Microscopic Mechanism Of Tight Oil And Gas Adsorption Based On Density Functional Theory

The tight oil and gas reserves are abundant,and the reservoir has the characteristics of small pore size,large specific surface area and strong interaction between fluid molecules and media.It is difficult for conventional percolation theory to describe the tight oil and gas flow.However,molecular simulation can better describe the interaction between fluid molecules and porous minerals.Adsorption is one of the important characteristics of the interaction between oil and gas molecules and porous media.Due to the complex pore structure and mineral composition of tight reservoirs,the computer simulation has incomparable advantages over the experimental method in the study of the microscopic adsorption of oil and gas.Through the analysis of the adsorption capacity of the tight oil and gas molecules on the molecular skeleton structure of rock mineral,this paper aims to explore the adsorption mechanism of tight oil and gas from the molecular level,which provides a strong basis for understanding the occurrence of tight oil and gas in the reservoir,and exploring the correlation between the adsorption performance and the mineral composition.Density functional theory is a quantum mechanical method to study the electronic structure of multi electron systems.Compared with other molecular simulation methods,density functional theory has the advantages of higher accuracy,and is very convenient in practical application.In this study,the adsorption of different oil and gas molecules(CH4,C2H6,CO2,N2 and C6H14)at different sites(top site,bridge site and hollow site)on different mineral components(CaCO3 and SiO2)surface has been simulated and calculated,by using the first principles method based on density functional theory.Through the analysis of the adsorption energy,the physical structure and the electronic density of state after structural optimization,the adsorption properties of tight oil and gas in the reservoir rock matrix were discussed.The results show that the adsorption capacity of hydrocarbon gas is stronger than that of non hydrocarbon gas.The adsorption capacities of CH4,C2H6,CO2,N2 and C6H14 molecules on CaCO3(100)and ?-SiO2(010)surface are from large to small in order of C6H14>C2H6>CH4>N2>CO2.The adsorption energies of C6H14 and C2H6 are less than-0.62eV,which is a weak chemical adsorption.The adsorption energies of CH4 and N2 are greater than-0.62eV,which is physical adsorption.CO2 on the surface of ?-SiO2(010)is exothermic adsorption,and it is endothermic adsorption on CaCO3(100)surface.The smaller the difference of the adsorption energies at the different high symmetry sites is,the better the fluidity of oil and gas molecules are.After adsorption,the bond length and bond angle of oil and gas molecules change slightly.The electronic density of state of CH4 is obviously shifted to the low energy region after adsorption,but the electronic density of state of CaCO3(100)surface and a-SiO2(010)surface are almost unchanged,which means that the adsorption of methane on the surface of calcium carbonate and silica is weak.In addition,the electronic density of states of C6H4 molecule,CaCO3(100)surface and ?-SiO2(010)surface appear to be lower in the bond state energy and higher in the antibonding state energy after adsorption,which indicates that C6H14 molecule has the covalent effect on the surface of the adsorption medium.Therefore,the tight oil and gas in the tight sandstone and tight carbonate matrix have the form of adsorption state.The adsorption of tight gas is weak physical adsorption and easy to change into a free state,while the adsorption capacity of tight oil is strong,it is chemical adsorption and easy to form stable flow.