[H-SC05] CCUS (Carbon Dioxide Capture, Utilization, and Storage) for Climate Mitigation

[HSC05-P07] Characteristics of pCO₂ variability in Osaka Bay 1: seasonal variability of covariance between pCO₂ and dissolved oxygen

*Keisuke Uchimoto^1,2, Yuji Watanabe^1,2, Ziqiu Xue^1,2 (1.Geological Carbon dioxide Storage Technology Research Association , 2.Research Institute of Innovative Technology for the Earth)

Keywords:CO2 leakage, CCS, offshore CO2 storage

One of public concerns about sub-seabed CO₂ geological storage is CO₂ leakage into the sea. When the sea is shallow, CO₂ would leak in the form of bubbles. CO₂ bubbles are, however, easy to dissolve in seawater, so that CO₂ concentration would increase in the event of CO₂ leakage. Thus, several methods to detect CO₂ leakage have been proposed based on variables regarding CO₂ concentration, e.g., partial pressure of CO2 (pCO₂), total dissolved inorganic carbon (DIC) and pH. The pCO₂-DO% covariance method is one of them. In the method, pCO₂ values exceeding the upper limit of a predicted interval of a linear regression of pCO₂ on the percentage dissolved oxygen saturation (DO%) are regarded as anomalously high values which may be induced by CO₂ leakage. To shed light on the relation between pCO₂ and DO%, and consequently improve the pCO₂-DO% covariance method, continuous observation of pCO₂, DO, and so on was carried out at 2 stations, in Port of Kobe and off KIX, in Osaka Bay for around 1 year. In this study, we analyzed data observed at the station in Port of Kobe. The data have shown negative correlation between pCO₂ and DO%. On the DO%-pCO₂ plane, there are two endmembers: hypoxic water with extremely high pCO₂ and extremely low DO% in summer and oxygen-rich water with pCO₂ lower than 400 μatm, which approximates to pCO₂ in the atmosphere, and DO% larger than 100 % in winter. In this station, the bottom layer is ventilated by the vertical mixing and by advection of oxygen-rich water from the western part of Osaka Bay. In summer, however, both the mechanisms do not work and so O₂ is depleted and CO₂ increases owing to respiration. In winter, on the other hand, photosynthesis as well as the ventilation makes pCO₂ decrease and DO increase. Interestingly, pCO₂ and DO% data do not simply oscillate between the two endmembers but cycle clockwise on the DO%-pCO₂ plane. Values of pCO₂ tend to be higher in fall than spring when DO% is the same value. It is inferred that the difference in pCO₂ values between fall and spring is caused by the difference in temperature between fall and spring, and the difference in the air-sea equilibrium timescale between CO₂ and O₂. This asymmetry between fall and spring makes false-positive occurrence frequent in fall when the pCO₂-DO% covariance method is used.

acknowledgement: This presentation is based on results obtained from a project (JPNP18006) commissioned by the New Energy and Industrial Technology Development Organization (NEDO).