Page 301 - Advanced Mine Ventilation

278 Advanced Mine Ventilation

16.3.3 Gob Well Spacing on the Longwall Face

It was earlier shown that the effective length of the gob producing most gas is limited
to 1.3 depth immediately behind the face. Gas removed by the bleeders is discounted
here to provide a little reserve capacity for the gob wells to handle peak emissions.
Knowing the gob well production capacity, the number of producing gob wells in
the gas emission space can be calculated by simply dividing the total emissions by
the production capacity of the gob well. The total number of gob wells for the panel
can be calculated by prorating this number by the ratio, the length of panel/the length
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of gas emission space. Table 16.5 shows the specific gob emissions as observed for
different widths of longwall panels in a highly gassy coal seam in Virginia. Specific
gob emissions tend to increase with the width of the panel.
The optimum width of a longwall panel is the width where gob wells are most effi-
cient in draining the gob gases and the total number of gob wells for the panel is the
minimum. The most efficient gob drainage is reached when the spacing between the
two adjacent gob wells (s) is equal to half the width (w) of the longwall panel. A ratio,
s/w that is less than 0.5, indicates inefficient methane drainage. To illustrate this point,
spacing of gob wells for longwalls with different widths but the same tonnage of
extraction is calculated. The following assumptions are made for this calculation:
1. The rate of extraction is the same for all longwall panels, one acre per day.
2. The longwall face is 10,000 ft long and needs two gob wells near the set up entry.
3. A gob gas capture ratio of 70%e80% will be achieved.
4. Specific gob emission is 30 MMCF/acre.
Table 16.6 shows estimated total number of gob wells for various widths of long-
wall panels.
Fig. 16.9 shows a plot of s/w against longwall face widths. The most efficient cap-
ture of gob gas is obtained when s/w ¼ 0.5 or the longwall width is 700 ft. Because the

Table 16.5 Specific Gob Emissions for Longwalls in Highly Gassy Seams

Width of Longwall Specific Gas
Face (ft) Emission (MMCF/acre)
450 25
600 30
750 33
900 36
1050 40

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An example: a longwall panel produces 30 MMCFD at a depth of 2000 ft. Most of the gas is produced
from an area 2600 ft immediately behind the face. A 12 in. diameter gob well will produce 4 MMCFD.
Hence, we need 6e7 gob wells over a length of 2600 ft. Therefore, the gob well spacing is 370e430 ft or
400 ft on the average. For a 10,000 long panel, total number of gob wells is at least 25.