The Molecular Behavior And Performance Of New Chemical Oil Displacement Agents

Polymers and surfactants are widely used in EOR (Enhance Oil Recovery) as flooding agents. The high viscosity of polymer solution can reduce the mobility ratio between oil phase and water phase through plugging ability and profile control effects, and enhance the oil displacement efficiency. The surfactant can also increase the oil displacement efficiency through decreasing the oil/water interfacial tension and increasing the displacement efficiency. In recent years, the partially hydrolyzed hydrophobically modified polyacrylamide HMHPAM as a kind of new flooding agent get more and more attention, owing to its thickening effect and interfacial activity. In this thesis, the Molecule Dynamics (MD) and Dissipative Particles Dynamics (DPD) were combined with dynamic interfacial tension, viscosity, zeta potential, Dynamics Light Scattering (DLS), Atomic Force Microscopy (AFM), and Transmission Electron Microscopy (TEM) to study the properties of HMHPAM. The relationship between the conformation of HMHPAM molecular structure and its solution properties was studied at molecular level. To provide the theoretical guidance for design and applicant of HMHPAM, the influences of molecular structure and environmental conditions on HMHPAM solution were also investigated. As another new flooding agent, the alkanolamide (DDA), which has both ultra low interfacial tension and plugging ability and profile control effects, was also studied. The flooding mechanism and displacement efficiency of these two flooding agents were investigated through a flooding model.This thesis is divided into five parts:1. The relationship between molecular configuration and phase behavior of HMHPAMThe influence of the polymer concentration, polymerization process, degree of hydrolysis, and the hydrophobic modified group of HMHPAM on the polymer solution were studied by DPD simulation. It was found that the comb structure which the hydrophobic modified groups of polymer molecular chain were distributed uniformly and randomly, were better than the block copolymer. The hydrophobic modified groups of HMHPAM can help the polymer form the net-work structure, which can enlarge the hydrodynamic radius of polymer chain and increase solution viscosity seriously. The hydrolysis groups of HMHPAM can also benifit the increase of the viscosity of polymer solution, because the electrical repulsion between different hydrolysis groups can stretch the polymer chain. So in HMHPAM system, the hydrogen bond between acrylamide groups, the hydrophobic interaction between hydrophobic modified groups and the electrical repulsion between hydrolysis groups decided the phase properties of HMHPAM. The strong hydrophobic interaction can lead to polymer curl up even separate out from solution, and the strong electrical repulsion can destroy the net-work structure of polymer. For HMHPAM, the optimal hydrolysis degree and hydrophobic degree were40%and2%, respectively.2. The Influence of Environment Conditions on the Properties of HMHPAMThe influences of pH, salinity and additive agent (sodium dodecyl sulfonate (SLS)) on the properties of HMHPAM by dissipative particle dynamics (DPD) simulation combined with viscometer and AFM observations (Atomic Force Microscopy) were studied. The effects of electrical shielding effect and water absorbing ability of salts and pH on the behavior of HMHPAM molecules were investigated through changing the interaction parameter between different beads. The conformation variation of polymer chain described by root-mean-square (RMS) end-to-end distance and simulation snapshot was compared with the change of solution viscosity. The hydrolyzed groups of HMHPAM were existed in the form of acrylic acid in acidic solution and acrylic acid anion in alkaline solution. The electrostatic repulsion between different acrylic acid anion groups can stretch the polymer chain and increase the solution viscosity, which resulted in the higher viscosity of HMHPAM solution in environment with pH>8. It was found that, the water absorbing effect of salts was the main reason seriously reduced the viscosity of HMHPAM solution, while the influence of electrical shielding of salts was not obvious. The added SLS can influence the interfacial activity and bulk phase thickening property of HMHPAM. HMHPAM can be absorbed onto the oil/water interface and reduce the oil/water interfacial tension of system. The added SLS reduced the interfacial tension obviously, which help to broaden the potential application of HMHPAM. In the HMHPAM solution, the SLS can enter the hydrophobic domains of polymer chain through hydrophobic interaction, which can enlarge the molecular chain of polymer and increase the system viscosity.3. The influence of inorganic cations on HMHPAMThe effects of Na+, Mg2+, Ca2+, Cr3+and Fe3+ions on the HMHPAM solutions was explored using the MD simulations and experimental methods. It was found that the influence of multivalent cations on the property of HMHPAM was not rely on the water absorbing ability of cations, but the direct interaction between the cations and polymer. The influence degree of cations on HMHPAM solution was Fe3+>Cr3+>>Ca2+>Mg2+>Na+. The Cr3+and Fe3+both have coordination interaction with acrylic anion groups and there would be about three acrylic anions groups bound with Cr3+or Fe3+. But the interaction degree of Cr3+or Fe3+with HMHPAM in solutions were found to be different, therefore their influence on properties of HMHPAM were dissimilar, trace amount of Cr3+can be regarded as cross-linker and increase the systems’ viscosity, but the Fe3+can only decreased the viscosity of polymer solution. The hydration ability of Mg2+was bigger than Ca2+, but Ca2+have more strong influence on capacities of polymer solution, which was reordered to the classical Hofmeister series. It was found that the hydration shell of Ca2+was loose and dehydrated easily which made it interact with acrylic anions directly. The Ca2+can form stable dipolar pair with different acrylic anions though strong electrostatic attraction and leads to the forming of big aggregations of polymer. While the dense hydration layer adhered to Mg2+strongly which made the Mg2+only enter the second hydration shell of acrylic anions and the formed Mg2+-HMHPAM dipolar pair was unstable, so the influence of Ca2+on the viscosity of HMHPAM solution was bigger than the Mg2+4. The interfacial property of DDAThe MD simulation and experimental methods were combined to study the interfacial behavior of DDA. It was found that the mechanism of DDA systems getting ultra low interfacial tension was ascribed to the strong hydrogen bond between the head groups of DDA and water molecules, form thick hydrated layer with water. The formed hydrated layer can be regarded as a barrier stopping the oil phase from directly contacting with water phase, which turns oil/water interface into oil/hydrophobic tail of DDA/hydrated layer of head group/water interface, and reduces the interfacial tension significantly. In addition, the interfacial charge has a great influence on the oil/water interfacial tension of DDA system. For the DDA solution, the added ionic surfactants introduced charge into the oil/water interface. The higher charge of surfactant head group always means the stronger electrostatic repulsion between each other, and the oil/water interface can contain some macropores with no surfactants covering. In the macropores domains, the water molecules and oil molecules contact directly and the interfacial tension increases seriously. So it can be concluded that the interface with low or zero charge would be beneficial for getting the ultra interfacial tension system. The mechnism was testified in the SLS/CTAB system.5. The oil displacement efficiency of the two types of new flooding agentThe flooding efficiency of hydrophobically modified polymer and non-ionic surfactant DDA system were studied by a sample flooding model. It was found that both of these two systems can increase the displacement efficiency, while the mechanisms were different. The HMHPAM solution has high viscosity, which can reduce the mobility ratio between oil phase and water phase, relief the crossflow effect of displacement fluid and increase the oil displacement efficiency. While the low interfacial tension of DDA system can promote desorption and dispersion of crude oil from rock surface, and increase the oil displacement efficiency as a result. In addition, as a kind of non-ionic surfactant, the DDA can lead to the dispersive crude oil aggregate again. The aggregated crude oil drops can be regarded as blocking agent which can prevent the fingering effect and crossflow effect of the displacement fluid in the process of oil displacement. Compared with other flooding agents, the non-ionic surfactant DDA has the best oil displacement efficiency.