## 1 Introduction
They also noted a clear reduction in breakdown pressure when a notch exists at the end of the wellbore, due to stress concentration [7]. Similarly, [PERSON] et al. [15] conducted experiments using 100 and 150 mm mortar samples (cubes) and found that the fracture initiation and breakdown pressure is depend on the orientation of the perforations in perforated wellbore. According to [PERSON] and [PERSON] [20], based on the model initially developed by the CSIRO, fracture initiation and breakdown pressures are not necessarily the same, and breakdown pressure can be equal to or greater than the initiation pressure. However, these two pressures can be clearly distinguished using a pressure vs. time plot, in which after the initiation pressure the plot deviates from the linear trend to a non-linear trend until the breakdown pressure is reached [20]. According to [PERSON] et al. [21], breakdown pressure and fracture morphology are dependent on the fluid type under identical stress state conditions. In relation to fracture morphology, some fracturing fluids, such as CO\({}_{2}\), SF\({}_{6}\) and water, consistently create simple fractures and others, as N\({}_{2}\) and air, consistently create fractures in a complex helical network [21]. According to the study of [PERSON] et al. [13] on the fracture aperture and permeability of artificially fractured granite samples, permeability and the fracture aperture start to reduce with time when the applied effective stresses are around 25 and 35 MPa, respectively. According to [PERSON] et al. [22], for low temperature (20 \({}^{\circ}\)C) isothermal flow, fractures tend to close monotonically due to the removal of bridging asperities, and around 60% reduction in fracture aperture after two months was observed in their tests. Importantly, some studies were carried out to investigate the accurate methods to perform the hydraulic fracturing tests and [PERSON] et al. (2017) documented the steps to conduct a valid hydraulic fracturing test in laboratory conditions. In most of the existing experimental work, fracturing has been performed using artificial materials such as concrete (or cement mortar), glass and Perspex, and few studies have considered the fracturing of real rock samples. Although in some cases the material properties are the same, other materials may not behave sufficiently like real rocks, which could affect the experimental results. Moreover, to investigate fracture permeability, artificially fractured samples (using the Brazilian tensile method or water-jet cutting, etc.) have been frequently used. However, the fracture surfaces and fracture apertures generated in artificially fractured rocks may vary greatly from those in hydraulically fractured rocks. In fact, in artificially fractured rocks, some of the rock particles may disintegrate from the rock surface, which leads to significant fracture apertures. A knowledge gap therefore exists in terms of the hydraulic fracturing process in real rock samples and also of the fracture permeability of hydraulically fractured rock samples. The aim of this study is to contribute to filling this gap by investigating the influence of confining pressure on the hydraulic fracturing process and the influence of both confining pressure and injection pressure on fracture permeability in hydraulically fractured rocks. Here, intact siltstone samples were used to conduct the fracturing and permeability experiments.