What's new in production

 Vol. 232 No. 12

Methane hydrates are the cholesterol of the oil and gas industry. They tend to accumulate in tight places and form deposits, causing pipes and valves to clog up with catastrophic results, particularly in deepwater drilling and production. Most of the research into hydrates in the petroleum industry has focused on stopping them from forming in the first place. Under certain pressures and temperatures, 400–1,200 psi and 32–40°F, methane and water combine into crystalline solids, and the process is hard to stop.

To recap the elementary part, under the previously described conditions, water forms geometric lattices that create pockets or cages into which light hydrocarbons and some light gas molecules can fit. There is a lot of natural gas in a given volume of hydrate. (For purposes of this article, “hydrate” is always methane hydrate, unless indicated otherwise.) One cubic foot of hydrate can contain 150–180 cf of methane. By comparison, a cubic foot of liquefied natural gas (LNG) equals about 600 cf of gas at atmospheric pressure.

Much has been written, in this magazine and other places, about the potential of hydrates as a source of usable methane. The resources are so abundant—estimates are as high of 7,300 Tcf—that it’s hard to imagine a future where this resource isn’t exploited. Very soon, tests will be conducted by the Department of Energy, in cooperation with Japan Oil, Gas and Metals National Corp. (JOGMEC) and ConocoPhillips in the Prudhoe Bay area of Alaska. The project, going by the wonderful name Ignik Sikumi (“fire in the ice”), will involve injecting CO2 into hydrate-bearing formations, forcing the hydrate to swap CO2 for methane and liberating the combustible gas. This and similar studies will determine the cost and practicality of unlocking these resources, both in permafrost and in deep ocean sediments.

Hydrates as a storage medium. To flip the issue completely on its head, what if you wanted to make your own hydrates? It’s certainly possible, but why would anybody want to? The answer lies in the economics and logistics of natural gas.

A large part of the world’s gas reserves are in the form of “stranded gas,” which for either economic or transportation reasons can’t find a market. A lot of that is associated gas. Usually, this stranded gas is either reinjected, vented or flared, because without access to pipelines, the natural gas cannot be made useful. Even though the practice has become illegal in some areas, and is highly restricted in others, it is estimated that 4.6 Tcf of gas is flared worldwide every year, the approximate annual consumption of Germany and France combined.

The social and environmental cost of this waste could be partially alleviated if the gas could be stored in some way. Liquid natural gas (LNG) is one option, but the necessary investment requires large deposits of gas, like 1–3 Tcf, to make it worthwhile. Making the gas into methane hydrate, on the other hand, would produce a more stable form, which could be stored in insulated containers and transported by truck or rail. And it’s quite possible that the economics could work, because it’s cheaper to make hydrate than LNG, and easier to store.

To make hydrates, you chill ice down to 250°K (–23°C), pretty darn cold, but readily achievable with modern refrigeration. The ice is then pulverized and sieved to produce very small uniform ice pellets. This is called “seed ice”.  At that temperature, the ice crystals don’t clump together. Natural gas is pumped into the vessel, and the pressure raised to around 25 Mpa (3,600 psi). The temperature is slowly raised to 290°K (17°C, called “room temperature” in Britain) where methane hydrates readily form, and absorb the maximum amount of methane. The temperature is dropped again back to 250°K, conditions that make it highly stable. In this form it can be transported by regular refrigerated trucks, as opposed to the highly specialized LNG containers.

The energy required to make hydrates isn’t free, but some of the stranded gas could be used to power the various pumps and generators the process requires.

Dry water. One limitation on making hydrates is time. The ice crystals absorb methane rather slowly, on a scale of hours. To make the process more practical, it would need to be faster. One intriguing possibility is dry water (DW). This seeming oxymoron is a substance discovered in 1968 that combines water and silica. The water is atomized into microscopic droplets, each of which is completely encased in microparticles of hydrophobic silica. The silica prevents the droplets from touching each other and forming a liquid. This is called an “inverse foam” by  scientists,  a free-flowing, fine-grained, apparently dry stuff that resembles talcum powder, but is 95% water. 

A couple of years ago, researchers demonstrated how it is possible to turn the water in DW into hydrates. Under comparatively moderate temperatures and pressures, 2.7 Mpa (290 psi) and 0°C, the dry water crystals absorb methane readily and more quickly. Also, the result is more stable, with higher tolerances for storage and transport.

Potentially, the same method could be used to sequester CO2 or to transport hazardous chemicals in a safer form. There  are many exciting possibilities for dry water—not bad for a substance that was once of interest only to the cosmetics industry (for combining water-dispersible pigments and extracts into powered products).

Lab results suggest that by adding a gelling agent to the water before creating the hydrates, the DW could be made recyclable, able to go through multiple cycles of absorption and release. This is a crucial factor in the economics.

Admittedly, all this is pretty speculative. But with stranded natural gas equal to a fourth of US consumption being flared in remote locations every year, the industry needs some good ideas. 

henry.terrell@gulfpub.com