"Economic Development and the Environment"
on the Sakhalin Offshore Oil and Gas Fields II

Copyright (C) 1999 by Slavic Research Center, Hokkaido University.
All rights reserved

Oil and Gas Development on the Sakhalin Island Shelf: An Assessment of Changes in the Okhotsk Sea Ecosystem

Alexander Leonov

Oil Transformation in a Marine Environment
Knowledge about the processes of oil decomposition in a marine environment serves as a scientific base on which we can construct a strategy for dealing with oil pollution of the seas and oceans. This knowledge determines the efficiency of chemical and microbiological measures to counteract oil spills. Oil from different origins differs in structure, and these differences increase when oil contacts water and air: the oil's structure begins to vary owing to the loss of part of its hydrocarbons which have minimum molecular mass, density and viscosity, as well as maximum volatility and solubility in the water. At the same time, the properties of oil remaining in a water environment vary in the opposite direction [53].
The complex transformations of oil and its products begin immediately after contact with a marine environment. The course, duration, and the results of the oil's transformation depend on the properties and structure of the oil and on the particular situation and parameters of the environment. The main features of the oil's transformation are: the dynamic (especially in the initial stages) and close interactions of physical, chemical and biological processes in the dispersion of all the oil's components down to their complete disappearance in the initial substrate.
Thus, it is possible to group the following major processes in the transformation of oil when it enters a marine environment [1, 40, 42, 52-66]:
Oil spilled on the sea surface is initially influenced by the action of gravitational forces and then is controlled by its viscosity and forces of surface tension. A one ton oil spill distributed in a 50 m radius has a thickness of up to 10 mm; the formation of a thinner film (less than 1 mm) covers an area up to 12 square kilometers [64]. As the crude oil spreads, it quickly loses its volatile and water-dissolved components, and the remaining viscous fractions retard the spilling process. The oil film drifts predominantly in the wind's direction with a speed equal to 3 to 4% of the wind's speed, frequently exceeding the rate of water motion [62]. In the course of time, the oil film on the surface becomes thinner and as it approaches critical thickness, (about 0.1 mm), it begins to breakdown into fragments, which are transported over extensive areas. In strong winds, the residues of the oil film are quickly dispersed in a layer of active mixing. Thus, the essential components of the oil turn into emulsified forms and are transported significant distances by currents.
This process is especially important for less dissolved saturated hydrocarbons. During contact with the air, the volatilization of hydrocarbons occurs from the water surface into the atmosphere [53]. During the first several days after an oil spill, a significant amount transforms into a gaseous phase. This amount can make up to 75, 40 and 5 % respectively for easy, mean and heavy oils [66]. The evaporation of low molecular alkanes, cycloalkanes and benzene are the quickest (from minutes up to hours). Polycyclic aromatic hydrocarbons (PAH) (anthracene and pyran types) do not transform into a gaseous phase; they remain in a water environment and are exposed to complex transformations as a result of oxidation, biodegradation and photochemical processes which usually result in the formation of more polar and dissolved compounds. A combination of meteorological and hydrological effects (the power and direction of the wind, waves and currents) determines the specific characteristics of the distribution and the subsequent state of oil in a marine environment.
The solubility of oil hydrocarbons depends on their molecular structure and mass. Aromatic hydrocarbons are mostly dissolved, actively passing into a water environment and behaving like truly dissolved substances [53]. Naphthene hydrocarbons seldom dissolve in water. As a rule, when a hydrocarbon's mass increases, its solubility in water is reduced. After oil enters a water environment, the relative enrichment of the dissolved fraction by the most dissolved low molecular aromatic and aliphatic hydrocarbons with their subsequent and rather fast volatilization and increasing of the contribution of less volatile (less dissolved) fractions of aromatic hydrocarbons take place. About 1 to 3% (sometimes up to 15%) of crude oil can pass into a dissolved state. First of all, it concerns the low molecular hydrocarbons of aliphatic order and aromatic structure, as well as polar compounds appearing as a result of oxidizing transformations of some initial petroleum fractions in the marine environment. The transition process into a dissolved state is spread over time and depends on the hydrodynamic and physiochemical conditions of the surface waters. The concentration of dissolved fractions under the oil film in the sea is made up of 0.1 up to 0.3 to 0.4 mg/l [61]. An excess of these concentrations is usually accompanied by the formation of decomposable oil-water emulsions.
Many parameters affect the formation of water-dissolved oil product fractions. The most important among them are: the oil type; the degree and duration of oil mixing with water; the ratio of mixed volumes of oil and water; and the sedimentation time required for the achievement of stable hydrocarbon distributions between water and petroleum phases.
Emulsification and dispersion
Emulsified oil is often the dominant form of chronic oil pollution. This fact is stipulated by the extended action of hydrodynamic factors (wind and others), by receipt of the oil into a marine environment in the form of emulsions, and by the presence of high molecular compounds in the oil pollution's structure (promoting self-emulsification). The formation of oil emulsions in a marine environment depends on the oil's structure and the water's turbulence. The most stable emulsions ("water oil" types) contain between 30 to 80% water. They are usually formed after strong storms in zones of heavy oil spills with an increase in nonvolatile fractions (for example, naphthenes) and can exist in a marine environment for more than 100 days as a peculiar "emulsion" of brown and other tones. The stability of emulsions increases with temperature decreases. "Oil in water" type emulsions are unstable because of the action of inter-surface tension forces which quickly reduce the oil's dispersion. This process can be slowed down with emulsifiers - surfactant substances with strong water-receptive properties. These substances are used for eliminating the consequences of petroleum pollution. Thus, stabilization of the petroleum emulsion, its dispersion in the formation of microscopic drops and the acceleration of oil decomposition in the water column takes place.
Hydrometeorological conditions are a determining influence on the fate of different oil products at all stages in their distribution in a marine environment. The role of hydrological and meteorological conditions is especially important in the first hours after oil enters a marine environment, when the oil still has low viscous volatility and dissolved fractions. Only in this period is the effective dispersion of oil products possible; small dispersed fractions will not be formed later [53].
Oil aggregates may be frequently found in a marine environment in the form of resinous and mazut lumps and balls (petroleum lumps, tar balls, pelagic tar). They are formed by about 5 to 10% of spilled crude oil and up to 20 to 50% of settled oil and oil products in the ballast and flush waters of tanker holds. The chemical structure of aggregates is rather changeable but its basis is usually made of asphaltic (up to 50%) and high molecular compounds of heavy oil fractions.
Chemical oxidation and destruction
The chemical oxidation of oil in a water environment begins only a day after its entering into the sea. The chemical oxidation of oil is often accompanied by its photochemical decomposition under the impact of an ultraviolet part of solar spectrum. This process is catalyzed by vanadium and is inhibited by sulfur. The final products of oil oxidation (hydroperoxides, phenols, carboxyl acids, ketones, aldehydes and others) usually possess increased solubility in water and increased toxicity.
Microbiological decomposition
Microbiological decay defines the final fate of oil products in a marine environment. There are about 100 species of bacteria and fungus capable of using oil products for their growth. Their number does not exceed 0.1 to 1% of the number of heterotrophic bacterial communities in clean water areas and this figure increases up to 1 to 10% in polluted water [54]. The mechanisms of oil hydrocarbon uptake by microorganisms are the subjects of special laboratory studies [40, 42, 63].
The ability of hydrocarbons to biodegrade depends on the structure of their molecules. Compounds of the paraffin order (alkanes) have this ability to a greater degree in comparison to aromatic and naphthene substances. The rate of microbiological destruction of hydrocarbons usually decreases as the complexity of their molecular structure increases. For example, the biodegradation rate is tens or hundreds of times lower for anthracene and benzo(a)pyrene than for benzene [59, 65]. The biodegradation rate of oil depends on the degree of oil dispersion, on the water's temperature, on the content of biogenic substances and oxygen, as well as on the species' structure and the number of the oil-oxidizing microflora [55, 60].
Oil-based drill solutions impregnated by drill slimes are rather stable in a marine environment. Experiments simulating natural conditions have shown that the biodegradation of oil-based drill waste after 180 days did not exceed 5%, whereas other drill solutions (prepared on the basis of fatty acid esters) were nearly completely degraded (99%) due to microbiological processes and physical-chemical decomposition [57].
Part of the oil (up to 10 to 30%) is sorbed in suspended matter and settles on the bottom of the seabed. Sedimentation occurs more in narrow coastal zones and in shallow water where the amount of suspended matter is significant and water mixing occurs more frequently. At a greater distance from the coast, sedimentation occurs extremely slowly, except for heavy oils. Suspended oil and its components are subjected to intensive chemical and biological decomposition. On the sea-bottom, the degree of oil decay is sharply reduced since the oxidizing processes slow down due to anaerobic conditions. The fractions of heavy oil accumulated in sediments may be stored there for months and years.
The ratio of dissolved and suspended forms of oil and its components in a marine environment varies in an extremely wide range depending on the particular combination of environmental factors, the structure, properties and the oil's origins. For example, in the Baltic Sea, this ratio varied in the range of 0.2 to 2.1 [52]. A study of oil sedimentation in the Caspian Sea showed that a significant amount of marine salt (possibly in the form of concentrated brine) was found adhering to suspended oil particles. In two samples, 0.3 and 0.1 mg of salt were found respectively in 4.4 and 2.1 mg of oil hydrocarbons [53].
The general conclusion from all the studied processes is that oil quickly loses its initial properties. It is divided into groups of hydrocarbons and fractions of different forms whose composition and chemical structure are considerably transformed. The content decreases owing to the dispersion and decay of the initial and intermediate compounds and the formation of carbon dioxide gas and water. The purification of a water environment polluted by hydrocarbons takes place once the indicated processes are completed.
Oil spill behavior in marine ice conditions
The oil transformation process in ice-covered water areas is slower due to the following factors: an increase of the viscosity of crude oil at low temperatures; restrictions in the oil's distribution owing its adsorption on the ice surface; accumulation in the porous stratum and interstices of the ice cover; and a slow down of the oil's decay by bacterial and photochemical processes in conditions of lower temperatures and restricted oxygen input [1].
During the spring-summer period, the migration of oil into ice capillaries varies from 1 to 49 sm/day. The mean rate of vertical movement of oil into ice is equal to 8 sm/day. Strong winds and currents break up the ice cover and allow the ice to drift. In the Bering Sea, for example, the typical rate of ice drift is 7.4 km/day, and it may increase up to 33-44 km/day during storms [67].