Investigation Of Quantum Dot-based Nano-fluid System For Inhibiting Asphaltene Deposition During CO₂ Flooding

Carbon dioxide capture,utilization and storage（CCUS）technology,as one of the important means to achieve carbon neutralization for China,which has large-scale potential for deep reduction of greenhouse gas emissions.Among them,CO₂-EOR technology has a broad application prospect,and the reservoir can be used as an ideal site for CO₂ storage.While realizing the long-term safe storage of CO₂,it can also enhance oil recovery and take into account both economic and environmental benefits.However,the problem of asphaltene deposition will occur in the process of CO₂ flooding,which will lead to the change of reservoir wettability,permeability reduction and even pore throat blockage,resulting in permanent damage to the reservoir.In recent years,nanoparticles have shown great application potential in the field of enhanced oil recovery,and are considered to be one of the effective solutions to solve the problem of asphaltene deposition.Carbon quantum dots（CQDs）,as a new type of carbon nanomaterials,whose size is less than 10 nm,have excellent properties such as good chemical stability,low toxicity and easy functionalization.It is an ideal material to solve the problem of asphaltene deposition during CO₂-EOR.Based on this,a quantum dot-based nanofluid system（CQDs nanofluid）with excellent size effect and good compatibility was developed,and the physical and chemical properties of CQDs were characterized by fluorescence image,Uv-Vis,TEM,FTIR and contact angle tests.The inhibition performance of CQDs nanofluid on asphaltene deposition was evaluated by asphaltene static and dynamic deposition experiments,and the microscopic mechanism of the system was revealed.The results shown that the size of carbon quantum dots synthesized by microwave method was distributed between 0.5～4 nm,the average particle size was 2.3 nm,and shown good oil solubility.The static deposition experiment of asphaltene shown that CQDs can effectively inhibit the aggregation behavior of asphaltene.The inhibition rate of CQDs nanofluid on asphaltene deposition was up to 55%.At the same time,the starting point of asphaltene precipitation can be delayed from 45 V%to 55 V%,and the average particle size of asphaltene aggregate can be reduced from 1200 nm to 450 nm.Microfluidic experiments shown that CQDs nanofluid can effectively inhibit the dynamic deposition of asphaltene,and the precipitation time of asphaltene deposition particles can be delayed from 30 min to 60 min.After asphaltene deposition 180 min,the particles were reduced from 28.8μm to 18.9μm.The inhibitory effect of CQDs nanofluid on asphaltene deposition was produced by the synergistic action of CQDs nanoparticles and their dispersants（JRL reagent,ester compound oil-soluble viscosity reducer）.JRL reagent can dissolve part of asphaltene aggregates,making their particle size from large to small.On the one hand,CQDs adsorbed around asphaltene particles to inhibit its deposition by weakening the interaction between asphaltene particles,on the other hand,CQDs entered the asphaltene aggregate phase to promote the further dispersion of aggregates.To sum up,the quantum dot-based nanofluid system developed in this study can effectively inhibit the accumulation of asphaltene,which can be used as a potential technical path to solve the problem of asphaltene deposition in the process of CO₂-EOR,and is expected to further enhance the oil recovery of low permeability reservoirs.