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CHAPTER 2

Mechanisms of Low-Salinity and

Smart Waterﬂood

ABSTRACT include ﬁnes migration, in situ generation of surfactant,
The enhanced oil production of low-salinity waterﬂood salting-in, multicomponent ionic exchange, electrical
(LSWF) and smart waterﬂood has been demonstrated double layer expansion, and pH increase. A brief
through experiments and ﬁeld-scaled tests for sandstone description of these mechanisms follows.
and carbonate reservoirs. Extensive studies have tried to
reveal the exact mechanism underlying the LSWF to Fines Migration
increase the oil recovery. A number of theories have Tang and Morrow (1999) proposed the ﬁnes migration
been proposed to describe the increasing oil production mechanism based on the experimental observations of
by LSWF. However, there is a controversy on the exact sandstone and the mechanism is schematically described
mechanism of the LSWF. Because of the inherent differ- in Fig. 2.1. The mechanism hypothesizes that the heavy
ence between sandstone and carbonate reservoirs, there polar components of crude oil adhere to the clay, which
is no universal mechanism to describe the LSWF process coats the pore walls of sandstone rock grain (Fig. 2.1A).
for both sandstone and carbonate reservoirs. Therefore, The crude oil has two potential behaviors in waterﬂood-
this chapter discusses the reliable mechanisms proposed ing process. Firstly, crude oil drops adhere to ﬁnes at pore
in both sandstone and carbonate reservoirs. walls and they remain as trapped oil fraction. Secondly,
the mixed-wet clay particles adhering crude oil are
Up to now, numerous mechanisms of low-salinity stripped away from the pore walls with the ﬂowing oil
waterﬂood (LSWF) and smart waterﬂood have been and the clay particles tend to be at the oil-water interface
proposed. Many studies have reported that wettability (Fig. 2.1B). It is explained that the behavior and stability
modiﬁcation as the dominant mechanism of LSWF for of mixed-wet ﬁnes depend on the balance between
sandstone and carbonate reservoirs. There are other dis- mechanical and colloidal forces. The mechanical forces
cussions of mechanism such as the clay particle- are capillary forces adhering crude oil to the ﬁnes and
plugging high permeable zone in sandstone reservoirs. viscous forces stripping clay from the pore wall. In addi-
Because the complex COBR interactions are involved tion, a mechanical resistance mitigates the stripping. The
in the process, the effects of the LSWF and smart water- colloidal forces between particles govern the stability of
ﬂood can be probably combined results of several colloids and also control the displacement of oil by
different mechanisms contributing together. Extensive changing electrical double layer. Reduction in salinity
studies have formulated a variety of theories to explain expands the electrical double layer in the aqueous phase
the mechanism of LSWF based on the observations of between particles and promotes the stripping of clay. In
experiments and ﬁeld test. No single theory has been addition, the mixed-wet clay at the oil-water interface in-
widely accepted. Some of most promising theories for hibits the residual oil trapping by snap-off. Therefore, the
LSWF and smart waterﬂood in sandstone and carbonate partial removal of mixed-wet clay particles from the pore
reservoirs are explained further in this chapter. wall potentially causes the locally heterogeneous wetting
and increases the oil recovery (Fig. 2.1C).

MECHANISMS IN SANDSTONE RESERVOIRS In Situ Generation of Surfactant
In spite of the extensive research studies of LSWF McGuire, Chatham, Paskvan, Sommer, and Carini
conducted over two decades, unanimous mechanism (2005) published the results of single-well chemical
has not been accepted to explain the experimental tracer test (SWCTT) for Alaska’s North Slope. The study
observations in sandstone. The proposed mechanisms suggested the in situ generation of surfactant by pH

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