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.
Methane
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.
Now
researchers at MIT, led by associate professor of mechanical
engineering Kripa Varanasi, say they have found a solution, described
recently in the journal Physical Chemistry Chemical Physics. The paper’s lead author is J. David Smith, a graduate student in mechanical engineering.
The
deep 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.
“The
oil 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.
These
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.
Inside
the 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 superhydrophobic 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.
The
study 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.
The
basic 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.
Richard
Camilli, an associate scientist in applied ocean physics and
engineering at Woods Hole Oceanographic Institution who was not involved
in this study, says, “The energy industry has been grappling with
safety and flow-assurance issues relating to hydrate formation and
blockage for nearly a century.” He adds that the issue is becoming more
significant as drilling progresses into ever-deeper water and says the
work by Varanasi’s team “is a big step forward toward finding more
environmentally friendly ways to prevent hydrate obstruction in pipes.”
The
research team included MIT postdoc Adam Meuler and undergraduate
Harrison Bralower; professor of mechanical engineering Gareth McKinley;
St. Laurent Professor of Chemical Engineering Robert Cohen; and Siva
Subramanian and Rama Venkatesan, two researchers from Chevron Energy
Technology Company. The work was funded by the MIT Energy
Initiative-Chevron program and Varanasi’s Doherty Chair in Ocean
Utilization.
Hydrate-phobic surfaces: fundamental studies in clathrate hydrate adhesion reduction
