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
 
 

2.8.2 Atmospheric Monitoring

CO2 Detectors: leakage of CO2 can be detected by analysing the air at the ground level. The measurements can be performed continuously by permanently installed detectors or intermittently with mobile CO2 detectors. Chemical CO2 sensors and infrared CO2 sensors are the most commonly used devices in the detection and measurement of CO2 concentrations. Chemical CO2 gas sensors have the advantage of consuming less energy compared to infrared detectors and are smaller in size. Short and long term drift effects and relatively low life times are the drawbacks of chemical sensors.

Infrared sensors are used to detect CO2 in a gaseous environment based on the characteristic absorption of CO2. The key components of an infrared sensor are an infrared source, a light tube, an interference (wavelength) filter, and an infrared detector. This type of monitoring is mostly used for initial assessment and to assure worker safety on site triggering automated alarm systems to warn at high CO2 levels. Higher sensitivities may be reached using the more sophisticated FTIR instruments.

Laser Systems: The laser systems use a laser beam with a wavelength of infrared light matched to CO2 absorption wavelength. The emitted light is absorbed by the CO2 in the air. If there is CO2 leakage to the surface, e.g. in the vicinity of an abandoned well, it can be detected by the attenuation of the emitted light. Concentration of CO2 in the air can be calculated by using the difference in the power of emitted light at the source and the detected light at the detector. Laser detection techniques offer more advantages than other gas detection methods: Laser technology does not suffer from interferences, except from other CO2 sources. It provides fast response and can measure a wide range of concentration values. Laser systems are more expensive compared to chemical and infrared sensors, but their ease of calibration and maintenance-free operation decreases the long-term cost of the equipment. In the field, open path laser detection can be used for measurements over short or greater distances (i.e. metre scale or tens of meters). Thus, traditional gas analysers and detectors are being replaced by laser systems based on the advantages they offer in measuring CO2 concentrations (Jones et al., 2009; Humphries et al., 2008).

An advanced leak detection system generates georeferenced CO2 concentration data along a path or route. The system incorporates a high sensitivity three-gas detector (CH4, total hydrocarbons, and CO2) with a Global Positioning System (GPS) with real-time mapping. This system is commonly applied to pipeline monitoring, transmission and liquid line monitoring, and landfill liner integrity monitoring via a ground or airborne vehicle. Detection of total gas composition can be used to separate leakage signal from processes that produce CO2. CO2 leakage by itself would displace all other gasses equally, whereas in-situ generation of CO2 by biologic action or combustion decreases oxygen. Similarly, open path lasers may be used for mapping of CO2 concentrations in the shallow atmosphere (e.g. Jones et al., 2009). However, this kind of mapping is sensitive to local meteorological conditions (especially wind blow) that may alter the atmospheric distribution of released gas. In consequence, precise location of gas vents on the ground may require the supplementary use of the chamber method.

Light Detection and Ranging (LIDAR): Light detection and ranging (LIDAR) is an optical remote sensing technology that measures properties of scattered light to find the range (or other information) of a distant target. Laser pulses are used to determine the distance to an object or surface. Similar to radar technology, which uses radio waves instead of light, the distance to an object is determined by measuring the time delay between transmission of a pulse and detection of the reflected signal. An open-path device uses a laser to shine a beam (with a wavelength that CO2 absorbs) over many meters. The attenuated beam reflects from a mirror and returns to the instrument for determination of the CO2 concentration. One instrument can sample a large area, if the beam can reflect from more than one mirror. The LIDAR technology is highly sensitive to aerosols and cloud particles and has many applications in atmospheric research and meteorology (Cracknell, 2007). Differential Absorption LIDAR is typically applied to detect atmospheric concentrations of CO2 above storage sites and in the vicinity of pipelines in R&D CO2 storage projects. It is a non-intrusive method to collect data in areas of limited access or containing potential physical or chemical hazards and it can penetrate vegetative canopy With this technique large areas can be covered in short time. In addition, LIDAR data collection is not limited to daylight hours, but appropriate weather conditions needed for operation since water absorbs or scatters laser pulses. The produced large data sets are difficult to store, manipulate, interpret and utilise.

Eddy Covariance: The Eddy covariance technique measures atmospheric CO2 fluxes at a height above the ground surface. These systems can detect CO2 fluxes over large areas in real time, along with micro-meteorological variables, such as wind velocity, relative humidity, and temperature (Anderson and Farrar, 2001; Baldocchi et al., 1996). Integration of these measurements allows derivation of the net CO2 flux over the upward footprint (either m2 or km2 scale, depending on tower height). Open-path systems tend to underestimate covariance due to sensor placement. Precipitation, winds from unfavourable directions, or extremely calm conditions can cause erratic, non-interpretable results (Baker, 2008). CO2 from many sources (vegetation, soil gas, industry, compressors, pipelines, etc.) may mask leakage signal because of the magnitude and temporal variability of these sources.