Capstone: CO₂ sequestration — Sleipner revisited

The Sleipner field in the Norwegian North Sea has been injecting CO₂ into the Utsira Sand aquifer since 1996. It is the world's longest-running industrial carbon capture and sequestration (CCS) project and the most-published 4D monitoring case study. Annual baseline + monitor seismic surveys track the CO₂ plume's migration and provide the evidence for regulatory containment claims.

Project setup

Sleipner CCS. 1 Mt/year CO₂ injection at 1000 m depth into a 200 m thick saline aquifer. Annual repeat 3D surveys (baseline 1994, monitors at 1999, 2001, 2002, 2004, 2006, 2008, 2010, 2013, 2018, 2022). Primary deliverable: a 4D difference volume per year showing the CO₂ plume extent and shape. Secondary: per-layer saturation maps through rock-physics inversion.

The pipeline

Why joint processing is the central theme

CO₂ reflectivity changes are SMALL (typically 3–7 % amplitude change at the reservoir horizon) but the 4D signal must be PRESERVED across multiple decades of repeat surveys. Any non-geology-related difference between surveys — wavelet mismatch, processing parameter drift, legacy vs modern acquisition — contaminates the plume map and can force a misinterpretation of containment status. Joint processing with shared parameters across all surveys is the only way to keep NRMS below 15 % consistently over the monitoring duration.

Regulatory dimension

• Containment evidence. The operator must demonstrate that injected CO₂ stays in the approved storage complex. 4D difference volumes are the primary evidence; NRMS QC certifies that the observed plume shape is real.
• Volume reconciliation. The plume volume from 4D must match (within uncertainty) the metered injected volume. A consistent 20 % mismatch suggests leakage — a regulatory event.
• Temporal consistency. Each year's monitor must be comparable with all previous monitors. The joint processing and 4D matching are not optional — they are the audit trail.

What Sleipner actually showed

Nine layers of sandstone within the Utsira aquifer act as independent traps; CO₂ rises and pools against each mudstone baffle. The 4D difference over 25 years shows the plume rising from the injection point, distributing upward through successive layers, and stabilising in the uppermost trap. The measured plume volume matches the metered injection within 3 % — a textbook containment success.

Where this goes next

§10.5 changes scale entirely: an ultra-high-frequency near-surface survey for engineering geotechnics where the depths and wavelengths are four orders of magnitude smaller than marine exploration.

References

• Yilmaz, Ö. (2001). Seismic Data Analysis (2 vols.). SEG.
• Virieux, J., Operto, S. (2009). An overview of full-waveform inversion in exploration geophysics. Geophysics, 74, WCC1.
• Castagna, J. P., Backus, M. M. (1993). Offset-Dependent Reflectivity. SEG.
• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge UP.