Experimental Study On Hybrid Nanofluid Alternating CO₂ Microbubble Flooding To Enhance Heavy Oil Recovery

There are big storage of heavy oil in complex reservoir such as offshore,thin layer and edge-bottom water reservoir.Heat injection measures such as huff and puff has poor applicability,because of the bulky equipment and the severe heat loss.Water alternating CO₂combined with the advantages of water flooding and gas flooding,is a good technology to solve the above problem.However,because of the high viscosity and high asphaltene content of heavy oil,there are gas channeling,gravity overlap and asphaltene precipitation over the injection process,lead to low vertical sweep efficiency and the permeability reduced.To further enhance complex heavy oil recovery,a variety of nanofluids are introduced in the process of enhance heavy oil recovery,and combined with the advantages of CO₂microbubble,suggested a method of hybrid nanofluid alternating CO₂ microbubble flooding to solve the problem of traditional method.Firstly nanofluid stability analysis was conducted through direct visual observation and particle size measurement to ensure the nanofluids did not change during the experiments.Then,the ability of nanofluids to reduce the oil-water interfacial tension was studied,along with the factors including the type and concentration of nanofluids.Then,the ability of nanofiuids to change the wettability of oil-wet sandstone surfaces to a water-wet state was studied,along with the factors including the type,concentration and exposure time of nanofluids,salinity and temperature.Then,the stronger adsorption of hybrid nanoparticles on sandstone surfaces was considered to be the underlying mechanism for the higher efficiency of hybrid nanofluids for the wettability shift than that of single nanofluids,and SEM technology verified the mechanism.Finally,based on the above results,the optimal hybrid nanofluids was screened to investigate the hybrid nanofluid alternating CO₂ microbubble flooding,followed with the factors including injection mode,type of injected fluids,size of CO₂ microbubble,pressure,slug size and slug ratio.The results indicate that nanofluid stability was reduced as the increase of nanoparticle concentration,salinity and temperature.At low nanoparticle concentration,salinity and temperature,hybrid nanofluid stability is acceptable.The ability of hybrid nanofluid to reduce the oil-water interfacial tension is stronger than single nanofluid,as the nanofluid concentration increase,the interfacial tension first reduce,and then almost the same.There is an optimal nanofluid concentration,which can reduce the interfacial tension to a minimum.The contact angle reduced as the increase of nanofluid concentration,exposure time and salinity.But at high nanofluid concentration and high salinity,the contact angle increases.Two adsorption kinetics models were applied to predict the measured contact angles at different concentrations and exposure times with good agreement.The stronger adsorption of hybrid nanoparticles on sandstone surfaces was considered to be mechanism for the higher efficiency of hybrid nanofluid for the wettability shift than that of single nanofluids.The recovery of hybrid nanofluid alternating CO₂ microbubble flooding is much higher than conventional WAG flooding,hybrid nanofluids enhanced higher recovery than CO₂microbubble.The upper water injection with the bottom gas injection,reduce the size of CO₂microbubble,increase pressure and reduce slug size are contributed to enhance ultimate recovery.In addition,the optimal slug ratio is approximate to 1:1.