## 3 Mesostructural Evolution of Sandstone Under Dry-Wet Cycles
### The Evolution of Porosity and Permeability of Jointed Sandstone  
The spaces in rocks that are not filled with particles, impurities and cementing substances are called the pore structure. The spatial volume and connectivity of the spaces can be characterized by porosity and permeability. In this section, the change of the porosity and permeability of sandstone specimens under the action of dry-wet alternation were observed by NMR technology. The mesostructure of sandstone was analyzed.  
The evolution of cumulative porosity of jointed sandstone with the dry-wet alternation number is depicted in Figure 4. This shows all the cumulative porosity curves consisted of three phases, namely the slow accumulation stage, the rapid accumulation stage and the steady development stage. To be specific, the cumulative porosity curves changed slightly when t = 0-0.1 \(\upmu\)s. Subsequently, the growth rate increased rapidly (0.1-100 \(\upmu\)s). After 100 \(\upmu\)s, the growth rate of the cumulative porosity curve decreased, while cumulative porosity stayed constant when it reached the maximum value. To explore the influence of the wet-dry cycle on the porosity of sandstone, the changes of the maximum accumulated porosity and permeability with the wet-dry cycle number were analyzed and are shown in Figure 5. In this figure, the accumulated porosity under different dry-wet cycles represents the peak value in Figure 4, and the permeability was calculated using the empirical formula (Coates model) according to the volume of movable and bound fluids of \(T_{2}\) spectrum [43]. The study found that both accumulated porosity and permeability increased with the increase of the dry-wet alternation number. To determine the relationship between the total porosity of jointed sandstone and the dry-wet alternation number, the explicit fitting expression was deduced:  
\[\Phi_{n}=p+q\ln(n+1) \tag{3}\]  
where \(p\) denotes the initial porosity, \(n\) is the number of dry-wet alternation cycles, \(\Phi_{n}\) is the total porosity corresponding to the dry-wet alternation number and \(q\) is the logarithmic fitting coefficient of the total porosity. In the experiment, \(p\) = 7.005% and the fitting coefficient is \(q\) = 0.372, yielding the correlation coefficient \(R^{2}\) is 0.993.  
On the basis of the experimental results, it was found that dry-wet alternation action promotes the development of porosity channels, resulting in damage to sandstone. When water infiltrates the sandstone, hydrophilic minerals such as potassium feldspar, albite and illite dissolve as the hydration reaction proceeds (Equations (4) and (5)); thus, the number of micropores increases gradually. Besides, the expansion of hydrophilic clay minerals leads to uneven stress inside the rock. Therefore, microcracks occur in the sandstone. Under the action of dry-wet alternation, the micropores and microcracks expand from the rock surface to the interior. As a result, the pore connectivity of rock samples is improved [44]. The more new pores and pore throats generated by dry-wet alternation action, the higher the total porosity and permeability. Although the variation trends of micropores, mesopores and macropores in jointed sandstone are different under dry-wet alternation conditions, the total porosity and permeability increase with an increasing dry-wet alternation number. The irreversible cumulative damage of jointed rock masses caused by dry-wet alternation action ultimately causes the significant weakening of mechanical properties [45].  
\[\text{CaCO}_{3}+\text{H}_{2}\text{O}=\text{Ca}^{2+}+\text{HCO}_{3}^{-}+\text{ OH}^{-} \tag{4}\]  
\[\text{CaMg}(\text{CO}_{3})_{2}+\text{H}^{+}=\text{Ca}^{2+}+2\text{HCO}_{3}^{-}+ 2\text{Mg}^{2+} \tag{5}\]