Preparation Of Silica Nanofluids And Spontaneous Imbibition Investigation For Enhanced Oil Recovery In Ultralow-permeability Reservoirs

Ultra-low permeability reserves are abundant,however,due to the feature of low permeability and small pore throat,matrix reserves of ultra-low permeability reservoirs can't be exploited efficiently.Imbibition is an oil recovery mechanism of great importance for ultra-low permeability reserves.Surfactant can play a role of improving the interface properties in nanoscale pores and throats.Due to nano-size effect,nanomaterials have attracted great concern in improving interface properties and regulating wettability to improve oil recovery.It's significant that the synergistic effect of surfactants and nanomaterials is exerted by the compounding of surfactants and nanomaterials to enhance imbibition effect in ultra-low permeability reservoirs.Silica nanoparticles were modified by surface modification,and the surface hydroxyl number and chemical structure of modified products were determined and analyzed.Surfactants which can stably compounded with silica nanoparticles were selected by dispersibility,salt tolerance and imbibition experiments.The dispersibility,stability,oil-water interface activity and wettability-alternation ability of nanofluids were evaluated.Afterwards,the law of nanofluids imbibition in ultra-low permeability cores was investigated,and the impacts of the concentration of silica nanoparticles and surfactants,core permeability,temperature,formation water salinity on oil displacement in imbibition experiments were researched.At last,the mechanisms of imbibition of nanfluids in low-permeability reservoirs were initially revealed by investigating the adsorption behaviors of nanofluids at solid-liquid and oil-water interface.After modification,benzimidazolyl has been successfully grafted on the surface of silica nanoparticles.Complex system consisting of 0.1 wt% ampholytic surfactant BSSB-12 and 0.1 wt% silica nanoparticles reached the highest oil recovery of 26.5% in imbibition experiments.Results showed that with the rise of nanoparticles and surfactant concentration,the ultimate oil recovery and oil displacement velocity enhanced.But when the concentration of surfactant was over than 0.1 wt%,the oil recovery would not rise any more.Oil recovery was positively related with cores permeability and temperature.The formation water salinity had no obvious effect on imbibition results of nanofluids.Interface adsorption behavior experiments showed that nanoparticles can enter the pore throat in ultra-low permeability cores and form an effective adsorption on the pore throat surface.Under the synergy of surfactant molecules and nanoparticles,the wettability of cores changed from oil-wet to water-wet,and oil-water interfacial tension reduced.The appearance of structural disjoining pressure between oil droplet and rock surface and the decreasing adhesion work between the crude oil and core surface caused that oil droplet was more easily detached from the surface of cores.Capillary force changing from resistance to motivator and reducing resistance from Jamin effect caused that oil droplet moved more easily in the pores and throats of cores.