Radioactive water tracers

Tracers are used in well to well tests to gather data about the movement and saturation of fluids in the subsurface. Radioactive tracers can be used to gather data about water or gas. This article discusses some of the commonly used radioactive water tracers.

Application

In most field studies, the tracer is expected to behave exactly as the water it is going to trace. Very few compounds will behave as passive tracers in all situations, but near-passive tracers will, in many applications, work satisfactorily. If the objective is to measure fluid communication exclusively, a near-passive tracer may be as good as a true passive tracer. Table 1 lists the most frequently applied radioactive tracers.

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  Table 1

The compound that best fulfills the passive tracer requirements is tritiated water (HTO). The passive water tracer mimics all movements and interactions that the water molecules do in the traced water volume. For instance, HTO can track the free movement in and out of dead-end pores because it is insensitive to coulombic forces set up by negatively charged rock surfaces. Other interactions can exchange with connate water in the rock pores or exchange with crystal water molecules. Thus, HTO sometimes seems to lag behind the injection water breakthrough as measured, for instance, by a salt balance (ionic logging). This has sometimes been interpreted that HTO is unstable at reservoir conditions and that it may be subject to isotope-exchange reactions of tritium with hydrogen in neighboring hydrogen-containing compounds, some of which are stationary. Isotopic exchange, however, is expected to be negligible.

Other noncharged tracers are methanol, CH₂TOH, and the other light alcohols. These tracers will behave qualitatively similar to HTO with respect to transport but have different interactions. Other organic molecules may be applied as radioactive water tracers and can be labeled either with tritium or with C. Larger alcohols, however, may have a partition coefficient that may cause a considerable retention.

Zemel measured partition coefficients of some alcohols. Over a limited range, the effect of temperature on the partition coefficient, K, can be represented by a semiempirical equation:

....................(1)

where T is the temperature in Kelvin, A is a constant related to the enthalpy, and B is a constant related to the entropy change. Table 2 gives B and A values for some alcohols.

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  Table 2

Anions are the more applicable electrically charged tracers; however, in laboratory experiments, clear evidence of ion exclusion can be seen (i.e., negatively charged species tend to be repelled from the negatively charged rock surfaces). As a result, the tracers tend to flow in the middle of the fluid-conducting pores and will not easily enter into dead-end pores or through narrow pore throats, which results in a smaller available PV for anions than for noncharged species. The production profile differs in reproducible ways from that of HTO.

Anionic tracers are represented here by thiocyanate or S¹⁴CN^-. Fig. 1 gives a typical production profile from flooding experiments. This profile is compared with the production profile of the simultaneously injected HTO. It is evident from the curve that the breakthrough of HTO occurs before that of S¹⁴CN^- and that the tail of the HTO profile is more pronounced. This profile difference is qualitatively the same for all near-passive anionic water tracers. The retention volume may be represented by the peak maximum value or the mass middle point (first moment, μ₁) for nonsymmetrical profiles. These values are found best by fitting the profile with an analytical function consisting of polynomials.

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  Fig. 1 – Production curve of S¹⁴CN compared with the production curve of HTO in a dynamic flooding laboratory test (carbonate rock) (after Bjørnstad and Maggio).

³⁵SCN is applicable only to small reservoirs because the half-life of ³⁵S is only 87 days. ³⁶Cl^- has shown to be an excellent tracer. There are no possibilities for thermal degradation, and it follows the water closely. The ³⁶Cl^- is a long-lived nuclide (3×10⁵ years), and the detection method is atomic mass spectroscopy rather than radiation measurements. The disadvantage is that the analysis demands very sophisticated equipment and is relatively time consuming.

For mono-valent anions, the retention factors (see Eq. 2) are in the range of 0 to -0.03, which means that such tracers pass faster through the reservoir rock than the water itself (represented by HTO). A compound such as ³⁵SO₄^2- may be applied in some very specific cases but should be avoided normally because of absorption.

....................(2)

Some anionic tracers may show complex behavior. Radioactive iodine (¹²⁵I^- and ¹³¹I^-) breaks through before water but has a substantially longer tail than HTO. Both a reversible sorption and ion exclusion seem to play a role here. ¹²⁵I^- and ¹³¹I^- have half-lives of 60 and 8 days, respectively, which makes the compounds less attractive as tracers in large reservoir segments.

Cationic tracers are, in general, not applicable; however, experiments have qualified ²²Na⁺ as an applicable water tracer in highly saline (total dissolved solids concentration > seawater salinity) waters. In such waters, the nonradioactive sodium will operate as a molecular carrier for the tracer molecule. Retention factor has been measured in the range of 0.07 (see Eq. 2 ) at reservoir conditions in carbonate rock (chalk). Accordingly, the tracer is delayed somewhat by sorption and ion exchange to reservoir rock but in a reversible manner. Wood reported the use of ¹³⁴Cs, ¹³⁷Cs, ⁵⁷Co, and ⁶⁰Co cations as tracers. The same cations also were injected as ethylene diamine tetra-acetic acid (EDTA) complex in a carbonate reservoir. The EDTA complexes were recovered completely in a 3-day push-and-pull test. For the cations, only the Cs⁺ were produced while the Co³⁺ never appeared in the producer; however, the Cs⁺ cations generally cannot be used. It will adsorb strongly on clay-containing rock. ⁵⁶Co(CN)₆³ is a stable complex that has been tried as tracer. Not all trials have been successful, and the compound is not normally applied. Especially at temperatures greater than 90°C, they should be avoided. The complex can be labeled with several isotopes of cobalt (⁵⁶Co, ⁵⁷Co, ⁵⁸Co, ⁶⁰Co) in addition to ¹⁴C.

Nomenclature

A	=	constant related to the enthalpy
B	=	constant related to the entropy change
K	=	partition coefficient
VS	=	retention volume of standard reference tracer, L3
VT	=	retention volume of tracer candidate, L3
T	=	temperature, T
β	=	retention factor

Noteworthy papers in OnePetro

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External links

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See also

Chemical water tracers

Radioactive gas tracers

Well to well tracer tests

PEH:Well-To-Well_Tracer_Tests