Page 45 - Synthetic Fuels Handbook

NATURAL GAS 33

natural gas is odorless, colorless, noncorrosive, and nontoxic. When vaporized it burns
only in concentrations of 5 to 15 percent when mixed with air (Sec. 2.4). Neither lique-
fied natural gas, nor its vapor, can explode in an unconfined environment. Since liquefied
natural gas takes less volume and weight, it presents more convenient options for storage
and transportation.

2.4 UNCONVENTIONAL GAS

The boundary between conventional gas and unconventional gas resources is not well
defined, because they result from a continuum of geologic conditions. Coal seam gas, more
frequently called coalbed methane, is frequently referred to as unconventional gas. Tight
shale gas and gas hydrates are also placed into the category of unconventional gas.

2.4.1 Coalbed Methane
Coalbed methane and gas hydrates (Sec. 2.4.2) are two relatively new resources that is
recognized as having plentiful supplies of methane and other lower boiling hydrocarbons
(Berecz and Balla-Achs, 1983; Sloan, 1997; Gudmundsson et al., 1998; Max, 2000; Sloan,
2000).
Coalbed methane (CBM) is the generic term given to methane gas held in underground
coal seams and released or produced when the water pressure within the seam is reduced
by pumping from either vertical or inclined to horizontal surface holes. The methane is
predominantly formed during the coalification process whereby organic matter is slowly
transformed into coal by increasing temperature and pressure as the organic matter is buried
deeper and deeper by additional deposits of organic and inorganic matter over long periods
of geologic time. This is referred to as thermogenic coalbed methane.
Alternatively, and more often (but not limited to) in lower rank and thermally immature
coals, recent bacterial processes (involving naturally occurring bacteria associated with
meteoric water recharge at outcrop or sub-crop) can dominate the generation of coalbed
methane. This is referred to as late stage biogenic coalbed methane.
During the coalification process, a range of chemical reactions take place which pro-
duce substantial quantities of gas. While much of this gas escapes into the overlying or
underlying rock, a large amount is retained within the forming coal seams. However, unlike
conventional natural gas reservoirs, where gas is trapped in the pore or void spaces of a
rock such as sandstone, methane formed and trapped in coal is actually adsorbed onto the
coal grain surfaces or micropores and held in place by reservoir (water) pressure. Therefore
because the micropore surface area is very large, coal can potentially hold significantly
more methane per unit volume than most sandstone reservoirs.
The amount of methane stored in coal is closely related to the rank and depth of the
coal; the higher the coal rank and the deeper the coal seam is presently buried (causing
pressure on coal) the greater its capacity to produce and retain methane. Because coal has
a very large internal surface area of over 1 billion square feet per ton of coal, it can hold on
average three times as much gas in place as the same volume of a conventional sandstone
reservoir at equal depth and pressure. In order to allow the “absorbed” gas to be released
from the coal it is often necessary to lower the pressure on the coal. This generally involves
removing the water contained in the coalbed. After the gas is released from the internal sur-
faces of the coal it moves through the coal’s internal matrix until it reaches natural fracture
networks in the coal known as cleats. The gas then flows through these cleats or fractures
until it reaches the well bore.