Page 211 - Geochemistry of Oil Field Waters

198 ORIGIN OF OILFIELD WATERS

Erosion

Erosion is the opposite of deposition (the processes are reversible), but
erosion must occur before deposition can proceed. The products of weath-
ering are eroded and transported to a new location by the action of water
and wind. The water serves to transport the majority of these products, and
it can transport them by dissolution, suspension, or pushing of larger
particles.

Transpor ta t ion mechanisms
Both wind and water can transport the products of weathering, however,
this discussion will consider only water. The transport mechanisms con-
sidered are chemistry, physics, and hydraulics.
Perhaps the primary solvents of weathered products are carbonated water,
organic acids, and sulfate solutions. Elements that dissolve readily in car-
bonated waters are lithium, sodium, potassium, magnesium, calcium, stron-
tium, iron, manganese, phosphorus, and others. The organic acids will
dissolve iron and manganese, while sulfate solutions will dissolve copper,
iron, and manganese compounds.
The chemistry of the water is a prime factor in the dissolution of the rock;
if the pH is acidic, the transition group metals are more likely to dissolve,
while if it is basic, elements such as silica are more likely to dissolve. Salts of
the alkali and alkaline earth metals will dissolve if the pH is either acidic or
basic; however, if the pH is above 10, some of the alkaline earths such as
magnesium will precipitate. The pH of the water is influenced by the dissolu-
tion of carbon dioxide. For example, as carbon dioxide dissolves in water,
the pH will change. The pH of pure water in equilibrium with carbon dioxide
can be calculated and is 5.65 (Hem, 1970). The pH is calculated using the
mass-law equations in which the activity of water is unity in dilute solution,
and h, = constant equal to the product of the activities of H+ and OH-.
Introduction of another phase such as calcite into the water carbon
dioxide system will change the pH. Garrels and Christ (1965) calculated that
such a system in equilibrium with the atmosphere will have a pH of 8.4.
Additional ions such as those found in ocean water will produce other pH
values. For example, if the system is ocean water in equilibrium with carbon
dioxide, the pH at each equilibrium step is approximately:

H20+C02 * H2C03 7PH 5)
H2 co3 A - HC03-+H+ (PH 6-31
HC03+H+ =+ 2H++C03-2 (pH 10.3)

Some chemicals when dissolved in water act as buffers, where a buffer is
defined as something that .produces an effect which inhibits a large pH
change when an acid or a base is added to the water. Therefore, as a water