Measurement method of borehole wall resistivity for coal mine gas extraction
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摘要: 针对煤矿瓦斯抽采用水平定向千米钻机钻孔孔壁状态检测问题，通过分析认为常用的伽马射线法、超声波法等测井技术难以应用于煤矿环境，因此设计了七电极径向电阻率测量方法，以测量瓦斯抽采钻孔孔壁电阻率，实现钻孔孔壁状态检测。介绍了七电极径向电阻率测量方法原理，推导了电阻率计算公式，分析了屏蔽电极间距、测量电极中心距、电极分布比3个参数对电阻率测量的影响。建立了三维仿真模型和二维轴对称仿真模型，研究了不同发射信号类型和电极分布参数下，测量电极周围电流的聚焦情况和电势分布，结果表明：直流和脉冲信号的穿透能力较弱，不能穿透孔壁，无法实现孔壁电阻率测量，而交流信号具有较好的穿透能力，可用于孔壁电阻率测量；电极分布比对电流聚焦效果有显著影响，当电极分布比为2.5~3时，发射电流聚焦效果较好，能取得较好的电阻率测量效果。根据上述结果确定了发射信号类型和电极分布参数，仿真分析了以空气、泥浆、岩层、煤层为介质时发射电流聚焦情况和电势分布，结果表明七电极电阻率测量方法对电极周围介质性质具有较强的分辨能力。根据仿真结果制作了实验电极，将电极安装在1节钻杆上，分别测量土壤、空气和煤碎粒的电阻率，所得测量值均在标准参考值范围内，验证了七电极径向电阻率测量方法能够实现钻孔孔壁电阻率测量，为煤矿瓦斯抽采用水平定向千米钻机钻孔孔壁状态检测提供了有效方法。Abstract: For borehole wall state detection of horizontal directional kilometer drilling rig used for coal mine gas extraction, it is considered through analysis that the commonly used logging technologies such as gamma ray method and ultrasonic method are difficult to be applied to coal mine environment. Therefore, a seven-electrode radial resistivity measurement method is designed to measure the resistivity of borehole wall of gas extraction borehole and realize the detection of borehole wall state. The principle of seven-electrode radial resistivity measurement method is introduced. The formula of resistivity calculation is deduced. And the effects of three parameters, namely, the distance between shielding electrodes, the center distance between measuring electrodes and electrode distribution ratio, on resistivity measurement are analyzed. A three-dimensional simulation model and a two-dimensional axisymmetric simulation model are established. The models are used to study the current focusing and potential distribution around the measuring electrode under different transmit signal types and electrode distribution parameters. The results show that the penetration capability of DC and pulse signals is weak. DC and pulse signals cannot penetrate the borehole wall and cannot realize the measurement of borehole wall resistivity. The AC signal has better penetration capability and can be used for the measurement of borehole wall resistivity. The electrode distribution ratio has a significant effect on the current focusing effect. When the electrode distribution ratio is 2.5-3, the focusing effect of the emitter current is better, and a better resistivity measurement effect can be obtained. According to the above results, the transmit signal types and electrode distribution parameters are determined. The focus of emission current and potential distribution are simulated and analyzed when air, mud, rock stratum and coal seam are used as media. The results show that the seven-electrode resistivity measurement method has strong capability to distinguish the properties of the media around the electrodes. According to the simulation results, the experimental electrode is made and installed on a drill pipe to measure the resistivity of soil, air and coal particles respectively. The measured values are all within the standard reference value range. It is verified that the seven-electrode radial resistivity measurement method can realize the borehole wall resistivity measurement. This study provides an effective method for the borehole wall state detection of horizontal directional kilometer drilling rig for coal mine gas extraction.
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图 1 七电极径向电阻率测量电极结构
Figure 1. Electrode structure of seven-electrodes lateral resistivity measurement
图 3 七电极径向电阻率测量仿真模型
Figure 3. Simulation models of seven-electrodes lateral resistivity measurement
图 4 不同发射信号下钻杆周围电势分布
Figure 4. Electric potential distribution around drill pipe under different excitation signals
图 6 不同分布比时发射电流聚焦效果与电势分布
Figure 6. Excitation current gathering effect and electric potential distribution under different distribution ratio
图 9 不同介质中电极表面轴向截线电势分布
Figure 9. Electric potential distribution on electrode surface in axial serif in different substances
表 1 七电极径向电阻率测量仿真模型中电极分布参数
Table 1. Electrode distribution parameters in simulation models of seven-electrodes lateral resistivity measurement
S 2.37 2.60 3.00 3.36 L0/cm 32 37 42 47 L1/cm 14 14 14 14 
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表 2 七电极径向电阻率测量仿真模型中被测介质参数
Table 2. Parameters of measured substance in simulation models of seven-electrodes lateral resistivity measurement
参数 空气 泥浆 煤岩 岩层 电阻率/(Ω·m) 设置值
正常值3×106
104~1063.3
3~3.5300
30~103105
10~103相对介电常数 1 16 4.5 4
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表 3 电阻率实际测量结果
Table 3. measured results of resistivity
参数 土壤 空气 煤碎粒 I/mA 4.87×10−3 2.21×10−3 3.75×10−3 Umean/V 1.716 0.274 0.484 ρ/(Ω·m) 8.91×104 3.132×104 326.15 ρ0/(Ω·m) 4×104~105 104~106 30~103
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