for CPH by E.R. Crain, P.Eng. in 2020. This
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TRACER LOGGING BASICS
Tracer logs are used to monitor hydraulic
fracture placement in a reservoir and as
a method of determining injection flow
processes that can be tagged are gravel-pack
placement, squeeze cementing, acid treatments, and lost
The most common radioactive tracers are the isotopes
110Ag (silver), 195Au (gold), 135I
(iodine), 192Ir (iridium), 124Sb
(antimony), and 46Sc (scandium).
MONITORING HYDRAULIC FRACTURE
In this case, a short-lived
radioactive element is added to the frac fluids or propping
material. Different elements may be introduced at different
times during the fracture placement so that fracture growth
can be assessed versus time. After placement, a gamma ray
log is run, starting some distance below to some distance
above the frac interval. If only one element was used, a
conventional gamma ray tool will suffice. For a
multi-element survey, a spectral gamma ray log is required.
The fracture height and position can be assessed by
observation of the gamma ray curves. The fracture may extend
above and below the perforated interval. Comparison of this
log with a competent petrophysical analysis will help
determine if the frac went up or down into unwanted
territory, for example a water zone below or a gas zone
above the desired oil zone.
Post-frac radioactive tracer log with natural gamma ray in
Track 1 recorded before stimulation. Tracer log in Tracks 2
and 3 shows some placement into both sets of perfs, but also
above upper perfs, possibly due to channel in cement behind
MONITORING FLUID FLOW
In this case, a specialized production
logging version of the gamma ray tool is used to
monitoring the reduction in tracer material as it moves down
the well. A slug of radioactive tracer is added to the
injection fluid. As the slug moves down the well, several
gamma ray logs are recorded at well defined time intervals.
The position 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 d