etection of the peak and the depth of the peak are
recorded. This process is repeated several times, resulting
in a succession of detections of the same slug (see Fig. 1).
As long as the peak progresses downward, there is flow in or
near the wellbore. Once the peak stops, there is no flow in
or near the wellbore below the stopping depth.
For each detection, the area under the trace
and above the common baseline of the traces is proportional
to the percentage of injection still in or near the wellbore.
Generally, only one gamma-ray detector is used for
slug tracking. Slug tracking gives the best overview of where
injection leaves the wellbore and whether, after exiting, any
injection travels in a channel close to the pipe.
The vertical distance (ft) between two successive peaks in total
flow divided by the time (minutes) between detection of the peaks
provides an accurate estimate of the average flow velocity of total
Multi-pass radioactive tracer log
in 800 bwpd injection well showing slug moving downhole with
decreasing peak amplitude as slug begins to enter perforations.
A velocity shot survey is used in
intervals where greater vertical resolution is desired. With the
tool stationary, a slug of tracer is ejected into the injection
flow. As it passes downward, the slug is first detected by the top
detector and then by the bottom detector, resulting in two traces on
the log. The time interval between the two peaks (travel time) is
inversely related to the velocity of the injection flow.
The ratio of the travel time in total flow to the
travel time at a selected position is the fraction of injection
still in the wellbore at the selected position. However, dividing
the separation between the detectors (ft) by the travel time
(minutes) does not produce the average velocity of flow, as the slug
cannot be uniformly mixed in the flow before it passes the
Two detectors are preferred for velocity shots. If
there is only a single detector, there can be timing errors between
initiating ejection of a slug and actual ejection downhole. These
timing errors contaminate the measured travel times.
TRACER SURVEYS IN PRODUCTION WELLS
Fewer applications of tracer logging occur in
production wells. In a true single-phase flow, a slug is tracked for
a while and then disappears uphole. Multiple slugs are used, one for
each producing interval under investigation. Usually, a well is
logged from a bottom, no-flow interval up to an interval of total
Because of the unusual circulation patterns that can occur in
multiphase flows, tracer results can be misleading