Page 347 - Reservoir Formation Damage

Inorganic Scaling and Geochemical Formation Damage 327

the addition of incompatible fluids during drilling, workover and improved
recovery processes and liberation of light gases, such as CH 4, CO 2, H 2S,
and NH 3, during pressure-drawdown. Changes in temperature and pressure
often cause the variation of the pH of the reservoir aqueous phase, which
in turn induces adverse processes such as the precipitation of iron and
silica gels (Kharaka et al., 1988; Rege and Fogler, 1989; Labrid and Bazin,
1993). Geochemical reactions can also be classified as homogeneous and
heterogeneous depending on whether the reaction occurs inside a phase
or with another phase, respectively. Geochemical reactions can also be
classified as reversible and irreversible. As explained by Lichtner (1985),
the rates of reversible reactions are independent of the surface area.
Reversible reactions can attain local equilibrium over a sufficiently long
period of time, at which time, the reaction rates terms vanish in the
transport equations. However, Lichtner (1985) adds that irreversible reactions
require kinetic or rate expressions, in terms of the pertinent driving forces,
that is chemical affinity, and/or the surface available for reactions.
Detailed geochemical description is a very cumbersome task and often
unnecessary and unjustified in view of the lack of the basic thermo-
dynamic and kinetic data required for description. Rather, geochemical
models are constructed to emphasize the chemical reactions of the important
aqueous species and minerals, which are essential for adequate description
of the rock-water interactions, and neglect all other reactions. This is done
to compromise between the quality of description and the effort necessary
to gather basic thermodynamic and kinetic data and to carry out the
numerical computations.
Among the various alternatives, the kinetic and equilibrium models are
extensively utilized. The kinetic models describe the rate of change of
the amount of mineral and aqueous species in porous media in terms of
the relevant driving forces and factors, such as deviation from equili-
brium concentration and mineral-aqueous solution contact surface. The
proportionality constant is called the rate constant. The equations formed
in this way are called the rate laws or kinetic equations. The equilibrium
models assume geochemical equilibrium between the pore water and the
minerals of porous formation. Since equilibrium can be reached over a
sufficiently long time, equilibrium models represent the closed systems
at steady-state conditions. Mathematically, the equilibrium models can be
derived from kinetic models in the limit of infinitely large rate constants.
Hence, rapid reactions reach equilibrium faster. Therefore, the equilibrium
models represent the limiting conditions and yield conservative predictions
(Schneider, 1997). Equilibrium models are particularly advantageous for
determining the mineral stability and graphical representations of the
mineral and aqueous species interactions (Bj0rkum and Gjelsvik, 1988;
Stumm and Morgan, 1996; Schneider, 1997).