In recent years, with the rapid increment of global energy consumption, shale gas has become an increasingly promising alternative energy resource because of its high efficiency and environmental friendliness. It is also abundant, with worldwide reserves of shale gas reaching up to 7299 trillion cubic feet. Unfortunately, it is hard to exploit these reserves as shale gas is confined in nanoscale pores, which are of very low permeability. In the United States of America, hydraulic fracturing and horizontal drilling technologies are popularly used to exploit shale gas. However, the hydrofracturing method can cause a series of problems, such as huge water consumption and contamination of underground water. Thus, understanding the transport mechanisms of shale gas in nanoconfinements and fi...[ Read more ]

In recent years, with the rapid increment of global energy consumption, shale gas has become an increasingly promising alternative energy resource because of its high efficiency and environmental friendliness. It is also abundant, with worldwide reserves of shale gas reaching up to 7299 trillion cubic feet. Unfortunately, it is hard to exploit these reserves as shale gas is confined in nanoscale pores, which are of very low permeability. In the United States of America, hydraulic fracturing and horizontal drilling technologies are popularly used to exploit shale gas. However, the hydrofracturing method can cause a series of problems, such as huge water consumption and contamination of underground water. Thus, understanding the transport mechanisms of shale gas in nanoconfinements and finding new ways to exploit shale gas are of vital importance, and may lead to significant changes to the energy consumption landscape. In this work, through molecular dynamics (MD) simulations, we use zeolite and quartz to simulate the structure of shale and explore several methods for exploiting shale gas.

First of all, the effects of temperature and pore size on the release of methane in zeolite nanochannels are investigated. The percentage of methane released at different temperatures for various zeolite structures is calculated. In the all-silica MFI (silicalite-1) zeolite, it is found that the release rate increases with increasing temperature at roughly a constant rate when the temperature is below 598 K. For higher temperatures, the release percentage reaches about 90% and remains roughly a constant. The release percentage for the other zeolite structures is greatly affected by the average pore size.

Secondly, the effects of external pressures on the release of methane through zeolite nanochannels are studied. The percentage of methane released under three types of pressure loadings, constant, sawtooth-shaped and sinusoidal, with various strengths and frequencies is obtained. As the pressure strength is increased, it is found that the release percentage first decreases and then increases significantly before finally approaching a constant. At sufficiently high pressures, the percentage of methane released under constant external pressures is about 65%, while it reaches over 90% for the sawtooth-shaped and sinusoidal pressures. The loading frequency for periodic external pressures appears to be unimportant compared with the effect of the pressure strength. Theoretical explanations of the release percentage are made based on the kinetic energy of methane molecules and the energy barrier inside the nanochannels, which are in good agreement with MD simulations.

Finally, the displacement of methane by carbon dioxide in silicon dioxide nanochannels is explored. The displacement of methane molecules in pores with four different diameters is calculated. Here, the silicon dioxide nanochannels are modeled as slit pores of shale. It is found that the release percentage of methane increases significantly when carbon dioxide is injected into the silicon dioxide nanochannels for all four pore diameters. As the interaction between carbon dioxide and the channel wall is stronger than that of methane, the carbon dioxide can replace the methane molecules and decrease their energy barrier, which makes it easy to release the methane molecules from the nanochannel and finally increase the release percentage of methane.

The percentage of released methane is also theoretically determined by the kinetic energy of the methane molecules and energy barrier inside the pores, which is in good agreement with the numerical results. The studies in this thesis offer insights into developing new methods for the exploitation of shale gas.