Patrick L. Delaune
Texaco Exploration and Production Technology Department
INTRODUCTION
Fluorescence has been used as a means of detecting oil while drilling since first introduced nearly sixty years ago. The technique, first disclosed by Bentz and Strobel in 1933, involved exposing the sample to a broad spectrum ultraviolet (UV) light and looking for fluorescence in the sample. The color and intensity of the fluorescence was observed to determine the presence of oil and to attempt to correlate oil sands. This technique for oil detection became an integral part of commercial mud logging services introduced in 1939. Still in use today, this visual technique has long been recognized to be highly subjective and inconsistent, making it unreliable as a means for detecting oil. The major limitation is the fact that many oils fluoresce outside the visible region and therefore go undetected using visual techniques.
Surface measurements of gas and oil from mud and drill cuttings are often the only direct physical evidence of the formation available to the interpreter. In the mid 1980s, Texaco began research and development to improve the accuracy and reliability of these surface measurements. One of the first results of this effort was a method to accurately determine the amount of oil in drill cuttings and cores using fluorescence. Called the Quantitative Fluorescence Technique (QFT), this method uses a portable fluorometer to measure the fluorescence of crude oil extracted from the sample. The intensity of the fluorescence, accurately measured by the fluorometer, is proportional to the amount of oil in the sample. The results are plotted as a function of depth to yield an oil concentration profile of the well.