Experimental investigation of the microstructural evolution of weakly cemented sandstone under seepage erosion

The coal-bearing strata in the Ordos Basin, China, formed in the Jurassic, and the associated sandstone aquifer is characterized by a short diagenesis time and weak cementation. Affected by seepage erosion, interstitial materials migrate inside the sandstone formation, resulting in changes in porosity and permeability over time and ultimately increasing the risk of mine water damage. Based on the combination of cast thin section (CTS) analysis, X-ray diffraction (XRD) testing, and nuclear magnetic resonance (NMR) testing, the changes in the porosity and pore structure parameters of medium-grained sandstone and coarse-grained sandstone under seepage were studied. On this basis, the relationship between porosity and seepage erosion time, porosity composition, and pore size was also established using the T2 spectrum distribution curve of weakly cemented sandstone samples obtained by NMR. The results show that the medium-grained and coarse-grained sandstone’s critical displacement pressures (DP) are approximately 0.5 MPa and 0.3 MPa, respectively. When the DP does not reach the critical value, the weak cementing material separates and moves inside the sample, leading to mutual conversion between pores of different sizes. Meanwhile, the total porosity changes only slightly. After DP exceeds the critical value, some weakly cemented substances can escape from the sample, significantly changing the microscopic pore structure inside the sample, increasing large-scale pores, enhancing water conductivity and increasing total porosity. These research results are of great theoretical significance for mine water disaster prevention and control in China’s western mining area.