Figure 1 – The Gas Hydrate “Pyramid.” Only a very small fraction of the gas hydrate present in sediment has any resource potential – but even that small percentage represents a huge potential in-place resource. Figure adapted from Boswell and Collett, 2006.
Gas hydrate, a crystalline compound of water and natural gas, has been touted as a vast potential energy resource for more than a decade – but realizing this potential has persistently remained beyond reach due to technical and economic hurdles.
A field program scheduled for 2011 on Alaska’s North Slope represents a major step toward commercialization.
Gas hydrate is a common component of sediments along continental margins and in Arctic regions where elevated pressures and low temperatures are present.
Hydrate concentrates natural gas so that the dissociation of a cubic foot of hydrate yields approximately 164 cubic feet of gas and 0.8 cubic foot of water. Hydrate may be present in either sands or shales, but hydrate-bearing sands are of particular industry interest as the hydrate is concentrated in the pore space of coarse sediments and thus has the best potential for commercial extraction.
While hydrate-bearing sands contain only a small fraction of the global hydrate volume, their resource potential is estimated to be in the many thousands of Tcf (figure 1).
Research programs undertaken in the Gulf of Mexico, the North Slope of Alaska, Canada, Japan and Korea have confirmed the presence of hydrate-bearing sands and have validated predictive models for hydrate exploration. This was abundantly demonstrated with the results of a Gulf of Mexico drilling and logging program undertaken by a consortium led by Chevron and the U.S. Department of Energy in April and May of 2009. The 21-day program drilled seven holes at three sites (Green Canyon 955, Walker Ridge 313 and Alaminos Canyon 21), representing a variety of geologic settings and a range of predicted hydrate saturations. LWD logging confirmed multiple hydrate accumulations and demonstrated the ability of direct detection techniques.
Additional field operations are being planned for the Gulf of Mexico, and South Korea was expected to conduct its second hydrate drilling program in April.
A gas hydrate assessment released by the U.S. Minerals Management Service in 2008 determined a mean estimate of 6,710 Tcf occurring in sand reservoirs, and a 2009 assessment released by the U.S. Geological Survey determined a mean estimate of 85.4 Tcf technically recoverable from sand reservoirs on the North Slope of Alaska.
Methods for converting the solid hydrate to its component natural gas and water include depressurization, thermal stimulation and dissolution. All of these methods have technical and economic issues, along with safety and environmental concerns.
An intriguing additional concept for gas hydrate development is the possibility of CO2exchange as a means of production, with CO2 injected into a methane hydrate reservoir, resulting in the production of methane gas and the long-term sequestration of the CO2 as hydrate.
Short-term hydrate production tests were conducted in Canada and Alaska from 2002 through 2007 and have demonstrated that natural gas can, in fact, be produced from hydrate-bearing sands. These tests provided data for computer models on the response of hydrate-bearing sands to depressurization and thermal stimulation, but as designed could not produce natural gas at rates that would draw industry interest.
The question remains: Can sustained natural gas production be achieved at commercial rates from hydrate-bearing sands?
Further, are there safety and environmental issues that need to be addressed before hydrate development can proceed?
Those questions will be addressed in early 2011 as BP Alaska Exploration and its partners on the North Slope undertake a long-term, industry-scale production test at Prudhoe Bay field. This program has substantial funding from the U.S. Department of Energy and has benefited from the technical support received from the USGS and numerous service companies and academic institutions.
With success from the Prudhoe Bay test, Japan plans an offshore hydrate production test as early as 2012. ConocoPhillips and the Department of Energy also are designing a field program for a test of the CO2 exchange technology on Alaska’s North Slope.
As gas hydrate moves toward commercial development, the Energy Minerals Division and its Gas Hydrate Committee are actively working to keep AAPG members informed of ongoing developments. AAPG Memoir 89, “Natural Gas Hydrates – Energy Resource Potential and Associated Geologic Hazards,” containing 39 papers that include all aspects of gas hydrates in nature, was released in January and is available online at EMD’s Gas Hydrates page and at the AAPG Bookstore.