Microcanonical ensemble molecular dynamics simulations of structure I methane hydrate is presented in this work to study the endothermic decomposition process. The mechanism of decomposition of methane hydrate as a function of time was explained at the molecular level. The initial temperature and pressure of the simulation were chosen so as to depict the natural gas hydrate in conditions of oceanic sediments. A more realistic strategy was developed to perform the microcanonical ensemble simulation of solid−liquid interface of hydrate and amorphous water. Two water models, SPC/E and TIP4P, were used for the simulations, and the results of the simulations were compared. Heat transfer calculations were performed on the adiabatic system, and an attempt has been made to fit the MD simulation results to the heat balance equations derived from the heat transfer calculations. Estimates of the properties at the macroscopic scale, like the equilibrium temperature of methane hydrate and rate of supply of hot water for sustained release of methane from solid hydrate phase, were determined. The equilibrium temperature obtained by the above method was found to be in agreement with the experimentally observed value. Both the SPC/E and TIP4P water models gave similar results.
One of the options suggested for methane recovery from natural gas hydrates is molecular replacement of methane by suitable guests like CO2 and N2. This approach has been found to be feasible through many experimental and molecular dynamics simulation studies. However, the long term stability of the resultant hydrate needs to be evaluated; the decomposition rate of these hydrates is expected to depend on the interaction between these guest and water molecules. In this work, molecular dynamics simulation has been performed to illustrate the effect of guest molecules with different sizes and interaction strengths with water on structure I (SI) hydrate decomposition and hence the stability. The van der Waals interaction between water of hydrate cages and guest molecules is defined by Lennard Jones potential parameters. A wide range of parameter spaces has been scanned by changing the guest molecules in the SI hydrate, which acts as a model gas for occupying the small and large cages of the SI hydrate. All atomistic simulation results show that the stability of the hydrate is sensitive to the size and interaction of the guest molecules with hydrate water. The increase in the interaction of guest molecules with water stabilizes the hydrate, which in turn shows a slower rate of hydrate decomposition. Similarly guest molecules with a reasonably small (similar to Helium) or large size increase the decomposition rate. The results were also analyzed by calculating the structural order parameter to understand the dynamics of crystal structure and correlated with the release rate of guest molecules from the solid hydrate phase. The results have been explained based on the calculation of potential energies felt by guest molecules in amorphous water, hydrate bulk and hydrate-water interface regions.
This study investigates, theoretically and experimentally, the effect of conventional and new chelating-agent-based acidizing fluid systems on carbonate formations. An initial assessment showed that an existing semi-empirical model used for hydrochloric (HCl) acid systems was not suitable for predicting wormhole growth and penetration for chelating agents. With recent advances to acidizing and stringent environmental regulations, chelating agents have begun to replace conventional acid systems for well stimulation. They are more environmentally acceptable and can work in high-temperature wells. One crucial aspect for successful well stimulation of carbonate matrix acidizing is acidizing models. Yet, there have been few attempts to model wormhole growth and penetration for these newer chelating agents. A semi-empirical mathematical model is proposed for chelating agents based on published experimental data. The results were compared with HCl acid systems. The predicted wormhole growth was in good agreement with laboratory data for a wide range of temperatures and concentrations. A comparative study with chelating-agent and HCl-acid systems is also discussed. Results showed that pore volume to breakthrough (PVbt) for the chelating-agent-based system increased more steeply compared to the HCl-acid-based system with injection rate. This would translate into higher pumping volumes in the field for a given wormhole penetration if not optimized. This could be attributed to the slower reaction rate of the chelating agent and might require adjustment in transport phenomena for optimized treatment design. The model developed in this work should help field engineers design treatments with optimum rates and volumes, helping minimize the cost of stimulation treatments. It should also further facilitate understanding of the wormhole formation mechanism for chelating agents.
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