Microwave-Induced Pore-Network Modification in Water-Bearing Tight Sandstone Revealed by Low-Field NMR

Water-bearing tight sandstone is highly sensitive to microwave irradiation because pore water enhances microwave absorption and local heating. However, how water saturation controls pore-fluid redistribution, pore-network evolution, and permeability enhancement remains unclear. Here, low-field NMR measurements ( T 2 distribution, T 1 – T 2 mapping, and MRI) were combined with gas permeability tests to investigate tight sandstone samples with different initial water saturations after microwave treatment. The results show a stage-dependent response. At short irradiation durations, microwave treatment mainly causes selective dehydration and redistribution of residual fluids. With longer irradiation durations, especially at higher water saturations, the dominant response shifts to pore-throat opening, microcrack generation, and pore-fracture network reconstruction. Permeability increased by up to 133.6%, with an average increase of about 65%, but the permeability response does not follow pore-size growth or porosity increase alone. Instead, permeability shows the strongest correlation with FFI/BVI ( R 2 = 0.8236), indicating that the expansion of connected movable pore space plays a dominant role in post-treatment flow behavior. These results show that water saturation strongly affects localized heating and the extent of pore-structure modification, and that microwave stimulation of tight sandstone is a connectivity-controlled process.