Volumetric Measurements of a Thin CO2-Saturated Layer Through Time at the Sleipner Field, North Sea

Abstract:

Monitoring of injected CO₂ at the Sleipner carbon storage project is primarily achieved through three-dimensional time-lapse seismic reflection surveys. Previous studies have shown that injected CO₂ is trapped in nine distinct thin layers within the reservoir. Whilst seismic reflection images obtained from the time-lapse surveys are adequate to measure changes in lateral extent of these layers through time, measuring the volume of CO₂ trapped within each layer has proven difficult. The vertical resolution of the seismic images is only sufficient to directly measure CO₂-saturated layers thicker than 8 m, yet the uppermost CO₂ layer of the reservoir is thought to be less than 10 m thick. This vertical resolution limit has hindered our understanding of the migration of CO₂ through time, both within individual layers and throughout the entire reservoir. Here we present a new method to measure the thickness of thin, CO₂-saturated layers by exploiting the repeatability of the time-lapse seismic experiments. Measurements of reflection amplitude from the top of the CO₂ layer are combined with measurements of relative changes in two-way-travel-time to the same reflector between surveys to give a unique estimate of CO₂ layer thickness. This method is applied to the uppermost layer at Sleipner. Synthetic testing allows quantitative estimates to be made of the uncertainty in the CO₂ thickness introduced by noise and uncertainties in the seismic data. By measuring CO₂ layer thickness on all time-lapse surveys, temporal changes in the volume of CO₂ in the top layer can be calculated. The flux of CO₂ into the top layer is observed to be growing at an increasing rate, and the implications for fluid CO₂ propagation throughout the reservoir are discussed. The relationship between the thickness of the CO₂ layer and the topography of the structural trap for the top layer is also explored.