Key Reports:

State-of-the-Art of Monitoring Methods to evaluate Storage Site Performance

1.4 Comprehensive monitoring concepts

A comprehensive monitoring concept shall meet the different monitoring purposes (see 1.1.1.) in the spatial dimension, and in the temporal dimension providing information on substances and processes of concern (see 1.3.4 and 1.3.5).

For practical purposes different compartments can be distinguished in the spatial dimension. These compartments fulfil different functions in storage operation and may comprise various subjects of protection. The individual compartments are accessible for installation and application of different monitoring techniques. Relevant compartments to be considered for setting up a comprehensive monitoring concept may include:

storage formation, including caprock,
secondary containment formations,
the overburden, including faults,
injection facilities, including wells,
the hydraulic unit, extending beyond the storage complex,
shallow, potable water aquifers,
the marine environment,
surface of the storage site and surrounding biosphere.

The practical delineation of a "storage complex" as defined in Art. 3 of the European CCSCarbon dioxide Capture and Storage Directive and the extent of it are a matter of ongoing debate. Depending on the position of the protagonists, it could be restricted to the first two compartments of the list above, or include the first five compartments. The term "surrounding environment" is not well defined by the EU CCS Directive either. However, it should include at least the area of the hydraulic unit. This list of compartments may be adapted to the local situations. For example, shallow potable groundwater resources and the marine environment may be mutually exclusive, or caprock and reservoir may be split up into separate compartments. The different compartments are partially nested, adjacent or interconnected. Though the compartments are fixed in space, the phases within these compartments migrate with time within the compartments and may change at a particular site within a compartment. In general, there will be an outward migration of different phases away from the injection well. In a saline aquifer these expanding zones are (Fig. 1-3):

supercritical CO2Carbon dioxide saturated reservoir near the injection wells (g),
partially saturated gas-water transition zone (g, f),
CO2Carbon dioxide dissolved in formation water (CO2Carbon dioxide_(aq)),
zone of brine displacement (q_f),
outer zone of the hydraulic unit, with negligible brine displacement but measurable pressure increase (Dp_f).

Fig _ 1_3 Fig. 1-3: Schematic illustration of expanding monitoring zones (dashed lines) and fixed features within different compartments (solid lines). Colours indicate monitoring intensity. An explanation of the labelling of zones is given in the text (Modified after may et al., 2011).

According to the various phases present in the zones, different monitoring techniques are required to record key parameters or proxy data as indicators for subsurface processes. monitoring intensity will follow these zones and is generally more intensive in the dynamic region surrounding injection wells and less intensive, at the margins of the hydraulic unit. However, areas of particular concern, such as potential pathways or valuable resources at the surface, may need special attention throughout all monitoring phases. For monitoring of fluid migration processes and pathways the relations between compartments and zones have to be taken into account. Provisions for obtaining the required data have to be specified in the site-specific monitoring plans.

On the time scale different phases can be distinguished, which also will require different levels of monitoring intensity (Figs. 1-4 and 1-5):

Baseline monitoring in the pre-injection period,
Standard operational monitoring during normal injection according to permit,
Intensified contingency monitoring during times of significant irregularities and following corrective measures,
Closure and post closure period, before transfer of liability to the competent authority,
Long-term monitoring after the transfer of liability (Art. 18.6, EUEuropean Union CCS Directive).

Monitoring intensities may be highest for baseline acquisition and during the injection phase in case of irregularities and consecutive corrective measures (Fig. 1-5; Tab. 1-3). The general, descriptive term "intensity" includes the frequency of measurements, the numbers of sampling points and methods applied. Apart from these peak times, monitoring intensity may be reduced if injection performance is according to plans. After the end of injection and transfer of liability, the monitoring efforts may be reduced to a level, which allows for detection of Leakages or significant irregularities. Slow geochemical processes, e.g. may lead to risks, long after site closure. If any leakages or significant irregularities are detected, monitoring shall be intensified again.

Fig _1_4 Fig. 1-4: Phases of CO2Carbon dioxide geological storage projects from a monitoring perspective.

Table 1-3. Level, scale and monitoring intensity considering the purpose and type of observations required.

Level	Scale	Intensity	Purpose	Observations
normal operation, after transfer of responsibility	regional	low	reconnaissance	indicative parameter, proxies
significant irregularity	restricted area	moderate	search and detection	direct measurements
leakage, negative impacts	local	high	characterisation	flux and magnitude determination

Fig .1-5 Fig. 1-5: Schematic illustration of variable monitoring intensity with time (after v. Goerne et al., 2010). The occurrence of an irregularity has been placed arbitrarily towards the end of the operational phase. This does not imply that a site has to be closed after such an incident.

Temporal and spatial scales can be combined to a generic table. Allocating monitoring purposes (see Section 1.1) on this table yields a matrix that can be used to generate comprehensive lists of monitoring tasks (Tab. 1-4). The arrangement of the compartments according to their occurrence with depth represents potential pathways for continuous transition from the actual storage formation up to the surface, where injection facilities are usually located.

For establishing site-specific monitoring concepts local settings and features must be well known and site-specific risks need to be addressed. Plans must be kept sufficiently flexible in order to react in cases of significant deviation from the predicted behaviour, either through more intensive monitoring efforts or by monitoring the effectiveness of corrective actions according to an associated safety concept.

May et al. (2011) proposed a Structured procedure for preparing site-specific monitoring plans, including the following steps:

Mapping of monitoring areas;
Classification of monitoring intensity;
Definition of monitoring tasks;
Selection of monitoring methods;
Specification of measurements and observations.

Within this procedure, the allocation of the purposes to the matrix (Tab. 1-4) can be used to verify the completeness of site-specific monitoring plans. Integrating various monitoring purposes and tasks helps reducing the number of methods required to provide all the information needed for safe, durable and environmentally friendly storage of CO2Carbon dioxide during the entire life-time of a project. Examples for site-specific monitoring plans are given in Chapter 4.

Tab. 1-4: Comprehensive, generic monitoring framework: monitoring purposes with regard to different compartments and project phases (may et al., 2011). Symbols in brackets indicate the need of case-specific considerations.

Tab2