Effect of the Cations (Na+, Ca2+, Fe2+, and Fe3+) on the Partially Hydrolyzed Polyacrylamide Shrinking by Molecular Dynamics Simulations

The presence of cations on injection fluids used during polymer flooding leads to viscosity losses of the polymeric solution and reduces its drag capacity. Thus, understanding the mechanisms of this chemical degradation is crucial to improving the efficiency of these treatments. This study focused on obtaining physical insights into the mechanisms involved in chemical degradation by molecular dynamics simulations. To do this, the interaction energies between a variety of cations present at polymer flooding (Na+, Ca2+, Fe2+, and Fe3+) and partially hydrolyzed polyacrylamide (HPAM) were calculated. First, several potentials for ion description were evaluated to guarantee a proper description of the ion hydration. Then, multiple simulations were carried out to understand the effect of each ion individually and the synergic effect of a mixture of ions (brine) on the HPAM chain shrinking. The radius of gyration of the HPAM chain was used as an evaluation parameter of the chain shrinking. The results indicate that multivalent cations have a stronger interaction with the polymer than the monovalent cations, exhibiting smaller interaction distances and higher interaction energies. These interaction energies are related to the ionic radius of the cations and their charge. Smaller cations get close enough to avoid the repulsion between charged monomers, being the Coulombic interactions the most important (two-third of the total interaction energy). Thus, the strongest interaction energies with the HPAM correspond to multivalent cations, which reduce the radius of gyration of the HPAM since they can interact with two carboxylate oxygen simultaneously. Interestingly was found a high dependence of the concentration of Fe3+ cations in the interaction with HPAM; at high concentrations, the cations cannot get close enough to interact with the polymer, but in low concentrations, the cations present the strongest interaction. These findings contribute to understanding the mechanisms that macroscopically are related to viscosity losses in the solution by the cation effect.