RADIOACTIVE TRACER LOGGING BASICS
Tracer logs are used to monitor hydraulic fracture placement in a reservoir and as a method of determining injection flow profiles. Other processes that can be tagged are gravel-pack placement, squeeze cementing, acid treatments, and lost circulation zones.

The most common radioactive tracers are the isotopes  ¹¹⁰Ag (silver), ¹⁹⁵Au (gold), ¹³⁵I (iodine), ¹⁹²Ir (iridium), ¹²⁴Sb (antimony), and ⁴⁶Sc (scandium).



MONITORING HYDRAULIC FRACTURE PLACEMENT
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 casing.
 

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 PetroWiki.com

TRACER SURVEYS IN INJECTION WELLS
By 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 injection.

Multi-pass radioactive tracer log in 800 bwpd injection well showing slug moving downhole with decreasing peak amplitude as slug begins to enter perforations.

VELOCITY SHOT SURVEYS IN INJECTION WELLS
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 detectors.

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 flow.

Because of the unusual circulation patterns that can occur in multiphase flows, tracer results can be misleading