For relatively pure water samples (type A), the procedure is as follows:

(1) Filter the sample through a lipophobic filter to remove any traces of dispersed oil droplets and any suspended particles.

(2) The oilfield samples usually contain high salt contents and chemical load. This raises the quenching effect profoundly during sample measurement, which introduces inaccuracy in the results. Though correction factors for chemical quenching can be applied to rectify the quenching effect, it is always desirable to avoid such complicated processes. For this reason, samples are distilled before measurement. All necessary glassware should be thoroughly washed and dried to avoid contamination. Therefore, transfer preferably 250 mL of the water filtrate into a 500 mL round flask.

(3) Assemble the distillation equipment as shown in Fig. 90.

(4) Distil with gentle boiling and collect distilled water in the sidearm of the glass equipment.

(5) Discard the two first fractions of the distilled water because these may contain volatile hydrocarbon components which have been dissolved in the original water sample and which may quench the scintillation process.

(6) Collect the third 8-10 mL sample for analysis. If, for any reason, the water sample cannot be analysed the same day that it is purified and distilled, the samples (vials) should be stored in the refrigerator until they are prepared for counting.

(7) Mix with 10-12 mL of an appropriate scintillation cocktail which is able to accommodate about 50% of water without a detrimental decrease in counting efficiency, for instance Ultima Gold.

(8) Store the counting sample for at least 1 h in the dark in order for any chemiluminescense or phosphorescence to die out before starting the counting sequence.

(9) Three background samples (‘dead’ water) and three standard samples with known activity on a given date are prepared and counted along with the produced water samples.

(10) If variable quenching is suspected in the produced water samples, a correction has to be made for the corresponding varyiable counting efficiency. Most modern liquid scintillation counting equipment provides an instrumental quench correction method. In case this is not the case, the quench correction can most easily be performed by the internal standard method. After having counted the samples, a known activity of a non­quenching tritium compound (standard solution) is added to each vial and counted again.

This gives the counting efficiency directly according to the formula:

where

Rx is the background corrected counting rate of the produced water sample;

Rx+s is the background corrected counting rate of the sample after addition of a known quantity of standard;

Ds is the disintegration rate of the added quantity of standard (Bq).

The activity of tritium in the original sample, Dx (Bq), can then be calculated by:

and the activity concentration, Dxc in Bq/L, in the produced water by:

where Vx is the volume of the water in the counting sample in millilitres.

For samples with a visible layer of oil (type B), remove the oil layer on top of the water with a pipette, then carry out the same procedure as outlined in point 1 above.

For the most difficult samples consisting largely of an oil-water emulsion (type C), the first step is to break the emulsion in order to sample any water dispersed in the oil. There are a variety of commercial emulsion breakers available (a chemical supplier can assist).

Then carry out the same procedure as for point 1 above, with the modification which takes into consideration the volume of water available after emulsion breaking may be limited, i. e. a few millilitres only. In that case, the distillation process may be omitted or performed in miniaturized equipment.