Study Of Mechanism Of Fluid Flow In Micro-Nano Pores Of Tight Oil Reservoirs

China's tight oil is rich in reserves,and the exploration and development progresses greatly in recent years.Tight oil becomes alternative energy sources of conventional reservoir in the situation that the latter is faced great difficulties to make increase of production.Micro-nano throats widely develope in tight reservoirs.Such throats lead to large decrease of permeability which is called micro flow effect.At present,the understanding of flow mechanism in micro-nano throat is insufficient.The characterization of seepage parameters of tight oil reservoir is not reasonable.Therefore,studies of flow mechanism in such throats and the characterization of seepage parameters in tight oil reservoir are of significance.Firstly,studies of water flow in micro-nano throats were conducted.Based on the method of Dissipative Particle Dynamics(DPD),water flow in nano sized quartz slits with width ranged 2-150 nm was simulated.The potential function of LJ-96 was introduced to characterize the interaction between water and solid rock.The molecular structure was optimized to minimize the amount of calculation,which enlarged the scale of discrete medium simulation.The result showed that there was a belt of higher density near solid wall.Fluid there was stagnant,which was also called boundary layer.For water flow in micro tubes,a micro flow experimental platform was established,and the flow experiments in tubes with radiu ranged 1.377-15.361?m were completed,and the impact of pressure gradient and tube radius on micro flow was studied.The result showed that flow rate of water in micro tubes was lower than theoretical value and showed nonlinear characteristics.The nonlinear characteristics were determined by flow rate displacement.The cause of nonlinearity and the conversion from nonlinear flow to linear flow was analyzed.The effect of boundary layer thickness on micro flow was analyzed.Secondly,flow characteristics of main component of tight oil in micro-nano throats were studied,such as n-hexane,n-octane and n-sixteen alkane.Base on DPD method,the flow of n-hexane in quartz slits of 2-15 nm was simulated.On improvement of the micro flow platform,the radius of flow experiment was reduced to 660 nm.The experiments flow of n-hexane,n-octane and n-sixteen alkane in tubes with radius of 660nm-4.840?m were conducted.The characteristics of flow rate,velocity,flow resistance and nonlinearity were analyzed.The conversion from nonlinear flow to linear flow of the alkanes was compared.The impact of pressure gradient,throat radius and fluid properties on micro flow was investigated.The result showed that the longer the molecular chain was,the higher the flow rate displacement,the lower the beginning pressure gradient of linear flow.Thirdly,in order to reveal the impact of fluid-solid interaction,the two phase flow of water and oil was simulated by DPD method.Two sets of fluid-solid interaction were introduced.The density profile and velocity profile of water and oil under different fluidsolid interaction was analyzed.In flow experiments,the investigation of impact of polarity of fluids on micro flow were carried out by using deionized water and isopropanol.The result showed that the thickness of boundary layer of water,whose polarity was higher,was thicker.Four different strengths of fluid-solid interaction were imposed for the flow of deionized water and n-decane to compare the difference of flow characteristics.Finally,on basis of the study of flow characteristics in micro throats,new method of characterizing the apparent viscosity of the fluid at micro-scale was optimized.Based on the assumpution of dynamic boundary layer,the velocity profile of water flow in micro slits and tubes was derived.Based on the assumpution of stagnant boundary layer,the thickness of boundary layer was regressed,considering radius,fluid viscosity and pressure gradient.Based on the characterization of thickness of boundary layer,the effective throat radius distribution of capillary bundle model was characterized,and the effect of original distribution of throats,the radius distribution,fluid properties and pressure gradient was analyzed.The effective permeability considering the thickness of boundary layer was characterized and applied to tight oil reservoir methematic model.The impact of boundary layer on tight oil reservoir seepage was investigated.In this paper,flow characteristics and nonlinear flow mechanism in micro-nano throats were clarified.The impact of fluid-solid interaction on micro flow was studied.Its effect on seepage of tight oil reservoir was further analyzed,which laid a theoretical foundation of the efficient development of tight oil reservoirs.