The Role of Induced Un-propped (IU) Fractures in Unconventional Oil and Gas Wells

Abstract

The term induced un-propped (IU) fractures refers to fractures created around the main propped fracture which are too small to accommodate any proppant. These could include natural fractures, and micro-fractures induced along bedding planes or along other planes of weakness. Based on production data, diagnostic methods and field observations it is becoming increasingly clear that induced un-propped fractures created during the hydraulic operation play a critical role in determining the success of fracture treatments. In this paper five independent pieces of evidence are presented to prove the existence of induced un-propped fractures in most wells under downhole conditions and to demonstrate their importance in production. These include: micro-seismic data, production history matching, tracer data, pressure communication between wells and finally calculations on the fate of the injected fracturing fluids. Examples are provided to clearly demonstrate how this information indicates the presence of IU fractures. Once the existence of these fractures has been demonstrated simulations are conducted to show what role these induced un-propped fractures play during fracturing and during short-term and long-term production. It is shown that shale properties and the rheology and rate of injection of fracturing fluids plays an important role in the spatial extent and width of IU fractures. In some shales these fractures play a dominant role while in others they may be less important. Finally recommendations are made for fracture design to account for the presence of these IU fractures. These recommendations have a large impact on important decisions such as well spacing, fracture spacing, fluid rheology, proppant loading, proppant size and other fracture design considerations. In many instances these recommendations would have been quite different had the presence of IU fractures not been recognized.