Flow Behavior And Mechanism Study On In-situ Emulsion Enhancing Oil Recovery In Surfactant/Polymer Flooding

Chemical flooding is an important technology to improve oil recovery in high water cut reservoir,which yield more than 1500×10~4t each year.Especially for the compound flooding containing surfactant,which will be one of the main technologies for the tertiary oil recovery after polymer flooding.However,emulsification is usually observed in compound flooding.Compare to the homogeneous solution,the existed dispersed phase makes the significantly different flow behaviors.Characterization of the flowing of in-situ emulsion with ultra-low interfacial tension is a hard work.Simultaneously,the commercialized petroleum sulfonate in pilots is different from the surfactants produced by fine chemicals.It is a critical issue to finding correct methods to scientifically evaluate the properties of emulsions.Moreover,judging the occurrence of emulsification in reservoir condition and using in-situ formed emulsion to further improve oil recovery in compound flooding are significance for high efficiency and stable production in petroleum industry.This paper firstly investigates the interaction in petroleum sulfonates/brine/oil system.The effect of salinity,carbon number,concentrations of surfactant and nanofluid on emulsion phase behavior are evaluated.The prediction of phase behavior is discussed through the HLD-NAC model.Then the process of in-situ formation of emulsion in deep reservoir is mimicked with the restricted porous media,and the pseudo-resistance coefficient is proposed to compare the influence of multiple factors on in-situ emulsification.The transport rheology of low tension emulsion is modified based on the power law fluid in-situ rheology.Moreover,four kinds of flow patterns are divided using the droplet generation microfluidic method.The dimensionless length of dispersed droplets containing multiple factors was determined,and the transition of different flow patterns in the restricted space are characterized by dimensionless parameters.Finally,the porous structure of fracture-matrix and porous media containing in-situ emulsion formation device are fabricated via the microfluidic etching technology.Laser confocal,nuclear magnetic resonance(NMR),visual models packed with graded sand and the full-size core displacement experiment are adopted to investigate the process and mechanism of surfactant/polymer flooding for enhancing oil recovery from pore-scale to core-scale.Considering these advantages of in-situ formation of emulsions,an inverse emulsion which can assist controlled release of polyacrylamides is developed and its application potential in the reservoir are evaluated.Our results show that the emulsion type transits from oil-in-water to bicontinuous phase to water-in-oil with the increase of salinity and carbon number of oil.The incremental surfactant concentration and the addition of functionalized nanoparticles contribute to the stability of the emulsion.The emulsification process can be directly combined with reservoir numerical simulation by calculating the oil-water solubilization ratio in the emulsion.The research on the flowing of low-tension emulsion in porous media indicates the emulsification of surfactant/polymer system presents stronger non-newtonian fluid properties.The dispersed process of emulsion flowing through porous media leads resistance increase,which makes the correction coefficient in the equivalent shear rate model less than the polymer solution with the same formula.The surfactant concentration has more effect than shear rate on the emulsification,while the oil saturation has a minimal effect.Based on the capillary number of continuous phase and the weber number of dispersed phase,the flow pattern of dispersed droplets with different fluid properties can be matched together.We establish the scale law of dimensionless sizes of dispersed droplets including two-phase flow rate ratio,viscosity ratio and capillary number of continuous phase.It has a higher fitting accuracy to the data in multiple literatures than the existing formula.Compared to sandstone,it is easier to emulsify for surfactant/polymer flooding in strong heterogeneity conglomerate reservoir.Adsorption of emulsions and“Jamin effect"are respectively mechanisms of emulsion to improve oil recovery of porous media and fracture-matrix.Fluid diversion could be only observed in the condition with properly matched sizes of emulsion droplets and pore throats,an incremental oil recovery of 25%over the SP flooding could be achieved by emulsion flooding if there was emulsion blockage.Otherwise,an incremental oil recovery of only 5%-6%could be achieved.The emulsion polymer contains swollen polymer micelles and gradually releases polymer after phase inversion,which causes its viscosity,viscoelasticity,and plugging capacity to increase with aging time.It has a higher recovery factor of 2.7%more than the conventional polymer under same injection viscosity.