Oil deposition in the area surrounding a blowout is one of the most visible consequences of the loss of control over the well flow. Less visible but equally serious are the short to medium term effects of oil coverage on the environment. Apart from the direct damage, studies indicate effects on e.g. tree growth to persist many years following the incident.

Hence oil fallout in the case of loss of well control, is a factor to be taken into account in decisions on well locations, emergency procedures, contingency planning etc. This requires an estimate of the area around the well likely to be affected by oil fallout, given the geomorphology of the terrain, prevailing winds and expected outflow conditions. Models to obtain such an estimate have been proposed but application of these models e.g. for environmental impact assessment, has been limited. In part this can be attributed to a lack of validation of the models, since this requires a combination of well effluent data, outflow conditions, meteorological data, a detailed terrain description, and fallout maps.

For the field case discussed here, all the necessary data for validation were assembled from various sources ranging from the well operator and environmental agencies to meteorological and cartographic institutes. The relation between the observed pattern of oil deposition around the well and factors such as outflow conditions and wind speed and direction is analyzed with a simple model. This demonstrates that the main factors determining the area affected by deposition of oil are the outflow conditions and wind direction.

To verify this result, a comparison with historical observations on oil fallout around gushers is made. Although more sketchy, the same patterns emerge from this comparison. This strongly suggests that the methods developed to explain the fallout pattern of the field case discussed can be used over a wide range of conditions to predict the area affected by a blowout. This prediction can serve several purposes such as site selection, definition of emergency procedures, environmental impact assessment and contingency planning.

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