4D Seismic Imaging of an Injected C02 Plume at the Sleipner Field, Central North Sea

Abstract

C02 produced at the Sleipner field is being injected into the Utsira Sand, a major saline aquifer. Time-lapse seismic data acquired in 1999, with 2.35 million tonnes of C02 in the reservoir, image the C02, plume as a number of bright sub-horizontal reflections. These are interpreted as tuned responses from thin (< 8 m thick) layers of C02 trapped beneath intra-reservoir shales. A prominent vertical 'chimney' of C02 appears to be the principal feeder of these layers in the upper part of the reservoir. Amplitude-thickness scaling for each layer, followed by a layer summation, indicates that roughly 80% of the total injected C02 is concentrated in the layers. The remainder is interpreted to occupy the feeder 'chimneys' and dispersed clouds between the layers. A prominent velocity pushdown is evident beneath the C02 accumulations. Velocity estimation using the Gassmann relationships suggests that the observed pushdown cannot readily be explained by C02 present only at high saturations in the thin layers; a minor proportion of low saturation C02 is also required. This is consistent with the layer volume summation, but significant uncertainty remains.

The Geomechanics of CO2 Storage in Deep Sedimentary Formations

“…If such CO 2 -induced shrinkage would occur in the lower parts of a caprock, the horizontal stress might be significantly reduced, and shrinkage cracks could occur. These processes have been mentioned associated with upwelling of CO 2 across thin shale layers at Sleipner (Chadwick et al 2004). Similar shrinkage effects as a result of mineral dissolution in carbonate-rich rock are also possible (Espinoza et al 2011).…”

Offshore CO 2 storage: Sleipner natural gas field beneath the North Sea

“…The plume is elliptical in plan, with a major axis increasing to about 4500 m by 2010, accompanied by development of a prominent northerly extension since 2004. A strong velocity 'pushdown' is evident on reflectors beneath the plume and a vertical column of markedly reduced reflectivity, up to 80 m in diameter, forms a 'seismic chimney' roughly above the injection point (Chadwick et al 2004b). …”

“…Early papers concentrated on quantification of the plume reflectivity and velocity pushdown with the aim of independently verifying the measured injected amount of CO 2 Chadwick et al 2004b;2005). A satisfactory match was obtained for the 1999 dataset, using a saturation model containing around 85% of the known injected CO 2 whilst maintaining a satisfactory match with the seismic data.…”

Quantitative evaluation of thin-layer thickness and CO2mass utilizing seismic complex decomposition at the Ketzin CO2storage site, Germany

“…Arts et al (2004a), Chadwick et al (2004) and Ghaderi & Landrø (2009) utilized seismic amplitude and time shift to estimate the layer thickness of CO 2 at the Sleipner project site, but if phase and amplitude are not well preserved during the data processing errors may be large. As an alternative, spectral decomposition is an effective time-frequency analysis tool for decomposing seismic signals into discrete-frequency components which are phase-independent.…”