2.1.3. !. Tracer classification

Reservoir tracers can be divided into two categories:

(1) Passive or conservative (or also, less precisely, termed ideal) tracers: The requirement is that the tracer shall passively follow the fluid phase or phase fraction into which it is injected without exhibiting any chemical or physical behaviour different from that of the traced component itself. In addition, the tracer must not perturb the behaviour of the traced phase in any way and neither must the fluid phase or its components perturb tracer behaviour. In petroleum reservoirs, passive (or in practice near-passive) tracers are used in studies of water flooding.

(2) Active (also, less precisely, termed non-ideal or reacting) tracers: The tracer behaves in a qualitatively predictable way and is used to measure a property of the system into which it is injected. The degree of active take-up is a quantitative measure of the property being determined.

Examples of active tracers include:

• Phase partition (with the potential to measure oil saturation in the water contact reservoir zone);

• Sorbtion onto rock, either reversibly or irreversibly (with the potential to measure ion exchange capacity of formation rock);

• Hydrolyzation (e. g. for measurement of water saturation, or temperature if the water saturation is known);

• Thermal degradation (to measure reservoir temperature away from wells);

• Microbial degradation (to measure microbial activity).

It is practical to divide the available interwell reservoir tracers into three types based on their different production mode, treatment and analytical methods:

(i) Stable isotope ratios;

(ii) Non-radioactive chemical species;

(iii) Radioactive atoms or molecules.

Zemel, in his book on Tracers in the Oil Field [9], argues that radioactive and non-radioactive chemical tracers are not necessarily different kinds of tracers, but that radioactive tracers are only radioactively tagged chemical tracers. He might as well have included tracers based on stable isotopic ratios in this argument by stating that isotopic ratio tracers are only chemical tracers labelled with a different stable isotope ratio. It is correct that the flooding properties and survivability in reservoirs are determined by the chemical properties of the tracer compound. It is not, however, generally valid that the same materials without a radioactive tag are also useful tracers. This all depends on the degree of their natural occurrence in reservoir fluids and on their detectability by non­radiochemical methods.

Tracers may also be classified as intrinsic or extrinsic:

Intrinsic tracers are molecules containing an isotope (radioactive or stable) of one of the molecules’ natural elements, which makes the labelled molecule particularly detectable by nuclear or mass spectrometric methods in systems where the dynamic characteristics and general behaviour of the non-labelled molecule are followed in a given medium. For example, in the case of water, there are three such labels: oxygen-18 (‘H218O) and deuterium (‘H2H16O) measured as isotopic ratios (18O/16O by S18O and 2H/1H by SD) by mass spectrometric techniques and tritium (1H 3H16O) measured by nuclear techniques (in practice, liquid scintillation counting). In this case, the water molecule is traced from the inside, in the confines of its nucleus. In this case, the water tracer will (in practice) follow all movements and reactions of the water itself.

Extrinsic tracers are made up of atoms or molecules supposedly to sharing the same dynamic characteristics and, in general, the same mass flow behaviour as the investigated medium. Falling into this category are all the substances that allow for tracing outside the molecular or ionic structure. For example, in case of water, 131I-, S14CN — and [60Co(CN)6]3- are examples of extrinsic tracers. They will not follow water in all its movements because of their charge and because they are basically a salt which will not, for instance, evaporate with the water.

Tracers may further be classified as artificial or natural:

Artificial tracers are generally defined as those tracers which are produced artificially and are deliberately introduced (injected) into the system under study. Most of the tracers employed in industrial applications, including geospherical tracing, are artificial tracers. They are further classified as radioactive and non-radioactive artificial tracers.

Natural tracers are those tracers that exist in nature, generated by nature itself. Such tracers are, for instance, the noble gas 222Rn that may be used to follow mass flow in extended open systems, isotopic ratios of hydrogen atoms (SD) to study, for instance, the movement of injected sea water in an oil reservoir, provided its SD value is sufficiently different from that of formation water, etc. Such tracers are mainly used to trace processes in environmental, geospherical, biological and agricultural studies and are not especially relevant for study of industrial processes in general.

The last class that will be mentioned here are activable tracers:

Activable tracers differ from other tracers only by the fact that they contain a chemical element with a special capacity (high activation cross-section) to be analysed in minute quantities by instrumental neutron activation analysis. As such, they are of special interest to the radiotracer specialists. These compounds are either organometallic covalently bound compounds or simply electrostatically bound chelate complexes. Their main advantage is that they do not pose any radiological hazards during operation and they have a practically infinite shelf

life in comparison to radiotracers. On the other hand, there is always a danger of contamination of the collected sample before activation.

In order to hinder sample contamination, all chemicals and mechanical components in contact with the liquid sample must be virtually free of activable element. These samples are activated with thermal or epithermal neutrons and sample measurements are carried out in a laboratory equipped with high resolution gamma spectrometers.

Metallo-organic compounds may also be analysed in trace quantities with other trace analytical techniques, for instance by inductively coupled plasma mass spectrometry. Such compounds may include other metals than those which are optimal for instrumental neutron activation analysis.