Molecular Dynamics Simulation Of Adsorption And Displacement Characteristics Of N-alkanes And CO₂ In Different Mineral Nanopores

Carbon capture,utilization and storage(Carbon Capture,Utilization and Storage,CCUS)technology is to capture the carbon dioxide generated in the production process,and then put into a new production process for recycling and storage.Among the many CCUS technologies,CO₂ enhanced oil recovery(CO₂-EOR)has received more and more attention because it can improve the recovery rate of crude oil while permanently storing greenhouse gases underground.However,the geological structure of unconventional reservoirs is very complex.In addition to containing a large number of nanopore structures,the shale composition is complex,and each rock may have more than one exposed crystal plane.In these nano-scale pores,the oil adsorption characteristics and transport laws are significantly different from the macro scale.Therefore,in order to better develop carbon dioxide flooding technology,this article need to study the adsorption characteristics of different rock crystal planes and oil,CO₂-EOR mechanism and other issues.In this paper,the molecular dynamics simulation method is used to study the effect of different exposed crystals on the oil adsorption characteristics of the rock,the mechanism of CO₂-EOR,and the effects of displacement pressure,pore size and oil filling rate on the displacement efficiency of carbon dioxide.First,this article studied the effect of the reference exposure surface and the edge exposure surface of the nanopore structure of rocks such as quartz,calcite,illite,montmorillonite,and kaolinite on the adsorption characteristics of the oil.This article compare the strength of the oil adsorption performance of different crystal faces by comparing the density peak of its adsorption peak,the interaction energy between the oil and the rock surface,and the angular distribution of the oil molecules in the adsorption layer.And through the two-dimensional density distribution of oil molecules in the adsorption layer,to understand the specific arrangement of oil molecules in the adsorption layer.It is found that the different exposed crystals of the rock have obvious differences in the adsorption capacity of the oil molecules,and the arrangement of the oil molecules near the rock wall is also very different.For different rock types,this article must also consider the different exposed crystal planes of the rock.Then this article studied the CO₂-EOR system in the pores of calcite and montmorillonite and explored the CO₂-EOR mechanism by analyzing the diffusion coefficient of oil molecules and the interaction energy between oil molecules and the wall surface.this article increased the system pressure by increasing the amount of carbon dioxide into the system and found that when the pressure is increased,the diffusion coefficient of the oil molecules will increase first,and when the pressure increases to a certain level,the diffusion coefficient of the oil molecules will decrease,which indicates that the oil Injecting carbon dioxide into the reservoir can increase the diffusion coefficient of oil molecules,but injecting too much carbon dioxide will reduce the diffusion coefficient of oil molecules.Besides,the study found that with the increase of pressure,the interaction energy of oil molecules with the wall gradually weakens and the oil molecules will gradually move away from the wall,which shows that the injection of carbon dioxide can weaken the interaction energy of oil molecules with the rock wall and make it off the wall.Finally,the effects of pore size,displacement pressure,and oil filling ratio on the displacement efficiency of CO₂-EOR transportation were studied.It was found that small pore size will lead to higher displacement efficiency,while large pore size will result in faster displacement rate;large displacement pressure can bring faster displacement rate and higher displacement efficiency,but at large pore size,the advantage of large displacement pressure will be weakened;the higher the oil filling ratio,the faster the displacement speed and the higher the displacement efficiency.