Influence of chemical composition on the fluorescence lifetimes of crude petroleum oils

Abstract

Fluorescence based methods are increasingly being used for the analysis of crude oils because they offer high speed, low cost, non-contact, and non-destructive testing options. The fluorescence of crude oils is due to the presence of a wide range of cyclic aromatic compounds, with the intensity and temporal behaviour of this fluorescence being directly related to chemical composition. In particular, we have found that the Fluorescence Lifetime (FL) measured at a range of different emission wavelengths is correlated with the concentrations of the aromatic and polar fractions of petroleum oils, and the density (API gravity).We have analysed 22 different crude oils from around the world with API gravities of between 10 to 50. The fluorescence steady-state emission spectra and fluorescence lifetimes at a range of emission wavelengths were measured for 380 nm excitation. It was found that the correlations between the chemical and physical characteristics of the crude oils and the measured fluorescence parameters are highly non-linear. Furthermore, there is a wide degree of scatter in the observed data for medium oils, which have similar physical properties but widely varying chemical compositions. We discuss these findings in the context of developing quantitative methods of analysis for crude petroleum oils based on fluorescence lifetime measurements.

Frequency Domain Fluorescence Lifetime Study of Crude Petroleum Oils

“…The fact that there are very large numbers of interacting fluorophores produces very complex decay behaviour. In previous work from this laboratory [9][10][11][12][13][14] the intensity averaged lifetime calculated from a multiple discrete lifetime model was used. However, it should be noted that while this model is not an accurate descriptor of the emitting species, it had utility in describing general trends in the photophysics of crude oils.…”

“…Previous studies in our laboratory using Time Correlated Single Photon Counting (TCSPC) methods have shown that one can correlate various aspects of oil composition with lifetime changes [9][10][11][12][13][14]. Unfortunately, these studies also showed that accurate quantitative measurements were not possible using intensity averaged lifetimes calculated from TCSPC data.…”

Time-Resolved Fluorescence Microspectroscopy for Characterizing Crude Oils in Bulk and Hydrocarbon-Bearing Fluid Inclusions

“…The physical details and gross chemical composition, and the geographical and source rock information of the oils analysed in this study have been published in detail elsewhere. 35,36 Table 1 outlines the most salient information and Fig. 1 shows a ternary plot of the gross chemical composition of the crude oils used.…”

“…27 Fluorescence decay data was collected over the 460 -700 nm wavelength range with a 20 nm step size, and the intensity-averaged fluorescence lifetimes (τ ) were calculated using the same criteria and methods as detailed in previous studies. 21,30,35,36 Results and Discussion:…”

“…In our laboratory, the use of fluorescence lifetime measurements for the characterization of crude petroleum oils is being investigated, with the objective of developing quantitative analytical methods for the prediction of both physical (API gravity) and compositional data such as aromatics, polars, alkane, and sulphur concentration. 21,30,35,36 In this work, we have conducted a detailed time-resolved fluorescence study on a set of 23 well-characterised, crude petroleum oils, from different geological sources, and on 6 hydrocarbon bearing fluid inclusions. The correlation between the compositional/physical data and the intensity-averaged fluorescence lifetime (τ ) of the bulk crude oils was examined with the goal of developing a quantitative analytical method for characterising the composition of the oils trapped within HCFI.…”