Fluorescence Technique Texaco's Exploration and P

Fluorescence Technique

Texaco's Exploration and Production Technology Department has developed two fluorescence techniques to enhance the detection of crude oil extracted from formation samples and to improve mudlogging as a formation evaluation tool. The first of these is a portable field method called the Quantitative Fluorescence Technique (OFT TM), which uses a single excitation wavelength and a short range of emission wavelengths. The intensity measurements are used to generate an oil concentration profile of the well. This method has proven successful in locating oil-bearing zones that were difficult to identify using conventional mudlogging techniques. The second technique uses a more sophisticated ultraviolet (UV) spectroscopic instrument and is called the Total Scanning Fluorescence Technique (TSF TM). This method generates a three-dimensional fluorescence spectra or fingerprint. The spectra results from the total fluorescence contribution from each of the fluorescing species in the oil. The unique chemical makeup of different crude oils gives spectral characteristics that can be used to predict oil composition or type. This method gives more information than conventional fluorescence techniques. This paper discusses the effectiveness of these fluorescence methods in identifying crude oil, even in the presence of similar fluorescing substances commonly added to drilling muds, such as base oils or diesel. It also includes a feasibility study that demonstrates the application of these methods in a North Sea well drilled with an oil-based mud. Other examples are cited to show how these methods worked in offshore wells in the Mediterranean, and an offshore well in the Gulf of Mexico.

Introduction

Fluorescence has been used as a means of detecting the presence of hydrocarbons in drill cuttings for many years. Mudloggers have used the color and intensity of fluorescence from rock cuttings as a basis for oil detection during the drilling process. Texaco's E and P Technology Department has developed the "Quantitative Fluorescence Technique", which has proven to be a more reliable method of detecting hydrocarbons in wells drilled with water-based muds. A feasibility study was pursued to examine whether or not the method could be applied to wells drilled with oil-based muds. The primary concern was the interference that the base oils could have on the fluorescence measurement of formation crude oil. Since oil based muds are formulated with oils containing small aromatic ring compounds, and these compounds are also found in crude oils, the resulting fluorescence will be a contribution from both components. Thus, the oil-based drilling fluid could completely mask the fluorescence due to the formation crude oil, especially for lighter crudes. It is the purpose of this paper to show that it is possible to detect native hydrocarbons extracted from drill cuttings whose rock matrix has been infiltrated.