The storage of CO 2 , if it is to achieve significant reductions in global emissions, would eventually have to be able to handle thousands of millions of tonnes globally. For several reasons, including the availability of sites and the need to be able to regulate and monitor the storage of CO 2 , only a limited number of storage facilities are likely to be used, each with capacity of up to hundreds of millions of tonnes of CO 2 . Geological formations, especially ones that have previously held oil or gas, should be well qualified to perform this task, as the amounts of fluid removed from the formation during oil
and gas production are typically similar to the volumes required. The fact that these reservoirs have held oil or gas over geological timescales gives confidence that they could hold CO 2 safely and securely, providing that the geological seal has not been compromised by the hydrocarbon extraction. Other geological formations, in principle, have even greater capacity, especially formations holding saline water since these have no other use. If the security of storage in such formations can be established, saline aquifers should make good stores for CO 2 .
There are few other natural systems suitable for holding even a fraction of the vast quantities of CO 2 produced by the world’s use of fossil fuels. One possible candidate is the deep ocean – if CO 2 can be injected at sufficient depth, it would have density greater than the surrounding ocean and so, in principle, should stay on the seabed (Ocean Storage, 1998). In
practice, the CO 2 would react with the seawater forming hydrates; these solid-like materials have only limited stability which could lead to gradual dispersion of the CO 2 into the ocean waters, eventually compromising the storage. Just as significantly, such injection of CO 2 into the oceans is not allowed under international agreements such as the London Convention (Hendriks et al ., 2005).
Another option for dealing with the captured CO 2 , which superficially has attractions, is to put it to use or make it into something else, for example a solid material or a chemical that could be sold.
An established way of using CO 2 is in enhancing oil recovery (EOR). This, and related newer uses in enhanced gas recovery and enhanced coal bed methane recovery, could provide a source of income to offset, partially, the cost of storage. However, the result of such injections would be to bring forward production and perhaps increase the overall extraction of hydrocarbons. This has caused some people to question whether CO 2 used in EOR (and the related techniques) should be accepted as a climate change mitigation measure. Nevertheless some countries have already recognised EOR projects as contributing to storage of CO 2 .
Although developing a saleable product from CO 2 may be possible, for example as a building material, the size of the demand for any such product means it could not provide the whole answer to the huge quantities of CO 2 produced by global combustion of fossil fuels. In any system for utilising CO 2 , or making it into something else, it is very important to consider fully all of the inputs and outputs in order to understand its true impact; to do this it is essential to select the appropriate system boundary for the scheme being studied (Freund et al ., 2005).
A related possibility is the production and disposal of a solid material made from CO 2 – this has attracted attention as a very secure method of storage. Natural minerals such as serpentinite and olivine have been identified as potential sources of Mg compounds that could be reacted with CO 2 to make such a stable material. To do this on an industrial scale, at a
rate commensurate with the production of CO 2 by a power plant, would require a chemical engineering process. Typically such a process would have two stages – in the first, the rock would be decomposed, perhaps to make Mg (OH) 2 ; in the second stage, the CO 2 would be reacted with the products of the first stage to make the solid product (Fagerlund et al ., 2012). Although this second stage may be exothermic, the first stage is likely to require input of energy, so the process would involve heat recovery to minimise the energy demand of the plant. Even with the best integration, extra energy would likely be needed; an obvious source of this would be fossil fuels but the consequence of using such an energy source would be increased greenhouse gas emissions, which would tend to offset or even cancel out the emissions reduction arising from capturing the CO 2 in the first phase. A way of avoiding this problem might be to use energy from a renewable source but the cost of setting up such a system in order to convert captured CO 2 into a solid would likely make it more attractive to utilise the renewable energy directly (for electricity production or another purpose).
Another problem with this concept is that such a process of CO 2 mineralisation would require intermediate materials which would have to be recycled and any losses made good. In addition, the cost of the plant would be substantial (Gerdemann et al ., 2007), making the overall storage cost much greater than the cost of injection into geological formations. Finally, it must be recognised that the volume of material produced would be substantially greater than the volume of mineral mined, presenting a waste disposal problem.
For these reasons, conversion into solids is unlikely to be answer. The storage option that has attracted most attention is geological storage of CO 2 , the subject of this book. Various steps have been taken in many countries to support the development and application of CCS using geological storage – demonstration projects have been monitored as the basis for learning, financing schemes have been put in place to encourage early application, regulations have been developed to ensure that CCS systems are safe and secure and achieve the goals that society requires. These and other developing
trends are discussed below.