Radiolabelled metal complexes as possible Interwell water tracers, - The case of [Co(Cn)6]3- and its separation from radiolabelled and simultaneously used SCN-

We have previously reported on experimentally derived properties for the [Co(CN)6]3- complex. This complex is of interest as a basic carrier of the five different radionuclides 56Co, 57Co, 58Co, 60Co and 14C. Here, the first four are gamma-emitters while the latter is a pure beta-emitter. This labelling offers the possibility of five different radiolabelled water tracers. In addition, the relatively high cross-section for thermal neutron capture of stable 59Co (37.2 barn), makes it very sensitive to neutron activation analysis. Thus, the non-labelled molecule may also be used as a water tracer. One prerequisite for its use as a water tracer in petroleum reservoirs is that it survives the reservoir conditions without substantial degradation or reactions. Thermal and chemical stability were measured in various actual chemical environments with various substrates present. The substrates were solid substances which the tracer is likely to contact during field operations. Typically, they are reservoir rocks like sandstone, calcite and clay and other solid substances like steel, corroded steel, grease and scale (BaSO4). For the complex [Co(CN)6]3- high degree of sorption was found on some forms of rust. In addition, thermal degradation seems to accelerate at temperatures higher than 90 deg. C. Dynamic experiments in porous media below 90 deg. C shows that this complex is a near ideal water tracer. However, it shows, as expected, some degree of anion exclusion behaviour due to the three negative charges. In real field situations it may be used in areas where 35SCN- or S14CN- have been applied simultaneously. Since the thiocyanate tracers have to be analysed by liquid scintillation counting (LSC) the presence of [Co(CN)6]3-, regardless of the radiolabel, will disturb the counting of the thiocyanate. For gamma-emitting labels the analysis may be performed by gamma spectrometry, but if the label on the [Co(CN)6]3- is 14C, there will be a mutual disturbance of the two radiolabelled complexes. Therefore, there is a need to develop radiochemical procedures to separate cobalthexacyanide and thiocyanate on tracer level in produced water. Results from attempts on this separation are reported here. Objectives. The aim in this study was to develop a method for quantitative analysis using 1000 mL seawater (as a mimic of produced water) containing a mixture of SCN- and [Co(CN)6]3- by enrichment procedures and preconcentration into samples small enough to be measured by LSC. The sample volume for LSC should not exceed 8-10 ml since this is normally the maximum water sample volume dissolved by modern scintillation cocktails (12 mL) for ordinary 22 mL scintillation vials.