The term induced unpropped (IU) fractures refers to fractures created around the main propped fracture that are too small to accommodate any proppant. These could include natural fractures and microfractures induced along bedding planes or along other planes of weakness. On the basis of production data, diagnostic methods, and field observations, it is becoming increasingly clear that IU fractures created during the hydraulic-fracturing operation play a critical role in determining the success of fracture treatments.
There has been a debate about whether certain shales are naturally fractured. The classic example of this is the Barnett Shale. Many authors have suggested that the Barnett Shale is highly naturally fractured, whereas others have argued that field observations in the Barnett can be explained on the basis of geologic lithofacies and heterogeneity. Observations made on cores clearly indicate the presence of fractures. Mineralization on the faces of these fractures indicates that they are not drilling-induced fractures but are instead native to the Barnett. Similar questions and discussions have arisen about the role of natural and induced fractures in other shale plays.
One reason that it is often difficult to resolve questions about natural fractures in shales is the extreme level of heterogeneity commonly observed in many types of shale. Differences in lithology and the highly laminated nature of many types of shale are apparent when viewing cores from source rocks such as the Eagle Ford Shale. In some instances, the boundary between the lithologically distinct layers is sharp and well-defined, but, in other instances, it is diffuse and displays its own characteristic lithologic gradation. Boundaries between lithologically distinct layers can often act as planes of weakness along which fractures can develop under stress....
The Role of Induced Unpropped Fractures in Unconventional Oil and Gas Wells
20 September 2016