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ANS hydrate test results promising Data from North Slope test well indicate hydrate saturation up to 75 percent but feasibility of gas production remains unknown Alan Bailey Petroleum News
In a paper presented on Oct. 16 to the Arctic Energy Summit Technical Conference Scott Digert, BP technical advisor, and Robert Hunter, ASRC Energy Services project manager, presented new results from the BP-operated Mount Elbert gas hydrate stratigraphic test well, drilled in mid-February on Alaska’s North Slope (see “North Slope gas hydrate well hits target” in the Feb. 25 edition of Petroleum News).
Gas hydrate consists of a white crystalline substance that concentrates natural gas by trapping methane molecules inside a lattice of water molecules (methane is the primary component of natural gas). The hydrate crystals remain stable within a certain range of temperature and pressure. But when decomposed the crystals yield about 164 times their volume in methane, thus making gas hydrate deposits a potential major source of natural gas. But the practicalities of producing gas from hydrate deposits are unknown, and the Mount Elbert well forms part of a multi-year project to determine whether or not gas hydrates can be developed on the North Slope.
BP, ASRC Energy Services, Ryder Scott Co., the U.S. Geological Survey, the U.S. Department of Energy, the University of Alaska Fairbanks and the University of Arizona have all collaborated in the North Slope project, with DOE footing the $4.6 million bill for drilling the Mount Elbert well.
Straddles the permafrost In the Milne Point area, where the Mount Elbert prospect is located, the zone in which the underground pressures and temperatures support gas hydrate stability straddles the base of a permafrost zone that extends to about 2,000 feet below the ground surface. Because of the potential complications of trying to develop gas hydrates in the permafrost, the research team has focused its efforts on gas hydrate prospects below the permafrost.
However, with the gas hydrate deposits believed to be formed at relatively shallow depths from gas migrating upwards from oil fields deep underground, the necessary juxtaposition of appropriate reservoir and seal rocks represented a significant uncertainty at Mount Elbert.
“We weren’t sure if these shallow formations would have sufficient charge of gas from the deeper oil fields, nor that they would have sufficient seal to retain this gas in the shallow sediments, because a lot of them are soft sediments,” Hunter said.
But, drilled to a depth of 3,000 feet from an ice pad in the Milne Point unit using Doyon Rig 14, the Mount Elbert well encountered gas hydrate deposits that the scientists working on the project had predicted from the interpretation of seismic data. That result confirmed the effectiveness of some seismic techniques developed earlier in the gas hydrate project.
“We were pleased to see that the interpretation held through and we encountered two approximately 50-foot hydrate bearing zones,” Hunter said. “… The hydrate was confirmed in both the primary sands.”
S Hydrate-bearing core uccessful core retrieval from the well resulted in the recovery of about 100 feet of hydrate-bearing core, the first significant hydrate core ever collected on the North Slope.
“We acquired about 430 feet of core, 100 feet of which was gas hydrate-bearing,” Hunter said.
Although the drillers used oil-based drilling mud chilled to 30 degrees Fahrenheit to minimize damage to the hydrates while acquiring samples and data from the well, time was of the essence in recovering hydrate bearing core — releasing the underground pressure on the hydrate and bringing samples to the surface would cause the hydrate to start to disassociate.
“We used the first ever wireline coring system on the North Slope with a conventional drill rig, which enabled us to pull up the core quite quickly about 24 feet at a time,” Hunter said.
The team cut the samples into 3-foot lengths and transferred them to a cold trailer for processing. Some hydrate samples were re-pressurized in methane or preserved in liquid nitrogen for later lab analysis, Hunter said.
Sample analysis is providing invaluable information about the precise nature of the hydrate deposits.
“This core clearly shows that hydrate is contained within the pore space of the sediment,” Hunter said.
And that implies that the hydrate deposits at Mount Elbert originally formed as a conventional gas field. Then, when the permafrost formed about 2 million years ago, the lowering temperatures would have caused the hydrates to crystallize within the rocks, Hunter said.
Wireline logs As well as taking core samples, the research team ran a suite of open-hole wireline logs in the well. One finding from the logging data was the extent to which the hydrates saturate the pore space in the reservoir rocks — the maximum saturation is about 75 percent, Hunter said.
The team also used a technique call modular dynamic testing, or MDT, to test the production characteristics of the hydrates in the well.
“This was the first in the world open-hole dual packer MDT program within hydrate-bearing sediments,” Hunter said.
And, in a new method of data acquisition, a tiny instrument measured the temperatures and pressures at the inlet of the MDT tool.
The testing showed a sharp decline in temperatures when the hydrates disassociated down hole — the disassociation reaction is known to absorb heat. And the pressure response within the well following shutdown of production testing indicated some blocking or choking of the reservoir, possibly as a result of the reformation of hydrate crystals or perhaps from the formation of ice, Hunter said.
That suggests that during a full production test at Mount Elbert it would be necessary to apply heat or use chemical methods to release gas, rather than just relying on reducing the pressure in the hydrate zone, Hunter said.
Next steps So, where to from here for the gas hydrate research project?
The Mount Elbert prospect is part of the Eileen trend, one of two major gas hydrate trends in the central North Slope. And each of those trends contains many individual gas hydrate prospects, such as Mount Elbert. The total gas in place in just the Eileen trend has been estimated at a whopping 33 trillion cubic feet. But, in the absence of established methods of hydrate production, there are currently no technically recoverable resources.
The research project hopes to find some way of extracting at least some of the vast hydrate resource.
“We think that the ultimate recoverability (from the Eileen trend) could be up to 12 tcf, but we need to pin down that range,” Digert said.
The next step will be the continued assessment of the Mount Elbert results, to evaluate whether there may be some viable method of gas production from the prospect. Armed now with the knowledge that the hydrates occupy pore spaces within the reservoir rocks and having data such as the hydrate production characteristics, the researchers will assess how the properties of the reservoir are likely to change during hydrate disassociation and, hence, evaluate strategies for production testing.
The various stakeholders in the project will then need to decide whether or not to proceed to the next project phase — that next phase would likely involve testing production methods at the prospect.
“Significant uncertainties remain and there are many things that we need to address before we fully understand hydrate productivity,” Hunter said.
But the Arctic sands of the central North Slope form the tiny tip of what could become a large pyramid of gas hydrate production possibilities.
“What we’re trying to do here … is use the Arctic as a natural laboratory to determine whether or not gas hydrate can become an unconventional resource, because that’s something that we do not yet know,” Hunter said.
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