PET-Derived Carbon Quantum Dots as Nano Tracers for Sandstone Reservoirs

Tracers play an important role in understanding fluid flow through subsurface porous media, such as reservoir connectivity and heterogeneity, in various applications including groundwater monitoring, CO2 sequestration, and oil recovery. However, commonly used tracers, like radioactive tracers, fluorescent dyes, and fluorinated compounds, are not environmentally benign. Previous studies have investigated the use of nanoparticles as potential subsurface tracers. However, these nanoparticles are unable to survive harsh conditions of high temperature and salinity often encountered in the subsurface and show high retention in porous media. Furthermore, studies on the transport of nanoparticles through porous media, particularly in the presence of another immiscible phase, are scarce in the literature. In this study, systematic experimental work was carried out to investigate the potential of carbon quantum dots (CQDs), synthesized from waste poly(ethylene terephthalate) (PET) bottles, as subsurface tracers for sandstone reservoirs. The synthesis process involved calcination, acid hydrolysis, and nitrogen doping. The calcination step breaks down the poly(ethylene terephthalate) (PET) polymer into smaller hydrocarbons. The acid treatment and nitrogen doping (via a hydrothermal process) lead to further carbonization, nucleation, growth, and surface passivation of CQDs, introducing various oxygen- and nitrogen-containing surface functional groups, thus impacting the hydrophilicity and aqueous stability of CQDs. The synthesized CQDs were characterized by using various analytical techniques such as diffraction light scattering, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, ultraviolet–visible spectroscopy, and photoluminescence spectroscopy. The FT-IR and XPS results confirmed the presence of various functional groups such as hydroxyl (-OH), carboxyl (-COOH), ester (-COOR), and amines (-NH2). These functional groups account for the hydrophilic nature of the CQDs. The TEM images showed that the average diameter of the CQDs was 4.24 nm. The fluorescence quantum yield of the CQDs was 16%. The zeta potential of CQDs was found to be −52.17 mV. The stability of aqueous suspensions of CQDs was studied in seawater and brines containing 10 wt % NaCl up to 60 °C. The effect of divalent calcium ions on the stability of CQDs suspension was also studied. Following this, the transport of CQDs through porous media was studied. These experiments were also performed in the presence of another immiscible phase (n-decane). The CQDs were stable in brines containing up to 10 wt % NaCl and at a temperature of up to 60 °C. Furthermore, the CQDs were also stable in seawater and the brines containing 10 wt % NaCl and 2 wt % calcium chloride at 60 °C. No noticeable partitioning of CQDs in n-decane and negligible adsorption on sand was observed. Transport experiments showed that these CQDs behaved similarly to a conservative tracer, showing negligible retention in porous media even in the presence of n-decane.