The molecular explanation for polyacrylamide viscosity reduction in high-salinity formation-water

Many oilfields report that the viscosity of polymers in high-salinity reservoirs will decrease significantly. In this paper, molecular dynamics simulations were conducted to investigate the molecular configuration and network of hydrolyzed polyacrylamide (HPAM) molecules in high-salinity formation-water and the viscosity changes from the microscopic dense to dilute phase. In addition, the viscosity of HPAM/formation-water solution was measured to verify and compare with simulation results. Simulation and experimental results show that the molecular network of the microscopic dense phase is essential for the apparent viscosity. The calculated apparent viscosity could decrease 37% as the net-shape molecular network of the microscopic dense phase is broken by calcium ions, which is similar to our experimental results. This paper improved our understanding of the mechanisms of polymer viscosity alteration in high-salinity formation-water and provided insights that can be used to improve the strategy of enhanced polymer flooding and the novel polymer gel formula.