Pore-scale numerical investigation into the impacts of the spatial and pore- size distributions of organic matter on shale gas flow and their implications on multiscale characterisation

摘要

Owning to their differences in surface properties and pore geometry, pores of predominant submicron sizes in organic and inorganic matter of gas shale incur different gas flow and transport behaviours. Those differences can manifest at sample scales differently depending on the spatial distributions of shale organic and inorganic matter, and the pore-size distributions in these types of matter. Therefore, understanding the impacts of variable configurations of them on the gas flow is essential to guide progressive subsampling in multiscale shale characterisation that is required for modelling shale gas flow at the first place. This article reports a pore-scale numerical investigation into the impacts of combinations of three end-member spatial arrangements of the organic matter and two contrasting sets of organic and inorganic pore-size distributions at variable organic fractions on gas flow using pore-network modelling. A unified pore-network flow model for shale gas that captures a comprehensive set of gas flow and transport mechanisms is developed in this work to calculate the effective gas apparent permeability at reservoir conditions. In terms of the mean permeability at each selected organic fraction, the largest differences are found to appear at a high fraction above 25% across all the arrangements, and the upper bound can reach more than two orders of magnitude greater than the lower bound. The results suggest that subsampling ought to focus on subdomains where organic fractions are high, organic and inorganic pores differ in size, and distinct flow-enhancing or baffling arrangements of organic matter are present.