 of the slug is seen as a large gamma ray peak
whose size is proportional to the flow rate. A reduction in
the size of the peak indicates a loss of fluid into the
formation. Fluid velocity can be calculated from the time
interval and the distance the peak has moved.
Tracer-loss measurements produce a type of radioactive
tracer log used mainly to give a general idea of fluid flow
in low flow-rate wells. In very low flow-rates, an
alternative technique has been used in which the gamma ray
detector is held stationary at some depth until the slug has
passed. The detector is then moved down to another depth to
observe the slug again. With these data, it is possible to
make quantitative estimates of fluid flow.
The typical tool has a reservoir to hold radioactive
material and a pump section at the top
with two gamma ray detectors below. Once downhole, a slug of
tracer is ejected by the pump. The radioactivity of the slug
is much greater than the natural reservoir radioactivity.
The most commonly used tracer material is an aqueous
solution of sodium iodide, which contains the
isotope iodine 131 with a half-life of 8 days.
The following Sections are based on an article on
TRACER SURVEYS IN INJECTION WELLS
tracking the progress of the slug down the wellbore, the
exits of injected flow from the wellbore can be determined,
as well as whether any of the injection, after exiting,
passes through a channel close to the pipe.
For slug tracking, the logging operator ejects a slug of
tracer from the tool at some distance above the
perforations. After ejection, the tool is run up and down
through the slug to ensure that the slug is uniformly mixed
across the wellbore cross section. Then the tool is lowered
quickly and an upward logging pass is made at constant
logging speed until the slug is detected. The time of
detection of the peak and the depth of the peak are
recorded. Then the tool is quickly lowered again, and
another upward logging pass is made at the constant logging
speed until the slug is detected and again the time
of detection 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 ejectio