Using an Old Underground Blowout as a Proxy for CO2 Leakage Scenarios

Based on seismic data acquired prior to an underground blowout in 1989, we find that it is reasonable to assume that there has been some gas migration into shallower sediments over a time period of 20 years. By studying the seismic datasets (several 2D lines and one 3D) acquired over this area, we can learn something about how fast gas migrates through shallow sediments close to the seabed. Since CO2 is denser and will probably flow slower than gas, we think it is possible to use this example as a proxy to estimate the lower bound for vertical transit times of CO2 through shallow sediments of the type we are studying. This knowledge we believe might be of importance in risk assessment associated with underground storage of CO2. However, the reservoir pressure and temperature conditions (that vary with depth) dictate the CO2 behaviour, whether it is supercritical with liquid-like densities or with gas-like viscosities. It is important to stress, therefore, that there are significant differences between CO2 in a supercritical phase and the assumed gas associated with an underground blow out from an oil well.