Page 172 - Geochemistry of Oil Field Waters

SULFUR 159

the free form. Most combined oxygen is ionic; however, it forms a covalent
molecule with hydrogen, namely water. It also forms complex oxy-salts with
various metals. The oxygen content of rocks decreases with depth.
The solubility of oxygen in water is primarily a function of temperature
and pressure, and surface waters at ambient conditions may contain 7.63
mg/l at 3OoC and 11.33 mg/l at 10°C (Hem, 1970). The amounts of
dissolved oxygen in subsurface petroleum-associated waters is usually low,
and in most in situ conditions it is undetectable because of the low redox
potential of the environment. It can cause corrosion problems in the well
pipes, but in most cases it is atmospheric oxygen that mixes with the pro-
duced brine during production operations that causes oxygen corrosion.

Sulfur

Sulfur is a member of the VI A group of elements and is widely dispersed
in sedimentary and igneous rocks as metallic sulfides. The crust of the earth
contains about 0.05 wt.% of sulfur (Fleischer, 1962). Free sulfur often is
related to volcanic activity and can be deposited directly as a sublimate.
Many commercial deposits, however, are associated with sedimentary
gypsum, and probably result from biogenic activity such as that. of anaerobic
bacteria. Large deposits of sulfur are found in caprocks of anhydrite over-
lying some salt domes.
Hydrogen sulfide, often found in oilfield waters, is formed by anaerobic
bacteria. One such species of bacteria is the Desulphouibrio, which obtains
its oxygen from sulfate ions, causing them to be reduced to hydrogen
sulfide.
Sulfur in surface water usually occurs in the form (S6) complexed with
oxygen as the sulfate anion S04-2. As previously mentioned, the conversion
of oxidized sulfur to a reduced form commonly involves a biogenic process,
and such a reduction may not occur unless these bacteria are present. The Eh
of subsurface oilfield brines usually is somewhat reducing, and the sulfur
species in such environments can include hydrogen sulfide (H2 S), sulfite
and thionates (S406-’). Detailed studies of the sulfur species in
subsurface brines have not been made, and it is likely that other forms of
sulfur are present in some brines. The temperature, pressure, Eh, pH, and
other constituents in solution all influence the types of dissolved sulfur that
occur in oilfield brines.
Shales, sandstones, and carbonates contain about 2,400, 240, and 1,200
ppm, respectively, of sulfur (Mason, 1966). Sea water contains 900 mg/l of
sulfur as sulfate, and subsurface oilfield brines contain from none up to
several thousand milligrams per liter. The amount of sulfate in the brine is
influenced by bacterial activity and by how much calcium, strontium, and
barium is present. If these three cations are present in relatively high concen-
trations, the amount of sulfate present will be low. However, some brines
containing high concentrations of magnesium and low concentrations of the
other alkaline earth metals may contain high concentrations of sulfate.