Page 236 - Fundamentals of Enhanced Oil and Gas Recovery

224 Mohammad Ali Ahmadi

Most of these mechanisms are related to each other. In this section, we will discuss
major mechanisms and their working conditions [45,55].

7.6.2.1 Fine Mobilization
Martin [56] and Bernard [57] observed that clay swelling and dispersion accompanied
by increased differential pressure and incremental oil recovery resulted. Kia et al. [58]
reported that when sandstones were previously exposed to sodium-salt solutions,
flooding these sandstones by fresh water resulted in the release of clay particles and a
drastic reduction in permeability. The permeability reduction was lessened, however,
when calcium ions were also present in the salt solution.
However, Lager et al. [51] reported that no fine migration or significant perme-
ability reductions were observed during numerous LS core displacement experiments
under reduced conditions and full reservoir conditions, although these core floods had
all shown increased oil recovery. Valdya and Fogler [59] reported that a gradual reduc-
tion in salinity kept the concentration of fines in the flowing suspension low, with for-
mation damage minimized or avoided. Soraya et al. [60] conducted several low
salinity water injection displacement experiments; they concluded that low salinity
did not have effect; however, they reported sand production in their experiments.

7.6.2.2 Limited Release of Mixed-Wet Particles
Tang and Morrow [37] found and described the mechanism of limited removal of
mixed-wet particles from locally heterogeneous pores in terms of wettability. They
concluded that when the salinity is reduced, the electrical double layer in the aqueous
phase between particles is expanded, and the tendency to strip fines is increased. The
stripped fines migrate and aggregate so that the oil coalesces and consequently, the oil
recovery factor increased. This mechanism combines the Derjaguin, Landau, Verwey,
and Overbeek (DLVO) theory and fine migration [45].

7.6.2.3 Increased pH and Reduced IFT Similar to Alkaline Flooding
McGuire et al. [44] proposed that LS mechanisms could be due to increased pH and
reduced IFT similar to alkaline flooding. This increase in pH is due to exchange of
hydrogen ions in water with adsorbed sodium ions [61]. Another related mechanism
is that a small change in bulk pH can impose a great change in the zeta potential of
the rock. When pH is increased, organic materials will be desorbed from the clay sur-
faces [62 64].
Austad et al. [63] proposed a hypothesis that cation exchange resulted in local pH
increase close to clay surfaces. Zhang et al. [48] reported that after LS brine injection,
a slight rise and drop in pH were observed. However, no clear relationship between
effluent pH and recovery was observed [45].