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

Fate of Dissolved Methane from Ocean Floor Seeps

Methane is an important greenhouse gas, with a global warming potential that is far higher than that of CO2. Methane from seafloor seeps, whether naturally occurring or in relation to petroleum infrastructure, has been suggested to be a significant contribution to greenhouse gas releases. Here, we consider the fate of methane from seeps on the Norwegian continental shelf by means of models for dissolution of methane from rising bubbles, mixing and biodegradation of dissolved methane, and mass transfer to the atmosphere. Laboratory experiments with tritium-labeled methane have been conducted to help determine the biodegradation rate of methane in natural seawater, and the results, together with literature data, have been used to guide the modeling. From the modeling study, we present results as a function of biodegradation half-life, treating this as a free parameter to reflect the considerable span in values reported in the literature. Considering three different locations on the Norwegian continental shelf, we find that if the biodegradation half-life of methane is in the range of a 9 to 16 days, as suggested by our experiments, then about 57-68% of the released methane will biodegrade in the water column from a seep at 65 m depth. For deeper locations of 106 and 303 m, we find respectively 75-83%, and more than 99% biodegradation.

Autonomous Large-Scale Radon Mapping and Buoyant Plume Modeling Quantify Deep Submarine Groundwater Discharge: A Novel Approach Based on a Self-Sufficient Open Ocean Vehicle

Groundwater discharge into the sea occurs along many coastlines around the world in different geological settings and constitutes an important component of global water and matter budget. Estimates of how much water flows into the sea worldwide vary widely and are largely based on onshore studies and hydrological or hydrogeological modeling. In this study, we propose an approach to quantify a deep submarine groundwater outflow from the seafloor by using autonomously measured ocean surface data, i.e., ²²²Rn as groundwater tracer, in combination with numerical modeling of plume transport. The model and field data suggest that groundwater outflows from a water depth of ∼100 m can reach the sea surface implying that several cubic meters per second of freshwater are discharged into the sea. We postulate an extreme rainfall event 6 months earlier as the likely trigger for the groundwater discharge. This study shows that measurements at the sea surface, which are much easier to conduct than discharge measurements at the seafloor, can be used not only to localize submarine groundwater discharges but, in combination with plume modeling, also to estimate the magnitude of the release flow rate.

Numerical Estimation of Gas Release and Dispersion from a Submarine Pipeline

Submarine pipeline gas releases and dispersions can cause safety concerns such as fire and explosion, which can cause serious casualties and property losses. There are many existing studies on the impacts of the horizontal diffusion distances of natural gas leakages from subsea pipelines, but there is a lack of research on the impact of influencing factors on vertical diffusion distances. Therefore, a diffusion model of natural gas leakage from a submarine pipeline is established by using the computational fluid dynamics method (CFD). The influence law and degrees of factors such as water depth at the leakage point, leak orifice size, leak pressure and the ocean current’s velocity on the leakages and vertical diffusion distances of natural gases from submarine pipelines are systematically investigated. The results show that the leaked natural gas jet enters the sea water to form an air mass, which rises continuously under the action of the pressure in the pipe and the buoyancy of the sea water. The gas mass breaks into smaller bubbles affected by the interaction between the gas–liquid two phases and continues to float up and diffuse to the overflow surface. It is also found that the ocean current’s velocity will affect the offset of leakage gas along the current direction; the depth of the leakage water, the pressure in the pipe and the leakage aperture will affect the time when the gas reaches the sea surface and the release area after a submarine pipeline’s leakage. The research results would help to support risk assessments and response planning of potential subsea gas release accidents.

Relative sensitivity of hydrodynamic, thermodynamic, and chemical processes for simulating the buoyant multiphase plume and surfacing flows of an oil and gas blowout

Deepwater hydrocarbon releases experience complex chemical and physical processes. To assess simplifications of these processes on model predictions, we present a sensitivity analysis using simulations for the Deepwater Horizon oil spill. We compare the buoyant multiphase plume metrics (trap height, rise time etc), the hydrocarbon mass flowrates at the near-field plume termination and their mass fractions dissolved in the water column and reaching the water surface. The baseline simulation utilizes a 19-component hydrocarbon model, live-fluid state equations, hydrate dynamics, and heat and mass transfer. Other simulations turn-off each of these processes, with the simplest one using inert oil and methane gas. Plume metrics are the least sensitive to the modeled processes and can be matched by adjusting the release buoyancy flux. The mass flowrate metrics are more sensitive. Both liquid- and gas-phase mass transfer should be modeled for accurate tracking of soluble components (e.g. C₁ - C₇ hydrocarbons) in the environment.

Dynamics of Live Oil Droplets and Natural Gas Bubbles in Deep Water

Explaining the dynamics of gas-saturated live petroleum in deep water remains a challenge. Recently, Pesch et al. [ Environ. Eng. Sci. 2018, 35 (4), 289-299] reported laboratory experiments on methane-saturated oil droplets under emulated deep-water conditions, providing an opportunity to elucidate the underlying dynamical processes. We explain these observations with the Texas A&M Oil spill/Outfall Calculator (TAMOC), which models the pressure-, temperature-, and composition-dependent interactions between oil-gas phase transfer; aqueous dissolution; and densities and volumes of liquid oil droplets, gas bubbles, and two-phase droplet-bubble pairs. TAMOC reveals that aqueous dissolution removed >95% of the methane from ∼3.5 mm live oil droplets within 14.5 min, prior to gas bubble formation, during the experiments of Pesch et al. Additional simulations indicate that aqueous dissolution, fluid density changes, and gas-oil phase transitions (ebullition, condensation) may all contribute to the fates of live oil and gas in deep water, depending on the release conditions. Illustrative model scenarios suggest that 5 mm diameter gas bubbles released at a <470 m water depth can transport methane, ethane, and propane to the water surface. Ethane and propane can reach the water surface from much deeper releases of 5 mm diameter live oil droplets, during which ebullition occurs at water depths of <70 m.