Example 1
Isotope and trace element fingerprinting is needed because of the inherent problems in obtaining representative formation-water samples from wells drilled using drilling fluids. During drilling, drilling fluids are circulated into the open hole via the drill-pipe and drill-bit. Thus, during circulation the drilling fluid invades permeable zones near the wellbore (Figure 1a). When a Drill-Stem-Test is conducted, the first fluid recovered/sampled is typically drilling fluid – this fluid may not be representative of the true formation-fluid in the zone of interest (Figure 1b). Only over time, is the drilling fluid displaced with true formation-fluid (Figure 1c). The problem is... how to know when sufficient time has elapsed to obtain a "true" uncontaminated sample of formation fluid.

Figure 1. How fluid samples are contaminated during drilling.

Example 2
Isotope fingerprinting can work when other chemical methods fail, because the isotopes of the hydrogen (H) and oxygen (O) are not affected by the addition of chemicals used to saturate drilling fluids. Thus, in many cases, isotope fingerprinting techniques can differentiate formation-fluids from drilling-fluids, when standard chemical methods can not.

Figure 2. Isotopic profiles from SE-Saskatchewan. The Total Dissolved Solids (TDS, g/L) for all the aquifers in the section is approximately 300 g/L. Stable isotopes of Oxygen and Hydrogen can be used to differentiate the different aquifers.

Drilling fluids are commonly made up using near surface waters. These near surface waters have characteristic isotopic compositions that are typically very low (e.g., -10 delta 18O per mil or lower). Drilling fluids usually retain their primary O and H isotopic compositions, even with in spite of the chemical additives. Thus, for example, in Figure 2, drilling fluids with a signature of –10 % are easy to differentiate from the various formation-waters in the section.

The advantages of chemical and isotope fingerprinting outlined above also render the latter especially well suited to identify the sources of co-produced water (e.g., in-zone as opposed to out-of-zone).

Example 3

Figure 3 – Environmental application of dissolved Bromine as a trace element. Total Dissolved Solids (TDS) plotted versus the chlorine to bromine ratio (Cl/Br) illustrates how the bromine content of deep formation waters is relatively high compared to surface saline lakes.

Bromine is extremely soluble in water, and is a highly-conservative tracer. In contrast to most major ions Br (and I, Cl) retains its primary compositions as these do not participate in water-rock reactions. The conservative nature of Br, I and Cl is of great importance in establishing the origin of co-produced waters. Due to low contents in surface waters the ratios between these tracers do not change during dilution rendering them very efficient in tracing shallow aquifer and surface water contamination with production brines.

Publications

Rostron, B.J., and C. Holmden, 2003. Regional variations in oxygen isotopic compositions in the Yeoman and Duperow aquifers, Williston Basin (Canada-USA). Journal of Geochemical Exploration, v. 78-79, p. 337-341.
Rostron, B.J., and C. Holmden, 2000. Fingerprinting formation-waters using stable isotopes, Midale Area, Williston Basin, Canada. Journal of Geochemical Exploration, v. 69-70, p. 219-223.
Rostron, B.J., C. Holmden, and L.K. Kreis, 1998. Hydrogen and oxygen isotope compositions of Cambrian to Devonian formation waters, Midale area, Saskatchewan. In: Proceedings of the 8th International Williston Basin Symposium, Regina, Saskatchewan, October 18-21, p. 267-273.
Benn, A.A., and B.J. Rostron, 1998. Regional hydrochemistry of Cambrian to Devonian aquifers in the Williston basin, Canada-USA. In: Proceedings of the 8th International Williston Basin Symposium, Regina, Saskatchewan, October 18-21, p. 238-246. [this reference could be deleted to save space]