The geological storage of carbon dioxide (CO2) is the process whereby CO2 captured from power plants or other industrial facilities is transported by pipeline to a suitable location and then injected under pressure into a deep geological reservoir formation, where it remains permanently trapped and prevented from entering the atmosphere. The processes by which it is retained in the subsurface are generally those that have trapped oil, gas and naturally generated CO2 for millions of years. The geological formations that can be utilised for this trapping have the same characteristics as those that are able to act as reservoir rocks for petroleum. They have good porosity and permeability and have an overlying sealing formation, which will prevent the trapped fluids migrating out of the storage reservoir and possibly escaping to the surface. In addition, because of the phase behaviour of CO2, efficient storage requires that they are stored at depths greater than 800 below the surface. Unlike oil and gas, which rely primarily on a three dimensional structural trap to prevent them from ultimately rising to the surface, there are additional trapping mechanisms for CO2. Given a sufficiently long migration path within a formation, CO2 will ultimately be rendered immobile by dissolution into the formation water, residual trapping and potentially, over longer time scales, mineralisation. As groundwaters at these depths are generally saline, this type of storage is often termed deep saline aquifer storage. A recent nationwide review by Commonwealth and State geological surveys, as part of the Carbon Storage Taskforce, rated the suitability of geological basins across Australia for geological storage of CO2. The most geologically suitable basins are the offshore Gippsland and North Perth basins but several onshore basins also rate highly. These include the Eromanga, Cooper, Bowen, Galilee, Surat, Canning and Otway basins. The Victorian Government has recently released area for greenhouse gas storage exploration in the Gippsland Basin and the Queensland Government in the Galilee and Surat basins. The aquifers within these basins provide groundwater for human consumption, agriculture, mining, recreation and groundwater dependent ecosystems. The Surat Basin also contains oil and gas accumulations that are being exploited by the onshore petroleum industry. Understanding the existing the groundwater's chemistry and the connectivity between aquifers in the context of its current use is essential in order to determine whether prospective aquifers could be used for geological storage of CO2 without compromising other activities. The potential risks to groundwater from the potential migration of CO2 and changes to groundwater properties that might be expected will also be discussed. Current data gaps include poor hydrogeochemical data coverage for the deeper aquifers and particularly limited data on trace metals and organics. A comparison with experiences learned from enhanced oil recovery using CO2 in North America and the CO2CRC's pilot CO2 injection project in Western Victoria will illustrate some of the unique differences and opportunities for geological storage of CO2 in Australia.

Oral presentation at "Groundwater 2010" conference, 31 October - 4th November 2010, Canberra