Geological storage or geosequestration is a key technology in CCS and offers the greatest potential for the permanent removal of CO2. It involves safely storing captured, liquefied, and transported CO2 deep underground.
Geological storage of CO2 requires certain characteristics of the rocks. The rocks need to have both sufficient porosity and permeability. The porosity determines the total CO2 that could be stored in the rocks while the permeability is a measure of how well the pores within these rocks are connected allowing the CO2 to flow through the rock. This rock formation needs to be overlain with a reservoir seal, which is basically a rock with very low permeability so that CO2 will not flow through it.
A common misconception is that these storage sites are like vast caves or caverns underground. In fact, they consist of tiny pores within the rock, which act like a sponge to house the liquefied CO2. These tiny pores add up to a huge volume, with the capacity to store millions of tonnes of CO2. They are overlain by a non-porous ‘cap’ rock that prevents the CO2 from escaping.
There are three main categories of geological storage options for CO2.
Saline water saturated rocks
These are underground formations of deep porous sedimentary rock, such as sandstone, that are saturated with salty water which is unfit for human consumption or agricultural use, and covered by a layer of impermeable cap rock (such as shale or clay), which acts as a seal. Once injected into the formation, the CO2 dissolves into the saline water in the reservoir rock.
CO2 storage in deep saline formations usually takes place at depths below 800m. At this depth, the CO2 will be at high enough pressures to remain in a liquid-like state.
Saline formations have the largest storage potential globally and a number of CO2 storage demonstration projects are proving their effectiveness to maximise storage capacity and containment.
In many ways, the saline formations are similar to oil and gas reservoirs except that they contain saline water instead of hydrocarbons.
Saline aquifers have been demonstrated as a storage option in the Sleipner, In-Salah and Snøhvit projects that each store approximately 1 Mtpa CO2. The Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is testing injection in a saline formation in Victoria and testing the associated monitoring technologies.
Oil and gas systems
The second main type of geological storage is depleted oil and gas fields. These have a proven ability to store hydrocarbons over millions of years, demonstrating very good reservoir characteristics. CO2 injection is already widely used in the oil industry for Enhanced Oil Recovery (EOR) from mature oilfields. The CO2 makes the oil easier to produce and is used in tertiary oil recovery for heavier oils. CO2 is used in EOR projects in North America, for example the Weyburn project in Canada and the Salt Creek project in Wyoming.
The CO2CRC Otway project has injected approximately 65,000 t in a depleted gas reservoir in Victoria between April 2008 and August 2009. This makes it the largest injection project using a depleted oil or gas field in the world.
Coal seam storage
This involves injecting CO2 into coal seams that cannot be mined economically, because they are too deep, too fragmented, or of very poor quality. There it displaces other gases, such as methane. The displacement effect means that coal seam CO2 injection could be most effective as part of the commercial production of coal seam methane (also known as coal bed methane), an increasingly important and relatively new energy source. Storage in coal seams is different since the CO2 is adsorbed in the coal matrix instead of being held within the pores of the rocks as in saline formations and oil and gas systems. The properties of the coal strongly influence whether CO2 will adsorb into it.
Other storage projects
In Australia, numerous studies have been completed to identify potential geological storage sites. These have been high level based on existing data. Atlases for geological storage have been produced for Queensland, NSW and Victoria. However, much more highly granulated data is required to understand the geological storage potential and opportunities in Australia.
The Australian Coal Association Low Emissions Technologies (ACALET) is supporting a number of projects that aim to provide a better understanding of the geological potential.
Data acquisition campaigns are occurring in most states of Australia. Four data wells have been drilled in NSW, with another four planned later this year that will provide regional data. In Queensland, 13 wells were drilled for the ZeroGen project within a small area that enabled detailed and site specific analysis. More wells are planned in Queensland to provide a broader geological overview. A preliminary well has also been completed in Western Australia and more wells are planned there in 2013.