Effects of PVP and NaCl on the decomposition of methane hydrate by MD simulation

Natural gas hydrate has been regarded as one of the most promising alternative energy, while wellbore collapse and gas intrusion often occur due to hydrate decomposition during the drilling process. Both NaCl and PVP can be used as the inhibitor for hydrate decomposition, but the inhibition decomposition mechanism of the combination of NaCl and PVP still remains to be unclear. In this study, the synergies of NaCl and PVP on the inhibition of hydrate decomposition were investigated through the molecular dynamics approach. The F4 order parameters, the RDF, and the hydrate configuration were used to characterize the dynamic decomposition process of hydrate. Results show that the combination exhibits the optimal performance of inhibition decomposition compared with NaCl or PVP alone. Meanwhile, nanobubble formation is investigated by the number density distribution of methane molecules. Results show that NaCl promotes the accumulation of methane molecules, while PVP attracts methane molecules. The aggregation level of methane molecules in the 15 wt% NaCl/PVP system is lower than in the 15 wt% NaCl system because of the steric hindrance generated by the PVP molecule. The MSD, the number of cages, the hydration free energy, and the shear viscosity were used to reveal the mechanism of the synergistic inhibition. Results show that the hydration-free energy and shear viscosity of the 15 wt% NaCl/PVP is 11.69 +/- 0.10 kJ/mol and 1.67 +/- 0.26 mPa.s. The mixtures of NaCl and PVP exhibit synergistic inhibition by enhancing the hydration free energy and the shear viscosity of the solutions. This work provides an insight into the effects of PVP and NaCl on hydrate decomposition and the formation of nanobubbles. And it also contributes to solving the problems of wellbore instability and gas invasion during drilling hydrate bearing sediments.