Synergistic effects between anionic and amphoteric surfactants on promoting spontaneous imbibition in ultra-low permeability reservoirs: Study of mechanism and formula construction

Abstract Faster spontaneous imbibition in ultra-low permeability reservoirs ensures higher productivity. Surfactants can promote spontaneous imbibition by creating a strong water-wet condition while keeping the oil-water interfacial tension (IFT) at an intermediate level. In this paper, A surfactant mixture composed of Alpha Olefin Sulfonate (AOS, an anionic surfactant) and Cocoamidopropyl-Hydroxysulfobetaine (CHSB-35, an amphoteric surfactant)CHSB-35 was proposed to promote the spontaneous imbibition rate of Chang 9 reservoir, a sandstone reservoir with high salinity, and ultra-low permeability. The synergistic effects between AOS and CHSB-35 as well as the functional mechanisms were clarified by conducting spontaneous imbibition tests. Experimental results show that when the total surfactant concentration locates in the range of 0.20–0.35 wt%, the oil-water IFT can be lowered to an ultra-low level at a salinity of 11.2 wt%. However, the spontaneous imbibition rate was found to be higher at a total surfactant concentration around 0.10–0.15 wt% with intermediate IFTs and contact angles around 35°. Our results indicated that once surfactant changes the rock wettability from an oil-wet state to a water-wet state, ultra-low IFTs are unnecessary or should be avoided in ultra-low permeability reservoirs. Spontaneous imbibition tests show that the oil production rate of AOS-CHSB-35 mixture is four times faster than simulated water and two times faster than each single surfactant.