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 35Cl(n,p)35S reaction. The 35S 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. Cogenerated 32P is eliminated with hydrochloric acid and the sulfur is reacted with potassium cyanide under heating and reflux with ethanol, K35SCN 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 35S 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 35SCN- 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.

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