An upscaling framework for predicting gas multiscale transport behavior in shale matrix-microfracture systems considering microscopic transport mechanisms

Predicting gas production in shale rock is challenging due to the complex multiscale processes ranging from nanometer to micrometer-scale, complex components including the resolved (e.g., microfractures) and the unresolved components (e.g., nanoscale pores in inorganic and organic matters), and microscopic transport mechanisms such as the slip effect, surface diffusion, and the real gas effect. To this end, a comprehensive upscaling framework is proposed in this paper to mimic the multiscale gas transport processes in shale matrix microfracture systems. Here, the microscopic transport mechanisms are considered by a modified nanopore model, and then, upscaled to the matrix scale by the generalized lattice Boltzmann model. Using the proposed upscaling framework, we analyze the variations of apparent permeabilities with pressure and matrix porosity and the enhancement of gas transport capability by microscopic effects (slip effect and surface diffusion) and microfractures (orientation and aperture). The results show that the improvement in apparent permeability due to the microfractures is ∼1.70 times that of the basic shale matrix, while the improvement due to microscopic effects (at a pressure drop from 20 MPa to 1 MPa) is only 1.03 times. This indicates that the microfractures significantly affect the gas transport compared to the reduced pressure. More importantly, the microfracture aperture substantially affects the transport performance, especially when aligned with the flow direction. Increasing the aperture from 2 μm to 3 μm leads to a ∼2.56-fold improvement in transport capability. These findings contribute to more accurately estimating gas transport performance in shale with microfractures.