Analysis of radiolabelled thiocyanate tracer in oil field brines

Abstract

Thiocyanate (SCN −) labelled with 14C or 35S is applicable for the tracing of the water flow in water-driven oil production. This work describes two improved methods for increasing the concentration of S 14CN − with sample sizes up to 1000 ml of sampled brine. The radioactivity detection is carried out by low-background liquid scintillation counting using standard-size counting vials of 20 ml. The first method is based on solvent extraction: two extraction/strip steps reduce the volume by the required factor of 100 down to 10 ml. The extraction agent is 0.5M tri-isooctylamine (TiOA) in Solvesso-150 and the stripping agent is 5M NH 3. A chemical yield of 70–75%, a somewhat unfavourable quenching factor and a moderate reproducibility lead to a detection limit L D of 0.04 Bq/1. The second method is based on anion exchange. 1000 ml tracer-containing brine may be loaded onto 6.1 ml of the anion-exchange resin AG1-x8 before significant breakthrough of the tracer. More than 98.4% is absorbed. By elution with 2.8M sodiumperchlorate, more than 99.5% of the absorbed tracer is concentrated in a sharp elution peak of 10 ml. With a total chemical yield of 98%, a lower quenching factor and higher reproducibility than for the solvent extraction method, an L D-value of 0.005 Bq/1 is obtained.