79
Hydrate investigation in the laboratory can prove difficult, due to the severe pressure conditions often required to mimic the natural blowout. Chen et al. (2014)
constructed a novel high-pressure water tunnel (HPWT) at the Colorado School of
Mines Center for Hydrate Research, where counterflowing conditioned seawater
was used to suspend high-pressure gas bubbles in a view window. Chen et al. (2014)
noted that Multiflash (Infochem 2012) and CSMGem (Ballard and Sloan 2004) –
common phase equilibrium tools in hydrate research – correctly identified the methane saturation required to achieve hydrate formation from 70 to 139 bar. At 173 bar,
hydrate formation was observed with as little as one fifth of the methane content
predicted by phase equilibrium calculators; the authors speculate that, if the chemical potential driving force toward hydrate formation outstrips the dissolution rate of
light hydrocarbons in the seawater, hydrate formation may be achievable. Chen
et al. (2014) further noted that, below the equilibrium saturation, the hydrate shell
was not observed to grow to completion, but rather may be characterized by dendritic needles or plates on the gas-water interface.
Within the above context, computational fluid dynamics approaches may provide
a tractable method by which to estimate the rate of hydrate formation considering
both (i) the chemical potential driving force for hydrate formation across the range
of bubble sizes generated and (ii) the rate balance in the near-field plume between
fresh seawater entering and partially saturated seawater exiting. In the limit that a
partial or complete hydrate shell is able to form on the gas bubble surface, the consequential rate of gas diffusion to the seawater will decrease, as demonstrated by
Davies et al. (2010). Although simplistic approaches may treat this effect as constant, the complexity of the above-described process should inform caution with
modeling efforts. That is, the thickness, porosity, and permeability of the hydrate
“shell” are distributed properties, due to both anisotropy in the extent of gas saturation within the plume and the well-known stochasticity (May et al. 2018) of hydrate
crystal nucleation. As such, the estimation of near-field behavior – including the rise
velocity of hydrate-encrusted bubbles – should ultimately yield a distributed output
to appropriately represent the consequences of hydrate formation.
5.7 Conclusion
Several correlations for the calculation of bubble or droplet rise velocities for both
“clean” and “contaminated” interfaces are available in literature. Surprisingly, even
though the environment during an oil spill is characterized by saltwater, multicomponent oil droplets/bubbles, suspended matter, and possibly gas hydrates, the experimental results for rising methane bubbles do not necessarily follow the correlations
for contaminated interfaces. Instead, the results scatter between the correlations for
clean and contaminated interfaces as upper and lower boundary, respectively. This
can be explained by initial shape deformations and the resulting rise trajectories of
the bubbles.
5 Behavior of Rising Droplets and Bubbles: Impact on the Physics of Deep-Sea…
Hydrate investigation in the laboratory can prove difficult, due to the severe pressure conditions often required to mimic the natural blowout. Chen et al. (2014)
constructed a novel high-pressure water tunnel (HPWT) at the Colorado School of
Mines Center for Hydrate Research, where counterflowing conditioned seawater
was used to suspend high-pressure gas bubbles in a view window. Chen et al. (2014)
noted that Multiflash (Infochem 2012) and CSMGem (Ballard and Sloan 2004) –
common phase equilibrium tools in hydrate research – correctly identified the methane saturation required to achieve hydrate formation from 70 to 139 bar. At 173 bar,
hydrate formation was observed with as little as one fifth of the methane content
predicted by phase equilibrium calculators; the authors speculate that, if the chemical potential driving force toward hydrate formation outstrips the dissolution rate of
light hydrocarbons in the seawater, hydrate formation may be achievable. Chen
et al. (2014) further noted that, below the equilibrium saturation, the hydrate shell
was not observed to grow to completion, but rather may be characterized by dendritic needles or plates on the gas-water interface.
Within the above context, computational fluid dynamics approaches may provide
a tractable method by which to estimate the rate of hydrate formation considering
both (i) the chemical potential driving force for hydrate formation across the range
of bubble sizes generated and (ii) the rate balance in the near-field plume between
fresh seawater entering and partially saturated seawater exiting. In the limit that a
partial or complete hydrate shell is able to form on the gas bubble surface, the consequential rate of gas diffusion to the seawater will decrease, as demonstrated by
Davies et al. (2010). Although simplistic approaches may treat this effect as constant, the complexity of the above-described process should inform caution with
modeling efforts. That is, the thickness, porosity, and permeability of the hydrate
“shell” are distributed properties, due to both anisotropy in the extent of gas saturation within the plume and the well-known stochasticity (May et al. 2018) of hydrate
crystal nucleation. As such, the estimation of near-field behavior – including the rise
velocity of hydrate-encrusted bubbles – should ultimately yield a distributed output
to appropriately represent the consequences of hydrate formation.
5.7 Conclusion
Several correlations for the calculation of bubble or droplet rise velocities for both
“clean” and “contaminated” interfaces are available in literature. Surprisingly, even
though the environment during an oil spill is characterized by saltwater, multicomponent oil droplets/bubbles, suspended matter, and possibly gas hydrates, the experimental results for rising methane bubbles do not necessarily follow the correlations
for contaminated interfaces. Instead, the results scatter between the correlations for
clean and contaminated interfaces as upper and lower boundary, respectively. This
can be explained by initial shape deformations and the resulting rise trajectories of
the bubbles.
5 Behavior of Rising Droplets and Bubbles: Impact on the Physics of Deep-Sea…