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Aerial view of the Otway Project in Australia. Image: CO2CRC. |
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A demonstration project on the southeastern tip of Australia has
helped to verify that depleted natural gas reservoirs can be repurposed for
geologic carbon sequestration, which is a climate change mitigation strategy
that involves pumping carbon dioxide deep underground for permanent storage.
The project, which includes scientists from
Lawrence Berkeley National Laboratory (Berkeley Lab), also demonstrated that depleted
gas fields have enough carbon dioxide storage capacity to make a significant
contribution to reducing global emissions.
Geologic carbon sequestration involves capturing
carbon dioxide from large stationary sources, such as coal-burning power
plants, and injecting it deep underground into rock formations that trap the
greenhouse gas. The technology holds promise as a way to curb climate change
because fossil fuels will likely remain cheap and plentiful for
decades to come.
Scientists are looking to depleted gas reservoirs
as a possible target for carbon sequestration because the reservoirs have a
proven ability to store gas. The same caprock that trapped natural gas for millions
of years can also trap carbon dioxide. Depleted gas reservoirs also provide
some of the infrastructure needed for injection, such as boreholes and a
pipeline network.
They’re also plentiful. A 2009 report by the
International Energy Agency Greenhouse Gas R&D Program estimates that 160
gigatons of capacity in depleted gas fields—matched to point sources—will be
available by 2050.
But the science of storing carbon dioxide in
depleted gas fields has needed real-world verification, which is why the CO2CRC
team started the Otway Project. The group developed computer models to track
the subsurface flow of carbon dioxide at the site. They conducted
risk assessment analyses and met with members of the surrounding community.
They also blanketed the site with a network of soil, groundwater, air, and
subsurface monitoring equipment.
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Geological cross-section of the Otway Project. Carbon dioxide-rich gas is extracted from the Buttress well (on the left), injected into the depleted gas field using CRC-1, and the Naylor-1 well houses the monitoring equipment installed by Berkeley Lab scientists. Faults are black lines. |
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As part of this effort, Freifeld and fellow
Berkeley Lab scientist Tom Daley traveled to Otway in 2007, before carbon
dioxide injection began, to help oversee the installation of fluid sampling
tools called U-tube samplers. The apparatus was developed at Berkeley Lab and
enables the collection of subsurface fluids at the same pressures that occur
deep underground, preserving the samples’ chemical integrity during the
collection process.
Three U-tube samplers were lowered deep into a
borehole that was once used to extract natural gas, but is now used for
monitoring purposes only. Once the operation began in 2008, technicians
collected U-tube samples almost weekly. These samples allowed scientists to
monitor the carbon dioxide as it filled the reservoir.
Freifeld and Daley also installed extremely
sensitive seismic and acoustic monitoring equipment in the borehole that
enabled technicians to "image" the movement of carbon dioxide within the
reservoir.
"We found what we expected. The carbon dioxide
largely replaced the volume previously occupied by the natural gas," says
Freifeld. "The reservoir had filled with water since the natural gas was
extracted, and we watched as the injected carbon dioxide pushed the water to a
level below our instruments."
Additional calculations conducted by the CO2CRC
team predict that between 56% and 84% of the space originally occupied by the
natural gas is now reoccupied by carbon dioxide. These findings help buttress
the conclusion that depleted gas fields have enough storage capacity to make a significant
contribution to reducing global emissions, the authors say in their Proceedings
of the National Academy of Sciences paper.
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