Modeling of biogeochemical consequences of a CO2 leak in the water column with bottom anoxia
Introduction
Carbon capture and storage (CCS) has emerged as a promising technology for reducing greenhouse gas emissions and reaching international climate goals. During and after injection into an offshore geological reservoir, monitoring is required to verify that the CO2 stays in the reservoir as intended, and to ensure that there is no negative impact on the surrounding environment (Blackford, 2014). While seismic methods are used to monitor the geological reservoir and the overburden, marine monitoring is intended to verify that there are no indications of leakage at the seabed or in the water column. Anomalies potentially related to leakage include changes in the chemical composition of the sediment pore water and in the water column, the appearance of seep-related features on the seafloor such as bacterial mats or pockmarks, microbubbles in the sediment, and bubble trains in the water column. A comprehensive CCS monitoring program should incorporate multiple technologies to target the different ways in which a leak-related anomaly may occur (Blackford, 2020).
Several recent and ongoing research projects contribute to knowledge building and maturing of marine monitoring strategies. The ECO2 project built a framework for environmental practices related to CCS (http://www.eco2-project.eu/). As part of the QICS project (Blackford, 2015), controlled releases of CO2 made it possible to understand and document the environmental impact of CO2 leakage, and to evaluate the detectability of leaks. Lately, the STEMM-CCS project (STEMM-CCS, 2019) covers both the marine environment and reservoir overburden sediment. Results include modeling of natural fluctuations in the CO2 concentration in seawater, which can be several orders of magnitude, both spatially and temporally and modeling of leakage scenarios and subsequent optimization of sensor locations and movement patterns for mobile sensor platforms (Blackford et al., 2017). The results allowed the influence of potential leaks on the pH distribution in the vicinity of the leak to be modelled (Vielstädte, 2019). A comprehensive controlled CO2 release experiment was also included in the ACT4storage (Acoustic and chemical technologies for environmental monitoring of geological carbon storage) project, where the focus was on evaluating the capabilities and limitations of different acoustic and chemical sensor technologies for marine monitoring of offshore CO2 storage sites.
While it is considered unlikely that sequestered CO2 will migrate from a geological reservoir and reach the seabed, it is important to study the environmental impact that a hypothetical leak might have and to have tools and solutions in place to detect a leak if it should occur. A potential CO2 leak may be accompanied or preceded by a release of pore water from the sediments. The evacuated pore water will have different characteristics compared to the bottom sea water, such as lower levels of dissolved oxygen, variations in salinity, etc. We can hypothesize that a leak will lead to anoxic conditions at the sediment-water interface, or potentially to formation of a bottom anoxic layer. In this work, we combine experimental data from a controlled CO2 release experiment with a biogeochemical model to better understand the chemical footprint of a hypothetical leak under anoxic conditions. Anoxic bottom conditions may either be formed during a leakage or be present prior to the leakage event. In this research, we analyze the potential changes in the carbonate system that can be affected by anoxic conditions, from point of view of their detectability with existing sensors. Such data is novel supplementary to the leak experiments conducted in normoxic conditions, i.e. STEMM-CCS. The controlled release experiment was carried out in a sheltered environment in the Oslo Fjord with bottom anoxia (the Horten Inner Harbor, Fig. 1) as part of the ACT4storage project. In the event of unintended CO2 migration from the reservoir reaching the seabed, the CO2 may enter the water column in gas phase or dissolved in seawater, or both. To mimic this situation, the controlled release experiment included releasing both gaseous CO2 and CO2 dissolved in seawater. During the 6-week period of the controlled release experiment, acoustic and chemical sensors mounted on seabed templates were programmed to continuously record data, capturing natural variability as well as simulated CO2 leak events. The goals of this experiment were to better understand the effects of a potential CO2 leak by quantifying background variations in relevant chemical and oceanographical parameters (CO2, O2, pH, salinity, temperature, and ocean currents), and to evaluate different technologies' ability to detect the changes in parameters connected with the simulated CO2 leak. A biogeochemical model was used to analyze biogeochemical consequences of a CO2 leak in the water column with anoxia. The simulations were in good agreement with the baseline observations and measured changes forced by the experimental leak and could be used as a reliable instrument to simulate natural variability and predict the effects of a potential CO2 leak.
Section snippets
Model description
To analyze the biogeochemical consequences of the leak, we used a 2 -dimensional vertical model consisting of a biogeochemical module, a transport module and a new bubble fate module described here. We combined the biogeochemical module of Bottom RedOx Model, BROM(Yakushev et al., 2017), with the 2-Dimensional Benthic-Pelagic model, 2DBP for vertical and horizontal transport of matter in the water column and upper sediments (Yakushev et al. 2020). In addition we use a new BROM family bubble
Observations
The Nearshore 2018 controlled release experiment was organized in the Horten Harbor, a small 2 km wide and 20 m deep bay in the Open Oslo fjord (Fig. 1). The sheltered environment and limited ocean currents is an advantage because it allows good control of the simulated leak with the applied sensors. Because of restricted water mixing, the bottom layer of the Harbor is anoxic, and the chemical composition of the seawater at depths below ∼ 13 m dramatically differs from what would be expected in
Discussions
Existing CCS storage sites are situated in seas with normoxic conditions, where potential leaks can lead to appearance of anoxic plumes that can occupy the bottom layers. The aim of this work was to study the implications of a CO2 leak in an anoxic environment. Suboxic and anoxic conditions are characterized by the presence of non-typical oceanic water biogeochemical processes including mineralization of organic matter with oxidized forms of N, Mn, Fe, S and consumption of oxygen for reduced
Conclusions
To study the biogeochemical consequences of a release of CO2 in the marine environment a FABM family set of models consisting of a transport model, biogeochemical model (including carbonate system processes block) and bubble fate (transport and dissolution) was applied. The model was used to study the consequences of a 2-h long discharge of CO2 with rate of 11 l/min. CO2 bubbles are dissolved shortly after termination of the leak, while changes in pH, pCO2 and TIC can be detected for several
Funding
This work has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No.654462 (STEMM-CCS) the Norwegian Research Council project “Acoustic and chemical technologies for environmental monitoring of geological carbon storage (ACT4storage)”. EY and EP were additionally supported by the Norwegian Foreign Ministry and its Arctic 2030 program (project PERMAFLUX), the FRAM High North Research center for Climate and the Environment under the Ocean
Author contributions
Conceptualization, E.Y.; methodology, E.Y.,A.B.; software, E.Y.; validation, E.Y.,A.B.; visualization, E.E.,E.P.; resources, E.E.; writing—original draft preparation, E.Y.,A.B.; writing—review and editing, E.Y.,A.B.,E.E.,C.T.,A.Z.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to acknowledge the work of Odd Arne Skogan who helped with sampling.
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These authors contributed equally