Monitoring is an essential element within the selection, planning, installation, operation and abandonment of CO2 geological storage sites. Monitoring provides fundamental data about a storage site's state and performance and thereby helps to meet the common public concern on potential impacts of CO2 geological storage on the environment, human health and assets. The main substances of concern are CO2 and, because of the large volumes, displaced formation water. Monitoring also includes the observation of impurities within the CO2 phase, substances mobilised in the subsurface, and geomechanical effects of CO2 injection.

Monitoring programmes implemented at current demo and industrial-scale projects are mostly restricted to the most effective monitoring methods (in terms of availability and cost) to comply with the legal and safety requirements. In contrast, a wide variety of monitoring tools are developed, adapted, tested and validated at natural release, research and pilot test sites.

Overall, well-advanced "deep" monitoring techniques are available for monitoring the performance of the storage reservoir and tracking the expansion and migration of the CO2 plume. At present, seismic monitoring is the dominant geophysical method for the observation of CO2 in saline aquifers and depleted oil reservoirs. Seismic methods allow, in most cases, mapping of the migration of the CO2 plume and, in combination with other measurements, can also provide reasonably accurate volume estimates.

Also, a number of established, reliable methods and tools exist for near-surface monitoring at CO2 storage sites. The available monitoring methods comprise different suites of techniques enabling i) large-scale surveys that contribute to baseline measurements and that can be used to detect eventual leakage pathways on a regional level, ii) rapid surveying of relatively large areas and derivation of essential results in a short time, iii) detailed small-scale verification and characterisation procedures for selected, confined areas using local knowledge to target possible spots of CO2 and fluid leakage. Reliable techniques exist that can distinguish CO2 from deep origins (geogenic or anthropogenic) from shallow, biogenic CO2. In case of leakage, rates can be quantified by detailed flux measurements. The resolution of the monitoring methods and, consequently, the capacity to detect fluid migration and irregularities, depends very much on local site conditions and the intensity of monitoring. High-resolution measurements obviously increase the detection capacity but require intensive and costly monitoring efforts.

The verification activities at five active CO2 storage sites showed that monitored site performance deviated from modelled predictions at all sites (Wildenborg et al., 2012). Hence, a key element of site-specific monitoring plans will be to establish relevant criteria that will allow discrimination between acceptable deviations from the permitted behaviour (which will only necessitate a model update without consequences on the performance prediction of the site) from deviations that represent significant irregularities (and require updates of the risk and the monitoring plan, and potentially give indications to take remedial actions, change the injection plan and eventually require major revisions of the numerical models) (Wildenborg et al., 2012). In addition, criteria are needed to evaluate convergence of predicted and observed site performance with time. Such a convergence reflects a sufficient understanding of the storage system, which is a prerequisite for long-term predictions and the transfer of responsibility of a storage site.

The following recommendations for monitoring CO2 storage site performance are based on extensive experience from groundwater observation, environmental monitoring, natural gas storage and hydrocarbon production, industrial CO2 storage and research pilot projects, the investigation of natural analogues and controlled CO2 release experiments:

  • Comprehensive, integrated, and flexible monitoring plans are needed in order to satisfy various monitoring needs during normal operation and for contingency monitoring. monitoring shall form an

FIG 5.5

Fig. 5-1: Schematic evolution of site-specific monitoring plans in relation to other elements of CO2 storage management

integral part of the overall site management and must be continuously improved as well as any associated activities, as illustrated in Fig. 5-1.

  • The development of tools and testing their application at ongoing storage projects under in situ conditions are needed in order to evaluate and provide monitoring technologies and concepts considering "new scientific knowledge, and improvements in best available technology", as required by the European CCS Directive (2009/31/EC).
  • Cost effectiveness measures, such as campaign optimisation or combination of various methodologies, should be considered. Additionally, it may be beneficial to increase the lifetime of sensors in order to save costs on the maintenance activities.
  • The thresholds for acceptable deviations from predictions and the demonstration of convergence must be specified prior to CO2 injection. They should include safety margins, taking into account uncertainties from site characterisation, performance predictions and monitoring accuracy. Monitoring plans should be designed in a way that provides appropriate information to verify the specified conditions.
  • All stakeholders, including the local population, should be involved in the definition of i) acceptable conditions, ii) significant irregularities and iii) site-specific threshold values. Furthermore, they should participate in the planning of the measures to be taken in the case where such values are exceeded.
  • The planning, operation, performance, and updating of monitoring activities, such as storage operation in general, should be conducted under independent supervision, e.g. a competent authority, that is not the permitting agency at the same time.
  • With respect to detection limits and uncertainties in quantification, the CO2 injected into a storage formation should be regarded as contained within the storage complex, providing that no indication of deviation has been observed by a reasonably extensive, sensitive and appropriate monitoring programme.
  • Concerning the detection of anomalies and the distinction of storage-related impacts from natural variations and phenomena, it is essential to integrate the results of near-surface and subsurface monitoring efforts in a thorough, systematic and plausible manner. Extensive site-specific knowledge is required because the resolution and sensitivity of many monitoring methods (and, hence, the capacity to detect irregularities or fluid migration) depend very much on local site conditions and the intensity of monitoring.
  • The comparison of monitoring results with baseline data and model predictions will be crucial for a quantification of effects. Extensive baseline monitoring is required for recording natural (e.g. seasonal) variations for relevant parameters that are needed for understanding processes and unravelling the controlling factors for these processes and the resulting variations. Baseline monitoring should start well before the first CO2 injection, as part of site characterisation, in order to record secular natural variations and have sufficient time for the interpretation of the recorded data, so that natural processes can be considered in the risk and monitoring plans.

Some of these issues are currently addressed in ongoing international research projects (e.g. RISCS, ECO2, CO2CARE, SiteChar) and national projects around the world. Once the full range of results is available, it will be possible to further refine monitoring strategies for future CO2 storage sites.