Page 352 - Reservoir Formation Damage

332 Reservoir Formation Damage

where 7V a is the total number of aqueous species involved; Nf and N*
denote the total number of aqueous and mineral reactions, respectively;
W/ and W r s represent the rates of the r th aqueous and mineral reactions,
respectively; v£. and v^. denote the stochiometric coefficients of the
species a in the aqueous and mineral reactions, respectively, and q a
represents the rate of species a addition per bulk formation volume by
means of direct injection of fluids through wells completed in the reservoir.

Ion Exchange and Adsorption Reactions

Ion exchange and adsorption are surface chemical or surface com-
plexation processes leading to the exchange of chemical species between
the aqueous solution and mineral surfaces present in geological porous
formations (Jennings and Kirkner, 1984; Lichtner, 1985; Kharaka et al.,
1988). Kharaka et al. (1988) explain the difference between ion exchange
and adsorption as following: "The ion exchange model treats the exchange
of cations or anions on a constant charge surface" and "the adsorption
model simulates the exchange process on a surface where the surface
charge is developed due to the ionization of surface sites at the solution-
surface interface." Therefore, adsorption is a more general concept and
ion exchange is a special case of adsorption (Lichtner, 1985; Sahai and
Sverjensky, 1998). Among the various surface complexation models,
Sahai and Sverjensky (1998) facilitate the triple-layer model (Yates et al.,
1974) for describing the electrical charge near mineral surfaces, as
described in Figure 13-2 (Sahai and Sverjensky, 1998) according to
Westall (1986). As indicated in Figure 13-2, this model considers the
mineral surface, referred to as the O-plane, for adsorption of hydroxide
ions and protons and at a short distance near the mineral surface, referred
to as the p-plane, for adsorption of electrolyte ions and the surface charge
is generated by adsorption of the electrolyte ions and protons (Sahai and
Sverjensky, 1998).
Clays present in geological porous formations have many active ion
exchange sites, a, occupied by various cations and cation exchange takes
place for replacement of the cations in the order of the replacing tendency
of Ca +2 > Mg +2 > K + > Na + (Li et al., 1997). The cation exchange capac-
ity (CEC) of rocks can be expressed as the total number of moles of
exchange sites a per unit mass of rock, Qf x, or per unit volume of rock,
w a, which are related by (Lichtner, 1985):

% (13-15)
Lichtner (1985) points out that "precipitation/dissolution reactions can
alter the exchange capacity of the porous medium by creating or destroy-