Observations of bubbles in natural seep flares at MC 118 and GC 600 using in situ quantitative imaging

Author Posting. © American Geophysical Union, 2016. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 121 (2016): 2203–2230, doi:10.1002/2015JC011452.This paper reports the results of quantitative imaging using a stereoscopic, high-speed camera system at two natural gas seep sites in the northern Gulf of Mexico during the Gulf Integrated Spill Research G07 cruise in July 2014. The cruise was conducted on the E/V Nautilus using the ROV Hercules for in situ observation of the seeps as surrogates for the behavior of hydrocarbon bubbles in subsea blowouts. The seeps originated between 890 and 1190 m depth in Mississippi Canyon block 118 and Green Canyon block 600. The imaging system provided qualitative assessment of bubble behavior (e.g., breakup and coalescence) and verified the formation of clathrate hydrate skins on all bubbles above 1.3 m altitude. Quantitative image analysis yielded the bubble size distributions, rise velocity, total gas flux, and void fraction, with most measurements conducted from the seafloor to an altitude of 200 m. Bubble size distributions fit well to lognormal distributions, with median bubble sizes between 3 and 4.5 mm. Measurements of rise velocity fluctuated between two ranges: fast-rising bubbles following helical-type trajectories and bubbles rising about 40% slower following a zig-zag pattern. Rise speed was uncorrelated with hydrate formation, and bubbles following both speeds were observed at both sites. Ship-mounted multibeam sonar provided the flare rise heights, which corresponded closely with the boundary of the hydrate stability zone for the measured gas compositions. The evolution of bubble size with height agreed well with mass transfer rates predicted by equations for dirty bubbles.Gulf of Mexico Research Initiativ

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, 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 <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

Baseline survey at proposed experiment site off Norway, 19-26 July, 2002

Årsliste 2002A survey of a proposed experiment area off the west coast of Norway was conducted in July 2002 for the planned International CO2 Sequestration Experiment. The survey consisted of echo sounder transects, ROV dives, CTD profiles, carbon chemistry analysis, 13C /bacterial production rates, sediment grab samples, scavenger trap deployments, long line fishing, fish trap deployments, trawling, zooplankton net hauls, and ADCP current measurements. Based on the survey results, a recommendation for an exact location for the upcoming field experiment has been made. Necessary background and baseline data for the design of the field experiment and sampling scheme were collected

Direct coupling of near-field and far-field models hones predictions of oil spill transport and fate from deep-sea blowout

Deep-water spills pose a unique challenge for reliable predictions of oil transport and fate, since live oil spewing out under very high hydrostatic pressure has characteristics remarkably distinct from oil spilling in shallow water. It is thus important to describe the complex thermodynamic processes occurring in the near-field, meters above the wellhead, and the hydrodynamic processes in the far field, up to kilometers away. However, these processes are typically modeled separately since they occur at different scales. Here we directly couple two oil prediction applications developed during the Deepwater Horizon blowout operating at different scales: the near-field Texas A&M Oilspill Calculator (TAMOC) and the far field oil application of the Connectivity Modeling System (oil-CMS). To achieve this coupling, new oil-CMS modules were developed to read TAMOC output, which consists of the description of distinct oil droplet “types”, each of specific size and pseudo-component mixture that enters at a given mass flow rate, time and position into the far field. These variables are transformed for use in the individual-based framework of oil-CMS, where each droplet type fits into a droplet size distribution (DSD). Here we used 19 pseudo-components representing a large range of hydrocarbon compounds and their respective thermodynamic properties. Simulation results show that the dispersion pathway for different droplet types varies significantly. Indeed, some droplet types are predicted to remain suspended in the subsea over months, while others accumulate in the surface layers. In addition, the biodegradation and dissolution rates of oil pseudo-components significantly alter the dispersion, denoting the importance of more biodegradation and dissolution studies of dispersed live oil at high pressure, with and without subsea dispersant injection (SSDI). This new modeling tool shows the potential for improved accuracy in predictions of oil partition in the water column, and of advancing impact assessment and response during a deep water spills