A review of surfactants as synergists in the field of enhanced oil recovery

The efficient development of oil resources is significant for alleviating the increasingly serious contradiction between oil supply and demand and ensuring national oil security. Surfactant flooding technology is one of the essential methods to enhance oil recovery and has been widely used in oil and gas development. The screening and development of traditional surfactants for oil displacement are mainly focused on reducing interfacial tension based on the theory of capillary number while ignoring the emulsification effect of surfactants. For different types of residual oil/remaining oil, the main contradictions faced by the target reservoir should be thoroughly analyzed to clarify whether the performance of surfactants used for oil displacement is mainly to reduce interfacial tension or strong emulsification capacity, and then screen surfactants suitable for reservoir characteristics and crude oil properties. In this article, the principle and application of surfactant flooding technology are reviewed from two aspects of interfacial tension and emulsification performance, the interrelationship between interfacial tension and emulsification capacity is explored, and the evaluation indicators and existing problems of surfactant used for oil displacement are analyzed in detail. Most of the evaluation methods for emulsification in the past were based on water extraction rate. A method for evaluating the emulsification ability of surfactants to crude oil is introduced. It is expected to provide scientific guidance for the screening and development of surfactants for oil displacement.