Oil Fallout in the Vicinity of an Onshore Blowout: Observations on a Field Case

Abstract

Summary Oil deposition in the area surrounding a blowout is one of the most visible consequences of the loss of control over 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 (e.g., reduced tree growth) that 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 limitation can be attributed to a lack of validation of the models because this requires a combination of well-effluent data, outflow conditions, meteorological data, a detailed terrain description, and fallout maps. For the field case discussed in this paper, all the data necessary 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 relationship demonstrates that the main factors determining the area affected by deposition of oil are the outflow conditions and the wind direction. To verify this result, a comparison with historical observations on oil fallout around gushers has been made. Although more sketchy, the same patterns emerge from this comparison, which 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. Finally, it is noted that this method also applies in less dramatic situations (e.g., predicting droplet deposition around vent stacks).