| There are a large number of nano-micron sized pores in the low permeability tight reservoir,with low permeability and large specific surface area.When the fluid flows in the nano-micron sized pores,the interface effect between the solid wall surface and the fluid is enhanced,and the micro force effect cannot be ignored.The flow performance is manifested by the increase of fluid viscosity near the solid wall,the significant electric double layer effect and the increase of the proportion of adsorbed water film in the pores,which will have a significant impact on the fluid flow pattern in the pores.However,the extent of the effect of microscopic forces within the pore at the nanometer scale on the oil-water two-phase seepage characteristics is still unclear,and it is difficult to give its influence on actual reservoir development and accurate simulation prediction.Therefore,the effect of microscopic force action on oil-water two-phase seepage characteristics by combining the DLVO theoretical model(van der Waals force and electric double layer force),diffusive electric double layer theoretical model and pore network simulation based on nanoscale mechanical property experiments are systematically investigated in this thesis.The main research results are displayed as follows:(1)Contact-mode based atomic force microscopy(AFM)is used to probe the morphological characteristics and force variations between the AFM probe and SiO2 in the deionized water and polymer solution environments on water-wet and oil-wet substrates,respectively.In the hydrophilic environment,when the distance between the AFM probe and the solid surface exceeds 8.3 nm,the forces calculated by DLVO theory are in good agreement with the measured microscopic forces;in the hydrophobic environment,when the distance between the AFM probe and the solid surface exceeds 10.6 nm,the forces calculated by DLVO theory are in good agreement with the measured microscopic forces.Therefore,the van der Waals and electric double layer forces play a dominant role in the microscopic forces at the solid-liquid interface after a certain distance.(2)A modified fluid viscosity model is used to characterize the effect of van der Waals forces on fluid flow,and a mathematical model of single-phase fluid flow in nanometer pores considering the effect of van der Waals forces is developed and extended to a three-dimensional quasi-static pore network model of oil-water twophase.The effects of van der Waals forces on the residual oil distribution and oilwater two-phase seepage characteristics under different wetting conditions are investigated.The results show that the effect of van der Waals force on the residual oil distribution under water-wet conditions is significant,while the effect on the residual oil distribution under oil-wet conditions is small;the influence of van der Waals force on the oil displacement efficiency increases and then decreases with the increase of oil-water contact angle.Among them,the strongest effect on oil displacement efficiency is for neutral wet conditions and the weakest effect is for oil-wet conditions.(3)A mathematical model of single-phase fluid flow within a nano-micron sized pore under pressure drive is constructed by combining the more generalized Poisson-Nernst-Planck equation and Navier-stokes equation for the problem that the flow of formation water in a nano-micron sized pore can generate electric double layer overlap.A finite element numerical simulation method is used to solve the model,and the numerical simulation results are verified using one-dimensional micro circular tube flow experiments.The effect of the electric double layer effect on the single-phase fluid flow within the nano-micron sized pores increases and then decreases with the increase of the pore radius.Based on the single-phase conductivity calculation model,the water-phase conductivity at the corner is obtained,and a three-dimensional quasi-static pore network model is established for oil and water phases considering the electric double layer effect.The effect of the electric double layer effect on the oil-water two-phase seepage law and the remaining oil distribution is clarified,and it is revealed that the electric double layer overlap is the fundamental reason for the effect of the electric double layer effect on the fluid flow in the nano-micron sized pore.(4)The physical simulation results of AFM contact-model measurement show that under hydrophilic conditions,a stable adsorbed water film is formed between the aqueous phase and the solid wall surface due to the combined effect of van der Waals and electric double layer forces.The DLVO theory is used to calculate the thickness of the adsorbed water film,and a three-dimensional quasi-static pore network model of oil-water two-phase considering the joint effect of van der Waals and electric double layer forces is established.The effects of the adsorbed water film caused by the combined effects of van der Waals and electric double layer forces on the fluid flow characteristics,oil-water two-phase seepage characteristics and residual oil distribution in the nano-micron sized pores are investigated.The results show that when the radius of individual pore is less than 10μm,the influence of adsorption water film on oil-water two-phase flow in nanometer pore is not negligible;with the decrease of average pore size and the increase of oil-water contact angle,the influence of adsorption water film on displacement efficiency gradually increases;the influence of adsorption water film on residual oil distribution is significant,in which the oil-water mixed residual oil is most affected by adsorption water film.Besides,the combined effect of van der Waals and electric double layer forces not only impedes the flow of water phase,but also reduces the flowable pore space of oil phase,which has an obstructive effect on fluid flow.The research results of this thesis can provide theoretical and technical guidance for the understanding of seepage characteristics and remaining oil recovery at the pore space scale. |
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