This method has been applied mainly for injected water. It is useful, especially where unsaturated water-wet rock may absorb short tracer pulses by water imbibition from the injected water front edge. Ideally, the method requires

continuous logging of the water injection rate and corresponding adjustment of the tracer dosage rate in order to maintain a constant concentration of tracer in the injected fluid. A simpler arrangement can be implemented if a constant water injection rate can be assured. An example of a tracer injection arrangement is shown in Fig. 14.

Typically, the tracer used is HTO and the concentration in the tracer container is 370 MBq/L. The tracer concentration in the injection water, and the corresponding rate of tracer dosage, depends on the detection limit in the analytical laboratory and on the expected fraction of traced injected water in the produced water. The design should be a tracer concentration at the top of the production curve (at ‘equilibrium’) corresponding to some 100-1000 Bq/L in the produced water.

There is an example of a field experiment with continuous injection of HTO tracer and a subsequent tracer pulse injection (125I). Such operations may be laborious and cumbersome, especially for offshore operations. They require access to tracer engineers on a semi-continuous basis; regular controls with the equipment and tracer solutions will have to be performed. In addition, long term storage and use of radioactive solutions give rise to some scepticism among petroleum rig personnel. This method should only be used if there are clear advantages over the pulse injection method.

In principle, this method may be used also for non-radioactive tracers, but the tracer reservoir volume has to be larger and the pump somewhat different from both that described in Fig. 12 (smaller than this) and that used in Fig. 14 (somewhat larger than this).