Characteristics Evaluation of Fluorescein Sodium as Fluorescent Tracer for Petroleum Wells

Abstract

Fluorescein sodium (FluNa) is an organic xanthene-derivative fluorochrome appearing as crimson powder. In the form of solution, fluorescein absorbs blue light and emits green-to-yellow fluorescence under the incidence of visible or ultra-violet light. In petroleum industry, it is normally used as fluorescent tracer to determine reservoir connectivity between injection and production wells. In this study, fluorescein concentration at 10 ppm was critical concentration that reversed relationship between fluorescence intensity and fluorescence concentration. Emission wavelength at maximum fluorescence intensity became a function of fluorescein concentration until 512 nm at 1 ppm and there was no longer reduction of emission wavelength beyond this value at lower concentration. Fluorescence of fluorescein could not be observed by naked eyes at concentrations below 100 ppb, whereas its detection limit by fluorescence spectrophotometer could be lowered to 1 ppb. Fluorescence spectra of fluorescein concentrations at 10 ppt and 100 ppt were similar to those of tap water, formation water, and deionized water. Therefore, detection limit of fluorescein was determined to be at 1 ppb. Fluorescence of fluorescein was photodegraded by light up to 30% when stored inside transparent glass bottles and being exposed to light for 7 days and fluorescence of the solution became darker with time. On the other hand, fluorescence intensities of FluNa solution stored inside amber-coloured glass bottles in both all-time concealment and normal room conditions were almost identical. Fluorescence spectra of the solution at pH of 3 had lower fluorescence intensity and shorter emission wavelength at maximum fluorescence intensity than the solutions at pH of 7 and 11 that were mostly identical. Fluorescence of fluorescein was not affected by temperature in the range of 30-70°C for 12 days. Hence, the storage conditions for fluorescein solution were recommended to be stored inside amber-coloured glass bottle to prevent photodegradation with pH 7 or above.