Investigation of multi-scale gas transport behavior in organic-rich shale

Gas flow in shale is controlled by different space scales and time scales due to its ultra-fine pore structure, high content of clay minerals, high content of organic matter and serious anisotropy. In this work, multi-scale gas transport in organic-rich shale is investigated by theoretical modeling and pore structure analysis. Characteristics of multi-scale gas transport in shale are sufficiently confirmed through study of multi-scale flow tube, multi-scale pore structure and multi-scale flow regime. The effect of Knudsen diffusion on non-linear gas flow in shale is analyzed. Contribution of Knudsen diffusion to apparent gas permeability is calculated to be greater at the lower pressure or permeability. Knudsen diffusion coefficient and its corresponding hydraulic radius are calculated to be lower at the lower permeability or higher pressure. Finally, an equivalent permeability formula is put forward to model the apparent permeability of shale at specific pressure, temperature and petrophysics. The increase multiples of permeability for gas slippage during the development of the studied shale gas reservoir is determined, indicating that the effect of non-linear flow on shale permeability measurement and well production forecast should be paid abundant attention. Results from this study are beneficial to comprehensively understand multi-scale gas transport in shale.