SIX hundred metres down in the black stillness of the Gulf of Mexico, Roger
Sassen gazes out through the Plexiglass dome of his submersible. In front of
him, held in the sub’s bright spotlights, is a mechanical arm. And gripped in
the arm’s shiny metal claw is a long plastic flask which Sassen has positioned
to capture the tiny silver bubbles of gas streaming up from a crack on the
seafloor. With a speed that startles him, the gas turns to a solid plug of pale
yellow crystals in the neck of the flask. This material, he knows, is
hydrate—an icy solid composed of methane and water that forms in the
crushing pressures of the deep sea. But to Sassen, this mass is a crystal ball,
and in it he sees a vision of a world powered by methane.
Huge deposits of the same frozen hydrate that Sassen created lie beneath the
seabed, on the margins of the world’s continents. While known oil reserves will
be gone in about 40 years, and natural gas a few decades later, global reserves
of methane hydrate—estimated to be more than 20 000 trillion cubic
metres—amount to twice the world’s predicted total energy reserves in the
form of coal, oil and natural gas. It is enough to supply the world’s energy
needs for centuries. “I think that eventually hydrate companies are going to
replace oil companies,” predicts Sassen, a professor at Texas A&M University
in College Station and former head of geochemistry research for Texaco and Getty
Oil. And Sassen aims to bring this fuel to the marketplace.
Like crude petroleum and natural gas, the methane that makes up the hydrates
has been formed from the remains of living things. For thousands of years,
bacteria in sediments deep below the seabed have been munching their way through
layers of organic-rich material, releasing methane as they feast. As the gas
bubbles upwards, it reaches water-bearing sediments. And where methane gas and
water meet, the intense pressures and low temperatures—just a few degrees
centigrade—encourage the formation of hydrate: molecules of methane
trapped inside dodecahedral-shaped ice cages
(see Diagram p 40).
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Geologists’ dizzying estimates of the volume of methane available from this
source—each cubic metre of hydrate yields more than 160 cubic metres of
methane—have caught the petroleum industry’s attention. But the problems
are finding, extracting and then transporting the gas. Unlike the gushing
oilfields of Texas or the Middle East, most of the world’s hydrate deposits are
spread over many thousands of square kilometres deep beneath the seabed. Hydrate
hunters need a way to locate the most promising deposits. And having found them,
you can’t just dig the hydrate out, because most of these deposits lie hundreds
of metres beneath the seabed. Finally, having extracted the gas, it has to be
transported to where it is needed.
One of the best techniques for surveying large areas of the seafloor is
seismic profiling. This involves beaming pulsed sound waves into the sediments,
and detecting the waves that reflect back from boundaries where the density or
porosity of the rock beneath change abruptly. Layers of hydrate are easily
spotted. The transition between hydrate-bearing sediments and those below them
containing only gas and water cause a very strong reflection. Above this
boundary, crushing pressure and low temperature force water molecules to form
hydrate. Below it, heat from the Earth’s interior prevents crystallisation and
the methane remains free. But while finding these deposits is one thing,
measuring the amount of hydrate that is present is more difficult.
Seismic profiling uses a mathematical model to translate the strength of
reflected sound waves directly into hydrate concentrations. The model relies on
assumptions about how fast sound waves travel through hydrate-bearing sediments
and how much sound energy is absorbed along the way. The only way to firm up
these assumptions is to bounce sound through hydrate-loaded sediments in the
lab. But you can’t just bring lumps of the stuff up from the depths. Expose a
chunk of hydrate to atmospheric pressure and it pops, fizzes and bubbles away to
nothing in front of your eyes.
To get around this, Bill Dillon and his colleagues at the US Geological
Survey in Woods Hole, Massachusetts, have devised a pressurised apparatus called
GHASTLI—the Gas Hydrate And Sediment Testing Laboratory
Investigation—which allows them to grow hydrates in different kinds of
sediment and measure how fast sound travels in them. With a good understanding
of the acoustic properties of hydrates, they should be able to come up with
better ways of finding the richest fields. Dillon’s aims are clear: “I would
like to be able to run a ship across the surface of the ocean and tell you with
remote sensing how much hydrate is down there,” he says.
Getting it out
Having found the biggest reserves of hydrate, the next problem is how to
extract methane from them. In most cases, simply drilling a hole in the hope
that the gas will flow out simply won’t do. The main problem is what oil people
call “poor reservoir conditions”. At the Blake Ridge, a giant hump of sediment
in the Atlantic Ocean off the coast of the Carolinas, the hydrate zone begins
200 metres below the crest of the ridge—itself in 2800 metres of
water—and continues down for another 250 metres. The Blake deposits may
hold a trillion cubic metres of gas, but the hydrate makes up only 5 per cent of
the sediment—most of which is essentially wet, clay-like mud. Drawing
methane from a well on the Blake Ridge would be like drawing a breath through a
big wad of cotton. “They’ve got a lot there, but it’s very dispersed,” says
Dendy Sloan, a leading hydrate researcher at Colorado State University in Fort
Collins. “Because it’s so dispersed, it’s questionable whether it could be
done.” To date, only a handful of sites have been studied in enough detail to
assess their potential as reservoirs.
Things are a lot easier when hydrates lie above natural gas reservoirs that
are already being tapped. The most productive natural gas fields are found in
porous sedimentary rock such as sandstone. These pressurised reservoirs behave
like an inflated tyre: drilling a well into the reservoir releases a hissing
flood of gas. There are some hydrate deposits on land known to occur in this
type of reservoir—the North Slope of Alaska, for example. There, where the
land slopes northwards towards the sea, methane hydrate occurs in six sandstone
beds that alternate with shale. The rock is 30 to 40 per cent hydrate by volume,
containing an estimated trillion cubic metres of methane.
Because the rock is porous, hydrate reservoirs on the North Slope could be
tapped with a method called depressurisation, says Timothy Collett, a hydrate
researcher with the US Geological Survey in Denver, Colorado. Sinking a well
into the free gas zone beneath the hydrate layer would lower the pressure in the
reservoir. This reduced pressure would allow the bottom of the hydrate “cap”
above it to decompose into gas, which would expand into the free gas zone below.
“If you have a significant natural gas field supporting the project, the
hydrates come along for the ride,” Collett says.
Earlier this year, the Japanese National Oil Corporation and the Geological
Survey of Canada drilled an exploratory well into gas hydrates beneath the
Mackenzie Delta in the far northwest of Canada, just across the border from
Alaska. Next year, the Japanese aim to start producing gas from hydrate
reservoirs in the East China Sea. India, too, is active in hydrate research, and
is selling offshore leases for gas hydrate exploitation.
But depressurisation doesn’t really tap the hydrate’s full potential.
Compared with the flow from conventional free gas reservoirs, depressurisation
would produce a mere trickle. “If you really want to get serious about producing
large volumes of gas from hydrate, you have to change its temperature,” Collett
says.
The idea would be to pump steam or hot water down the well into the frozen
hydrate. Something similar is already done routinely to recover stickier grades
of crude petroleum. One technique is known as the “five spot”. Four wells are
drilled in a square with a fifth in the centre. Hot liquids pumped in at the
corner wells will decompose the hydrate and push it toward the middle well.
According to theoretical studies, Collett says, this method could produce
millions of cubic metres per day, comparable to the best natural gas fields.
Having mined the hydrate, the final problem is getting the methane to where
you need it. Most hydrate deposits are beneath the seabed, and undersea
pipelines like those that bring natural gas ashore are extremely costly. “Gas is
very cheap at a wellhead, but once you move it somewhere it gets really
expensive,” Collett says.
In the Gulf, exploratory drilling has now reached beyond 3000 metres. But
with greater depth come greater hazards, Sassen says. The lower slopes of the
Gulf are unstable and irregular, which creates the risk of setting off
underwater landslides that would not only destroy the hydrate wellheads, but
would tear billion-dollar pipelines apart as well. “The hydrate can actually
cement the sediments together,” Collett says. Take the hydrates out of the
sediments, and they “could fail in a catastrophic landslide”.
This is one of the reasons that prompted Sassen to venture into the depths in
his submersible in 1995. He was determined to show that the conditions that
formed the hydrate in the first place could be used to convert the gas back into
compact, solid pellets. So on the night before the dive, he cut scratches on the
inside of the plastic flask to provide nucleation sites for the hydrate
crystals. And as he discovered, the reaction is fast and simple: at pressures of
more than 40 atmospheres, and if the water is not too warm, gas plus water
equals hydrate.
Easy as ice cream
Converting methane back into solid hydrate pellets would do away with the
need for expensive and vulnerable pipelines. The conversion would be as easy as
making ice cream. Just swirl gas and cold seawater together at the right depth,
and it turns into mushy hydrate. Sassen envisages transporting the hydrate in
torpedo-shaped dirigibles or gas blimps 300 metres long, made from a rigid frame
covered with light, strong material. A plant on the seabed could fill the blimps
with pellets, and a small refrigeration unit would keep them cool. Sassen
believes the gas blimp could then be towed by a submarine tug to shallower
waters, where it would dock, storing the methane for later use.
Sassen thinks gas-to-hydrate conversion could also allow untouched deep-water
reserves to be exploited. He points out that existing deep-water platforms could
be converted to produce hydrates when they are no longer needed for conventional
production. “I do not wish to understate the engineering problems involved,”
Sassen says, “but it appears easier than getting to Mars.”
But Collett thinks there may be a better solution already under development.
It’s a chemical process called methane liquefaction, which converts methane gas
into a kerosene-like liquid that is stable at room temperature. The methane is
first partially burnt to form a mix of hydrogen and carbon monoxide called
syngas. This is then exposed to a catalyst—typically iron or
cobalt—which encourages larger hydrocarbon molecules to form. Collett
suggests that ships or floating drilling rigs could convert the gas at sea, and
transfer the liquefied material directly to tankers. The downside is the low
efficiency of the process: 35 per cent of the gas is lost, mostly at the initial
heating stage.
Despite the hurdles still to be overcome, even the sceptical Sloan—wary
of what he calls “blue sky” schemes such as Sassen’s gas blimps—can’t
ignore the siren call of gas hydrate. “There’s a hell of a lot out there,” he
says. “You just have to be intelligent about getting it out.” Joe Curiale, a
geochemist at Unocal, an energy company based at Sugar Land, Texas agrees: “As
oil gets used up, the methane out there will become more attractive.”
But we may not have too much choice. “I think people can see the writing on
the wall,” Sassen says. “It’s obvious that hydrocarbons are getting harder to
find and they’re costing more.” Asked if he can imagine a future without energy
from gas hydrate, Sassen has a ready answer. “Sure. Go back to the Stone
.”
- For more information see:
http://abacus.er.usgs.gov/hydrates/index.html