Study On Enhanced Recovery By Water Displacement At Pore Scale

Petroleum is an important transportation-driven energy source.China has an imbalance in the production and import of petroleum resources.Oil production can be improved through enhanced oil recovery technology,which can balance the gap between oil production and imports.With the increasing maturity of microfluidic chip processing technology,scholars at home and abroad can study the effects of capillary force,viscous force,and gravity on the water flooding process at the pore scale.However,traditional gravity flooding research methods are not conducive to characterizing the mutual influence of gravity,capillary force,and viscous force on the two-phase flow in the field of gravity flooding,or there is no quantitative characterization of residual oil saturation changes in gravity flooding.Therefore,the experiments and numerical simulations in this paper are used to discuss the effects of gravity,viscous force,and capillary force on the two-phase flow of water flooding.In this paper,the microfluidic flow control board is used to control the flow rate of the injected fluid.The two-phase displacement process and the pressure at both ends of the porous medium are recorded through the microfluidic flow sensor and camera,which aims to explore the trend of residual oil saturation and trapped oil mass at a single flow rate.In addition,the phase field method is used to study the water flooding process of horizontal/gravity flooding in this paper.Fluid pairs of different physical properties(density,viscosity),average velocity of injection and gravity are set to control the relative magnitude of viscous force,capillary force and gravity.Experimental and numerical simulation studies have found that:(1)During the displacement process with a single flow rate,viscous fingering produces a large area of trapped fluid mass,which leads to a decrease in the recovery rate of n-decane solution.As the amount of fluid mass increases,the residual oil saturation gradually decreases.(2)For the water-oil horizontal displacement process,the undisplaced oil occupies most of the residual oil saturation during capillary fingering.When the mainstream path breaks through the outlet end of the porous medium,the other flow branches will stop continuing to flow forward.At this time,the pressure in the main flow path decreases,causing oil to flow back into the previously occupied channel.In the process of viscous fingering,trapped oil mass is the main mechanism of residual oil formation.The residual oil saturation is inversely proportional to the number of capillaries.The effect of enhancing the viscous force(increasing the flow rate)or using a fluid with a favorable viscosity ratio can play a role in enhancing the recovery factor.The characteristic length of the interface is linearly proportional to the water saturation.The breakthrough time of water in porous media decreases as the number of capillaries increases.Increasing the flow rate and using unfavorable viscosity ratio fluid pairs will aggravate the premature breakthrough of the flow path in the porous medium and reduce oil recovery.(3)During the water-oil gravity flooding process,compared with horizontal flooding,the front edge of gravity flooding will break through the outlet of the porous medium earlier at a low flow rate.After the mainstream path breaks through the outlet end,the pressure drops sharply,and priority flow is formed.Under the unfavorable viscosity ratio,when the capillary number is low and the gravity number is high,the undisplaced oil accounts for most of the residual oil saturation,and as the capillary number increases and the gravity number decreases,large areas of trapped oil begin to appear The amount of undisplaced oil is reduced.When the capillary number is high and the gravity number is low,the residual oil type is mainly oil trapped in a small area.The residual oil saturation has an inversely proportional linear relationship with the number of log capillaries,and the density ratio has a limited effect on the gravity flooding process.Gravity flooding exacerbates the instability of the water-oil flooding process at a low capillary number,causing the displacement front to break through the outlet end of the porous medium prematurely.The residual oil saturation is directly proportional to the logarithmic gravity.When the average injection velocity is high,the effect of gravity on the water-oil flow is small.A single dimensionless number cannot well characterize residual oil saturation.The combined dimensionless number considers the influence of gravity,capillary number,viscosity ratio,and density ratio on residual oil saturation,and can well describe the change trend of residual oil saturation.