Page 351 - Advanced Mine Ventilation

328 Advanced Mine Ventilation

in diameter. Powerful exhaust fans capable of a suction of 5e10 inches of mercury are
needed to capture up to 80% of gob gas emissions.
The total cost of degasifying a longwall panel in a highly gassy coal seam is approx-
imately $11 million or $3.52/ton. Coal seam degasification is needed for mine safety
and high productivity, but in highly gassy mines, it becomes quite expensive. In these
mines, the processing and marketing of coal mine methane becomes necessary to
defray the cost [2].

20.4 Gas Production From Coal SeamsdA Stand-Alone
Business

Modern coal mines cover a large lease of 100,000e300,000 acres. A mineable coal
seam of 6 ft thickness can be overlain and underlain by several thinner (3e5 ft) coal
seams giving a total coal thickness of 30 ft to a mineable depth of 3000 ft. A simple
calculation shows that the coal lease has a gas reserve of 2e6 TCF assuming a gas con-
3
tent of 500 ft /ton. Assuming a low recovery factor of 40%, the recoverable gas vol-
ume is 0.8e2.4 TCF. The gross revenue from this gas is $2.4 to $7.2 B even at a low
cost of $3/MCF for natural gas. In author’s opinion, “Every large coal mine lease is a
small to medium size gas field.” A typical example to produce 100 MMCFD of gas
with an economic analysis is presented to show that marketing coalbed methane
(CBM) can not only defray the cost of degasification but also substantially increase
the net profit from the combined mining and gas production venture. A highly gassy
mine can generate a net profit of $50e60 million/year at $5/MCF.

20.4.1 Reserve Estimate
The reserve classification for natural gas wells is divided into three categories:

1. Proved reserves: They are the estimated volumes that geological and engineering data
demonstrate with reasonable certainty to be recoverable in the future with existing operating
and economic conditions. These reserves are further divided into (1) producing reserve, (2)
nonproducing reserve, (3) shut-in reserve, and (4) undeveloped reserve [3].
2. Probable reserves: It is less certain than the proved reserves and can be estimated with a de-
gree of certainty sufficient to indicate they are more likely to be recovered than not.
3. Possible reserves: They are less certain than probable reserves and can be estimated with a
low degree of certainty, insufficient to indicate whether they are more likely to be recovered
than not.
The gas contained in a coal lease will fall in the category of proved reserve because
the entire property is drilled on a 500-acre spacing to withdraw a complete core to a
depth that contains all coal seams. Proximate analysis, gas content measurements,
and other reservoir property measurements are carried out to prove the coal and gas
reserves.
The gas in place, GIP, is given by Eq. (20.1)