Development and Application of a Radioactive Sulfur Tracer for Use in Waterflood Studies
Authors: Moreira Rubens Martins , Pinto Amenonia Mario Ferreira 
Published: December 2007

Due to the multiplicity of injection and production wells in waterflooded oilfields, tracer studies may require the use of several tracers for the water phase. Tritiated water is usually the radiotracer choice, but other tracers must be available. Is not a novel radiotracer but is has been synthesized by tedious sequences of chemical oxidation and reduction steps, involving the manipulation of rather high activities. An alternative path is proposed aiming at easing this burden.

Basically, potassium chloride is irradiated in an evacuated ampoule by neutrons inside a nuclear reactor, radiosulfur being generated by the [35]Cl(n, p)[35]S reaction. The [35]S are liberated from their sites in the KCl crystal lattice by heating at 500°C, carrier is added and sulfur is extracted with trichloroethylene; contact with air or water is avoided. Co-generated [32]P is eliminated with hydrochloric acid and the sulfur is reacted with potassium cyanide under heating and reflux with ethanol, K[35]SCN being obtained.

The tracer was submitted to a bench test pushing a bank of its solution through a Berea sample and its performance compared satisfactorily with the tritiated water benchmark.

A field test was performed making a simultaneous injection of both radiosulfur and tritium and the results compared. Unexpectedly the [35]S response showed itself ahead of the tritium. The reasons for that are still being scrutinized but it might be that a phenomenon of ionic repulsion occurred in which the [35]SCN- were repelled by the negatively charged inner surfaces of the smaller micropores within the reservoir. Further tests to clarify this point, using more refined analytical procedures are being planned.

Graphs showing the measured responses of the lab and field tests are included.

It is hoped that this new labeling method will contribute to waterflood tests by broadening the multitracer range available. Additionally it props the chemical synthesis and the radioanalytical supports of the tracer techniques applied in reservoir characterization.

Introduction

Any technique aiming at the improved knowledge of the injection fluid behavior in oil reservoirs will certainly have a large economical significance. Thus the injection of tracers in reservoirs has become widespreadly used in order to quantitatively evaluate the performance of enhanced recovery processes as well as to supply information about many aspects regarding the behavior of the injected fluid and the reservoir characteristics. Such information could hardly be obtained by the sole use of other methods currently used in oil production. Whenever the reservoir is flooded with water during secondary recovery, the best tracer is tritiated water, which consists of a water molecule in which at least one of the hydrogen atoms has been substituted by its isotope tritium. This radiotracer emits a very soft beta radiation, which nonetheless can be accurately detected and quantified after suffering a huge dilution within the reservoir. Other radioactive tracers are even more easily detected, but tritiated water has the obvious advantage of behaving exactly as normal water does, i.e. it traces with absolute fidelity the water path and transit times. Presently tritiated water is the first choice when a tracer is to be performed for waterflood evaluation. However, it often happens that such evaluations require the definition of the contribution at different injection wells to some producer well of interest. In such cases distinct tracers need to be simultaneously injected in the contributing wells and be discriminated in the produced outflow. Radiotracers are again advantageous in this connection since they can be easily differentiated and quantified through a single counting of the samples of the produced water, usually with little or no previous treatment. This application has prompted the efforts of many investigators after the development of other suitable substances tagged with radioactive isotopes. However large the number of natural or artificial radioisotopes known, only a quite restricted number of chemical compounds labeled with them can comply with the strict requirements demanded by reservoir applications.