Page 300 - Advanced Mine Ventilation

Postmining Degasification of Coal Mines 277

16.3.2.1 The Size of Gob Well and Gas Production Capacity

Coal industry in the United States uses gob wells with diameters ranging from 4 to
12 in. Moderately gassy mines use 4e7 in. diameter casings but highly gassy mines
use bigger casings. The most popular size is 12 in. in diameter. The cost of drilling
larger diameter gob wells increases exponentially.
All gob wells are assisted in gas production by a well-designed blower. Lampson
blowers (Series 600, 800, and 1200) are commonly used blowers and have a good
track record. Operating at a suction pressure of approximately 1e3 psi the gas produc-
tion volumes are as follows:

Casing Diameter Gas Production
6 in. 1e2 MMCFD
9 in. 2.5e3.2 MMCFD
12 in. 4.5e5.0 MMCFD
15 in. 7.0e8.0 MMCFD

Assuming an average of 70% methane in the gob gas, a single 12 in. diameter gob
well can remove 3e4 MMCFD of methane from the gas emission space.

16.3.2.2 Distance of Gob Wells From the Tailgate
For best efficiency, all gob wells must be located between the centerline of the long-
wall panel and the tailgate. On an average, best results are obtained when the gob wells
are located about 100 ft from the centerline toward the tailgate as shown in Fig. 16.8.
Gob wells located on the headgate side of the longwall gob produce significantly
lower amount of gas and thus are very inefficient.

Figure 16.8 Optimum layout of gob wells on a longwall face.