Sonja Bhatia

Ph.D. Thesis

Fibre optic applications for dissolved carbon dioxide monitoring of marine geologic sequestration sites.

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Marine-based Carbon Capture, Utilisation, and Storage (CCUS) has the potential to reduce the rate at which CO2 is added to the atmosphere thereby contributing to the mitigation potential for anthropogenic climate change. There has, however, been public and regulatory resistance to the technology due to the the possibility of leakage, which could occur within a 5 km radius of the injection site. The risks of marine CCUS could be better characterised using distributed sensor networks to monitor possible leakage areas.

Laboratories at the Universities of Toronto and Victoria demonstrated that Long Period Grating Fibre Optic Sensors (LPGs) can detect CO2 in laboratory conditions. These sensors have the potential for cost-effective, distributed sea oor deployment in sensor networks, but certain in situ deployment challenges must be overcome to move beyond the laboratory testing stage. In situ, LPGs will be susceptible to interference from non-target species and to biofouling. As such, sensor signal may not be uniquely responsive to CO2, and other substances or factors could contribute to noise, or could even mimic a seep or leak. Furthermore, sediment dynamic and pore-water water column exchange of total alkalinity and DIC may in uence both background variability and signal in the marine context.

The key research contributions of this thesis are as follows: (1) a characterisation of the effect of biofouling on the response of LPGs for both single and multi-species biofilms,(2) a defined set of concentrations of carbonate and non-carbonate species in application environments of interest and an estimate of the likelihood of interference with LPG signal as sensitivity improves, (3) a characterization of the impact in background variability in the key parameters of the carbonate system, (4) a discussion of the signal-to-noise ratio (SNR) for LPGs used in monitoring sea oor geologic sequestration sites, and (5) an evaluation of the technology readiness level of LPGs. Both laboratory experiments and field studies were conducted to ascertain the eect of biofilm growth on sensor response. These studies showed that biofouling would likely contribute increase the central wavelength of the attenuation band within the rst 30 h of LPG immersion in seawater. Hydrochemical modelling work using Phreeqc 2.18 revealed that carbonate species and non-carbonate species were likely to be present in concentrations below the detection limits of o-the-shelf LPGs. Finally, the contribution of these hydrochemical and biological variables to noise was calculated, and the type of signal escaping CO2 would generate was used to estimate SNR under dierent seepage and leakage scenarios. The estimated SNRs showed significant variability for seeps and leaks of the same scale under various pressure and temperature conditions. More work is needed to test LPGs under temperature and pressure conditions likely to be experienced in sea oor environments, and also in settings where hydrate coatings may form lms that prevent the LPG from contacting the uid to be measured.

Keywords: hydrochemistry, biofouling, fibre optic sensing, marine monitoring, carbon capture, utilisation and storage (CCUS), long period grating (LPGs), marine geochemistry, carbon dioxide sensing, dissolved carbon dioxide
Pages: 174
Supervisor: David Risk