So-called 'gas chimneys' are likely to provide the main geological risk for out-of-reservoir CO2 migration in thick post-rift overburden successions such as typify the central and northern North Sea. Here we postulate that, in the North Sea, such chimneys formed in the geological past, with a likely peak activity at the end of the ice-age, and are currently rather dormant. With this postulate we set a bound on possible bulk migration rates considering both advective and diffusive flow and based on a hypothetical CO2 storage site at 800 m depth. Calculated migration velocities into the overburden, by either advection or diffusion, are very low, at less than one metre per thousand years. Consequently flux rates are also very low, several orders of magnitude below the leakage thresholds that have been suggested as ensuring effective mitigation performance. Time-lapse seismic reflection data from the Sleipner storage site, which is located beneath some small chimney features, show no evidence of CO2 migration into the overburden. This cannot prove the postulate, because the time interval spanned by the seismic surveys is just a few years, but it is nevertheless consistent with it.
Chadwick, R. A., Williams, G. A., & Noy, D. J. (2017). CO2 Storage: Setting a Simple Bound on Potential Leakage through the Overburden in the North Sea Basin. In Energy Procedia (Vol. 114, pp. 4411–4423). Elsevier Ltd. https://doi.org/10.1016/j.egypro.2017.03.1597