Engineering Aspects of Geological Sequestration of Carbon Dioxide

Abstract

Abstract Injection into geological formations is a technical option for decreasing carbon dioxide emissions to the atmosphere. Several key factors influence the performance and long-term integrity of injection into deep saline formations. Accurate representations of the physical properties of carbon dioxide have compiled and use to determine the theoretical storage density at typical sub-surface conditions, and the amount of energy that would be expended in injecting the carbon dioxide (whether pure or mixed with other gases), leading to an estimate of the net reduction of emissions. Detailed computer simulations indicate that the distribution of the injected carbon dioxide is initially dominated by gravity segregation, relative permeability effects and the permeability anisotropy of the saline formation. Post-injection, important effects occur which are not taken into account in conventional petroleum reservoir simulation. Convective mixing is predicted to occur, which greatly accelerates the dissolution of carbon dioxide in the saline formation water. This unusual phenomenon arises from the increase in the density of brine when saturated with carbon dioxide. Complete dissolution of the injected carbon dioxide is predicted to occur over hundreds to thousands of years, depending significantly on the vertical permeability of the formation and the geometry of the top seal. Reactions between the carbonated water and the formation also increase the rate of dissolution. If the chosen site is not a structural trap but a gently dipping regionally extensive formation, then the carbon dioxide can migrate up to tens of kilometres from the injection site, depending on the angle of dip and the horizontal permeability. However, convection and dissolution can still keep the carbon dioxide confined. This theoretical analysis has been developed as part of the Australian Petroleum CRC's GEODISC program, and is being used to assess the suitability of proposed sites for carbon dioxide sequestration. Introduction Projected emissions of "greenhouse gases", principally carbon dioxide, are predicted to cause significant changes in average global temperature and sea-levels1, which could have negative consequences for people in many parts of the world. Scenarios for stabilising atmospheric carbon dioxide at reasonable levels will eventually require substantial cuts in overall emissions over the next few decades1,2. If usage of fossil fuels is to continue at current levels while avoiding undesirable climate change, technical means need to be found to reduce the carbon dioxide emitted to the atmosphere in the production and consumption of fossil fuels. One possible solution is to store or "sequester" CO2 emissions in a form where they will not reach the atmosphere. A large industrial source, such as a natural gas processing plant or power station, might produce 107 to 109 kg of CO2 per year over the operating life of the facility. The only disposal options that appear to have sufficient capacity are either the deep ocean or underground3. The latter option can encompass a variety of situations e.g. depleted oil and gas reservoirs, in which CO2 can be used for enhanced recovery, deep unmineable coal seams, where the CO2 can be stored while enhancing methane production, or deep saline formations. The latter option is particularly suitable in the Australian context, since sites with large sequestration capacity can be identified in most of Australia's petroliferous sedimentary basins4, and so the focus of this paper is on sequestration in deep saline formations.