Reactivation mechanism of natural fractures by hydraulic fracturing in naturally fractured shale reservoirs

• A non-planar propagation model of hydraulic fracture is introduced.
• A dynamic model to calculate the stress intensity factor of natural fracture-tip is proposed.
• Reactivation mechanism of a natural fracture after crossing by a hydraulic fracture.
• A hydraulic fracturing test on shale outcrop with variable injection rates is demonstrated.

Hydraulic fracturing technique has been commonly used in the development of shale reservoir aiming to produce from complex network of fractures. One of the key parameters that affects the hydraulic fracture complexity is the natural fracture in the shale reservoir. Hydraulic fractures being trapped by natural fractures propagate non-planar fracture in three dimensions. Therefore, conventional hydraulic fracture model cannot include the non-planar characteristics of these fractures. In case a hydraulic fracture crosses a natural fracture, the dependence of the natural fracture reactivation on injection rate remains poorly understood. A non-planar propagation model for hydraulic fracturing in naturally fractured shale reservoirs was developed based on PKN model. A dynamic model was derived to calculate the stress intensity factor for a natural fracture being crossed by a hydraulic fracture based on the theory of rock fracture mechanics. Based on the experimental data from Longmaxi shale reservoir, the fluid pressure threshold to activate the natural fractures was obtained for different approaching angles. The effects of fluid pressure on the propagation angle of natural fracture were discussed. According to different initial injection rates and approaching angles, the time and minimum injection rate were determined to adjust the pump, which can improve the complexity of hydraulic fracture. True tri-axial hydraulic fracturing test with rapidly variable injection rate was conducted on the shale outcrop from Longmaxi formation. Experimental results proved that increasing the injection rate at the right time can reactivate the natural fractures, which can induce a network of fractures in shale reservoirs.