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
 
 

2.8.5 Vegetation stress and changes

CO2 or brine leaks from underground storage sites may have significant impacts on local ecosystems in the shallow subsurface, the sea floor, and within the water column that could provide useful indicators. Detection techniques require initial surveys to establish baseline conditions above storage sites. Confidence in leakage detection will require improved understanding of how plant populations change in composition, quantity and health, as conditions change. One of the reasons for plant stress can be an increased CO2 concentration in the soil. Typically, the baseline CO2 concentration in the soil is expected to be only a few percent. Higher concentrations can kill plants through asphyxiation and soil acidification. The deterioration of the vegetation may indicate CO2 leakage. Locating such anomalies in the vegetation will help to identify pathways for CO2 leakage to the surface (Male et al., 2010).

The change in vegetation can be monitored by periodic visual inspections, or by imaging systems installed on platforms (Rouse et al., 2010). Satellite technology and planes can also be used to monitor the vegetation stress locations employing techniques such as colour infrared ortho-imagery and aerial photography. These approaches allow rapid large-area surveys. In offshore locations ecosystem stress monitoring is more difficult, but similar to onshore locations, changes in the flora and fauna around abandoned subsea wells can indicate CO2 leakage at the sea bottom.

Direct monitoring of ecosystem health provides confidence that the storage system is not causing damage, reduces the risk in case of leakage, and allow to decide if observed changes are the result of CO2 injection or not. However, ecosystem sensitivity towards CO2 leakage varies with species and setting, which may lead to methods to be insensitive (false negatives). Furthermore, many other factors may lead to ecosystem stress and to abundance changes that must be followed up using other techniques (false positives). In addition, there may be a time-shift between the occurrence of the leakage and the occurrence (and measurement) of changes in ecosystem health induced by a leakage. Data interpretation is complicated by a lack of quantitative data on the effects on marine and terrestrial ecosystems of excess CO2 from leaking storage sites (West et al., 2005).

Colour Infrared Transparency Film: This technology utilises three sensitised film layers that reproduce infrared as red, red as green, and green as blue, due to the way the dyes are coupled to these layers. All three layers are sensitive to blue so the film must be used with a minus blue (i.e., yellow) filter. Vegetative health can be determined from the relative strengths of green and infrared light reflected; this shows in colour infrared (CIR) as a shift from red (healthy) towards magenta (unhealthy). CIR aerial photos of specific project sites can be taken from an aircraft or by satellite to determine vegetative health in the vicinity of the project site as an indicator of a possible CO2 leakage pathway. Using a combination of wavelengths provides a better understanding of events occuring on the earth's surface. However, the presence of water interferes with the quality of the image due to absorption of near infrared wavelengths (appears black on the image).

Thermal Hyperspectral Imaging: Hyperspectral imaging collects and processes information from across the electromagnetic spectrum as a set of images. Each image represents a range of the electromagnetic spectrum, also known as a spectral band. These images are then combined and form a three dimensional hyperspectral cube for processing and analysis. Sensors may be airborne, satellite mounted, or hand held. Like CIR, hyperspectral imaging is an excellent tool in assessing vegetative integrity around an injection site. In Aerial hyperspectral imagery an entire spectrum is recorded at each point, the operator needs no prior knowledge of the sample, and post-processing allows all available information from the dataset to be mined. Data can be acquired over a relatively large area quickly and efficiently. Airborne or satellite deployment can image the whole area, even the poorly accessible on the ground. For data processing, fast computers, sensitive detectors, and large data storage capacities are needed.