The microbially-induced calcium carbonate precipitation (MICP) technique has shown great robustness in dealing with soil and groundwater contamination problems. A typical result of the implementation of MICP technique is a change in the pore structure. In this study, the effects of MICP on the pore structure of yellow sandstone from the Zigong area, Sichuan, China under different conditions, (e.g., temperature, pH, and calcium ion concentration) are investigated using LF-NMR resonance. The pore network of yellow sandstone is accurately measured using the peak area of the T₂ spectral signal. The distribution of calcium carbonate in the pores of the yellow sandstone is characterized by the magnitude of the T₂ signal variation. The results show that the precipitation of calcium carbonate caused by MICP tends to be deposited in relatively large pores. However, the calcium carbonate precipitates in the smaller pores at a higher temperature. A higher pH considerably enhances the precipitation, and the alkaline environment tends to cause the precipitation of the calcium carbonate in the large pores. Although the amount of produced calcium carbonate continuously increases as the MCIP process continues, which is expected, the production efficiency decreases steadily.

Clay is widely encountered in nature and directly influences seepage behaviors, exerting a crucial impact on engineering applications. Under low hydraulic gradients, seepage behaviors have been observed to deviate from Darcy’s law, displaying a non-linear trend. However, the impacts of clay content on non-linear seepage behavior and its pore-scale mechanisms to date remain unclear. In this study, constant-head seepage experiments were conducted in sand–clay porous media under various hydraulic gradients. Low-field nuclear magnetic resonance (LF-NMR) technology was utilized to monitor the bound-water and free-water contents of sand–clay porous media under different seepage states. The results show a threshold hydraulic gradient (i₀) below which there is no flow, and a critical hydraulic gradient (i_cr) below which the relationship between the hydraulic gradient (i) and seepage velocity (v) is non-linear. Both hydraulic gradients increased with clay content. Moreover, the transformation between bound water and free water was observed during the seepage-state evolution (no flow to pre-Darcy or pre-Darcy to Darcy). As the hydraulic gradient reached the i₀, the pore water pressure gradually overcame the adsorption force of the bound-water film, reducing the thickness of the bound-water film, and causing non-linear seepage behavior. When i₀ < i < i_cr, the enlarging hydraulic gradient triggers the thinning of bound water and enhances the fluidity of pore water. Moreover, the increasing clay content augments the bound-water content required for the seepage state’s change.

Adsorption reaction in unsaturated porous media is of great importance for soil and groundwater remediation. In this study, the influence of the Peclet number (Pe) and water saturation on adsorption behavior at liquid-liquid interfaces was quantitatively investigated. The pore-scale reactive transport in unsaturated porous media was directly simulated. The Navier-Stokes equations, the surface transfer and adsorption reaction equations, and the advection-diffusion equation (ADE) were coupled to obtain the flow and concentration fields. The results showed that water saturation had a significant influence on the complexity of the flow field. A nonmonotonic relationship was found between water saturation and the uniformity of the flow field. Peclet number had little influence on the maximum adsorption. On the other hand, the adsorption time showed a nearly linear relationship with the Peclet number and increased with increasing Peclet number. Additionally, a nonlinear relationship was found between water saturation and the maximum adsorption. As water saturation increased, the maximum adsorption tended to increase to a peak and then decrease. The peak of the maximum adsorption occurred at , , and the shortest adsorption time was observed at . However, the difference in adsorption times for saturations of 0.458 and 0.698 was not significant and was only about 20 PV difference.