CO2-induced seismic time shifts at Sleipner

Abstract

Since 1996, [Formula: see text] has been injected into the Utsira Formation above the Sleipner Øst field in the North Sea. During the injection period, the [Formula: see text] distribution in the reservoir has been monitored using eight repeated time-lapse seismic surveys. This seismic monitoring has shown that the [Formula: see text] is visible as thin bright reflectors within the Utsira Formation, interpreted to represent [Formula: see text] flooded layers. We show a detailed analysis of the thin [Formula: see text] layers, with respect to amplitude and time shift development. For a fixed location that becomes [Formula: see text] flooded, the amplitude first increased strongly and a small pull-up of the trough amplitude timing was observed. This response is attributed to interference between top and base of the layer, as long as layer thickness is beneath tuning thickness. This pull-up is an effect induced by wavelet distortion and not related to [Formula: see text] in overlying layers. With time the amplitude decreases and the pull-up is reduced to zero. This is attributed to the layer being thick enough for top and base to be separately resolved. Using our time-lapse observations we have estimated this wavelet distortion effect as a function of time of first flood, which has enabled us to correct for it, and estimate time shifts caused by [Formula: see text] in overlying layers. Thickness estimates derived from these time shifts complement previous amplitude derived estimates of [Formula: see text] thickness.