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Floor Gas Emissions and Gas Outbursts 293

Fig. 15.8 shows a simultaneous degasification of both development sections and
longwall faces.

17.4.2 Highly Gassy Coal Seams (Depth 1500e3000 ft)

These coal seams cannot be properly degassed by in-mine horizontal wells alone. The
most economic method to degas these coal seams is to do it in two stages:
Stage 1. Drill the entire property on a grid pattern vertically. A spacing of 40 acres/well may
be more economic, but gassy coal seams may need a vertical well at 20 acre spacing. All coal
seams (overlying or underlying the mineable coal seam) that are a source of gas are hydro-
fractured as discussed by Thakur [1]. The hydrofracture of the working seam is so designed
that future development headings and the longwall face both are degassed. This is accom-
plished by keeping the longwall panel width smaller than 750 ft and creating a hydrofracture
of at least 1000 ft in length. Typically 50% of the gas in situ can be drained in 5e10 years.
Stage 2. Drill the longwall panels on close intervals (100e150 ft) horizontally from the
development sections. This will remove another 20%e30% of in situ gas resulting in
70%e80% of total degasification. Horizontal drilling will also reveal any geological anom-
alies in the longwall panels, such as faults and washouts. These horizontal boreholes can also
be used for dust control by water infusion.

17.4.3 Highly Gassy Coal Seams Deeper than 3000 ft

Mining coal from a depth greater than 3000 ft is expensive and generally not advisable.
However, some working mines are 4000e4500 ft deep [7]. The permeability of such
coal seams is very low (less than 0.1 md), and a different technique is needed for
proper degasification of these coal seams. The entire areas occupied by such deep
and highly gassy coal seam needs to be drilled horizontally from surface as shown
in Fig. 17.7.
Thakur [1] has proposed this technique for commercial production of gas from deep
coal seams. The technique has been extensively used in northeast United States for gas
production from Marcellus Shale at a depth of 8000 ft. The drilling process is
described here briefly. For details reference should be made to the original work
[1]. A drilling pattern as shown in Fig. 17.7 can degas an area of 2300 acres. It is
an expensive process, but the revenues from gas sales can defray the cost of degasifi-
cation. Improved productivity (tons/man-day) and safety of mine workers are added
benefits.

17.4.3.1 Horizontal Drilling from the Surface
The technology for horizontal wells drilled from the surface has been developed in the
past 15 years. It is an improvement on the in-mine horizontal drilling procedure. It is
mainly used for commercial gas production from shallow or deep coal seams. It is
much more expensive than in-mine horizontal drilling. Wells in shallow coal do not
need hydrofracking because the natural permeability of the coal is high. In deep