Hydraulic fracture propagation in naturally fractured reservoirs: Complex fracture or fracture networks

All reservoirs are fractured to some degree. Depending on the density, dimension, orientation and the cementation of natural fractures and the location where the hydraulic fracturing is executed, pre-existing natural fractures can impact hydraulic fracture propagation and the associated flow capacity. Understanding the interactions between hydraulic fracture and natural fractures is crucial in estimating fracture complexity, stimulated reservoir volume (SRV), drained reservoir volume (DRV) and completion efficiency. However, what hydraulic fracture looks like in the subsurface, especially in unconventional reservoirs, remain elusive, and many times, field observations contradict our common beliefs. In this study, a global cohesive zone model is presented to investigate hydraulic propagation in naturally fractured reservoirs, along with a comprehensive discussion on hydraulic fracture propagation behaviors in naturally fractured reservoirs. The results indicate that in naturally fractured reservoirs, hydraulic fracture can turn, kink, branch and coalesce, and the fracture propagation path is quite complex, but it does not necessarily mean that fracture networks can be created, even under low horizontal stress difference, because of strong stress shadow effect and flow-resistance dependent fluid distribution. Perhaps, 'complex fracture', rather than 'fracture networks', is the norm in most unconventional reservoirs.