Geoscience Australia - Carbon Capture and Storage
Narrator: Concern about global warming as a result of the increase in greenhouse gas emissions has become a major community issue.
Carbon dioxide is a key greenhouse gas, and since the industrial revolution the atmospheric concentration of carbon dioxide has increased by almost 40 per cent. This increase is mainly due to the burning of fossil fuels, and in Australia over 80 per cent of our electricity is generated from fossil fuels - mainly coal. Carbon dioxide is also a significant by-product of many industrial processes, such as the production of iron, steel and cement.
Carbon Capture and Storage is one of a range of solutions that can help reduce greenhouse gas emissions by capturing much of the carbon dioxide at the source of emissions and permanently storing them deep underground.
In a conventional coal-fired power station, carbon dioxide is released directly into the atmosphere with other flue gases. Power stations equipped with carbon capture technologies however, will be able to separate out much of the carbon dioxide at the source of the emissions.
The captured carbon dioxide gas is then highly compressed turning it into a supercritical fluid: a state where it has the density of a liquid, but flows like a gas. The supercritical carbon dioxide is then piped under pressure to a suitable injection site for permanent underground storage. Suitable geological storage sites are found in sedimentary basins where layers of sediments have been laid down over millions of years.
Some sedimentary rocks like sandstones are porous meaning that there is a lot of space between the sand grains that can be filled with fluids such as fresh or salty water, oil, natural gas or carbon dioxide. The pores are also interconnected allowing fluids to slowly move through the permeable rock. Around 30 per cent of the total volume of some sandstones or reservoir rocks can be filled with these fluids. Other rocks such as mud stones and shales are much less permeable than sandstones, their structures restricting fluids from flowing through them. These rocks are good sealing layers or cap rocks trapping fluids in the reservoir rocks below.
From studying sedimentary basins around the world, geologists know there will be suitable sites for permanently storing carbon dioxide as oil, gas and naturally occurring carbon dioxide have been trapped in these structures for tens or hundreds of millions of years. Back at the surface, the supercritical carbon dioxide is injected under high pressure into the sandstone reservoir rocks more than 1,000 metres underground where a number of physical and chemical processes then take place.
First, the injected carbon dioxide moves slowly up through the pores of the sandstone until it is physically trapped under the cap rock layer and spreads out beneath it. Secondly, as the main plume moves through the rock, some of the carbon dioxide is left behind and becomes trapped in the pore spaces of the sandstone. This is called residual trapping. The third process occurs over a few hundred years as the carbon dioxide slowly dissolves into the salty water within the rocks. This new solution is slightly denser than the surrounding water and moves slowly down through the rock strata. The carbon dioxide is now chemically as well as physically trapped in the reservoir. Finally, some of the carbon dioxide will react with the salty water and surrounding rocks to form new minerals.
Careful site selection extensive modelling and ongoing monitoring of the storage site will ensure that the carbon dioxide has been permanently stored and is behaving as expected. Techniques such as time lapse seismic imaging can be used to track the migrating carbon dioxide plume in the reservoir rocks.
Deep monitoring wells allow us to monitor the pressure in the storage area and to sample sub-surface water and gases, while a network of water bores is used to monitor the quality of underground water above the reservoir. At the surface, soil, water and atmospheric monitoring takes place to detect any changes from baseline carbon dioxide levels.
As we continue to explore and develop cleaner renewable energy technology such as solar, geothermal, wind, hydro and tidal power, fossil fuel such as coal, oil and natural gas, will still be required to supply our baseline energy needs for the coming decades. Carbon capture and storage together with increased take up of renewable energy is a part of the solution to greenhouse gas emissions, especially in Australia where most of our electricity comes from fossil fuels.
Carbon capture and storage uses long established technologies from the petroleum industry to permanently store carbon dioxide in rocks just like those, which have stored oil and natural gas for millions of years. Several commercial scale geological storage projects are operating around the world today.
Carbon capture and storage has the potential to make the use of fossil fuels, and specific industrial processes, carbon dioxide emissions free.