Chemical Pre-Flush and Additive Strategies to Enhance CO$_2$ Solubility and Injectivity in Deep Saline Aquifers: Mechanisms, Evidence, and Field Readiness
Deep saline aquifers offer the largest global capacity potential for geologic carbon storage, yet storage performance is frequently constrained by coupled limitations on \COtwo{} dissolution (solubility trapping) and injectivity under high-salinity conditions. Elevated ionic strength suppresses \COtwo{} solubility via ``salting-out'' effects. Meanwhile, near-wellbore dry-out during injection of dry or undersaturated \COtwo{} can concentrate brine and precipitate salts, reducing permeability and injectivity. Mineralogy and \COtwo{}-brine-rock wettability further control multiphase flow topology, interfacial area for dissolution, capillary trapping, and compatibility of additives with reservoir surfaces. This review critically evaluates chemical pre-flush and additive-enabled conditioning strategies intended to (i) increase local aqueous \COtwo{} dissolution capacity, (ii) mitigate or prevent dry-out-driven salt precipitation and near-wellbore formation damage, and (iii) enhance \COtwo{}-brine contact through mobility control. We synthesize and compare evidence for four strategy families: (1) freshwater and engineered low-salinity (LoSal) pre-flush slugs and intermittent re-wetting; (2) chelating agents (e.g., EDTA/GLDA/DTPA) that complex multivalent cations and alter dissolution/precipitation pathways; (3) surfactant-derived \COtwo{} foams, including \COtwo{}-soluble surfactants designed for foam generation at the displacement front with achievable injectivity; and (4) nanoparticle-assisted and hybrid foam systems that extend foam stability under harsh salinity and temperature.