关键词: 致密储层, 渗透流动, 渗吸—渗透耦合机理, 采收率, 水相矿化度

Abstract: The tight reservoirs are characterized by low porosity, low permeability, as well as a complex interplay among pores, natural fractures, and artificial fractures. It is generally believed that spontaneous imbibition caused by capillary action is the primary driving force for oil displacement in tight reservoirs. However, variations in the water phase salinity within the reservoir can trigger more complicated imbibition-osmosis coupling effects, making it evident that the true flow characteristics of oil and water phases in the reservoir cannot be adequately explained only through imbibition mechanisms. Therefore, a systematic study was performed on the oil-water flow characteristics in tight reservoirs induced by salinity changes. Firstly, the impact of salinity on interfacial tension (IFT) and contact angle was investigated, followed by a quantitative analysis of the imbibition dynamics and efficiency under varying salinity conditions. Subsequently, the fluid flow in tight reservoir cores induced by salinity changes were characterized using improved imbibition-osmosis experiments. Combined with theoretical analysis, the study reveals the underlying mechanisms of osmosis flow and imbibition-osmosis coupling effect, and preliminarily probes into the contributions of the fluid flow caused by salinity variation between the inner and outer core to oil displacement efficiency. The experimental results indicate that as the salinity increases, the hydrophilicity of rock enhances. Additionally, the salinity variation between the internal and external water phases of the rock matrix also affects the contact angle. With an increase in salinity, the oil-water IFT initially keeps stable, then increases rapidly, and finally stabilize again. Furthermore, with the increasing of salinity, the imbibition-based oil displacement efficiency first increases and then decreases, peaking at 17.42 % when the salinity reaches 1×10⁵mg/L. The salinity gradient between the internal and external water phases within the rock core can induce an osmosis effect that triggers the flow of water and oil. When the internal salinity level of a core sample exceeds the external level, the core undergoes a "re-absorption" phenomenon, known as "forward osmosis". In contrast, when the internal salinity level is lower than the external level, the core exhibits a "water expulsion" phenomenon, referred to as "reverse osmosis". Forward permeation can significantly enhance oil recovery, achieving a 33.14 % to 48.66 % contribution to the total recovery. For tight reservoirs, the goal of high-efficient development can be realized through reasonable adjustment of the aqueous salinity.

Key words: tight oil, osmosis flow, imbibition-osmosis coupling mechanism, oil recovery, water salinity