Quantum Dots as Tracers in Geothermal and EGS Reservoirs

Abstract

Colloidal nanocrystal quantum dots are small crystallites of semiconductors in the size range of 1 to about 20 nm and composed of a few hundred to several thousands of atoms. As a result of quantum size effects and strongly confined excitons, quantum dots display unique size and shape-related electronic and optical properties. In particular, they can be made to fluoresce over a wide range, including the visible and near infrared (NIR) regions of light – regions where geothermal and EGS reservoir waters possess very little interference. In this paper, we describe the excellent sensing/tracing ability of colloidal nanocrystals, which is rooted in their unique structure. While the inorganic semiconducting nanocrystal core delivers tunable emission colors (ranging from the visible to the NIR range), the surface chemistry of colloidal quantum dots can independently be adjusted by the choice of ligands to optimize their interaction with the sensing environment (e.g., hydrophilic/hydrophobic, chemically functional groups, positively/negatively charged surface, etc.). Thus, quantum dot tracers could be designed to possess all of the qualities of the conventional conservative tracers (i.e. the naphthalene sulfonates), or be converted to reactive tracers depending on the surface treatment. Quantum dot tracers would then be available for several new EGS-tracing applications that are currently unavailable using conventional geothermal tracers including the use of these novel tracers to characterize near-wellbore and interwell fracture-surface area resulting from hydraulic stimulation processes.