(129e) Multi-Scale Discussions on Mass Transport in Gas Shale Reservoirs

Fluid mechanics of natural gas in organic-rich shale involves nano-scale phenomena which could lead to potential non-Darcian effects during gas production. In general, these are low-Reynolds number and non-continuum effects and, more importantly, pore-wall dominated multi-scale effects. In this study we introduce a new lattice Boltzmann method to investigate these effects numerically in simple pore geometries. The standard method has been developed in the 1980s to overcome the weaknesses of lattice gas cellular automata and has emerged recently as a powerful tool to solve fluid dynamics problems, in particular in the areas of micro- and nano-fluidics. The new approach takes into account molecular-level interactions using adsorptive/cohesive forces among the fluid particles and defining a Langmuir-slip boundary condition at the organic pore walls. The model allows us to partition mass transport by the walls into two components: slippage of free gas molecules and hopping (or surface transport) of the adsorbed gas molecules. Using the standard two- and three dimensional lattices, low-Reynolds number gas dynamics is simulated in a one-hundred nanometer model organic capillary and later in a bundle of smaller size organic nanotubes. The results point to the existence of a critical Knudsen number value for the onset of laminar gas flow under typical shale gas reservoir pressure and temperature conditions. Beyond this number the predicted velocity profile shows that the mechanisms of slippage and surface transport could lead to molecular streaming by the pore walls which enhances the gas transport in the organic nanopores. The work is important for development of new-generation shale gas reservoir flow simulators and it can be used in the laboratories for organic-rich shale characterization.