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Providing coverage of Alaska and northern Canada's oil and gas industry
April 2017

Vol. 22, No. 17 Week of April 23, 2017

Gas hydrate: an enticing resource

USGS expert overviews research into a potential major future energy source and the challenge of establishing economic production

Alan Bailey

Petroleum News

Gas hydrates, solids in which gas molecules are held inside a lattice of water molecules, are known to exist in vast quantities around the world. And, with the gas in the material usually being methane, the primary component of natural gas, people have long wondered about the possibility of using the hydrates as a source of natural gas, for fuel and other purposes.

On April 18 Tim Collett, an international expert on gas hydrates from the U.S. Geological Survey, talked to the Alaska Geological Society about the current status of gas hydrate research, and about the opportunities and challenges for developing hydrates as an energy resource. Huge quantities of hydrates are known to exist around the base of the permafrost under Alaska’s North Slope.

Stability zone

The hydrates are only stable within a certain range of relatively low temperatures and high pressures - move the temperature and pressure outside that range and the hydrates disassociate into water and gas. But, as a solid within its stability range, the hydrate can capture huge volumes of gas. One unit volume of solid hydrate holds about 160 unit volumes of free gas, Collett said.

Methane has been captured in the form of hydrate in many parts of the world, typically in sediments under the oceans, where temperatures are cold, or under the permafrost in Arctic regions. Because most oceans are very cold at depths below around 400 meters, sub-ocean hydrates are even found in tropical regions such as offshore India, Collett commented.

Vast quantities worldwide

There are known to be vast quantities of hydrates in existence worldwide. In fact, it is estimated that hydrates hold twice as much carbon as all of the world’s oil, gas and coal resources, Collett said. However, much of the hydrate resource cannot realistically be developed. For example, much submarine hydrate exists as grains scattered through clay dominated muds.

Interest in hydrate development is now focusing on deposits where the hydrates permeate sand reservoirs, either under the seafloor or under the permafrost, Collett said.

Collett said that testing of gas hydrate production dates back as far as the 1970s, when production from a gas field in western Siberia evaluated the concept of hydrates overlying the gas in the field disassociating and thus bolstering gas production. However, modern hydrate research began with a science-driven ocean drilling program that started in 1995. And, as interest in gas hydrates has grown, the North Slope of Alaska has proved a particularly effective region for research, given the relative ease of access and the proximity of existing infrastructure.

U.S. hydrate research has also focused on the Gulf of Mexico, where extensive gas hydrate resources exist at shallow depths beneath the seafloor.

Elsewhere in the world, Japan has a very active research program, offshore in the Nankai Trough and the Sea of Japan. There have been two very successful gas hydrate drilling projects offshore India, and there are other projects offshore Korea and on the Malaysian ocean shelf. There is also some research into permafrost gas hydrates on the Tibetan Plateau.

The U.S. Geological Survey conducted an assessment of technically recoverable gas hydrates under the North Slope, estimating the possibility of producing perhaps 85 trillion cubic feet of gas from the resource in the region. The Bureau of Ocean Energy Management has assessed how much hydrate may be in place offshore.

Essentially, gas production from a hydrate deposit involves moving the hydrate out of its stability zone, typically by lowering the pressure in the hydrate reservoir, or by raising the temperature.

Hydrate test wells

A key step in research into the potential for gas hydrate production came in 2002 when hydrate cores were recovered from the Mallik well on Canada’s Mackenzie River Delta. Testing of that well indicated that production through the application of heat to the reservoir appeared ineffective. However, gas production through pressure reduction did prove successful - in 2007 and 2008 scientists returned to the well and conducted a six-day production test, Collett said.

On the North Slope the USGS has been actively researching techniques for locating gas hydrate deposits in the subsurface and assessing the scale of the resources. However, the agency has also been working with the oil industry and the Department of Energy in conducting research into potential hydrate development. Japanese interests have also been involved in the North Slope research.

A major milestone came in 2007 with the drilling by BP of the Mount Elbert gas hydrate test well in the Milne Point unit into one of two major North Slope gas hydrate accumulations, Collett said. The researchers in the Mount Elbert project gathered gas hydrate cores from the well and also used a technique called modular dynamic testing to evaluate the production characteristics of the hydrates that the well penetrated.

Reservoir permeability

One surprising result from this project was a discovery that the permeability of the hydrate deposit was higher than expected, a finding that is encouraging from the perspective of gas production from the hydrates though pressure reduction, Collett said. Given the solid nature of the hydrates, the researchers had assumed that the hydrates would completely clog the reservoir. However, it turned out that only 80 percent of the pore spaces between the sand grains of the reservoir were actually occupied by hydrate - about half of the remaining 20 percent of the pore space was filled with water.

In 2011 ConocoPhillips drilled the Ignik Sikumi gas hydrate test well in the Prudhoe Bay unit. A prime purpose for drilling this well was to test the concept of gas production from hydrates through the substitution of carbon dioxide for methane in the hydrate lattice. Carbon dioxide fits better into the lattice structure than does methane. So, if carbon dioxide is flushed through the hydrate, carbon dioxide should replace some of the methane, thus causing both the production of methane gas and the sequestering of waste carbon dioxide in the solid subsurface gas hydrate, Collett explained.

After conducting a carbon dioxide exchange experiment, the researchers allowed gas to flow from the Ignik Sikumi well, out to 63 days, Collett said. An interesting result from this test was a confirmation of the relationship between hydrate gas production and the temperature of the hydrate reservoir - the disassociation of the hydrate absorbs heat, thus causing the remaining hydrate to cool, as gas production continues. Thus, the continuing production of gas from hydrates will involve the balancing of the relationships between reservoir pressure, reservoir temperature and production rates, Collett explained, commenting that cooling could ultimately result in water freezing into reservoir-clogging ice.

Gulf of Mexico

There has also been significant gas hydrate research in the Gulf of Mexico. An industry led project investigated drilling into hydrate deposits in the gulf and characterized the hydrate resources in sand reservoirs. The Department of Energy has now taken over that project, with the University of Texas conducting a drilling project to collect hydrate cores.

Offshore Japan, the second of two subsea hydrate production tests has just begun. After previous production testing in 2013, the current testing is aimed at mechanical issues relating to well completions, Collett said.

In research offshore both Japan and India a recent development has been the recovery of pressure cores, drilling cores preserved in containers at the pressures experienced in the actual hydrate reservoirs. Research using these cores has revealed new information about the permeability of the reservoirs, suggesting the existence of permeabilities higher than previously thought, Collett said.

Production estimates

Using the range of likely hydrates reservoir permeabilities, scientists have estimated potential gas production rates from hydrate wells ranging from 500,000 cubic feet to 4.5 million cubic feet per day offshore India, Collett said. Modeling of possible production from the Ignik Sikumi well in Alaska suggested a rate peaking at about 4 million cubic feet per day. And there have been estimates of potential offshore production rates as high as 40 million cubic feet per day.

But production tests so far have only demonstrated the production of very small volumes of gas over short time periods, Collett emphasized. The next step towards realizing full-scale hydrate production is a long-term test involving multiple wells. Discussions are currently taking place between the USGS, the National Energy Technology Laboratory and the North Slope operators about the possibility of carrying out a test program of this type, Collett said.

And possible commercial production of hydrates is still a number of years into the future, perhaps in the late 2020s or early 2030s, with Japan hoping to achieve commerciality by 2025.

Production challenges

The challenges with commercial hydrate production will revolve around the need to balance the necessary heat flow into the hydrate reservoir to maintain the reservoir temperature along with gas production from the reservoir and the depressurization needed to cause the hydrate to disassociate. Essentially, given the depressurization, artificial lift of the gas resource will be required from the start of production, and the production of released water along with the gas will become a major economic factor. On the other hand, water flow into the reservoir could help with the temperature maintenance issue. Estimates suggest that gas from hydrates could cost twice as much to produce as gas from a conventional resource, Collett said.

But, given their current relatively high cost of gas, countries such as Japan and India see potential economic benefit in hydrate development. And on the North Slope, gas from hydrates could have value as a fuel or for maintaining reservoir pressures in oil fields.






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