Simulating and Quantifying Multiple Natural Subsea CO₂ Seeps at Panarea Island (Aeolian Islands, Italy) as a Proxy for Potential Leakage from Subseabed Carbon Storage Sites

Abstract

Carbon dioxide (CO2) capture and storage (CCS) has been discussed as a potentially significant mitigation option for the ongoing climate warming. Natural CO2 release sites serve as natural laboratories to study subsea CO2 leakage in order to identify suitable analytical methods and numerical models to develop best-practice procedures for the monitoring of subseabed storage sites. We present a new model of bubble (plume) dynamics, advection-dispersion of dissolved CO2, and carbonate chemistry. The focus is on a medium-sized CO2 release from 294 identified small point sources around Panarea Island (South-East Tyrrhenian Sea, Aeolian Islands, Italy) in water depths of about 40–50 m. This study evaluates how multiple CO2 seep sites generate a temporally variable plume of dissolved CO2. The model also allows the overall flow rate of CO2 to be estimated based on field measurements of pH. Simulations indicate a release of ∼6900 t y–1 of CO2 for the investigated area and highlight an important role of seeps located at >20 m water depth in the carbon budget of the Panarea offshore gas release system. This new transport-reaction model provides a framework for understanding potential future leaks from CO2 storage sites.

Major CO2 blowouts from offshore wells are strongly attenuated in water deeper than 50 m

“…The similarity in CO 2 leakage conditions at different depths carries over to the estimation of bubble size and size distribution as shown in Table . Note first that the volume‐median bubble diameter ( d 50 ) is approximately 0.5 mm for both cases, which is quite small relative to the 3–8 mm bubble diameters observed at natural gas seeps . The small bubble sizes in our simulations result from the large CO 2 blowout flow velocity and related turbulence.…”

“…Integral approaches are simple, robust, and computationally efficient and have been used for decades in both single and multiphase contexts . TAMOC has been extensively tested and validated on both laboratory and field measurements of various kinds of fluid and gas releases . Inputs to TAMOC include the CO 2 leakage rate; the diameter of the orifice out of which the CO 2 is leaking; the water depth, temperature, and salinity of the seawater at the leak point; the temperature and salinity profiles in the water column; and the background large‐scale cross current in the water column.…”

“…The concentration of component i just outside the bubble wall, C s,i , can be approximated by the solubility of component i in seawater . C i eq is calculated in our TAMOC simulations as the concentration that results from dissolution of the bubbles in the volume of water containing the bubble plume . In other words, the equilibrium seawater concentration of component i rises as bubbles dissolve, thereby decreasing the concentration gradient driving mass transfer.…”

“…16 In the area of offshore GCS feasibility and risk assessment, the evaluation of impacts of minor (0.1-1 kg s -1 ) and medium (1-10 kg s -1 ) leakage rates over large areas is a very active research area. [17][18][19] A hypothetical major CO 2 blowout scenario in the North Sea was evaluated for its impacts on ocean acidification and large-scale dissolved CO 2 circulation rather than on the details of ebullition, dissolution, or emission at the sea surface. 20 We have previously carried out high-flow-rate single-well CO 2 blowout simulations for onshore and offshore wells 21,22 without analysis of attenuation by the water column.…”

“…And well blowouts are known to occur during CO 2 injection for enhanced oil recovery . In the area of offshore GCS feasibility and risk assessment, the evaluation of impacts of minor (0.1–1 kg s –1 ) and medium (1–10 kg s –1 ) leakage rates over large areas is a very active research area . A hypothetical major CO 2 blowout scenario in the North Sea was evaluated for its impacts on ocean acidification and large‐scale dissolved CO 2 circulation rather than on the details of ebullition, dissolution, or emission at the sea surface .…”