Page 348 - Reservoir Formation Damage

328 Reservoir Formation Damage

Because of the highly intensive numerical computations involved, the
geochemical models of the rock-water interactions are usually imple-
mented by computer-coded software. The geochemical computer software
is constantly evolving and becoming more robust and accurate as a result
of the advancement in computer technology, availability of accurate
thermodynamic data, and development of efficient numerical solution
methods. The engineers responsible for developing operational strategies
and procedures for scale control in petroleum reservoirs should rely on
such software. However, efficient use of the ready-made software requires
some familiarity with the fundamental concepts, theories, and methods
involved in the treatment and formulation of geochemical reactions. This
information is usually provided with the user's guide and/or by relevant
publications. In the following, a brief review of the description and
graphical representation of aqueous and mineral species reactions and
various approaches to geochemical modeling are presented.

Reactions in Porous Media

The various chemical reactions occurring in the pore space can be
classified into the groups of homogeneous and heterogeneous reactions
(Lichtner, 1992). The reactions occurring within the aqueous fluid phase
are called the homogeneous or aqueous reactions. The reactions of the
aqueous phase species with the solid minerals of porous formation,
occurring at the pore surface, are called the heterogeneous or mineral
reactions. A convenient treatment of the geochemical reactions can be
achieved by grouping the various reacting solute species into the primary
and secondary sets of species (Kandiner and Brinkley, 1950; Lichtner,
1992). The primary set of species is formed by selecting a minimum,
critical number of reacting aqueous species, S a, necessary for adequate
description of the homogeneous and heterogeneous reactions. Thus, all
other species form the set of the secondary species. The secondary species
are derived from the primary species by means of the equations of the
relevant chemical reactions.

Aqueous Phase Reactions

Lichtner (1992) classifies the homogeneous reactions into three cate-
gories: (a) ion pairing/exchange reactions, (b) complexing reactions, and
(c) redox reactions. The aqueous phase reactions are generally rapid
relative to the mineral reactions (Liu et al., 1997). The rapid rates of
aqueous phase reactions require kinetic descriptions with significantly
large rate constants. Thus, for all practical purposes, these reactions can
be assumed instantaneous and a transport controlled, local chemical