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Booklet-Geologic Storage of CO₂

What Is Geosequestration?

The fossil fuels, coal, oil and natural gas, currently supply around 85% of the world's energy needs. The International Energy Agency predicts that fossil fuels will continue to be heavily used for many years to come.

The burning of fossil fuels is a major source of excess CO₂, the gas that has most contributed to the increased concentration of greenhouse gases in the atmosphere.

There is an urgent need to reduce the atmospheric concentrations of greenhouse gases that are likely to produce rapid, human-induced climate change.

We can decrease our greenhouse gas emissions through increased energy efficiency, switching to lower carbon-intensive fuels, making greater use of renewable energy and through geosequestration, the long-term geological storage of CO₂.

Capturing CO₂

The capture of CO₂ from a stationary source, such as, a power plant involves separating and purifying CO₂ from the bulk of the flue gas stream rather than allowing it to be released to the atmosphere. The purified CO₂ stream is then available for geological storage.

The main sources suitable for CO₂ capture are: industrial processes, electricity generation, and in the future, hydrogen production from fossil fuel sources.

Industrial processes that lend themselves to CO₂ capture include natural-gas processing, ammonia production, and cement manufacture, however the total quantity of CO₂ produced by these processes is relatively small. A far larger source of CO₂, accounting for one-third of total CO₂ emissions in Australia, is fossil-fuelled electricity generation. Research is underway on the capture of CO₂ from this source.

Technologies for capturing CO₂ from electricity generation fall into three categories: post-combustion, pre-combustion, generally referred to as Integrated Gasification Combined Cycle or IGCC, and oxyfuels (in which a power plant's fuel is burnt in oxygen rather than air).

A range of capture equipment and technologies can be used in these processes. Research is underway into the four main technologies namely:

• Absorption, where CO₂ is selectively absorbed into liquid solvents;
• Membranes, where CO₂ is separated by semipermeable plastic or ceramic membranes;
• Adsorption, where CO₂ is separated using specially designed solid particles; and
• Low Temperature Processes, where separation is achieved by chilling and/or freezing the gas stream.

While the capture of CO₂ for geosequestration is a relatively new concept, CO₂ capture for commercial markets has been practised in Australia and overseas for many years. CO₂ is captured from natural gas wells in south-east South Australia, near Mt Gambier and in southern Victoria, near Port Campbell. The CO₂ is then used for various commercial processes including carbonation of beverages and dry-ice production.

In the United States CO₂ capture at power plants using chemical absorption solvent has been practised since the late 1970s, with the captured CO₂ being used for enhanced oil recovery. There are plans in the United States to build the world's first integrated gasification combined cycle plant, known as FutureGen, that will not only produce electricity but also hydrogen fuel, with the CO₂ generated in the process being captured and sequestered.

Following capture, CO₂ is usually transported from the source, such as a power station, to the geological storage site in a compressed form via a pipeline. It can also be transported by truck, rail or ships depending on the location of both the source and the geological storage site and injected via pipeline deep underground.

Storing CO₂

Geological storage of CO₂ secures the gas deep underground in a geological reservoir. In addition to the careful selection of a suitable geological reservoir, a comprehensive monitoring system is required initially to ensure that the gas is safely contained. Geological reservoirs into which CO₂ can be injected include depleted oil and natural gas fields, and deep saline formations.

Since the stored CO₂ will be less dense than the water in and around the reservoir rocks, it needs to be geologically trapped to ensure that it does not reach the surface. The exact trapping mechanism depends on the geology.

In depleted oil and gas reservoirs geological traps contain the CO₂. In some cases these are anticlines, in other cases fault traps.

In the case of deep saline formations with no distinct geological traps, an impermeable caprock above the underground reservoir is needed and the CO₂ is contained by the groundwater flow. This is known as hydrodynamic trapping.

Solubility and mineral trapping are two other very important mechanisms. Solubility trapping involves the dissolution of CO₂ into the saline water in the reservoir. In mineral trapping the CO₂ reacts with minerals in the rocks to form stable carbonate minerals. The first commercial-scale project dedicated to CO₂ storage in a geological reservoir has been in operation at the Sleipner West Field since 1996. Sleipner West is a natural gas field with a high concentration of CO₂. It is operated by Statoil and located in the North Sea about 250 km off the coast of Norway.

CO₂ is usually transported from a source, such as a power station, to the geological storage site in a compressed form via a pipeline. It can also be transported by truck, rail, or in the case of a geological storage site, deep beneath the seabed, by ocean tanker.

It is injected from a tanker, truck or pipeline deep underground into the geological reservoir.

For further information on geosequestration see Cooperative Research Centre for Greenhouse Gas Technologies. Additional information on energy can be found at the Department of Primary Industries