Page 168 - Geochemistry of Oil Field Waters

ALUMINUM 155

Aluminum

Aluminum is the third most abundant element in the earth’s crust, but its
concentration in natural waters usually is less than 1 mg/l. The ionic radius
of trivalent aluminum is 0.57 a (Goldschmidt, 1958), and it usually behaves
as a cation when 6-coordinated with oxygen compounds. However, when
4-coordinated, it usually acts like the central atom of an anion. The
4-coordination usually, but not exclusively, is associated with minerals
formed at high temperatures, but the 6-coordination is associated with
minerals formed at low temperatures, which includes most sediments in the
petroleum environment.
The clay minerals illite, kaolinite, and montmorillonite often contain
about 13.5, 21, and 11% aluminum, respectively. Quartzites, sandstones,
limestones, and shales contain about 0.7, 3.0, 0.6, and 10% aluminum,
respectively. During weathering silica will leach out and leave aluminum
hydroxide behind (Pirsson and Knopf, 1947), and sedimentation processes
leave only about 0.4 mg/l aluminum in sea water.
According to Hem (1970), the cation AP3 predominates in solutions with
a pH of 4.0 or less. Above pH 4.5, polymerization gives rise to an aluminum
species with a gibbsite (aluminum hydroxide) structural pattern. Above pH
7.0, the dissolved form is the anion A1 (OH),-.
The pH of the water is the main control of the amount of alumium that is
likely to be present in natural waters. A water with a pH less than 4.0 may
contain 1% or more of aluminum; for example, waters associated with acid
mine drainage. Oilfield waters contain trace amounts to more than 100 mg/l
of aluminum.

A 1 ha 1 in ity

Alkalinity is defined as the capacity of a solution to neutralize an acid,
usually to a pH of 4.5. A solution with a neutral pH of 7.0 may have a
considerable amount of alkalinity; therefore, alkalinity is a capacity function,
in contrast to pH, which is an intensity function. The alkalinity-pH ranges
originally coincided with methyl orange and phenolphthalein color end
points. The potentiometric titration produces more accurate alkalinity
results, and it utilizes an end point where the most abrupt pH change occurs
while specific increments of a standard acid are added.
Alkalinity usually is caused by the presence of bicarbonate, carbonate, or
hydroxyl ions in a water; however, the weak acids such as silicic, phosphoric,
and boric can contribute titratable alkalinity species. Carbon dioxide, which
is dissolved in circulating waters as bicarbonate or carbonate as a result of
the carbon cycle, is the prime source of alkalinity in shallow ground waters.
However, in deep subsurface brines, additional carbon dioxide probably is
dissolved as a result of diagenesis of inorganic and organic compounds.
Most oilfield waters contain no hydroxyl ions, and most of them contain