Page 46 - Geochemistry of Oil Field Waters

SPECIFIC GRAVITY 35

Method of determination

Procedure. To determine the resistivity of the petroleum-associated water,
filter the sample to remove oil and transfer the sample to the cell or cell
container and place it in a water bath. Allow sample sufficient time to adjust
to bat!i temperature, and measure resistance of sample and record the
temperature to nearest 0.1Oc.

Calculation. The resistivity calculation is dependent upon the type of cell
and bridge used, but in general the following formula will apply:

D2 V
R, =-
4LXT
where R, = resistivity of water, am; V = difference in potential between
potential-measuring electrodes, V; I = current flowing through the cell, A;
D = inside diameter of potential-measuring electrodes, m; and L = distance
between potential-measuring electrodes, m.
Because D and L are constant for any one cell and I is held constant for
most waters, these values can be combined into a single constant, K, and the
following simplified equation used :

R, = KV

Calculated resistivity
The resistivity of petroleum-associated waters often is calculated using the
laboratory analysis (Dunlap and Hawthorne, 1951). The concentrations of
the ionic constituents are used in the calculation method.
Dunlap and Hawthorne (1951) caution users of their calculation method
that the sulfate factor 0.50 may give unreliable results if the water contains
appreciable concentrations. of sulfate. If the sulfate concentration exceeds
2,500 mg/l, a factor of 0.40 will give a better calculated resistivity value.

Specific gravity

Specific gravity is the ratio of the weight of a given volume of material to
the weight of an equal volume of some other material used as a standard
(Mellon, 1956, p.306), and pure water is the usual standard for liquids and
solids. Depending upon the accuracy desired, the specific gravity of a
petroleum-associated water can be determined with a pycnometer, specific
gravity balance, or hydrometer. Because any oil in or on the sample will
interfere with the specific gravity determination, the sample should be
filtered.