Abstract
CO2 separated from natural gas produced at the Sleipner and Gudrun fields is being injected into the Utsira Sand, with around 18 million tons currently stored. Time-lapse 3D seismics have been deployed to monitor development of the CO2 plume. The 2010 seismic survey resolved, for the first time in 3D, the topmost CO2 layer into distinct reflections from its top and base. Seismic velocity is diagnostic of CO2 layer properties and a forensic interpretative approach is adopted to determine spatial velocity variation in the topmost CO2 layer. Velocity is obtained by equating absolute layer thickness, derived by subtracting a constructed flat CO2 – water contact from the topographical relief of the reservoir top, to the temporal separation of the layer top and base reflections, with appropriate correction for wavelet interference effects. Layer velocities show a systematic and robust spatial variation between a northern area with a mean velocity of 1371 ± 122 ms−1 and a central area with a much higher mean velocity of 1638 ± 103 ms−1. Recent fluid flow simulations of the topmost CO2 layer have shown that incorporating a high permeability channel in the model reservoir significantly improves the history-match. This high permeability channel corresponds remarkably closely to the low seismic velocities mapped in the northern area, with higher layer velocities of the central area interpreted as more argillaceous, less permeable overbank deposits. The new velocity analysis therefore provides independent support for including deterministic permeability heterogeneity in predictive fluid flow modelling of Sleipner.
Highlights
Injection and geologyLarge-scale underground storage of anthropogenic carbon-dioxide is a key technology for keeping cumulative man-made emissions of greenhouse gases within safe limits (IPCC, 2005; ETI, 2015)
Rock physics using the natural range of sand properties observed from Utsira Sand well logs indicates that the velocities of CO2 layers could range from < 1400 to > 1500 ms−1 for high CO2 saturations, with higher velocities at intermediate CO2 saturations (Fig. 7), the latter supported by recent experimental data and calibrated rock physics from the Utsira core (Falcon-Suarez et al, 2018)
Seismic velocity is a key diagnostic of CO2 layer properties but deriving velocities directly from seismic data has proved very challenging at Sleipner due to the very thin layers, associated strong tuning effects, and lack of borehole calibration
Summary
Large-scale underground storage of anthropogenic carbon-dioxide is a key technology for keeping cumulative man-made emissions of greenhouse gases within safe limits (IPCC, 2005; ETI, 2015). It is a key element in negative emissions technologies such as bio-energy with CCS (BECCS) and direct air capture and storage of CO2 (DACCS) (CCC, 2019). The CO2 injection operation at Sleipner in the Norwegian central North Sea commenced in 1996 and is the world’s longest-running industrial-scale storage project (Baklid et al, 1996). CO2 separated from natural gas produced at the Sleipner Vest field, and since 2014 the Gudrun field, is being injected into the Utsira Sand, a regional saline aquifer of late Cenozoic age (Fig. 1a). Injection commenced in 1996, at a roughly constant rate of 0.8 to 0.9 million tons per year, with around 17 million tons of CO2 stored by the end of 2017 (Eiken, 2019)
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