Experiments should be carried out to examine the potential for tracer distribution between the water and oil phases. Three different methods are:

(1) Static batch experiments, where phase mixing and separation takes place in a mixing apparatus and where samples can be extracted from each phase for analysis of tracer concentration. Equipment for this purpose ranges from the simplest, such as the separation funnel, to thermostatized equipment illustrated in Fig. 36 and to more complicated autoclaves where pressures can be applied if deemed necessary.

Thermostatized water і n

Magnetic stirrer

FIG. 36. Experimental set-up for studies of radiotracer partitioning between seawater and stock tank oil at temperatures <90 C and at ambient pressure.

(2) Dynamic experiments with continuous phase mixing followed by phase separation as exemplified by the flow injection apparatus shown in Figs 37 and 38.

(3) The dynamic or chromatographic method, where a small tracer pulse is forced through a porous medium with known oil saturation at moderate linear flow rates (25-50 cm/d) together with the standard reference non­partitioning tracer, HTO. Difference in tracer transportation time is a measure of the degree of partitioning. Various forms of flow rigs can be used for this purpose. One such piece of equipment is illustrated in Fig. 39.

Heating cabinet

FIG. 37. Flow equipment used for measurement of partition coefficients of tracers between water (brines) and oils at ambient pressure and moderate temperatures (<90 XI).

Method (1) will give the true equilibrium partition coefficient, as will method (2) if the length of the mixing coil is sufficient to ensure full equilibrium transfer. The mixing coil may be empty (capillary) or filled with a packing material (static mixer). Method (3) will give an ‘effective’ partition coefficient because it includes kinetic effects such as diffusion rates and rate of exchange between phases (across liquid boundaries) and diffusion into the bulk volume as

30 50 70

Temperature (°С)

the tracer pulse passes by. This latter may give values that are more representative for the situation in the reservoir where the tracer (in most examinations) is transported as a pulse through the porous medium. The best situation occurs when the results from both experiments match.

The degree of partitioning (partition coefficient) is expressed by:

C

K = -^ (16)

C

^tr, w

where Ctr, o and Ctr, w are concentration of the tracer in the oil phase (o) and the water phase (w), respectively.

This quantity is directly derived in method (1) above by the counting of water and oil samples. Since C is proportional to the disintegration rate:

Ctr, o = Ro /so (17)

and

Ctr, w Rw/sw

where є is the counting efficiency and R is the count rate in the oil and water phases. Thus,

In dynamic experiments, the practical partition coefficient K’ is derived on the basis of the recorded tracer production curve (or chromatogram). This curve is established by sampling the fluid effluent from the chromatographic column and counting by liquid scintillation counter and/or gamma spectroscopy. K’ can be calculated from Eq. (20):

K,=v; — Vw) ■ a — S0)

Vw ■ S o

where

Vtr is the retention volume of the tracer candidate, i. e. the volume from the

start injection to the peak maximum of the tracer production curve (which may be found by curve fitting);

Vw is the retention volume of the water represented by the non-partitioning

standard reference water tracer HTO;

So is the oil saturation or fraction of oil volume occupied by oil;

K « 0 for passive water tracers. Compounds with K > 0 are of interest for measurement of the remaining oil saturation.

A typical result for a passive water tracer is that the degree of partitioning into the oil phase is approximately 0, as illustrated for S14CN — with HTO as a control in Fig. 39.