Gas in tight sand and shale exists in underground reservoirs with microdarcy (μD) or even nanodarcy permeability ranges; these reservoirs are characterized by small pore throats and crack-like interconnections between pores. The size of the pore throats in shale may differ from the size of the saturating fluid molecules by only slightly more than one order of magnitude. The physics of fluid flow in these rocks, with measured permeability in the nanodarcy range, is poorly understood. Knowing the fluid flow behavior in the nano-range channels is of major importance for both simulation studies and calculations of the relative permeability of gas in tight shale gas systems. In this work, a lab-on-chip approach for direct visualization of the fluid flow behavior in nanochannels was developed using an advanced single-molecule imaging system combined with a nano-fluidic chip. Displacements of two-phase flow in 100 nm depth channels were characterized. Specifically, the two-phase gas slippage effect was investigated. Under experimental conditions, the gas slippage factor increased as the water saturation increased. The two-phase flow mechanism in nano-scale channels was proposed and proved by the flow pattern images. The results are crucial for permeability measurement and gas slippage factor determination for unconventional shale gas systems with nano-scale pores.
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