PET-Derived\nCarbon Quantum Dots as Nano Tracers for\nSandstone Reservoirs

Tracers\nplay an important role in understanding fluid\nflow through\nsubsurface porous media, such as reservoir connectivity and heterogeneity,\nin various applications including groundwater monitoring, CO<sub>2</sub> sequestration, and oil recovery. However, commonly used tracers,\nlike radioactive tracers, fluorescent dyes, and fluorinated compounds,\nare not environmentally benign. Previous studies have investigated\nthe use of nanoparticles as potential subsurface tracers. However,\nthese nanoparticles are unable to survive harsh conditions of high\ntemperature and salinity often encountered in the subsurface and show\nhigh retention in porous media. Furthermore, studies on the transport\nof nanoparticles through porous media, particularly in the presence\nof another immiscible phase, are scarce in the literature. In this\nstudy, systematic experimental work was carried out to investigate\nthe potential of carbon quantum dots (CQDs), synthesized from waste\npoly(ethylene terephthalate) (PET) bottles, as subsurface tracers\nfor sandstone reservoirs. The synthesis process involved calcination,\nacid hydrolysis, and nitrogen doping. The calcination step breaks\ndown the poly(ethylene terephthalate) (PET) polymer into smaller hydrocarbons.\nThe acid treatment and nitrogen doping (via a hydrothermal process)\nlead to further carbonization, nucleation, growth, and surface passivation\nof CQDs, introducing various oxygen- and nitrogen-containing surface\nfunctional groups, thus impacting the hydrophilicity and aqueous stability\nof CQDs. The synthesized CQDs were characterized by using various\nanalytical techniques such as diffraction light scattering, Fourier\ntransform infrared spectroscopy, X-ray photoelectron spectroscopy,\ntransmission electron microscopy, ultraviolet–visible spectroscopy,\nand photoluminescence spectroscopy. The FT-IR and XPS results confirmed\nthe presence of various functional groups such as hydroxyl (-OH),\ncarboxyl (-COOH), ester (-COOR), and amines (-NH<sub>2</sub>). These\nfunctional groups account for the hydrophilic nature of the CQDs.\nThe TEM images showed that the average diameter of the CQDs was 4.24\nnm. The fluorescence quantum yield of the CQDs was 16%. The zeta potential\nof CQDs was found to be −52.17 mV. The stability of aqueous\nsuspensions of CQDs was studied in seawater and brines containing\n10 wt % NaCl up to 60 °C. The effect of divalent calcium ions\non the stability of CQDs suspension was also studied. Following this,\nthe transport of CQDs through porous media was studied. These experiments\nwere also performed in the presence of another immiscible phase (<i>n</i>-decane). The CQDs were stable in brines containing up\nto 10 wt % NaCl and at a temperature of up to 60 °C. Furthermore,\nthe CQDs were also stable in seawater and the brines containing 10\nwt % NaCl and 2 wt % calcium chloride at 60 °C. No noticeable\npartitioning of CQDs in <i>n</i>-decane and negligible adsorption\non sand was observed. Transport experiments showed that these CQDs\nbehaved similarly to a conservative tracer, showing negligible retention\nin porous media even in the presence of <i>n</i>-decane.