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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.
AlAbbad, Mohammed A. (Saudi Aramco) | Sanni, Modiu L. (Saudi Aramco) | Kokal, Sunil (Saudi Aramco) | Krivokapic, Alexander (Institutt for Energiteknikk) | Dye, Christian (Institutt for Energiteknikk) | Dugstad, Øyvind (Restrack) | Hartvig, Sven K. (Restrack) | Huseby, Olaf K. (Restrack)
Summary The single-well chemical-tracer test (SWCTT) is an in-situ test to measure oil saturation, and has been used extensively to assess the potential for enhanced oil recovery (EOR) or to qualify particular EOR chemicals and methods. An SWCTT requires that a primary tracer be injected and that a secondary tracer be generated from the primary tracer in situ. Typically, a few hundred liters of ester is injected as primary tracer, and the secondary tracer is formed through hydrolysis in the formations. The ester is an oil/water-partitioning tracer, whereas the in-situ-generated alcohol is a water tracer. During production, these tracers separate and the time lag of the ester vs. the alcohol is used to estimate oil saturation in the near-well region. In this paper, we report a field test of a class of new reacting tracers for SWCTTs. In the test, approximately 100 cm of each of the new tracers was injected and used to assess oil saturation. In the test, ethyl acetate (EtAc) was used as a benchmark to verify the new tracers. This paper reviews the design and implementation of the test, highlights operational issues, provides a summary of the analyzed tracer curves, and gives a summary of the interpretation methodology used to find oil saturations from the tracer curves. Briefly summarized, we find the Sor measured by each of the novel tracers to compare with that from a conventional SWCTT. To validate stability and detectability of the tracers, a mass-balance assessment for the new tracers is compared with that of the conventional tracers. A benefit of the new tracers is the small amount needed. Methodological advantages resulting from using small amounts include the possibility to inject a mix of several tracers. Using several tracers with different partitioning coefficients enables probing of different depths of the reservoir. In addition, the robustness of SWCTTs can be increased by using several tracers, with different reaction rates and temperature sensitivity. The field trial also demonstrated that the new tracers have operational advantages. One benefit is the possibility to inject the new tracers as a short pulse of 10 minutes. Other benefits are that the small amounts needed reduce operational hazards and ease logistical handling.
Figure 1.1b – Reservoir evaluation by material balance with measured Sor. A reliable in-situ measurement of Sor simultaneously defines the target for enhanced oil recovery (EOR) and allows estimation of the potential bypassed (mobile) oil in the field. This moveable oil is the target for infill drilling and/or flood sweep efficiency improvements. Because Sor varies greatly with formation type, oil/water properties, and other variables that are not completely understood (e.g., wettability changes caused by water flood practices), Sor measurements range from 10% to 45%. There is no reliable way to predict Sor with acceptable accuracy for most reservoirs.
This paper reviews various applications of partitioning tracers in the petroleum industry. While non-partitioning tracers are routinely used for flow characterization and source identification, partitioning tracers have been under-utilized largely because of a lack of publicity and credibility. In theory, partitioning tracing is potentially applicable whenever a phase boundary, for instance, gas-oil, oil-water, and water-rock, exists. Partitioning between phases will slow down the partitioning tracers in a phenomenon known as chromatographic retardation, from which fluid saturations and surface properties can be deduced. properties can be deduced. Single-well tracer testing to determine residual oil saturation to waterflood S orw constitutes the most common application of partitioning tracers. More than 200 tests have been run since its first invention In 1971. In the meantime, to overcome model inadequacies, new features are continually incorporated into the simulators, making the simulators extremely difficult to run. To get around the simulation problems, a mass balance method was proposed for direct calculation of S orw from the hydrolysis rate. Based on the same principle, an internally calibrated method involving a new class of compounds which can undergo hydrogen exchange with water is being investigated. With recent advances in instrumentation and the introduction of the chromatographic transformation technique. several successful interwell tests, mostly by Esso Resources Canada Limited (ERCL), to determine residual oil saturations in watered-out and gas-saturated reservoirs have been reported. The technique involves direct comparison of the partitioning and non-partitioning tracer profiles with no need for simulation. Under ideal conditions, S or can be determined by layers. In a different approach, full-field simulation of a multi-well test has also been employed to estimate oil saturation distribution. These tests clearly demonstrate that the valuable information on S or justifies the incremental cost of including a partitioning tracer with the non-partitioning tracer in any routine tracing projects. projects. In-situ miscibility measurement by ERCL is another interesting application. Separation of partitioning tracers injected with solvent is a measure of deviation from the first contact miscibility condition FCM. Other unproven techniques including determining trapped gas saturation during a foam flood, relative permeability ratio measurement and direct logging of gamma emitting partitioning tracers through casing to determine S or partitioning tracers through casing to determine S or vertical distribution are also reviewed in the paper.
Inert non-partitioning tracers have been widely used in the petroleum industry to tag the injected fluid as well as to characterize the flow path. The tracer results provide a guideline to pattern balancing and reconfiguring provide a guideline to pattern balancing and reconfiguring for better sweep and more effective utilization of the injected fluid. Tracer response can also be indicative of in-fill drilling potential. In addition to their use in reservoir management, non-partitioning (in particular, radioactive) tracers are also employed routinely in drilling/completion and injection profile logging.