Spontaneous Emulsification in Tight Shale Reservoirs Driven by Crude Oil Composition and Formation Water Chemistry: Implications for Enhanced Oil Recovery

Emulsification phenomena have been observed during shale oil production from the Lucaogou Formation in Jimsar Sag of northwest China, yet the underlying mechanisms remain poorly understood despite their significant impact on oil recovery. In this study, we employ a visual micro-displacement system to investigate the pore-scale emulsification behavior of crude oil and water, revealing how emulsions form and evolve under confinement. Our results show that oil-water spatial distribution strongly affects the emulsification mode: upon water channel breakthrough, oil predominantly disperses as water-in-oil (W/O) or oil-in-water (O/W) emulsions, with the dominant type governed by the local water saturation and capillary forces. High water saturation in pore throats favors the formation of O/W emulsions, while low water saturation in oil-wet pore networks promotes W/O emulsions.The dispersed phase originates from wall-adhered or pore-throat-trapped oil. At microscale, high-curvature boundaries induce local variations in interfacial tension and capillary pressure, promoting oil breakup into micro-droplets and facilitating emulsion generation. The native crude oil from the Jimsar Sag exhibits high viscosity and elevated contents (24%-44%) of resins and asphaltenes, all enhancing emulsion stability. The formation water is characterized by high-pH, bicarbonate-calcium type composition, while the crude oil contains abundant acidic compounds (acid number >1.4), leading to in-situ formation of natural surfactants at oil-water interfaces. We further demonstrate that during shut-in (soaking) periods, spontaneous emulsification in the pore network contributes to an enhanced oil recovery. Even after the shut-in effect diminishes, additional surfactant injection could re-activate emulsification, enabling further production gains. These findings offer new mechanistic insights into reservoir emulsification processes and present a scientific basis for designing improved shale oil recovery strategies tailored for complex reservoir chemistries and microstructures.