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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.
The accurate evaluation of reservoir-performance characteristics in the secondary recovery of petroleum by water flooding requires use of a water tracer that may be injected into water-input wells and detected at oil-production wells to supplement data obtained from core analyses, wellhead tests, and subsurface measurements. Radioactive iodine has been used successfully as a water tracer in field tests to determine: (1) relative rates and patterns of flow of injected water between water-input and oil-production wells and (2) zones of excessive water entry into oil-production wells.
Laboratory evaluations of potential water tracers, previous tracer studies, the value of using a radioactive tracer, general field procedures, and the use of surface and subsurface instruments for the detection of the emitted gamma radiation, are summarized. Data from the field tests are presented graphically and discussed in detail.
It is concluded that the radioactive-tracer method, using radioactive iodine, may be used successfully to measure either the relative rates and patterns of flow or zones of excessive water entry into wells under conditions of comparatively rapid transit time between wells.
Extensive use is made of data obtained from core analyses, wellhead tests, fluid characteristics, and subsurface measurements in evaluating the sweep efficiency of water injected into oil sands for the recovery of oil. Theoretical flow rates and patterns may be calculated from those data using radial-flow formulas, if it is assumed that the physical conditions in the productive formation are homogeneous. Unfortunately, homogeneous conditions rarely, if ever, exist in oil-productive formations. The use of a tracer that may be injected into an oil sand and detected quantitatively, or even qualitatively, at offsetting oil-production wells provides basic data that may be used in determining more accurately the subsurface rates and patterns of flow of injected water between wells than is possible by theoretical calculations based on assumed conditions. Consideration of the data obtained by using a water tracer assists in the application of remedial measures to water-input wells, such as plugging of channels, or selective plugging of highly permeable zones, thereby effecting a more uniform flood and a greater ultimate oil recovery.
Almost since the emergence of petroleum engineering as a distinct discipline, radioactive tracers have been used to study the in-situ placement and flow of various subsurface processes. Since very few techniques exist for determining the placement or flow of materials around wellbores, the use of tracers which emit gamma radiation capable of penetrating the casing and being detected by wireline conveyed instruments is an obvious and logical undertaking. Indeed, properly conducted radioactive tracer studies help provide a safe and cost-effective means of determining the location and placement of many types of treatments and procedures frequently performed on wells.
Radioactive tracers have been used to study nearly all aspects of drilling, completion, and production/injection in the petroleum, geothermal, and underground injection profiling. Moreover, advancements in handling and injection techniques have improved not only the tracer log interpretations, but the safety and environmental aspects as well.
A successful radioactive tracer study requires careful coordination and cooperation among all of the involved parties: tracer contractor, pumping service operator, wireline contractor, and well operator. This paper will discuss the methodology required to conduct reliable downhole tracer studies. Special attention will be devoted to the chemistry and mechanics of the tracer materials and their suitability to the various applications involved. Also, the techniques required to transport and inject these materials in the field without contaminating the associated pumping equipment and tubulars will be cited. Additionally, the logging instruments and techniques required for accurate in-situ detection will be discussed. Finally, the environmental considerations and recommended procedures for returning a well to normal operation after a tracer study will be covered.
Due to the inherent inaccessibility of the downhole environment, radioactive tracers have presented one of the few viable means of analyzing the placement and flow of various processes and materials. Indeed, gamma radiation is one of the few phenomena which can penetrate casing and wellbore fluids and be easily detected in-situ.
Subsequently, gamma ray emitting tracers have been used as a diagnostic tool in the petroleum industry for quite some time. Production logging practices have routinely included the use of 131Iodine to quantify borehole fluid injection profiles since about 1955, and the use of radioactive tracers for a myriad of subsurface applications was described in vivid detail in 1965. Today, the use of radioactive tracers in the petroleum industry is so widespread that firms specializing in the preparation, handling, and analysis of tracers have come into existence, and petroleum tracer technology has become a mature service sector offering numerous applications.
Injection Profile Studies - By far, the most common use of radioactive tracers in the petroleum industry is wireline conveyed injection profile studies.
ABSTRACT The results of a group of a oil-well fracture treatments using radioactive sand and gamma surveys to determine the disposition of the sand are discussed. The results show the method to be of value in locating of sand in location other than the intended zone of treatment are shown. The problem of interpreting the gamma ray surveys is briefly discussed. A method for following radio into a well is shown to be a semi-quantitative Interpretation of the surveys is given. INTRODUCTION It is the purpose of this paper to discuss techniques for using radioactive tracers to locate fractures, channels and leaks, and permeable regions in oil wells A large number of wells have been treated with materials containing radioactive tracers The examples to be discussed were chosen to represent as wide a variety of circumstances and results, both good and bad, as possible DESCRIPTION OF THE METHODS Two different materials containing radioactive tracer were used One material, selected originally for locating fractures, is radioactive sand. Propping sand, identical in all respects to the round sand used routinely in fracture work, was coated with a radioactive tracer with a relatively short half life and which emits an energetic gamma ray The tracer is not removed from the sand by crude oil, salt water, or the emulsifying agents in general use in fracture work The density, gram size, and other physical properties of the sand are not altered in any way which would affect their deposition in fractures. Sand was chosen as the carrier for the radioactive tracer because it is the material considered which most certainly goes into the fracture and which does not migrate after it is in place. This assumption is still considered essentially correct However, sand is sometimes found in the bottom of the well making the results of these tracer surveys more difficult to interpret The procedure for handling the radioactive sand at the well location is simple and requires very little special equipment. The sand on which the tracer has been deposited is delivered to the base camp or well location in closed metal cans. Each can contains 50 lb of radioactive sand The gamma radiation from these cans is of the order of 5 to 10 milliroentgens per hour at the surface of the cans Storing a number of the cans together does not greatly increase the radiation because of the self-shielding effect of the relatively large bulk of sand. The sand is mixed with non-radioactive sand in a ratio of about 200 lb of radioactive sand to 1,800 lb of non-radioactive sand. This mixing may be done either in a rotovoy or with the sand blender Either or both of these pieces of equipment are standard on any fracture operation. After this sand has been mixed, the radiation is reduced to insignificance so far as the physiological hazard is concerned Sand, blended as described, is used in fracturing operations exactly as it would be if it were not radioactive
Abstract Tracer technology is an efficient and effective monitoring and surveillance tool with many useful applications in the oil and gas industry. Some of these applications include improving reservoir characterization, waterflood optimization, remaining oil saturation (Sor) determination, fluid pathways, and connectivity between wells. Tracer surveys can be deployed inter-well between an injector and offset producer(s) or as push-and-pull studies in a single well. Tracers can be classified several ways. (a) Based on their functionality: partitioning and passive tracers. Partitioning tracers interact with the reservoir and thus propagate slower than passive tracers do. The time lag between the two types can be used to estimate Sor, to ultimately assess and optimize EOR operations. (b) Based on their carrying fluid: water and gas tracers. These can be used in IOR or EOR operations. All gas tracers are partitioning tracers and the most common are perfluorocarbons; they are thermally stable, environmentally friendly, have high detectability and low natural occurrence in the reservoir. On the other hand, water tracers are passive tracers and the most commonly used ones are fluorinated acids. (c) Based on radioactivity: radioactive and non-radioactive tracers. Selecting a tracer to deploy in the field depends on a number of factors including their solubility, fluid compatibility, background concentration, stability, detectability, cost, and environmental impact. This paper provides an overview of various tracer applications in the oil and gas industry. These will include the single-well tracer test (SWCT), inter-well tracer test (IWTT), nano tracers, gas tracers and radioactive tracers. Their use will be highlighted in different scenarios. Field case studies will be reviewed for all types of tracers. Lessons learnt for all the applications, including what works and what does not work, will be shared. Specific cases and examples will include the optimization of waterflood operations, remaining oil saturation determination, flow paths and connectivity between wells, and IOR/EOR applications. The current state-of-the-art will be presented and novel emerging methods will be highlighted. This paper will showcase how the tracer technology has evolved over the years and how it shows great potential as a reservoir monitoring and surveillance tool.