Use of Gamma Ray-Emitting Tracers and Subsequent Gamma Ray Logging in an Observation Well To Determine the Preferential Flow Zones in a Reservoir
Authors: Gesink J.C.J. , van den Bergen E.A. , de Monchy A.R. 
Published: December 2007

Introduction
The labeling of the injection water with a radioactive or chemical tracer has been applied successfully to establish interwell flow patterns in waterfloods and EOR projects. Samples taken from the production wells surrounding the injector are analyzed quantitatively for the presence of tracer. Information about preferential flow and lateral sweep can be obtained. Most reservoirs exhibit stratification, with different sands having different flow properties. This will result in different breakthrough times properties. This will result in different breakthrough times for the injected water in the producing wells. Since the tracer concentration of the sample taken periodically from the well represents an average value, there is, in general, no possibility to indicate the layer(s) from which the tracer originates. The subsurface flood performance can be determined in situ if an observation well is drilled at a suitable location between the injector and the producer. The injection water is labeled with a radionuclide that emits gamma rays powerful enough to be detected through the casing (e.g., powerful enough to be detected through the casing (e.g., cobalt-60). With a conventional gamma ray logging tool the tagged injection water can be detected by comparing the log periodically run after the injection with the base log (before injection). The development of vertical sweep around the wellbore can be established. Thief zones and zones unaffected by the flood can be identified.

Tracer Selection and Well Logging Response
Any tracer that is to be used in this technique for indicating the vertical sweep has to meet certain general requirements.Losses because of adsorption in the formation rock must not occur. Very positive results were obtained with the application of cobalt hexacyanides in several field tests. These tracers show no tendency to get adsorbed and are stable under most reservoir conditions.To produce a clear, unambiguous response on a logging tool in a nonproducing observation well, the radio-nuclide used should emit gamma rays powerful enough to be detected through the casing.The radioactive lifetime (as indicated by the half-life) of the tracer should be at least of the same time scale as the project.
Table 1 lists a number of potentially suitable nuclides, their half-lives, yield, and energy of principal gamma rays. The properties of naturally occurring potassium-40 are included in the table as reference. potassium-40 are included in the table as reference. The total amount of a radioactive tracer to be injected to produce a detectable signal on the logs is determined, in principle, by two factors:the tracer concentration Co in the surroundings of the observation wellbore, which causes a signal of sufficient intensity to be detected above the background in the particular formation, andthe total volume, V, in which the tracer is diluted.
In practice, both the upper limits for Co and the total activity practice, both the upper limits for Co and the total activity to be injected, Co x V, often are dictated by standards of radiological safety. Especially in those experiments where a producing well is used for sampling purposes, the design of a test must guarantee that the radioactivity levels in the produced water will not exceed standards for water as currently recommended by the Intl. Commission on Radiological Protection (ICRP). A simple relation between the signal from the detector and a given concentration of a nuclide in the formation around the detector is:
(1)
where
Co = activity per unit volume of the formation, Bq/m [curie/cu ft],= linear attenuation coefficient in theformation for the photons emitted by thenuclide, m [ft ], and= constant representing detector propertiesand physical properties of the nuclide, m [sq ft].
JPT
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