Study On Imbibition And Oil Displacement Mechanism Of Nano-Fluid In Low Permeability Sandstone Reservoir

Low permeability sandstone reservoir is an important oil and gas resource in China,which has poor petrophysical property and high heterogeneity,leading to a large amount of crude oil remains in the pore matrix after water injection.Nano-fluid is a new type of functional fluid,it can improve the recovery efficiency of matrix by changing the properties of reservoir rocks and fluids through adsorption.This paper takes the low permeability sandstone reservoir in Yanchang Z area of Ordos Basin as the research object,use surfactants to modify nano-Si O₂particles to improve the dispersion of nano-Si O₂ particles in aqueous solutions.To study the effect of Si O₂ nano-fluid in improving core oil absorption and drainage,the research content includes the following four aspects:（1）Evaluated the nano-fluid dispersion stability,reduce the oil-water interfacial tension and change the ability to change the wettability to screen the types of surfactants.The experimental results show that the nano-Si O₂ particles modified by anionic surfactants（SDBS and APEC-Na）and nonionic surfactants（OP-10 and APG）have good dispersion stability,and the SDBS modified nano-Si O₂ particles can effectively reduce the oil-water interfacial tension and reservoir is changed to water-wet from oil-wet.Thus,the best nano-fluid system was determined:0.05 wt%SDBS+0.05 wt%nano-Si O₂ particles.（2）Evaluated the nano-fluid reduction of oil-water interfacial tension,wetting performance and emulsifying performance.The results show that the performance of nano-fluid is better than SDBS solution.Based on the good salt resistance,temperature resistance and less damage to reservoir,the static imbibition and dynamic imbibition effects of nano-fluid were evaluated.The results show that:compared with SDBS solution,nano-fluid can improve the static and dynamic imbibition recovery by 7.05 and 4.25 percentage points,respectively.（3）Studied the law of imbibition and oil displacement of nano-fluid in low-permeability sandstone reservoirs,explore the effects of displacement velocity,permeability,wettability,water saturation and fracture on the core recovery of nano-fluid displacement.The results show that the optimal displacement velocity of the nano-fluid is 0.1 m L·min^-1;for low permeability sandstone cores with permeability in the range of 5×10^-3-15×10^-3μm²,the permeability is positively correlated with the recovery percent;the core with neutral wetting and low water saturation is more conducive to imbibition and oil displacement of nano-fluid;the higher the viscosity of crude oil,the lower the recovery rate of nanofluid displacement.（4）Analyzed the force characteristics of nanoparticles in reservoir microchannels,and calculated the interaction energy between the nanoparticles and the reservoir wall,from the two aspects of nano-fluid adsorption at the oil-water interface and the solid-liquid interface,combining with microscopic visualization displacement experiment,the mechanism of nano-fluid to improve the effect of imbibition and oil drainage is described.The results show that the effective adsorption of nanoparticles at the oil-water interface and solid-liquid interface can change the wettability of oil-wet cores,reduce oil-water interfacial tension and adhesion work,improve the imbibition power and reduce the flow resistance,which is conducive to the migration and discharge of crude oil from the rock pore throat.