Methodological Issues

Geological conditions vary widely and only a few published studies of monitoring programmes that identify and quantify fugitive anthropogenic carbon dioxide emissions from geological storage operations currently exist (Arts et al. 2003, Wilson and Monea 2005; Klusman 2003a, b, c). Although the Summary for Policymakers of the SRCCS suggests that properly selected geological storage sites are likely to retain greater than 99 percent of the stored CO2 over 1000 years and may retain it for up to millions of years, at the time of writing, the small number of monitored storage sites means that there is insufficient empirical evidence to produce emission factors that could be applied to leakage from geological storage reservoirs. Consequently, this guidance does not include Tier 1 or Tier 2 methodology. However, there is the possibility of developing such methodologies in the future, when more monitored storage sites are in operation and existing sites have been operating for a long time (Yoshigahara et al. 2005). However a site-specific Tier 3 approach can be developed. Monitoring technologies have been developed and refined over the past 30 years in the oil and gas, groundwater and environmental monitoring industries (also see Annex 1). The suitability and efficacy of these technologies can be strongly influenced by the geology and potential emissions pathways at individual storage sites, so the choice of monitoring technologies will need to be made on a site-by-site basis. Monitoring technologies are advancing rapidly and it would be good practice to keep up to date on new technologies.

Tier 3 procedures for estimating and reporting emissions from CO2 storage sites are summarised in Figure 5.3 and discussed below.

Figure 5.3 Procedures for estimating emissions from CO2 storage sites

Figure 5.3 Procedures for estimating emissions from CO2 storage sites

In order to understand the fate of CO2 injected into geological reservoirs over long timescales, assess its potential to be emitted back to the atmosphere or seabed via the leakage pathways identified in Table 5.3, and measure any fugitive emissions, it is necessary to:

(a) Properly and thoroughly characterise the geology of the storage site and surrounding strata;

(b) Model the injection of CO2 into the storage reservoir and the future behaviour of the storage system;

(c) Monitor the storage system;

(d) Use the results of the monitoring to validate and/or update the models of the storage system.

Proper site selection and characterization can help build confidence that there will be minimal leakage, improve modelling capabilities and results, and ultimately reduce the level of monitoring needed. Further information on site characterisation is available in the SRCCS and from the International Energy Agency Greenhouse Gas R & D Programme (IEAGHG 2005).

Monitoring technologies have been developed and refined over the past 30 years in the oil and gas, groundwater and environmental monitoring industries. The most commonly used technologies are described in Tables 5.1-5.6 in Annex I of this chapter. The suitability and efficacy of these technologies can be strongly influenced by the geology and potential emissions pathways at individual storage sites, so the choice of monitoring technologies will need to be made on a site-by-site basis. Monitoring technologies are advancing rapidly and it would be good practice to keep up to date on new technologies.

A range of modelling tools is available, some of which have undergone a process of code inter-comparison (Pruess et al. 2004). All models approximate and/or neglect some processes, and make simplifications. Moreover, their results are dependent on their intrinsic qualities and, especially, on the quality of the data put into them. Many of the physico-chemical factors involved (changes in temperature and pressure, mixing of the injected gas with the fluids initially present in the reservoir, the type and rate of carbon dioxide immobilization mechanisms and fluid flow through the geological environment) can be modelled successfully with numerical modelling tools known as reservoir simulators. These are widely used in the oil and gas industry and have proved effective in predicting movement of gases and liquids, including CO2, through geological formations.

Reservoir simulation can be used to predict the likely location, timing and flux of any emissions, which, in turn, could be checked using direct monitoring techniques. Thus it can be an extremely useful technique for assessing the risk of leakage from a storage site. However, currently there is no single model that can account for all the processes involved at the scales and resolution required. Thus, sometimes, additional numerical modelling techniques may need to be used to analyze aspects of the geology. Multi-phase reaction transport models, which are normally used for the evaluation of contaminant transport can be used to model transport of CO2 within the reservoir and CO2/water/rock reactions, and potential geomechanical effects may need to be considered using geomechanical models. Such models may be coupled to reservoir simulators or independent of them.

Numerical simulations should be validated by direct measurements from the storage site, where possible. These measurements should be derived from a monitoring programme, and comparison between monitoring results and expectations used to improve the geological and numerical models. Expert opinion is needed to assess whether the geological and numerical modelling are valid representations of the storage site and surrounding strata and whether subsequent simulations give an adequate prediction of site performance.

Monitoring should be conducted according to a suitable plan, as described below. This should take into account the expectations from the modelling on where leakage might occur, as well as measurements made over the entire zone in which CO2 is likely to be present. Site managers will typically be responsible for installing and operating carbon dioxide storage monitoring technologies (see Annex 1). The inventory compiler will need to ensure that it has sufficient information from each storage site to assess annual emissions in accordance with the guidance provided in this Chapter. To make this assessment, the inventory compiler should establish a formal arrangement with each site operator that will allow for annual reporting, review and verification of site-specific data.

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