Impact of Perforation on Hydraulic Fracture Initiation and Extension in Tight Natural Gas Reservoirs

In the exploitation of tight natural gas reservoirs, increasing the number of fractures to enlarge the stimulated reservoir volume is the key point. Furthermore, high breakdown pressure is the primary factor that causes the difficulty of hydraulic fracturing treatment in a tight gas reservoir. Both of these factors are related closely to the perforation strategy. However, the impact of perforation on hydraulic fracture initiation and extension in a tight gas reservoir has not yet been researched in an experimental way. We designed a piece of equipment that we refer to as a mini-sized true tri-axial hydraulic fracturing simulation system (8cmx8cmx10cm) for the experiments. By using this equipment, we could simulate hydraulic fracturing using downhole drilling cores, which were acquired from the Erdos basin tight gas reservoir. We mainly examined the influence of the horizontal stress difference and spacing, density, and depth of perforation on the fracture patterns. Experimental results show that, in specimens without perforation, the number of fractures is less than that of the perforation specimens, and the direction of fractures is uncontrollable. In specimens with perforations, fractures initiate at the perforation points, which is conducive to forming fractures uniformly. The perforation spacing and horizontal stress difference strongly influence the interaction among fractures. The interaction among fractures is mitigated as the perforation spacing or the horizontal stress difference increases. The depth of perforation affects the fracture initiation. Factures are easier to initiate and propagate in deeper perforations. The existence of perforation could effectively reduce the breakdown pressure by approximately 18%.