Page 62 - Geochemistry of Oil Field Waters

50 ANALYSIS OF OILFIELD WATERS

added when the yellow color of free iodine has been almost eliminated by
the sodium thiosulfate titration, and the titration should be continued until
one drop changes the solution from a light blue to colorless. (Subsequent
blue recoloration should be disregarded.)
If no hydrogen sulfide or other interfering substances are present, the first
six steps of the determination may be eliminated, using only the part of the
procedure starting with the addition of the alkaline iodide solution.

Calculation. The dissolved oxygen content of the water is determined by the
following equations:
x-( Y-1)
v= 200
200 w
u=-
V
where U = dissolved oxygen content, ppm; V = volume of sample titrated,
ml; W = volume of 0.025N sodium thiosulfate required, ml; X = volume of
sample bottle, ml; Y = total volume of all reagents added, ml; and 1 = the 1
ml of acid added, which does not change the effective oxygen-tested volume
of the sample because it is added after all the oxygen has been absorbed.
The factor used to take into account the volume of reagents added may
involve a slight error, because it is based on the assumption that the reagents
contain no dissolved oxygen.

Carbon dioxide
Petroleum-associated waters containing carbon dioxide and bicarbonate or
carbonate will contain a weak acid H2C03 or its salt, which buffers the
solution. This combination controls the pH of waters in the range of about
pH 4.5-8.0. Such buffering is caused by the presence of slightly dissociated
acids or bases, and when H+ or OH- ions are added they first convert the
undissociated acid or base to its salt or vice versa.
Loss of carbon dioxide will disturb the carbon dioxide-bicarbonate-
carbonate buffer systems. For example, the pH probably will change and
precipitation of calcium carbonate or other compounds may occur. An in-
crease in carbon dioxide will shift the. carbon dioxide-carbonate-
bicarbonate equilibria, allowing more material such as calcite to go into
solution.
Bacterial reduction of sulfate can cause the amount of dissolved carbon
dioxide and hydrogen sulfide in petroleum-associated waters to be quite
high. Several hundred milligrams per liter of C02 can be present in such
waters. Knowledge of the amount of carbon dioxide in solution is useful in
carbonate equilibria studies (Garrels and Christ, 1965) and in water com-
patibility studies (Watkins, 1954).