Page 33 - Geochemistry of Oil Field Waters

22 ANALYSIS OF OILFIELD WATERS

TABLE 3.11
Accuracy data on oilfield brine samples for boron

Sample Concentrations of boron found (mg/l)

Taylor Paluxy
(added 10 mg/l boron) (added 15 mg/l boron)
20.2 35.5
20.2 35.4
20.1 35.2
20.1 35.2
20.3 35.3
20.3 35.2
20.4 35.1
Average 20.2 35.3
20-2 35.3
Percent recovery ] x 100 = 100.0 ] x 100 = 100.3
[ 10.2 + 10 [ 20.2 + 15

The precision and accuracy data are valuable in determining that the
analyst and the method are capable of generating valid data. Once this is
proven, the data can be used to evaluate systematic performance. This can be
done by using spiked samples about 10% of the time to determine that the
accuracy is favorable, and evaluating replicate samples to determine that the
precision is favorable.

Preliminary sample treatment
The following determinations should be made in the field immediately
after sampling:
(1) temperature (in "C), (2) pH, (3) dissolved oxygen, (4) resistivity, (5)
acidity, (6) alkalinity, (7) sulfide, and (8) carbon dioxide.
If possible, the oilfield water sample should be filtered immediately after
sampling in the field. A preferred method-is to use pressure filtration
through a 0.45-micrometer (pm) membrane filter. A liter of filtrate usually is
sufficient and the following determinations can be made on aliquots: (1)
iodide, (2) bromide, (3) chloride, (4) selenium, (5) sulfate, (6) nitrogen, (7)
phosphate, (8) silica, (9) boron, (10) potassium, (11) sodium, and (12)
lithium.
If a field-filtered sample cannot be provided, a laboratory-filtered sample
may be substituted with slightly less confidence in the reported data.