Improvement of Polymer Flooding of a Viscous Oil by Addition of an Alkali

Abstract

Abstract Polymer flooding has been applied successfully in many water-wet, viscous oil reservoirs. When the reservoir is intermediate-wet or oil-wet, the polymer dosage has to increase to keep the displacement front stable, which adds to the cost. Alkali can generate in-situ soap, which lowers interfacial tension and may change wettability; therefore, alkali-polymer (AP) flood can be considered as an alternative way of improving recovery without increasing polymer concentration. This work studies the effect of alkali addition to polymer floods in displacing viscous oils from sand packs and carbonate packs of various wettability. Sessile drop contact angle (SDCA) and water drop penetration time (WDPT) measurements were conducted to evaluate the wettability before and after alkali treatments. The floods were conducted in a custom-designed 2-D visualization cell. The results show that the alkali was able to reduce the IFT between the viscous oil and water to 0.07 dyne/cm. SDCA and WDPT results showed that the sand pack with an initial water saturation (Swi) tends to be water-wet, while that without Swi tends to be intermediate-wet. The carbonate pack was tested to be strongly oil-wet even at Swi. Alkali was effective in making the intermediate-wet sand more water-wet, but has a limited effect on the oil-wet carbonate. Coreflood results show that in water-wet sand packs, the AP flood with a viscosity ratio of 10 results in a stable displacement with an incremental oil recovery of 7% OOIP compared to the straight polymer flood of the same viscosity ratio. In intermediate-wet sandstone packs, the AP floods were unstable, and early breakthrough was observed. However, the oil recovery of AP floods was still 5% higher than the straight polymer flood at 2 PV. AP flooding in oil-wet carbonate packs recovered 18% additional oil at 2 PV injection than the polymer flood at the same viscosity ratio. This study suggests that the efficiency of polymer foods can be improved by addition of an alkali.