## 3 Hydraulic Fracturing Experiment
### Experimental Setup  
In order to meet the needs of physical simulation experiments for fracturing shale cylindrical specimens, a physical simulation test system for fracturing shale cylindrical specimens was constructed in this study using a conventional triaxial rock mechanics experimental machine, and the overall structure is shown in Figure 20. The test system is mainly composed of a rock stress servo loading system, fracturing fluid injection system, and acoustic emission detection system. Among them, the rock stress servo loading system is mainly used to provide the stress state of the deep shale and simulate the real stress environment of the formation. The fracturing fluid injection system is mainly used to inject fracturing fluid into the specimen simulation wellbore with constant pressure/flow rate and realize the real\(-\)time acquisition of pump pressure and discharge volume. The acoustic emission detection system is mainly used to detect the microfracture signal during the shale hydraulic fracturing process and obtain the real\(-\)time hydraulic fracture initiation and spatial distribution characteristics. Figure 19: Crack pattern at the end of injection when the laminar tensile strength is 0.5 MPa. Figure 20: Shale cylindrical specimen fracturing physical simulation test system. (1) Rock stress servo loading system  
The rock stress servo loading system is mainly composed of a fully digital electrohydraulic servo rock triaxial test system produced by Beijing Zhongchuang Test Instrument Technology Co. The system mainly includes: a rock core high-pressure triaxial chamber, servo hydraulic source, servo booster, portal rigid frame, radial/axial strain gauge, heating system, and digital acquisition system, and so on. The system has a maximum output axial force of 2000 kN and a maximum working circumferential pressure of 140 MPa, which can better meet the requirements of stress size for deep shale fracturing; meanwhile, it is equipped with a specimen heating and constant temperature system with an upper limit temperature of 100 \({}^{\circ}\)C. The maximum allowable specimen size is \(\Phi\)100 mm \(\times\) 200 mm, and it is equipped with an axial/radial strain gauge with 0.0001 mm resolution and 0.3% measurement accuracy. (2) Fracturing fluid pumping system  
The fracturing fluid pumping system is mainly used to provide a constant flow of high-pressure fracturing fluid. In order to provide a more accurate flow control than most reciprocating pumps, this test system uses a Teledyne Isco 260 HP high-precision high-pressure plunger pump as the power source of fracturing fluid (Figure 21), leakage of 0.50 \(\mu\)L/min, maximum output pressure of 65.5 MPa, measurement accuracy of 0.1%, and equipment operating temperature of 5-40 \({}^{\circ}\)C.  
(3) Acoustic emission detection system  
The acoustic emission detection system is mainly composed of five modules: host, sensor, preamplifier, acquisition card, and AEWin signal acquisition and analysis software, as shown in Figure 22. The selection of an acoustic emission sensor has an important impact on the detection results. The acoustic emission probe selected in this test is the SR40M-type low-frequency narrow-bandwidth probe produced by Beijing Shenghua Xingye Technology Co. (Beijing, China). Because the acoustic emission sensor has high output impedance and weak output current, it is not suitable for long-distance transmission, so it needs to connect a preamplifier to amplify the voltage signal of the sensor several times before outputting to the acquisition card. The acoustic emission preamplifier used in this test is a 2/4/6-type amplifier manufactured by Physical Acoustics Corporation (PAC), with an adjustable gain of 20, 40, and 60 dB.