Factors Influencing The Formation And Decomposition Of Methane Hydrate Based On Molecular Dynamics Simulation

Natural gas hydrate is the most important new energy source in the 21st century,the abundant reserves and clean use play an important role in alleviating China’s future energy crisis.The formation and decomposition kinetics of gas hydrates is an important theoretical basis for the safe development and efficient use of natural gas hydrates.However,the results of traditional macroscopic kinetic experiments still cannot effectively analyze the microscopic mechanism of gas hydrate formation and decomposition processes.Based on molecular dynamics simulation,the formation,decomposition and CO2 replacement process of methane hydrate were studied,and the formation,decomposition and replacement mechanism of hydrate were revealed from the molecular level.The main research contents include the following three aspects:(1)Kinetic simulation of the formation of methane hydrate.This part of the simulation is carried out using the open source dynamics software GROMACS.The water molecule uses the TIP4P(Transferable Intermolecular Potential 4 Points)model,the methane uses the OPLS-aa(Optimized Potentials for Liquid Simulations-all atom)model,and the NaCl uses the force field parameters developed by Smith.Firstly,the formation process of methane hydrate in pure water and NaCl solution was simulated,and the stable methane hydrate structure was obtained,the clustering nucleation model of Sloan was verified;The growth rate was about 0.56A/ns in the methane aqueous solution containing hydrate crystals,finally obtained 12 complete crystal nucleus structures,and in 3.4wt%NaCl solution,due to the electrostatic attraction of Cl-and the hydration of Na+,affecting the distribution of water molecules and causing salting out effect,The growth rate of the object is about 0.36A/ns,and finally 9 nucleuses are obtained and contain defects.The growth of hydrate is very sensitive to temperature,not sensitive to pressure,and the temperature causes a greater change in the diffusion coefficient of water molecules.The effect of methane concentration(molar ratio:0.098,0.11,0.127,0.138)on the growth of hydrate was studied,it was found that high concentration limits the initial rate of hydrate formation,but promotes the formation of hydrates in the later stage of growth,in a system with a molar ratio of 0.138,the growth rate of hydrate is 0.75A/ns;in the process of methane hydrate formation,the most influential effect is the concentration of methane in aqueous solution,followed by temperature,and finally the pressure.(2)Decomposition kinetics simulation of methane hydrate.This part is simulated by the molecular dynamics software Material Studio.The CVFF(Consistent Valence Force Field)force field and the ensemble NVT(Canonical Ensemble)are selected,and the water molecule select SPC(simple point charge)model.The decomposition model of methane hydrate was used to verify the decomposition model of Kim;The high temperature caused the hydrogen bond to vibrate violently,at the same time,the high temperature was beneficial to break the liquid membrane structure and shorten the reaction stagnation time,the decomposition rate of methane hydrate at normal pressure was related to temperature(278K,283K,288K,293K)increased,the rate of decomposition of hydrate in aqueous solution at 293K was 0.066A/ps;by studying the effect of NaCl solution concentration(2.5wt%,5wt%,10wt%,20wt%)on the decomposition process,it is found that the high concentration of NaCl solution can promote its decomposition,but the concentration is too high,due to the hydration of ions and the hydrophobic action of methane,the decomposition rate will decrease,and the decomposition rate of 10wt%NaCl solution is the fastest,which is 0.067A/ps;for different types of salt solution,Ca2+ has a strong destructive force on the cage structure,the decomposition rate of 5wt%CaC12 solution is 0.062A/ps,the absorbed energy is 13605.58kCal/mol,and the decomposition rate of 5wt%NaCl is 0.053A/ps,although the decomposition rate is moderate,but the absorption energy is less,it can also be used as a hydrate decomposition solution;the ability of different types of salt solution to decompose methane hydrate from strong to weak is:CaCl2>LiCl>KCl>MgCl2>AlCl3>NaCl.(3)Molecular dynamics simulation of CO2 displacement methane hydrate.This part of the simulation is carried out using the open source dynamics software GROMACS.The CO2 uses the EPM2(a simple site-based intermolecular potential model)force field model,the force field model of CH4,water molecules and salt ions is identical to the generated part.The displacement model of Ota was verified by the kinetic simulation of CO2 displacement of methane hydrate;the CO2 displacement methane hydrate was carried out from the interface to the interior of the hydrate,but the formation of CO2 hydrate layer hindered the replacement process;at 20MPa,the higher the temperature(260K,270K,280K),the better the replacement,at 260K,CO2 will form a new hydrate on the basis of the nucleus provided by methane hydrate;the higher the pressure(5MPa,10MPa,20MPa)at 280K,the more stable the methane hydrate,the stronger the tendency of CO2 to form new hydrates;the higher the CO2 concentration(molar ratio:0.098,0.12,0.126),the higher the methane substitution rate;in the 3.2wt%NaCl solution equivalent to the salinity of deep sea water,the displacement can still be carried out,and the methane escape rate increases;the simulation finds that the control rate of the entire replacement process and the replacement rate are the concentration of guest molecules in the solution and the newly formed CO2 hydrate layer,how to make CO2 continuously through the solid layer to replace CH4 is the focus of future research.The results of this paper will provide a reference for the analysis of the microscopic mechanism of hydrate formation and decomposition,and lay the foundation for the development and transportation of natural gas hydrates.