Page 44 - Geochemistry of Oil Field Waters

RESISTIVITY 33

RESISTIVITY. .hm-n.ttrl
Fig. 3.1. Plots of resistivity of aqueous solutions containing various concentrations of
sodium chloride.
should be determined at a known constant temperature. The sample should
be freshly filtered and free of oil. Nonionized silica and other materials in
suspension in an oilfield water can affect the resistivity determination, but in
general such interferences can be ignored. Cell polarization can be trouble-
some with highly mineralized waters and will vary directly with the current
that flows between the electrodes and inversely with the frequency of the
current. High input voltage to the bridge or low cell resistance (highly
mineralized waters) increases the likelihood of polarization. Cell resistance
can be increased by increasing the cell constant.

Reagents. The necessary reagents are standard potassium chloride solutions
of l.OOON, O.lOOON, and 0.01OON (use only certified reagent-grade KC1 that
has been oven-dried to constant weight at 110OC); chromic-sulfuric acid
cleaning solution; platinizing solution (dissolve 3 g of chloroplatinic acid and
0.02 g of lead acetate in 100 ml of water); and a 10% aqueous sulfuric acid
solution.

Equipment. The necessary resistivity measurement equipment includes a
Wheatstone bridge; resistivity cells, either dip or pipet type, with platinum