Page 43 - Geochemistry of Oil Field Waters

32 ANALYSIS OF OILFIELD WATERS

values below this level and the results multiplied by the correct dilution
factor.
To obtain maximum accuracy and precision the following precautions
should be observed:
(a) Protect the Lucite standard from scratches, nicks, and fingerprints.
(b) While calibrating the instrument, use a constant orientation of the
Lucite standard.
(c) Use a homogeneous sample in the sample cuvette; do not take readings
until finely dispersed bubbles have disappeared.
(d) Dilute samples containing. excess tubidity to some value below 40
JCU; take reading, and multiply results by correct dilution factor.

Suspended solids analysis

To determine the composition of the suspended solids they can be
removed by filtration using a 0.45-pm membrane or less porous filter. The
filtered solids can then be subjected to chemical analysis. To determine the
exact composition of the solids may require the filtration of a large sample
in order to procure enough solid material. The heavy-metal content can be
determined by subjecting a portion of the sample to an emission spectro-
metric analysis; X-ray diffraction can be used to determine which, if any,
clays are present; extraction with organic solvents followed by infrared mass
spectrometric, chromatographic, and gas chromatographic analysis will give
an indication of organic compounds present; thermogravimetric analysis will
provide clues; wet chemical analysis can be used to determine many of the
anions; and X-ray fluorescence can be used to determine some of the anions.

Resistivity

The resistivity of petroleum-associated waters is used in electric log inter-
pretations (Wyllie, 1963), and for such use the values must be adjusted to
the formation temperature. This can be done by referring to curves such as
those shown in Fig.3.1, which gives resistivity values for sodium chloride
solutions. The resistivity of a formation water will not be exactly the same as
that of a pure sodium chloride solution of equal dissolved solid (DS) con-
tent, but for practical purposes the assumption that the resistivities are
approximately equal is satisfactory.
It is possible to calculate the resistivity from water-mineral analysis by
using methods such as those developed by Dunlap and Hawthorne (1951) or
Jones (1944). The calculated values are less accurate and usually lower than
the directly measured resistivities. The direct-measurement method is essen-
tially the electrical resistance of a cube of oilfield water. In well-logging
practice, the edge of the cube considered is 1 m in length. Therefore, resis-
tivity of an oilfield water is expressed in ohm-meters (am).
Temperature has a profound effect on resistivity; therefore, all resistivities