Carbonated Alkaline Fluid (CAF): A CO₂ Mineralization Approach for Sustainable Produced-Water Reinjection

Escalating atmospheric carbon dioxide (CO₂) intensifies the need for scalable mitigation. This work introduces a surface-based workflow that couples CO₂ mineralization with produced water (PW) treatment, converting PW into carbonated alkaline fluid (CAF)─a chemically modified brine engineered to tune fluid–fluid and rock–fluid interactions and upgrade reinjection water quality. PW is carbonated with CO₂ and then alkalinized with sodium hydroxide (NaOH), thereby raising the pH (∼10.2) and reducing divalent cations (Mg²⁺/Ca²⁺) via brucite (Mg(OH)₂) and calcite (CaCO₃) precipitation, thus tailoring the residual brine chemistry and immobilizing CO₂ primarily as CaCO₃. The workflow was evaluated using inductively coupled plasma–mass spectrometry (ICP–MS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), zeta potential, interfacial tension (IFT), contact angle, and spontaneous imbibition. ICP–MS confirmed removal of divalent ions from the produced water matrix─Ca²⁺ decreased by 26% and Mg²⁺ by 60%─in agreement with XRD/SEM-EDS identification of calcite and brucite (∼26 wt% brucite of the XRD-detectable crystalline fraction) formed during the NaOH step. By depleting Ca²⁺/Mg²⁺ and elevating pH, the electrical double layer expands and ion bridging weakens, shifting the brine–rock zeta potential from +5.3 to −10.4 mV and flipping the wettability from 157° (oil-wet) to 48° (water-wet). The pronounced drop in IFT (22.6 → 13.8 mN/m) is consistent with higher pH and in situ generation of carboxylate surfactants from weakly acidic oil species. In Indiana limestone, oil recovery increased from 5.5% with PW to 25.8% with CAF─an absolute gain of +20.3 percentage points, reflecting reduced capillary retention and enhanced spontaneous imbibition. Because reactions occur in compact, modular surface units, the scheme avoids subsurface storage risk, simplifies permitting, and coproduces potentially saleable calcite and brucite alongside a reusable injectate, offering a practical path to lower-carbon, higher-efficiency water reinjection in carbonate formations.