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

 
1. INTRODUCTION
 
2. MONITORING TECHNIQUES
 
3. MONITORING CONCEPTS – STATUS QUO
 
4. SETTING UP A SITE-SPECIFIC MONITORING PLAN
 
5. CONCLUSIONS AND RECOMMENDATIONS
 
6. REFERENCES
 
 
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
 
 
» 2.8.1 Soil and Seabed Gas Monitoring
 
2.8.2 Atmospheric Monitoring
 
2.8.3 Tracers – natural and introduced
 
2.8.4 CO2 detection in shallow subsurface
 
2.8.5 Vegetation stress and changes
 
2.8.6 Biological monitoring
 
 

2.8.1 Soil and Seabed Gas Monitoring

Gas composition and isotopic signatures: Chemical composition of gases collected at soil and subsoil depth (or sea bed samples in offshore wells) can be used to quantify CO2 concentrations at a certain depth (usually 1-2 m) or concentration profiles (by depth) and assess whether CO2 originates from natural or non-biologic sources (e.g. fossil fuel combustion). Numerical simulation studies of leakage and seepage demonstrate that CO2 concentrations can attain high levels in the shallow subsurface even for relatively moderate CO2 leakage fluxes (Oldenburg and Unger, 2003).

Soil pore gas concentrations and isotopic composition can be measured using a variety of techniques, including drive points (geoprobes), infra-red gas analysis (see 2.8.2), gas chromatography, and mass spectrometry.

The soil gas technique provides accurate measurements of CO2 concentration at a particular location, but data interpretation depends on the sampling grid. The spatial resolution must be considered. From studies on natural analogues, it is known that leak points could be small and localised. A higher sampling density is achievable increasing the costs and decreasing the speed of ground coverage. In general, the application of soil gas monitoring is a more time consuming and expensive method than surface gas monitoring for CO2 leakage detection (e.g. Klusman, 2011).

Distinct isotopic signatures can be used to detect CO2 leakage (e.g. Klusman, 2011). Reaction of CO2 with the formation water results in more acidic water with increased dissolved inorganic carbon (lowering the δ13C value of bicarbonate). The more acidic brine drives calcite and dolomite dissolution, resulting in higher pH values, increasing δ13C ratios of bicarbonate, and increased Mg2+, Ca2+, dissolved inorganic carbon concentrations and total alkalinity in the water. The net result is an average field-wide δ13C (HCO3-) decrease with time (Shevalier et al., 2005). monitoring of these isotopic signatures at or around the abandoned wellbore can indicate leakage of CO2.

Flux Accumulation Chambers: An accumulation chamber with an open bottom (cm2 scale) is placed either directly on the soil surface or on a collar installed on the ground surface. Air contained in the chamber is circulated through, e.g. an Infrared Gas Analyser, and the rate of change in CO2 concentration in the chamber is used to derive the flux of CO2 across the ground surface at the point of measurement (Norman et al., 1992). Advanced techniques include using other trace gases, such as radon, as proxies for determining and differentiating gas fluxes from depth (Baubron, 2005). These chambers quantify the CO2 flux from the soil at a small, predetermined area. This technology can quickly and effectively determine CO2 fluxes from the soil. It allows collection of high quality gas sample, from which naturally occurring tracers, such as isotopes or noble gasses, or introduced tracers can be detected.

Alternatively, the CO2 flux at a given locality can be evaluated using monitoring probes that are set in the ground at different levels and/or at the surface.

Flux is assumed to be more closely related to leakage rate than is concentration. However, monitoring a large area requires many installations. Also, soil gas flux has a strong seasonal and other temporal variability that has to be understood in order to provide leakage estimates. For this, baseline measurements are essential. Soil flux measurements are not effective if the water table is close to surface or if the soil is wet or frozen.

(Sea)Water chemistry and hydroacoustic techniques: For offshore CO2 storage sites and in onshore surface water bodies, the chemical analysis of the sea/lake bed sediment samples, water samples (Annunziatellis et al., 2009; Schuster et al., 2009), combined with indirect monitoring techniques such as ecosystems stress monitoring of the sea bed and hydroacoustic techniques, can be jointly used for detecting CO2 leakage on the sea (or lake) floor.

Hydroacoustic techniques comprise a variety of possible sources (e.g. single beam and multibeam echosounders, sidescan sonar systems) all with broad frequency contents of thousands to tens of thousands of Hz. These hydroacoustic techniques allow detection of gas bubble streams resulting from a potential CO2 leakage. In more detail, these techniques are capable of detecting individual bubble streams in the water column, tracing them to the seabed and estimating gas fluxes. For example, observation of bubble streams above the abandoned Wells or in their close vicinity may indicate CO2 leakage directly through the abandoned wellbore or from the wellbore to the surrounding formations due to casing and cement failure (von Deimling et al., 2010). In particular, multibeam methods can be used to rapidly and efficiently survey lakes and larger offshore areas.