> Blue crude. The blue coloration of this unusual variety of Gulf of Mexico crude oil is caused by strong fluorescence under ambient light from a high concentration of perylene, a polychromatic hydrocarbon. Typically, oils are brown, and their color, as measured by optical spectroscopy, is their degree of “brownness.”
As the molecular complexity of hydrocarbons
increases beyond ethane, the frequency signature is more complex. Thus, the group comprising propane, butane and pentane—the C3-5 group—is combined for analysis. Liquid hydrocarbons include the hexane and heavier hydrocarbons— the C6+ group. Optical absorption of water covers a broad
spectrum in the NIR range and overlaps many of the hydrocarbon peaks. The presence of water can mask other fluids, especially CO2, from the detector. Hydrocarbon fluorescence results from the
Oilfield Review Autumn 09 FluidsLab Fig. 8
aromatic fraction of crude oils, and its color and intensity are characteristics of the oil type (above right). Ultraviolet (UV) light and fluores- cence have been used by the oil industry for many years. At one time a black light, or UV light, was common on wireline logging units, primarily for core analysis and detection of trace amounts of hydrocarbon in formation fluid samples when mostly filtrate was recovered. Mud loggers still use black lights to detect fluorescence in cuttings.
ORWIN09/10-FluidsLab Fig. 8
> Hydrocarbon fluorescence. Chromophores are molecules that absorb light; fluorophores, a subset of chromophores, absorb light and then fluoresce. For crude oil, virtually all chromophores and fluorophores have some aromatic carbon. Graphite is an aromatic carbon in large ring systems and is correspondingly black. In the visible light spectrum, light-absorbing heavy oils appear dark, and lighter oils have less color because they absorb less light (top). Under UV radiation (bottom), the heavy oils produce a dull, reddish brown fluorescence. Light oils appear blue and produce fluorescence with greater intensity. Being clear, the lightest oil absorbs little visible light and some UV radiation, and thus fluoresces, but at a low level.
The InSitu Fluorescence sensor allows the measurement of fluorescence to be made down- hole. Although it retains some of the early appli- cations, this sensor offers new utilities, including fluid-phase detection and oil typing. One applica- tion of the fluorescence measurement is the detection of retrograde condensation, also known as retrograde dew, a condition that can occur upon pressure reduction with each stroke of the pumpout tool.19 A recent innovation using fluorescence is fluid typing in emulsions.20
Emulsions often form 18. Mullins, reference 4: 74.
19. Retrograde condensation is the formation of liquid hydrocarbons in a gas when the pressure drops below the dewpoint pressure. It is called retrograde because some of the gas condenses into a liquid under isothermal conditions instead of expanding or vaporizing
Oilfield Review Autumn 09 FluidsLab Fig. 9
ORWIN09/10-FluidsLab Fig. 9
in sample acquisition of heavy oils because the asphaltenes in the oil act as a surfactant for both formation water and water-base mud (WBM) filtrate. When these emulsions form, significant light scattering occurs, making optical density measurements difficult to interpret. In the labo- ratory, centrifuges and chemicals are used to demulsify the liquids and analyze the oil portion. This approach is not always successful nor is it an option downhole. The fluorescence measurement, however, unlike the optical density measurement, is rela-
when pressure is decreased, as would be the case for a single-phase fluid.
20. Andrews AB, Schneider MH, Cañas J, Freitas E, Song YQ and Mullins OC: “Methods for Downhole Fluid Analysis of Heavy Oil Emulsions,” Journal of Dispersion Science and Technology 29, no. 2 (February 2008): 171–183.
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