The National Energy Technology Laboratory has published some new results from methane hydrate testing carried out in 2011 and 2012 in the Ignik Sikumi test well on Alaskaís North Slope. According to an article in the latest edition of NETLís Fire in Ice publication, the results shed light on the potential use of injected carbon dioxide as a means of producing natural gas from methane hydrate deposits, while also demonstrating that producing gas by depressuring the deposits may work more easily than previously thought.
Methane hydrate is a naturally occurring solid that traps concentrated volumes of methane, the primary component of natural gas, in an ice-like lattice of water molecules. The material, which is only stable within a certain range of temperatures and pressures, is known to exist in huge quantities around the base of the permafrost under the North Slope. A viable means of commercially producing natural gas from the material could add massive volumes of recoverable natural gas to the more conventional gas resources available on the Slope.
Multi-year researchResearch into both the extent of the methane hydrate deposits in Arctic North America and elsewhere around the world, and into the technical and commercial feasibility of producing gas from the hydrates, has been in progress for a number of years.
In 2002 a test in a methane hydrate test well in northwestern Canada attempted methane production through the application of hot water to the hydrate reservoir but found this technique to be ineffective. However, another test demonstrated that the hydrates could be disassociated through depressurization without the artificial application of heat.
In 2007 BP, the U.S. Department of Energy and the U.S. Geological Survey drilled the Mount Elbert methane hydrate stratigraphic test well at Milne Point on the Alaska North Slope. Tests in this well demonstrated the possibility of depressuring the hydrates and thus releasing methane by extracting free water from the hydrate reservoir.
And in 2008 a new test in the Canadian well succeeded in producing about 13,000 cubic meters of gas over a six-day period using depressurization.
In April 2011 ConocoPhillips drilled the Ignik Sikumi methane hydrate test well to a depth of 2,597 feet in the Prudhoe Bay unit on the North Slope. The well passed through the permafrost layer and extended below the base of methane hydrate deposits known to exist at the well site. A team involving ConocoPhillips, the U.S. Department of Energy, and the Japan Oil, Gas and Metals National Corp. then conducted tests using the well.
Carbon dioxide injectionA primary purpose of the Ignik Sikumi test was to try injecting carbon dioxide into the methane hydrate, in hopes that the carbon dioxide would displace methane from the hydrate lattice, thus serving the dual purpose of both producing natural gas and sequestering carbon dioxide coming from the Prudhoe Bay oil field. The test also involved depressurization of the hydrates, along the lines of what had been tried in the Canadian well a few years earlier.
Laboratory experiments had demonstrated the use of carbon dioxide to displace methane in hydrates, but nobody knew whether this type of technique would work in the field. Successful use of the technique could enable the exploitation of hydrate deposits without any impact on the mechanical stability of the deposits, and without the formation of pore-clogging ice or secondary hydrates as a consequence of disassociation-induced cooling.
According to the report that NETL has now published, the Ignik Sikumi tests actually involved injecting a mixture of nitrogen and carbon dioxide into the hydrates, along with chemical tracers to track where the injected material ended up. A flowback of material from the well at pressures above the stability pressure for pure methane hydrate was then conducted for one-and-half days to evaluate the effect of the injected mixture. This test was followed by a 30-day flowback in which a pump reduced the downhole pressure to levels below the hydrate stability threshold, thus inducing gas production through hydrate disassociation.
The tests demonstrated that, while pure carbon dioxide tends to react with free water in a methane hydrate reservoir to form carbon dioxide hydrate, rather than displacing methane from the existing hydrate, the injection of a carefully designed gas mixture, such as the mixture of nitrogen and carbon dioxide used in the Ignik Sikumi well, can be effective in methane production, the new report says.
Responsive to pressureThe depressurization test, the longest conducted to date as a part of methane hydrate research, showed methane production that was highly responsive to the wellís bottomhole pressure. And, importantly, the heat exchange as a consequence of cooling of the hydrates during disassociation proved more favorable than anticipated, thus pointing to the possibility of using more aggressive pressure reduction techniques, the new report says.
While more laboratory studies and field tests will be needed to better understand the gas exchange process that was tested in the Ignik Sikumi well, the next field tests should involve the use of both an injection well and a production well the NETL report says. Commercial viability would depend on factors such as the well configuration, the injection method and the gas mixture injected. It is clear that gas injection and exchange will remain valuable in association with methane hydrate development strategies that are based primarily on reservoir depressurization, the report says.