Modelling oil entrapment in sea ice on the basis of 3d micro-tomographic images

Oil entrapment in drifting sea ice may lead to release of oil far away from spilling locations, when the ice finally melts at the end of its drift. The entrapment process hence needs to be understood properly to evaluate the environmental risks of oil spills in ice-covered waters. However, field studies of oil-in-ice processes are sparse, while laboratory tests do not necessarily represent nat-ural sea ice in terms of growth conditions, time scales and, in particular, microstructures. In this study we consider the role that the microstructure of sea ice may play for oil entrapment. We discuss oil migration into the sea ice pore space on the basis of 3-d images of young winter sea ice, obtained by X-ray micro-tomography. Simulating fluid flow through these images we obtain the vertical distribution of sea ice permeability and pore size distributions, the basic parameter for the prediction of migration of oil into the ice. We compare the analysis with published results from laboratory experiments of oil entrapment in laboratory-grown ice of similar age. Our anal-ysis suggests that care must be taken when interpreting laboratory experiments, due to limited oil amounts released. Furthermore, we perform a similar analysis of older and thicker summer ice that has similar porosity yet coarser pores. The potential of the old ice to entrap oil is expected to be an order of magnitude larger than indicated by results with young laboratory-grown ice. Due to the importance of sea ice microstructure to predict oil entrapment and transport, future approaches should include models of sea ice microstructure evolution with growth and melt conditions