78
The dissolution of components of liquid (live) oil is governed by the pressuredependent oil-to-water partitioning coefficient that is thoroughly discussed in Chap.
8. When gas bubbles are present inside methane-(super)saturated oil droplets as
explained above, a certain amount of the dissolved methane will leave the droplet by
dissolution into the surrounding water; but the large share of methane will rather
enter the bubbles due to energy reasons. In turn, the oil acts as a mass transfer barrier for the gas molecules much like at the surface of oil-coated bubbles. Overall, the
common practice regarding oil and gas plumes as a combination of liquid-only oil
droplets and pure gas bubbles is questionable. There is no explicit reason why the
gaseous and liquid hydrocarbon phases should evolve and rise separately. Actually,
an oil droplet that contains free gas or an amount of oil that is rising at the surface
of a coated gas bubble rises much faster than an oil droplet containing the same
amount of oil (but no gas), because of the different particle size and average particle
density.
Deepwater blowout conditions further deliver a nearly ideal environment for the
formation of clathrate hydrates, which are icelike solids and are formed when
molecular cages of water enclathrate low-molecular-weight species; guest molecules can include methane, ethane, propane, isobutene, and carbon dioxide. The
internal diameter of the molecular water cages provides a geometric constraint on
the species that can be successfully enclathrated, where the severity of the highpressure and low-temperature conditions depends on the availability of guest species. Sloan and Koh (2007) discuss the array of hydrate structures – based in
combinations of cages with distinct internal diameters – that are found in both
nature and laboratory applications; structures I and II (respectively, “sI” and “sII”)
are the most common, where sII hydrate is typically generated from the light hydrocarbon composition found in conventional oil and gas reservoirs.
While hydrate equilibrium is typically identified by a boundary in temperaturepressure space, this representation assumes both water and hydrate-forming guests
are available in excess. Deepwater blowouts invoke a unique constraint on equilibrium, as the continuous seawater is depleted in methane. As such, hydrate cannot
immediately form upon a rising natural gas bubble until the local seawater has
reached equilibrium with the light hydrocarbon components. Subsea currents can
reach the order of 0.1 m/s, so the successful formation of hydrates fundamentally
requires seawater to be entrained within the plume at sufficient residence times to
approach equilibrium. In regions where the subsea currents are effectively mitigated – either due to mechanical placement of oil and gas equipment or through
boundary effects near the ocean floor – hydrate formation may also be achievable.
It should be noted that, in the context of a deepwater blowout, most models predict
gas bubble diameters significantly larger than 100 microns; visual experiments from
the Colorado School of Mines (Taylor et al. 2007; Brown and Koh 2016; Davies
et al. 2010) have demonstrated that the formation of hydrate at the boundary of a
hydrate guest phase (e.g., methane) and water will result in an initial “shell” of
approximately 50 microns in thickness. In a bubble swarm, this hydrate shell will
enclose residual high-pressure gas inside, where further hydrate growth requires
diffusion of either water and/or gas across the shell.
S. Pesch et al.
The dissolution of components of liquid (live) oil is governed by the pressuredependent oil-to-water partitioning coefficient that is thoroughly discussed in Chap.
8. When gas bubbles are present inside methane-(super)saturated oil droplets as
explained above, a certain amount of the dissolved methane will leave the droplet by
dissolution into the surrounding water; but the large share of methane will rather
enter the bubbles due to energy reasons. In turn, the oil acts as a mass transfer barrier for the gas molecules much like at the surface of oil-coated bubbles. Overall, the
common practice regarding oil and gas plumes as a combination of liquid-only oil
droplets and pure gas bubbles is questionable. There is no explicit reason why the
gaseous and liquid hydrocarbon phases should evolve and rise separately. Actually,
an oil droplet that contains free gas or an amount of oil that is rising at the surface
of a coated gas bubble rises much faster than an oil droplet containing the same
amount of oil (but no gas), because of the different particle size and average particle
density.
Deepwater blowout conditions further deliver a nearly ideal environment for the
formation of clathrate hydrates, which are icelike solids and are formed when
molecular cages of water enclathrate low-molecular-weight species; guest molecules can include methane, ethane, propane, isobutene, and carbon dioxide. The
internal diameter of the molecular water cages provides a geometric constraint on
the species that can be successfully enclathrated, where the severity of the highpressure and low-temperature conditions depends on the availability of guest species. Sloan and Koh (2007) discuss the array of hydrate structures – based in
combinations of cages with distinct internal diameters – that are found in both
nature and laboratory applications; structures I and II (respectively, “sI” and “sII”)
are the most common, where sII hydrate is typically generated from the light hydrocarbon composition found in conventional oil and gas reservoirs.
While hydrate equilibrium is typically identified by a boundary in temperaturepressure space, this representation assumes both water and hydrate-forming guests
are available in excess. Deepwater blowouts invoke a unique constraint on equilibrium, as the continuous seawater is depleted in methane. As such, hydrate cannot
immediately form upon a rising natural gas bubble until the local seawater has
reached equilibrium with the light hydrocarbon components. Subsea currents can
reach the order of 0.1 m/s, so the successful formation of hydrates fundamentally
requires seawater to be entrained within the plume at sufficient residence times to
approach equilibrium. In regions where the subsea currents are effectively mitigated – either due to mechanical placement of oil and gas equipment or through
boundary effects near the ocean floor – hydrate formation may also be achievable.
It should be noted that, in the context of a deepwater blowout, most models predict
gas bubble diameters significantly larger than 100 microns; visual experiments from
the Colorado School of Mines (Taylor et al. 2007; Brown and Koh 2016; Davies
et al. 2010) have demonstrated that the formation of hydrate at the boundary of a
hydrate guest phase (e.g., methane) and water will result in an initial “shell” of
approximately 50 microns in thickness. In a bubble swarm, this hydrate shell will
enclose residual high-pressure gas inside, where further hydrate growth requires
diffusion of either water and/or gas across the shell.
S. Pesch et al.