Crude oil blends vary tremendously in their chemical composition, depending on the geographical location of their origin and the particular compounds mixed with the petroleum products. Surfactants, often added to aid transport, will affect physical properties when oil is spilled. Hydrocarbons are by far the most abundant compounds in crude oils, accounting for 50-98% volume. All crude blends contain lighter "fractions" (similar to gasoline) of hydrocarbons as well as heavier tars and wax-like hydrocarbons.
Alaskan North Slope (ANS) crude blends are Group III oil products and considered medium grade. The BP ANS crude from Pump Station #9 has a relatively high viscosity (23.9cSt @50 F) and an API gravity of 29.6. (The American Petroleum Institute gravity, or API gravity, is a measure of how heavy or light a petroleum liquid is compared to water.)
ANS crude blends tend to emulsify quickly, forming a stable emulsion (or mousse). The rate of emulsification, while difficult to model, is known to be accelerated by wind mixing, and is thought to be related to the blend’s wax content. The above-mentioned BP blend of ANS is thought to form a mousse after it experiences about 14% evaporation of its lighter ends.
From 15-20% of this product evaporates in the first 24 hours of a spill, depending on the wind and sea conditions, and very little oil is dispersed into the water column. The weathered oil then starts to form a stable mousse with up to 75% water content (thereby increasing the slick volume four-fold), and it undergoes dramatic changes in its physical characteristics.
The viscosity of the oil-in-water mixture increases rapidly and the color usually turns from a dark brown/black to lighter browns and rust colors. As the water content of the emulsion increases, weathering processes (e.g., dissolution and evaporation) slow down.
Behavior of Weathered ANS Crude Blends
The "sticky" mousse behaves differently from a fluid and may react to additional weathering forces by forming a surface skin, creating a non-homogenous material with a crust of slightly more weathered mousse surrounding a less weathered core.
As the mousse is subject to increased mixing from energetic wave action, the crusts can be torn or ruptured and the less weathered mousse released. The continued exposure of weathered mousse to wave action continues to stretch and tear patches of mousse into smaller bits, resulting in a field of streaks, streamers, small patches, and eventually small tarballs.
Cleanup and Spill Response Considerations
While organisms are not at high risk from crude oil dispersed into the water column, stranded crude tends to smother organisms. In birds, it can cause mortality from ingestion during preening as well as from hypothermia from matted feathers.
The oil-in-water emulsion is very sticky and makes cleanup and removal more difficult. When stranded on the shoreline, the degree of adhesion varies depending on the substrate type, e.g., this mousse will not penetrate far in finer sediments.