Enhanced Oil Recovery by Low Salinity Surfactant: Effect of Ions on Core and Pore Scales

Abstract

Enhanced oil recovery (EOR) by surfactants is a well-established method in the petroleum industry. In a surfactant flooding, oil is mobilized and recovered by reduction of the interfacial tension (IFT) between oil and water phases or changes in the rock wettability. It is experimentally proved that surfactant injection can lead to low residual oil saturation (<10%) at ultralow IFT (<0.001mN/m). The chemicals used to obtain this low oil saturation are usually mixture of surfactant, cosurfactant, solvent and alkali. The process is complex and involves a challenging design.

It has been also found that reduction in the water salinity and changes in the ionic composition of the injected water in a water flooding may also increase the amount of recovered oil. The method is often called low salinity water (LSW) flooding. The circumstances and mechanisms under which oil recovery can be achieved by LSW flooding have yet to be found. However, the mechanisms are believed to change the wettability.

It has been also proved that LSW has synergy when combined with EOR by surfactant injection. The importance of water salinity and composition is a common point in both surfactant and LSW flooding. The combined low salinity water and surfactant injection is called low salinity surfactant (LSS) flooding.

Lower surfactant adsorption to the rock is a major advantage of the LSS flooding compared with the conventional surfactant flooding with ultralow IFT. On the other hand, higher IFT is a disadvantage of the LSS compared with conventional surfactant flooding. However, it is possible to reduce the IFT of the LSS solutions with crude oils by changing the ratio of calcium to sodium (Ca2+/Na+) cations in the surfactant solution. Impact of the ionic composition on fluid-fluid interactions (mainly IFT) has been well documented whereas the impact on the rock-fluid interactions (mainly wettability) has to be studied further. This study was the main focus of this dissertation.

State-of-the-art pore scale in situ imaging by X-ray microtomography in addition to the ordinary coreflooding experiments were used to investigate the effect of ionic composition in wettability alteration, oil recovery and trapping in LSS injection. The observations proved that the wettability alteration occurs in LSS injection, and calcium-contained surfactant solution performs better when the rock is initially oil wet, whereas surfactant solution with only NaCl performs better in water wet rock. Experimental evidences in pore scale have been provided in this dissertation. Performance of EOR by LSW and LSS seems to be predictable by studying the impact of ionic composition on IFT (only in LSS), initial wettability and possible interplay of initial wettability and interfacial rheology.