The Study Of Microscopic Seepage Mechanism For Polymer Flooding

During the development of oilfield, the problems of Non-Newtonain fluid are often encountered which usually exhibit shear-thinning (pseudo-plastic) behaviour in solutions. The bulk macroscopic properties of these solutions, mainly their viscosity/shear-rate dependency, are well understood and characterized using established models. Conventional methods can not predict the shear-shinning behaviour from the bulk properties because of the double complexities of the porous media and Non-Newtonain fluid. Compared with the traditional method, the network modeling, which captures the geometry and topology of sands and sandstones, could be employed to study the flowing of Non-Newtonain fluid in porous media.Based on oil-water two-phase network simulation model, the microscopic simulation model for polymer flooding was provided together with polymer seepage mechanism (tackifying, diffusion, adsorption, trapping, retention, shear degradation). By means of 3-D network model, the organic unity of reservoir rock pore structure and the rheology of fluid are obtained. Through the microscopic simulation, the seepage mechanism of non-newtonian single phase and multi phase flow are studied essentially. Laws of remaining oil after polymer flooding are statistically analyzed. On the basis, laws of distribution of retained polymer and its influence factors are studied which provided necessary basis for the study of the starting parameters after polymer flooding and its influence factors.The non-newtonian single phase flow simulation showed that compared with conventional methods, network model could represent the topology structure of the porous media and fit the results of single phase flow well without re-scaling. The non-newtonian multi phase flow simulation showed that the water-phase relative permeability decreased as the pressure gradient increased, when it reached a certain value the water-phase relative permeability is increased to the Newtonian one as the pressure gradient continued to increase. The forms of remaining oil distribution were divided into single grain/island shape,fleck shape, network shape and oil-water mixed shape. Microscopic simulation showed that the retained oil of network shape decreased significantly while the volume ratio of single grain/island shape,fleck shape and oil-water mixed shape increased. Under simulated conditions, the retained polymer account for 61.7% of the injected polymer. Pore radius and shape factors are the main influencing factors. The concentration of retained polymer in pores is inversely proportional to the product of the pore radius and the square root of shape factor. The study of starting parameters of remaining oil after polymer flooding showed that the remaining oil can be started by increasing the viscosity of the injecting fluid which needed to be increased to a high one. The remaining oil saturation can be decreased significantly by injecting a slug with low interfacial tension. Compared with polymer flooding, the method of injecting a slug with low interfacial tension after polymer flooding has the same mechanism with the polymer flooding, and the laws of the statistic distribution of the remaining oil are consistent. When the interfacial tension changed after polymer flooding, the distribution of remaining oil changed greatly.