Page 182 - Geochemistry of Oil Field Waters

PHYSICAL PROPERTIES 169

Fig. 5.19 can be used to predict that ferrous ions are the more common form
of dissolved iron and that ferric ions will precipitate in an oxidizing environ-
ment if the pH is above 1. Similar diagrams can be drawn for other con-
stituents.
Some water associated with petroleum is “connate” water, and Fig. 5.19
indicates that such water has a negative Eh; this has been proven in various
field studies (Buckley et al., 1958). The Eh of some petroleum-associated
waters in the Anadarko Basin ranged from -270 mV to -300 mV (Collins,
1969a).
Knowledge of the Eh is useful in determining how to treat a water before
it is injected into a subsurface formation (Ostroff, 1965). For example, the
Eh of the water will be oxidizing if the water is open to the atmosphere, but
if it is kept in a closed system in an oil-production operation, the Eh should
not change appreciably as it is brought to the surface and then reinjected. In
such a situation, the Eh value is useful in determining how much iron will
stay in solution and not deposit in the well bore.
Organisms that consume oxygen cause a lowering of the redox potential.
In buried sediments, it is the aerobic bacteria that attract organic con-
stituents which remove the free oxygen from the interstitial water. Sedi-
ments laid down in a shoreline environment will differ in degree of oxidation
as compared to those laid down in a deep-sea environment (Pirson, 1968).
For example, the Eh of the shoreline sediments may range from -50 to 0
mV, but the Eh of deep-sea sediments may range from -150 to -100 mV.
The aerobic bacteria die when the free oxygen is totally consumed; the
anaerobic bacteria attack the sulfate ion, which is the second most important
anion in the sea water. During this attack, the sulfate reduces to sulfite and
then to sulfide; the Eh drops to -600 mV; H2S is liberated, and CaC03
precipitates as the pH rises above 8.5 (Dapples, 1959).

The term pH means the logarithm (base 10) of the reciprocal of the
hydrogen-ion concentration, and the pH of pure water at 25OC is 7.0, which
means that there is lo-’ ‘mole per liter of H+ in solution. When other
constituents are solubilized by water, the pH probably will change because
the chemical equilibrium shifts as new ions combine with H+ or OH-. The
presence of slightly dissociated acids or bases will tend to buffer the solu-
tion, and the addition of H+ or OH- will shift the pH only a small amount
until the acids or bases are changed to salts.
The pH of oilfield waters usually is controlled by the carbon dioxide-
bicarbonate system. Because the solubility of carbon dioxide is directly
proportional to temperature and pressure, the pH measurement should be
made in the field if a close-to-natural-conditions value is desired. The pH of
the water is not used for water identification or correlation purposes, but it
will indicate possible scale-forming or corrosion tendencies of a water. The