Effect of spontaneous emulsification on oil recovery in tight oil-wet reservoirs

Abstract

Surfactants improve oil recovery from tight oil-wet formations through wettability alteration and interfacial tension (IFT) reduction. The popular perspective believes that wettability alteration induced capillary-driven spontaneous imbibition is the major mechanism of Enhanced Oil Recovery (EOR) in tight oil reservoirs. Therefore, surfactant with functions of wettability alteration while guarding a relatively high IFT should be chosen as the best candidate. However, ultra-low IFT may change the phase behavior and flow-pattern within the porous media to improve the oil recovery through emulsification.Therefore, in this paper, we investigated the effect of spontaneous emulsifying surfactants on the oil recovery from tight sandstones through spontaneous imbibition experiments and the results are compared with that of regular surfactants The results manifested that the spontaneous-emulsifying surfactants with ultra-low IFT can improve oil recovery from oil-wet cores, and the performance surpassed that of regular surfactants through wettability alteration. It is because a spontaneous-emulsifying surfactant is capable of accelerating the oil dispersion and reducing the residual oil saturation by emulsifying the oil phase. Partly, it diffuses out from the porous media in the form of the microemulsion, and partly small oil droplets with ultra-low IFT may coalesce into larger ones so that it is easy for them to deform and to be produced by buoyancy. Therefore, comparing with the traditional viscous flow, the change of the flow pattern due to the addition of spontaneous-emulsifying surfactants further enhanced the recovery in term of producing rate and final recovery factor.