Hydrocarbon Show Analysis By Spectral Quantification Of Fluorescence Examples From Onshore Otway Basin.
Honours Degree, 2000
University of Adelaide
The technique of quantifying hydrocarbons shows in oilfield cuttings by measuring the light spectrum of the fluorescence generated under ultraviolet (UV) illumination was suggested by Peter Boult of Boral Energy Resources Limited (BERL). An initial trial on cuttings from the Penley-1 well by Lisa Ryan was successful in showing the potential of the technique. Peter Boult then established an Honours project between BERL and the NCPGG, undertaken by John Hobday. The project was not satisfactorily finalised by the student but this summary attempts to encapsulate the work done.
AimsThe project as devised had two main aims:-
- Devise a field technique of oil extraction from cuttings which allows quantification of the fluorescence signature generated when the extract is illuminated by UV light.
Apply the extraction and fluorescence techniques to a selection of holes from the South Australian portion of the Otway Basin in an attempt to identify oil migration along faults and fractures.
The successful trial by Ryan involved extracting oil from cuttings using acetone then recording the response under UV by photographing the dimple plates used for the extraction. The photographs, or more particularly the negatives, were examined under a petrological microscope which had an image analyser attached. The colour of the fluorescing rims on the dimple plates was assessed in terms of their red-blue-green (RGB) reflectivities and intensities. The total intensity was presumed to be proportional to the amount of hydrocarbon extracted from each sample and the colour of the fluorescing extract was indicative of the source and maturity of the hydrocarbon.
The follow up project (this report) looked at standardising the techniques in terms of amount of sample necessary, finding the most efficient extractant, finding the best method of recording the results and investigating the most suitable ways of presenting the results.
Results and ConclusionsA considerable part of the project was spent on resolving the first aim, that of devising a suitable technique. Trichloro ethane (TCE) is no longer used as an oil extractant in field operations because of its carcinogenic nature. A search was made for a suitable replacement that would be both efficient and safe. In discussions with organic chemists, the following possibilities were discarded:-
- Benzene (carcinogenic)
- Toluene (carcinogenic)
- Methanol (poisonous, absorbed through the skin)
- Acetone (too volatile)
Initial experiments were done using mixtures of dichloro methane (DCM) and ethanol as a mixture of these two compounds should dissolve and extract both non-polar and polar organic compounds. While various mixtures of DCM and ethanol did extract from samples known to contain hydrocarbons (the samples from Penley-1), the reactions were very slow and incomplete. Despite reservations about its volatility, acetone appeared to be the most suitable extractant.
Extracts were prepared from various size fractions from cuttings samples to find the optimum sampling strategy. About two grams of a mixture of dust and aggregated cuttings gave the best results.
The extracts on a dimple plate were viewed directly under a petrological microscope with in-built UV illumination but this was not successful as such microscopes have and orange filter included in the light train to prevent damage to the eyes of the observer. This filter removed most of the information inherent in the fluorescent signature.
A series of trials were conducted to optimise the photographic recording of the fluorescence induced under UV. Many of these trials failed either through uneven illumination from a small UV source or through excessive reflected glare of the short (blue) wavelengths from the white surface of the dimple plate. It became necessary to return to the conditions used in the original successful trial where the dimple plates were photographed by Steve Battey at Challenger Core Services. The controlling factors on successful photography were the use of bright, even UV illumination and a Kodak 2E Gelatin filter. No such filter was available in Australia within the time frame of the project so that all photography had to be done at Challenger.
The initial successful trial measured the fluorescence response from negatives of the photos taken in order to view the image in transmitted light. This unfortunately provided some confusion in interpretation of the true colours of the fluorescence. To avoid this confusion, diapositive (slide) film was used to allow measurement of the true colours in transmitted light.
With all the recording parameters in place, the second aim of the project could be tackled. Cuttings were sampled from the wells Jacaranda Ridge-1, Pyrus-1 and Zema-1. Intervals within these holes had the hydrocarbons extracted, photographed under UV illumination and the fluorescing intervals analysed in terms of the spectral response and intensity. These data are presented in the appendices to the thesis.
The next problem to be solved was the presentation of the extensive data set. Each sample has four values collected: the total intensity of the fluorescence and the intensity of transmission of the red, green and blue wavelengths. In an attempt to reduce the number of data sets in any given display, it was decided to quantify the colour by the ration of the red and green transmission values. Blue was omitted because of the known high background levels from reflection off the dimple plates, despite the use of the gelatin filter. The red/green ratio was plotted against depth down the holes sampled but the results were extremely noisy. Other plots, mainly involving various smoothing parameters were tried with little success.
A second type of plot was prepared, a "cigar-style" plot. In this, values of red, blue and green are cross plotted against intensity on the one chart. There is no resolution against depth but different oils should be visible, discriminated by changes in slope of the cigar plots. The noisy and elevated background of the blue data is immediately visible but at least the background value can be estimated and removed from consideration. Significant blue intensities are those which plot in a separate cloud above the background.
It became apparent on the Red, Green and Blue plots against depth that there was a certain amount of cyclicity in the results. High intensities repeated every six metres. This observation, combined with the fact that many samples showed fluorescence even where none was expected, suggests that much of the fluorescence observed is the result of contamination. Pipe dope, the grease applied to the threaded joins between the sections of drill pipe, is known to fluoresce and it appears that this material has found its way into many of the samples analysed.
It may be possible to manipulate the data to look for true anomalies through the background noise inherent in the samples and induced by the technique, but the project stopped before that work was undertaken.
The accompanying report is incomplete in several aspects but it does provide a summary of the establishment of the technique and does contain the raw data collected from three wells. The analysis of those data needs to be done again with the benefit of the exercises already tried.
Summary by Dr N M Lemon.