Gas Hydrate in Nature: The Ice That Burns

Ice-like methane hydrate made in the laboratory sustains a flame as its gas escapes. USGS.

Gas hydrate is a natural, ice-like solid that forms when water and gas combine under high pressure and moderate temperature. It is widespread in seafloor sediments below about 300–500 meters (984–1,640 feet) of water and in permafrost. Several countries are studying it as an energy resource, and scientists are investigating how environmental change may affect it.

What it is

• Cages of water: water molecules form cages that trap gas — usually methane, the natural gas piped to homes, sometimes also ethane, carbon dioxide or hydrogen sulfide.
• Concentrated: in the most common crystal structure, 1 cubic inch of hydrate leaves up to 0.8 cubic inch of water and 180 cubic inches of methane when it breaks down at room pressure and temperature.
• Vast: first studied because it clogged oil and gas pipelines, natural hydrate has drawn international interest since the 1980s. It holds an estimated 1,800–12,400 gigatons of carbon — 3–24.8 × 10¹⁵ cubic meters (106,000–876,000 trillion cubic feet) of methane. The U.S. used about 27.49 trillion cubic feet of natural gas in 2016.

A, methane hydrate crystals formed in Indian Ocean sediment (the 10-micrometer scale bar is about a quarter of a human hair's width). B, white hydrate and gray sediment recovered from below the Gulf of Mexico seafloor. USGS.

Under the sea

• Nearly 99 percent of the world's gas hydrate is in sediments of deep continental margins, usually under 500 m (about 1,640 ft) of water or more.
• Its source: microbes turn organic carbon in the sediments into methane; that methane, older microbial methane, or gas rising from deep reservoirs combines with pore water to form hydrate below the seafloor. Hydrate that forms at seeps on the seafloor itself is not a significant part of the global total.
• Where: the hydrate zone thins upslope and thickens downslope to hundreds of meters. Usually less than 5 percent of the sediment holds hydrate, often in particular layers or fractures.
• U.S. margins: hydrate occurs on all of them. The Bureau of Ocean Energy Management estimates an average 21,444 trillion cubic feet of methane in hydrate in the northern Gulf of Mexico, 21,702 on the Atlantic margin and 8,192 on the Pacific margin (excluding Alaska).
• Drilling: for more than two decades, expeditions have studied it — best in the northern Gulf of Mexico and off Oregon, Vancouver, India, Japan, South Korea and China.

Gas hydrate in permafrost (left) and in deepwater marine sediments (right); the orange-labeled places are most sensitive to climate. Methane can be oxidized in sediments, the ocean and the air. USGS.

Where gas hydrate has been recovered, inferred from seismic data, or drilled. USGS.

In permafrost

Only a small fraction of hydrate lies in thick permafrost — ground frozen through by deep cooling in ice ages over the past few hundred thousand years. In the U.S., most continuous permafrost is on Alaska's North Slope; some that formed on land in the last ice age was drowned by rising seas over the past 15,000 years and now lies under the shallow Arctic shelf. There, pure methane hydrate is stable in the lowest frozen sediments and the top of the unfrozen layers below. It probably formed when gas and water froze in place during ice-age cooling, much of the gas having risen from deep reservoirs. It has been studied most in northern Alaska and Canada's Mackenzie Delta.

Finding it

• Seismic surveys map a bottom-simulating reflector (BSR) marking the base of the hydrate stability zone; where it appears, hydrate lies above — though hydrate also occurs where there is no BSR.
• Electromagnetic surveys: like ice, hydrate resists electric current, so these surveys, combined with seismic imaging, are very effective at finding it in some marine settings.
• Boreholes give direct access locally, with instruments that measure sediment layers, gas and hydrate amounts, and fractures.

A USGS seismic image about 300 kilometers off Delaware (2014): the bottom-simulating reflector cuts across sediment layers; hydrate may lie above it and gas bubbles below. Water depth at left is 2,850 meters. D. Hutchinson, USGS.

As an energy resource

Hydrate packs a lot of methane into compact deposits shallower and more accessible than most conventional gas, and hydrate-bearing sands are the main target because they can hold high concentrations; existing technology suits extracting methane from them.

• Not yet commercial: as of 2017, no gas was produced commercially from hydrate, so there are no known reserves. Short research tests have run in permafrost in Canada and the United States and offshore Japan and China; longer tests (months to over a year) are planned, and some countries hope for commercial production within decades.
• How: the usual method lowers the pressure in the sediments, making hydrate break down (dissociate) into gas and water, using existing technology. Dissociation won't run away once started — sediments sometimes need heating to keep gas flowing — but large volumes of water must be managed.

The gas hydrate resource pyramid: the fine-grained marine sediments at the base hold the most methane but are unlikely to be exploitable; sands near the top are the best targets. After Boswell and Collett (2011); USGS.

And the environment

• Stability: hydrate is stable only within set ranges of temperature and pressure, so warming air and oceans may break some down. Most released methane likely stays in the sediments; methane that escapes into the ocean is usually turned into carbon dioxide by microbes, making the water more acidic.
• How much reaches the air: about 5 teragrams (1.1 × 10¹⁰ pounds) of methane a year is attributed to hydrate breakdown — about 1 percent of the 555 teragrams emitted each year from all sources. Whether hydrate is adding methane to the atmosphere now, or will this century, is a key research question.
• Where it's breaking down: likely since at least the late 20th century, on upper continental slopes at 300–800 m (1,000–2,600 ft) and around the Arctic where rising seas flooded old permafrost. Most hydrate is buried deeply enough to stay stable for centuries or more under most warming scenarios.
• Slope failures: hydrate often lies beneath submarine landslide scars, but there's no evidence it causes them. Breaking down, it raises pressure in sediments, which can prime slopes to fail if an earthquake or heavy sediment loading triggers them; layers of strong, hydrate-cemented sediment over weak, gas-charged sediment may also play a part.

What's next

Several countries are pursuing commercial methane production from hydrate. The USGS joins national and international expeditions and assesses how much methane the deposits hold; the next production tests will likely monitor for gas escape, subsidence and produced water. Many deposits are still unexplored, and a key environmental challenge is fingerprinting hydrate methane to track it through ocean and air, and pinpointing which deposits are vulnerable over decades to millennia.

Gas hydrate drilling and production-test expeditions, completed and planned, as of late 2017. USGS.

Sources

Based on Gas Hydrate in Nature, by Carolyn Ruppel, USGS Fact Sheet 2017–3080, U.S. Geological Survey (DOI), citing Boswell and Collett (2011), the Bureau of Ocean Energy Management (2012), Collett and others (2009), Ruppel and Kessler (2017) and the U.S. Energy Information Administration (2017); a work of the United States government in the public domain. The figures come from the fact sheet's PDF, which also restores the exponents and units lost in the import; a photograph taken by Oceaneering Inc. is left out.