CO₂ Capture and Mineralization in Seawater Using HMDA for Simultaneous Carbon Management and Oil Recovery

This study evaluates hexamethylenediamine (HMDA)-assisted CO₂ mineralization in engineered seawater (SW) formulations and quantifies its implications for enhanced oil recovery (EOR) in carbonate reservoirs. Carbonated smart SW (SSW) systems containing 0.3, 0.6, and 1.0 wt % HMDA were prepared and assessed for mineral precipitation yield, precipitate morphology, brine-rock and brine-oil electrokinetics, interfacial tension (IFT), wettability alteration, and spontaneous imbibition performance. The 0.6 wt % formulation (SSW-06H) produced the highest solids yield (16.29 g/L) and generated predominantly halite-rich recovered solids with well-defined crystalline features (SEM/XRD), indicating substantial NaCl crystallization associated with the NaCl-rich SW matrix and NaOH-induced Na⁺ enrichment, thereby shifting the treated brine toward a lower effective salinity regime. Electrokinetic measurements demonstrated systematic zeta-potential shifts at both limestone-brine and oil–brine interfaces, consistent with strengthened electrostatic repulsion and wettability states favorable for storage-compatible displacement. All HMDA-conditioned systems reduced IFT, while the strongest wettability alteration was observed for SSW-03H. Integrating interfacial reconditioning with salinity control, SSW-06H achieved the highest spontaneous imbibition recovery (48.9%) compared with unmodified SW (6.7%). Mechanistically, CO₂ initially dissolves in the HMDA-SW system to form bicarbonate and amine-associated carbon species under carbonated conditions. Subsequent NaOH addition increases alkalinity, shifts carbonate speciation, and may promote carbamate/carbonate-rich ionic environments that modify brine chemistry, reduce effective salinity, and improve EOR-relevant interfacial behavior. The results show that HMDA-enabled CO₂-conditioned brines can convert captured CO₂ into a functional lever for brine respeciation and targeted salt removal, induce wettability shifts conducive to improved displacement, and translate CO₂ utilization into measurable recovery gains. Optimizing HMDA dosage in carbonated brines provides a practical pathway to couple CO₂ management with EOR in carbonate reservoirs while supporting reduced life-cycle carbon intensity and subsurface storage objectives.