Shale gas migration modeling considering pore scale and fracture density

Unconventional shale gas provides a potential resource for future energy consumption. Shale gas is trapped in pore radii from several nanometers (nm) to 1 millimeter (mm), while the pore throat radii of the order of molecular mean free path. The collision between methane molecular and pore surface may become ignorable. Two kinds of flow regimes were proposed. The Darcy law and Fick diffusion were coupled as macro flow regime to simulate gas flow in shale gas pores, while the slippage effect, Knudsen diffusion were applied as micro flow regime, and numerical analysis was performed. The pore size and perforation number, as well as fracture density were considered in the numerical model. The cases for the micro flow regime with pore radius of 5 nm, macro flow regime with fracture distance ranging from 5m to 30m were employed, and the change of perforation number were considered in the numerical simulation to reflect the optimal design for the gas extraction. Numerical results showthat gas production rate is predominately determined by the density of induced fractures. Gas production rate based on slippage effect is not significantly higher than traditional convection when the pore radius is 5 nm. Diffusion plays an important role in gas production for macro flow regime at low gas pressure, while slip flow is predominant for the micro flow regime. Gas production is more considerable when considering the intermittent time during gas extraction, which results in pressure redistribution.