## 2 Sealing Method  
The proposed wedge-shaped metal buckle can function well when subjected to high temperature and high pressure (Figure 2a). This sealing structure consists of caps and pedestals. The cap and pedestal can be well paired to form a complete set of seals. Two types of sealing assembly methods are proposed in this paper: single-seal and multi-seal types. Both sealing types require the placement of a central fracturing pipe for water injection. The outer diameter of the central fracturing pipe should be smaller than the inner diameter of the borehole to facilitate the placement of sealing structures. Therefore, there is a certain gap between the central fracturing pipe and the borehole. For the single-seal type, a pedestal and a cap are sequentially placed inside the central fracturing pipe, forming a wedge-shaped structure. The cap is tightened by a metal fixing rod, providing a certain amount of pre-tightening force to achieve a tight fit between the pedestal and the cap. This single-seal type is relatively simple, but the disadvantage is that it is prone to sealing failure of leaking water after the rock is first hydraulically fractured, affecting further expansion of the fracture. For the multi-seal type, a certain length of metal segmented pipe is required between each sealing structure to achieve partial or full-length sealing. Finally, a metal fixing rod is used above the top cap to provide pre-tightening force for the sealing elements. Therefore, the central fracturing pipe, sealing structure, and the rock surrounding the borehole form a unified sealing entity. The liquid enters through the central fracturing pipe to the bottom and accumulates hydraulic pressure energy at the bottom, facilitating the hydraulic fracturing process. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**NO.**} & **Temperature** & **Rock Dimension-** & **Sealing Methods** & **Sample Types** & **Applied Stress/MPa** & **Ref.** \\ \hline
1 & 20–200 & \(300\times 300\times 300\) & Pre-located during sample preparation & Concrete & 6–10 & [14] \\
2 & 20–300 & \(\mathit{\phi}22.5\times 45\) & Open hole & Granite & 0–60 & [15] \\
3 & 20–120 & \(300\times 300\times 300\) & Open hole & Granite & 10–30 & [3] \\
4 & 20–150 & \(\mathit{\phi}50\times 100\) & Open hole & Granite & 5–10 & [16] \\
5 & 20–200 & \(300\times 300\times 300\) & Steel casing grouted by sealant & Granite & 40 & [17] \\
6 & 20–50 & \(300\times 300\times 300\) & Steel casing grouted by epoxy & Granite & 4.1–12.5 & [18] \\
7 & 20–200 & \(100\times 100\times 100\) & Chemical sealant & Granite & 5–10 & [19] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Hydraulic fracturing tests under high temperatures using various sealing methods. The sealing structure could be series-connected (i.e., multi-seal type) to perform hydraulic fracturing tests of large rock samples under HTHP, such as \(q\)200 \(\times\) 400 mm, 300 \(\times\) 300 \(\times\) 300 mm\({}^{3}\), and 1000 \(\times\) 1000 \(\times\) 1000 mm\({}^{3}\), which will help to uncover the multiple hydraulic fracturing mechanisms (Figure 2b). In this study, the length of the steel ring is determined as 30 mm for granite samples with a dimension of \(q\)200 \(\times\) 400 mm. The 30 mm length refers specifically to the experimental dimensions discussed in this paper. For other specimen sizes, the dimension can be appropriately scaled up or down. The main reason for using a 30 mm spacer is to conserve sealing material. Additionally, based on the extensive experiments we have conducted, a 30 mm spacer was found to be suitable. Specifically, the easily available copper is employed to manufacture this sealing material due to its excellent ductility, large thermal expansion coefficients, and cost-effectiveness. In the process of hydraulic fracturing, conventional sealing materials and technologies become inapplicable under HTHP conditions.