## 3 Test Results and Analysis
### The Results of the NMR Experiment  
As the rock underwent soaking, certain minerals were either dissolved or regenerated. Notably, calcium carbonate was first dissolved and subsequently reformed as a secondary mineral. This process induces alterations in the microscopic pore structure and strength of the sample. Building on this phenomenon, the research team conducted NMR experiments to investigate the corrosion process of the sample. Figure 4 illustrates the T\({}_{2}\) spectrum pore size distribution results over different months. Rock is recognized as a material with porous properties. In the context of NMR experiments, relaxation time refers to the duration required for the magnetization vector of the nucleus to return to an equilibrium state in the presence of an external magnetic field. For fluids contained within rock masses, relaxation times are typically categorized into three types: volume relaxation, surface relaxation and diffusion relaxation [34]. When the rock is subjected to a uniform magnetic field, the influences of volume relaxation and diffusion relaxation can generally be regarded as negligible. Consequently, the transverse relaxation time can be represented in a simplified approximate form [35]. \[\frac{1}{T_{2}}\approx\frac{1}{T_{2S}}=\rho_{2}\frac{S}{V}=\rho_{2}\frac{F_{S} }{r} \tag{3}\]  
In the equation, \(\rho_{2}\) represents the transverse relaxation rate, and its units can be either \(\upmu\)m/s or nm/s [36]. \(S\) denotes the surface area of the pore, while \(V\) indicates the volume of the pore. The \(F_{S}\) value is closely associated with the shape of the pores. The shape factor for spherical pores is 3, for columnar pores it is 2 and for layered pores it is 1; all of these shape factors are dimensionless. Furthermore, r refers to the characteristic length corresponding to the pore shape [37]. When normalizing the NMR spectral curves of sandstone, it can be inferred from the principle of relaxation time [38] that the transverse relaxation time is approximately equal to the radius of the pore [39; 40]. Therefore, in the normalized accumulation curve, the relaxation time can serve as an approximate reflection of the pore radius of coarse sandstone. Based on the results of the NMR T2 spectrum, the pore size range can be categorized into three groups: pores with a diameter ranging from 0.0002 \(\upmu\)m to 0.07 \(\upmu\)m are termed micropores, those with a diameter from 0.07 \(\upmu\)m to 1 \(\upmu\)m are referred to as mesopores, and pores with a diameter of 1 \(\upmu\)m or larger are classified as macropores, with those up to 26 \(\upmu\)m also falling under this classification. Utilizing this classification criterion, coarse sandstone samples subjected to different corrosion times can be categorized accordingly. As illustrated in the results presented in Figure 5, it is observed that as the soaking time increases, the percentage of micropores initially decreases before increasing again, the percentage of mesopores first rises and then declines, while the percentage of macropores shows a preliminary increase followed by a decrease. \begin{table}
\begin{tabular}{c c c c c} \hline
**Soaking Time/m** & **Compressive Strength/MPa** & **Elasticity Modulus/GPa** & **Poisson Ratio** & **Permeability/mD** \\ \hline
0 & 152.68 & 11.64 & 0.301 & 1.00 \\
1 & 134.97 & 9.53 & 0.228 & 1.32 \\
2 & 121.8 & 8.21 & 0.200 & 2.09 \\
3 & 148.86 & 11.08 & 0.289 & 1.04 \\
4 & 138.98 & 10.71 & 0.244 & 1.08 \\ \hline \end{tabular}
\end{table} 
Table 3: Mechanical parameters of samples with different soaking times. The stress change curve of sandstone under varying soaking time conditions can be categorized into three distinct stages: the dissolution stage, the mineralization stage and the secondary dissolution stage, as illustrated in Figure 6. The analysis of the figure reveals that during the dissolution stage, the compressive strength of coarse sandstone decreases rapidly.