Page 66 - Advanced Mine Ventilation

Estimation of Ventilation Air Quantity 49

4.4 Moderately Gassy Coal Seams

A typical layout for a longwall face operating in moderately gassy coal seams is shown
in Fig. 4.4.

4.4.1 Premining Degasification
Moderately gassy coal seams generally need to be degassed in advance of mining.
Fig. 4.5 shows a degasification scheme with in-mine horizontal boreholes. Boreholes
drilled parallel to the development headings degas them, and cross-panel boreholes
drilled into the longwall panels degas the longwall panels. If these boreholes are drilled
promptly and produced efficiently, nearly 50% of the in situ gas can be drained before
mining. The outbye boreholes can be 1000 ft apart, but inbye boreholes should be
spaced closer.

4.4.2 Postmining Degasification
A gob well must be installed within 500 ft from the setup entry and, based on local
experience, additional gob wells should be installed at 30- to 60-acre spacing. Usually
the first gob well is the best producer, but other gob wells also help in controlling
methane emissions. With proper planning and blowers on the gob wells to assist in
methane drainage, 50 to 60% of total gob methane emissions can be captured. The
gob well diameter ranges from 6 to 9 in., and gas production is always assisted with
well-designed blowers.

4.4.3 Ventilation Layout and Quantities

A preferred ventilation layout for longwall panel mining moderately gassy coal seams
is shown in Fig. 4.4. An internal return, in this case, is a necessary requirement; other-
wise, ventilation air quantities shown in Table 4.2 may be difficult to provide.

4.5 Very Gassy Coal Seams

A typical layout for a longwall face operating in a very gassy coal seam is shown in
Fig. 4.6.

4.5.1 Premining Degasification
Very gassy coal seams must be drained several years ahead of mining with vertical frac
wells. These frac wells can be put at about 20-acre spacing. They can also drain
methane from overlying coal seams and reduce gob emissions. Frac wells drilled about
5 years ahead of mining can drain nearly 50% of gas contained in coal, but this may not
be enough to sustain a high rate of extraction. Additional degasification is done with