Page 70 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

52 David A. Wood and Bin Yuan

tends to lower the acid number of the oil and lower its negatively charged
components. Sulfate is the most effective ion in terms of its ability to alter
the wetting properties in carbonates (Shariatpanahi et al., 2011); its pres-
ence increases the water-wetness of the system. Austad (2013) suggests
22
that it is the combined effect of the presence of SO 4 ions and a
decrease in the NaCl concentration of the injected fluid, which is impor-
tant in altering wettability in carbonates and making LSWF effective.

This combined effect is also temperature sensitive with 90 C to 110 C
proposed as the optimum range, because although high temperatures are
beneficial, they also lead to a reduction in sulfate ion concentrations in
the formation fluids, due to sulfate mineral precipitation.
In contrast to carbonates, the surface of silicate minerals, particularly the
clays, is negatively charged. The permanent negative charge of clay minerals
enables them to act as cation exchangers with a variable affinity to
exchange with specific cations (Austad, 2013); that affinity increases in
1 1 1 21 21 1
the following order Li , Na , K , Mg , Ca , H . As the
cations adsorb onto the clay, the clay surfaces become more oil-wet, a
process that is sensitive to the pH of the formation water (Madsen and
Lind, 1998). As clays are normally not uniformly distributed throughout
an oil reservoir, the clay-rich local areas may be less water-wet than the
clay-poor zones.
Tang and Morrow (1999), in experiments on the Berea sandstone,
established that adsorption from crude oil, the presence of potentially
mobile fines, and initial water saturation were all requirements for an
increase in oil recovery to result from a decrease in salinity of the injected
water. Lager et al. (2008) concluded that pH induced IFT reduction or
emulsification and fines migration were effects rather than the driving
mechanisms of LSWF, which, based on their experiments with sandstone
cores, they concluded was primarily cation exchange (i.e., MIE).
However, they noted for LSWF to be effective, the system also needs to
involve oil with polar components (i.e., acids and/or bases), the indige-
nous formation water needed to have meaningful concentrations of diva-
21 21
lent cations, i.e., Ca ,Mg , and salinity of the injection fluid should
be between 1000 ppm and 2000 ppm, but could work with salinities up
to about 5000 ppm. The effective salinity level of the LSW seems to
21 21 1
depend on the divalent (mainly Ca ,Mg ) versus monovalent (Na )
ionic composition of the injection fluid, consistent with the findings of
21
Yildiz and Morrow (1996). In a MIE mechanism, it is the Ca and
Mg 21 ions which are adsorbed from the LSW by the rock matrix until it