## 5 Conclusions  
Hydraulic fracturing is a promising technique for the extraction of unconventional natural gases, including shale gas and tight gas, from deep geological formations. However, gas production is entirely dependent on the reservoir's permeability, which is governed by the fracture network and fracture pattern created by the hydraulic fracturing process. Precise understanding of this fracture  
Figure 15: Variation of fracture aperture for hydraulically fractured rock specimen with 7 MPa confining pressure (**a**) with injection pressure and (**b**) with confining pressure. network creation is therefore vital for an effective hydraulic fracturing process. This understanding can only be gained based on thorough knowledge of intact and fracture permeability in reservoir rock. Therefore, the aim of this study was to understand the effect of reservoir depth (confining pressure) on the breakdown pressure of reservoir rock while understanding the influences of fluid injection pressure and reservoir depth (confining pressure) on the intact and fracture permeability of the reservoir rock mass. The results enable the following major conclusions to be drawn:  
* The reservoir flow ability for gas movement may reduce with increasing injection pressure and reservoir depth due to the Klinkenberg phenomenon and pore structure shrinkage, respectively. This confirms that, under field conditions, an additional pressure gradient may not directly enhance gas production while depth is a key factor. * During the hydraulic fracturing process, three main pressure development regions can be identified over time; (1) initial pressure development; (2) wellbore pressurization stage; and (3) failure stage. This will be a guide to the identification of final failure in advance, which could help with other investigations such as the monitoring of ground responses in the investigation of micro earthquakes. * The breakdown pressure of the reservoir rock linearly increases with increasing reservoir depth (confining pressure). For example, increasing the confining pressure applied on the tested siltstone from 1 to 7 MPa caused the rock mass breakdown pressure to be increased from 2.5 to 18.2 MPa, a large increment compared to the confinement increment. The derived relationship can be used to predict the breakdown pressures in siltstone reservoir rocks at certain depths. * Interestingly, hydraulic fracturing seems to occur more often along radial directions in shallow reservoirs and diametrical directions in deep reservoirs. For example, in the hydraulic fracturing experiments, the fracture created was mostly diametrical at greater confinement (7 MPa) and radial at lower confinement pressure (1 MPa), and only a partial fracture could be seen (the fracture length was only up to around 58 mm) under low confinements. This is due to the release of a greater amount of energy at greater confining pressures, resulting in greater breakdown pressures. This shows the effectiveness of deep reservoirs for hydraulic fracturing compared to shallow reservoirs. * This full fracturing of the reservoir rock at greater depth offers greater flow characteristics to the reservoir rock, enhancing productivity. For example, in the fracturing tests, the fracture permeability of the partially and fully fractured rock samples compared to an intact rock sample was about 10 times and \(10^{6}\) times, respectively. However, an increase of confining pressure from 5 to 25 MPa at 1 MPa injection pressure caused a permeability reduction of about 24, 56 and 81% for intact, partially fractured and fully fractured samples, respectively. This shows the greater sensitivity of greatly fractured reservoir rock to a change in possible overburden stress caused by many possible activities, including building construction and extraction. However, the possible long-term influence on gas production from fractured reservoirs can be minimised with the use of applications such as proppants in hydraulic fracturing.