Abstract
New analysis of the baseline (pre-injection) seismic data at Sleipner has revealed large-scale, roughly north-trending, channelled ‘fairways’ at a range of stratigraphical levels in the Utsira Sand. The baseline data also reveal localised stratigraphical ‘point discontinuities’ within the reservoir, some of which show evidence of having provided vertical conduits for earlier natural gas flow. The repeat time-lapse seismic datasets, where finer details of reservoir geology are illuminated by the reflective CO2, show smaller-scale, north-trending and sometimes sinuous channels within the larger channel fairways. They also show a number of vertical CO2 pathways within the CO2 plume, corresponding to the point discontinuities seen on the baseline data.Reservoir flow models were set up with flow properties constrained only by the observed levels of CO2 accumulation in the reservoir and the arrival time of CO2 at the reservoir top just prior to the first repeat seismic survey in 1999. The initial model with laterally homogeneous sand units separated by thin semi-permeable mudstones achieved only a moderate match to the observed time-lapse seismic data. Subsequent flow models, progressively incorporating higher permeability vertical CO2 pathways through the mudstones and large-scale channel fairways within the reservoir sands, yielded a progressive and marked improvement in the history-match of key CO2 layers within the plume. Crucially, no layer-specific model calibration was employed to achieve this improvement.New geophysical measurements from Utsira Sand core have recently become available. These measurements provide important constraints on rock physics models of CO2 and brine mixtures in the Utsira Sand. An empirical Brie fluid mixing law for intermediate fluid saturations provides a good fit to the new laboratory data, allowing measurements of CO2 saturation to be converted into seismic velocity. This rock physics model was used to convert CO2 saturation distributions predicted by the most realistic reservoir model into a seismic velocity model of the CO2 plume. Synthetic seismic reflectivity profiles generated using this velocity model show a striking resemblance to the observed time-lapse seismic data, both in terms of plume layer reflectivity and also of time-shifts within and beneath the CO2 plume. This provides confidence in the fidelity of the preferred reservoir model solution.These results represent a significant breakthrough in the understanding and modelling of CO2 plume development at Sleipner. We emphasise that the improvements were brought about not by fine tuning reservoir properties to fit the observed time-lapse data, but simply by incorporating geological permeability features that can reasonably be inferred from the baseline seismic data.
Highlights
CO2 separated from natural gas produced at the Sleipner field in the North Sea (Norwegian block 15/9) is being injected into the Utsira Sand, a regional saline aquifer of late Cenozoic age (Fig. 1)
The time-lapse seismic data collected at Sleipner shows that CO2 migration is being controlled by enhanced permeability pathways, both horizontal (‘fairways’) and vertical (‘chimneys’)
These data indicate that Gassmann fluid substitution with Brie mixing and e = 3–5 (Fig. 22) should provide a satisfactory approxima tion of the seismic velocity - saturation relationship in the Sleipner CO2 plume and this was used to produce a velocity profile along the line of section together with matching density
Summary
CO2 separated from natural gas produced at the Sleipner field in the North Sea (Norwegian block 15/9) is being injected into the Utsira Sand, a regional saline aquifer of late Cenozoic age (Fig. 1). Lindeberg et al (2001) and Van der Meer et al (2000) published 3D flow models of the CO2 plume which produced reasonable replica tions of the CO2 plume as it was interpreted from the first time-lapse seismic datasets in 1999 Both models had homogenous sand units, with the shape of individual CO2 layers controlled by arbitrarily adjusting the topography of the mudstone topseal to each layer. The precise nature of vertical fluid flow pathways through the reservoir remains enigmatic They might reflect the lateral impersistence of the thin intra-reservoir mudstones, localised by-pass features due to sand injection, or even fractured gas chimneys. Given the degree of parameter uncertainty in sand permeability and layer-seal topographies, we have elected to assess the nature of geologically realistic reservoir permeability heterogeneity and its influence on the broader development of a growing CO2 plume
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