Page 246 - Geochemistry of Oil Field Waters

23 0 ORIGIN OF OILFIELD WATERS

Ion exchange

Ion exchange reactions on clay minerals are reversible and they follow the
law of mass action. The number of exchange sites governs the reaction, and
other important factors include temperature, pressure, solution concen-
trations, and bonding strength of exchangeable ions. Ion exchange between
clay minerals and a brine will stop when equilibrium is attained.
As the waters move in their subsurface environment, their dissolved ions
have a tendency to exchange with those in the rocks. There are two extreme
types of adsorption in addition to intermediate types of adsorption. The
extreme types are: (1) a physical adsorption or Van der Waals adsorption
with weak bonding between the adsorbent and the constituent adsorbed; and
(2) a chemical adsorption with strong valence bonds.
Cations can be fixed at the surface and in the interior of minerals. These
fixed cations can exchange with cations in the water. Under the right
physical conditions of the adsorbent, similar exchange can occur with the
anions. Some of the constituents in formations that are capable of exchange
and adsorption are argillaceous minerals, zeolites, ferric hydroxide, and cer-
tain organic compounds.
Particle size influences the exchange rates and capacities if the solids are
clays such as illite and kaolinite. The rate increases with decreasing particle
size. However, if a larger mineral has a lattice, the exchange can easily occur
on the plates. The concentration of exchangeable ions in the adsorbent and
in the water is important. More exchange usually occurs when the solution is
highly concentrated.
According to Grim (1952), the replacing power of some ions in clays is:

(1) In NH, , kaolinite:
Cs > Rb > K > Ba > Sr > Ca > Mg> H > Na > Li
(2) In NH, , montmorillonite:
Cs > Rb > K > H > Sr > Ba > Mg> Ca> Na > Li

These two clays often are present in sedimentary rocks and the replacing
order indicates that lithium and sodium are more likely to be left in solution,
while cesium and rubidium are more likely to be removed from solution.
Fig. 7.6 is a plot of the chloride content versus the lithium content of
some oilfield waters taken from the Smackover formation. The .lithium
enrichment results at least in part from exchange reactions on clays. Lithium
has a small radius, a low atomic number, a larger hydrated radius than
sodium, and a larger polarization than sodium. Because of these, its replacing
power in the lattices of clay minerals is low (Kelley, 1948). Other ions such
as barium, strontium, calcium, magnesium, cesium, rubidium, potassium, and
sodium will preferentially replace lithium in clay minerals, thus releasing
lithium to solutions. Furthermore, the solubility products of most lithium