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Premining Degasification of Coal Seams 249

Table 15.1 Gassiness of Coal Seams

3
Category Gas Content (ft /t) Depth (ft)
Mildly gassy <100 <600
Moderately gassy 100e300 600e1500
Highly gassy 300e700 1500e3000

15.1.2 Specific Gas Production
The second reservoir parameter that is equally important is the “specific gas production
rate” of the coal seam. It is a measure of how much gas will be produced per day if an
opening (a horizontal borehole or a vertical slit) is created in the coal seam by a given
degasification technique. It is measured in MCFD/100 ft (e.g., 15 MCFD/100 ft for
Pittsburgh coal seam). This parameter is a combination of reservoir pressure, perme-
ability, porosity, gas content, and diffusivity of the coal seam.

15.1.3 Specific Gob Gas Production

The third reservoir parameter of importance is the “specific gob gas production” for the
mine which is a measure of total gas production per unit area of the gob and is
measured in MMCF/acre. It represents the total influx of gases into the gob from all
overlying and underlying coal seams that are disturbed by longwall mining. For
moderately and highly gassy mines in the United States, the specific gob gas produc-
tion ranges from 8 to 30 MMCF/acre.

15.2 Premining Degasification

All premining degasification techniques can be classified in three broad categories.
• In-mine horizontal drilling
• Vertical wells with hydraulic fracturing
• Horizontal wells drilled from surface
In very deep coal seams (deeper than 3000 ft), the horizontal boreholes can also be
hydraulically fractured to enhance gas production and expedite degasification. Selec-
tion of a particular technique depends primarily on the gas contents and other reservoir
properties. Intensity of drilling is dictated by the specific gas production rate.
These three degasification techniques will be briefly described here. They are dis-
cussed in detail by Thakur [1] and a reference can be made for additional information.

15.2.1 In-Mine Horizontal Drilling

This is by far the cheapest and yet the most effective way of degasifying a coal seam
prior to mining. The author [2] developed this technique, which can drill a 3- to 4-in.