Relationship between capillary water absorption mechanism and pore structure and microfracture of red-layer mudstone in central Sichuan

The pore-microfracture structure, water-rock interaction and capillary water absorption mechanism of red-layer mudstone were comprehensively studied. The mineral composition was determined using X-ray diffraction, and the pore-microfracture structure was observed using scanning electron microscopy. Mercury intrusion porosimetry and nitrogen physisorption were used to obtain pore throat size distribution information. The effects of mineral composition and pore structure on adsorption and desorption behaviour were investigated using water vapour physisorption. The wettability of the mudstone surface was characterised by contact angle measurement, atomic force microscopy. The capillary water absorption coefficient and capillary water absorption height of mudstone were quantitatively studied using capillary water absorption experiments. The results showed that the clay mineral content of mudstone was 46.8%, and the pores were mainly micropores and transition pores, which were very dense. The water vapour physical adsorption isotherm of mudstone showed that the average water-holding rate of mudstone was 31.47%. In the process of desorption of mudstone, the “ink-bottle” effect limited the desorption of water under medium and high relative humidity. The macroscopic contact angle of the mudstone surface was less than 29.79°, indicating mudstone’s strong hydrophilic property. The driving force of capillary water absorption of mudstone should consider the capillary force, osmotic pressure and gravity. The hydration fracture of the mudstone was the dominant channel of its capillary water absorption. Understanding the relationship between the water absorption mechanism of mudstone capillarity and pore-microfractures is necessary for revealing the disaster mechanism of the red-layer mudstone.