الملخص الإنجليزي
Most of the global oil reserves are held in the complex and heterogeneous carbonate
reservoirs. Surfactant flooding is one of the key techniques to enhance oil recovery from
these reservoirs owing to its capabilities in altering wettability and improving oil
displacement through interfacial tension (IFT) reduction. However, performing surfactant
flooding is highly limited by its economic feasibility. Anionic carboxylate surfactants are
inexpensive and have attractive potentials in harsh reservoir conditions of high
temperature and high salinity. The synthesis of carboxylate surfactant with various
amount of ethylene oxide (EO) and/or propylene oxide (PO) units and different
hydrocarbon (HC) length creates a versatile alkyl propylene/ethylene (alkoxy)
carboxylates (AEC). Previous studies confirmed the positive effect of combining low
salinity water (LSW) and surfactants on enhancing oil recovery over using LSW alone.
Therefore, the objective of this study was to investigate the effect of low salinity on the
behavior of commercially available carboxylate surfactants with various AEC structures
on IFT reduction, wettability alteration and enhancing oil recovery in carbonates at
representative reservoir temperature. The experimental approach for screening out the
surfactants included the following: compatibility, phase behavior, IFT reduction, contact
angle (CA), and spontaneous imbibition tests. These tests were conducted at temperature
of 75 °C and three salinities (~ 200 g/L, ~ 20 g/L, and ~ 2 g/L). Surfactants with high
hydrophilicity nature were compatible at all salinities. Lowering salinity resulted in
improving the compatibility of most surfactants. A total of 15 compatible surfactant
systems were subjected for further testing. In phase behavior test, only two surfactants
produced Winsor Type III at high salinity. These surfactants are characterized by
intermediate to long HC chain. Phase behavior changed from Winsor Type III to Winsor
Type I with reducing the salinity. The remaining AEC surfactants had low solubilization
capacity of oil and produced Winsor Type I or no interaction at different salinities. The
minimum IFT in the order of 10-2 mN/m was achieved by a branched surfactant with a
long HC chain and high hydrophilicity at high salinity. Surfactants containing PO units
showed higher IFT reduction at low salinity. A surfactant with long HC chain and low
hydrophilicity produced the highest IFT reduction (10-2 mN/m) at low salinity. CA
measurements revealed that all tested surfactants were able to alter the wettability of oil wet calcite toward neutral to weak oil-wet, except one surfactant which altered the
wettability from strong oil-wet to strong water-wet with final CA of around 30°. This
surfactant had the shortest HC chain and highest hydrophilicity. Best performing
surfactants in terms of IFT and CA reduction were considered for spontaneous imbibition
tests. Highest oil recovery (86%) was achieved by the branched AEC surfactant that
showed greatest IFT reduction, highest micro-emulsion formation capabilities, and
neutral-wet state at high salinity. Comparing the results of this study with previous studies
shows that combining AEC surfactants with LSW resulted in fair oil recovery, where
better performance of AEC surfactants was obtained at high salinity. The performance of
AEC surfactants at different salinities were highly affected by their structure.
