Ira Leifer

The bubble mechanism for methane transport from the shallow sea bed to the surface: A review and sensitivity study

Bubbles transport methane (CH4) released from the sea bed to the surface while exchanging gas with the surrounding aqueous environment. The fraction of CH4 released at the surface depends upon the release depth, bubble size, dissolved gas concentrations, temperature, surface-active substances, and bulk fluid motions—particularly the upwelling flow. A bubble model was developed to examine the sensitivity of bubble-mediated CH4 transport to these parameters. A strong sensitivity to several environmental parameters was found that must be accurately measured in order to correctly predict bubble-mediated CH4 transport. Strong sensitivities (one or more orders of magnitude, depending upon initial bubble size and depth) were found for both aqueous CH4 concentration and upwelling flows. The equations used to model bubble-mediated gas transfer incorporate parameterizations of bubble motion and gas transfer. A review of the literature on the experimental measurements and theory used to derive these parameterizations is presented, and the approaches used in geophysics and chemical engineering bubble models, are described. This research seeks to provide all the necessary parameterizations and theoretical background to allow modeling of CH4 bubble streams for diverse marine conditions.

Dynamics of the gas flux from shallow gas hydrate deposits: interaction between oily hydrate bubbles and the oceanic environment

Decomposition of methane hydrates on the continental margins is a potentially significant source of atmospheric methane, but the input depends upon the poorly understood fate of the hydrocarbon bubbles rising from the sea floor. During a field trip to the Gulf of Mexico, three different seepages were imaged and analyzed. Three different imaging techniques were tried (side, front, and back illumination), of which back illumination produced the best results. The images were analyzed and the size-dependent bubble distribution, mass flux, and rise speeds determined. The total observed gas flux was 62.3×10−3 mol s−1, primarily methane, of which a single vent produced seven times the next largest vent. Of this major vent, 50% of the bubble mass was contained in the largest bubbles, r>5500 μm. The vertical velocities demonstrated that these bubbles were heavily contaminated with oil, which was also corroborated by bubble shape and oscillation observations.

A Study on the Temperature Variation of Rise Velocity for Large Clean Bubbles

Abstract A series of microphysical laboratory experiments studying the hydrodynamics of single bubbles were conducted to measure the variation of rise velocity, VB, with temperature, T, and radius, r. Bubbles with an equivalent spherical radius between 377 and 4500 μm were studied for T varying between 0° and 40°C. While for nonoscillating bubbles VB increases with T; due to the significance of oscillations, VB decreases with T for oscillating bubbles, in conjunction with an increase in trajectory oscillations with T. Using observations from this study and data from other researchers, a three-part parameterization of VB(r, T) is proposed with transitions at Re = 1 and the onset of oscillations, where Re is the Reynolds number. The T for the transition to oscillatory behavior was found to vary linearly with r. An empirical parameterization of VB(r, T) for oscillatory and nonoscillatory bubbles that correctly incorporates the effect of T is presented.

Natural marine seepage blowout: Contribution to atmospheric methane

The release of methane sequestered within deep-sea methane hydrates is postulated as a mechanism for abrupt climate change; however, whether emitted seabed methane reaches the atmosphere is debatable. We observed methane emissions for a blowout from a shallow (22 m) hydrocarbon seep. The emission from the blowout was determined from atmospheric plume measurements. Simulations suggest a 1.1% gas loss to dissolution compared to ∼ 10% loss for a typical low-flux bubble plume. Transfer to the atmosphere primarily was enhanced by the rapid upwelling flows induced by the massive discharge. This mechanism could allow methane suddenly released from deeper (>250 m) waters to contribute significantly to atmospheric methane budgets. Copyright 2006 by the American Geophysical Union.

Temporal variation in natural methane seep rate due to tides, Coal Oil Point area, California

Two large steel tents (each 30 m by 30 m), open at the bottom to the seafloor, capture ∼16,800 m3 d-1 (594 MCF) of primarily methane from a large natural hydrocarbon seep, occurring a kilometer offshore in 67 m of water. The gas is piped to shore where it is metered and processed. The seep flow rate was monitored hourly for 9 months. Our results show that the tidal forcing causes the flow rate to vary by 4-7% around the mean. These results are the first quantitative documentation of the effect of tides on natural gas seepage in relatively deep water. Time series analyses of the 9 month record clearly show four principal tidal components with periods of 12.0, 12.4, 23.9, and 25.8 hours. High tide correlates with reduced flow, and low tide correlates with increased flow. The correlation indicates that each meter increase of sea height results in a decrease of 10-15 m3 hr-1 or 1.5-2.2% of the hourly flow rate. The observed changes are best accounted for by a pore activation model, whereby gas is released from small pores at low pressures but is inhibited at higher pressure. Pressure-dependent gas solubility changes are a less likely cause of flow variation. Our study implies that sea level differences, on a tidal timescale, can significantly change the gas seepage rate from sediments. Lower sea level in the last hundred thousand years would presumably allow higher gas loss from the sediment, assuming sufficient gas present, because of reduced hydrostatic pressure at the sediment-sea interface. The magnitude of this long-term change cannot be extrapolated from our tidal data. Copyright 2001 by the American Geophysical Union.

Modifications of the local environment by natural marine hydrocarbon seeps

The bubble gas partial pressures, dissolved gas and oil, and fluid motions within the rising bubble streams of three shallow ( 108times) than atmospheric equilibrium values; at two of the seeps it was slightly supersaturated with respect to the bubbles partial pressure. This indicates that the rate limiting step for methane, CH4, transfer into the water column was not bubble gas transfer, but rather turbulence transfer of water out of the saturated bubble stream to the bulk ocean. Strong upwelling flows were observed, as well as elevated fluorescence indicative of dissolved oils. At the bottom, bubbles are circa 90% CH4, but at the surface they were circa 60% CH4, 30% air, and 10% higher hydrocarbons.

Optical Measurement of Bubbles: System Design and Application

Affordable high quality charge-coupled device (CCD) video cameras and image processing software are powerful tools for bubble measurements. Because of the wide variation between bubble populations, different bubble measurement systems (BMSs) are required depending upon the application. Two BMSs are described: a mini-BMS designed to observe the background bubble population from breaking waves, and a large-BMS designed to noninvasively determine the time-resolved bubble distribution inside dense bubble plumes and near the interface, as are details of the analysis techniques. Using the two systems in conjunction with each other allowed size distributions over the range 15-5000-μm radius to be obtained. The BMSs were designed for application to breaking-wave bubble plumes in the field or laboratory. Distributions measured by both BMSs in aerator-generated plumes agreed very well for the overlapping size range. Also presented are observations of bubble plumes produced by breaking waves in a large wind-wave flume, and calibration experiments showing the effect on measured bubble size due to blur induced by slow shutter speeds.

Bubbles generated from wind‐steepened breaking waves: 1. Bubble plume bubbles

Measurements of bubble plumes from paddle-amplified, wind stress breaking waves were made in a large wind-wave channel during the LUMINY experiment in fresh (but not clean) water. Bubble plumes exhibited considerable variability with respect to dynamics, bubble size distribution, and physical extent. A classification scheme was developed, and time-and size-resolved bubble population distributions were calculated for each plume class. Comparison of the bubble distributions suggested that there were two primary types of bubble plumes, termed dense and diffuse on the basis of the ability to optically obscure the background. Diffuse plumes at injection were weakly size-dependent to 1000-μm radius, with a steep decrease for larger bubbles. Dense plumes were multimodal with a steep decrease for small bubbles, a second peak at 1700-2000-μm radius, and a steep decrease for larger bubbles. Because of this peak, large bubbles contributed to total plume bubble volume much more significantly for dense plumes than diffuse plumes. Void fractions of diffuse plumes were greater at maximum penetration than for dense plumes. Finally, the injection and rise phases were approximately equal in time. Copyright 2006 by the American Geophysical Union.

Biochemical effects of petroleum exposure in hornyhead turbot (Pleuronichthys verticalis) exposed to a gradient of sediments collected from a natural petroleum seep in CA, USA

Concentrations of serum/plasma estradiol, biliary fluorescent aromatic compounds (FACs), levels of hepatic CYP1A expression, and DNA damage were measured in sexually mature hornyhead turbot (Pleuronichthys verticalis) exposed in the laboratory for 7 days to a gradient of sediments collected from a natural petroleum seep in the Santa Barbara Channel. Coal oil point (COP) sediments were homogenized and divided into four treatments containing 0 (sediment from the Orange County Sanitation Districts reference location), 33, 66, and 100% (COP) sediments. Sediment concentrations of 20 PAHs ranged from below the detection limit for the 0% COP sediment treatments to 105 microg/g in the 100% treatments with lower molecular weight compounds predominating. Concentrations of biliary FACs were not linear with COP treatment but levels of hepatic DNA damage increased linearly with increasing concentrations of high molecular weight PAHs. Hepatic CYP1A expression was elevated only in the 100% treatments. A reduction of plasma estradiol in male and female fish was observed in all COP exposures. These results demonstrate that acute sediment-only exposure of flatfish to naturally-derived PAHs elicits alterations in biochemical endpoints indicative of PAH bioavailability and adverse effects with different sensitivities.

Turbine tent measurements of marine hydrocarbon seeps on subhourly timescales

[1] Three turbine seep-tents simultaneously measured marine seep gas fluxes with high time resolution (0.2 s) at multiple locations. Tents were inverted polyvinyl cones, 2-m diameter, 1-m tall, and weighted on their lower skirt edges. Rising gas bubbles induce vertical fluid motions, which were measured by laboratory-calibrated turbines in chimneys on top of the tents. Initial deployment was at an active seep area in the Coal Oil Point seep field, in the Santa Barbara Channel, California. The three tents simultaneously collected data for continuous time periods of 2 hours in both the morning and afternoon. Seabed temperature and pressure were acquired every 3 s over the same time periods as the flux measurements from a conductivity temperature depth, CTD, mounted on one tent. Results strongly suggest that oceanic swell had a significant forcing effect on the flux at a subhourly timescale. There was an inverse relationship between effect of swell height on the flux and flux. Swells from 1 to 4 m height and periodicities of 7 and 12 s caused variations of � 1% to 4% from the average flux. Proposed mechanisms to explain the observations are diffusion with surrounding sediments, termed gas charging, swell induced changes in fracture size, termed fracture forcing, and swell induced vent activation/deactivation, termed pore activation. On the basis of the seep frequency response, we propose pore activation was dominant.