Sleipner: The ongoing challenge to determine the thickness of a thin CO2 layer

Sleipner is the world's longest-running CO2 storage project. Since injection commenced in 1996 almost 1 million tonnes per year have been injected with more than 16 million tonnes successfully stored by 2016. A comprehensive programme of time-lapse 3D seismic monitoring has been carried out, providing unrivalled imaging of the CO2 plume as it has developed and migrated in the storage reservoir. The plume has a tiered structure comprising a number of thin layers of CO2 of the order of a few metres thick. Determination of accurate layer morphology is key to understanding details of fluid flow processes in the plume which is necessary to demonstrate future storage security. Migration of the topmost layer of CO2, trapped directly beneath the reservoir topseal, determines the longer-term storage performance at Sleipner and here we focus on mapping its travel-time (temporal) thickness. Our primary approach is to use spectral analysis to determine tuning frequencies across the layer and from these to derive temporal thickness. These range from zero at the layer edges to around 16 ms in the central parts of the layer and correlate closely with the base topseal topography. Uniquely, results are then compared with those from other published approaches including amplitude analysis, temporal shifts and direct measurement of temporal spacing on the latest high-resolution seismic data. It is clear that the spectral methods provide robust determination of temporal thickness well below the tuning thickness, and, taken in suitable combination with the various other methods, can provide reliable determination of temporal thickness across the range from close to zero to well above the tuning thickness where explicit layer resolution is obtained. Application of an appropriate layer velocity allows true layer thicknesses to be determined and layer volumetrics to be estimated.