Application Of Nanofluid In EOR And Its Mechanism

Nanofluids are kind of dispersion system with nanoparticles dispersed uniformly and stably.And they have been widely used in different fields.It has already been proved that nanofluids with proper nanoparticles type and concentration could enhanced the oil recovery though sandpack experiments in laboratory.The factors which affect the nanofluid stability were studied firstly,including surfactants types,nanoparticle concentration and surface property.The surfactants included CTAB,AOT and TX-100;And the nanoparticles were SiO₂ nanoparticles（25-50 nm）with different surface hydrophobicity.Surfactant adsorbed on the SiO₂ surface and changed the surface wetting ability and surface charges accordingly.The nanoparticles were dispersed stably in the solution by electrostatic repulsion,and space repulsion after surfactant adsorption.According to the parameters of dispersed particle size,zeta potential and solution turbidity,we chose that0.05 wt%SiO₂ nanoparticles with 2.0 mM surfactant as the most stable formula.We have prepared stable nanofluids based on the primary results.The micromechanism of nanofluid flooding during oil recovery could be studied though the wettability alteration of the core wall,the interface tension changes between oil and water,and the peeling of oil film during displacement.An oil droplet was fixed on a quartz slice and the slice was displaced upside down on a transparent cell filled with nanofluid.The oil droplet shape and three-phase contact angel among oil,water and SiO₂ slice were observed with a contact angle instrument,in order to simulate the interaction between crude oil and nanofluid in reservoir.The result showed that the three-phase contact angle of oil droplet in nanofluids with AOT and TX-100 changed more than that in nanofluid with CTAB.And we observed the max three contact angel of oil droplet in the nanofluids with hydrophobic particles.The stripping of oil droplet is driven by the dynamic spreading of nanofluids on solid surface.For the flooding fluid without nanoparticles couldn’t peel the oil,the adsorption of nanoparticles on the rock surface might be a key factor in oil droplet stripping.we have established an ideal model to study the interaction between nanoparticles and rock wall.An AFM probe was modified with a SiO₂ beads on the cantilever to measure the adherence force between the bead and the silicon surface with AFM force mode.The experimental result showed that the hydrophilic nanoparticles were easy to adsorb on oil film in nanofluids,while the hydrophobic nanoparticles were easy to adsorb on the rock wall.So the hydrophobic nanofluid had more opportunity to spread on the rock surface and strip the oil droplet simultaneously.Finally,we assessed the displacement ability of nanofluids through core flooding experiments.The result showed that when the core have a higher permeability,the nanofluid with AOT had a higher recovery efficiency than AOT solution only,and the nanofluid with TX-100 showed a weak effect than TX-100 solution.It also showed that the nanofluids with hydrophobic particles had a higher oil recovery than the nanofluids with hydrophilic particles when added AOT or TX-100.As for the core with lower permeability,nanofluids had a weak effect on oil recovery,since the nanoparticles was easy adsorbed on the core wall and blocked the throats,and the nanoparticles hardly permeated into the core.