Low salinity effects (LSE) improved
recovery could be obtained
by reducing salinity and/or modifying the specific ions of injected
brines. Low salinity waterflooding (LSW) is described as a certain
low-cost and eco-friendly oil recovery technique, when referring the
enhanced oil recovery (EOR) potential by laboratory observations and
field pilots in recent decades. Various underlying mechanisms behind
LSE were proposed, including saponification, multi-ionic exchange
(MIE), electric double-layer (EDL) expansion, fines migration, salt
in effect, osmotic effect, and so on. However, first, to our best
knowledge, there is not yet a certain mechanism recognized by all
experts. Second, LSE mechanisms of sandstones were always taken for
granted to be applied directly to carbonates, without considering
discrepancies in mineral compositions and pore structures. Third,
spontaneous imbibition performances were usually concluded as the
indication of wettability alteration, without considering the straightforward
effects on imbibition caused by low salinity brine. Review results
reveal LSE necessary conditions and influences. Necessary conditions
should be satisfied to trigger LSE, including oil phase with polar
components, connate water, injected brine key ions composition and
low salinity, clay minerals, and initial wettability without the best
performance. Their sole or mutual interactions influence LSE functions.
Mechanisms could be classified into two types: solid–liquid
and liquid–liquid. These (e.g., EDL expansion, MIE, fines migration)
caused by solid–liquid could be attributed to wettability alteration,
while microdispersion, osmosis effect, and snap-off belong to liquid–liquid
interfacial factor. Sandstones mainly consist of quartz and clay,
initially tending to behave as initial water wetness, while carbonates
mainly consist of calcium carbonate or magnesium carbonate, mainly
behaving as initial oil-wet. Therefore, LSE in carbonates are contributed
to by mechanisms related with wettability alteration, while LSE in
sandstones are mainly related to other factors. The findings of this
study can help with better understanding of LSE and speeding up our
further endeavors to explore its potential in EOR.