Nanoscale pores in shale make the interaction of fluid molecules and pore walls more important than in conventional reservoirs, where the pores are orders of magnitude larger. Vast experimental observation in recent literature shows higher-than-expected liquid flow in nanopore systems. Therefore, conventional flow equations such as Darcy’s law may not be valid for shale systems because of the difference in the controlling physics of liquid flow. Two main characteristics of liquid flow in shale are pore geometry and liquid slip at the inner pore wall. By looking more closely at SEM images of shale samples, many angular- to slit-shaped geometry have been observed. The behavior of liquid flow in noncircular pores significantly deviates from the Hagen–Poiseuille flow equation used in circular pores. Additionally, liquid flow in microscale pores reasonably assumes a no-slip boundary condition on pore inner walls, which needs to be modified to slip boundary condition in nanoscale pores. In this study, we examined numerous SEM images of shale samples to summarize different pore shapes and sizes. We then used computational fluid dynamic (CFD) modeling for various geometric and slip conditions to develop a generalized flow equation to capture pore geometry and slip effect. The developed flow equation is used in pore-network modeling simulation to study effective liquid permeability in a shale system. Our study confirms that the assumption of simplified circular pore causes apparent permeability to be significantly overestimated. The discrepancy between the realistic multigeometry pore model and the simplified circular pore model becomes more pronounced when pore sizes reduce and liquid slip on the inner pore wall is taken into account.
Fuel – Elsevier
Published: Sep 15, 2016
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