Page 137 - Geochemistry of Oil Field Waters

Chapter 4. INTERPRETATION OF CHEMICAL ANALYSES OF OIL-
FIELD WATERS

Water analyses may be used to identify the source of a water. In the
oilfield one of the prime uses of these analyses is to determine the source of
extraneous water in an oil well, so that casing can be set and cemented to
prevent such water from flooding the oil or gas horizons. In some wells a
leak may develop in the casing or cement, and water analyses are used to
identify the water-bearing horizon so that the leaking area can be repaired.
With the present emphasis on water pollution prevention, it is very impor-
tant to locate the source of a polluting brine, so that remedial action can be
taken.
Comparisons of water-analysis data are tedious and time-consuming; there-
fore, graphical methods are commonly used for positive, rapid identification.
A number of systems have been developed, all of which have some merit.

Calculating probable compounds

The hypothetical combinations of dissolved constituents found in waters
are commonly calculated by combining the positive and negative radicals in
the following order:
calcium bicarbonate
magnesium sulfate
sodium chloride
potassium nitrate

Calcium is combined with bicarbonate, and if more calcium is available
than that consumed by bicarbonate, it is combined with sulfate, chloride,
and nitrate until exhausted. Conversely, any excess bicarbonate is combined
with magnesium, sodium, and potassium until consumed. Other radicals can
and should be added for most petroleum reservoir waters. These include
lithium, strontium, barium, iron, borate, phosphate, bromide, and iodide.
They can be grouped in the appropriate column and then in the calculations
each positive and negative radical is totally combined, the next following
radical is combined until both the cations and anions are exhausted. If the
analysis is correct, the cations and anions will be present in approximately
equivalent amounts.
To calculate the hypothetical combinations, the reacting values of the
positive and negative radicals or ions are calculated as follows: reacting