**6.1 Limiting entry fracturing**

This technique limits the number of perforations and their diameter while injecting a large volume of fracturing fluid that causes increasing the bottom hole pressure on a large scale. Therefore, the fracturing fluid is forced to shunt into limited entries creating new fractures as shown in **Figure 6** [85, 86]. The main advantages of this technique are a relatively simple operation, short operation time, the fact that multi-fractures are created in a single operation which is environmentally favorable for reservoir protection. However, this technique has some limitations including high perforation back pressure, difficult to control any single fracture, and fractures which may not form in perforations of long interval horizontal well.

An example where limited entry fracturing technology was applied in horizontal well is Zhao 57-Ping 35 of Daqing Oil Field [84]. The well was divided into 4 sections each containing 19 perforations, and an isolating packer was set above the kickoff point. Using two simultaneous pumping facilities, a total fracturing fluid volume of 374.3m3 with an average sand ratio of 35.6% was injected at a rate of 7.5 m3 /min. The fracture initiation pressure was 30.5 MPa, four fractures were created, and the total fracture span was 400 m. The entire operation took 79 minutes. This treatment achieved success allowing the production after fracturing to increase 20–30 times and reach the production level of 4 vertical wells.

#### **6.2 Staged fracturing technique**

As limited entry fracturing cannot operate on all the target layers at one time, staged fracturing technique is used when the horizontal section is long and many layers are targeted for fracturing. Staged fracturing creates many fractures by utilizing packers and/or other segmenting materials. Operating a section by section at the time, one fracture is created in every section. The key points to achieve staged fracturing are tools and technique that fulfill the treatment requirements.

There are three types of staged fracturing techniques often used: the bridge plug fracturing, through coiled tubing fracturing with straddle packer and gel

**Figure 6.** *Technique of the limited entry fracturing of a horizontal well [84].*

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

**Figure 7.**

*Staged fracturing mechanism of horizontal wells [84].*

complex-slug fracturing as shown in **Figure 7**. Contrary to packer separation, the gel complex-slug fracturing avoids the risk of downhole tool stuck, but in the latter, the fracture initiation points are difficult to control.

An example where gel staged fracturing technology was applied in well Saiping-1 of Changqing Oil Field where four fractures were created. The process is briefly described as the following: perforating the end of horizontal well section, followed by first fracturing treatment, running a production test, and temporary plugging the first section by sand filling gel plug and, next, repeating the process in perforating the second, third, and fourth sections followed by a formation pressure and production tests.
