Numerical simulation study of rock breaking mechanism by high voltage electric pulse
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摘要: 高压电脉冲钻井技术目前已成为一种新型高效的岩石破碎方法，同时也是目前钻井提速领域的研究热点。为了推动高压电脉冲的破岩机理研究，建立了单对电极破碎红砂岩的多物理场耦合电击穿二维数值模型，通过电流场、电击穿场和电路场的耦合再现均质红砂岩中等离子体通道的产生，分析了电极倾角、电压、电极间距对岩石电击穿(岩石内部等离子体通道的形成)的影响规律。研究表明：等离子体通道由靠近放电电极顶端局部区域开始萌生，并朝着岩石局部介电强度薄弱处发展；随着加载脉冲电压值的增大，电击穿发生时刻逐步降低，岩石模型的等效失效体积逐渐增大；在保证岩石能被电击穿的前提下，增大电极间距能提高高压电脉冲的破岩效率；放电电极的电极倾角逐步增大的过程中，岩石等效失效体积出现明显的波动，其极值多数出现在电极倾角35°~55°的范围之内。为了进一步推动高压电脉冲破岩的工业化应用，在二维模型基础上构建了红砂岩的多物理场耦合动态电击穿三维数值模型，再现了电极钻头破岩过程中岩石内部破碎坑的状貌；与此同时，选用自主设计的同轴型电脉冲钻头开展了电击穿室内实验，电击穿室内实验结果与仿真实验结果相互印证。Abstract: High-voltage electric pulse(HVEP)drilling has become a new and efficient rock breaking method, which is also the research focus in the field of drilling speed increase. To probe into rock breaking mechanism of high voltage electric pulse, this study establishes a two-dimensional numerical model of multi-physical field coupling electric breakdown of single pair of electrodes. The model reproduces the generation of plasma channels in homogeneous red sandstone from the coupling of current field, electric breakdown field and circuit field. This paper analyzes the effect of electrode pair angle, voltage and electrode spacing on rock electrical breakdown(that is, the formation of plasma channel in rock). The model includes the circuit structure parameters of pulse tool, the occurrence and development of electrical breakdown, and the relationship between electrical breakdown intensity and time. Results indicate that the plasma channel begins to sprout from the partial area near the top of the discharge electrode and develops towards the partial weak dielectric strength. With the voltage value of loading pulse increasing gradually, the time of electrical breakdown decreases gradually; comparatively, the equivalent failure volume of rock model increases gradually, and there is a positive correlation between them. On the precondition that the rock can be electrically broken, increasing the electrode spacing can improve the rock breaking efficiency of high voltage electric pulse. The equivalent failure volume of rock shows significant fluctuations during the gradual increase of electrode inclination angle of discharge electrode, and its extreme value mostly appears in the range of electrode inclination angle of 35°~ 55°. To further promote the industrial application of high-voltage electric pulse rock breaking, this paper proposes a three-dimensional numerical model of multi-physical field coupling dynamic electric breakdown of red sandstone based on two-dimensional model, reproducing the appearance of the fracture crater in the rock during the rock breaking process with electrode bit. At the same time, the self-designed coaxial electrode bit is selected for experiments of electric breakdown, and the laboratory experimental results of electric breakdown confirm the simulation experimental results.
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Key words:
- high voltage electric pulse /
- rock breaking /
- electrode bit /
- electric breakdown /
- plasma channel
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图 1 高压电脉冲破岩的两种基本方式
Figure 1. Two basic ways of rock breaking by high voltage electric pulse
图 2 高压电脉冲破岩过程的三个阶段
Figure 2. Three stages of high voltage electric pulse rock breaking
图 3 二维岩石电击穿数值模型
Figure 3. Two-dimensional numerical model of rock electrical breakdown
图 5 加载不同脉冲电压岩石模型电击穿时刻与等效失效体积变化
Figure 5. Variation of electrical breakdown time and equivalent failure volume of rock model loaded with different pulse voltages
图 6 不同脉冲电压、电极间距下，电极的岩石电击穿时刻和岩石等效失效体积变化
Figure 6. Rock breakdown time and equivalent failure volume of electrodes with different electrode spacing under various pulse voltages
图 7 不同脉冲电压、电极倾角下，电极的岩石电击穿时刻和岩石等效失效体积变化
Figure 7. Rock breakdown time and equivalent failure volume of electrodes with different electrode inclination angles under various pulse voltages
图 8 电极钻头三维动态电击穿模型的示意图
Figure 8. Three-dimensional dynamic electrical breakdown model of electrode drill
图 9 三维电极钻头动态电击穿砂岩破碎坑状貌
Figure 9. Dynamic electric breakdown of sandstone fracture crater by three-dimensional electrode bit
图 10 高压电脉冲室内电击穿实验平台
Figure 10. Experimental platform for electrical breakdown of high-voltage electrical pulse
图 11 电击穿实验后砂岩试样破碎坑状貌
Figure 11. Fracture crater appearance of sandstone sample after electric breakdown experiment
表 1 电极、水、岩石和绝缘套筒材料属性
Table 1. Material properties of electrode, water and insulating sleeve
材料 相对介电常数 电导率/(S·m-1) 比热容/[J·(kg·K)-1] 密度/(kg·m-3) 导热系数/[W·(m·K)-1] 水 80 0.001 25 4 180 1 000 0.59 电极 1 5.7×107 385 8 960 400 绝缘材料 4 0 1 700 1 150 0.26 岩石 5 4×10-5 750 2 600 3.14 
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表 2 仿真实验各正交参数和变量的详细信息
Table 2. Specifics of orthogonal parameters and variables in simulation experiment
变量种类 具体参数 电极倾角θ/(°) 0 10 20 30 40 50 60 70 80 90 电压U0/kV 100 110 120 130 140 150 电极间距Ld/mm 10 12.5 15 17.5 20
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表 3 砂岩岩样的材料参数
Table 3. Material parameters of sandstone samples
参数项 砂岩 参数值 单位 杨氏模量 35 GPa 泊松比 0.2 — 相对介电常数 5 — 电导率 4×10-5 S/m 密度 2 500 kg/m3 孔隙率 12% — 空间体积能密度阈值 5.5 kJ/m3
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