Page 163 - Geochemistry of Oil Field Waters

150 INORGANIC CONSTITUENTS AND PHYSICAL PROPERTIES

Iron, cobalt, and nickel possess atomic radii that differ only about 2% or
less, so that the crystal chemistry of the three are related. The divalent ions
of nickel, magnesium, cobalt, and iron have similar ionic radii; consequently,
their chemistries in the sequence of isomorphous crystallization of mixtures
are similar. The trivalent ions of iron and cobalt are similar in size, but the
high oxidation potential of cobalt prevents much replacement (Goldschmidt,
1958).
The solubility of iron compounds in ground waters is a function of the
type of iron compound involved, the amounts and types of other ions in
solution, the pH, and the Eh. According to Larson and King (1954), 100
ppm of ferrous iron can stay in solution at pH 8 and pH 7; the theoretical
maximum is about 10,000 ppm. The effects of many other ions, plus tem-
perature and pressure differentials, such as those common to oilfield waters,
have not been thoroughly studied. When a ground water in which ferrous
iron is dissolved contacts the atmosphere, the following reaction can occur:

2Fe2+ + 4HCO3- + H2 0 + 1/2 02 + 2Fe(OH), + 4C02

Sandstone contains iron oxide, iron carbonate, and iron hydroxide, and
shales and carbonate rocks contain oxides, carbonates, and sulfides of iron.
Oilfield waters with characteristic low redox potentials dissolve some iron
from the surrounding rock. The iron occurs in such waters at two levels of
oxidation, ferrous or ferric.
Knowledge of the amount and type of iron compounds in oilfield waters
is used to estimate the amount of corrosion that is occurring in the produc-
tion system, and to determine the type of treatment required if the water is
to be used for waterflooding. This knowledge also enables determination of
the Eh of the in situ water, because the Eh can be calculated from the Fe+2
and Fe+ values.
Shales, sandstones, and carbonates contain about 47,200, 9,800, and
3,800 ppm, respectively, of iron (Mason, 1966). Sea water contains about
0.01 mg/l, and subsurface brines contain from traces to over 1,000 mg/l of
iron.

Copper
Copper is a member of the VIII group of elements, and it is character-
istically thiophile; the largest concentrations of it are found in various sulfur
compounds. The earth’s crust contains about 0.01 wt.% of copper (Fleischer,
1962). Its compounds are dissolved easily during weathering, if the pH of the
solution is less than 4.5. Many of the water-soluble copper compounds are
salts of organic acids such as acetic, citric, and naphthenic. Much of the
copper that is dissolved is precipitated afterward as sulfide. Traces of copper
remain in the oceans, but its content is kept low because of the adsorption
on, or combination with, marine organisms. Miholic (1947) presented an age