Geological and Technical Foundations of Offshore CO₂ Storage in Depleted Reservoirs

Depleted offshore oil and gas reservoirs are compelling carbon dioxide (CO₂) storage targets because their traps and seals are field-proven and their geology is unusually well constrained by decades of wells, cores, and 3D seismic. Offshore deployment also reduces onshore land-use conflicts and can repurpose platforms, pipelines, and selected wells after CO₂-service qualification, trimming costs and schedules. Technically, prior depletion creates pressure headroom; most candidate intervals lie ≥800-1000 m where CO₂ is supercritical, and many reservoirs (e.g., Miocene sandstones) offer high injectivity; challenges include higher marine logistics costs and the need for robust remote monitoring. Case studies underline both potential and practice: Sleipner (Norwegian North Sea) has stored >15 Mt with repeated 4D seismic confirming containment beneath a thick shale caprock, whereas Snøhvit (Barents Sea) required an early shift to a secondary interval after faster-than-expected pressure buildup, illustrating the value of contingency planning. Emerging Gulf of Mexico hubs target high-porosity sandstones but should accommodate growth faults and salt tectonics through conservative pressure envelopes and geomechanical screening. A disciplined workflow is essential, integrating legacy data, applying screening thresholds, retrofitting infrastructure for CO₂ service, and designing injection within safe pressure limits, sometimes paired with brine production. This review highlights the central role of measurement, monitoring, and verification (MMV)particularly time-lapse seismic, continuous downhole pressure-temperature, and targeted seabed geochemistryas a novel contribution. These approaches are emphasized not only for conformance and containment assurance but also for adaptive operational control. Priority research needs include methods to integrate extensive legacy data sets into predictive models and tighter coupling of real-time monitoring with simulation to guide decision-making. With rigorous site selection, sound engineering, and vigilant MMV, depleted offshore reservoirs can deliver high-capacity, secure CO₂ storage at the scales required for meaningful emissions reduction.