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Well-to-well tracer tests contribute significantly to the reservoir description, which is essential in determining the best choice of production strategy. Direct dynamic information from a reservoir may be obtained, in principle, from three sources: production history, pressure testing, and tracer testing. The value and importance of tracer tests are broadly recognized. Tracer testing has become a mature technology, and improved knowledge about tracer behavior in the reservoir, improved tracer analysis, and reduction of pitfalls have made tracer tests reliable. Many tracer compounds exist; however, the number of suitable compounds for well-to-well tracers is reduced considerably because of the harsh environment that exists in many reservoirs and the long testing period. Radioactive tracers with a half-life of less than one year are mentioned only briefly in this chapter because of their limited applicability in long-term tests. Tracers may be roughly classified as passive or active. In principle, a passive tracer blindly follows the fluid phase in which it is injected. Interpretation of tracer-production curves must account for this. The results from the application of active tracers may give information about fluid saturation and rock surface properties. This information is especially important when enhanced-oil-recovery techniques that use expensive fluids such as surfactants, micellar fluids, or polymers are considered. In the last 50 years, many tracer studies have been reported and even more have been carried out without being published in the open literature. Wagner pointed out six areas in which tracers could be used as a tool to improve the reservoir description. Many companies apply tracer on a routine basis. The reservoir engineer's problem generally is a lack of adequate information about fluid flow in the reservoir. The information obtained from tracer tests is unique, and tracer tests are a relatively cheap method to obtain this information. The information is an addendum to the general field production history and is used to reduce uncertainties in the reservoir model. Tracer tests provide tracer-response curves that may be evaluated further to obtain relevant additional information. Primarily, the information gained from tracer testing is obtained simply by observing breakthrough and interwell communication.
The value and importance of tracer tests are broadly recognized. Tracer testing has become a mature technology, and improved knowledge about tracer behavior in the reservoir, improved tracer analysis, and reduction of pitfalls have made tracer tests reliable. Many tracer compounds exist; however, the number of suitable compounds for well-to-well tracers is reduced considerably because of the harsh environment that exists in many reservoirs and the long testing period. Radioactive tracers with a half-life of less than one year are mentioned only briefly in this chapter because of their limited applicability in long-term tests. Tracers may be roughly classified as passive or active.
Abstract Due to the multiplicity of injection and production wells in waterflooded oilfields, tracer studies may require the use of several tracers for the water phase. Tritiated water is usually the radiotracer choice, but other tracers must be available. Is not a novel radiotracer but is has been synthesized by tedious sequences of chemical oxidation and reduction steps, involving the manipulation of rather high activities. An alternative path is proposed aiming at easing this burden. Basically, potassium chloride is irradiated in an evacuated ampoule by neutrons inside a nuclear reactor, radiosulfur being generated by the Cl(n, p)S reaction. The S are liberated from their sites in the KCl crystal lattice by heating at 500°C, carrier is added and sulfur is extracted with trichloroethylene; contact with air or water is avoided. Co-generated P is eliminated with hydrochloric acid and the sulfur is reacted with potassium cyanide under heating and reflux with ethanol, KSCN being obtained. The tracer was submitted to a bench test pushing a bank of its solution through a Berea sample and its performance compared satisfactorily with the tritiated water benchmark. A field test was performed making a simultaneous injection of both radiosulfur and tritium and the results compared. Unexpectedly the S response showed itself ahead of the tritium. The reasons for that are still being scrutinized but it might be that a phenomenon of ionic repulsion occurred in which the SCN- were repelled by the negatively charged inner surfaces of the smaller micropores within the reservoir. Further tests to clarify this point, using more refined analytical procedures are being planned. Graphs showing the measured responses of the lab and field tests are included. It is hoped that this new labeling method will contribute to waterflood tests by broadening the multitracer range available. Additionally it props the chemical synthesis and the radioanalytical supports of the tracer techniques applied in reservoir characterization. Introduction Any technique aiming at the improved knowledge of the injection fluid behavior in oil reservoirs will certainly have a large economical significance. Thus the injection of tracers in reservoirs has become widespreadly used in order to quantitatively evaluate the performance of enhanced recovery processes as well as to supply information about many aspects regarding the behavior of the injected fluid and the reservoir characteristics. Such information could hardly be obtained by the sole use of other methods currently used in oil production. Whenever the reservoir is flooded with water during secondary recovery, the best tracer is tritiated water, which consists of a water molecule in which at least one of the hydrogen atoms has been substituted by its isotope tritium. This radiotracer emits a very soft beta radiation, which nonetheless can be accurately detected and quantified after suffering a huge dilution within the reservoir. Other radioactive tracers are even more easily detected, but tritiated water has the obvious advantage of behaving exactly as normal water does, i.e. it traces with absolute fidelity the water path and transit times. Presently tritiated water is the first choice when a tracer is to be performed for waterflood evaluation. However, it often happens that such evaluations require the definition of the contribution at different injection wells to some producer well of interest. In such cases distinct tracers need to be simultaneously injected in the contributing wells and be discriminated in the produced outflow. Radiotracers are again advantageous in this connection since they can be easily differentiated and quantified through a single counting of the samples of the produced water, usually with little or no previous treatment. This application has prompted the efforts of many investigators after the development of other suitable substances tagged with radioactive isotopes. However large the number of natural or artificial radioisotopes known, only a quite restricted number of chemical compounds labeled with them can comply with the strict requirements demanded by reservoir applications.
Tracers are used in well to well tests to gather data about the movement and saturation of fluids in the subsurface. Radioactive tracers can be used to gather data about water or gas. This article discusses some of the commonly used radioactive water tracers. In most field studies, the tracer is expected to behave exactly as the water it is going to trace. Very few compounds will behave as passive tracers in all situations, but near-passive tracers will, in many applications, work satisfactorily.
Interwell tracer tests are widely used. This article reviews some of the studies reported in open literature. The selection introduces different problems that have been addressed, but the original papers should be studied to obtain a more detailed description of the programs. The Snorre field is a giant oil reservoir (sandstone) in the Norwegian sector of the North Sea. Injection water and gas were monitored with tracers, 18 and the resulting tracer measurements are discussed in this page.