NMR-based analysis of the effect of moisture migration on sandstone pore structure under alternating wetting and drying conditions

The wetting-drying (W-D) cycle is a type of water–rock interaction. The pore structure of rock, such as shape, size, distribution and pore throat, affects fluid storage and transport. Fractal theory and experimental research on the evolution characteristics of pore damage during the wet-dry erosion process are highly important for determining W-D damage. The mass and velocity of liquid migration are related to the pore size, porosity, fluid properties, etc. Experimental data show that the water absorption quality and velocity in rocks decrease with the number of wet-dry cycles. At the same test time, the mass and velocity of the SI water absorption method are smaller than those of the FI method. Under these two conditions, the amount and rate of water absorption represent the degree of water–rock interaction. Considering the pore evolution during the wet-dry cycling, an equation describing the motion of liquid in porous media was derived based on the imbibition-type separation model. The experimental data are in excellent agreement with the calculated values of the model. Permeability characteristics can affect the area and degree of rock deterioration as well as the development rate of pores and microcracks. Based on the interaction between permeability and pores, quantitative analysis of the weakening process (local damage) of rocks under W-D cycles can provide good reference indicators for evaluating the stability of geotechnical engineering.