Review of low salinity waterflooding mechanisms: Wettability alteration and its impact on oil recovery

Abstract

Low salinity waterflooding (LSW) has attracted attention of numerous researchers as a promising technique for enhanced oil recovery, yet its application is still limited because of incomplete mechanisms and unreliable predictions. This paper summarizes relevant mechanisms for predicting responses of LSW. Since the wettability alteration is generally regarded as the most important one among possible LSW mechanisms, two aspects of predicting LSW responses are reviewed: prediction of wettability alteration and effects of wettability alteration on oil recovery. For the first aspect, the importance of more quantitative prediction of wettability alteration is addressed because experimental results have suggested that the optimal wettability may not be water-wet as imagined. From the relation between wettability and interaction forces in the oil-brine-rock systems, mechanisms of wettability alteration are reviewed along with quantitative calculation. The extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which has been often used to calculate the contact angles, is not available to obtain an accurate contact angle anymore. For the second aspect, an overview of the optimal wettability for oil recovery is given with relevant pore-scale fluid mechanisms. Though experiments found the maximum oil recovery from weakly water-wet or neutral wet rocks, the results were not elaborate enough to reveal the relation between the contact angle and the oil recovery. The pore-scale simulation is a powerful tool to investigate such relations but so far it still needs further developments on scaling and validations with experimental data.