Experimental study of hydraulic fracturing initiation and propagation from perforated wellbore in oil shale formation

Perforation hydraulic fracturing is essential in developing unconventional oil and gas resources. This study scales the critical parameters to simulate operations at the field scale, using a true triaxial hydraulic fracturing experimental system to mimic actual far-field stress conditions. The research simulates various scenarios of perforation and fracturing fluid parameters, which significantly influence fracture propagation behaviour and fracture network morphology. The results showed that the initiation pressure required for fracturing with perforation is 20.91% lower than that of open-hole fracturing. Multiple fractures, including bedding and branch fractures, exhibit cross-propagation behaviour, forming barrier-type morphology. Induced by perforation, hydraulic fracture initiation and propagation occur along the direction of maximum geo-stress, but unreasonable orientation and reduction in perforation length can also increase initiation pressure. The flow rate and viscosity of fracturing fluid are positively proportional to the initiation pressure. A high flow rate of 100 mL/min facilitates the generation of bedding fractures, enhances the bedding fractures connectivity and increases fracture network complexity. However, the increase in viscosity forces hydraulic fracture to penetrate the bedding plane directly and diminishes the complexity of the fracture network. Compared to the pressure build-up phase, fluctuations in the fracturing curve during the propagation phase are significantly related to the complexity of the fracture network. Small-scale structural surfaces developed within the strata can become part of the hydraulic fractures and increase the fracture network complexity. Additionally, the hydraulic fractures are captured and hindered by large-scale structural surfaces, forming a single fracture morphology. The findings significantly aid in predicting fracture propagation and fracture network morphology in oil shale exploitation.