Phase Equilibrium Calculation Method and Phase Equilibrium Curve Characterization of Natural Gas Hydrates Under the Action of Polymer Additives in Cement Slurry Filtrate: Based on Molecular Dynamics Simulation

Polymer additives in well cement slurry filtrate would affect the stability of natural gas hydrate (NGH), which could lead to formation collapse and cause marine disasters. It is necessary to clarify the critical conditions for the stability of NGH, i.e., NGH phase equilibrium. LAMMPS software and the TIP4P model were used to develop a method for calculating NGH phase equilibrium. Based on the single functional groups and combined functional groups of polymer additives, the potential energy, angular order parameter (AOP) of water molecules, and NGH phase equilibrium temperatures under different pressures were calculated, and a phase equilibrium curve was characterized. Results show that amide groups promote NGH decomposition more strongly than carboxyl and sulfonate groups, with a 1.5% dodecylamide system causing NGH phase equilibrium temperature to decrease by 1.68–2.77 K. AM/AA promotes NGH decomposition more strongly than AA/AMPSNa, AM/AMPSNa, and AA/AMPSNa/AM, with a 1.5% AM/AA system causing NGH phase equilibrium temperature to decrease by 2.33–3.56 K. To ensure the safety of well cementing and marine environments, the contents of amide groups and carboxyl groups should be reduced when developing polymer additives for cement slurry used in NGH formation cementing.