The Micro-flow Theory In Low-permeability Porous Media Reservoir

China has abundant resources of low permeability reservoirs, but limited by the current theory and technology development, the recovery can only reach about 20%. This is far below the indicators of the middle and high permeability layer after water flooding - chemical flooding, whose recovery can arrive at 60 %. There are many reasons affect the low recovery in low-permeability reservoirs. According to the definition of recovery, affected efficiency and micro oil displacement efficiency determine the recovery. The micro oil displacement efficiency in low-permeability reservoir is affected by many factors such as pore structure, fluid property of displacement and so on. It is critical to study the flow regulation of different fluids in porous media and the limit factors of micro oil displacement efficiency in low permeability reservoir.Low-permeability reservoir has complex pore structure, small throat and large pore throat ratio, and non-Darcy phenomenon is the main characteristic of low permeability reservoirs. The flow features, the micro-force and its leading factors changed significantly because the liquid is affected by the rock wall while flowing in the sprue. In this paper, based on previous research results of many micro-fluid flow and non-Darcy flow regulation, the micro-mechanism of water displacing oil and the micro force in porous media are studied; and the fluid viscosity coefficient equation in the micro sprue with the interaction of solid and liquid and the two-dimensional flow control equations and numerical equations are given; using the calculation results, the flow field distribution maps of two types of flow channel model, that is, contraction-expansion and expansion-contraction sprue are plotted, the fluid flow characteristics are analyzed under the conditions of different model parameters, and the fluid flow regulation is considered for two cases, that is, considering the role of solid-liquid interaction and without considering the role of solid-liquid interaction; the effective porosity conformance efficiency is determined by the stress method. Research results obtained are as follows:Through the research on the forces of the oil droplets in the porous media, the relationship of forces among the fluids during the whole development of reservoir and the influence of it on the distribution of resident oil are analyzed. Especially, considering the action of buoyancy on the oil droplets in different steps of development of the reservoir, Darcy's Law is used to study on the influence of the buoyancy of fluid on vertical distribution of resident oil. The result shows that the pressure gradient of the central region between the oil & water wells is far less than that near the well bore region, but the difference of buoyancy and gravity does not change significantly between oil & water wells, resulting in the angle's becoming larger between the direction of the force that the oil droplets suffered and the horizontal direction. The drift trend upward of oil droplets increased, and it is easy to form the residual oil at the top of the reservoir.In order to be compared with the flow behaviors of the fluid in boundary layer when considering the action between the solid and the liquid, the governing equations and the numerical finite difference equations of the 2-dimentional flow of Newtonian fluid are established, and through the numerical calculation of the convergent-divergent and the divergent-convergent model of flow path and the parameters of these two models, figures of the velocity, the stress and the flow line distribution are plotted. The result shows that under the condition of the same pressure gradient at the inlet, the velocity, the stress and the stream function distribution in the flow path are controlled by the parameters of the model. The calculating result of the divergent-convergent model of flow path with the same radius of pore, different radius of pore throat and different pore length is analyzed, and it is founded that the pore throat ratio and the radius of throat are the main factors which influence the distribution of flow field, while the influence of pore length is relatively little; as to the convergent-divergent model of flow path, with certain pressure gradient at the inlet and pore radius, the influences of the pore throat ratio, the radius of throat and the throat path length on the distribution of flow field in flow path are not obvious.On the basis of the study for solid and liquid microscopic force between molecules,the solid-liquid interaction was considered to be the main factors of the micro porous channel boundary layer of the formations. When fluids flow in extremely small radius of flow channels, the fluid boundary layer become fluids, its viscosity which was different from the conventional liquid viscosity was expressed as the increasing viscosity on the macro, and its value equaled the sum of fluid phase viscosity and solid-liquid effects caused by the additional viscosity. Through the force analysis of the molecules of the boundary layers, a boundary layer expression of fluid viscosity coefficient was given, and its value was a function of the distance from the solid interface and the nature of the solid-liquid surface. Using the expression, a two-dimensional boundary layer flow control equation and its simplified equation were set up, and the circular and rectangular cross-section velocity profile capillary distribution of the approximate solution was acquired as well. By divergence of the two-dimensional boundary layer equations using finite difference method, a numerical equation of the two-dimensional boundary layer equations was set up. Through the programmed computational procedures, the numerical solution of the circular section which considered the solid-liquid capillary flow velocity distribution was given firstly. Compared with the approximate solution above, we found that their speed distribution coincide, therefore we proved the correctness of the boundary layer numerical equation which considered the solid-liquid interaction and boundary conditions; secondly, using the numerical boundary layer equation, two types of pores: contraction-expansion, expansion-contraction, the velocity, stress and flow line fields have been calculated and the flow field distribution maps of different model parameters were acquired. In contrast with the non-boundary layer fluid flow characteristics, we knew that when considering the solid-liquid interaction, the fluid flow rate decreased in magnitude, the effective affected area of fluids within pores reduced, throat radius diminished, and the performance of solid-liquid effect became more significant.Using the relationship among the geometry of pore model and the hydrodynamic parameters derived from the real reservoir conditions in Daqing Oilfield, the lower limit of the yield stress of oil in micro-pore was determined. According to the calculation results of stress in the expansion-contraction sprue and the limit value of this yield stress, the effective displacement efficiency can be calculated. The result shows that the bigger the pore throat ratio, the lower the displacement efficiency.