Radiotracer and marker studies

4.1.1. Unsealed radioactive materials used as tracers and markers

The oil and gas industry uses unsealed radioactive solids (powder and granular forms), liquids and gases to investigate or trace the movement of other materials, even within closed and sometimes inaccessible pipework and vessels [4, 7]. Most of these radiotracers can be detected and/or measured easily by their emissions. To achieve the objectives of a study, the physical form of the radiotracer is selected or manufactured so as to be consistent with the materials to be studied and its decay characteristics need to be appropriate [30]. Typical properties of a physical radiotracer include:

(a) Capability to follow the material under investigation but not display the same chemical behaviour as, or react with, other material in the system under investigation;

(b) Stability of form such that it will not degrade in the high temperatures, pressures or corrosive media into which it is introduced;

(c) Minimal radiotoxicity, i.e. dose per unit activity intake;

(d) Half-life compatible with the investigation schedule so as to minimize residual contamination in the system or product;

(e) Suitable radiation emissions making it readily detectable;

(f) Initial activity that is as low as reasonably achievable (ALARA), taking into account the radiotracer’s half-life, the anticipated activity at the measurement locations and the detection limits of the techniques employed.

Alpha emitters are not easily detected and are generally unsuitable as radiotracers. Beta emitters, including ³H and ¹⁴C, may be used when it is feasible to use sampling techniques to detect the presence of the radiotracer, or when changes in activity concentration can be used as indicators of the properties of interest in the system. Gamma emitters, such as ⁴⁶Sc, ¹⁴⁰La, ⁵⁶Mn,

24Na, ¹²⁴Sb, ¹⁹²Ir, ⁹⁹Tc^m, ¹³¹I, ¹¹⁰Ag^m, ⁴¹Ar and ¹³³Xe are used extensively because of the ease with which they can be identified and measured. They are readily traced or followed by detectors placed outside the system. They allow the use of non-invasive procedures that involve minimal or no disruption to production.

4.1.2. Examples of the upstream use of radiotracers and markers

Radiotracers are used during completion, stimulation and recovery enhancements to determine that procedures have been carried out satisfac-torily. Some examples are described below.

As cement is mixed for a well completion, a glass ampoule, containing scandium oxide incorporating 750 MBq of ⁴⁶Sc as powdered glass, is released into the slurry tank just before the initial batch of cement is to be pumped downhole. By releasing the radioactive material directly into the cement, the contamination of equipment and the risk of spillage are minimized. The tank is monitored as the slurry is pumped to the bottom of the string and the grout rises to fill the annulus. As pumping continues, a logging tool is lowered down the well through the displacement fluid to detect and monitor the progress of the plug of radiotracer rising up the annulus until its appropriate position is reached.

To evaluate whether a fracturing process to stimulate the flow has penetrated rocks in the pay zone, plastic pellets coated with approximately 10 GBq of ¹¹⁰Ag^m are added to a proppant during the ‘frac job’. When the fracturing work is complete and when surplus fluids have been removed from the well so as to prevent their solidification in the tubing string, the job is assessed by lowering a logging tool down the well to detect and map out the movement and final positions of the injection fluids and proppants.

To indicate the flow rate of the well fluids, radiotracer ‘spikes’, comprising ⁹⁹Tc^m and ¹³¹I solutions, are released from logging tools into production wells and the time taken for them to traverse the known distance

between two radiation detectors is determined. When radiotracers are injected along with waterflood and gas drives, it is possible to identify the flow patterns,

‘thief’ zones, channelling, flow rates of injected fluids in the reservoir and the relationship(s) between injector and producer wells. The activities of the nuclides injected are significant (Fig. 20) — up to 1 TBq of ³H and ¹⁴C labelled compounds — but the activity concentrations of samples obtained at the producer wells are very low.

In order to aid the detection of any spillage of solutions of these ‘soft’

beta emitters, they are sometimes spiked with a short half-life gamma emitter such as ⁸²Br, which will need measures to minimize external exposures at the injection well. ‘Hard’ beta emitters, such as the gaseous radiotracer ⁸⁵Kr, generate bremsstrahlung and also need measures to minimize external exposures.

FIG. 20. Tritiated water for injection as a radioactive tracer (courtesy:

Scotoil Group plc).

4.1.3. Examples of the downstream use of radiotracers and markers

Flow rate measurement is one of the most common applications of radiotracers. It is used to calibrate installed flow rate meters, measure the efficiency of pumps and turbines, investigate flow maldistribution and heat transfer problems and make plant or unit mass balances. The two methods in widest use rely on pulse velocity and dilution flow measurements.

The pulse velocity method [31] relies on the injection of a sharp pulse or spike of gamma emitter into the process stream. The flow needs to be turbulent and completely fill the pipe bore. Downstream, at a distance sufficient to ensure a good lateral mixing of the radiotracer with the process stream, two radiation detectors are positioned, separated by an accurately measured distance (L). As the radiotracer passes, the response of each detector is registered and the mean transit time (T) measured. Knowing the mean internal cross-sectional area (A) of the pipe bore, the mean linear flow velocity (L/T) can be calculated and converted to volume flow rate (V=LA/T).

The dilution flow method does not need the flow to be full bore or be confined within a closed circuit [32]; the flow can be in open channels, ditches, sewers or rivers. A known activity concentration (C) of radiotracer is introduced at a known constant rate (U). Downstream, at a distance that allows complete lateral mixing, samples are taken and the activity concentration (S) measured. The volume flow rate (V) is very much greater than the injection rate (U) and may be calculated (V=CU/S).

Often, a leak may be inferred from flow rate measurements. In other circumstances, leaks may be detected directly, for example, when radiotracer seeps from a pipeline either above or below ground level.

Residence time measurements have also served to detect leaks across feed–effluent exchangers associated with catalytic reactors. A radiotracer is injected at the inlet of the vessel and a detector provides a signal to record the time of its entry. Another detector at the vessel outlet is used to measure the instantaneous concentration of tracer leaving the vessel. The response or ‘C curve’ of this detector represents the residence time distribution of material in the vessel. A long residence time indicates excellent mixing in the vessel and a short residence time indicates poor mixing (plug flow). The presence of a subsidiary peak prior to the main peak in the residence time distribution curve may indicate a leak across the exchanger. Mean residence time of materials in chemical process vessels and the distribution of residence times both influence the output and quality of the product. Analysis of C curves provides quanti-tative information relevant to the design of mixing characteristics of full size plant.

4.2. SAFETY OF UNSEALED RADIOACTIVE MATERIALS