2.2.3.I. Pulse injection

There are two main procedures for tracer pulse injection:

(i) Integral (topside) injection at the well head where the tracer enters all available perforated zones and is injected into the reservoir according to the injectivity in the various zones.

(ii) Downhole injection where different tracers may be injected in different isolated zones.

FIG. 6. Drum lined with shock absorbant material for transportation of the container

Topside radiotracer bypass injection: This is the simplest, cheapest and most frequently used injection method. The practical implementation procedure depends on the tracer to be injected, i. e. the procedure is simplest for beta emitters and somewhat more cumbersome for strong, high energy gamma emitters. The techniques range from mechanical crushing of tracer-containing glass vials in the injection stream to controlled pump operated injection and soluble solid state tracer slug injection.

Below, a well-proven technique for water radiotracer injection is described. The tracer mixture is prepared in a 100 mL flow through high pressure (rated to 500 bar) steel cylinder fitted with high pressure valves at both ends (total liquid volume ~ 70 mL). For beta tracers, a small quantity of a short lived gamm emitter (often 131I) is added for monitoring purposes.

Figure 7 illustrates typical small-sized injection equipment fitted into a suitcase shaped transportation container which may be carried by hand. The equipment is coupled to the main injection line as a bypass at two positions across a throttle valve. The function of the throttle valve is to set up a pressure difference between the two positions. The tracer container is connected to the equipment as shown. There are possibilities for pump operation of preparative fluids (inlet for chemicals) before tracer injection. The tracer is injected by by passed injection water driven by the pressure difference, and without any use of pumps. The injection efficiency is monitored by external gamma detectors in cases where a small quantity of a gamma emitting tracer is added to the tracer container. A typical injection time is <10 s for >99% of the tracer. However, rinsing continues

	Thrott e va ve
03 Valve 2
Manometer
Valve A( f
Valve D
Va ve C
gj Valve E
Va ve F
Valve I
Bottle valve G	Bottle valve H	Drain
Tracer bott e

Lead shield

FIG. 7. Small-sized injection apparatus for topside pulse injection of radiotracers.

for 60 min to clean out any remaining traces of activity from the injection apparatus.

For some tracers, it is advantageous to apply an extra non-radioactive molecular carrier in the injection phase (e. g. for 125Г and radiolabelled [Co(CN)6]3-). These tracers are injected using the same injection apparatus but now connected only to the main flow line at the outlet connection point. The injection is performed by pump operation where injection water containing carrier and other tracer preserving chemicals is pumped from an injection water reservoir.

Topside radiotracer ‘crushing’ injection: Figure 8 shows equipment for crushing injection of tracer. A 20 mL tracer glass vial containing a beta emitting tracer is loaded into the holder of the injector.

The equipment is then installed at the well head of the injector (Fig. 9). By operation of the hand wheels (A and B), the tracer vial will be crushed by the central bar and the tracer thereby released into the well. As with the bypass injection described above, a small quantity (a few megabecquerels) of a gamma emitting tracer, for instance 131I-, is utilized as a second tracer to monitor the injection process and confirm successful injection.

Pump operated radiotracer topside pulse injection: Pulse injection may also be carried out by means of pumping. When a gamma emitting tracer is used,

Connection tube 1

Connection

FIG. 8. Crushing injection equipment.

the radiation load to the operators has to be monitored in order to minimize any dose.

In this case, the transportation cage with the original lead shield may also be used during injection by leaving the radioactive liquid inside, connecting a slim flow line to the bottle and injecting the tracer by means of a high pressure suction pump. Useful equipment for this purpose is illustrated in Fig. 10.

FIG. 10. Piston pump system for injection of gamma emitting tracers such as [0Co(CN)6]3 . It can also be used for beta emitters such as HTO and S14CN-.

Pump operated topside pulse injection of non-radioactive tracers: As mentioned previously, non-radioactive tracers are mainly liquid (aqueous) solutions of weak acids or salts. Over long distances, it is most convenient to transport the tracer compound in dry form (Fig. 11) and perform, whenever possible, the dissolution operation at the well site.

Some of the tracer compounds have limited water solubility. Weak acids may need addition of a base such as NaOH or KOH in order to promote dissolution. Finally, several hundred litres may be required for injection, even for an ordinary-sized reservoir section. Hence, injection requires higher capacity pumps than previously described. Figure 12 gives an example of both the tracer solution container and the pneumatically operated pumps used for injection.

This type of tracer injection may take a few hours depending on the volume of the tracer solution. In principle, this represents a square injection pulse but it may be regarded as an instantaneous pulse injection when compared with the transit time through the reservoir.

FIG. 11. Example of tracer chemicals in dry form in transportation containers.

FIG. 12. Injection equipment for non-radioactive tracer solutions, here shown on the deck of an offshore production platform in the North Sea.

Downhole pulse injection: Downhole injection offers several advantages over topside injection:

• Removal of the danger for contamination of topside equipment. This may be especially important for radiolabelled [Co(CN)6]3- and possibly other complexes which may react chemically in the injection tubing.

• For stratified reservoirs, each zone may be uniquely labelled with a special tracer. This makes it possibile to examine vertical permeability in reservoirs and detect any extensive sealing.

• For horizontal wells which cover an extensive lateral reservoir section, zone injection is absolutely desirable for optimal information.

FIG. 13. Downhole tracer injection tool.

Downhole injection is not yet in general use although field tests have been successfully carried out [11]. A few attempts have been made to construct tools for general application, one of which is illustrated in Fig. 13 [10]. It is mainly constructed for vertical and deviated wells. It can be lowered into the well by a wireline which makes signal transfer possible. The tool is remotely operated from topside by PC control. It is based on the principle of a moving arm sealing onto the perforated section of the well through which the tracer solution is pumped at low speed during somewhat reduced rate of ordinary water injection. The tool is not yet in operation due to high cost of operation.

Lately, it has become technically possible to position downhole injection tools in horizontal wells by means of a well tractor. Combined with inflatable packers on the same line, sections may be isolated for specific tracer injection. Such injection equipment is composed of general and readily available components. It is therefore technically possible to conduct zone injection, but the first large scale field experiment has yet to be carried out. Some well completions are constructed to allow water injection into selected isolated zones. These completions can be used for selective zone tracer injection although the addition of tracer itself is carried out topside.