The Basic Research On Bionanofluid For Improving Oil Recovery In Low-permeability Reservoirs

With the rapid development of Chinese economy,most of high-quality reservoirs have begun showing depletion in oil recovery while the demand for crude oil is increasing.As a result,low-permeability reservoirs have become an important resource of energy supply.In the process of exploring more efficient and environmentally friendly oil recovery methods for low-permeability reservoirs,nanomaterials have attracted many researchers demonstrating their potential application in energy sectors because of their unique physical and chemical properties.In the present study,we prepared a new nanofluid with Si O2 nanoparticles and widely used commercial biosurfactant to evaluate the displacement process of nanofluid in low-permeability reservoirs and construct a brand-new nanoflooding system.The study discussed the effect of nanofluid on rock wettability,oil recovery efficiency and wateroil distribution to analyze the mechanisms of nanoflooding.The major findings and conclusions are shown as follows:(1)A serious of experiments were conducted to research the influence factors depending on the stability of the Si O2 nanoparticles-biosurfactant suspension.The result revealed that biosurfactant at concentration below the critical micelle concentration stabilized the suspension and the Si O2 nanoparticle concentration should maintain below1000 mg/L.(2)This study investigated the effect of nanofluid on wettability alteration of rock surfaces,depending on the oil displacement.Wettability alteration is one of the most important parameters to increase oil recovery from low-permeability reservoirs but the mechanism is lacking of a uniform interpretation.In this study,the adsorption of nanoparticles precipitated rock surface to water-wet state.It is beneficial for the spreading and diffusion of injection water.In addition,the wettability alteration could reduce the adhesion of crude oil to rock surface and promoted the peeling of adsorbed oil.The neutral wetting of reservoir with the maximum capillary force guaranteed the greatest oil displacement efficiency.For oil-wet reservoirs,nano-EOR process had a greater potential for oil recovery from oil-wet cores than that from water-wet and neutral cores.(3)According to the micromodel test,characteristics of small pore size and complex strata structure bring the various distributions of residual oil in reservoir in pore spaces,corresponding to different oil displacement mechanisms.In this study,the effect of nanofluid on enhanced oil recovery(EOR)is investigated by analyzing the change of oilwater area before and after displacement.The experimental result revealed that nanofluid could enhance oil-displacing efficiency and enlarge swept area of injecting water,reducing residual oil saturation.(4)There are many factors affecting nanoflooding efficiency,including the components of nanofluid,reservoir permeability,injection scheme and nano-treated time.The experimental results showed that “Si O2-rhamnolipid” system achieved a higher oil recovery and the concentration of nanoparticles was decided by the permeability of reservoir.A high particle concentration would impede the final oil recovery when the permeability of the core was too low because of the blockage of nanoparticles.Moreover,exposure time of 24 h for nanofluid was conducive to remove residual oil from rock surface,improving oil recovery.And cyclic nanofluid injection could provide a higher oil recovery than the continuous injection scheme.(5)Nuclear magnetic resonance(NMR)techniques,coupled with mercury intrusion porosimetry(MIP)test were conducted to reveal that nanofluid could alter the oil-water distribution in low-permeability reservoir which was conductive to EOR.For general-low permeability cores(?50 m D),the pore size was relatively large and the nanofluid easily flowed through the area of water breakthrough and the residual oil in small pore space was hardly swept.In comparison,for ultra-low permeability cores(?0.1m D),the residual oil was mobilized by nanofluid but accumulated at the junction of small pores and large pores.When nanofluid injected in extra-low permeability cores(?5 m D),the distribution of residual oil saturation reduction was relatively balanced.That is to say,there were obvious enhanced oil recovery in both macroporous and small porous areas.(6)In the process of nanoflooding,nanoparticles and biosurfactant showed a significant synergistic effect on EOR.The unique surface properties of nanoparticles could not only cause wettability alteration by nanoparticle adsorption,but also peel the adsorbed oil off rock surface by structural disjoining pressure.Biosurfactant could not only stabilize the nano-suspension system,but also reduced the water-oil interfacial tension.It was conducive to promoting the dispersion of oil droplets in displacing water and reducing the difficulty of peeling crude oil by nanoparticles.