21st May 2010.
‘Methane ice’ has been hitting the headlines in the wake of the Deepwater Horizon drilling rig disaster in the Gulf of Mexico, and the complex issues relating to methane hydrate formation and occurrence have been brought to the attention of a wider public as a result of BP’s effort to contain the oil spill and limit the associated damage.
To researchers interested in methane as a greenhouse gas, methane hydrates are significant for their potential role as agents of catastrophic climate destabilisation. To the oil and gas industry, they represent both challenge and opportunity. Challenge in their capacity to clog-up gas pipelines and other equipment and to endanger drilling operations, and opportunity in that according to some estimates, up to 5 x 1015 m3 of methane (1) could currently be locked up as hydrates in permafrost regions and ocean sediments.
There is no consensus yet (May 2010) as to the events leading to the gas explosion which wrecked the Deepwater Horizon drilling rig and killed eleven men. The main companies involved, BP, Transocean and Halliburton, are refraining from speculation pending investigations. Robert Bea, a civil engineering professor at the University of California, Berkeley and an oil industry consultant, has suggested that methane hydrates in the sub-sea sediments, destabilised as a result of drilling-related operations, could have contributed to conditions which ultimately led to an explosive mixture of high pressure oil and gas surging up the well and triggering the fatal explosion on the drill floor. No-one disputes that methane hydrates are present in the formations through which the well was drilled and that they constitute a serious and recognized drilling hazard. However, given that the hydrate-containing sediments occur at 3,000-5,000 ft* below the sea floor in the region of the well, and the well was drilled much deeper, down to the target oil and gas reservoir at 18,000 ft subsea, the reservoir is the more likely (?) source for the gas which triggered the fatal explosion. The technical complexities involved in drilling a well to these depths in over 5000 ft of ocean mean that the exact sequence of events may take some time to become clear.
The role of methane hydrates in scuppering one of BP’s plans to contain the oil leaking from the well wreckage on the sea floor, is, by contrast, unambiguous. The idea, basically, was to place a large steel container over the main leak, in order to gather up the oil and subsequently pump it into a tanker. However, gas leaking from the well combined with sea-water to form hydrates which blocked the container as it was being manœuvered into place, and prevented it from being successfully deployed due to the hydrate-induced increased buoyancy. To mitigate against further problems with hydrates, BP will be injecting methanol at appropriate points into the equipment used in further attempts to gather up the leaking oil, using the best available information on the imperfectly understood processes of hydrate formation and dissociation.
*All depths are quoted in feet in line with common US/UK oil industry practice.
(1) Milkov A. V. (2004) Global estimates of hydrate-bound gas in marine sediments: how much is really out there. Earth Sci. Rev 66:183–197, (doi:10.1016/j.earscirev.2003.11.002).
Drilling rig image source: Flickr.com cy esp