Page 350 - Advanced Mine Ventilation

Economics of Coal Mine Degasification 327

If the total cost of in-mine drilling, including all of the underground pipeline costs,
all vertical frac wells, and all other gob wells, is added and then divided by the tons of
coal in the longwall block, the result is the cost of coal seam degasification per ton of
coal. An annual clean coal production of 3.1 million tons is assumed.

20.3.1 Estimated Cost for Mildly Gassy Coal Seams (Gas Content
3
Less Than 100 ft /ton)
3
Premining degasification: For coal seams with gas contents less than 100 ft /ton, there
is generally no need for premining degasification.
Postmining degasification: Two gob wells are recommended for the longwall panel.
The first gob well should be installed within 1000 ft of the setup entry and the second
one installed in the middle of the panel.
The total cost is $100,000 or $0.03/ton.

20.3.2 Estimated Cost for Moderately Gassy Coal Seams (Gas
3
Content 100e300 ft /ton)
Premining degasification: The longwall panel should be drilled horizontally at 1000 ft
intervals, and development boreholes should be drilled to degas development sections.
Total in-mine drilling footage for a typical panel may total 25,000 ft.
Postmining degasification: In moderately gassy coal seams, a proposed longwall
panel may need 5e6 gob wells. The diameter and size of exhaust fans will depend
on local conditions.
The total cost is approximately $1.55 million or $0.50/ton.

20.3.3 Estimated Cost for Highly Gassy Coal Seams (Gas Content
3
Over 300 ft /ton)
Premining degasification: Highly gassy coal seams must be drained several years
ahead of mining with vertical frac wells (wells that have been hydraulically fractured).
These frac wells can be placed at about a 20-acre spacing. Frac wells drilled about
5 years ahead of mining can drain nearly 50% of the in situ gas prior to mining, but
this may not be sufficient. Additional degasification with in-mine horizontal drilling
can raise the gas drained to nearly 70%. Horizontal boreholes are drilled
200e300 ft apart to a depth of 900 ft. Assuming a 200 ft interval, nearly 45,000 ft
of horizontal drilling and about 15 vertical frac wells may be needed to properly degas
the panel.
Postmining degasification: Because of very high gas emissions from the gob, the
first gob well must be installed within 50e100 ft from the setup entry. Subsequent
gob wells may be drilled at a 6- to 15-acre spacing, depending on the rate of mining
and the gas emission per acre of gob. In the US states of Virginia and Alabama,
two states with some highly gassy coal seams, gob wells are generally 9e12 inches