Study On Preparation Of Active Nanofluid And Its Imbibition Application In Ultra-low Permeability Reservoirs

Nanomaterials have been widely used in the oil production field due to their unique nanometer size effect.Nanomaterials can enhance oil recovery by decreasing oil/water interfacial tension and changing reservoir wettability.Some water-based nanofluids for enhanced oil recovery(EOR)have been developed.Surfactants are primarily used as dispersants of nanomaterials.Nevertheless,the interaction between surfactants and nanomaterials is non-covalent interaction that is instable in a complex reservoir environment.Nanomaterials are easily flocculated and cause formation damage when the interaction between surfactants and nanomaterials is destroyed.It is a challenge to develop a kind of water-based nanomaterials with inexpensive price,interfacial activity and good dispersion.In this work,a series of silica-based nanoparticles and carbon-based nanoparticles with interfacial activity possessing hydrophilic surface functional groups(carbon chain with carboxylate)and hydrophobic surface functional groups(carbon chain)was prepared through surface modified method and in-situ growth method.For tracing the location of nanoparticles in core pores during laboratory experiment,fluorescent materials were added into the structure of nanoparticles to make them own fluorescent properties.Silica-based nanoparticles based on silica sol modified by using oleic acid(OASNPs)and fluorescent carbon-based nanoparticles made from acetic acid(C2NPs)were optimally selected by studying the water-dispersible and the ability to reduce oil/water interfacial tension of the prepared nanoparticles.With quantum dot as core,fluorescent silica-based nanoparticles modified by using oleic acid(OAFSNPs)were prepared.The morphology and surface chemical structure of OAFSNPs were same as that of OASNPs.Effects of different factors on particle size,Zeta potential and light transmittance of OASNPs,OAFSNPs and C2NPs in aqueous solution were studied.The change rules of particle size and surface charge of OASNPs,OAFSNPs and C2NPs in their dispersion process were clarified.The dispersion mechanisms of OASNPs,OAFSNPs and C2NPs were revealed.Due to the electrostatic repulsion and steric repulsion,OASNPs,OAFSNPs and C2NPs could be stably dispersed in an aqueous phase.C2NPs nanofluid was selected as chemical agent for EOR in ultra-low permeability reservoirs through the evaluation standard of temperature resistance and salt resistance.C2NPs could be stably dispersed in the aqueous phase without any mechanical equipment.The adsorption property of C2NPs at the interface of two phase and the spreading property of C2NPs nanofluid at oil/water/solid three-phase region were clarified.The adsorption of C2NPs at the oil/water interface was controlled by diffusion and an elastic interface film was formed at the oil/water interface.The oil/water interfacial tension was reduced due to the adsorption of C2NPs at the oil/water interface.Effects of different factors on the ability to reduce oil/water interfacial tension and spreading property of C2NPs nanofluid were studied.The adsorption rule of C2NPs at the oil/water interface and the spreading rule of C2NPs nanofluid at oil/water/solid three-phase region were clarified.The adsorption of C2NPs at the oil/water interface and solid surface were proved by fluorescence imaging technology which is based on the fluorescence properties of C2NPs.C2NPs nanofluid exhibited excellent ability to reduce oil/water interfacial tension,alter wettability and spread.The oil recovery rule of C2NPs nanoflid was clarified and an oil recovery model of C2NPs nanofluid was established for oil recovery prediction of ultra-low permeability reservoirs.The oil recovery mechanism of C2NPs nanofluid was revealed.Reservoirs could be developed efficiently using C2NPs nanoflid when reservoir permeability is more than 0.1×10^-3?m².The oil in pore with size of larger than 0.046?m could be discharged by C2NPs nanoflid.Oil recovery using C2NPs nanofluid was increased by 10%than using purchased silica nanofluid and SDS surfactant solution.Oil recovery rate using C2NPs nanofluid was double that using purchased silica nanofluid and SDS surfactant solution.Furthermore,oil recovery using C2NPs nanofluid with a low concentration(0.05 wt%)was as high as 38.9%.Transport rule of oil and nanofluid in the core during the spontaneous imbibition process was explored through the technologies of nuclear magnetic resonance and fluorescence imaging.Capillary force leaded the spontaneous imbibition of C2NPs nanofluid in ultra-low permeability core.In the process of spontaneous imbibition,C2NPs nanofluid firstly entered micropores and mesopores and the oil in the micropores and mesopores was discharged.C2NPs nanofluid entered macropores and the oil in the macropores was discharged in the middle and later periods of spontaneous imbibition.Oil recovery contribution from mesopores and oil recovery of mesopores were highest.