The effect of surfactants on weathering of surface oil slicks: an experimental study to improve modelling of surfacing oil after subsea dispersant injection (SSDI)

Abstract Subsea dispersant injection (SSDI) has become a widely adopted contingency method since its first large-scale application during the Deepwater Horizon oil spill in 2010. In Norwegian waters, where offshore oil and gas production occurs at shallower depths (100–400 m), a significant volume of chemically dispersed oil could surface due to the limited depth and high plume buoyancy from uncompressed gas. This study investigates the weathering behavior of surface oil slicks resulting from SSDI, focusing on the effects of residual surfactants. Laboratory experiments conducted at SINTEF with five oil types of varying properties revealed that residual surfactants from SSDI in surface slicks lead to: (1) reduced interfacial tensions (IFTs), (2) decreased water uptake, (3) lowered emulsion viscosity, and (4) increased dispersion of surface oil. These effects collectively reduce the persistence of surface oil and potentially minimize stranding following a subsea blowout with SSDI. These findings were used to develop multivariate regression models predicting maximum water uptake and emulsification kinetics as functions of oil properties and surfactant concentration. These models are integrated into an operative Oil Spill Contingency and Response (OSCAR) model to enhance the prediction of surface oil slick behavior after SSDI treatments. The findings indicate that incorporating the effects of residual surfactants into oil spill models can significantly improve the accuracy of SSDI effectiveness assessments, particularly in regions with shallow waters and high gas-to-oil ratios. This research provides valuable insights for optimizing SSDI operations and improving oil spill response strategies.