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Chemical water tracers
Tracers are used in well to well tests to gather data about the movement and saturation of fluids and hydrocarbons in the subsurface. Chemical tracers can be used to gather data about water or gas. This article discusses some of the commonly used chemical water tracers for well to well tests. Chemical tracers can also be used in a single well configuration to estimate residual oil saturation or connate water saturation.
Application of several nonradioactive chemical tracers has been reported in the literature. Table 1 lists the most frequently used chemical water tracers. The most-applied nonradioactive anion is the thiocyanate anion. It has a low natural background in the reservoir, and a detection limit in the range of 1 μg/L (1 ppb) can be obtained by electrochemical detection after separation on a high-pressure liquid chromatograph. In the reservoir, this tracer will behave as the radioactive labeled S14CN- or as the 35SCN-.
Nitrate (NO3-), bromide (Br-), iodide (I-), and hydrogen borate (HBO3-) also have been applied as tracers.  These can be analyzed by ion chromatography or by high-pressure liquid chromatograph. A minimum detection limit in the range of 25 to 1 ppb is reported for the different compounds. This detection limit will not be sufficient in many situations. It is also a problem for several of these compounds that the background concentration normally will be too high. Nitrate is the cheapest of the tracers, but its background may be in the range of 500 ppb, which means that an excessive amount of tracer is needed. Examples of organic molecules are:
- Rhodamine B
The most-applied chemical water tracers are fluorinated benzoic acids. A large suite of mono-, di- and tri-fluorinated benzoic acids have been qualified as tracers. Because their thermal stability is variable, the compounds must be selected with care, especially for high-temperature reservoirs. Trifluoromethylbenzoic acids also may work as tracers; however, these compounds interact with the oil phase to a larger degree and retention of these compounds is observed.
The partitioning of acids to the oil phase is generally low. The partitioning will depend on the pH and at lower pH, when a larger portion of the compounds is in the undissociated form, higher partitioning can be expected. Fig. 1 shows laboratory test results in which the 4-FBA is compared with tritiated water in a packed-column flooding test. The two tracers are injected simultaneously and the residence time distribution measured. Both tracers are produced at the same time, and only a marginal retention of the 4-FBA is recorded. This experiment is performed by an oil saturation of approximately 20%. In most practical applications, this retention can be neglected.
Fig. 1 – Response curve of 4-FBA compared with the response curve of tritiated water in a laboratory flood (after Bjørnstad and Maggio).
There is active research to identify new tracers. Among the new tracers are deuterated compounds. Deuterated fatty acids probably will work well as tracers but, because of production costs, their applicability is limited. A new group of potential tracers are shorter DNA fragments. These compounds have two major advantages:
- An extremely low detection limit
- They exist in an almost unlimited number of distinguishable variations
It is, however, uncertain if it is possible to produce modifications that can be qualified with respect to flow behavior and stability.
- Zemel, B. 1995. Tracers in the Oil Field, 43. Amsterdam, The Netherlands: Developments in Petroleum Science, Elsevier Science.
- Bjørnstad, T. and Maggio, G.M. 2002. Radiotracer Applications in Industrial Processing, Oil & Geothermal Reservoirs. Intl. Atomic Energy Agency, Vienna, Austria
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