Molecular Design Pathways of Phenanthroline-Based Interwell Water Tracers for Oilfield Applications

Interwell water tracer tests are critical for characterizing reservoir connectivity and flow behavior during waterflood and enhanced oil recovery operations. Existing tracers, however, are often limited by analytical complexity and challenges in scaling to multiwell deployments. This work presents a molecular design strategy for phenanthroline-based, lanthanide-sensitized interwell tracers by systematically examining how backbone functionalization governs photophysical performance, chemical robustness, analytical detectability, and reservoir-relevant transport behavior. A focused library of phenanthroline derivatives was synthesized and evaluated to decouple the roles of chelating and solubilizing functionalities. Photophysical characterization shows that while the phenanthroline antenna is broadly preserved across the derivative set, efficient Eu3+ sensitization and extended excited-state lifetimes require incorporation of a 2,9-dicarboxylate coordination motif. Sulfonation alone does not enable effective ligand-to-metal energy transfer. In contrast, combining sulfonation with dicarboxylate chelation in 5-((3-sulfopropyl)amino)-1,10-phenanthroline-2,9-dicarboxylic acid (SA-PDCA) produces strong Eu3+ emission, favorable excited-state stabilization, and improved aqueous compatibility. SA-PDCA is detected optically at ppb-level concentrations and is efficiently recovered from produced-water matrices via solid-phase extraction with substantial retention of signal intensity. Hydrothermal stability testing demonstrates that substituent identity and attachment mode strongly influence chemical robustness under high-temperature, high-salinity conditions. Coreflood experiments conducted under coinjection conditions showed that SA-PDCA exhibited transport behavior comparable to dipicolinic acid, with near-quantitative apparent recovery in carbonate rock. Collectively, these results establish SA-PDCA as a promising lead tracer candidate and define generalizable design principles for advanced reservoir surveillance applications.