The main objective of this report is to identify and review site selection and characterisation methods. This report presents and discusses all the steps required to assess the capacity, performance and integrity of a site. Simulation of CO2 storage in an underground formation requires a complex multi-disciplinary effort, with the analysis of a number of interacting processes, including geology, multi-phase flow and transport, geochemistry and geomechanics. A site characterisation first calls for the geological characterisation and modelling of the site at basin and reservoir scales and the modelling of flow and transport mechanisms so as to simulate the short-term to mid-term behaviour of the storage. As well as hydrodynamic effects, geomechanical effects generated by the injection of a large volume of fluid in the subsurface have to be modelled over a long period. Modelling geochemical and biological processes is essential to understand the geochemical feedback on the reservoir properties and the trapping mechanisms that will occur. All these skills and knowledge are required to assess potential environmental impacts and risks. The estimation of the economical viability of the project is also essential to decide whether a geologically suitable storage site can actually be developed for CCS. In parallel with the technical aspect of characterising the site, public perception and acceptance appears to be a potential major impediment to deployment of CCS and so social activities towards local communities have to be performed at a very early stage.

It is widely accepted that prolific burning of fossil fuels has raised the amount of CO2 in the atmosphere to levels at which it is contributing to climate change and that de-carbonising energy is necessary to avert catastrophic and irreversible change. Carbon Capture and Storage (CCS) is a technology that could contribute significantly to reduced CO2 emissions to the atmosphere. It works by removing CO2 from the pre- or post-combustion exhaust gas of power stations and other industrial processes and injecting the CO2 into underground geological reservoirs of porous rock for permanent storage. While not eliminating society's dependence on fossil fuels, it provides a bridging solution to mitigate the problem while renewable energy Sources are developed to large-scale implementation and the acceptability of nuclear power into the future is resolved.

The selection and characterisation of potential CO2 storage sites are probably the most important steps for ensuring the safety and integrity of a CO2 storage project and are essential in developing a CCS project. In essence, a site selection process should demonstrate that the site has: sufficient capacity to store the expected CO2 volume; sufficient injectivity for the expected rate of CO2 capture and supply; and sufficient containment to store safely the injected CO2 for the period of time required by the regulatory authority, so as not to pose unacceptable Risks to the environment, human health or other uses of the subsurface.

Guidelines have been published by several bodies on the necessary steps and process involved in selecting and managing a storage site within whatever regulatory environment applies (WRI, 2008; CO2CRC, 2008; NETL, 2010a; NETL, 2010b). It is not the purpose or intention of this report to repeat those, but rather to focus in detail on geoscience aspects of site selection.

Types of storage sites

There are three main types of reservoir for geological storage of carbon dioxide as a fluid: depleted oil and gas fields, saline aquifers and coal beds. Research is also being directed at storing CO2 by forming solid carbonate minerals by combining CO2 with reactive rocks with high Fe, Mg and Ca content, such as mafic and ultramafic igneous rocks - this latter form of storage is not considered in this report.

A considerable amount of understanding, experience and technology developed by oil and gas operations is directly applicable to storage site characterisation and selection. CO2 can 'replace' oil and gas in fields that have been depleted, or it can be used to prolong oil or gas production from fields that are still active. Enhanced Oil Recovery (EOR) and Enhanced Gas Recovery (EGR) are processes in which CO2 is injected into a reservoir to increase the amount of hydrocarbons extracted, thus providing an economic benefit whilst also potentially storing CO2. The main requirement is to ensure that injected CO2 is not produced with the oil or gas and the EOR/EGR project becomes a CO2 storage project. Depleted oil and gas fields have the obvious attraction as storage sites that containment at the site has already been demonstrated by the retention of hydrocarbons for millions of years. It is important to ensure, however, that extraction of the hydrocarbons has not damaged the integrity of the reservoir or seal by pressure reduction and that extraction Wells do not provide potential leakage pathways for CO2. Another major advantage of depleted Reservoirs over saline Aquifers is that there will be large amounts of geological and engineering data already available for site characterisation.

Saline Aquifers are sedimentary rock units in which pore space is saturated with saline water that is unsuitable for consumption or irrigation. Such units are widely distributed and can be of very large volume and extent. They therefore have the potential to provide large storage capacity in areas without depleted hydrocarbon Reservoirs. However, because they have not previously had an economic or resource value, they are generally much less-well understood than hydrocarbon Reservoirs and so assessment of their CO2 storage potential carries more uncertainty regarding containment security and fluid flow properties.

Carbon dioxide storage in coal beds is through adsorption onto the coal surfaces rather than filling of pore space. CO2 is preferentially adsorbed and thus displaces methane (CH4) from the coal. As with EOR, this process can be used to produce coal bed methane and so CO2 storage can be combined with hydrocarbon production. In fact, as methane has a higher greenhouse effect than CO2, any CO2 coal storage projects must include methane production and use, to avoid emission to the atmosphere and result in greenhouse gas emission reduction.

Trapping mechanisms

For CO2 storage involving partial filling of pore space, it is necessary to inject and store CO2 in its dense supercritical form, in order to maximise use of the available porosity. The critical point for CO2 is at 31.1°C and 7.38 MPa; this equates to a depth of approximately 800 m at typical crustal temperatures.

The CO2 trapping mechanisms include:

  • physical trapping in structural and stratigraphic traps, in which the CO2 is contained in closures produced by the geometrical arrangement of reservoir and seal rocks and faults;
  • residual trapping, in which some portion of migrating CO2 remains trapped in pore spaces by capillary forces;
  • solubility trapping, in which some portion of the CO2 dissolves into the formation water;
  • Hydrodynamic Trapping, in which, although the dissolved and free CO2 migrates with formation water through the reservoir, very long residence times mean it is effectively stored permanently;
  • mineral trapping, in which the CO2 precipitates as new carbonate minerals and so is permanently stored with high security;
  • adsorption trapping, in which gaseous CO2 adsorbs onto the surface of coal.

Estimating the volumes and proportions of CO2 that would be trapped by each of these mechanisms is a key part of a site characterisation. Increased understanding of Trapping mechanisms is an important area for scientific research.

Site selection

The first stage of a site selection process for CO2 storage is a screening of national or regional geology to identify large areas of potentially suitable sedimentary basins. Basins can be assessed and ranked using criteria such as size, depth, stratigraphy (reservoir-seal pairs or potentially injectable coal seams), seismicity, geothermal characteristics, accessibility, proximity to CO2 Sources etc. Basin identified as having potentially suitable assets for CO2 storage can then be assessed at basin and sub-basin scale to locate possible closures and traps, the distribution of reservoir-seal pairs at suitable depths, or coal seams, using existing data such as geological maps, seismic surveys and well data.

Prospective storage sites can be ranked for the following factors:

  • Storage Capacity. A simple estimate can be made of storage capacity from the area of the identified trap, thickness of the reservoir below the critical depth and the porosity, and this compared to the likely CO2 supply that the site may need to accommodate. Not all of the total pore space in the reservoir can be filled with CO2 and key parameter in capacity estimates is the efficiency or utilisation factor, the fraction of the pore volume that can be occupied by or will retain injected CO2. This is a function of the fluid already present in the reservoir, pore size and shape, grain mineralogy and reservoir heterogeneity at all scales. Efficiency factors can vary widely from site to site and have a major effect on capacity calculations. Values used are typically around 40 % for depleted gas fields, and range 0.1 - 6 % for saline aquifers; establishing a reliable value for a site before injection of CO2 begins is clearly important and a challenge for future research. The efficiency factor can be maximised by careful injection strategy and well planning.
  • Injectivity Potential. Reservoir characteristics, such as permeability, porosity and pressure will control the rate at which CO2 can be injected into the reservoir. In practise, injectivity can be increased by extending the length of Wellbore within the reservoir by drilling horizontal wells and/or by increasing the number of wells.
  • Containment. For a seal/caprock to be effective for storage, it must be laterally continuous and sufficiently thick over the proposed injection reservoir, with low vertical permeability and high capillary entry pressure. An effective seal can be demonstrated by a pressure or salinity differential, or a history of trapping oil or gas. The size and spacing of faulting is also a factor, but it is particularly important to assess whether faults are likely to be sealing or migration pathways. The migration distance over which CO2 can travel in the reservoir will affect the probability of the more secure trapping mechanisms: residual, hydrodynamic or mineralisation.

    It is also important to ensure that any existing wells or other artificial breech of the seal will also trap CO2 and not provide an escape pathway.
  • Site Logistics. Economic and logistical factors will cotrol whether a geologically suitable storage site can actually be used. Excessively deep wells or long pipelines may make a site uneconomical. On the other hand, clusters of CO2 sources sharing pipeline and storage site facilities can make a project more economical. Cooperation among projects at a regional scale will be required to benefit from shared facilities and avoid problems from using the same storage complex.
  • Existing Natural Resources. Competing use of the same underground space, or sterilisation of alternative underground resources that could potentially be compromised by CO2 storage, such as oil and gas, mineable coal, potable water, a geothermal energy source, may require national or regional policies on relative importance of the conflicting interests. Proximity to population centres, national parks or other protected sites, could limit surface operations, either because of legislation or because of negative public reaction.

A key consideration in any project is the selection of the stage to begin the outreach process to best avoid delays caused by negative reaction from communities around potential locations for CO2 storage. General consensus from studies and experience seems to be that early is better, to open lines of communication and develop community understanding before fear of the unknown or manipulation by alternative agendas get embedded.

Once a potential storage site has been identified by the basin-scale assessment described above, it has to be evaluated through a detailed site characterisation, to add quantitative confidence that the site will geologically store the required quantity of CO2 to the level of security and for the period required by the regulatory authority. The geoscience aspects of a detailed site characterisation as well as the economic aspects are described in the following chapters of this report, which comprise best practice recommendations from international studies and working groups in CO2 storage site selection. In addition, key references are listed for societal aspects:

  • Chapter 2 - Geological characterisation of the site - This chapter describes the creation of a geological model with which to assess the volume, injectivity, storage efficiency and lifetime of the reservoir; potential leakage and how to avoid or mitigate it; and the long-term behaviour and fate of the stored CO2 and displaced brine.
  • Chapter 3 - Flow modelling - This chapter describes the modelling of flow and transport mechanisms and the numerical models that are used. The purposes of fluid flow simulations are also illustrated on some examples of CO2 injection pilots.
  • Chapter 4 - Reactive flow modelling - This chapter presents an overview of reactive flow modelling (solute transport modelling). The state of current knowledge within geochemical and solute transport modelling is presented as well as an overview of what has to be modelled and for how long. The state of the art of chemical and solute transport modelling and its applications status, concentrating on reactive flow modelling, as well as the important role played by available data are discussed.
  • Chapter 5 - Coupled geomechanical and flow modelling - This chapter presents the scope of geomechanical modelling and the data required to assess the long-term performance of CO2 storage. The different issues of geomechanical modelling are then presented and illustrated on case studies. Finally, coupling methods are presented.
  • Chapter 6 - Environmental impact and risk - This chapter presents an overview of the risk process that determines both the consequences and likelihood of an event and that is the input for good monitoring and mitigation plan.
  • Chapter 7 - Economic analysis - This chapter presents the cost associated to CO2 storage emphasising the great uncertainties on these costs and their site dependency.
  • Chapter 8 - Public perception and acceptance - This is widely perceived to be a potential major impediment to deployment of CCS. Concerns over safety, permanence of storage and adverse impacts on environment, health and property prices need to be carefully managed at local and national scales. How, when and by whom the CCS message should be delivered are likely to vary from site to site depending on local cultural factors. Therefore, this report does not seek to be prescriptive, but rather, presents references to major studies on the issue.