Radioactive tracer logging
The radioactive tracer-logging tool has a reservoir to hold radioactive material and a pump section at the top. For injection-well logging, two gamma ray detectors below the reservoir and pump are preferable. Some tools employ only one detector, but this is less desirable. The tool includes the circuitry to amplify and transmit the detector counts to the surface, for recording.
Most natural radioactivity underground is from the decay of isotopes of potassium, thorium, and uranium. These materials concentrate in the shales, where they register approximately 100 API units on the gamma-ray log. Once downhole, a "slug" of tracer is ejected by the pump, under surface control. The activity owing to the ejected slug is much greater than the natural background activity. 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.
Modes of logging
There are two modes of logging with radioactive tracers:
· Slug tracking
· Velocity shot
For slug tracking, the logging operator ejects a slug of tracer from the tool. 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 again. 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.
Fig. 1 – A radioactive slug-tracking record from a water-injection well taking 600 BWPD.
By visual inspection of the area under the traces in Fig. 1, nearly all of the injection reaches Depth D, and the injection leaves the wellbore between D and the bottom of the perforation set. A convenient measure proportional to slug area is the product of the slug’s height, above a common baseline, and the slug’s width at half-height. Numbers from the areas agree with those shown on the left side of Fig. 1 as derived from the travel times. Slug A is not yet mixed in the flow. Slug area has the advantage of being insensitive to variations in fluid velocity, allowing the approach to be extended to traced slugs moving behind casing. Notice in Fig. 1 that activity also is detected below the perforations. A slug was ejected below the perforations. Upward logging passes showed that the peak of this slug was stationary. Therefore, there is no wellbore flow below the perforations, and the activity below the perforations is attributable to tracer channeling downward behind the pipe.
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 upward logging passes are made at high line speed. A constant logging speed should be used, and the same speed should be used for all passes.
Provided the ejected slug is uniformly mixed in the flow by movement of the tool up and down through the slug after ejection, 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. Such velocities are listed on the left of Fig. 1. The most frequently used tagging material for water is an aqueous solution of sodium iodide, which contains the isotope of iodine, I-131. The 8-day half-life is ideal. In solution, the iodine does not stick to rock surfaces; instead, with continued injection, the iodine is washed from the rock surfaces and carried away from the near-wellbore, beyond detection by the logging tool.
From inspection of slug spacing in Fig. 1, it is evident that slug tracking has limited vertical resolution. Furthermore, because 90% of the detected gamma rays originate within 1 ft of the detector, the tracer tool’s depth of investigation is also limited and is much less than that of the temperature tool. Because of the limited depth of investigation, tracer that is channeling after exiting the wellbore must be close to the pipe to be detectable. Not all channels can be detected by the tracer tool. The same is true for fractures.
A velocity-shot survey is used in intervals where greater vertical resolution is desired. To perform a velocity shot, the logging operator stations the tool so that the detectors are at chosen locations. Then, 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 (see Fig. 2). On the chart, time increases from the bottom upward. Thus, the top detector gives the lower trace. The time interval between the two peaks (travel time) is inversely related to the velocity of the injection flow. The chart speed should be such that there are 5 to 10 vertical chart divisions between the peaks in total flow, although such is not the case in the figure.
Fig. 2 – Example of a radioactive tracer velocity-shot survey.
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.
For detection of flow behind pipe, many logging operators prefer velocity shots. One detector is stationed within the perforations, while the other is stationed above or below the perforations to see if any flow channels up or down after exiting the wellbore. One difficulty with velocity shots is that they investigate only a very limited part of the total injection interval. In some circumstances, the results from velocity shots indicate the presence of a channel when in fact there is none. Whenever the results from velocity shots indicate an integrity problem, it is better to switch over to slug tracking, which investigates the overall injection interval as well as the wellbore above the interval.
Fewer applications of tracer logging occur in production wells. In a true single-phase flow, there is an appropriate tracer, whether the flow is water, oil, or gas. Because 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 in these flows can be misleading.