In this work, a special device that can simulate plane-strain condition is designed to observe the fracture initiation and propagation directly in a rock plate specimen. The confining pressures are used to simulate the influences of in-situ stresses, and the pre-cracks are for perforations. An opening device can switch hydraulic pump pressure on the boundary of the center hole, which produces a tensile stress on the pre-cracks and initiates their propagation. It needs to note that the influences of fluid are not considered in the experiments, due to the limit of difficulties of sealing issues. Several experiments were conducted using the device under different confining pressures and perforation angles. The experimental results showed that when the confining pressures and the perforation angles were small, the fractures initiated from the pre-cracks and propagated along the maximum confining stress. As the confining pressures and perforation angles increased, the fractures initiated from center hole of rock specimen rather than pre-cracks. Based on the maximum tensile stress criterion, the numerical analyses matched the experimental results well.
Hydraulic fracturing treatments have become an indispensable part of stimulating in shale gas and other unconventional reservoirs. However, they can be greatly influenced by the selected perforating parameters. Especially, the initiation and propagation of fracture in the near wellbore region will influence the development of fractures, and finally the treatments of hydraulic fracturing [1, 2]. Thus, the identification of fracture path during the hydraulic fracturing will provide the fundamental information that can help to improve the hydraulic fracturing treatments.
Many researchers have studied the influence factors on the fractures evolution near wellbore, and the results showed that perforating parameters, cement properties and confining pressure are the key factors [3-6]. For instance, the initiation pressure and hydraulic fracture reorientation path are controlled by the perforation orientation . When the confining stress and perforating angle increased, the initiation from perforations became difficult and finally initiated from weak cemented position.