Study Of Effect Of Gravity On Two-phase Flow In Porous Media At Pore Scale

Two-phase flow in porous media is commonly found in engineering and natural systems,such as enhanced oil recovery,soil environmental remediation,fuel cell energy storage technology and carbon dioxide capture and storage in geological reservoirs,biochemistry,etc.Therefore,the study of two-phase flow dynamics has received extensive attention at home and abroad.However,most of the studies on two-phase flow mechanisms by domestic and foreign scholars have been conducted in the horizontal injection direction,and there is no more comprehensive study on how gravity will affect the flow characteristics of the displacement process.In this paper,the interaction between viscous forces,gravity and capillary forces is modified by controlling the injection direction as well as the injection flow rate.A micro-fluidic visualization system was used to observe the flow state of the two-phase displacement process in homogeneous porous media,and the relationship between the dimensionless number and the flow state,as well as the recovery factor,was analyzed under different conditions.The micro-particle imaging velocimetry(micro-PIV)system was used to observe the two-phase displacement process at pore-scale,comparing the advancement characteristics of the two-phase interface during upward and downward displacement,the formation of trapped residual oil,and the Haines jump phenomenon,and the experimental analysis of the velocity field to quantify the fluid interactions.The combined results of the two experiments at pore-scale two-phase flow mechanism together lead to the following conclusions.(1)Under the vertical-upward,vertical-downward and horizontal injection methods,a stable displacement front can be obtained at large injection flow rates,and the flow stability gradually deteriorates as the injection flow rate decreases,forming the fingering front.In the downward displacement process,there are two kinds of displacement states at low flow rates,stable and unstable,and the frequency of unstable displacement state increases with decreasing flow rates.Compared with the horizontal and upward displacement process,the recovery factor of downward stable displacement state is the highest,and the recovery factor of downward unstable displacement state is lower,and the overall recovery factor of horizontal and upward displacement process is not much different.(2)The critical capillary numbers for the stable state transitions of upward,horizontal and downward displacement processes are 1.4×10^-5,4.44×10^-5 and 1.4×10^-4,respectively.A mathematical model is developed to predict the displacement state transitions under different conditions based on the pore filling theory and the critical capillary numbers.The empirical equations for the prediction of the recovery factor of two-phase displacement applicable to different injection models are constructed by combining the capillary number,gravity number,Bond number and viscosity ratio,which provide references for the study of two-phase displacement processes in porous media and practical engineering applications.(3)The front propulsion during upward and downward displacement is different.In the downward displacement,gravity acts as the propulsive force to intensify the fingering front,leaving a large area of trapped residual oil in the porous medium,while in the upward displacement,gravity acts as the drag force to hinder the advance of the fingering front,causing the flow path from the bottom of the fingering front to flow horizontally,or even backward.The Haines jump phenomenon is found in the upward and downward displacement processes.The Haines jump velocity in the unstable displacement state is 3-12times the average velocity of the flow path,while the Haines jump velocity in the stable displacement state remains 2-2.7 times the average velocity of the flow path,and the Haines jump phenomenon affects the velocity of a wide range of flow paths.(4)The fluid-fluid and fluid-solid interactions are visualized by velocity and vorticity fields,and it is found that the flow velocity in the middle of the flow path is the largest due to the viscous effect,and the flow velocity near the skeletal particles is the smallest.With the center of the flow path as the symmetric boundary,the left and right sides of the flow path show opposite vorticity distribution,and the vorticity at the corner is significantly larger than that in the flow path,and the magnitude of the vorticity increases with the increase of the injected flow rate.The non-zero velocity distribution is measured near the interface between the trapped oil phase and the water phase,which is only one order of magnitude smaller than the velocity at the center of the flow path,and it is found that the fluid in the dead pore is not completely stationary,but shows a smaller vorticity distribution,which indicates that the fluid-fluid interaction and momentum transfer are not negligible.