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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.
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.
This paper presents the first results of an ongoing experimental work at IFE for developing new tracers to be used for estimation of remaining oil saturation. The methodology is based on the chromatographic theory, which states that various components flowing through porous media will be delayed according to their partition coefficient to the immobile phase. Two experiments, one with gas tracers and the other with water tracers, are presented here, demonstrating the applicability of the methodology in the laboratory scale. Moreover, a numerical model (ITRC-SIM module) developed at IFE especially for simulating the tracer flow in hydrocarbon reservoirs is validated against the experimental data and used to demonstrate the use of tracers in small and field scale generic cases.
Tracers are inert chemical compounds widely used to improve the description of hydrocarbon and water reservoirs. Water tracers are divided into two categories: the ideal (or passive) tracers, which always follow the aqueous phase, and the partitioning tracers, which partition between the aqueous and the oil phase. Ideal tracers are used mainly in interwell tracer tests in order to obtain information about the flow patterns and to establish the velocity field and the well continuity. The tracer data can also be used to upgrade a reservoir model (1). Partitioning tracers, which are the subject of this paper, are used in hydrocarbon reservoirs in both single well tracer tests and inter-well tracer tests to estimate the residual oil saturation usually prior to enhanced oil recovery applications (2). The gas tracers are always partitioning between the oil and gas phases.
The most commonly applied water tracers are tritiated water (HTO), thiocyanade and some mono fluoro benzoic acids. The most commonly used gas tracers are tritiated methane, SF6 and several perfluorocarbons with low partitioning to oil phase.
In this paper, the first results of an experimental evaluation of new gas and water partitioning tracers for the estimation of remaining oil saturation in hydrocarbon reservoirs are presented. A numerical model, (ITRC-SIM) built with advanced physical and numerical features, has been developed and validated against the experimental data. Finally, the application of partitioning tracers in oil reservoirs is discussed in small and field scale generic cases.
Partitioning Tracers for Estimation of Residual Oil Saturation
When ideal and partitioning water tracers are injected simultaneously in an oil reservoir, the ideal tracers will flow only in the water phase adopting the velocity of this phase. On the other hand, the molecules of the partitioning tracers are moving back and forth between the water and oil phase. Consequently the partitioning tracer molecules are flowing with the water velocity when they are in the water phase and the oil velocity when they are in the oil phase. As a result of this chromatographic effect there is a net time delay on the arrival of the partitioning tracers at the production well compared with the arrival time of the ideal tracers. This time delay depends on the partitioning coefficients (which can be measured in the laboratory) and the oil saturation. The same methodology can be applied in a gas-oil system with the use of two gas tracers with different partitioning coefficients.
Abstract This paper discusses the applications of gas and water tracers in the Central Fault Block (CFB) of the Snorre Field in the North Sea. An extensive tracer program was initiated in 1993 to improve the understanding of the flow dynamics in the field. The CFB of Snorre has been produced by WAG injection since 1994. The tracer program has involved 3 injectors and 4 producers. Tracers applied were perfluorinated hydrocarbons (PFCs) and SF6 as gas tracers. As water tracers tritiated water (HTO), chemical SCN- and radioactive S14CN- have been applied in addition to the new tracer 4-fluoro benzoic acid (4-FBA). Results from this integrated tracer study have improved the understanding of fluid flow and WAG injection efficiency in the reservoir. The tracer program will be continued and will also be expanded to other fault blocks.