Atomistic Insight into Transportation of Nanofluid in Ultra-confined Channel
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Recently, a renewed interest arises in the application of nanofluids, nanoparticles (NPs) contained base fluid, for enhanced oil recovery (EOR) due to the potential of changing the fluid properties, wettability alternation of rocks, increasing the mobility of capillary-trapped oil, etc. Understanding dynamics of fluids flow in confinement, especially nanopores, is crucial for the design of flooding fluids for EOR. In this thesis, the transportation of nanofluids into ultra-confined capillary has been explored by atomistic simulation. The properties of capillary, wettability of NPs and external pressure are scrutinized to investigate their influence on dynamic transportation process. The goal is to reveal microscopic mechanism of transporting NPs into porous media, and to provide the guidance for designing new petroleum application-oriented NPs. The influence of NPs on spontaneous water imbibition into ultra-confined channels is studied initially. The results indicate that the addition of NPs decreases the displacement of fluids into the capillary dramatically, and the relationship between displacement and time can be described by l(t) ~ t1/2. By combining the dynamic process of imbibition, the water contact angle in the capillary and the relationship of displacement (l) and time (t), a competitive mechanism of NP effect on spontaneous imbibition is proposed. Then, spontaneous water-oil displacement process in ultra-confined capillary controlled by surface wettability of NPs, hydrophobic, mixed, hydrophilic and Janus NPs, are investigated. The results show that the presence of NPs modulates the fluid-fluid meniscus and hinders displacement process compared with NP-free case. From the perspective of motion behavior, hydrophilic NPs disperse in water phase or adsorb on the capillary, while hydrophobic and mixed-wet NPs are mainly distributed in the fluid phase. The NPs dispersed into fluids tend to increase the viscosity of fluids, while the adsorbed NPs contribute to wettability alteration of solid capillary. Different from NPs with uniform surface properties, Janus NPs self-assemble at fluids interface and aggregate at three-phase contact region, which modify the interfacial tension and three-phase contact angle. Via capillary number calculation, it is uncovered that the increase of fluids viscosity is responsible for hindered spontaneous displacement process by hydrophobic and mixed NPs. Wettability alteration of capillary induced by adsorbed NPs is dominating the enhanced displacement in the case of hydrophilic NPs. The capillary pressure is the key factor driving the displacement process for nanofluids with Janus NPs. Meanwhile, the realistic model including modified silica NPs and various oil components influence on the displacement of fluid flow into silica nanopore is simulated to study the distinct from the ideal system. Moreover, forced two-phase displacement process in hydrophobic and hydrophilic capillaries is investigated. The results indicate that the threshold capillary pressure for water transporting into capillary is linearly proportional to the interaction between capillary and water phase. According to the effect of pumping pressure on water-oil displacement process in hydrophobic and hydrophilic capillaries, two different modes are identified. Finally, three kinds of NPs, hydrophobic, Janus and hydrophilic NPs, are proposed as the candidates for the different type of capillary to displace oil phase. Our researches not only uncover the displacement mechanism of oil recovery process in reservoirs, but also propose the potential NPs for varied reservoirs, which is significant for understanding and designing the NPs or other chemicals for EOR.