Methane Bubbles Research Articles

Estimation of parameters of gassy layer in sediment in shallow water using measurement of angular dependence and spectrum of reflection coefficient of wide-band signals

Method of estimation of gassy layer parameters in sediment (sound speed and thickness) on the base of measurement of angular and frequency dependencies reflection coefficient of wideband signals is presented. Experiments were carried out in Lake Kinneret (Israel), which is characterized by remarkable organic content in sediment producing methane bubbles. Direct chemical, biological analysis and usage of frozen cores show comparatively narrow layer (a few tens of cm) changing in dependence on season. Principal point for acoustical method is low sound speed in bubble layer (up to 300–400 m/s) and big reflection coefficient from the corresponding half-space (0.6–0.7 for vertical incident). Due to narrow gassy layer, there should be half wavelength resonances at the corresponding frequencies. In experiments, wideband signals were used (500–2500 Hz) and receiving system: single hydrophone (1 m from source) and vertical line array. Estimation of layer’s parameters was carried out using measured reflection coefficient as a function of frequency and angle of reflection. Resonance frequencies were 800 Hz and 1600 Hz. The corresponding analysis of data give values of thickness 25–30 cm, with the sound speed in layer 400–450 m/c. Results are compared with experimental data obtained by direct measurements. [Work was supported by ONRG and ISF.]

Methane Bubble Growth and Migration in Aquatic Sediments Observed by X-ray μCT.

Methane bubble formation and transport is an important component of biogeochemical carbon cycling in aquatic sediments. To improve understanding of how sediment mechanical properties influence bubble growth and transport in freshwater sediments, a 20-day laboratory incubation experiment using homogenized natural clay and sand was performed. Methane bubble development at high resolution was characterized by μCT. Initially, capillary invasion by microbubbles (<0.1 mm) dominated bubble formation, with continued gas production (4 days for clay; 8 days for sand), large bubbles formed by deforming the surrounding sediment, leading to enhanced of macropore connectivity in both sediments. Growth of large bubbles (>1 mm) was possible in low shear yield strength sediments (<100 Pa), where excess gas pressure was sufficient to displace the sediment. Lower within the sand, higher shear yield strength (>360 Pa) resulted in a predominance of microbubbles where the required capillary entry pressure was low. Enhanced bubble migration, triggered by a controlled reduction in hydrostatic head, was observed throughout the clay column, while in sand mobile bubbles were restricted to the upper 6 cm. The observed macropore network was the dominant path for bubble movement and release in both sediments.

A three-dimensional CFD study of the hydrodynamic behavior of equal and unequal-sized in-line methane bubbles at high pressure

Abstract The hydrodynamic behavior of multiple bubbles rising upward is a field of ongoing research since various aspects of their interaction require further analysis. Shape deformation, rise velocity, and drag coefficient are some of the uncertainties to be determined in a bubble upward flow. For this study the predictions of the three-dimensional numerical simulations of the volume of fluid (VOF) CFD model were first compared with experimental results available in the literature, serving as benchmark cases. Next, 28 cases of pairs of equal and unequal-sized in-line pairs of bubbles moving upwards were simulated. The bubble size varied between 2.0–10 mm. Breakthrough of the present study is the small initial distance of 2.5 R between the center of the bubbles. To provide a more practical nature in this study material properties were selected to match methane gas and seawater properties at deepsea conditions of 15 MPa and 4 °C, thus yielding a fluid-to-bubble density ratio λ = 7.45 and viscosity ratio n = 100.46. This is one of the few studies to report results of the coalescence procedure in this context. The hydrodynamic behavior of the leading and trailing bubbles was thoroughly studied. Simulation results of the evolution of the rise velocity and the shape deformation with time indicate that the assumption that the leading bubble is rising as a free rising single one is not valid for bubbles between 2.0–7.0 mm. Finally, results of the volume of the daughter bubble exhibited an oscillating nature.

On the Theory of Accumulation of Hydrocarbons in a Dome Used to Eliminate a Technogenic Spill at the Bottom of the Ocean

Consideration is given to the operation of a dome separator installed at the bottom of the World Ocean and intended to eliminate the effects of technogenic spill of hydrocarbons. It is assumed that oil and gas (methane) escape from the damaged well on the ocean floor. Under the conditions of stable existence of a hydrate, a hydration sheath is formed on the surface of methane bubbles. To collect the entering hydrocarbons, a dome from a soft polyurethane shell is installed above the spill site, inside which the hydrocarbons are accumulated. In the regime of steady-state operation of the dome, provision is made for pumping out of hydrocarbons for transportation and further utilization. In the work, temperature fields of the hydrocarbon layers accumulated in the dome are investigated.

Modeling of rising methane bubbles during production leaks from the gas hydrate sites of India

ABSTRACTNatural gas hydrates is considered as a strategic unconventional clean hydrocarbon resource in the energy sector. Understanding the behavior of the rising methane gas bubbles during production leaks from the deep marine gas hydrate reservoirs well head is essential for environmental impact studies and to design environmental monitoring systems. Numerical model for quantitatively characterizing the vertical dissolution pattern of the wellhead released methane gas bubbles is analyzed for three potential gas hydrate locations in India. Simulation results indicate that the methane bubbles with diameter of 10 mm can transport methane gas till 650, 800, and 750 m from the seabed in the Krishna–Godavari(KG), Mahanadi and Andaman basins respectively. Results brought out that potential well head damage during methane hydrate production at 1050 m water depth could release up to 28 m3 of methane gas, in which 50% of the molar mass shall get dissolved within 40 m of water column from the seafloor.

Methane Bubble Size Distributions, Flux, and Dissolution in a Freshwater Lake.

The majority of methane produced in many anoxic sediments is released via ebullition. These bubbles are subject to dissolution as they rise, and dissolution rates are strongly influenced by bubble size. Current understanding of natural methane bubble size distributions is limited by the difficulty in measuring bubble sizes over wide spatial or temporal scales. Our custom optical bubble size sensors recorded bubble sizes and release timing at 8 locations in Upper Mystic Lake, MA continuously for 3 months. Bubble size distributions were spatially heterogeneous even over relatively small areas experiencing similar flux, suggesting that localized sediment conditions are important to controlling bubble size. There was no change in bubble size distributions over the 3 month sampling period, but mean bubble size was positively correlated with daily ebullition flux. Bubble data was used to verify the performance of a widely used bubble dissolution model, and the model was then used to estimate that bubble dissolution accounts for approximately 10% of methane accumulated in the hypolimnion during summer stratification, and at most 15% of the diffusive air-water-methane flux from the epilimnion.

Measurements of methane bubble size distribution from a remotely operated platform in the presence of oil

Subsurface releases of crude oil and methane gas may occur naturally or due to manmade, sometimes catastrophic, events. To determine the quantity of oil and gas being released, measurements can be performed on both the crude oil droplets and methane bubbles to determine their sizes and concentration in the water column. An acoustic backscatter measurement, in the 20–110 kHz range, was developed using transducers mounted on a commercial ROV insonifying an intentional subsurface oil and gas release in a large wave tank. Bubble size distributions for methane mixed in a plume of crude oil were obtained by performing an inversion on acoustic backscattering measurements after accounting for transducer properties and sound transmission through the water column. Due to the rapid nature of the measurements, monitoring of the evolution of the plume in time was also possible. Distributions were consistent with independent measurements using laser scattering techniques.

Oxygen Ebullition From Lakes

AbstractThe exchange of oxygen between lakes and the atmosphere is assumed to be driven by diffusion. Here we show that lakes can emit significant amounts of O2 by emerging gas bubbles—a process called ebullition. We found very high proportions of 17 ± 10% O2 (maximum 34%) in emerging gas bubbles in two shallow eutrophic reservoirs. In the studied reservoirs, O2 emission by ebullition was of similar magnitude as diffusive O2 fluxes. By reanalyzing previous studies, we show that the process is ubiquitous and probably quantitatively relevant in many places. We present evidence that O2 in bubbles originates both from photosynthetic oxygen production and hence bubble formation in the oxic water and from stripping by emerging methane bubbles. Ebullition can turn lakes undersaturated in respect to the atmosphere into a net O2 source. Neglecting O2 ebullition leads to an overestimation of lake internal respiration.

Influence of gas hydrate formation on methane seeps at the bottom of water reservoirs

It is shown by numerical modeling that the height of gas flares above underwater methane seeps depends strongly on water parameters. A simulation model of the dynamics of a rising bubble was used. Along with gas exchange through the bubble wall, the model takes into account gas hydrate formation, whose rate is determined by turbulent heat exchange with water. Calculations were performed for depths of 250 to 1500 m with an initial bubble diameter of 0.2 to 1.5 cm. It is shown that at the water temperature of the Arctic seas, rising methane bubbles transform into gas hydrate ice within the first 1-2 m of their path. At the same time, when the water temperature is higher than the hydrate formation temperature near the sea bottom or close to it, the bubbles travel a distance of tens or hundreds of meters before complete dissolution or complete hydration.

A new model for hydrodynamics and mass transfer of hydrated bubble rising in deep water

Hydrate shell growth characteristics have a significant influence on the migration process of a gas bubble rising in deep water, which has raised concerns across of broad range of environmental fields including marine chemistry, global warming, and ocean carbon dioxide disposal. In this work, a new coupled model of the behaviors of hydrated bubbles is developed. This model considers the bubble hydrodynamics, gas dissolution, hydrate nucleation time, and dynamic growth of the hydrate shell. In combination with published experimental data, the present model proposes new mass transfer correlations involving hydrated bubble dissolution and water permeation across the hydrate shell to describe the effects of the bubble interface and the hydrate shell structure on the mass transfer processes. Using the proposed model, the rising process of methane bubbles released at depths of 1000–2000m in the California coast is simulated. The simulated results show that the hydrate nucleation time decreases rapidly as the water depth increases. Furthermore, the formed hydrate shell can reduce the gas bubble rising velocity and shrinkage rate, because it alters the mechanical force balance and gas dissolution rate. This work adds further insights into quantitatively characterizing the upward migration and mass transfer of a hydrated bubble in deep water.

An enhanced bubble size sensor for long‐term ebullition studies

AbstractMethane ebullition from freshwater sediments is a significant source of greenhouse gas to the atmosphere. Methane bubble size determines the fraction of methane that dissolves into the water column instead of entering the atmosphere, but current methods to measure bubble sizes in situ are limited. Previously, we reported the design for a novel optical bubble size sensor capable of measuring methane bubble sizes in situ. Here, we report sensor enhancements to allow for continuous long‐term sensor deployment, uninterrupted data collection under high ebullition flux, and improved bubble volume measurement accuracy. The enhanced design includes a data and power cable running from the sensor to a custom data buoy housing batteries and an SD memory card. Batteries and SD cards are replaced every 2–4 weeks through the data buoy, allowing for continuous, undisturbed sensor deployment. In addition, sensor parameters have been tuned to reduce error from particle intrusion and fluctuations in raw signals that were observed during transit of some bubbles through the sensor. Field data show that on average the sensor measures cumulative bubble volumes equaling approximately 86% of collected gas volumes. We discuss potential mechanisms of remaining error, some of which may be due to zooplankton intrusion, and suggest directions for future sensor enhancements.

Methane Ebullition in Temperate Hydropower Reservoirs and Implications for US Policy on Greenhouse Gas Emissions.

The United States is home to 2198 dams actively used for hydropower production. With the December 2015 consensus adoption of the United Nations Framework Convention on Climate Change Paris Agreement, it is important to accurately quantify anthropogenic greenhouse gas emissions. Methane ebullition, or methane bubbles originating from river or lake sediments, has been shown to account for nearly all methane emissions from tropical hydropower reservoirs to the atmosphere. However, distinct ebullitive methane fluxes have been studied in comparatively few temperate hydropower reservoirs globally. This study measures ebullitive and diffusive methane fluxes from two eastern Washington reservoirs, and synthesizes existing studies of methane ebullition in temperate, boreal, and tropical hydropower reservoirs. Ebullition comprises nearly all methane emissions (>97%) fromthis study's two eastern Washington hydropower reservoirs to the atmosphere. Summer methane ebullition from these reservoirs was higher than ebullition in six southeastern U.S. hydropower reservoirs, however it was similar to temperate reservoirs in other parts of the world. Our literature synthesis suggests that methane ebullition from temperate hydropower reservoirs can be seasonally elevated compared to tropical climates, however annual emissions are likely to be higher within tropical climates, emphasizing the possible range of methane ebullition fluxes and the need for the further study of temperate reservoirs. Possible future changes to the Intergovernmental Panel on Climate Change and UNFCCC guidelines for national greenhouse gas inventories highlights the need for accurate assessment of reservoir emissions.

Correlation of aspect ratio and drag coefficient for hydrate-film-covered methane bubbles in water

Understanding the law of hydrate-film-covered methane bubbles (MHF-bubbles) rise is a prerequisite for predicting the formation and plugging of hydrate formation in deep water wellbore under bubble flow conditions. The aspect ratio and the drag coefficient are two key parameters for predicting the rising velocity of MHF-bubbles. In this work, an experimental facility is built to simulated deep-water wellbore environment. To effectively process the experimental data, a theoretical model for the rising velocity of a single MHF-bubble was established. The motion and deformation of single MHF-bubbles rising were investigated by injecting methane bubbles into water at 6MPa and 4°C. Diameters in vertical and horizontal axes of MHF-bubble, bubble size were recorded by high-speed photography. An image processing software was used to measure aspect ratio and terminal rising velocity. The results showed that due to the formation of the hydrate film the MHF-bubble is less likely to deform, and its aspect ratio remains relatively stable during ascent and decreases as the equivalent diameter increases. The drag coefficient decreases in the region where the equivalent diameter of the bubble is less than 1.5mm and increase along with equivalent diameter in the range 1.5–7mm. The experimental results of drag coefficient and aspect ratio were compared with correlations available in literature. The comparison showed that these correlations do not give fully satisfactory results in predicting the drag coefficient and aspect ratio of MHF-bubble rising in water. As a result, two new empirical correlations were proposed to correlate the aspect ratio as a function of Eötvös number (Eo) and the drag coefficient as a function of Morton number (Mo), Eötvös number (Eo), Reynolds number (Re). It was found that the values predicted by new empirical correlations are generally in good agreement with the experiment results. The results of this study have great significance to research into the problem of natural gas–liquid multiphase flow under high pressure and low temperature.

Sonar gas flux estimation by bubble insonification: application to methane bubble flux from seep areas in the outer Laptev Sea

Abstract. Sonar surveys provide an effective mechanism for mapping seabed methane flux emissions, with Arctic submerged permafrost seepage having great potential to significantly affect climate. We created in situ engineered bubble plumes from 40 m depth with fluxes spanning 0.019 to 1.1 L s−1 to derive the in situ calibration curve (Q(σ)). These nonlinear curves related flux (Q) to sonar return (σ) for a multibeam echosounder (MBES) and a single-beam echosounder (SBES) for a range of depths. The analysis demonstrated significant multiple bubble acoustic scattering – precluding the use of a theoretical approach to derive Q(σ) from the product of the bubble σ(r) and the bubble size distribution where r is bubble radius. The bubble plume σ occurrence probability distribution function (Ψ(σ)) with respect to Q found Ψ(σ) for weak σ well described by a power law that likely correlated with small-bubble dispersion and was strongly depth dependent. Ψ(σ) for strong σ was largely depth independent, consistent with bubble plume behavior where large bubbles in a plume remain in a focused core. Ψ(σ) was bimodal for all but the weakest plumes. Q(σ) was applied to sonar observations of natural arctic Laptev Sea seepage after accounting for volumetric change with numerical bubble plume simulations. Simulations addressed different depths and gases between calibration and seep plumes. Total mass fluxes (Qm) were 5.56, 42.73, and 4.88 mmol s−1 for MBES data with good to reasonable agreement (4–37 %) between the SBES and MBES systems. The seepage flux occurrence probability distribution function (Ψ(Q)) was bimodal, with weak Ψ(Q) in each seep area well described by a power law, suggesting primarily minor bubble plumes. The seepage-mapped spatial patterns suggested subsurface geologic control attributing methane fluxes to the current state of subsea permafrost.

Ecology, adaptation, and function of methane-sulfidic spring water biofilm microorganisms, including a strain of anaerobic fungus Mucor hiemalis.

Ecological aspects, adaptation, and some functions of a special biofilm and its unique key anaerobic fungus Mucor hiemalis strain EH11 isolated from a pristine spring (Künzing, Bavaria, Germany) are described. The spring's pure nature is characterized by, for example, bubbling methane, marine‐salinity, mild hydrothermal (~19.1°C), sulfidic, and reductive‐anoxic (Eh: −241 to −253 mV, O2: ≤ 0.1 mg/L) conditions. It is geoecologically located at the border zone between Bavarian Forest (crystalline rocky mountains) and the moor‐like Danube River valley, where geological displacements bring the spring's water from the deeper layers of former marine sources up to the surface. In the spring's outflow, a special biofilm with selective microorganisms consisting of archaea, bacteria, protozoa (ciliate), and fungus was found. Typical sulfidic‐spring bryophyta and macrozoobenthos were missing, but many halo‐ and anaerotolerant diatoms and ciliate Vorticella microstoma beside EH11 were identified. Phase contrast and scanning electron microscopy revealed the existence of a stabilizing matrix in the biofilm formed by the sessile fungal hyphae and the exopolysaccharide substance (EPS) structures, which harbors other microorganisms. In response to ecological adaptation pressure caused by methane bubbles, EH11 developed an atypical spring‐like hyphal morphology, similar to the spiral stalk of ciliate V. microstoma, to rise up with methane bubbles. For the first time, it was also demonstrated that under strict anaerobic conditions EH11 changes its asexual reproduction process by forming pseudosporangia via hyphal cell divisions as well as switching its metabolism to chemoautotrophic bacteria‐like anaerobic life using acetate as an e‐donor and ferrihydrite as an e‐acceptor, all without fermentation. EH11 can be suggested to be useful for the microbial community in the Künzing biofilm not only due to its physical stabilization of the biofilm's matrix but also due to its ecological functions in element recycling as well as a remover of toxic metals.

Experimental observations of acoustic backscattering from spherical and wobbly bubbles

Methane bubbles released from the seafloor transport gas through the water column to the atmosphere. Direct or optical methods via underwater vehicles are often used for quantifying methane gas flux in the water column, however these methods are time consuming and expensive. Acoustic measurements, using split-beam and multibeam echo sounders that are readily available on most sea vessels, provide a more efficient method for determining methane gas flux. These acoustic methods typically convert acoustic backscatter measurements of bubbles to bubble size using analytical models of bubble target strength. These models assume that bubbles have uniform shape; however it has been shown that bubbles with a radius greater than 1 mm, which have large Eötvös and/or large Reynolds number, are non-spherical. To investigate the error associated with assuming large bubbles are spherical, a 6 m deep tank experiment was conducted to compare calibrated target strength measurements of both small spherical and large wobbly bubbles to existing acoustic scattering models. Bubble sizes observed in this experiment ranged from a fraction of 1 mm to 6 mm in radius. This experiment used a broad range of frequencies (10-300 kHz) to cover typical echo sounder frequencies utilized in field measurements of natural methane seeps.

The effect of sampling effort on estimates of methane ebullition from peat

AbstractWe investigated the effect of sample size and sampling duration on methane bubble flux (ebullition) estimates from peat using a computer model. A field scale (10 m), seasonal (&gt;100 days) simulation of ebullition from a two‐dimensional (2‐D) structurally varying peat profile was modeled at fine spatial resolution (1 mm × 1 mm). The spatial and temporal scale of this simulation was possible because of the computational efficiency of the reduced‐complexity approach that was implemented, and patterns of simulated ebullition were consistent with those found in the field and laboratory. The simulated ebullition from the peat profile suggested that decreases in peat porosity—which cause increases in gas storage—produce ebullition that becomes increasingly patchy in space and erratic in time. By applying different amounts of spatial and temporal sampling effort, it was possible to determine the uncertainty in ebullition estimates from the peatland. The results suggest that traditional methods to measure ebullition can equally overestimate and underestimate flux by 20% and large ebullition events can lead to large overestimations of flux when sampling effort is low. Our findings support those of field studies, and we recommend that ebullition should be measured frequently (hourly to daily) and at many locations (n &gt; 14).

Low-frequency active acoustic response of underwater bubble plumes

Active, low frequency acoustics may provide a method of locating and identifying methane bubble plumes over a large area by exciting radial and column resonances. In this work, a proof of concept experiment was conducted by measuring the active, low frequency response of an air bubble plume at the Lake Travis Test Station at the University of Texas at Austin. Several flow rates, bubble sizes, and plume widths were investigated. The test consisted of insonifying the plumes at two different standoff distances using a low-frequency chirp from 300-2500 Hz. The plume response was much longer in time than originally expected due to resonant ringing. The long-time response was modeled with a finite element model, which predicted the column resonance. This behavior may be able to be exploited for long-range bubble plume identification. [Work supported by ExxonMobil.]

Interaction of gas bubbles and oil droplets in subsea oil and gas blowouts – a new development of VDROP-J model.

Abstract (2017-194) The presence of methane bubbles in the oil and gas blowout could greatly reduce the oil droplet sizes. Bubbles tend to introduce energy into the system and separate oil droplets from each other. The interaction of oil droplets and gas bubbles in the near field of a blowout was investigated numerically using the VDROP-J model, whose droplet size distribution (DSD) was thoroughly calibrated. For this purpose, a new numerical scheme has been developed in VDROP-J to account for the interaction of gas bubbles and oil droplets in the blowout, giving simultaneous simulation of bubble and droplet size distribution along the discharged plume. Validation shows improvement of the model compared with the one without considering the gas bubble and oil droplet interactions. Effects of gas volume fraction on the droplet formation are also investigated. This new development will enhance the knowledge in subsea oil and gas blowouts.

On the migration of a single gas bubble in water

A theoretical model of a single gas bubble rising in open water is considered. It is revealed that methane bubble rising is accompanied by the formation of the hydration shell on its surface under the thermobaric conditions of the stability of hydrates. Numerical solutions for two limiting cases were obtained and analyzed, when the formation rate of the hydrated crust on the bubble surface is limited by the intensity of heat removal released in the process of hydrate formation by the surrounding liquid or the diffusion resistance of the gas hydrate crust to the transfer of hydrate-forming components. The comparison of numerical results with experimental data showed that the scheme of the diffusion transfer of hydrate-forming components through the crust describes most adequately the process of the growth of the gas hydrate particle observed in experiments of methane bubbles rising in sea water. It is established that argon bubble rising under the corresponding thermobaric conditions can occur without the formation of the hydrate on its surface. The migration of the gas bubble is accompanied by its dissolution in water. Numerical estimates for the values of the argon diffusion coefficient in water and reduced diffusion coefficients of gas (methane) and water through the hydrate crust are obtained from the conditions of matching theoretical and experimental data from the change of the argon bubble radius and the gas hydrate particle.