Page 169 - Geochemistry of Oil Field Waters

156 INORGANIC CONSTITUENTS AND PHYSICAL PROPERTIES

no carbonate ions, but they do contain bicarbonate ions. Some oilfield
waters in the Rocky Mountain area are alkaline and contain both primary
and secondary alkalinity, where primary alkalinity is that associated with the
alkali metals and secondary alkalinity is that associated with the alkaline
earth metals. For example, the Green River formation waters that are in or
near trona beds may contain more than 20,000 mg/l of carbonate and 5,000
mg/l of bicarbonate. Most oilfield waters from other areas contain from
about 100 to 2,000 mg/l of bicarbonate.

Acidity

The basis of acidity is the solvated hydrogen ion H30+, which is found in
nature. Volcanic emanations produce HF, HCl, and H2 SO4, probably for-
med by reactions between water and constituents associated with the
magma. Waters associated with peat may contain organic acids, rain waters
may contain carbonic acid, and waters associated with reducing conditions
and anaerobic bacteria may contain H2 S.
Acidity, as contrasted to alkalinity, is the capacity of a solution to
neutralize a base, usually from below pH 4.5 to pH 7.0. Most oilfield brines
normally do not contain acidity. New wells or reworked wells often are
acidified or "acidized" with a strong mineral acid or a combination of
mineral and organic acids. This treatment causes the produced water to
contain a certain amount of acidity until all of the acid is neutralized or
diluted. Because of the large quantities of acids used in some treatments, it
may take 6 months or more for the water produced from a treated well to
return to normal. Organic acids and organic acid salts commonly are found
in oilfield waters, and the concentration ranges from trace amounts to more
than 3,000 mg/l.

Silica

Silicon is the second most abundant element in the earth's crust, which
contains about 27 wt.% of it (Fleischer, 1962). It always occurs in a com-
bined form. Most of the silicon compounds involve structures with oxygen,
and there are about a thousand silicate minerals in the earth's crust; however,
those which are predominant are relatively few in number.
The solubility of silica in water is a function of temperature, pressure, pH,
and other ions in solution. Most silica in natural water probably is in the
form of monomolecular silicic acid, H4 Si04 or Si(OH)4. Collins (196913)
studied the solubility of a serpentine in solutions of calcium chloride and
sodium chloride at temperatures from 30" to 200°C and pressures from 176
to 1,055 kg/cm2. The solubility calculated as silicon molarity in solution
increased with increasing concentrations of sodium chloride, increasing pres-
sure, and increasing temperature up to about 125°C. Between about 125"
and 2OO0C, the solubility decreased with increasing temperature. The solu-