Abstract:The primary fracturing in the second member of Kongdian Formation of Cangdong Sag in Dagang Oilfield exhibits significant effects during the initial stage, but energy and output decline rapidly in later stages, and the produced fluids are severely emulsified, resulting in a reduction of shale oil recovery, increasing difficulty of shale oil development, gathering, and transportation. This study aims to investigate the emulsification characteristics of shale oil with water in-depth and identify their influencing factors, thereby revealing the reasons and mechanisms behind shale oil emulsification, and thus proposing solutions for issues related to thickening due to this phenomenon. The findings reveal that the average wax content of shale oil in the second member of the Kongdian Formation exceeds 20%, with a pour point above 35°C. Moreover, the high content of heavy hydrocarbons favors the occurrence of the emulsification phenomenon. Nitrogen-containing polar compounds (i.e. asphaltenes and resins) and wax crystals facilitate the formation of water-in-oil (W/O) emulsions under flow conditions, resulting in thickening attributed to these emulsification processes. The viscosity of shale oil increases first and then decreases with the increase in water content. It reaches a maximum when water content ranges from 50% to 70%, attaining viscosities that are 3.23 to 7.45 times higher than that of dehydrated shale oil. The stability of W/O emulsion formed through the emulsification process of shale oil and water is enhanced by increasing emulsification temperature, increasing salinity of the water phase, and intruding fracturing fluid composition. Additionally, O/W/O multiple emulsions will be formed when emulsified shale oil is re-emulsified, further increasing the viscosity of the shale oil system. However, introducing a small amount of PSG surfactants during this re-emulsifying process can reverse the configuration from W/O back into an O/W structure for the resultant emulsion, thus reducing the viscosity of shale oil and improving its fluidity. Therefore, it is necessary to consider the physical and chemical properties of shale oil when conducting water injections and energy enhancement for shale oil reservoirs. Simultaneously, the development of high-activity surfactants for shale oil stimulation will be a focal point for subsequent process design and product research and development.