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properties of the oil released, and the release scenario (i.e., water depth and pressure,
rate of release, temperature, etc.). These differences in characteristics affect the partitioning of the oil into dissolved and droplet phases, droplet sizes, the related movement, and the physical behavior of the different oil phases. These in turn influence
the processes that affect the fate of the chemical components contained therein. Oils
targeted during subsea drilling ideally have a high API gravity to recoup investment
promptly as the refining of crude oil to gasoline requires fewer steps for high API
gravity oils compared to low API gravity oils (American Petroleum Institute 2011).
However, there is large variation in API gravity between wells. For example, oil
released during the DWH spill was relatively light, with an API gravity of 37, compared to the 1969 Santa Barbara release, which came from a reservoir with an API
gravity of approximately 26, resulting in a density closer to water (Fingas and
Fieldhouse 2004). These differences can affect the buoyancy of the oil, as the lighter
oils need to be relatively more weathered before they reach and exceed the density
of water and begin to sink.
The individual components of the oil can be grouped into four categories based
on chemical properties: saturates, aromatics, resins, and asphaltenes. Of these, the
saturates and aromatics are of greatest concern in terms of human and ecological
toxicity, while the asphaltenes play a major role in the emulsification of oil as the
lighter oil components are lost to weathering (Fingas and Fieldhouse 2004).
Chemicals in the saturated and aromatic fractions are more soluble and mobile in
the environment to varying degrees and are more amenable to degradation. Resins
and asphaltenes are relatively insoluble and recalcitrant.
At the point of the release in the subsea, petroleum will segregate into different
phases based on chemical properties as well as temperature and pressure.
Components can remain in the liquid oil (droplets), form a gas phase, form hydrates,
or dissolve into the aqueous phase. These phases are affected differently by transport processes and can result in the differential transport of oil components. The
physical processes governing this transport are discussed in detail in Lehr and
Socolofsky (2020).
6.4 Fate of Oil and Gas: Understanding Where Oil Goes
The high pressures at the point of a deep-sea release result in the turbulent expulsion
of oil and gas mixtures, as fast-rising gas bubbles interact with the oil to increase
dispersion and decrease droplet size. In cases of minimal dispersion due to turbulence, oil will appear at the surface close to the blowout, undergoing minimal alteration (Gros et al. 2014). If the oil is dispersed, either via a high-energy release or
through the application of chemical dispersants near the release point, the oil will
form small microdroplets. These droplets alter the fate of the oil, changing both the
transport of the droplet itself and the fate of the specific chemical components.
Despite the oil being lighter than water, microdroplets are effectively neutrally
buoyant, as the rise of the droplet through the water column is opposed by the
K. J. Murray et al.
properties of the oil released, and the release scenario (i.e., water depth and pressure,
rate of release, temperature, etc.). These differences in characteristics affect the partitioning of the oil into dissolved and droplet phases, droplet sizes, the related movement, and the physical behavior of the different oil phases. These in turn influence
the processes that affect the fate of the chemical components contained therein. Oils
targeted during subsea drilling ideally have a high API gravity to recoup investment
promptly as the refining of crude oil to gasoline requires fewer steps for high API
gravity oils compared to low API gravity oils (American Petroleum Institute 2011).
However, there is large variation in API gravity between wells. For example, oil
released during the DWH spill was relatively light, with an API gravity of 37, compared to the 1969 Santa Barbara release, which came from a reservoir with an API
gravity of approximately 26, resulting in a density closer to water (Fingas and
Fieldhouse 2004). These differences can affect the buoyancy of the oil, as the lighter
oils need to be relatively more weathered before they reach and exceed the density
of water and begin to sink.
The individual components of the oil can be grouped into four categories based
on chemical properties: saturates, aromatics, resins, and asphaltenes. Of these, the
saturates and aromatics are of greatest concern in terms of human and ecological
toxicity, while the asphaltenes play a major role in the emulsification of oil as the
lighter oil components are lost to weathering (Fingas and Fieldhouse 2004).
Chemicals in the saturated and aromatic fractions are more soluble and mobile in
the environment to varying degrees and are more amenable to degradation. Resins
and asphaltenes are relatively insoluble and recalcitrant.
At the point of the release in the subsea, petroleum will segregate into different
phases based on chemical properties as well as temperature and pressure.
Components can remain in the liquid oil (droplets), form a gas phase, form hydrates,
or dissolve into the aqueous phase. These phases are affected differently by transport processes and can result in the differential transport of oil components. The
physical processes governing this transport are discussed in detail in Lehr and
Socolofsky (2020).
6.4 Fate of Oil and Gas: Understanding Where Oil Goes
The high pressures at the point of a deep-sea release result in the turbulent expulsion
of oil and gas mixtures, as fast-rising gas bubbles interact with the oil to increase
dispersion and decrease droplet size. In cases of minimal dispersion due to turbulence, oil will appear at the surface close to the blowout, undergoing minimal alteration (Gros et al. 2014). If the oil is dispersed, either via a high-energy release or
through the application of chemical dispersants near the release point, the oil will
form small microdroplets. These droplets alter the fate of the oil, changing both the
transport of the droplet itself and the fate of the specific chemical components.
Despite the oil being lighter than water, microdroplets are effectively neutrally
buoyant, as the rise of the droplet through the water column is opposed by the
K. J. Murray et al.