Effect of low salinity and structural variation of anionic carboxylate surfactant on IFT reduction, wettability alteration, and oil recovery in tight carbonates
Nawraa Ali Abdulkarim Al-Sayegh, Rashid S. Al-Maamari
كلية الهندسة-جامعة السلطان قابوس · عمان
Most of the global oil reserves are held in the complex and heterogeneous carbonatereservoirs. Surfactant flooding is one of the key techniques to enhance oil recovery fromthese reservoirs owing to its capabilities in altering wettability and improving oildisplacement through interfacial tension (IFT) reduction. However, performing surfactantflooding is highly limited by its economic feasibility. Anionic carboxylate surfactants areinexpensive and have attractive potentials in harsh reservoir conditions of hightemperature and high salinity. The synthesis of carboxylate surfactant with variousamount of ethylene oxide (EO) and/or propylene oxide (PO) units and differenthydrocarbon (HC) length creates a versatile alkyl propylene/ethylene (alkoxy)carboxylates (AEC). Previous studies confirmed the positive effect of combining lowsalinity 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 thebehavior of commercially available carboxylate surfactants with various AEC structureson IFT reduction, wettability alteration and enhancing oil recovery in carbonates atrepresentative reservoir temperature. The experimental approach for screening out thesurfactants included the following: compatibility, phase behavior, IFT reduction, contactangle (CA), and spontaneous imbibition tests. These tests were conducted at temperatureof 75 °C and three salinities (~ 200 g/L, ~ 20 g/L, and ~ 2 g/L). Surfactants with highhydrophilicity nature were compatible at all salinities. Lowering salinity resulted inimproving the compatibility of most surfactants. A total of 15 compatible surfactantsystems were subjected for further testing. In phase behavior test, only two surfactantsproduced Winsor Type III at high salinity. These surfactants are characterized byintermediate to long HC chain. Phase behavior changed from Winsor Type III to WinsorType I with reducing the salinity. The remaining AEC surfactants had low solubilizationcapacity of oil and produced Winsor Type I or no interaction at different salinities. Theminimum IFT in the order of 10-2 mN/m was achieved by a branched surfactant with along HC chain and high hydrophilicity at high salinity. Surfactants containing PO unitsshowed higher IFT reduction at low salinity. A surfactant with long HC chain and lowhydrophilicity produced the highest IFT reduction (10-2 mN/m) at low salinity. CAmeasurements 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 thewettability from strong oil-wet to strong water-wet with final CA of around 30°. Thissurfactant had the shortest HC chain and highest hydrophilicity. Best performingsurfactants in terms of IFT and CA reduction were considered for spontaneous imbibitiontests. Highest oil recovery (86%) was achieved by the branched AEC surfactant thatshowed greatest IFT reduction, highest micro-emulsion formation capabilities, andneutral-wet state at high salinity. Comparing the results of this study with previous studiesshows that combining AEC surfactants with LSW resulted in fair oil recovery, wherebetter performance of AEC surfactants was obtained at high salinity. The performance ofAEC surfactants at different salinities were highly affected by their structure.