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.
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.