Advances and challenges in multiscale characterization of pore structure in unconsolidated sandstone reservoirs

Unconsolidated sandstone reservoirs are important hydrocarbon targets, but their weak cementation, clay sensitivity, stress-dependent grain contacts, and strong heterogeneity complicate pore-structure characterization. This review summarizes qualitative and quantitative characterization techniques, including scanning electron microscopy (SEM), field-emission scanning electron microscopy (FE-SEM), focused ion beam–scanning electron microscopy (FIB-SEM), X-ray micro-computed tomography (micro-CT), nitrogen adsorption, mercury intrusion porosimetry (MIP), constant-rate mercury injection (CRMI), and nuclear magnetic resonance (NMR). The operating principles, characterization capabilities, scales, advantages, and limitations of these techniques are compared. Electron microscopy provides high-resolution morphological information, with representative spatial resolutions of approximately 3–20 nm for conventional SEM and approximately 1 nm for optimized FE-SEM; micro-CT enables non-destructive three-dimensional visualization over larger sample volumes, adsorption and mercury-intrusion methods characterize pore-size and pore-throat distributions, and NMR provides fluid-sensitive information on pore space and petrophysical properties. However, sample preparation, weak-framework disturbance, scale dependence, and representative elementary volume selection can affect the reliability of results obtained from unconsolidated sandstone. No single technique can characterize the complete pore system across all relevant scales; complementary application and cross-validation of multiple methods are therefore necessary. Unlike broader reviews of general reservoir rocks, tight sandstone, or shale, this review provides a method-oriented synthesis focused on how weak cementation, preparation-induced disturbance, scale dependence, and representative-volume constraints govern method applicability and data interpretation in unconsolidated sandstone reservoirs.