Page 350 - Reservoir Formation Damage

330 Reservoir Formation Damage

Mineral Reactions

The reactions of the aqueous phase species with the solid mineral
matter of the porous matrix are referred to as the mineral reactions. Most
mineral reactions are typically hydrolysis reactions. The interactions of
minerals and aqueous species are generally slow relative to the aqueous
phase reactions (Lichtner, 1992). Their reaction kinetics are controlled by
the external mineral surface area contacting the aqueous phase. The
mineral surface area is determined by the sizes of the grains of the porous
formation. The rates of mineral reactions are gradual and, therefore,
require kinetic descriptions with finite reaction rate constants.
Consider a porous formation containing a total of A^ different mineral
species and undergoing a total of Nf different chemical reactions between
its minerals species and the aqueous phase species. s = \,2,...,N s denotes
the index for the participating mineral species of the porous formation.
denotes the total number of aqueous species involved in the r th
N r
heterogeneous reaction. M r s denotes the s th mineral species undergoing
the r th heterogeneous reaction. S a:ai = l,2,...,N r denotes the various
aqueous species involved in the r th mineral reaction. Then, the reactions
between porous formation minerals and aqueous species can be typically
represented by (Lichtner, 1992, Liu et al., 1996):

= l,2,...,N* r
(13-8)

or simply by

a=l
(13-9)

s
where v m denotes the stochiometric coefficients associated with the
aqueous phase species per one participating mineral species. Note that
s
v m is negative for the reactants and positive for the products.
Applying the mass action law (Prigogine and DeFay, 1954), and
assuming that the activity of the solid minerals is equal to one, the
chemical equilibria between the minerals of the porous formation and
the aqueous species of the r th reaction can be expressed in terms of the
equilibrium saturation solubility product as (Walsh et al., 1982):