## 4 Results and Discussion
### Effect of Confining and Injection Pressures on Intact and Fracture Permeability
This can be explained using the effective stress phenomenon, where increasing confining pressure causes the effective stress acting on the rock mass to increase, which causes the rock pore space to shrink, resulting in reduced permeability [37]. For example, according to Figure 12b, at 1 MPa injection pressure, when the confining pressure is increased from 5 to 25 MPa, siltstone permeability reduces by about 24%. However, at 4 MPa injection pressure, when the confining pressure is increased from 5 to 25 MPa, the permeability reduces only by around 12.5%. This indicates that the effect of confining pressure on the N\({}_{2}\) permeability of intact siltstone is reduced with increasing injection pressure, probably due to the effective stress reduction caused by the greater pore pressure induced by greater injection pressures. However, this is useful in the hydraulic fracturing process, because according to the findings, the intact permeability of the reservoir is little affected by the reservoir depth if a sufficient injection pressure is maintained, which reduces the chance of fluid leakage to the reservoir. Figure 12: Variation of permeability for intact rock specimen (**a**) with injection pressure; and (**b**) with confining pressure. The effect of injection and confining pressures on fractured siltstone permeability was then considered and the results are shown in Figures 13 and 14. The effect on a siltstone sample fractured at low confinement (1 MPa) is shown in Figure 13 and at high confinement (7 MPa) in Figure 14. According to Figure 13, similar to intact siltstone, the permeability of fractured siltstone at 1 MPa confinement reduces with increasing injection pressure. For example, when the injection pressure is increased from 1 to 4 MPa, siltstone permeability reduces by around 22% at 5 MPa confining pressure and around 8% at 25 MPa confining pressure (Figure 13a). The same Klinkenberg slip flow effect is responsible, because the sample which fractured at low confinement had some intact (non-fractured) portion for gas flow, which would have been subjected to the slip flow effect. In relation to the confinement effect on the siltstone sample fractured at low confinement (1 MPa), Figure 13b exhibits the similar influence of confinement on this fractured siltstone at low confinement to that in the intact specimen, as permeability reduces with increasing confining pressure, regardless of injection pressure due to the increased effective stress. As explained earlier, the permeability of the fractured sample at 1 MPa varied similarly to that of the intact sample with changing injection and confining pressures. This is because, in this fractured siltstone at low confinement (1 MPa was considered here), the fracture exists only up to around 58 mm along the length of the sample and the remaining part is the rock matrix. Therefore, the injecting fluid flows through both the fracture and the rock matrix, and their two different porosity systems. As a result, both porosity systems should be considered when analysing the permeability values for this partially fractured rock sample [45]. However, since fracture permeability is always much greater than matrix permeability, due to the easy flow paths through fractures, the final permeability of the partially fractured rock specimen is always governed by the matrix permeability. As a result, the permeability variation with injection and confining pressures for the intact sample and the fractured sample at 1 MPa confining pressure are comparable, because in both samples the final permeability values are governed by the rock matrix. In respect of the effect of injection and confining pressures on the N\({}_{2}\) permeability of fractured siltstone under much high confinement (7 MPa), according to Figure 14a, the permeability also reduces with increasing injection pressure, but at a much greater rate compared to the intact and partially fractured siltstone. However, the rate of reduction seems to be more significant at low confining pressures.