Isotopic exchange of tritium (³H) and ¹⁴C in gas/liquid systems

The application of tracers to follow the flow of injected fluids (water or gas) in petroleum and geothermal reservoirs have shown to be highly successful. Direction of flow, flow velocities, sweep efficiency and, for geothermal reservoirs, the effectiveness of heat extraction are monitored. Some of the applicable tracers are also radiolabeled, for instance with tritium (³H or T) and/or ¹⁴C. So-called conservative tracers (not degraded by environmental factors) are required. The most trustworthy conservative tracer compounds are those consisting of a molecule of the traced phase where one of the atoms has been exchanged with a radioactive isotope of the same element. Examples are HTO for water and CH₃T or CH₃CH₂T for lean hydrocarbon gas. Also, the radionuclide ¹⁴C can be used by exchange with natural non-radioactive carbon isotopes. Such a tracer compounds are called intrinsic radiotracers. However, during contact of such compounds with reservoir fluids one mechanism which may contribute to loss of tracer is the exchange of the radioactive label (T or ¹⁴C) with non-radioactive hydrogen and carbon isotopes in the reservoir fluids. This process is called isotope exchange and may be composed of several mechanisms. The aim of the present study has been limited to examine the potential for isotopic exchange, as a sink mechanism, for the gases tritiated and ¹⁴C-labelled methane and ethane to water and oil in petroleum reservoirs. The highest exchange rate found is 3,2 % of tritium from radiolabeled ethane to water at 150 °C after 6 months. All factors evaluated, this level of exchange is judged acceptable for the labelled compounds to be considered as conservative tracers for gas injection in oil reservoirs.