Study On Percolation Behavior Of Polymer Flooding At The Pore Scale

Polymer flooding, as a mature technique of enhancing oil recovery, has been industrially used in China, especially in Daqing Oilfileds and has been playing an important role in the stable and high production of oil. Many scholars have made research and obtained significant knowledge in its percolation theory and mechanism of oil displacement. However, in the field of micro-percolation heory and the mechanism of oil displacement, the main technical approach of the current studies is through laboratory experiment. In this paper, through a great deal of research from home and abroad, research was made in the percolation law of non-Newtonian power law fluid and viscoelastic fluid as well as two-phase percolation rule of polymer flooding. Through analysing the stress blance of a single oil drop on the oil wet rock, the mechanism of polymer oil displacement agent was explored and the reason of how polymer fluid could enhanced oil recovery was discovered. The research findings showed as follows:Suppose that power law fluid had similar distribution of Contorovich variational velcoity. Through calculus of variations, the flow drawdown relation of power law fluid in the triangle and rectangle capillaries was established. The result through calculus of variations showed higher precision solution in the bigger capillaries and fluid rheological parameter, compared with Matlab numerical solution and correlated document. Besides, the flow drawdown relation could be used in pore scaling porosity network analog and capillary grouping modeling. On the basis of the flow drawdown relation of power law fluid in a single capillary, calculation methods of effective permeability in a single capillary and capillary grouping were given, which showed that the effective permeability in the circular capillary and tube bundle was among the top in the case of the same cross-section area.On the basis of mechanical energy equilibrium principle and the work of Binding, shear viscosity, extension viscosity and the first direct stress difference were used to describe rheological parameter of viscoelastic polymer fluid. Flow drawdown formulas of polymer fluid in the convergent, convergent divergent circular and sinusoidual capillaries were established. When flow velocity was higher than a certain critical value, pressure drop caused by extensional stress was higher than that caused by shear stress. As to the polymer fluid in this paper, the effect caused by the first direct stress difference on the total pressure drop was too small to ignore.If porous media was considered as sinusoidal capillaries, the percolation model to describe the flow behavior of viscoelastic fluid is established. Compared with the laboratory experiment done by Koshiba, this paper revised the model so that it could better represent the extension character during the flow of viscoelastic polymer fluid in the porous media.Oil displacement process of Newtonian fluid under the water wet condition was simulated through conventional capillary bundle model and Interacting capillary bundle model raised by Dong. Conventional capillary bundle model negleted the physical characteristics during the course of percolation and had its defects when simulating flow in the water wet reservoir. However, Interacting capillary bundle model could well fill the gap. This paper spreaded the application of the model raised by Dong to oil/water two-phase flow of non-Newtonian power law fluid and damped least square method was used to get the solution. During the displacement of non-Newtonian power law fluid, power law exponent and consistency coefficient decreased, which made the gap of frontal velocity get bigger in different sizes of capillaries and made the displacement tend to non-uniform. This is qualitatively in accordance with the macroscopic model raised by Wu Yushu.Through interfacial rheometer CIR-100, interfacial viscosity, interfacial viscosity modulus and interfacial elasticity modulus could be measured. They all decreased and then increased as the temperature rose. Besides, they all increased as the polymer molecular weight and the concentration of polymer fluid rose. With the increasing salinity of polymer fluid, they decreased. Integral momentum balance equation proposed by Slattery is used to study effect of interfaical parameters on polymer flooding process, results shows that for values of interface tension less than the critical value the displacement veloctiy of residual oil will increase as the applied pressure gradient increases and since the increasing of interfacial viscosity in polymer flooding the displacement velocity of residual oil will decrease.Fluorescence analysis of poymer flooding showed that polymer fluid could greatly decrease the oil film remaining oil. As to this type of remaining oil, the dynamic model of a single oil film attaching to the surface of rocks was established in the reservoir flow field.The driving force of the oil film was the integration of shear stress and direct stress on the surface of the oil film when the oil displacement agent flowed through. And resistance force was the wettability hysteresis when the oil film got deformed and the binding force of the surface of rock to oil film. When the driving force was equal to resistance force, critical flow velocities at which the microscopic oil film started to move were given during the water flooding and polymer flooding. The analysis showed that the polymer fluid had high extension viscosity, which could produce extension stress that was higher than shear stress 1~2 order of magnitudes. It benefited the start of the oil film. However, during water flooding, the extension stress could be neglected and the shear stress could hardly exfoliate the oil film from the surface of the rock.