Multi-scale Gas Flow in Shale Pores with Water Films

Abstract

This study first puts forward an analytical model for calculating gas velocity profiles and predicting gas apparent permeability enhancement factors in shale nanometer scale characteristic dimensions of different geometries (slit pores and circular pores). The proposed model considers the presence of a mobile high-viscosity water film through modified boundary conditions at a liquid-solid interface and a gas-liquid interface on the basis of the governing equations for pressure-driven flow, finding good agreements with experimental data and validating that a mobile high-viscosity water film enhances gas flow capacity. A mobile bulk water layer is introduced on the basis of the above derived model to explain the case of high water saturation. Next, water-gas phase behaviors are studied in shale rocks with a wide range of pore size distributions rather than single nanopores based on the fractal theory. Also, the research focus will then be switched from the pore scale to the reservoir scale. With a specific pore distribution, gas-water relative permeability can be calculated accordingly, which is a necessity for shale gas simulation study. Finally, with the introduction of the previously derived gas apparent permeability model and the relative permeability curves, reservoir simulation is conducted to evaluate the shale gas production performance. This study has been extended to the case of multiphase flow in shale nanopores and shale rocks, providing a better explanation of the fluid flow pattern in actual reservoir conditions.