During the massive oil spill from the ruptured Deepwater Horizon well in 2010, it seemed, at first, like there might be a quick fix – a containment dome lowered onto the broken pipe to capture the flow so it could be pumped to the surface and disposed of properly. But that attempt quickly failed, because the dome almost instantly became clogged with frozen methane hydrate.
hydrates, which can freeze upon contact with cold water in the deep ocean, are
a chronic problem for deep-sea oil and gas wells. Sometimes these frozen
hydrates form inside the well casing, where they can restrict or even block the
flow, at enormous cost to the well operators.
researchers at MIT, led by associate professor of mechanical engineering Kripa
Varanasi, say they have found a solution, described recently in the journalPhysical Chemistry Chemical Physics. The paper’s lead author
is J. David Smith, a graduate student in mechanical engineering.
sea is becoming “a key source” of new oil and gas wells, Varanasi says, as the world’s energy demands
continue to increase rapidly. But one of the crucial issues in making these
deep wells viable is flow assurance – finding ways to avoid the buildup of
methane hydrates. Presently, this is done primarily through the use of
expensive heating systems or chemical additives.
and gas industries currently spend at least $200 million a year just on
chemicals” to prevent such buildups, Varanasi says; industry sources say the
total figure for prevention and lost production due to hydrates could be in the
billions. His team’s new method would instead use passive coatings on the
insides of the pipes that are designed to prevent the hydrates from adhering.
hydrates form a cage-like crystalline structure, called clathrate, in which
molecules of methane are trapped in a lattice of water molecules. Although they
look like ordinary ice, methane hydrates form only under very high pressure: in
deep waters or beneath the seafloor, Smith says. By some estimates, the total
amount of methane (the main ingredient of natural gas) contained in the world’s
seafloor clathrates greatly exceeds the total known reserves of all other
fossil fuels combined.
pipes that carry oil or gas from the depths, methane hydrates can attach to the
inner walls – much like plaque building up inside the body’s arteries – and, in
some cases, eventually block the flow entirely. Blockages can happen without
warning, and in severe cases, require the blocked section of pipe to be cut out
and replaced, resulting in long shutdowns of production. Present prevention efforts
include expensive heating or insulation of the pipes or additives such as
methanol dumped into the flow of gas or oil. “Methanol is a good inhibitor,” Varanasi says, but is
“very environmentally unfriendly” if it escapes.
Varanasi’s research group began looking into the problem before the
Deepwater Horizon spill in the Gulf of Mexico.
The group has long focused on ways of preventing the buildup of ordinary ice – such
as on airplane wings – and on the creation of super-hydrophobic surfaces, which
prevent water droplets from adhering to a surface. So Varanasi decided to explore the potential for
creating what he calls hydrate-phobic surfaces to prevent hydrates from
adhering tightly to pipe walls. Because methane hydrates themselves are
dangerous, the researchers worked mostly with a model clathrate hydrate system
that exhibits similar properties.
produced several significant results: First, by using a simple coating, Varanasi and his
colleagues were able to reduce hydrate adhesion in the pipe to one-quarter of
the amount on untreated surfaces. Second, the test system they devised provides
a simple and inexpensive way of searching for even more effective inhibitors.
Finally, the researchers also found a strong correlation between the hydrate-phobic
properties of a surface and its wettability – a measure of how well liquid
spreads on the surface.
findings also apply to other adhesive solids, Varanasi says – for example,
solder adhering to a circuit board, or calcite deposits inside plumbing lines –
so the same testing methods could be used to screen coatings for a wide variety
of commercial and industrial processes.