| Enhanced oil recovery and contaminated aquifer remediation, the two topical subjects are paramount facing energy and environment concerns. Multi-phase displacement in porous media, as the intrinsic process in the two areas, has received wide attention from engineering fluid researchers and computer simulations. At present, some macroscopic properties, such as distribution and saturation of fluids in porous media, the relationship between capillary pressure and fluid saturation, and the fluid permeability have been intensively studied. However, due to the complex microscopic structure of porous media and un-visibility of the fluid flow, the physical infrastructure and mathematical description for the flow of fluids through porous media have not been completed or reached consensus. Therefore, network model, tube bundle model, and lattice-Boltzmann method, etc. are developed to help people to describe and understand the multi-phase flow in porous media at the pore-scale or molecular level. Among these models and methods, a single capillary tube is popular to investigate capillary effect on micro-scale fluid displacement. Its core foundation, the Washburn equation has been extended to immiscible liquid-liquid displacement from single (liquid) phase displacement. Consider the situation that fluids of variable wetting properties always coexist in nature and practice, for example, when miscible or partially miscible gases such as dioxide carbon gas, nitrogen, natural gas, and stack gas flooding are employed in tertiary oil recovery, and when non-aqueous phase liquid pollutants enters into unsaturated aquifer, it would better be abstracted to an oil/water/gas three-phase system. Thus, it is necessary to verify whether the established two-phase displacement Washburn equation is suitable to describle immiscible three-phase flow or not.In the present dissertation, the extended Washburn equation for the immiscible continuous oil/water/gas three-phase displacement were established based on the theory and condition of the two-phase Washburn equation. A delicate experimental method for conducting the oil/water/gas three-phase displacement at micro scale was invented. Effects of flow pattern and pore scale on the behavior of the continuous three-phase flow was investigated using the model system. Furthermore, the role and influence of solid wettability, oil composition, mineral salt, and surfactant were evaluated by introducing simulated systems. In addition, multi-tube displacement experiment was set up for the first time, and the impact of external existence of tubes on the displacement of oil by water was also discussed. The main contents of the present dissertation are followings.1. The Verification of Two-Phase Displacement Washburn Equation for Application in Immiscible Oil/Water/Gas Three-Phase Displacement at Micro ScaleFirtst of all, the relationship between friction resistance, shear stress and viscosity of viscous fluid in laminar flow were obtained by using slab model. Then, the relationship between shear stress and mean velocity for laminar flow in a circular tube were also obtained. Based on Newton's second law and momentum equation, the forces for immiscible liquid-gas and liquid-liquid displacement were analyzed and the theoretical basis for Washburn equation that is commonly used in literature was verified. Finally, the original Washburn equation was modified to suit the immiscible liquid-liquid-gas three-phase displacement, i.e. oil-water-gas flow and water-oil-gas flow. The relational expressions of the relationship between external pressure and velocity, and of the pressure gradient for each liquid phase were developed.2. Experimental Approach for Investigating Immiscible Oil/Water/Gas Three-Phase Displacement at Micro ScaleUpon the established theory, experimental approach for immiscible three-phase displacement in micro-scale capillary was developed by using the decane/water/nitrogen system in capillaries of0.92?m radius. Compared to water-oil two-phase displacement, the continuous oil-water-gas flow reversed the spontaneity for the displacement of oil by water in water-oil two-phase system. In order to force the displacement of oil by water in the oil-water-gas flow, the external pressure was required to surmount the capillary pressure that was the difference between water-gas and oil-water interfaces. In contrast, spontaneous displacement velocity of the continuous water-oil-gas flow was significantly increased by comparison with water-oil two-phase flow and it was attributed to the promotion of oil-gas interface.3. Effects of Flow Pattern and Pore Size on Immiscible Oil/Water/Gas Three-Phase Displacement at Micro Scale Using the Model SystemConsider the effects of the fluid viscosity and capillary shape, pure water and decane with approximate viscosities, chemically inert nitrogen and quartz capillaries with high purity were used as the model system. Through the results of the spontaneous displacement velocities, the relationships between external pressure and displacement velocity, capillary pressure and displacing fluid saturation, and pressure gradient and displacement velocity in the oil/water/gas three-phase experiments conducted in capillaries of1-10?m, the effects of flow pattern and pore size were systematically discussed. Especially, the value of dynamic contact angles during spontaneous displacement, transformation and change of force, work and energy between the system and outside, and the crucial effect of pore size on the pressure gradient-displacement velocity relation have been thoroughly discussed.4. Role and Influence of Solid Wettability, Oil Composition, Mineral Salt, and Surfactant in the Oil/Water/Gas Three-Phase Displacement at Micro ScaleHydrophobic capillaries, simulated oil with2wt%crude oil, injected water and compound surfactant solution was respectively employed in the three-phase displacement experiments, and the role and influence of solid wettability, oil composition, mineral salt and surfactant in the oil/water/gas flow at micro scale was systematically discussed. It was found that all investigated factors would influence the three-phase flow behaviors by changing the wettability of capillary inner wall, which could be concluded from the results of the spontaneous displacement velocity, imbibition curves, drainage curves and capillary hysteresis during controlled displacement. In all the three-phase displacement experiments, the relationships between displacement velocity and pressure gradient were almost the same with the model system, revealing the main factor influencing the relationship was still the pore size.5. Preliminary Conduct of the Multi-Tube Experiments and Study on the Effect of Multiple Tube on the Displacement of Oil by WaterUpon the available experimental device and method for single-tube experiment, the parallel tube bundle models mentioned in literatures were really implemented in laboratory for the first time. It was found that under the condition that multiple tubes of different radii co-existed, it was necessary to pressurize step by step to improve the displacement of oil. Thus, it could avoid gas slug or multi-fluid slug generating in thicker tubes after being powerfully once pressured which would lead to the flow linger. Meanwhile, the distribution of pore size should be considered when choosing the external pressures. Comparisons among single-tube, double-tube and triple-tube experiments of the displacement of oil by water showed that both capillary pressure and the relationship between displacement velocity and pressure gradient would vary as other capillaries existed outside.The present dissertation was focused on the liquid-liquid-gas three-phase flow at micro-scale. However, the liquid-gas-liquid three-phase flow should consider the compressibility of gas and new liquid phase emerging at the original liquid-gas interface, both of which would make the related process more complicated and require more detailed theoretical treatment to Washburn equation. Consider the situation that dioxide carbon gas flooding, nitrogen gas flooding, natural gas flooding, and stack gas flooding are often employed in tertiary oil recovery, the effect of the gas type on the displacement behavior is worth further investigating. All the fluids employed here were Newtonian fluids while polymer solution etc. non-Newtonian fluids are also often used to enhance oil recovery. It is necessary to introduce more viscoelastic parameters and make Washburn equation suitable for more fluids of variable viscoelasticity. Additionally, the multi-tube experiments in this paper was the initial attempt upon the available experimental device and further improvement should be made to the device or practical method for convenience. Above uncompleted work will receive attentions and interest from fluid reasearchers. |
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