Study On Mechanism And Applicability Of Low Salinity Waterflooding Based On DLVO Theory

In recent years,numerous experimental studies and field trials of sandstone and carbonate rocks have proven that low salinity waterflooding can alter the wettability of rock and enhance oil recovery effectively for both secondary and tertiary stages of waterflooding.It has become the focus of research in petroleum industry owing to the enormous advantages of high efficiency in displacing oils,ease of injection into oil-bearing formations,easy access and operation of water,low investment and pollution,all of these advantages make it a more cost-effective compared to other enhanced oil recovery（EOR）methods.There are many EOR mechanisms for low salinity waterflooding,among which the wettability alternation is considered as the main mechanism behind low salinity waterflooding,but what factors mainly control the wettability alternation is still uncertain.Other scholars have proposed that rational use of fine migration can effectively change the physical properties of reservoir rock and injection profile,thus achieving equilibrium displacement and enhancing oil recovery.Based on the Derjaguin-Landau-Verwey-Overbeek（DLVO）theory and Electric Double Layer theory of colloid stability,the mathematical model of Crude Oil/Brine/Rock（COBR）disjoining pressure and clay particle force are established.The effect of injected water salinity,ion valence and velocity on the disjoining pressure and clay particle force are analyzed from the microcosmic view in this paper.Metastable wetting-water film thickness,contact angle of water on rock and particle migration concentration are calculated by combining the Young-Laplace equation,interaction potential theory and maximum concentration of attached fine particles equation,which can accurately predict the change of rock wettability and reservoir permeability with water injection properties.Thus,the characterization relationship between water injection properties and reservoir rock/fluid physical property parameters is established.In order to explore the main mechanism of wettability alternation and find the reservoir numerical simulation method,the history matching and numerical simulation of coreflooding experiments are carried out.Aiming at the problem of interlayer interference in vertically heterogeneous reservoir,the numerical simulation of low-salinity waterflooding is carried out in high water-cut stage.According to the geological conditions of the actual block of the 5²⁺³sandstone reservoir in the west of the 7th district of Gudong Oilfield,a conceptual model of multi-layer heterogeneous reservoir is established,which coupled the wettability alternation and fines migration EOR mechanisms of low salinity waterflooding,then the influence of low salinity waterflooding slug injection parameters on development effects is studied.Finally,combined with the oilfield example,the development effect of the optimized scheme in the actual reservoir block is comprehensively evaluated.The results show that lowering the salinity of injected water and the concentration of divalent cations can increase the electrostatic repulsive force between the COBR interface and clay particles,resulting in a thicker stable water film and causing the migration of clay particles,thus leading to a more water-wet rock surface and the decrease of reservoir permeability.By altering the oil-water relative permeability parameters,residual oil saturation and calculating the disjoining pressure,the experimental data of coreflooding and rock contact angle measurement can be well matched,which fully proved that the double layer expansion is the main mechanism behind the wettability alteration of low salinity wateflooding.After carrying out low-salinity waterflooding in high water-cut stage,the injection profile is adjusted and the interlayer interference is relieved,therefore achieving equilibrium displacement and improving the cumulative oil recovery about 3%beyond conventional seawater flooding.This study can provide preliminary theoretical support for the optimization of low salinity waterflooding compatibility system,reservoir applicability screening and development potential evaluation to obtain the maximum oil recovery.