Page 129 - Geochemistry of Oil Field Waters

OTHER METHODS 117

magnesium. The excess of equivalents per million of anions over cations is
assumed to be sodium plus potassium, and this practice includes all the
errors of the analysis plus the undetermined ions in the combined sodium
plus potassium value. The ions are converted to milliequivalents per liter
(me/l) by dividing each ion concentration (mg/l) by its milliequivalent
weight (mglme) to give the milliequivalents per liter for each ion determined.
After adding the milliequivalents per liter for both the anions and the
cations, the difference is multiplied by the milliequivalent weight of sodium
to give the calculated milligrams per liter of sodium.
Procedure. The calculation method is demonstrated as follows:

Anions: chloride (50,000 mg/1)/(35.5 mg/me) = 1,410 me/l
sulfate (1,290 mg/1)/(48.0 mg/me) = 27 me/l
bicarbonate (204 mg/1)/(61.0 mg/me) = 3 me/l
Total anions 1,440 me/l

Cations: calcium (5,900 mg/1)/(20.0 mg/me) = 295 me/l
magnesium (2,000 mg/1)/(12.1 mg/me) = 164 me/l
Total cations 459 me/l

Determination of sodium:
(1,440 - 459 me/l) x 23.0 mg/me = 22,600 mg/l

Dissolved solids

The dissolved solids determination can be used to estimate the accuracy of
the resistivity determination. The specific gravity determination, and the
evaporation method can be used to double check the calculated total dis-
solved solids. Theoretically, if all the dissolved solids are accurately deter-
mined, their sum will equal the weight of the residue left after evaporation
of the water. The dissolved solids include all the solid material in solution
which is ionized, or which is not ionized but does not include suspended
material, colloids, or gases.
The residue method involves evaporating a filtered sample to dryness fol-
lowed by drying the residue in an oven at 180°C for 1 hour. The cooled
residue is weighed and the total dissolved solids are calculated; e.g., if 100 ml
of brine is evaporated and the residue weighs 3.0 g, then the dissolved solids
equal 30,000 mg/l. The evaporation method is subject to errors when
hygroscopic material such as calcium chloride is in the water, as is usually
the case in oilfield waters.
The calculation method simply involves adding the sum of all the analyzed
constituents as follows: