Page 71 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

Low-Salinity Water Flooding: from Novel to Mature Technology 53

is fully saturated in those ions which lead to the incremental oil recovery
benefits (Lager et al., 2008). If the rock matrix is already saturated in
Ca 21 and Mg 21 ions before LSWF begins, the MIE mechanism will not
be effective.
Morrow et al. (1994) reported on spontaneous imbibition experiments
to characterize wettability effects of fluids in porous media. Buckley and
Liu (1998) distinguished four mechanisms by which crude oil compo-
nents may adsorb on high-energy mineral surfaces within a porous rock,
and the effect of different oil compositions in this regard. The four
mechanisms are:
• Polar interactions: These prevail in oil-wet systems where there is no
water film between oil and the bulk of the mineral grains
• Surface precipitation: If the oil is a poor solvent for the asphaltenes
(e.g., high API/low density), its wetting alteration capabilities tend to
be enhanced
• Acid/base interactions: In the presence of water, both the solid and oil
interfaces become charged, either behaving as acids (giving up a pro-
ton and becoming negatively charged) and bases (gaining a proton and
a positive charge). For monovalent salt solutions at low concentrations,
pH of the fluid impacts these interactions
• Ion-binding: Divalent or multivalent ions can at both oil and solid
water interfaces or bridge between them, with calcium ions playing a
key role in wettability impacts
Buckley and Liu (1998) also identified three different properties of
crude oils that in combination influenced their ability to alter the wetta-
bility of a reservoir: oil gravity (API), acid number, and base number.
This highlights the role which crude oil composition plays in the
effectiveness of LSWF in a particular reservoir.

2.10 EXAMPLE FIELD FIELD-SCALE TESTS AND
OUTCOMES OF LSWF

In contrast to core water-flood experiments, relatively little multi-
well, multiyear, field-wide production data specifically focused on evalu-
ating the effects of LSWF are published. Robertson (2007) evaluated
water-flood data from public records for several oil fields from the
Powder River basin of Wyoming (USA) which were flooded with LSW