# Experimental Study on High-Energy Gas Fracturing Artificial Coal
## 4 Test Results and Analysis  
### Borehole Wall Stress Test Results and Analysis  
Three simulated coal samples were selected for high-energy gas fracturing tests. In this paper, three specimens with a total of six borehole wall stress measurement points, of which four valid pieces of data were obtained. Two of the data sets were invalid due to destructed PVDF pressure sensors, sticking, etc. To minimize the effect of the PVDF pressure sensor on the borehole wall stress test data, the average of the valid data at the measurement points was used as the final test results, as shown in Table 5. To facilitate the analysis, the distinctive parts of the curve were intercepted for analysis, and some of the test results are shown in Figure 4.  
The pressure-time curves of the borehole middle and bottom in the role of high-energy gas are given in Figure 4. It can be seen from the Figure that the pressure-time wave of the borehole middle and bottom in the role of high-energy gas have the same trend. They show a rapidly rising pressure-rising phase, a slowly rising pressure-stabilizing phase, and a non-linearly declining pressure-releasing phase. After the fracturing agent is excited, a large amount of high-temperature and high-pressure gas is generated, which rapidly expands and impacts the borehole wall, causing the pressure of the borehole wall to rise rapidly. At the same time, stress waves inside the coal are induced, and initial cracks appear on the borehole wall under the action of the stress waves. As the borehole wall starts to crack, the high-energy gas wedges into the cracks, causing it to develop steadily. The amount of gas produced by the fracturing agent is greater than the amount of gas entering the coal, so the pressure at the borehole wall continues to rise, but the rate of rise slows. With the development of fracture until penetration, the coal ruptures and gas is rapidly released, and the pressure at the borehole wall drops rapidly to below 1 MPa. The pressure-time curve has obvious fluctuations in both the rising and falling sections, which are related to the PVDF pressure sensor's characteristics, the non-uniform flow of  
Figure 4: Pressure–time curve of the borehole wall: (a) Borehole middle, (b)Borehole bottom. Figure 3: Strain test(**a**) Strain brick, (b) Burial process, (e) Diagram of strain brick arrangement. gas in the borehole, and the unstable gas production rate of the fracturing agent due to pressure variations in the borehole. The statistical results of the borehole wall pressure tests are given in Table 6. From the table, it can be seen that the pressure rise time of the borehole wall in the role of high-energy gas is a dozen ms, and the maximum pressure is several tens to hundreds MPa. The borehole wall pressure under the action of explosive blasting can reach thousands or even tens of thousands MPa in a few microseconds [28]; hydraulic fracturing is a static load, and the peak pressure is generally below 100 MPa [29]. This means that the peak pressure and loading rate of the borehole wall in the role of high-energy gas are less than that of explosive blasting and greater than that of hydraulic fracturing. Explosive blasting has a strong load dynamic impact, hydraulic fracturing produces static loads, while the load produced by high-energy gas has both the dynamic action of stress wave and quasi-static action of gas, and the quasi-static effect of the gas is dominant. In addition, the pressure distribution in the borehole is uneven during the high-energy gas fracturing process, and the peak pressure decreases from top to bottom. Two reasons can explain this phenomenon: (1) The slow response of the fracturing agent and the fracturing agent excitation point is in the upper part, so the high-energy gas acts on the upper part of the borehole wall first, which makes the upper part of the borehole crack first. After the cracks appear in the upper part of the borehole wall, the gas enters the coal continuously.