Page 47 - Synthetic Fuels Handbook

NATURAL GAS 35

organic carbon content is low (<1 percent), and oxygen is abundant, aerobic bacteria use
up all the organic matter in the sediments. But where sedimentation rates and the organic
carbon content are high, the pore waters in the sediments are anoxic at depths of only a few
centimeters, and methane is produced by anaerobic bacteria.
The presence of clathrates at a given site can often be determined by observation of a
bottom simulating reflector (BSR), which is a seismic reflection at the sediment to clathrate
stability zone interface caused by the different density between normal sediments and sedi-
ments laced with clathrates.
The size of the oceanic methane hydrate reservoir is not well defined and estimates
of its size have varied considerably over a wide range. However, improvements in
understanding the nature of the gas hydrate resource have revealed that hydrates only
form in a narrow range of depths (such as in the area of continental shelves) and typi-
cally are found at low concentrations (0.9–1.5 percent by volume) at sites where they
do occur. Recent estimates constrained by direct sampling suggest the global inventory
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lies between 1 × 10 and 5 × 10 m of gas.
The potential for hydrate development is high and commercial development of natural
gas hydrates could cause a shifting in energy supply geopolitics because of the amount of
natural gas sequestered in hydrate form. If only a small part of the gas can be produced,
especially adjacent to countries that are presently energy importers, there will be major eco-
nomic and geopolitical consequences that would dramatically change the present energy
supply system.
2.4.3 Shale Gas
Large continuous gas accumulations are sometimes present in low permeability shale,
(tight) sandstones, siltstones, sandy carbonates, limestone, dolomite, and chalk. Such
gas deposits are commonly classified as unconventional because their reservoir charac-
teristics differ from conventional reservoirs and they require stimulation to be produced
economically.
The tight gas is contained in lenticular or blanket reservoirs that are relatively imperme-
able, which occur downdip from water-saturated rocks and cut across lithologic boundar-
ies. They often contain a large amount of in-place gas, but exhibit low recovery rates. Gas
can be economically recovered from the better quality continuous tight reservoirs by creat-
ing downhole fractures with explosives or hydraulic pumping. The nearly vertical fractures
provide a pressure sink and channel for the gas, creating a larger collecting area so that the
gas recovery is at a faster rate. Sometimes massive hydraulic fracturing is required, using
half a million gallons of gelled fluid and a million pounds of sand to keep the fractures open
after the fluid has been drained away.
In the United States, unconventional gas accumulations account for about 2 Tcf of gas
production per year, some 10 percent of total gas output. In the rest of the world, however,
gas is predominantly recovered from conventional accumulations.

2.5 COMPOSITION

Natural gas is a combustible mixture of hydrocarbon gases (Mokhatab et al., 2006).
The principle constituents of natural gas, in varying amounts depending upon the
source (Table 2.1) (Speight, 1993, 2007b) are methane (CH ), ethane (C H ), propane (C H ),
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4
6
3
2
butanes (C H ), pentanes (C H ), hexane (C H ), heptane (C H ), and sometimes trace
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7
4
10
5
6
12
amounts of octane (C H ), and higher molecular weight hydrocarbons. Some aromatics
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8