# Experimental Study on High-Energy Gas Fracturing Artificial Coal
## 4 Test Results and Analysis
### Test Block Destruction Characterization  
Figure 7 shows the destruction pattern of the test block under the action of high-energy gas. It can be seen from the Figure that the cracks of the test block under the action of the high-energy gas show a random distribution; all the cracks are penetrated, without secondary cracks, and the number of microcracks around the borehole is small. Both main cracks and microcracks are extended outward approximately along the radial direction of the borehole. The fracture surface of the fragments is mostly irregular and rough, and some of the fragments are spalling off, which is consistent with the characteristics of tensile destruction. The destruction characteristics of the borehole wall show that the surface of the borehole wall is intact, which means that there is no crush zone in the test block under the action of high-energy gas. The fragments produced by the action of high-energy gas were mainly large pieces, and no fragment-popping phenomenon occurred during the test. Figure 8 shows the SEM scanning results of the specimens before and after the high-energy gas fracturing test. It can be seen from Figure 8a that the pore surface of the specimen is relatively flat, and the closed and semi-closed pores were well developed before the high-energy gas fracturing test. After the high-energy gas fracturing test, the closed and semi-closed pores in the specimen were penetrated, and obvious microcracks appeared. With the development of microcracks, more secondary microcracks appeared. It can be concluded that the test block is not broken to a high degree under the action of high-energy gas, but it has good fracturing and improving permeability effect. The main reasons are as follows:  
1. High-energy gas fracturing of coal rock is divided into two processes, namely, the stress wave dynamic loading stage and the gas quasi-static loading stage. Firstly, the stress wave causes a dislocation pile-up in the internal medium of the specimen, which promotes the penetration of closed and semi-closed pores and the formation of  
Figure 6: Stress wave attenuation law under high-energy gas action. Figure 7: (**a**) Destruction pattern of the upper surface of test block. ; (**b**) Destruction pattern of the borehole. ; (**c**) Destruction pattern of the side of test block. the initial macro cracks. Then comes the quasi-static loading phase of the high-energy gas, which has a less dynamic shock effect, and its main effect is to wedge into the initial fracture caused by the stress wave to steadily drive the fracture expansion, thus can forming a large fracture zone. 2. The stress wave loading rate has an essential effect on the fracture toughness of the material. The fracture toughness of most materials under conditions of shock loading is lower than the static fracture toughness. In the static loading range, the fracture toughness remains relatively stable; in the medium loading range, the fracture toughness decreases with the increasing speed of the applied load. In explosive blasting and high-energy gas fracturing, the loading rate is generally medium loading or impact loading. From Section 3.2, it is known that the strain rate of coal in the role of high-energy gas is low, and the low strain rate means that the stress wave is loaded slowly, which means that the fracture toughness of coal during high-energy gas fracturing is greater than that of explosive blasting, so the test block is not broken to a high degree in the role of high-energy gas. 3. The smaller the stress wave loading rate, the fewer the number of cracks near the borehole. Explosive blasting produces stress waves that load at a much greater rate than high-energy gases, so more work can be undertaken in the same amount of time with explosive blasting. This can result in greater synthetic displacement of the media at the opening of the fracture zone and the formation of more cracks near the borehole. In addition, these cracks may divide to expend the energy of the stress wave.