Effect of Mineralogy on the Tracer-like Ability of PET-Derived Carbon Quantum Dots

Most previous studies examining carbon quantum dots (CQDs) as potential subsurface tracers have been confined to sandstone reservoirs, often in the absence of another immiscible phase. Furthermore, such studies for limestone rocks are scarce, where the behavior of CQDs deviates from that of conservative tracers. Additionally, the effect of mineralogy on the transport of CQDs has not been thoroughly explored. In this study, we address these critical gaps by investigating the transport of poly(ethylene terephthalate) (PET)-derived CQDs through sandstone and limestone rocks, both in the presence and absence of oil. We also examined the effect of different mineral compositions, including kaolinite and calcite, on the conservative tracer-like ability of these CQDs. The core flooding experiments were conducted on sandstone and limestone rocks at 60 °C with injection brine containing up to 10 wt % NaCl to simulate practical reservoir conditions. No noticeable adsorption of CQDs was observed in the sandstone rocks. However, some adsorption was observed in the limestone rocks. The effect of injection brine salinity and pH on CQD adsorption in limestone cores was examined. Transport studies confirmed that CQDs behave as conservative tracers in sandstones, both in single-phase and in the presence of crude oil at residual oil saturation with no noticeable retention. Increasing the salinity of the injection solution reduced CQD retention in limestone cores, presenting a simple alternative to adding sodium carbonate, which often causes geochemical reactions and scaling issues.