## 4 Results and Discussion
### Effect of Confining and Injection Pressures on Intact and Fracture Permeability
For example, when the injection pressure is increased from 1 to 8 MPa, the N\({}_{2}\) permeability of the fractured sample reduces by around 74% for 15 MPa confining pressure, 64% for 20 MPa confining pressure, and 62% for 25 MPa confining pressure. This shows that the reduction of fully fractured siltstone permeability with increasing injection pressure reduces with increasing profining pressure. The reduction of permeability with increasing confining pressure observed in the fully fractured sample is related to the alteration of fracture aperture, as shown in Figure 15. According to Figure 15a, when the injection pressure is increased from 1 to 8 MPa, the fracture aperture increases by 49% for 15 MPa confining pressure, 64% for 20 MPa confining pressure and 65% for 25 MPa confining pressure. According to Equation (6), the permeability of fractured rock increases with increasing fracture aperture. This is counter to the observed permeability reduction with increasing injection pressure, because according Figure 15a, the fracture aperture increases with increasing injection pressure. This contradictory behaviour is due to another effective factor that negatively affects fracture permeability. This is believed to be related to the Klinkenberg slip flow effect, which causes permeability to be reduced with increasing gas pressure. It should be noted that the full fractures created have quite low fracture aperture values (<16 \(\upmu\)m), and therefore the fracture probably acts as micropores that are subject to the slip flow effect. According to Figure 15b, the fracture aperture considerably reduces with increasing confinement due to the effective stress effect, and this causes some permeability enhancement. This explains the reduction of permeability with increasing confinement. Similar behaviour for fractured permeability has been reported by many researchers for various other reservoir rocks, including [PERSON] [46], [PERSON] et al. [47], [PERSON] et al. [13] and [PERSON] [48]. According to [PERSON] [48], fluid flow through a fracture is a function of pore fluid pressure and confining pressure, and fracture permeability reduces with increasing effective pressure. Therefore, it is apparent that under higher confining conditions such as 20 or 25 MPa, the fracture aperture can be small as 6 \(\upmu\)m and this can greatly influence the permeability of the reservoir, regardless of the fractures generated. Therefore, it is necessary to use proppants (small particles such as ceramic or sand) in field conditions to prop open the fractures generated after releasing the fracturing fluid pressure. Otherwise, the confining stress will reduce the fracture aperture, resulting in reduced permeability and low gas production. It should be noted that the observed fracture aperture is based on 54 mm diametrical specimens and this could change with the sample size and under field conditions. For example, under field conditions, there is no specific boundary for the rock mass to limit the fracture and therefore fracture will propagate until it reaches the fracturing stress level in the particular location. This will lead to a greater fracture aperture near the wellbore and a significantly small fracture aperture at the tip (end) of the fracture, which could not be observed in laboratory conditions with small specimens. However, the observed trends such as reduction of fracture aperture with increasing confining pressure will still be valid under field conditions. Figure 13: Variation of permeability for fractured sample with 1 MPa confining pressure (**a**) with injection pressure and (**b**) with confining pressure. In the case of intact and fractured rock permeability, partially fractured rock permeability is about 10 times greater than intact rock permeability, and fully fractured rock permeability is about \(10^{6}\) times greater than intact rock permeability. This shows that fracturing can significantly improve the permeability of and hence increase gas production from deep geological formations.