Sediments in northern Gulf not right for methane gas hydrate formation

June 1, 2005
Forward-looking energy companies that have been considering the possibility of exploiting methane gas hydrates in the northern Gulf of Mexico in the coming decades should think again.

Forward-looking energy companies that have been considering the possibility of exploiting methane gas hydrates in the northern Gulf of Mexico in the coming decades should think again. This is the conclusion of research scientists from Rice University, the Georgia Institute of Technology, and the Scripps Institute of Oceanography, all of which have been engaged in a detailed study of the region.

As known natural gas reserves are depleted, producers have been hopeful that gas hydrates, which are found in abundance along some continental margins, could be a potential energy source for a power hungry world. This could still be the case elsewhere, but not in the northern Gulf of Mexico, say the scientists.

Marine sediments in the northern Gulf are likely too warm and salty to hold the amount of methane gas hydrates originally thought to exist on the ocean floor there.

Researchers reported high-resolution geophysical and geochemical data for two representative sites off the coast of Louisiana that suggest previous estimates for the region should be revised sharply downward, according to their paper published in the March 15 issue of the journal Geophysical Research Letters (GRL).

“We found that conditions are not favorable for the formation of methane gas hydrates at these sites because of the geology of the north Gulf of Mexico, which consists of salt domes that one can think of as mushroom clouds of salt that rise buoyantly through sediments,” said Carolyn Ruppel, associate professor of geophysics in the Georgia Institute of Technology’s School of Earth and Atmospheric Sciences and lead author on the paper.

Researchers investigated methane gas hydrates in the northern Gulf of Mexico off the coast of Louisiana in the fall of 2002. (Above photo courtesy Carolyn Ruppel. Inset photo courtesy of Carlos Santamarina)
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“The thermal properties of salt make the sediments hotter there, and the heat, coupled with the presence of the salt in pore spaces, makes it harder to form gas hydrates,” said Ruppel.

Researchers continue to analyze their data to get a quantitative estimate of the gas hydrates at these sites, but the deposits are likely thin or non-existent, Ruppel said.

Methane gas (i.e., natural gas) is produced by the decomposition of organic material in the sediment or by thermal processes similar to those responsible for the formation of oil. As the methane diffuses through the sediment, it combines with water at the low temperatures and high pressures beneath the ocean to produce an ice-like solid.

Methane gas hydrates exist along continental shelves worldwide, most in sediments tens to hundreds of meters below the floor in waters more than 500 meters deep. These hydrates exist as disseminated deposits, chunks several centimeters across, and sometimes as concentrated layers.

In the northern Gulf of Mexico, previous research on potential methane gas hydrates assumed homogeneous conditions (e.g., the same temperature and geology) and did not consider the impact of salt on hydrate formation, researchers noted.

“The methods we used are very good at helping us understand the conditions in the sediment and make a prediction of what’s there,” said Georgia Tech’s Ruppel. “We found conditions that are not compatible with published estimates that imply large methane gas hydrate deposits in the northern Gulf.”

Ruppel and her colleagues took a multidisciplinary approach, using overlapping methods to characterize the two sites they studied. They used high-resolution seismic equipment to image the sea floor and to find conduits through which fluids could flow. They also analyzed water samples from cores of sediment extracted from the sea floor. They developed chemical profiles that revealed, for example, salt and sulfate concentrations.

“When you put all this together, you get a good idea of the conditions in the gas hydrate reservoir - that is, the sediments that contain gas hydrates,” Ruppel added.

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Gas hydrates continue to be touted as a potential new source of natural gas. The US Geological Survey estimates that gas hydrates off the US coast or in Alaskan permafrost could contain 300 times the amount of methane available from conventional sources. But producing methane from gas hydrates faces some daunting challenges.

If you could extract these hydrates out of the sea floor, you’d have a concentrated form of natural gas. But a key question is whether it would take more energy to extract the gas hydrates than the gas may provide.

Aside from the difficulty of deep-sea operations, mining the hydrates could destabilize the ocean floor or even trigger the runaway destabilization of the hydrates, say scientists. The methane might be tapped by pumping heated liquid into the hydrate deposits to dissociate and recover the gas, but this would be an energy-intensive operation.

Another alternative would be to drill through the hydrate layers into pools of free gas below - a potential hazard. And methane production presumes the ability to identify large hydrate deposits - something that is no certainty.

Scientists are also studying gas hydrates because they may contribute to global warming and could represent a threat to deep-sea petroleum exploration and production. At present, the scientific community lacks a clear understanding of how gas hydrates form in sediments and how their formation affects the stability of the sea floor.

If methane hydrates under the sea floor become destabilized for whatever reason - either petroleum production or climatic change - massive landslides could occur on the sea floor, says Carlos Santamarina, a professor in the School of Civil and Environmental Engineering at Georgia Tech.

Increased drilling in deeper waters where gas hydrates are likely to be found may pose potential hazards for drilling personnel, says Santamarina.

“If you are drilling into the gas hydrate, you have to worry that the hydrate could suddenly dissociate, leading to collapse of the sediment supporting the drill stem,” he added.

Perturbations of the sea floor can produce still bigger problems. Major sea floor slides can cause tsunamis, large oceanic waves that can bring catastrophic damage to low-lying coastal areas.

Beyond energy interests, methane gas hydrates may also play a role in global warming. Even slight warming of ocean waters by a few degrees could free significant amounts of methane, a potent greenhouse gas.

Ruppel noted that if even a portion of the methane released from the hydrates gets out of the oceans and into the atmosphere, it could exacerbate global warming and lead to a synergy between destruction of hydrates, release of methane, and climate change.

Scientists are also studying gas hydrates at Blake Ridge, off the South Carolina coast, and off the coasts of Oregon and British Columbia. They are focusing on hydrates as a potential energy source and also the safety issues related to drilling.

Petroleum companies are interested in understanding these issues better, and the US Department of Energy is funding much of this work through a joint industry project with ChevronTexaco Corp.

“Ultimately, these studies around North America and the world will shed more light on how much hydrate is out there,” said Georgia Tech’s Ruppel. “I hope it will get us closer to answering the question about whether hydrates are a viable energy source. . .If we do learn it’s a viable resource, then we’ll have to face a new set of issues on how to actually produce energy from this resource.” OGFJ