because of great
depth (z3000 ft) and consequent reduced permeability.
In summary, we need to concentrate on only three criteria:
1. Depth and gas content all coal seams deeper than 600 ft with a corresponding gas content of
3
200 ft /t or more. Depth and gas content are highly correlated (see Chapter 15).
2. Any coal seam that shows a high rate of gas emission and high pressure gradient. The first is
due to coal pulverization and high diffusivity coefficient or sorption time; the second is a
1
function of low permeability and reservoir pressure. A sorption time of less than 1 day
(as defined by Thakur [1]) should raise a warning flag. Likewise, a permeability of less
than 1 md and a reservoir pressure of 200 psi should signal a warning.
3. The structure and strength/toughness of coal. Chinese research puts great emphasis on it. In
general, weaker coal with a compressive strength of less than 3000 psi should raise a concern.
Most deep coal seams display this tendency.

17.4 Prevention of Gas Outburst

It is clear from the historical review of global gas outburst events that only moderately
gassy and highly gassy coal seams are liable to gas outburst (refer to Table 15.1). There
is really one technique for the prevention or, at least, minimizing the risk of gas
outburst in all coal mines, and it is a proper degasification of the coal seam(s) prior
to mining. Chapter 15 discusses all methods for premining degasification, but it will
3
be further detailed here. A well-degassed coal seam contains less than 100 ft /t gas
at a pressure of less than 100 psi.

17.4.1 Moderately Gassy Coal Seams (Depth 600e1500 ft)
Such coal seams have good permeability, and in-seam horizontal drilling or horizontal
wells from surface will adequately degas the coal seam and minimize the risk of any
gas outburst.
Fig. 15.7 shows the way to degas a development heading in advance of mining.

1
Sorption time is the characteristic time in which a coal sample gives out 63.21% of total gas contained in it.