Numerical Prediction of the Behavior of CO2 Bubbles Leaked from Seafloor and Their Convection and Diffusion near Southeastern Coast of Korea

Among various carbon capture and storage technologies to mitigate global warming and ocean acidification due to greenhouse gases, ocean geological storage is considered the most feasible for Korea due to insufficient inland space to store CO2. However, the risk of CO2 leakage and the behavior and environmental effects of the leaked CO2 need to be assessed for its successful implementation. Therefore, the behavior of CO2 bubbles/droplets dissolving into the surrounding seawater and the diffusion of dissolved CO2 by ocean flows should be accurately predicted. However, finding corresponding research has been difficult in Korea. Herein, the behavior and convection-diffusion of CO2 that was assumed to have leaked from the seafloor near the southeastern coast of Korea were numerically predicted using a multi-scale ocean model for the first time. In the simulation region, one of the pilot projects of CO2 ocean geological storage had started but has been temporarily halted. In the ocean model, hydrostatic approximation and the Eulerian–Lagrangian two-phase model were applied for meso- and small-scale regions, respectively. Parameters for the simulations were the leakage rate and the initial diameter of CO2. Results revealed that all leaked and rising CO2 bubbles were dissolved into the seawater before reaching the free surface; further, the change in the partial pressure of CO2 did not exceed 500 ppm during 30 days of leakage for all cases.

Binary Time Series Classification with Bayesian Convolutional Neural Networks When Monitoring for Marine Gas Discharges

The world’s oceans are under stress from climate change, acidification and other human activities, and the UN has declared 2021–2030 as the decade for marine science. To monitor the marine waters, with the purpose of detecting discharges of tracers from unknown locations, large areas will need to be covered with limited resources. To increase the detectability of marine gas seepage we propose a deep probabilistic learning algorithm, a Bayesian Convolutional Neural Network (BCNN), to classify time series of measurements. The BCNN will classify time series to belong to a leak/no-leak situation, including classification uncertainty. The latter is important for decision makers who must decide to initiate costly confirmation surveys and, hence, would like to avoid false positives. Results from a transport model are used for the learning process of the BCNN and the task is to distinguish the signal from a leak hidden within the natural variability. We show that the BCNN classifies time series arising from leaks with high accuracy and estimates its associated uncertainty. We combine the output of the BCNN model, the posterior predictive distribution, with a Bayesian decision rule showcasing how the framework can be used in practice to make optimal decisions based on a given cost function.

The effects of elevated carbon dioxide levels on a Vibrio sp. isolated from the deep-sea

INTRODUCTION

The effect of oceanic CO2 sequestration was examined exposing a deep-sea bacterium identified as Vibrio alginolyticus (9NA) to elevated levels of carbon dioxide and monitoring its growth at 2,750 psi (1,846 m depth).

FINDINGS

The wild-type strain of 9NA could not grow in acidified marine broth below a pH of 5. The pH of marine broth did not drop below this level until at least 20.8 mM of CO2 was injected into the medium. 9NA did not grow at this CO2 concentration or higher concentrations (31.2 and 41.6 mM) for at least 72 h. Carbon dioxide at 10.4 mM also inhibited growth, but the bacterium was able to recover and grow. Exposure to CO2 caused the cell to undergo a morphological change and form a dimple-like structure. The membrane was also damaged but with no protein leakage.

Multi-scale modeling of CO2 dispersion leaked from seafloor off the Japanese coast

A numerical simulation was conducted to predict the change of pCO(2) in the ocean caused by CO(2) leaked from an underground aquifer, in which CO(2) is purposefully stored. The target space of the present model was the ocean above the seafloor. The behavior of CO(2) bubbles, their dissolution, and the advection-diffusion of dissolved CO(2) were numerically simulated. Here, two cases for the leakage rate were studied: an extreme case, 94,600 t/y, which assumed that a large fault accidentally connects the CO(2) reservoir and the seafloor; and a reasonable case, 3800 t/y, based on the seepage rate of an existing EOR site. In the extreme case, the calculated increase in DeltapCO(2) experienced by floating organisms was less than 300 ppm, while that for immobile organisms directly over the fault surface periodically exceeded 1000 ppm, if momentarily. In the reasonable case, the calculated DeltapCO(2) and pH were within the range of natural fluctuation.

Experimental investigation of the rising behavior of CO2 droplets in seawater under hydrate-forming conditions

In a laboratory-based test series, seven experiments along a simulated Pacific hydrotherm at 152 degrees W, 40 degrees N were carried out to measure the rise velocities of liquefied CO2 droplets under (clathrate) hydrate forming conditions. The impact of a hydrate skin on the rising behavior was investigated by comparing the results with those from outside the field of hydrate stability at matching buoyancy. A thermostatted high-pressure tank was used to establish conditions along the natural oceanic hydrotherm. Under P-/T-conditions allowing hydrate formation, the majority of the droplets quickly developed a skin of CO2 hydrate upon contact with seawater. Rise rates of these droplets support the parametrization by Chen et al. (Tellus 2003, 55B, 723-730), which is based on empirical equations developed to match momentum of hydrate covered, deformed droplets. Our data do not support other parametrizations recently suggested in the literature. In the experiments from 5.7 MPa, 4.8 oC to 11.9 MPa, 2.8 degrees C positive and negative deviations from predicted rise rates occurred, which we propose were caused by lacking hydrate formation and reflect intact droplet surface mobility and droplet shape oscillations, respectively. This interpretation is supported by rise rates measured at P-/T-conditions outside the hydrate stability field atthe same liquid CO2-seawater density difference (delta rho) matching the rise rates of the deviating data within the stability field. The results also show that droplets without a hydrate skin ascend up to 50% faster than equally buoyant droplets with a hydrate skin. This feature has a significant impact on the vertical pattern of dissolution of liquid CO2 released into the ocean. The experiments and data presented considerably reduce the uncertainty of the parametrization of CO2 droplet rise velocity, which in the past emerged partly from their scarcity and contradictions in constraints of earlier experiments.

Fate of rising CO2 droplets in seawater

The sequestration of fossil fuel CO2 in the deep ocean has been discussed by a number of workers, and direct ocean experiments have been carried out to investigate the fate of rising CO2 droplets in seawater. However, no applicable theoretical models have been developed to calculate the dissolution rate of rising CO2 droplets with or without hydrate shells. Such models are important for the evaluation of the fate of CO2 injected into oceans. Here, I adapt a convective dissolution model to investigate the dynamics and kinetics of a single rising CO2 droplet (or noninteracting CO2 droplets) in seawater. The model has no free parameters; all of the required parameters are independently available from literature. The input parameters include: the initial depth, the initial size of the droplet, the temperature as a function of depth, density of CO2 liquid, the solubility of CO2 liquid or hydrate, the diffusivity of CO2, and viscosity of seawater. The effect of convection in enhancing mass transfer is treated using relations among dimensionless numbers. The calculated dissolution rate for CO2 droplets with a hydrate shell agrees with data in the literature. The theory can be used to explore the fate of CO2 injected into oceans under various temperature and pressure conditions.

Hydrate composite particles for ocean carbon sequestration: field verification

This paper reports on the formation and dissolution of CO2/seawater/CO2 hydrate composite particles produced during field experiments in Monterey Bay, CA using a CO2 injector system previously developed in the laboratory. The injector consisted of a coflow reactor wherein water was introduced as a jet into liquid CO2, causing vigorous mixing of the two immiscible fluids to promote the formation of CO2 hydrate that is stable at ambient pressures and temperatures typical of ocean depths greater than approximately 500 m. Using flow rate ratios of water and CO2 of 1:1 and 5:1, particulate composites of CO2 hydrate/liquid CO2/seawater phases were produced in seawater at depths between 1100 and 1300 m. The resultant composite particles were tracked by a remotely operated vehicle system as they freely traveled in an imaging box that had no bottom or top walls. Results from the field experiments were consistent with laboratory experiments, which were conducted in a 70 L high-pressure vessel to simulate the conditions in the ocean at intermediate depths. The particle velocity and volume histories were monitored and used to calculate the conversion of CO2 into hydrate and its subsequent dissolution rate after release into the ocean. The dissolution rate of the composite particles was found to be higher than that reported for pure CO2 droplets. However, when the rate was corrected to correspond to pure CO2, the difference was very small. Results indicate that a higher conversion of liquid CO2 to CO2 hydrate is needed to form negatively buoyant particles in seawater when compared to freshwater, due primarily to the increased density of the liquid phase but also due to processes involving brine rejection during hydrate formation.

CO2 hydrate composite for ocean carbon sequestration

Rapid CO2 hydrate formation was investigated with the objective of producing a negatively buoyant CO2-seawater mixture under high-pressure and low-temperature conditions, simulating direct CO2 injection at intermediate ocean depths of 1.0-1.3 km. A coflow reactor was developed to maximize CO2 hydrate production by injecting water droplets (e.g., approximately 267 microm average diameter) from a capillary tube into liquid CO2. The droplets were injected in the mixing zone of the reactor where CO2 hydrate formed at the surface of the water droplets. The water-encased hydrate particles aggregated in the liquid CO2, producing a paste-like composite containing CO2 hydrate, liquid CO2, and water phases. This composite was extruded into ambient water from the coflow reactor as a coherent cylindrical mass, approximately 6 mm in diameter, which broke into pieces 5-10 cm long. Both modeling and experiments demonstrated that conversion from liquid CO2 to CO2 hydrate increased with water flow rate, ambient pressure, and residence time and decreased with CO2 flow rate. Increased mixing intensity, as expressed by the Reynolds number, enhanced the mass transfer and increased the conversion of liquid CO2 into CO2 hydrate. Using a plume model, we show that hydrate composite particles (for a CO2 loading of 1000 kg/s and 0.25 hydrate conversion) will dissolve and sink through a total depth of 350 m. This suggests significantly better CO2 dispersal and potentially reduced environmental impacts than would be possible by simply discharging positively buoyant liquid CO2 droplets. Further studies are needed to address hydrate conversion efficiency, scale-up criteria, sequestration longevity, and impact on the ocean biota before in-situ production of sinking CO2 hydrate composite can be applied to oceanic CO2 storage and sequestration.

Experimental determination of the fate of rising CO2 droplets in seawater

Direct oceanic disposal of fossil fuel CO2 is being considered as a possible means to moderate the growth rate of CO2 in the atmosphere. We have measured the rise rate and dissolution rate of freely released CO2 droplets in the open ocean to provide fundamental data for carbon sequestration options. A small amount of liquid CO2 was released at 800 m, at 4.4 degrees C, and the rising droplet stream was imaged with a HDTV camera carried on a remotely operated vehicle. The initial rise rate for 0.9-cm diameter droplets was 10 cm/s at 800 m, and the dissolution rate was 3.0 micromol cm(-2) s(-1). While visual contact was maintained for 1 h and over a 400 m ascent, 90% of the mass loss occurred within 30 min over a 200 m ascent above the release point. Images of droplets crossing the liquid-gas-phase boundary showed formation of a gas head, pinching off of a liquid tail, and rapid gas bubble separation and dissolution.

The sequestration switch: removing industrial CO2 by direct ocean absorption

This review paper considers direct injection of industrial CO2 emissions into the mid-water oceanic column below 500 m depth. Such a process is a potential candidate for switching atmospheric carbon emissions directly to long term sequestration, thereby relieving the intermediate atmospheric burden. Given sufficient research justification, the argument is that harmful impact in both the Atmosphere and the biologically rich upper marine layer could be reduced. The paper aims to estimate the role that active intervention, through direct ocean CO2 storage, could play and to outline further research and assessment for the strategy to be a viable option for climate change mitigation. The attractiveness of direct ocean injection lies in its bypassing of the Atmosphere and upper marine region, its relative permanence, its practicability using existing technologies and its quantification. The difficulties relate to the uncertainty of some fundamental scientific issues, such as plume dynamics, lowered pH of the exposed waters and associated ecological impact, the significant energy penalty associated with the necessary engineering plant and the uncertain costs. Moreover, there are considerable uncertainties regarding related international marine law. Development of the process would require acceptance of the evidence for climate change, strict requirements for large industrial consumers of fossil fuel to reduce CO2 emissions into the Atmosphere and scientific evidence for the overall beneficial impact of ocean sequestration.