Seasonal dependence of false-positives in detection of anomalous pCO2 using the covariance method with dissolved oxygen in monitoring offshore CO2 storage sites

Abstract

To elucidate the dependence of frequency of false-positives on the season for the covariance threshold based on partial pressure of CO₂ (pCO₂) and the percentage dissolved oxygen saturation (DO%) for monitoring offshore CO₂ storage, the deviation of observed pCO₂ from a linear regression of pCO₂ on DO% was investigated. The analyzed data were continuously observed for nearly 1 year in a semi-enclosed bay. Observed pCO₂ tended to be higher than the linear regression in summer and fall, when false-positives would be more frequent. It is inferred that the difference in the air-sea equilibrium timescale between CO₂ and O₂ as well as high water temperature cause the higher pCO₂ in fall while the higher pCO₂ in summer is associated with hypoxic water. Continuous observations showed features of pCO₂ and DO% variations more clearly than multiyear sporadic observations. This study provides data on anomalies necessary for developing strategies for marine monitoring.

Keywords: CCS; Dissolved oxygen; Leakage; Marine monitoring; Offshore CO(2) storage; pCO(2).