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How
much stress can a poor rock stand?
Berkeley engineers test how water temperature affects geothermal
energy
By Sally Stephens
In the heart of Sonoma County's lush wine country, just
34 miles northeast of San Francisco, engineers have tunneled under
the Russian River to carry treated wastewater from the city of
Santa Rosa to the Geysers geothermal fields — the largest
producer of geothermal energy in the world.
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| Before firing up their
4,000-pound pressure chamber, Steve Glaser, left, and Jeff
Moore prepare to hoist the device some five feet into the
air, where it will dangle as it is carefully wrapped in an
insulating blanket, allowing researchers in the rock mechanics
lab to work around it safely. Peg Skorpinski photo |
Steam-driven turbines at the Geysers produce enough electricity
to meet the daily needs of some one million people throughout
the state of California. But, for the past 15 years, the amount
of steam available to fuel the geothermal plants has been decreasing,
as the reservoir beneath the Geysers dries up, a victim of overpumping.
As a result, energy production has slipped and is in danger of
grinding to a halt.
In search of solutions, plant operators at the Geysers, in partnership
with the Lake County Sanitation District, came up with an innovative
plan to restore and maintain adequate levels of underground water
to ensure continued energy production. They decided to inject
tertiary-treated wastewater, which is completely safe to drink,
from the Lake County communities surrounding Clear Lake into the
ground at the Geysers -- 30 miles to the south -- to replenish
the underground reservoirs.
For the past four years, some 7.8 million gallons of water each
day have been injected at the Geysers. With the addition of the
Santa Rosa wastewater project, expected to be completed this year,
an additional 11 million gallons of water will be added each day.
But while all the operational details to replenish the water reserves
seemed in place, according to Steven
Glaser, Berkeley professor of civil and environmental engineering,
one crucial element had been overlooked.
"I was shocked when a colleague told me that nobody really
knew what happened when cooler water, like the injected wastewater,
hit very hot rocks," says Glaser. "Will the fracturing
of the hot rock that inevitably results from the cold water injection
disrupt or block the movement of steam or increase it?" he
wondered. "Despite years of water injection in geothermal
fields like the Geysers, no one knows. Certainly, you don't want
to damage your reservoir."
| "Nobody
really knows what happens when cooler water, like the injected
wastewater, hits very hot rocks." |
So, Glaser assembled a research team, secured funding from the
U.S. Department of Energy and Shell International Exploration
and Production Co. (they are interested in drilling under high
temperatures), designed new lab equipment, and began a series
of controlled laboratory experiments testing rocks similar to
those found in the Geysers.
Working with graduate student Jeff Moore, Glaser assembled a unique
testing device now located along one wall of his rock mechanics
lab. Inside the testing device, flat bladders filled with pressurized
oil squeeze a rock sample up to 1,500 pounds per square inch (psi),
matching pressures found deep underground. Glaser and Moore saturate
the pressurized, heated sample with steam, up to 300 psi, pumping
the temperatures up to more than 200 degrees Celsius -- almost
400 degrees Fahrenheit. Finally, they inject water at room temperature
to a well-point drilled into the rock. Then they wait, and watch.
Glaser and his colleague Frank Morrison, professor of civil and
environmental engineering, monitor the movement of the cooler
water through the hot rock, recording any fracturing. Sensitive
acoustic sensors arrayed around the rock sample inside the heater
detect the telltale sounds that indicate fracture has occurred.
"Every time rock cracks," says Glaser, "it generates
sound waves, little snaps, crackles, and pops."
"The sensors, as they stand right now," Glaser adds,
"are not going to like 200 degrees Celsius. We have to find
different materials, with higher heat resistance, out of which
to make them." Using a technique that triangulates signals
from multiple sensors, the engineers should be able to create
a three-dimensional map of the inside of the rock, showing the
placement and size of new fractures.
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| It takes about three
hours to heat up Glaser's pressure chamber. Gauges control
the temperature and pressure for the water injection system
used to put the rock samples through their thermal paces.
Peg Skorpinski photo |
But it's not enough to know where new fractures occur. Glaser
and Morrison want to know how the cold water flows through and
interacts with the hot steam already in the rock.
To that end, the team will measure changes in the rock sample's
ability to transmit electrical currents, or its resistivity, as
the cold water moves through the heated rock sample. "Minerals
in rock act like insulators," Morrison explains. "Current
in rock flows through any water inside it." Resistivity increases
when rock is cold, and decreases when it is hot, since cold water
is less likely to conduct a current. Thus changes in resistivity
should indicate where areas of cool water and hot steam are as
they flow through the rock. "I've measured resistivity on
blocks of rocks before," Morrison adds, "but not under
conditions of such high temperatures and pressures, or with injected
water."
These conditions, which match those occurring deep underneath
the Geysers, as well as the substantial size of the rock samples
used (10.25 inches on a side) distinguish Glaser's experiment
from other rock tests. "Usually test rock samples are about
the size of your thumb," he says. "At Stanford, for
example, they do tests on thumb-sized rock samples that they can
heat to 105 degrees Celsius. Our experiment will run more than
twice as hot.
"Once the experiment is up and running this spring, it will
help heat the notoriously chilly lab in the Davis Hall annex,"
jokes Glaser, adding that he is concerned future testing could
be jeopardized because of the pending annex demolition.
Preliminary tests will be done on a sample of Berea sandstone,
a well-understood rock often used in petroleum testing. Then,
as the real testing begins, the team will collect rock samples
at the Geysers. "You consult a geologic map, drill some cores,
and find out where on the surface there are outcroppings of rock
similar to what's in the steam-producing zone underground,"
says Moore.
Glaser's one-of-a-kind testing device will also study fracturing
in hot dry rocks as well as those filled with steam. Engineers
in Japan and Europe regularly inject water into the hot dry rocks
deep in geothermal areas there, hoping to generate steam to produce
electricity. But, as with the Geysers, more testing must be done
under realistic conditions. Glaser's device also mimics conditions
encountered during petroleum drilling, and tests what happens
when engineers inject steam to clean up polluted underground sites.
The results of Glaser's and Morrison's tests will become
even more important once Santa Rosa's wastewater begins flowing.
Knowing what's really happening underground as the chilly
water hits hot rocks filled with steam will enable geothermal
plant operators around the world to continue to generate desperately
needed electricity for years to come.
Written by Sally Stephens, a freelance astronomy
writer based in San Francisco. Formerly a staff scientist and
editor of Mercury, she co-authored The Sporting Life,
a book on the physics of sports.
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