There is abundant evidence (Le Quere et al. 2018; IPCC 2018, 2022; National Academies Science Engineering Medicine 2021) that if we are to avoid dangerous climate change, we must make deep cuts in emissions by 2050 or earlier. For some, perhaps many, that equates to ceasing all use of all fossil fuels by 2035 if not sooner. But as is evident from the previous discussion, the global trends in the production and use of fossil fuels are currently not compatible with the agreed Paris Agreement objective of keeping the global temperature rise to 1.5 °C or below, nor with the target of net zero carbon by 2050 or sooner. So, despite the predictions of the end of coal or oil or gas, it is likely that they will still be part of the global energy mix by 2050 and it would be wise to address this in future clean energy options through permanent storage of the associated CO2 emissions, not instead of more renewable energy, but in addition to more renewable energy.
CO2 emissions can be captured and permanently stored at scale (IPCC 2005), or on a more modest scale, can be incorporated into useful “long-lived” product such as cement or other building products (Hitch and Dipple 2012; Kliejne et al. 2022). Typically, the CO2 is reacted with rocks such as serpentinites, containing calcium or magnesium, to produce a carbonate product (Sanna et al. 2014). Ideally the reactive rock is a ground-up product of a current mining operation (to avoid the cost and the energy penalty involved in rock crushing and grinding) and such material could be regarded as a “reserve”, in that its quantity, chemistry and reactivity is known and it is in a location where it could potentially be used. There are also large resources of reactive rock that could be mined in the future if it became commercially feasible to do so and where there is a market for the products. Use of CO2 should be encouraged where it is economically and logistically feasible, and where the CO2 sequestration is long term. Carbonate mineralisation could make a valuable contribution to decreasing CO2 emissions to the atmosphere, be a useful carbon offset or create negative emission. But it is unlikely to operate at the scale necessary to have a major impact on the global emissions profile.
Carbon capture and geological storage (CCS) is seen by many as a major mitigation option (IPCC 2005; Cook 2012) and its technical feasibility is well established (GCCSI 2021). But is the available geological storage resource likely to be sufficient to meet the mitigation need to cut CO2 emissions by billions of tonnes of CO2 per annum? Globally yes, according to the IPCC (2005), although this conclusion was based primarily on theoretical storage potential, with a significant level of uncertainty. Like mineral or energy resources, realisation of that potential and upgrade of a storage resource to a storage reserve is dependent on parameters such as the level of geological knowledge, commercial viability, environmental impact, social acceptance and government approvals. It also depends on distance of the storage site from the CO2 source, for the greater the distance to transport the CO2 (by pipeline, though ship transport is possible) the greater the cost of the transport infrastructure. It is possible to overcome the “tyranny of distance” through economies of scale, and a number of low emission nodes and networks are being considered for Europe, North America and Australia. But obviously, the starting point has to be that the geology is suitable, which generally means porous and permeable rocks (such as a sandstone) at the right depth (800 m or more), overlain by an impermeable seal (such as a mudstone) in a structurally simple location (limited faulting and fracturing) that is tectonically stable.
This set of attributes, usually referred to as site characterisation, has been defined by Cook (2006) as “The collection, analysis and interpretation of subsurface, surface and atmospheric data (geoscientific, spatial, engineering, social, economic, environmental) and the application of that knowledge to judge, with a degree of confidence, if an identified site will geologically store a specific quantity of CO2 for a defined period of time and meet all required health, safety, environmental and regulatory standards”.
The McKelvey box, with its various modifications (SPE 2007, 2011, 2018), can be used to represent the CO2 mitigation potential of geological storage capacity (Fig. 2). Terms such as operational, contingent and prospective capacity have been readily and usefully incorporated into the scheme. Together, they offer the opportunity to standardise the assessment of this resource and future reserves. The conceptual McKelvey box (as in Fig. 2) along with a probabilistic approach can be usefully applied. Total pore volume can be subdivided into discovered and undiscovered pore volume, which in turn can be classified as either commercial or sub-commercial. Operational storage capacity (discovered pore volume that is considered commercial) can be separated into proven reserves with a 90% probability of commercial use (1P), proven plus probable reserves with 50% probability of commercial use (2P) and proven plus probable plus possible, with a 10% probability of commercial use (3P), following standard petroleum industry nomenclature (SPE 2007, 2011). Other more speculative categories include contingent storage capacity (discovered pore volume which is expected to be commercial in the future) and prospective storage capacity (undiscovered pore volume which might become commercial at some future time). The prospect of contingent and prospective storage capacities achieving commerciality requires a probabilistic approach identifying the high, low and best estimates, in much the same way that a probabilistic approach is taken to oil and gas resources and reserves.
Storage capacity (Fig. 3) can also be represented as a resource pyramid (CSLF 2007; Kaldi Gibson-Poole 2008) with the best-known and highest-quality capacity at the apex of the pyramid and the poorly known or poor-quality capacity at its base. Theoretical storage capacity—the physical limit of what the geological system can store—is represented by the entire pyramid. Effective storage capacity is a subset of the theoretical capacity. Practical storage capacity is a subset of the effective capacity, which is obtained by considering other technical, regulatory, infrastructure and other constraints. The matched storage capacity involves detailed matching of large stationary CO2 sources with well characterised geological storage sites.
To what extent has this been done to date? There are a number of sites that have been well characterised, and their storage capacity risked and assessed. An excellent example of a mature project is provided by the Sleipner Project (Furrea et al. 2017); CarbonNet provides a developing example (Barker and Mendes da Costa 2018). The value of the accessible (in situ) pore space may need to be discounted on the basis of the “source-sink” distance, which impacts on transport costs. The injectivity of a reservoir impacts on how many injection wells are needed to inject, for example a million tonnes of CO2 per annum. Heterogeneity exerts a major influence on capacity and the extent to which a CO2 plume will spread (Benson et al. 2018).
The value of a storage resource or reserve can be calculated and risked if there is a market for carbon through, for example the EU Emissions Trading System, which over the past year has had a carbon price ranging from 40 to 90 euros per tonne of carbon. In the USA, a tax initiative (45Q) encourages geological storage, by offering a tax credit of US $55 for each tonne of anthropogenic CO2 stored in a saline aquifer. This provides a different but equally valid basis for monetising a storage site. Alternatively, if there is no national basis for placing a dollar value on a storage site, and no 45Q, then the value of the contingent or prospective storage capacity (Fig. 2) can be based on a carbon price arrived at by agreement between the CO2 producer (steel plant, power station etc.) and the CO2 mitigator (the company taking on responsibility for geologically storing the CO2).
So, whilst there are new geological parameters to consider, the established principles of reserves and resources can be applied to sites or geological formations or structures suitable for CO2 storage, a resource that is likely to be increasingly in demand. The same principles can be applied to underground storage of hydrogen and this is discussed later.