Key Reports:

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

2. MONITORING TECHNIQUES

A number of established, reliable methods and tools exists for near-surface monitoring at CO2 storage sites regarding i) gas monitoring, ii) biomonitoring (micro- and macrocosm), iii) ecological monitoring (populations and systems). Well-established deep subsurface technologies are also available that give information about the amount and the migration of CO2 underground. For example, seismic measurements are at present the dominating geophysical methods for monitoring CO2 injection in saline aquifers and depleted hydrocarbon reservoirs. The method allows, in most cases, detailed mapping of the migration of the CO2 plume, and reasonably accurate volume estimates may be achieved by using appropriate assumptions.

The various monitoring techniques have their specific advantages and shortcomings in terms of sensitivity, reliability, capability, e.g. for point vs. wide area measurements or continuous vs. discontinuous measurements. These aspects are introduced and discussed in the relevant Sections that cover the various monitoring compartments. For example, to provide an early warning of CO2 migration to shallower depths, monitoring can be performed in wells in the subsurface. Monitoring in injection or observation wells typically involves low background variability; however, often results in small/weak signals. Shallow subsurface technologies are able to detect and quantify amounts of CO2 that have leaked into the shallow overburden, soils or the seabed or, ultimately, the oceans or atmosphere. In contrast to measurements in the shallow subsurface where background variability is typically moderate, the high background variability noted at the surface is a major challenge for surface/water monitoring technologies.

For a comprehensive monitoring, various techniques are needed with very different characteristics combining i) continuous and discontinuous techniques, since a leak may vary with time and thus might be missed by one-off sampling, as well as ii) point and wide-area techniques, since large areas need to be covered rapidly because storage sites can cover many km², but targets (leaks) may be rather small.

In this chapter state-of-the-art and emerging monitoring techniques are introduced and their applicability, shortcomings and detection limits will be discussed in the context of monitoring of identified risks of geological CO2Carbon dioxide storage. This collation of techniques is done compartment-wise, i.e. distinguishing techniques:

to monitor the extension and migration of the CO2Carbon dioxide plume in the storage reservoir,
to track potential CO2Carbon dioxide leakage out of reservoir considering neighbouring aquifers (saline and freshwater) and the overburden including faults;
to detect potential impacts such as surface uplift, induced seismicity, fault reactivation,
to assess the sealing of abandoned wells and ,
to detect potential leakage and monitor potential impacts in near-surface eco-compartments.

In addition to the techniques' specific characteristics, special reference will be given to various "boundary conditions" to be considered when selecting monitoring tools such as location of the site (onshore/offshore), site accessibility (depending on land-use, topography, wells), volume to be monitored (considering depth, spread, pressure footprint).

An overview of potential CO2Carbon dioxide monitoring techniques and their applicability for monitoring of deep or shallow processes, for locating the CO2 plume, monitoring of fine scale processes, detection and quantification of a leakage was given by Pearce (Fig. 2-1). These authors group the potential monitoring techniques as techniques for primary and secondary use.

Fig _2_1 Fig. 2-1: Potential CO2Carbon dioxide monitoring techniques and their applications (from Pearce et al., 2005); ESP = Electric spontaneous potential; VSP = Vertical Seismic Profiling; EM = Electromagnetics; ERT = Electrical Resistance Tomography; IR = Infrared detector; NDIR = Non-dispersive infrared spectrometer.

For the purposes of tool selection for site-specific monitoring plans, monitoring methods can be grouped into three categories, based on application, function, and stage of development:

Primary Technology - A proven and mature monitoring technology or application.

Secondary Technology - An available technology that can provide insight into CO2Carbon dioxide behaviour and that will help refine the use of primary technologies.

Additional Technology - A technology which is research-related and might answer fundamental questions concerning the behaviour of CO2Carbon dioxide in the subsurface and which might have some benefit as a monitoring tool after testing in the field.

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

2. MONITORING TECHNIQUES

in depth

2.1 CO2 plume migration in the storage reservoir

2.2 Surface uplift

2.3 Induced seismicity and mechanical reaction of overburden

2.4 Faults

2.5 Abandoned wells

2.6 Overlying and adjacent aquifers

2.7 Freshwater aquifers

2.8 Near surface eco-compartments