Mechanisms of microfracture impact on reservoir quality and gas saturation in tight sandstones

Microfractures are believed to significantly influence gas storage and flow in tight sandstones, yet there is a continued need for quantitative methods to better understand their impact on reservoir quality and gas saturation. We integrate imaging techniques and petrophysical tests to assess how different types of microfractures influence porosity and permeability in tight sandstones. Gas accumulation in tight sandstones was simulated at the core scale using a gas-charging instrument developed from NMR technology, and the changes in gas saturation during the charging process were quantitatively measured; numerical simulations were conducted based on a porous media model constructed via digital core technology and the phase-field method. Overall, microfractures contribute minimally to the porosity. A cross-linked network of structural and circumgranular microfractures with sufficient apertures significantly enhances matrix permeability. Dissolution microfractures display small apertures and limited propagation, and do not form cross-linked networks, resulting in a negligible contribution to permeability. Bedding-plane microfractures typically develop in parallel orientations and contribute little to vertical permeability. During the gas charging process, capillary pressure controls the sequence of pore filling and the spatial arrangement of gas in tight sandstones. Microfractures are considerably larger than the pore throats, allowing water displacement at lower gas entry pressures during gas invasion. However, because tight sandstones contain a significantly higher proportion of small pore throats, the presence of a few microfractures has a negligible impact on overall gas saturation. These findings enhance our understanding of how microfractures influence reservoir quality and the processes controlling fluid migration and storage in tight sandstone gas reservoirs, offering critical insights for the assessment of reservoir quality and gas saturation in tight sandstone exploration.