Summary

Radioactive tracer logging has been used for many years to measure injection profiles. In this study, we analyzed the tracer loss and velocity shot methods of radioactive tracer logging to develop guidelines for improved interpretation of these logs. In addition, tracer logging was simulated experimentally in a test pipeline to check assumptions made in the theoretical analysis. Among the conclusions drawn were that the tracer loss method is imprecise because of poor depth resolution and nonuniform mixing of tracer and that the velocity shot method is subject to significant error when fluid is exiting between detector locations. Finally, a new radioactive tracer logging method is proposed that may offer some advantages over current practices.

Introduction

Radioactive tracer logging is the most commonly used technique for determining injection profiles in water injection wells. Two methods currently are applied - the tracer loss method, in which a slug of tracer is logged through repeatedly as it moves down the wellbore, and the velocity shot method, which consists of measuring the transit time of a slug of tracer between two detectors at several depth locations. In this study, we examined these two logging methods theoretically and experimentally to determine the accuracy and sources of errors in each method. In addition, a new tracer logging method that appears to offer some advantages over the tracer loss and velocity shot logs is proposed.

Lichtenberger1 has shown that the tracer loss method has poor depth resolution and thus should be used only as a gross indicator of fluid loss locations and amounts. Our analysis confirmed this feature of the tracer loss technique, particularly when the tracer is passing locations of fluid exit. The experiments showed that further errors occur in tracer loss analysis because of nonuniform mixing of the tracer. The velocity shot method also was found to have significant errors when fluid exit occurs between detector locations.

The proposed new logging method consists of ejecting two pulses of tracer into the wellbore and monitoring the distance between the pulses as they move down the wellbore. It is shown that this method is relatively insensitive to wellbore cross-sectional area, and is not affected by fluid exits between measurement locations. In the experimental evaluation of the two-pulse method, the responses measured compared closely with that predicted.

In this paper, the theoretical basis and sources of error for each of the three logging methods are presented. The experiments performed to evaluate tracer logging then are discussed. Finally, results of the experiments are compared with the theoretical predictions.

Theory

Two radioactive tracer logging methods are in common use today - the tracer loss method and the velocity shot method. In the tracer loss method, a single slug of tracer material is ejected into the wellbore above all zones of fluid loss; the tracer concentration then is measured as a function of depth by passing a gamma ray detector repeatedly through the tracer as the tracer slug moves down the wellbore. The velocity shot method consists of measuring the transit time of a slug of tracer between two points (usually between two gamma ray detectors but sometimes between the ejector and one gamma ray detector). Velocity shot measurements are repeated at various location in the wellbore.

Tracer Loss Method

A tracer loss log is analyzed by calculating the area under the gamma ray intensity vs. depth curve. Then the volumetric flow rate at any measurement location, Qi, is calculated from

  • Equation 1

where Q100 and A100 are the volumetric flow rate and area under the gamma ray curve above all zones of fluid exit, and Qi and Agr are the volumetric flow rate and area under the curve at any other measurement location. A mass balance on the radioactive tracer shows that this analysis is correct, assuming:

  1. the gamma ray intensity measured is proportional to the tracer concentration in the wellbore,

  2. the tracer is mixed uniformly in the wellbore fluid, and

  3. no part of the tracer slug is opposite a zone of fluid exit.

In this paper, we will not consider errors caused by the gamma ray measurement itself, though these can be significant, as shown by Wiley and Cocanower.2

Tracer Loss Method

A tracer loss log is analyzed by calculating the area under the gamma ray intensity vs. depth curve. Then the volumetric flow rate at any measurement location, Qi, is calculated from

  • Equation 1

where Q100 and A100 are the volumetric flow rate and area under the gamma ray curve above all zones of fluid exit, and Qi and Agr are the volumetric flow rate and area under the curve at any other measurement location. A mass balance on the radioactive tracer shows that this analysis is correct, assuming:

  1. the gamma ray intensity measured is proportional to the tracer concentration in the wellbore,

  2. the tracer is mixed uniformly in the wellbore fluid, and

  3. no part of the tracer slug is opposite a zone of fluid exit.

In this paper, we will not consider errors caused by the gamma ray measurement itself, though these can be significant, as shown by Wiley and Cocanower.2

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