## 3 Mesostructural Evolution of Sandstone Under Dry-Wet Cycles
### The Variation of Pore Size Distribution and Throats in Jointed Sandstone  
The pore structure of rock can be divided into pores (spaces surrounded by particles) and throats (channels between pores) according to the size and interrelationships of pore space, which represent the reservoir performance and the permeability of rocks, respectively. The pore diameter and pore throat are important indexes, which influence the porosity and permeability of rock samples. To further study the mesostructure of the sandstone, the pore diameter and throats of jointed sandstone under different dry-wet alternation conditions were investigated; the evolution of the mesostructure of sandstone was studied in this part. Figure 4: Variations of accumulated porosity of rock specimens with time under different numbers of dry–wet alternation cycles. Figure 5: The total porosity and permeability versus the number of dry–wet alternation cycles. Using NMR technology, the measurements of pores with different diameters were obtained (i.e., the pore size distribution of the jointed rocks). The results are depicted in Figure 6. We found three main peaks, corresponding to pore sizes of 0.001-0.1 \(\upmu\)m (micropores), 0.1-10 \(\upmu\)m (medium pores) and 10-100 \(\upmu\)m (macropores), respectively. What is more, the first main peak value gradually decreased as the dry-wet process went on, while the second and third main peak presented the opposite trend, and the growth rate of the second main peak was more obvious. According to the classification criteria of the internal pores in concrete [25], the pores in jointed sandstone were further divided into five grades: micropores (G\({}_{\text{I}}\)), small pores (G\({}_{\text{II}}\)), mesopores (G\({}_{\text{III}}\)), macropores (G\({}_{\text{IV}}\)) and larger pores (G\({}_{\text{V}}\)) (Table 2). The proportion of pores with different pore size ranges under different dry-wet alternation action is presented in Figure 7. With the increase of the dry-wet alternation number, the proportion of G\({}_{\text{I}}\) pores fluctuated slightly, staying at about 25%. The proportion of G\({}_{\text{II}}\) pores decreased from 39.107% to 34.512% with an increasing dry-wet cycle. The proportion of G\({}_{\text{V}}\) pores was the smallest, and the value was close 0. In addition, the proportions of G\({}_{\text{III}}\) pore and G\({}_{\text{IV}}\) pore increased with an increasing dry-wet alternation number. These phenomena also indicate that the G\({}_{\text{II}}\) gradually changed to the range of a high-grade pore size. \begin{table}
\begin{tabular}{l l l l l} \hline
**Grade** & **Symbol** & **Name** & **Pore Diameter \(r_{\text{p}}\)** & **Remarks** \\ \hline I & G\({}_{\text{I}}\) & Micropore & \textless{0.01 \(\upmu\)m} & Containing 0.002–0.01 \(\upmu\)m transitional pores, partially cementing pores \\ II & G\({}_{\text{II}}\) & Small pore & 0.01–0.1 \(\upmu\)m & Containing 0.01–0.1 \(\upmu\)m transitional pores, partially cementing pores \\ III & G\({}_{\text{III}}\) & Mesopore & 0.1–10 \(\upmu\)m & Containing 0.1–100 \(\upmu\)m capillary opening \\ IV & G\({}_{\text{IV}}\) & Macroropore & 10–100 \(\upmu\)m & Containing 0.1–100 \(\upmu\)m capillary opening \\ V & G\({}_{\text{V}}\) & Larger pore & \textgreater{}100 \(\upmu\)m & Containing air pores, visible to the naked eye \\ \hline \end{tabular}
\end{table} 
Table 2: Classification of pores of rock specimens. Figure 6: Pore size distribution of jointed rock samples under different dry–wet cycles. The results above indicate that the pore diameter of \(\mathrm{G_{II}}\) continued to expand to \(\mathrm{G_{III}}\), then the pores continued to expand and gradually turned to \(\mathrm{G_{IV}}\). According to the studies conducted by [PERSON] et al. [46], during wetting, when the water infiltrates into the sandstone, the soluble minerals, including feldspar and calcite, are dissolved, and the space occupied by the soluble minerals before becomes a cavity. In addition, the dissolution of minerals will weaken the bond between the skeleton particles.