Texaco's Exploration and Production Technology (EPTD) Division has been developing a crude oil identification method to improve surface logging techniques (formerly known as mud logging). Mudlogging is a commercial service designed to locate with respect to depth the occurrences of hydrocarbons in a well while drilling.

One of the areas of development is a crude oil fingerprinting technique using Total Scanning Fluorescence (or TSF). The technique is a very sensitive method of detecting a wide range of poly-aromatic hydrocarbon compounds (PAH), as well as the mono-ring aromatics. It has proven to be a very powerful tool for crude oil identification and for differentiating crude oil fluorescence originating from mud additives and natural fluorescing substances.

In this technique, oil is extracted using organic solvents from formation samples such as drill cuttings, core chips, or fluid samples such as drilling mud filtrates, and formation waters. The spectral pattern, or "fingerprint", is obtained by exciting a sample using multiple wavelengths over the entire UV range and measuring the fluorescence intensities at discrete emission wavelengths. This results in a 3-D excitation-emission matrix largely dependent on the aromatic components present in the oil. This gives a fluorescence spectra that can be used to identify crude oils and other petroleum-based hydrocarbons as long as aromatic species are present.

The major advantage of a multiple wavelength scan over a single wavelength scan is the increased amount of fluorescence data per sample and its sensitivity for a wide range of PAH compounds. Also, most of the naturally fluorescing PAH found in crude oils are not as volatile or as reactive as the straight or branched-chain hydrocarbons. Hence, the aromatic ring compounds have been found to be relatively more stable to weathering than alkanes and the fluorescence fingerprint is a reliable representation of the aromatic composition of the whole oil.

When a crude oil being identified is contaminated with other fluorescing species such as mud additives, (i.e. base oils, diesel) spectral overlaps between the two components can either alter the fingerprint or completely mask the spectra of the crude oil. The two components can be distinguished by the process of spectral subtraction and the crude oil can still be identified. An example of the process is shown in Figure 1. The poster will present different case studies that will demonstrate the value of fluorescence data in crude oil identification particularly in surface-logging applications.

Figure 19

The spectra of diesel (center left is subtracted from a spectra of crude oil contaminated with diesel (top). The spectral difference of the two spectra (bottom left) has identical peaks to the spectra obtained of the crude oil when scanned separately (bottom right).

Figure 19

The spectra of diesel (center left is subtracted from a spectra of crude oil contaminated with diesel (top). The spectral difference of the two spectra (bottom left) has identical peaks to the spectra obtained of the crude oil when scanned separately (bottom right).

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