The study of enhanced displacement efficiency in tight sandstone from the combination of spontaneous and dynamic imbibition

Spontaneous imbibition is critical in the oil recovery process for low permeability and tight reservoirs, which is driven by the capillary pressure. In the waterflooding process, dynamic imbibition has been determined as an important means of oil recovery for ultra-low permeability reservoirs. It functions significantly to the oil recovery in cyclic water injection. Unfortunately, some mechanisms are not fully understood within the spontaneous and dynamic imbibition such as the effect of pore size on imbibition and their collaboration on oil recovery. In this paper, three micro mechanisms were studied to understand their effects on oil recovery efficiency: spontaneous imbibition, dynamic imbibition and the combination of spontaneous and dynamic imbibition. The Nuclear Magnetic Resonance (NMR) technique was applied to investigate the sandstone formation of Yanchang group in Ordos basin. First, we analyzed the T2 spectrum characteristics of tight sandstone, by which the pore size was grouped into three along with the high-pressure mercury injection data. Second, we studied the quantitative effect of imbibition time and interfacial tension on the oil recovery efficiency of tight sandstone, with respect to various pore sizes during the spontaneous imbibition process. Simultaneously, the effect of interfacial tension on dynamic imbibition was evaluated at various pore sizes by means of displacement experiments and low-field NMR. Third, we proposed the methodology of the utilization of spontaneous and dynamic imbibition to enhance the displacement efficiency in tight sandstone, based on their characteristics. Meanwhile, the displacement characteristics were compared at various pore sizes from the three imbibition types. A feasible workflow was provided for field applications to enhance oil recovery by means of spontaneous and dynamic imbibition. This research fills the gap of waterflooding in tight sandstone and low permeability reservoirs and improves the waterflooding technique.