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CHAPTER 3

Modeling of Low-Salinity and

Smart Waterﬂood

ABSTRACT incorporating the mechanisms of LSWF and smart
This chapter discusses the numerical simulations waterﬂood in sandstone and/or carbonate reservoirs.
of low-salinity waterﬂood (LSWF) incorporating the
proposed mechanisms for sandstone and carbonate
reservoirs. Because the geochemical reactions including GEOCHEMISTRY
aqueous reaction, mineral reaction, ion exchange, and Equilibrium Thermodynamics
surface complexation and electrokinetics in the crude In a natural system, the complete chemical equilibrium
oil/brine/rock system are responsible for underlying is rarely achieved, especially, where biological processes
mechanisms, they are implemented in the numerical are involved (Drever, 1997). However, the calculation
simulations. In addition, a number of approaches of equilibrium conﬁdently approximates the real
have been proposed to model the mechanism equilibrium thermodynamics and indicates the direc-
mimicking the historical results of experimental works tion of the chemical reactions of systems. The Gibbs
and a couple of ﬁeld trial tests. This chapter discusses free energy of the system is a useful tool to indicate
the important features of geochemistry to support the the status of the chemical reactions. For a system at
numerical modeling of LSWF process. It also describes constant pressure and temperature, the Gibbs free
a variety of numerical studies, which have developed energy is deﬁned with temperature, pressure, and
the empirical and mechanistic modeling of LSWF enthalpy as shown in Eq. (3.1).
process both in sandstone and carbonate reservoirs.
G ¼ U þ pV TS ¼ H TS (3.1)
In the previous chapter, the various mechanisms of where G is the Gibbs free energy, U is the internal en-
low-salinity waterﬂood (LSWF) and smart waterﬂood ergy, p is the pressure, V is volume, H is the enthalpy,
have been formulated based on the experimental works T is the temperature, and S is the entropy.
and ﬁeld-scaled tests. Because of the different All chemical reactions spontaneously tend to reach
mineralogy and surface charge between sandstone and the equilibrium in a state of minimum Gibbs free
carbonate reservoirs, different mechanisms have energy. A chemical reaction, not at equilibrium, releases
been proposed in sandstone and carbonate reservoirs. the energy and moves toward the equilibrium state. The
Understanding the mechanisms of LSWF and smart change of Gibbs free energy indicates whether the
waterﬂood is essential to the development of numerical chemical reactions are under spontaneous process or
modeling of LSWF and smart waterﬂood. Extensive already in the equilibrium state. The change in the
attempts for the numerical modeling include the Gibbs free energy is deﬁned as in Eq. (3.2). Following
comprehensive geochemical reactions, electrokinetics, the deﬁnitions of Gibbs free energy change and equilib-
and empirical and mechanistic modeling of LSWF rium state, a spontaneous process involves the negative
mechanism, mainly wettability modiﬁcation. This change of Gibbs free energy. When the change of Gibbs
section presents the geochemistry, which contributes free energy is zero, the system is at the equilibrium.
to the proposed mechanisms of LSWF and smart water-
ﬂood. The detail explanations of geochemistry could be DG ¼ DH TDS (3.2)
found in the references (Appelo & Postma, 1999; where DG is the change in the Gibbs free energy, DH is
Bethke, 1996; Drever, 1997). Following sections the change in the enthalpy, and DS is the change in
describe the empirical and mechanistic modeling the entropy.

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