Mechanism-Resolved CO₂ Storage Capacity in Depleted Oil and Gas Reservoirs: A Pressure-Constrained No-Double-Counting Framework with a Niger Delta Benchmark.

Depleted hydrocarbon reservoirs are good targets for geological CO₂ storage, but estimates of the amount of capacity available can be over optimistic if structural, residual and dissolved trapping are assumed to be additive. In this study, a mechanism-resolved framework is created in which these inventories are mutually exclusive at each reporting time, and are constrained by a different pressure-limited mass to be injected. Specific expressions are derived for reservoir types that account for the connate water, residual hydrocarbons, structural accessibility, aqueous solubility and Land-type capillary trapping. A reduced-order pressure model is coupled with well deliverability and a bottom-hole-pressure ceiling and the post-injection transfer equations conserve the total retained mass by construction. A nominal injection target of 20 Mt over 20 years, for a representative Niger Delta (depleted sandstone) sample, gives the result of 19.521 Mt. since the pressure limit of 28 MPa is set to operate near the end of the injection cycle. At cessation, the structural/mobile amount is 11.62 Mt, the residual amount is 5.47 Mt, the dissolved amount is 2.34 Mt and the mineralized amount is 0.10 Mt. A 3,000-realization uncertainty analysis gives P10, P50, and P90 stored masses of 10.61, 19.11, and 20.00 Mt. The framework offers an audit-trail basis of screening that ensures trapping physics and pressure operability are distinct and double counting is avoided.