Microwave–water effects on hydrocarbon mobility in lacustrine shale reservoirs

Microwave irradiation provides a promising in-situ strategy for improving shale oil recovery, yet the role of abundant residual water retained after hydraulic fracturing in microwave-induced nanopore reorganization remains insufficiently constrained. To address this gap, this study employs water vapor adsorption, contrast-matched small-angle neutron scattering (CM-SANS), and SANS-based multifractal characterization to quantify water uptake and nanopore accessibility in Chang 7 lacustrine shale from the Ordos Basin and to evaluate microwave-induced pore-structure evolution in dry and water-saturated samples. The results show that 36.11%–56.61% of the nanopore volume within the 2–100 nm range is accessible to water, with preferential water occupancy in clay-related pores within the 2–3 nm interval. Water-saturated microwave irradiation increases pore volume and mean pore diameter, promotes pore enlargement and coalescence, increases pore number density in the 3–100 nm range, and reorganizes nanopore connectivity. These findings demonstrate that retained water is not merely a passive pore fluid but an active microwave-absorbing phase that participates in shale nanopore modification. By linking retained-water accessibility, microwave-induced pore-structure reorganization, and multifractal nanopore evolution, this study provides new nanoscale insights into water-assisted microwave stimulation and its potential to create more favorable transport pathways for shale oil migration.