Quantifying the potential exposure hazard due to energetic releases of CO2 from a failed sequestration well

Wells are designed to bring fluids from depth to the earth's surface quickly. As such they are the most likely pathway for CO(2) to return to the surface in large quantities and present a hazard without adequate management. We surveyed oil industry experience of CO(2) well failures, and separately, calculated the maximal CO(2) flow rate from a 5000 ft depth supercritical CO(2) reservoir. The calculated maximum of 20,000 tonne/day was set by the sound speed and the seven-inch well casing diameter, and was greater than any observed event. We used this flux to simulate atmospheric releases and the associated hazard utilizing the National Atmospheric Release Advisory Center (NARAC) tools and real meteorology at a representative location in the High Plains of the United States. Three cases representing a maximum hazard day (quiet winds <1 m s(-1) near the wellhead) and medium and minimal hazard days (average winds 3 m s-1 and 7 m s-1) were assessed. As expected for such large releases, there is a near-well hazard when there is little or no wind. In all three cases the hazardous Temporary Emergency Exposure Levels (TEEL) 2 or 3 only occurred within the first few hundreds of meters. Because the preliminary 3-D model runs may not have been run at high enough resolution to accurately simulate very small distances, we also used a simple Gaussian plume model to provide an upper bound on the distance at which hazardous conditions might exist. This extremely conservative model, which ignores inhomogeneity in the mean wind and turbulence fields, also predicts possible hazardous concentrations up to several hundred meters downwind from a maximal release. (C) 2008 Elsevier Ltd. All rights reserved.