Page 256 - Geochemistry of Oil Field Waters

24 0 ORIGIN OF OILFIELD WATERS

in Gulf Coast shales (Burst, 1969). As a result of this transformation, the
montmorillonite-type minerals lose interlayer water. Laboratory experiments
at elevated temperatures and pressures indicate that montmorillonite loses its
interlayer water and transforms into illite in the presence of potassium-
enriched water (Khitarov and Pugin, 1966). The structural variations of the
expandable minerals in clays also are apparently influenced by the potassium
content of the associated waters. This indicates that oilfield waters tend to
become depleted in potassium content where this reaction occurs.
Reactions between brines and minerals to form silicates that account for
the depletion of dissolved alkali metals are:

3A1, Si2 O5 (OH), + 2K+ * 2KA13 Si, Ol0 (OH), + 3H2 0 + 2H+
KA13Si030,0(OH)2 + 6Si02 + 2K+ + 3KA1Si308 + 2H+
A12 Si205 (OH), + 4sio2 + 2Na+ =+ 2NaA1Si308 + H2 0 + 2H+

These reactions account not only for the depletion of potassium or
sodium, but also for a decrease in pH because of the release of hydrogen
ions. The decrease in pH enables the water to dissolve metallic metals, to
convert bicarbonate to carbon dioxide, or to convert bisulfide to sulfide. The
Smackover brines often contain relatively high concentrations of sulfide.
Several investigators have attempted to determine what mechanism is
responsible for the increased concentration of calcium and depletion of mag-
nesium relative to sea water in many subsurface brines. Chave (1960) and
Von Engelhardt (1960) compared ocean water with subsurface brines con-
taining high concentrations of calcium, and demonstrated that dolomitiza-
tion cannot account for all of the calcium in the brine solutions. Von
Engelhardt (1960) noted that even the formation of chlorite utilizing magne-
sium with exchange of sodium and calcium does not account for all of the
soluble calcium; however, exchange reactions with other clays were not con-
sidered. Kramer (1963) assumed that calcium was more abundant in ancient
oceans, but White (1965) found this relation to be untenable and suggested
that shale-membrane filtration accounts for increased concentrations of
calcium in some brines. Additional data are needed before more definite
conclusions can be made. The amounts and ratios of calcium and magnesium
vary from one formation water to another as well as within one formation at
different geographic areas. Mineral formation, exchange reactions, leaching,
and shale-membrane filtration all can alter the composition of the brine.
However, in a specific area, one type of reaction may predominate.

Mem brane-concen tra ted brines

Essentially the postulate that clays and shales act as membranes to filter
dissolved solids from waters results from the fact that synthetic membranes
are used to desalinate waters by reverse osmosis. Conceivably, compacted
clays and shales may perform as imperfect semipermeable membranes. Solu-