that traps or contains molecules. The word clathrate is derived from the
and completely envelop the guest molecule, but in modern usage clathrates also include
Methane clathrates are common constituents of the
shallow marine geosphere and they occur in
deep sedimentary structures and form
outcrops on the ocean floor. Methane hydrates are believed to form by the precipitation or crystallisation of methane migrating from deep along
geological faults. Precipitation occurs when the methane comes in contact with water within the sea bed subject to temperature and pressure. In 2008, research on Antarctic
Vostok Station and
EPICA Dome C ice cores revealed that methane clathrates were also present in
deep Antarctic ice cores and record a history of
atmospheric methane concentrations, dating to 800,000 years ago. The ice-core methane clathrate record is a primary source of data for
global warming research, along with oxygen and carbon dioxide.
Methane clathrates in continental rocks are trapped in beds of
sandstone or
siltstone at depths of less than 800 m. Sampling indicates they are formed from a mix of thermally and microbially derived gas from which the heavier hydrocarbons were later selectively removed. These occur in
Alaska, Siberia, and Northern Canada.
In
2008, Canadian and Japanese researchers extracted a constant stream of natural gas from a test project at the
Mallik gas hydrate site in the
Mackenzie River delta. This was the second such drilling at Mallik: the first took place in
2002 and used heat to release methane. In the 2008 experiment, researchers were able to extract gas by lowering the pressure, without heating, requiring significantly less energy. The Mallik gas hydrate field was first discovered by
Imperial Oil in
1971–1972.
Experts caution that environmental impacts are still being investigated and that
methane—a
greenhouse gas with around
25 times as much
global warming potential over a 100-year period (GWP100) as carbon dioxide—could
potentially escape into the atmosphere if something goes wrong. Furthermore, while
cleaner than coal,
burning natural gas also creates
carbon emissions.
When
drilling in oil- and gas-bearing formations submerged in deep water, the reservoir gas may flow into the well bore and form gas hydrates owing to the low temperatures and high pressures found during deep water drilling. The gas hydrates may then flow upward with drilling mud or other discharged fluids. When the hydrates rise, the pressure in the
annulus decreases and the hydrates dissociate into gas and water. The rapid gas expansion ejects fluid from the well, reducing the pressure further, which leads to more hydrate dissociation and further fluid ejection. The
resulting violent expulsion of fluid from the annulus is one
potential cause or contributor to the "kick". (Kicks, which can cause blowouts, typically do not involve hydrates: see
Blowout: formation kick).
Measures which
reduce the risk of hydrate formation include:
- High flow-rates, which limit the time for hydrate formation in a volume of fluid, thereby reducing the kick potential.
- Careful measuring of line flow to detect incipient hydrate plugging.
- Additional care in measuring when gas production rates are low and the possibility of hydrate formation is higher than at relatively high gas flow rates.
- Monitoring of well casing after it is "shut in" (isolated) may indicate hydrate formation. Following "shut in", the pressure rises while gas diffuses through the reservoir to the bore hole; the rate of pressure rise exhibit a reduced rate of increase while hydrates are forming.
- Additions of energy (e.g., the energy released by setting cement used in well completion) can raise the temperature and convert hydrates to gas, producing a "kick".
