Preparation Of Amphiphilic Janus SiO₂ Stabilized Styrene-Acrylate Microsphere Emulsion And Its Oil Displacing Performance

With the increasing difficulty of oil extraction,enhancing oil recovery through oil displacement technology is of great significance to ensure China’s energy security.Acrylamide-based polymer microspheres are one of the main oil displacement materials used in China.However,its synthesis process requires the use of a large number of oily solvents,high production costs,environmental pollution,poor injectability in the application process,temperature and salt resistance and other reservoir compatibility performance is poor.In this study,we proposed that using water instead of oil as the dispersion medium to prepare sizecontrolled nanoscale polymer microspheres with miniemulsion polymerization in order to reduce the production cost and improve the environmental performance of the flooding agent for low permeability reservoirs;designed and synthesized a series of amphiphilic Janus SiO2 nanoparticles,and use them as the flooding agents to improve the injection performance with their small size effect and improve the oil displacement ability with their ultra-high surface activity;six kinds of styreneacrylate polymer core-shell microspheres emulsion stabilized by Janus SiO2 were synthesized with the amphiphilic Janus SiO2 nanoparticles as emulsifier based on Pickering emulsion polymerization method.As an oil displacement agent,polymer microspheres and free SiO2 nanoparticles in aqueous dispersions synergize to exert the role of oil flooding,maximizing the effect of improving oil recovery.Also,reducing the use of surfactants is conducive to the sustainable development of oilfield exploitation.The main contents are as follows:(1)An oil-in-water styrene butyl acrylate polymer microsphere(P(St-BA))emulsion was prepared with styrene(St)and butyl acrylate(BA)as the main monomers.The results showed that when the amount of surfactant sodium dodecyl sulfate was 2.0 wt%,the amount of co-emulsifier n-butanol was 2.0 wt%,the amount of crosslinker N,N-methylenebisacrylamide was 1.0 wt%,the amount of initiator ammonium persulfate was 3.0 wt%,and the amount of functional monomer 2-acrylamide-2-methylpropanesulfonic acid was 2.0 wt%,the spherical latex particles of P(St-BA)had a particle size of about 250 nm and a single peak distribution.The application results showed that the spherical structure and dispersion performance of P(St-BA)were not significantly affected by the treatment of 90℃and 100 g/L mineralized water.The P(St-BA)could be used for deep plugging and profile control in medium and high permeability cores,and the recovery was increased by more than 23%,which had a good recovery enhancement ability.(2)Six kinds of amphiphilic Janus SiO2 nanoparticles were designed and synthesized based on Polyacrylamide/polystyrene(PAM/PS),polyacrylamide/polymethylmethacrylate(PAM/PMMA),sodium polystyrene sulfonate/polystyrene(PSSS/PS),sodium polystyrene sulfonate/polymethylmethacrylate(PSSS/PMMA),polyacrylic acid/polystyrene(PAA/PS)and polyacrylic acid/polymethylmethacrylate(PAA/PMMA).The results showed that the morphologies of the series of amphiphilic Janus SiO2 nanoparticles were regular spherical and uniform in size.The average particle sizes were 50 nm、70 nm、70 nm、55nm、70 nm、60 nm,respectively;the surface tensions were 53.7 mN/m,50.9 mN/m,54.4 mN/m,53.4 mN/m,59.8 mN/m,62.5 mN/m,respectively;the critical micelle concentrations were:0.071 g/L,0.078 g/L,0.049 g/L,0.045 g/L,0.185 g/L,and 0.035 g/L.Hydrophile-lipophile balance(HLB)values were 14,15,15,15,16,16,respectively.The application experiments showed that compared to the 6.7%increase in the recovery of unmodified SiO2 nanoparticles,the recovery increases of PAM/PS Janus SiO2,PAM/PMMA Janus SiO2,PSSS/PS Janus SiO2,PSSS/PMMA Janus SiO2,PAA/PS Janus SiO2,PAA/PMMA Janus SiO2 nanoparticles were 8.6%、11.6%、9.4%、11.2%、11.0%and 15.0%,respectively.(3)A series of amphiphilic Janus SiO2 nanoparticle-stabilized P(St-BA)microspheres composite emulsion was prepared by Pickering emulsion polymerization using six kinds of amphiphilic Janus SiO2 particles as stabilizers and St and BA as main monomers.The results showed that a series of composite emulsion latex particles were well dispersed core-shell spheres with particle sizes of 300 nm,470 nm,300 nm,500 nm,430 nm,420 nm,respectively.When dosage of PAM/PS Janus SiO2,PAM/PMMA Janus SiO2,PSSS/PS Janus SiO2,PSSS/PMMA Janus SiO2,PAA/PS Janus SiO2,PAA/PMMA Janus SiO2 were 0.3%,0.7%,0.7%,0.5%,0.7%,0.7%,respectively,the composite emulsion had a relatively high conversion rate and solid content,a low gelation rate and a relatively stable system with a small particle size.The application results showed that except for the PAA/PMMA Janus SiO2 nanoparticle-stabilized composite emulsion,the spherical structure and dispersion performance of the other five kinds of composite emulsions showed good temperature resistance and salt resistance at high temperature of 90℃ and 100 g/L mineralized water;PAM/PS Janus SiO2,PAM/PMMA Janus SiO2,PSSS/PS Janus SiO2,PSSS/PMMA Janus SiO2 stabilized P(St-BA)had better centrifugal stability and frost resistance than other two kinds of composite emulsions.The results of single-core flooding experiments showed that PAA/PS Janus SiO2-stabilized P(St-BA),PSSS/PMMA Janus SiO2-stabilized P(St-BA)and PAM/PS Janus SiO2-stabilized P(St-BA)exhibited excellent recovery enhancement performance,with recovery enhancement exceeding 30%.(4)The preferred three kinds of composite emulsion were used for further study.The macroscopic displacement experiments on a dual-parallel core were carried out.The experimental results showed that the reservoir adaptation range of microspheres with single particle size was limited,while composite emulsion combined in pairs could expand the range of adaptation to reservoir.The PAM/PSPSSS/PMMA Janus SiO2 stabilized P(St-BA)had the widest reservoir adaptation range,specifically for reservoirs with permeability of about 10×10-3 μm2 and permeability gradient range of 2-8;using periodic injection method with the injection volume of 0.5 PV and the injection concentration of 0.5%,this combined system could achieve a recovery enhancement of more than 30%in the cores with different permeability gradations.(5)The oil displacement mechanism of styrene-acrylate polymer microspheres stabilized by amphiphilic Janus SiO2 nanoparticles was studied.The polymer microspheres expand the range of water flooding and mobilize the remaining oil in the reservoir with high oil saturation through pressure reduction and profile control;the free amphiphilic Janus SiO2 nanoparticles in the aqueous dispersion system promote the detachment of oil film from the rock surface by changing the wettability of the rock surface,reducing the interface tension of the oil-water and wedge extrusion effect.The deep-profile control and the oil recovery enhancement of heterogenous low-permeability reservoirs are achieved through the synergistic effect of the polymer microspheres and Janus SiO2 nanoparticles.This study provides theoretical basis and technical support for the development and application of chemical flooding technology.