Environmental Engineering Reference
In-Depth Information
Unconventional gas
Conventional gas reserve estimates are dwarfed by those for unconventional
gas - around nine hundred trillion cubic metres. Unconventional refers not
to the chemical make-up of the gas, which remains basically methane (CH4),
but the use of unconventional means to extract it. This involves special drilling
and stimulation techniques to release the gas from the formations in which it
is found.
Tight gas sands
These are like ordinary gas reservoirs of gas, but are less
permeable and therefore harder for the gas to flow through. North America
has led development of tight gas sands: this region has the largest share (38
percent) of tight gas sands reserves. US production of tight gas has steadily
increased since the early 1990s.
Coalbed methane
This is natural gas, usually obtained from coal deposits
that are too deep or too poor quality to be mined. Again, the US has led
development of coalbed methane (around 10 percent of gas production), but
in Canada, Australia and India coalbed methane gas also ranges between five
to seven percent of overall production.
Shale gas
Large amounts of gas are sometimes contained in layers of
impermeable shale rock. Recent advances in hydraulic fracturing, which
involves creating fractures in the shale layers and then propping them open
with a permeable sand, have made possible large-scale extraction of shale
gas, especially in the US. But even with fancy new techniques, the share of the
shale gas in place that can be extracted is much lower (8-30 percent) than
for conventional gas fields (60-80 percent). A repeat of the US success with
unconventional gas is doubtful, particularly bearing in mind that the more
crowded communities in Europe are far less likely to accept the intrusion of
heavy, noisy equipment, large demands for water and waste-water treatments
- all of which are tolerated in Texas and the lower Mississippi valley - to
extract shale gas.
Methane or gas hydrates
This is one of the largest - and definitely
strangest - forms of unconventional gas. Methane, when combined with
water at low temperatures and high pressure, creates a white crystal-like
substance called methane hydrate, huge amounts of which underlie the
oceans and polar permafrost. These lattice-type structures look remarkably
like ice, but burn if they meet a lit match. No body knows how to extract
them safely, but certain energy-resource-poor countries, such as Japan and
India, are very interested in their extraction. There has been no intentional
commercial production of gas hydrates. But in Alaska and Siberia some wells
have been drilled through gas hydrates to reach conventional gas reservoirs,
and as production lowers the pressure in these reservoirs, it is thought some
of the gas hydrates have dissolved into the reservoirs below and thus been
pumped out. Of course, methane is an extremely potent GHG and the release
of methane hydrates due to melting Arctic permafrost could be one of the
most dangerous and exponentially damaging side-effects of global warming.
