Quantification of gas storage and transport in organic-rich shale is important in determining natural gas production rates and reserves. However, laboratory measurements are challenging, due to very tight nature of the rock, and have large uncertainties due to presence of multiple mechanisms of gas storage and transport at multiple scales. The emphasis of this thesis is on understanding of storage and transport mechanisms and their interplay inside organic nano-capillaries. An atomistic modeling and molecular simulation approach is presented in investigating supercritical methane behavior in model carbon nanotubes representing nano-capillary.
Equilibrium Monte Carlo simulations show a non-uniform methane density profile across the diameter of the capillary. The results show excess methane at the central portion of the capillary indicating deviations from Langmuir adsorption model. Amount of excess methane is dependent on the competition in between the fluid-wall and fluid-fluid interactions.
To study the transport behavior of methane in nano-capillary, we performed nonequilibrium Molecular Dynamics simulations based on a moving piston model. The piston model allows us to study steady-state transport across the diameter of nanotube in order to understand the effects of adsorbed methane on transport under reservoir conditions. The results show that the adsorbed phase is not only mobile but also contribute significantly to total mass flux. The contribution of the adsorbed-phase is profound in smaller capillaries. Simulations of transport with different sizes of capillaries show that the adsorbed-phase transport velocity is independent of capillary size, but strongly dependent on the pressure drop across the capillary. This allows us to quantify the adsorbed-phase velocity into an adsorbed phase mobility factor
