What is CCS?

Carbon dioxide capture and geological storage (CCS) is one of several key greenhouse gas mitigation technologies . Burning fossil fuels such as coal, natural gas and oil releases carbon dioxide (CO₂) and other greenhouse gases (GHG) to the atmosphere adding to the potential for climate change. Australia and China are both heavily reliant on fossil fuels, particularly coal, to meet our energy needs. Our use of fossil fuels for energy results in a large percentage of our total greenhouse gas emissions coming from electricity production, manufacturing and refining and other energy intensive industries. Our abundant supply of fossil fuels and the increasing domestic demand for continued low cost energy means that the use of this energy source for electricity generation will increase. CCS is a key technololgy for near-term clean, sustainable energy in a portfolio that also includes a suite of low emission energy technologies such as solar and wind energy, geothermal energy, hydroelectric power, and nuclear power.

Geological storage is the process of capturing CO₂ from stationary emission sources such as power stations or industrial facilities and injecting it as a dense, liquid-like fluid into deep geological formations, thus preventing it from entering the atmosphere. Sedimentary rocks, particularly sandstones, contain large volumes of fluids (these include: water, hydrocarbons, CO₂, and other gases) held in microscopic voids or pores between rock grains. These pores can form up to 30% of the rock volume. Where the pores are interconnected the rock has permeability, that is, fluids can flow through it. Deep in the geological section, rocks like sandstones are usually filled with highly saline water that moves very slowly over millions of years. They are called deep saline reservoirs, and they are the "containers" proposed for storing greenhouse gases because they are too deep and too saline for any other practical use.

CO₂ injected into a saline reservoir becomes trapped in the rock through a number of mechanisms. Initially the CO₂, which is less dense than water, rises buoyantly through the reservoir until it meets a barrier – an impermeable cap rock (the seal, or "lid", to the reservoir) such as a mudstone or shale. The CO₂ will accumulate under the cap rock and spread out laterally beneath it. Some of the CO₂ will be caught in pores between grains of rock, and will not move any further. Over time, a significant portion of the rest of the CO₂ will dissolve into the saline formation water and be stored in solution while some of the CO₂ and water will react with minerals in the rock to precipitate new minerals. Geological storage sites are carefully selected and characterised to ensure that a suitable cap rock is present to prevent CO₂ from migrating out of the designated reservoir.

The most suitable reservoir and cap rocks are found in sedimentary basins, and particularly in hydrocarbon producing basins. In general, deep saline reservoirs have the greatest potential capacity to store CO₂, because they are widespread, large, and presently not used for other purposes. Depleted oil and gas fields may also be used to store CO₂ and CO₂ may be used to enhance oil recovery from oil fields. Researchers are also investigating the possibility of storing CO₂ in deep coal seams, basalts, shales, as CO₂ hydrates beneath the sea floor, and through mineral carbonation. Studies of hydrocarbon accumulations around the world have shown that fluids (oil, gas, water, CO₂) have remained trapped in deep geological formations and structures for tens to hundreds of millions of years. This gives us confidence that injected CO₂ can be securely stored in similar geological settings for similar amounts of time. Demonstrating the security and safety of storage before, during and post injection is of particular concern to government, industry and the public. Potential points of leakage include faults, cap rocks, and pre-existing petroleum wells. The former two are mitigated through good geological understanding of an injection site, while the latter is mitigated through careful design and engineering. In addition, both new and existing techniques are being used to track CO₂ in the subsurface, including seismic imaging, down-hole pressure measurement and gas and water sampling, and shallow aquifer groundwater sampling. Surface monitoring techniques such as atmospheric and soil gas sampling will ensure that in the unlikely event that any CO₂ migrates to the surface it will be detected and remedied immediately.

Capture, injection and geological storage of CO₂ is an established process in the petroleum industry and is already occurring at commercial scale (more than 1 million tonnes CO₂ per year) at several locations. These include Statoil's Sleipner and Snohvit gas fields in the North and Barents Seas respectively, BP's gas project at In Salah in Algeria, and the enhanced oil recovery project at the Weyburn and Midale fields in Canada. In addition, over 50 million tonnes of CO₂ are transported over more than 3000 km of dedicated CO₂ pipelines and injected each year for enhanced oil recovery in North America.

In Australia, one of the largest research storage projects in the world, the CO₂CRC's pilot CO₂ injection project in the Otway Basin in Victoria, has injected 65,000 tonnes of CO₂ into a depleted gas field, and a further injection project into a saline reservoir is planned. In Queensland, ZeroGen is developing a 530 MW IGCC power station with planned capture and storage of 60-100 million tonnes of CO₂. The Gorgon natural gas venture offshore Western Australia will be the world's largest CO₂ capture and storage project when it begins, storing 125 million tonnes of naturally occurring CO₂ separated from the produced gas. There are a number of other projects in various stages of planning or implementation worldwide.