Offshore CCUS & Storage

1. The Decision Problem

Offshore carbon capture, utilisation, and storage (CCUS) projects represent capital-intensive, multi-decade commitments where subsurface uncertainty directly governs project bankability and regulatory licensing. Developers, storage operators, and consenting authorities face three interdependent decision hurdles:

1. Injectivity & Capacity Verification: Ensuring the deep saline aquifer or depleted reservoir can sustain planned injection rates (e.g., $1.5\text{–}5.0\text{ Mt/year}$) without inducing unexpected pore pressure escalation.
2. Caprock & Fault Seal Containment: Proving that the primary caprock (shale/evaporite) and bounding fault systems will maintain capillary and mechanical integrity under effective stress shifts.
3. Lifecycle MRV Strategy: Designing an audit-ready Measurement, Reporting, and Verification (MRV) regime that satisfies international bodies (such as OSPAR and the London Protocol) while controlling lifecycle operational expenditure.

                          Offshore CCUS Evidence Architecture
                          ═══════════════════════════════════
   Water Column       [ USV Acoustic Sniffers ] + [ Benthic pH Sensors ]
        │
   Seabed (0m)        [ Permanently Trenched Optoelectronic DAS Cables ]
        │
   Overburden         [ High-Resolution 3D Seismic Velocity Model ]
        │
   Caprock (-1800m)   [ Geomechanical Breakthrough Pressure: 19.2 MPa ]
        │
   Storage Reservoir  [ Supercritical CO2 Plume Tracking & Microseismics ]

2. Multidisciplinary Evidence Pathway

Step 1: Storage Complex Characterisation & Ground Modeling

• Ultra-High-Resolution 3D Seismic: Full-waveform inversion (FWI) resolving deep fault networks, stratigraphic pinch-outs, and overburden gas blanking zones.
• Geomechanical Core Analysis: High-pressure, high-temperature (HPHT) triaxial stress testing to determine rock compressibility, Young’s modulus, and Poisson’s ratio under supercritical $\text{CO}_2$ exposure.
• Seismic-to-Well Tie: Micro-stratigraphic correlation matching regional wireline log signatures with high-frequency acoustic impedance profiles.

Step 2: Pre-Injection Environmental Baseline

• Water Column & Seabed Chemistry: High-precision CTD profiles, dissolved inorganic carbon (DIC) baseline measurements, and autonomous seabed pCO₂ sensor calibration.
• Benthic Ecology Habitat Mapping: Multi-beam backscatter analysis paired with environmental DNA (eDNA) sampling to establish pre-injection biological baselines.

Step 3: Operational 4D MRV & Anomaly Escalation

• Distributed Acoustic Sensing (DAS): Utilizing permanent seabed optical fibers for on-demand 4D time-lapse seismic imaging without expensive repeated streamer mobilization.
• Micro-Seismic Network: Continuous subsurface passive listening with event detection threshold ($M_w < 0.5$) to verify zero fault slip during high-rate injection.

3. Decision-Ready Deliverables