## 4 Results and Discussion  
### Effect of Confinement on Breakdown Pressure  
The different injection pressures over time in intact siltstone specimens at various confinements, when the flow rate was maintained at a constant rate of 5 mL/min, are shown in Figure 4a. According to the figure, injection pressure generally increases over time until it reaches the breakdown pressure of the sample. This significant pressure development occurred due to the low permeability of siltstone, and the injection flow rate (5 mL/min) was therefore much higher than the flow penetration rate through the rock matrix, which created an injection pressure development until the final failure. Figure 4a exhibits three main regions in the pressure development, regardless of confinement, and these are clearly shown in Figure 5. The first stage is the initial pressure development stage, at which pressure development with time is comparatively small and almost horizontal, because water more easily enters the accessible pores in the sample until they fill, giving only a small chance to fluid remaining in the upstream to undergo pressure development. However, in the second wellbore pressurization stage, most of the nearby pores are already filled and saturated. Therefore, the injected water cannot easily enter the pores and accumulates in the upstream with the developing pressure there. At the final failure stage, the sample breaks, creating a sudden pressure drop, because sample failure occurs, creating many fractures that offer much easier flow paths to the fluid. This is consistent with the similar pressure development trends observed by [PERSON] et al. [7] for concrete specimens.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Confining Pressure (MPa)** & **N\({}_{2}\) Injection Pressure (MPa)** \\ \hline
5 & 1 \\
5 & 2 \\
15 & 6 \\
20 & 4 \\
25 & 6 \\
20 & 10 \\ \hline
25 & 6 \\
20 & 10 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Permeability test conditions with different confining and injection pssures.  
Figure 4b shows the effect of confinement on the breakdown pressure of the siltstone samples, according to which the breakdown pressure of the siltstone specimens almost linearly increases with increasing confining pressure. This is consistent with the findings of [PERSON] and [PERSON] [30], who studied granite specimens. According to Figure 4b, when the confining pressure is increased from 1 to 7 MPa, breakdown pressure increases from 2.5 to 18.2 MPa, (about 7 times increase from 1 MPa). Each breakdown pressure was then cross-calculated according to the HW criterion (using Equation (2) and the tensile strength of the tested siltstone, 7.3 MPa) and the results are shown in Figure 4b. According to this figure, the experimental breakdown pressure values are much lower than the values of the pressure. The results are shown in Figure 4c. The results are shown in Figure 4d. The results are shown in Figure 4e. The results are shown in Figure 4e. The results are shown in Figure 4f.  
calculated using the HW criterion, probably due to the water-softening effect, where water molecules weaken the clay mineral and rock bonding, creating reduced tensile strength characteristics and therefore lower breakdown pressures [31]. Moreover, depending on the fracturing method, fluid pressure acts on the entire well surface and induces a high tensile force due to higher surface area. This shows the inapplicability of the HW criterion for wettable fluid injection into siltstone without the well casing. However, the linear relationship between the breakdown pressure and confining pressure seems to remain the same for both experimental and HW results.