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Chemical gas 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 gas tracers for well to well tests. Chemical tracers can also be used in a single well configuration to estimate residual gas saturation.
As early as 1946, Frost reported the use of helium as a tracer under gas injection. The background of the noble gases in the reservoir is, however, generally too high, which makes nonradioactive noble gases unattractive as tracers. These gases can be applied only when the dilution volume is very small. Table 1 lists the most frequently applied chemical gas tracers.
One group of tracers that has found wide application and has become the most widely applied of the chemical gas tracers today is the perfluorocarbon (PFC) group of molecules. PFCs have excellent tracer properties such as:
- High stability
- Chemical inertness
- High detectability
The most frequently applied compounds are :
- Perfluorodimethylcyclobutane (PDMCB)
- Perfluoromethylcyclopentane (PMCP)
- Perfluoromethylcyclohexane (PMCH), and 1,2- and 1,3-perfluorodimethylcyclohexane (1,2-/1,3-PDMCH)
Table 2 lists some of their properties. PFCs are liquids with a density of approximately 1.8 g/mL at standard conditions; therefore, they can be injected into the reservoir with high-pressure liquid pumps. 
One technique for analyzing PFCs is gas chromatography (GC) in combination with an electron-capture detector. The electron-capture detector is extremely sensitive to perfluorinated hydrocarbons and especially the cyclic compounds.
Senum reported a method to analyze the PFC content in a hydrocarbon gas from a production stream. The gas contained in pressure bombs is flushed through a capillary absorption tube sampler (CATS) filled with activated carbon that absorbs the PFCs. The PFC-containing pellets are desorbed thermally, and the gas is directed through a combustion system composed of a precolumn, catalysts, and traps to remove the hydrocarbons before the PFCs enter the main separation column on the GC/electron-capture detector system to determine the amount of each tracer.
Another applicable technique is GC in combination with mass spectroscopy.  This technique distinguishes tracers from other compounds not only by chromatographic separation but also by molecular mass. This reduces background noise, which is essential to obtaining a low limit of detection. The technique employing CATS has made the collection and logistics of gas samples much more efficient. Shipment and handling of high-pressure sampling cylinders are expensive and complicated. Sampling on CATS made a large improvement, making gas tracing on remote locations easier to operate. This technique allows analysis of tracer quantities in low 10−13 L/L concentrations. Because of the very sensitive analytical techniques, the amount of tracer needed even in large reservoirs is only a few kilograms. The CATS technology is applicable only to PFC tracers and has not been developed for the other gas tracers.
In addition to the PFC tracers, SF6 is a frequently applied chemical tracer. SF6 can be measured in very low concentration on electron-capture and mass-spectroscopy detectors. The tracer is stable at reservoir conditions, and it is relatively cheap. This compound is a gas at standard conditions and must be injected with gas booster pumps. The PFCs and SF6 have a very high global-warming potential; therefore, finding an alternative tracer is needed.
Another group of compounds that have good tracer properties is freons; however, because of their ozone-depleting character, these compounds are rarely applied. Hydrocarbon gases, in which hydrogen is substituted with the deuterium isotope, will work as a tracer, but because of high production costs, these compounds are not in common use.
- Frost, E.M. 1946. Helium Tracer Studies in the Elk Hills, California, Field. BM-R1-3897, US Bureau of Mines, (June 1946).
- Dugstad, Ø. 1992. An Experimental Study of Tracers for Labelling of Injection Gas in Oil Reservoirs. PhD dissertation, U. of Bergen, Norway.
- Senum, G.I., Cote, E.A., D'Ottavio, T.W. et al. 1989. Hydrocarbon Precombusting Catalyst Survey and Optimization for Perfluorocarbon Tracer Analysis in Subsurface Tracer Applications. BNL-42769, Brookhaven Natl. Laboratories Report Series, (May 1989), 34.
- Galdiga, C.U. and Greibrokk, T. 2000. Ultra trace detection of perfluorocarbon tracers in reservoir gases by adsorption/thermal desorption in combination with NICI-GC/MS. Fresenius. J. Anal. Chem. 367 (1): 43–50.
- Galdiga, C.U. et al. 2001. Experience With the Perfluorocarbon Tracer Analysis by GC/NICI-MS in Combination With Thermal Desorption. Tracing and Tracing Methods, Nancy, France, 2001, Recent Progress en Génie des Procèdes (May 2001) 15 (79).
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