Carbon Capture and Storage Feasibility in Depleted Offshore Gas Reservoirs
DOI:
https://doi.org/10.5281/zenodo.22803104Keywords:
carbon capture and storage, depleted gas reservoir, offshore geological storage, caprock integrity, CO2 injectivity, storage capacity estimation, monitoring measurement and verification, techno-economic analysis, North Sea basinAbstract
Depleted offshore natural gas reservoirs are widely regarded as attractive early targets for geological carbon
storage because their trapping geometry, seal integrity and reservoir properties were already proven by decades of
hydrocarbon production, yet quantitative techno-economic assessments that combine capacity estimation,
injectivity modelling and monitoring costs at basin scale remain scarce. We assessed the storage feasibility of a
six-reservoir cluster of depleted Rotliegend-type sandstone gas fields in a North Sea-style siliciclastic basin,
spanning depths of 2050-3400 m, porosities of 12-22%, and net gas pay of 20-90 m, using a probabilistic volumetric
method constrained by material-balance-derived pore volume vacated during historical gas production, a caprock
risk screen based on seal thickness, lithology and legacy well penetrations, and coupled reservoir-wellbore
injectivity simulation. The basin-wide P50 static storage capacity was estimated at 207 million tonnes of CO2,
ranging from 9 Mt in the deepest, tightest reservoir to 71 Mt in the shallowest, highest-permeability unit, with
injectivity indices spanning 1.8 to 14.7 tonnes per day per bar of injection overpressure across the cluster. A
25-year injection programme targeting 4 million tonnes per year, phased across the six reservoirs, would utilise only
48% of basin P50 capacity, leaving a substantial margin against the estimated one-third downside in P10 capacity.
Levelized storage cost, excluding capture and onshore transport, ranged from 9.8 to 34.7 dollars per tonne of CO2
across reservoirs, driven primarily by injectivity-limited well count and by remediation of three legacy wells with
degraded cement integrity in the poorest-quality reservoir. Monitoring, measurement and verification costs,
comprising baseline characterisation, repeat 4D seismic, seabed geochemical and geodetic sensing, and 20 years
of post-closure surveillance, added an estimated 1.1-1.6 dollars per tonne over the full project life. These results
indicate that a multi-reservoir depleted gas field cluster can provide materially cheaper and lower-risk storage than
saline aquifer alternatives, but that legacy well integrity, not pore volume or seal thickness, is the binding constraint
on the weakest reservoirs, a finding with direct relevance to basin-scale CCS hub sequencing decisions.
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