Molecular Dynamics Simulation Study On The Interaction Of Polyacrylamide With ?-silica And Calcium Carbonate

At present,the applition of polyacrylamide(PAM)in industry is increasing year by year,especially in oil and gas exploitation.PAMs become an important raw material for water shutoff materials,oil displacement agents and fracturing fluid additives.Therefore,the structure of PAM in strata has attracted wide attention.Many theories believe that PAM can adsorb on rock surface,but the interaction energy between PAM and rock can not be obtained by traditional experimental methods.In this paper,the interface models of PAM/a-Si02,PAM/H2O/?-SiO2,PAM/CaCO3 and PAM/H2O/CaCO3 systems were constructed using Material Studio software.The interaction energy between PAM and a-Si02(CaCO3),radial distribution function and concentration distribution curve of PAM were analyzed by molecular simulation thoroughly.The mechanism of the interaction between PAM and rocks was also studied.The research contents are as follows:(1)In this paper,the interaction energy between PAM and ?-SiO2,the radial distribution function and the concentration distribution curve of PAM were studied under vacuum.The effect of PAM's degree of polymerization on the interaction between PAM and ?-SiO2 was analyzed.The results show that when the degree of polymerization of PAM is less than 40,the interaction energy between PAM and ?-SiO2 increases with the increase of degree of polymerization due to the mass.Additionally,when the degree of polymerization reaches 50,the molecular chains curl up and the adsorption site of ?-SiO2 decrease,so the interaction energy decreases.Until the degree of polymerization is greater than 50,the configuration of PAM do not change,which cause the interaction energy between them to remain unchange.(2)The model of PAM/H2O/?-SiO2 system was constructed,and the effects of concentration,temperature and simulated time on the interaction between PAM and ?-SiO2 were investigated.It turns out that when the concentration of PAM increases,the number of water molecules adsorbing on the surface of ?-SiO2 decreases,which leads to the exposure of more adsorption sites.Therefore,the interaction energy between PAM and ?-SiO2,increases.When the temperature increases,the interaction energy between PAM and ?-SiO2 increases first and then decreases.This is mainly due to the desorption of water molecules from the surface of?-SiO2 which causes the increase of the interaction energy between PAM and ?-SiO2.And when the temperature is too high,water molecules drive PAM upward,which weakens the interaction energy between PAM and ?-SiO2.In addition,when the simulation time exceeds 1000 ps,the PAM/H2O/?-SiO2 system tends to be unstable.(3)The interaction energy between PAM and different CaCO3 surfaces were calculated.The interaction mechanism between PAM and CaCO3 surfaces was analyzed by the dynamic equilibrium structure of PAM/CaCO3 system,radial distribution function,concentration distribution curve and mean square displacement.The results show that when the crystal surface index of CaC03 is(0 1 1),the concentration of PAM on the surface of CaCO3 is the highest,which lead to the increase of the interaction energy between PAM and CaCO3.(4)The model of PAM/H2O/CaCO3 system was constructed.The interaction energy between PAM and CaCO3 and the radial distribution function of PAM and CaCO3 were calculated.The results show that the interaction energy between PAM and CaCO3 increases first and then decreases with the increase of temperature.What's more,when the temperature is 450 K,the interaction energy between PAM and CaCO3 is the largest.This is mainly because at 450 K,water molecules break away from the surface of CaCO3.There are more adsorption sites of CaCO3 for PAM to adsorb on.In summary,the interaction of PAM and ?-SiO2 and the interaction of PAM and CaCO3 are systematically studied by molecular simulation,which provides a good basis on experimental research.