Quantitative determination of bitumoids in rocks with luminescence spectroscopy

UDK: 543.42:622.276
DOI: 10.24887/0028-2448-2021-10-28-31
Key words: bitumoid, spectroscopy, luminescent analysis, crude oil, asphaltenes, optical absorptivity, concentration
Authors: I.I. Bachurin (Gubkin University, RF, Moscow), I.N. Evdokimov (Gubkin University, RF, Moscow), A.P. Losev (Gubkin University, RF, Moscow; RTE Nedra-Test LLC, RF, Moscow)

Looking for digitization of the visual luminescence-bituminological analysis (LBA), we have carried out experiments on quantitative determination of bitumoids in oil extracts by luminescence spectroscopy. A collection of Persian Gulf oil (D-3) with asphaltene content 7.6% was prepared, luminescence spectra were studied in a concentration range of 0.04–0.64 ml per 1 ml of chloroform (7–11 reference etalons). Chloroform of a spectroscopic grade was used. The luminescence spectra were measured with a Varian Cary Eclipse fluorescence spectrophotometer. A scanning range of the luminescence spectra was within 400–700 nm, an excitation wavelength – 360 nm. It was found out that for the D-3 oil, quantitative analysis by luminescence intensity was valid for concentrations of asphaltenes up to the 10th etalon (optical density of the solutions was less than 0.5), further analysis of more concentrated solutions gave errors due to high values of optical density. A valuable power-law regression of "intensity - oil concentration" was obtained. It was shown that at low optical densities of solutions, luminescence intensity is proportional to concentration of fluorescent substance (linear dependence), while the general relation is a power-law expression. Diagrams of luminescence intensity on concentration for different crude oils, published in the middle of last century with the first description of standard LBA, coincide with the experimental one. It was concluded that quantitative LBA may be implemented using modern digital luminescence spectrophotometers and portable analyzers; the operating range of the method pointing the numbers of reference etalon solutions was calculated. For condensates and light oils, the entire LBA collection may be used for quantitative analysis; for medium oils, instrumental analysis is valid within etalons 1–13; while for bitumens – only within etalons 1–7.


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