Page 46 - Synthetic Fuels Handbook

34 CHAPTER TWO

Gas derived from coal is generally pure and requires little or no processing because it
is solely methane and not mixed with heavier hydrocarbons, such as ethane, which is often
present in conventional natural gas. Coalbed methane has a slightly higher energy value
than some natural gases. Coal seam gas well productivity depends mostly on reservoir
pressure and water saturation.
To recover coalbed methane, multi-well patterns are necessary to dewater the coal
and to establish a favorable pressure gradient. Since the gas is adsorbed on the surface
of the coal and trapped by reservoir pressure, initially there is low gas production and
high water production. Therefore, an additional expense relates to the disposal of coalbed
water, which may be saline, acidic, or alkaline. As production continues, water produc-
tion declines and gas production increases, before eventually beginning a long decline. In
general, however, coal seam gas recovery rates have been low and unpredictable. Average
per-well conventional gas production in a mature gas-rich basin is about five times higher
than average per-well coal seam gas production. Thus, several times as many wells have
to be drilled in coal seams than in conventional gas accumulations to achieve similar gas
recovery levels.

2.4.2 Gas Hydrates
In addition to coalbed methane (Sec. 2.4.1), another relatively new, and possibly large
source of methane that can be expected to extend the availability of natural gas, is methane
hydrate (also called gas hydrate or methane ice) (Berecz and Balla-Achs, 1983; Sloan,
1997; Gudmundsson et al., 1998; Max, 2000; Sloan, 2000). Their production technologies
have only recently been developed and these sources are now becoming economically
competitive.
A gas hydrate is a molecule consisting of an ice lattice or cage in which low molecular
weight hydrocarbon molecules, such as methane, are embedded.
The two major conditions that promote hydrate formation are thus: (a) high gas pressure
and low gas temperature, and (b) the gas at or below its water dew point with free water
present.
Gas hydrates are common constituents of the shallow marine geosphere and occur
both in deep sedimentary structures, and as outcrops on the ocean floor. Methane
hydrates are believed to form by migration of gas from depth along geologic faults,
followed by precipitation, or crystallization, on contact of the rising gas stream with
cold sea water.
At high pressures methane hydrates remain stable at temperatures up to 18°C and the
typical methane hydrate contain one molecule of methane for every six molecules of water
that forms the ice cage, but this ratio is dependent on the number of methane molecules that
fit into the various cage structures of the water lattice. One liter of solid methane hydrate
can contain up to 168 L of methane gas.
Methane hydrates are restricted to the shallow lithosphere (i.e., less than 2000 m depth).
Furthermore, necessary conditions are found only either in polar continental sedimentary
rocks where surface temperatures are less than 0°C; or in oceanic sediment at water depths
greater than 300 m where the bottom water temperature is around 2°C (35°F). Continental
deposits have been located in Siberia and Alaska in sandstone and siltstone beds at less
than 800 m depth.
The methane in gas hydrates is dominantly generated by bacterial degradation of organic
matter in low oxygen environments. Organic matter in the uppermost few centimeters of
sediments is first attacked by aerobic bacteria, generating carbon dioxide, which escapes
from the sediments into the water column. In this region of aerobic bacterial activity
sulfates are reduced to sulfides. If the sedimentation rate is low (<1 cm per 1000 years), the