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280 Advanced Mine Ventilation

Table 16.7 Coal Gas Capture With Vertical Gob Wells

Total
Methane
Emissions Gob Wells Capture
Coal Seam Gassiness (MMCFD) Per Panel Ratio (%)
Pittsburgh (PA and WV) Moderately 4e8 5e6 40e50
Lower Kittanning (PA) Mildly 2e4 3e5 30e50
Pocahontas #3 (WV) Moderately 5e6 5e8 30e50
Pocahontas #3 (VA) Highly gassy 25e30 20e35 65e80
Blue Creek/Mary Lee (AL) Highly gassy 15e20 10e20 60e70

16.5 Gob Well Production Decline

Most gob wells are produced with a vacuum pump. The pump capacity is always
matched with the anticipated gas production. Available vacuum pumps (also
commonly called blowers) can handle a flow from 100 MCFD to 5 MMCFD and
create a vacuum of a few inches of WG to 120 in. of WG. Lampson 650 and 850
are commonly used. The initial gas production may be erratic, but it settles down in
10 days. The decline of production follows a power law given by Eq. (16.1).

n
Q ¼ At (16.1)
where Q is the total production in MCF, A is the initial production in MCFD, t is the
time in days, and n is an exponent with a value of 0.8 0.1, a characteristic of the gob
emissions space.
Production declines for four gob wells are shown in Fig. 16.10 for longwall panels
in the Pittsburgh seam of West Virginia, Lower Kittanning of Pennsylvania, and Poca-
hontas #3 seam of West Virginia and Virginia. The deeper gob well in Pocahontas #3
seam is about 2000 ft deep.
For design purposes, it was earlier assumed that the gob well will cease produc-
tion when the longwall face has retreated 1.3 depth (ft), but in reality, they
continue to produce some gas until the face has moved beyond 3000 ft regardless
of the depth.
A longwall panel in a highly gassy mine becomes a source of low-cost gas. A
typical 1000 10,000 ft panel with a specific gob production of 30 MMCF/acre
will produce 6.9 BCF gas. Even at a low price of $3/MCF, it can create revenue
of over $20 million. If this gas is marketed, it can defray the cost of degasification
and ventilation of the mines. More of economics will be discussed later in the
book in Chapter 20. Methane capture from sealed gob areas will also be discussed
in Chapter 20.