Pore structure variation characteristics of a Chinese local mudstone before and after the first cycle of wetting and drying

• The first wetting-drying cycle induces a substantial increase in pore volume and pore connectivity in mudstone, as revealed by micro-CT. • Porosity and fractal dimension across individual mudstone slices increase both in magnitude and fluctuation amplitude after the initial WD cycle. • Pore and pore-throat size distributions conform to Gaussian curves, with the smallest pores and throats showing the greatest relative increase. • WD transforms angular, flat pores into rounder and more regular shapes; sphericity is positively correlated with pore radius, providing insight into mudstone deterioration and failure mechanisms.