**3.1 Creation of the multi-fracture network**

The classical hydraulic fracturing theory indicates that the main formed fracture is a symmetric bi-wing plane extending parallel to the direction of maximum principal stress. However, field hydraulic fracturing treatment is completely different as complex fracture networks take place where the main fracture and other smaller branch fractures simultaneously extend in the fracture propagation zone [9–11].

Microseismic mapping shows that hydraulic fracturing in shale forms a multifracture network system [12–15] which consists of complex fractures as shown in **Figure 2** [16]. It was concluded from the mapping that natural fractures' direction was to the northwest and the propagation of the induced hydraulic fractures

direction was to the northeast where they intersected with natural fractures. This led to many crosscutting linear features and formed a complex fracture. Based on fracture extension characteristic in shale reservoirs, hydraulic fractures are classified into four major types [16]: single plane bi-wing fracture, complex multiple

#### **Figure 2.**

*Multi-fracture network extension in shale reservoirs during hydraulic fracturing (after Warpinski et al. 2008 [16]).*

**Figure 3.** *The hydraulic fracture classification complexity (after Warpinski et al. 2008 [16]).*

*A Review of Fracturing Technologies Utilized in Shale Gas Resources DOI: http://dx.doi.org/10.5772/intechopen.92366*

fracture, complex multiple fracture with open natural fractures, and complex fracture network as shown in **Figure 3**.

Confirming field observation from seismic mapping, simulation experiments [17–22] show that induced hydraulic fracture presents three types of extensions when intersecting with natural fractures: crossing the natural fractures, extending along the natural fractures or crossing, and extending along at the same time. It was concluded that fracture network would highly form during fracturing process of naturally fractured formations [23]. Moreover, several laboratory experiments confirmed that fracture network exists [24, 25] and found that the fracture network would easily form under low fluid viscosity injection [26, 27]. Other observations proposed that multi-fracture networks in shale reservoirs area are key to increase stimulated reservoir volume (SRV) where treatment success relies on whether hydraulic fracture could extend to form multi-fracture network [28–30].
