Measurement method of borehole wall resistivity for coal mine gas extraction

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.