Page 175 - Geochemistry of Oil Field Waters

162 INORGANIC CONSTITUENTS AND PHYSICAL PROPERTIES

very soluble; therefore, chloride is usually not removed from solution except
during freezing or evaporation processes and in hyperfiltration, as water
moves through some types of clay beds (White, 1965).
Shales, sandstones, and carbonates contain about 180, 10, and 150 ppm,
respectively, of chloride. Sea water contains about 19,000 mg/l of chloride,
the principal anion in the sea. The chloride content of the hydrosphere is
much greater than can be accounted for by weathering of rocks, and it has
been postulated that the primordial atmosphere may have been rich in
chlorine compounds. The volcanic emission of chlorine gases appears a more
plausible explanation, however.
Oilfield brines usually contain relatively high concentrations of chloride;
in some brines the concentration may be 200,000 mg/l or more. Chloride
usually is the predominant anion in oilfield brines. Table 5.1 illustrates how
its concentration can increase in an evaporite-associated brine. Evaporation
probably is the only geochemical process which appreciably affects the
chloride content of the seas.

Bromine

Bromine is a member of the VII A group of elements and it behaves
somewhat similarly to chlorine. The crust of the earth contains about 0.0005
wt.% of bromine (Fleischer, 1962). It usually occurs as the ion bromide Br-,
and it does not form its own minerals when sea water evaporates (Valyashko,
1956). It forms an isomorphous admixture with chloride in the solid phases.
The order of crystallization (see Table 5.1) is halite (NaCl), sylvite (KCl),
carnallite (MgC12 -KC1*6H2 0), and/or kainite (MgS04 *KC1=3H2 0), and at
the eutectic point, bischofite (MgCl? -6H2 0). Each of these chlorides
entrains bromide in the solid phase. This distribution accounts for the rela-
tive enrichment of bromide in the liquid phase because with each crystalliza-
tion more bromide is left in solution than is entrained in the solid phase.
Mun and Bazilevich (1962) reported that, in fresh-water lakes, bromide
accumulates in the muds, that its concentration is proportional to the
organic-matter concentration in the sediments, and that it is not influenced
by the pelitic fraction. In muds of salt lakes, the higher the bromide concen-
trations in the brine, the higher it is in the muds. In general, the bromide
content in the pore solutions increased with depth, but the bromide content
in muds decreased with depth, owing to more complete decomposition of
organic bromine compounds.
Bromide is two to three times more concentrated in carnallite than in
sylvite and five to ten times more than in halite (Myagkov and Burmistrov,
1964). Apparently, concentration and dilution are responsible for the com-
plex distribution of bromide in rocks of a carnallite zone. The determining
factor in the replacement of chloride by bromide is the mineral composition
rather than the bromide concentration in the brine.