Molecular Simulation On The Dissociation Mechanism Of Hydrate With Inhibitors

Chemical injection is one of the most important ways to exploit gas hydrate.Meanwhile,hydrate blockage is a big risk for the safe production of oil and gas.Addition of chemical inhibitors is an effective way to prevent gas hydrate formation.Macroscopic experiments can provide data to support the selection on specific inhibitors,but it is difficult to reflect the process of hydrate decomposition and to study the mechanisms of the inhibitors’ function on the micro-level.The utilization of molecular simulation is feasible to solve the problem,and provides a new method to evaluate the inhibitors.In this study,the hydrate-inhibitors system models were established,and the dissociation of SI methane hydrate were studied in the presence of different inorganic salts（NaCl,KCl and CaCl₂）and alcohols（methanol,ethanol）with different concentration from 265 K to 273 K by molecular dynamic simulations.The structural parameters,kinetic parameters and energy change were analyzed for the different simulation series in the inhibitor-hydrate system.The influences of the type,the concentration of inhibitors and the temperature/pressure on the dissociation were studied.Moreover,the effect of different inorganic ions on the stability of crystal structure,adsorption of kinetic inhibitors（PVP,PEO,PVCap）on hydrate surface,and influence on dissociation were considered.Finally,the microscopic mechanisms of inhibitors were explored.Simulation results showed that,in the inorganic solution,the film formed by the decomposed waters molecules could delay the dissociation process,and thus the hydrate decomposed at a inconstant rate.Inorganic ions could shorten the stagnation-time.Hydrate dissociation rate was proportional to the concentration of salts.The absorbed energies were different for different salt types in dissociation process,and the value of KCl was the largest among all.When the concentration of CaCl₂ reached 30wt%,the energy value was very low.The damage abilities of ions on hydrate cell were different,Ca²⁺>2K⁺>2Cl^->2Na⁺.In alcohol solutions,film structure was too loose to delay the diffusion of water.When the concentration was 20wt%-30wt%,the dissociation rate reached maximum.The values of dissociation heat decreased with an increasing concentration when the concentration was low and gradually increased when the concentration reached 30wt%.Unlike temperature,the volume of the confined space generated little effect on the decomposition process.The concentration and molecular weight of the kinetic inhibitors could affect the interaction between the hydrate surface and the inhibitors,and PVCap had the biggest decomposition ability on hydrate.Moreover,the structure of short-range（<2.5 ?）water molecules became more regularly arranged in presence of kinetic inhibitors.