Carbon quantum dots used as tracers in ecological, hydrogeological monitoring and reservoir management

Abstract

Tracer investigations is a direct and one of the most reliable methods to determine presence/source of subsurface connectivity, which have found use in geological exploration for study of reservoir fluids’ behavior and diagnostics of horizontal wells without resorting to well logging, as well as in environmental studies for evaluation of leak integrity of waste pits and search for pollution sources. In oil industry, the efficacy of tracer investigations is determined by the tracer characteristics. Tritium used to be applied on a wide scale in previous years, has all necessary characteristics to provide for accuracy and reliability of results, however, it does not meet the requirements regarding nuclear safety, so its application is restricted. Tracer systems in current use, which are organic dyes (fluorescent, ionic, alcoholic), also have a number of drawbacks, namely, a limited product range, a complicated procedure for quantitative identification of tracers. Besides, bright colors of fluorescent tracers considerably restrict hydrogeological surveys and might affect applicability of fresh water resources. To improve efficiency of upstream operations, it is important to find optimal tracers for reservoir studies to have real-time data about reservoir fluids’ behavior, inflow of reservoir fluids to wells, including horizontal wells. The synthesized carbon quantum dots specimens allowed to expand the range of tracers. The laboratory tests determined the efficiency of carbon quantum dots in comparison with the known fluorescein tracers. The paper presents a number of key indicators determined in the course of experiments such as minimal measurement limit (luminescent emission intensity), Stokes shift (difference between waves of excitation and emission), and results of core analyses. The synthesized carbon quantum dots made possible to expand the applicability of tracers and to improve performance as compared with conventional tracers. Results of laboratory experiments demonstrate good potential of the synthesized carbon quantum dots as tracers for reservoir management and ecological monitoring.