Laboratory studies of the properties of in-situ burn residues

Abstract

This study investigated the physical and chemical properties of the residue from in-situ burns of thick oil slicks. It involved burning small-diameter slicks of oil on water and analyzing the properties of the residues. The objective of the work was to identify the burn conditions that might produce residues that sink. Eight oils were selected for the project: (1) Alaska North Slope crude; (2) Alberta Sweet Mixed Blend crude; (3) Arabian Heavy crude; (4) Arabian Light crude; (5) Bonny Light crude; (6) Iranian Heavy crude; (7) Mayan crude; and (8) automotive diesel. Burn tests were conducted on all eight oils when fresh and on two of the oils when weathered. Experiments involved burning slicks of three thicknesses (5, 10, and 15 cm) on saltwater at room temperature (15°C). Residue density, water content, pourpoint, and viscosity were measured. Samples of parent oils and burn residues were fractionated into three boiling point ranges, and each was analyzed to quantify total saturates, aromatics, resins, and asphaltenes. The results showed that the residues from burns of thicker slicks of heavier crudes and weathered crudes may sink in fresh water or saltwater once they cool to ambient temperatures, whereas burn residues of lighter oils may not sink. Burn residues of all crudes were more dense than their parent oils and were solid or semisolid. Residue density was related to the density of the parent oil, the state of weathering, and slick thickness. Chemical analysis showed that the burn residues were composed almost exclusively of the higher boiling point (HBP) fraction; virtually all of the lower boiling point fraction and almost all of the middle boiling point fraction had been removed. Most, but not all, of the HBP fraction, which included all of the asphaltenes and resins, was preserved in the burn residue. The in-situ burning process appears to be neither a pure equilibrium flash vaporization nor a pure batch distillation, but rather a process lying somewhere between the two ideals. The results of the burns of automotive diesel contrasted strongly with those of crude oils. Diesel burns were far more efficient than those of crude oils, leaving only a few millimeters of residue regardless of the thickness of the original slick. The chemical composition of the residue and its properties were changed only slightly from those of the parent oil.