Oil wells blow-out events are characterized by the uncontrolled releases in atmosphere of multiphase hydrocarbon mixtures. The upper well section under blowout conditions is always characterized by the presence of annular flow with the most part of the oil transported in form of droplets. A further atomization occurs at the discharge, typically in critical flux conditions. The droplets ejected at sound velocity in the atmosphere exchange momentum with the gas jet rising at different elevations depending on the size, the jet momentum and the meteorological conditions. The granulometric distribution of the droplets also strongly affects the following aerosol dispersion and fallout. The aerosol dispersion follows two different mechanisms: Gaussian dispersion for the small droplets (with a spreading phenomenon strongly dependent on diffusion effects), ballistic dispersion (with a spreading dependent on stochastic variations in wind direction and speed). During the fallout phase a not negligible amount of oil mass evaporates changing both the droplets dimension and the oil compositions. The typically long duration of the blowout event makes a short term approach (with constant meteorological condition during the event) not realistic. A long term approach, taking into account the typical meteorological classes distribution and persistency in the involved area has to be considered for the correct prediction of the oil fallout to the ground. This process generates to the ground surface an areal source term for the following infiltration phenomenon. These areal sources are characterized by wide extensions and significant distances from the well (more than 10 km in some cases).

Most of the complex processes and phenomena, which are described above, result not completely treated in literature and specific models had been developed and implemented in a in-house software (3 ) as described in this paper. The overall model has been validated with real blowout contamination data.

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