Radioactive gas tracers

Tracers are used in well to well tests to gather data about the movement and saturation of fluids and hydrocarbons in the subsurface. Radioactive tracers can be used to gather data about water or gas. This article discusses some of the commonly used radioactive gas tracers.

Application

Several authors report the use of radioactive gas tracers in oilfield applications. The tracers most frequently applied have been tritiated methane, tritiated ethane, and ⁸⁵Kr. In addition, the use of tritiated butane, ¹²⁷Xe, ¹³³Xe, and tritiated hydrogen gas (HT) has been reported. Table 1 lists the most widely applied radioactive tracers.

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  Table 1

The tritium-labeled compounds may be measured directly in the gas phase by proportional counter techniques. To obtain the low detection limit normally required in well-to-well tracer studies, it is normal to convert the gas to water by combustion. The produced water then is counted by a liquid scintillation technique. The detection limit of the water produced from this process depends on the instrumentation but can be less than 1 Bq/L. Typically, approximately 5 to 10 mL of water must be produced to obtain the low detection limits. The water, mixed with a liquid scintillator, is counted for a few hours. The same technique can be applied for the different tritium-labeled hydrocarbon gases. If several tritium-labeled compounds are present in the same sample, it will be necessary to perform a chemical separation of the compounds before oxidation to produce the water. This separation complicates the analysis considerably; therefore, it is not common to use several of these tritiated compounds in the same reservoir segment.

The chemical properties of tritium-labeled hydrocarbon compounds are equal to their nonlabeled homologues in almost all practical situations, which means that their behavior in the reservoir will be the same as the nonlabeled hydrocarbons. The partition coefficients of these compounds, therefore, can be found by ordinary pressure-volume-temperature (PVT) packages like those applied in phase-equilibria calculations. The partition coefficient will influence the residence time in the reservoir and the concentration of the tracer in the production line. It is important to take into account partitioning both in the evaluation of flow behavior in the reservoir and in the understanding of sampling quality. Because of partitioning to the oil phase, some of the tracers will be in the oil phase at the sampling point. Calculation of total recovery of the tracer then requires partition coefficient and gas/oil ratio (GOR) at sampling conditions. This problem will be more significant for heavier hydrocarbons than for the lighter ones.

All the hydrocarbons that are labeled with tritium can also be labeled with ¹⁴C. These molecules are, in general, more expensive and their analyses are more complicated. From a general demand for reducing the application of long-lived radioactive tracers, the ¹⁴C-labeled compounds are less attractive than the tritium-labeled compound.

Other alternatives are the noble gases. The noble gases are virtually inert against chemical reaction. ⁸⁵Kr has a half-life of 10.76 years and is a beta emitter with an energy of 687 keV. The two xenon isotopes, ¹²⁷Xe and ¹³³Xe, are also inert noble gases that may be applied in special situations in which rapid response is expected. The half-lives are 36.4 and 5.25 days, respectively.

For CO₂ as injection gas, ¹⁴CO₂ will be an ideal tracer. CO₂ will interact with the formation water, which must be considered when ¹⁴CO₂ is used as a tracer. A few papers have reported application of HT.9 Hydrogen gas is reactive, and the tracer is lost in the reservoir.

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See also

Chemical gas tracers

Radioactive water tracers

Well to well tracer tests

PEH:Well-To-Well_Tracer_Tests