USGS estimates ANS holds 85.4 tcf of gas hydrates

There are 85.4 tcf of undiscovered, technically recoverable natural gas resources in gas hydrates on the Alaskan North Slope, reported the US Geological Survey on Oct. 18.

Nick Snow
Washington Editor

WASHINGTON, DC, Oct. 21 -- There are 85.4 tcf of undiscovered, technically recoverable natural gas resources in gas hydrates on the Alaskan North Slope, reported the US Geological Survey on Oct. 18.

The US Department of the Interior agency said scientists recently completed the first assessment of an area extending from the National Petroleum Reserve-Alaska (NPR-A) on the west through the Arctic National Wildlife Refuge (ANWR) on the east, and from the Brooks Range on the south, to the state-federal offshore boundary 3 miles into the Arctic Ocean off the northern coast.

The 55,894 sq miles consists mostly of federal, state, and Alaskan native lands, USGS noted in a new report. "Approximately 35 years ago, Russian scientists made what was then a bold assertion that gas hydrates, long a curiosity of physical scientists, should occur in nature. Since then, the USGS and others have built a strong foundation supporting the conclusion that gas hydrates are a global phenomenon containing potentially huge volumes of gas in terrestrial Polar regions and the deepwater portions of most continental margins," it said.

It explained that gas hydrates are naturally occurring, ice-like solids in which water molecules trap gas molecules in a cage-like structure known as a clathrate. Although many gases form hydrates in nature, methane hydrate is the most common by far, it added.

The report said that when USGS conducted the first systematic assessment of US in-place natural gas hydrate resources in 1995, the study suggested that the amount of gas there greatly exceeds the volume of known conventional gas resources. Recognizing gas hydrates' importance as a potential energy resource, USGS and the US Bureau of Land Management agreed in 2002 to assess the volume of hydrates that could be produced in northern Alaska, it said.

A producible resource
"For the first time, the USGS has assessed gas hydrates—a traditionally unconventional source with no confirmed production history—as a producible resource occurring in discrete hydrocarbon traps and structures," it pointed out.

The assessment's primary purpose was to conduct a geology-based analysis of gas hydrates' occurrence within northern Alaska to determine the role gas hydrates may play in the US energy future, according to USGS.

The project included three concurrent phases. The first focused on the Eileen gas hydrate occurrences overlying the Prudhoe Bay, Kuparuk River, and Milne Point oilfields. The second set out to identify and characterize potential gas hydrate accumulations within ANWR and NPR-A and on state-managed land between the Colville and Canning rivers. The third involved a systematic, geology-based appraisal of the volume of gas that technically can be produced from gas hydrates on the North Slope.

The report said the Northern Alaska Gas Hydrate Total Petroleum System includes Cretaceous and Tertiary reservoir rocks, which have been divided into three assessment units, from oldest to youngest: the Nanushuk formation, the Tulavak-Schrader Bluff-Prince Creek formations, and the Sagavanirktok formation. Only gas hydrates lying below permafrost were assessed. Free gas potentially trapped below the gas hydrate stability zone was not assessed.

"Also, as part of the gas hydrate petroleum system assessment, geochemical analysis of known gas hydrate occurrences revealed a link between gas hydrate accumulations and more deeply buried conventional oil and gas occurrences in which methane migration from depth has charged the reservoir rocks in the gas hydrate stability zone," the report continued.

Standard methods
Researchers used standard geology-based USGS methods originally developed to assess conventional oil and gas resources, it said. In order to use this approach, scientists analyzed three-dimensional seismic data acquired by the oil and gas industry to document that North Slope gas hydrates occupy limited, discrete volumes of rock bounded by faults and downdip water contacts.

"For the most part, it appears that the seismically-imaged gas hydrate accumulations were once conventionally trapped free-gas accumulations that were converted to gas hydrates at the onset of cold arctic conditions at the beginning of the Pleistocene Epoch," USGS said.

While the evaluation assumed that the assessed hydrocarbon resources can be produced by existing conventional technology, the report added that the production potential of northern Alaska's known and seismically inferred gas hydrate accumulations has not been adequately field-tested, although it has been the focus of a research effort led by the US Department of Energy.

"Although verified by only limited field testing, numerical production models of gas hydrate-bearing reservoirs at the Milne Point and Prudhoe Bay oilfields suggest that gas can be produced from gas hydrates with existing technology. Among the various techniques…depressurization appears to be the most promising method," it said.

Of the 85.4 tcf, which represents a mean estimate, the USGS report said that about 20.6 tcf, or 24%, is in the Sagavanirktok formation; 28 tcf, or 33%, is in the Tulavak-Schrader Bluff-Prince Creek formations, and 36.9 tcf, or 43%, is in the Nanushuk formation.

"Given that relatively few wells have penetrated the expected gas hydrate accumulations in these three assessment units, there is significant geologic uncertainty, which is reflected in the range of estimates," it said. The new mean estimate is lower than the 590 tcf reported in the 1995 assessment because it deals only with technically recoverable amounts instead of total in-place gas hydrates, it pointed out.

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