In order to qualify a water tracer on the basis of dynamic behaviour as passive (‘very good’) or near-passive (‘good’), the type of job the tracer is expected to fulfil needs to be carefully defined. If the job is to measure fluid communication exclusively, a near-passive tracer may work as well as a true passive tracer.

Non-charged tracer species: A true passive water tracer is one that mimics all movements and interactions that the water molecules undergo in the traced water volume. In practice, the only radioactive compound that fulfils this requirement is radioactive water (HTO). Movements can, for instance, be the free movement in and out of dead-end pores insensitive to columbic forces set up by negatively charged rock surfaces. Interactions can include exchange with connate water in the rock pores or exchange with crystal water molecules. Thus, it can sometimes be observed that HTO seems to lag behind the injection water breakthrough as measured, for instance, by salt balance (ionic logging) or that the HTO production profile is somewhat more skewed. In the literature this has been incorrectly interpreted to HTO instablility under reservoir conditions, and that it may be subject to isotope exchange reactions of tritium with hydrogen in neighbouring hydrogen-containing compounds, some of which are stationary.

Other non-charged radiotracers include, for instance, tritiated methanol (CH2TOH) and the other radiolabelled light alcohols. These will behave qualitatively similar to HTO with respect to the diffusive and convective parts, but differ as regards interactions.

Anionic tracers: Of electrically charged tracers, anions represent the more applicable ones. In laboratory experiments, however, ion exclusion is observed, i. e. negatively charged species tend to be repelled from the negatively charged rock surfaces. As a result, the tracers tend to flow in the middle of the fluid-conducting pores. They will not easily enter into dead-end pores or through narrow pore throats. This results in a somewhat smaller available pore volume for anions than for non-charged species. In laboratory experiments, the production profile differs in reproducible ways from that of HTO, but in full-sized field experiments this difference is not that obvious.

Anionic tracers are represented by S14CN-. A typical production profile is given in Fig. 3. This profile is compared with the production profile of the simultaneously injected HTO. The difference in shape is enhanced by subtracting the normalized HTO profile from the normalized S14CN- profile. The result is given in Fig. 4. It is evident from the curve that the breakthrough of HTO precedes that of S14CN — and that the tail of the HTO profile is more pronounced. This profile difference is qualitatively reproduced for all near-passive anionic water tracers and illustrates the phenomenon which has become known as anionic exclusion. The flow rate is increased, for instance, by a factor of 10 (i. e. to 200 cm/d), the first down-dip in the normalized difference curve disappears and the breakthrough of the anionic tracer precedes that of the reference tracer, HTO.

On the basis of such curves, retention factors may be derived from Eq. (4) and the production profiles found by such experiments.

1 + P = Vt/Vs (4)

where

P is the retention factor;

VT is the retention volume for the tracer candidate;

VS is the retention volume for the standard reference tracer.

The retention volume may be represented by the peak maximum value or the mass mid-point (first moment (p1)) for non-symmetric profiles. These values are best found by fitting the profile with an analytical function consisting of polynomials.

For monovalent anions, the retention factors are in the range 0.0 to -0.03, indicating that such tracers pass faster through the reservoir rock than the water itself (represented by HTO).

Some anionic tracers may show complex behaviour. Radioactive iodine (125I- and 131I) breaks through before water but has a substantially longer tail than HTO. Both reversible sorption and ion exclusion seem to play a role here.

Cationic tracers: These are in general not applicable. However, experiments have qualified 22Na+ as an applicable water tracer in saline (greater than sea water salinity) waters. In such waters, the non-radioactive 23Na+ will operate as a molecular carrier for the tracer molecule. The retention factor has been measured in the range of P « 0.07 at reservoir conditions in carbonate rock (chalk) [10]. Accordingly, the tracer is somewhat delayed by sorption on, and ion exchange with, the reservoir rock, but in a reversible fashion.

In the literature there is also a report on the successful use of 134Cs+ and 137Cs+ in a carbonate reservoir [11]. This tracer cannot, however, be generally used as it will adsorb strongly (and irreversibly under ordinary reservoir conditions) on clay-containing rocks. There is also the reported use of other cationic species such as 60Co3+ and other cobalt isotopes, but these compounds have never been produced back.