## 4 Results and Discussion
### Effect of Confinement on Fracture Pattern and Failure Surface  
The effect of confinement on the fracture pattern and failure surface of water-fractured samples was then analysed, based on the images taken before and after fracturing. According to the failure patterns, when the confining pressure is 1 to 3 MPa, the fracture is radial (the fracture occurs only on one side of the sample) and when the confining pressure is 5 to 7 MPa, the fracture is diametrical (the fracture continues through the diameter of the sample). Figure 6 shows the fracture patterns observed in samples which failed at 1 MPa and 7 MPa confinements. The reason for the different failure patterns under low and high confinements can be explained using the negative effective stress principle. According to Figure 4, the breakdown pressures are around 2.5 and 18.2 MPa at 1 and 7 MPa confining pressures respectively, and the corresponding negative stresses are around 1.5 and 11.2 MPa, respectively. See Equation (11) [32]:  
\[\sigma_{\textit{eff}}=\sigma_{c}-\sigma_{p} \tag{11}\]  
where \(\sigma_{\textit{eff}}\) is the effective stress, \(\sigma_{c}\) is the confining pressure or stress, and \(\sigma_{p}\) is the pore pressure of the sample. This equation can only be used based on two main assumptions: the rock grains are incompressible and the yield stress in the grain material is independent of the confining pressure [33], both of which are applicable to siltstone. As shown in Figure 7, the effective net force acting on the surface of the wellbore in the outward direction is always proportional to the negative effective stress, because the surface area of the wellbore remains constant throughout the fracturing process. As previously stated, the negative effective stress is positively correlated with the confining pressure. Therefore, the net force acting on the wellbore surface is high at higher confining pressures. This means that greater force and higher energy can cause a complete diametrical fracture through the wellbore. Moreover, for all the confining pressures, the fracture is always along the wellbore, along the vertical plane, due to the greater axial stress (20 MPa) applied compared to the confining pressures. Since fractures are always created perpendicular to the minimum stress direction, here the fracture occurs along the vertical plane. Similar results have been shown by other researchers, including [PERSON] et al. [21], [PERSON] et al. [8] and [PERSON] et al. [7] for concrete and artificial materials such as polymethyl methacrylate. Moreover, in relation to the fracture width or aperture, fractured samples under 1 and 7 MPa confining pressures exhibited fracture widths of around 12 \(\upmu\)m and 29 \(\upmu\)m, respectively, upon the release of confinement pressure. This demonstrates that the fracture width increases with increasing confining pressure, which is important for the hydraulic fracturing process. Based on this observation, wider fracture generation can be expected from the hydraulic fracturing process in deeper geological formations compared to shallow reservoirs. However, it should be noted that in the present study the fracture aperture was measured after releasing the confinement, which would be much higher than the fracture aperture in field conditions under the application of great confinements. Interestingly, not only the fracture width, but also the length was found to be confining pressure-dependent (Figure 6). According to Figure 6, a hydraulically fractured sample at 7 MPa confining pressure splits into two parts along the wellbore. This means that at the final breakdown, the fracture has been propagated throughout the sample, splitting it into two hemi-cylindrical parts. However, in the hydraulically fractured sample at 1 MPa confining pressure, the fracture propagated only up to 58 mm along the sample (13 mm beyond the wellbore), which is only about 64% and 29% compared to the total height of the sample and the sample height without the wellbore, respectively.