煤层水平井中随钻电磁波仪器影响因素分析及电阻率模拟计算

Influential factors of the electromagnetic wave instrument while drilling in coal seam horizontal wells and resistivity simulation calculation

为掌握煤层水平井中随钻电磁波仪器探测影响因素,通过有限元数值模拟研究顶底板围岩电阻率、仪器偏心、煤层井眼垮塌和煤层厚度等因素对电阻率测量值的影响,分析高阻煤岩地层条件下幅度比和相位差计算的电阻率响应规律。在此基础上,建立三层地质数学模型,模拟不同发射频率情况下随钻电磁波仪器钻进煤层时,幅度比和相位差电阻率计算解析解和数值解的差异,以及煤层相对介电常数对幅度比和相位差计算的影响。模拟结果表明:幅度比和相位差计算的电阻率解析解和数值解符合度很高,但当电阻率大于100Ω·m时,幅度比电阻率已经不能反映煤层的真实电阻率,所以在实际处理解释过程中用相位差电阻率要好些;高阻煤层不同发射频率情况下,电阻率数据主要对煤层电阻率敏感,对介电常数不敏感,只有在超高频时,介电常数才会对电磁波传播造成较大影响。

In order to figure out the influential factors of the detection by the electromagnetic wave instrument while drilling in coal seam horizontal wells,we studied the impacts of roof and floor surrounding rock resistivity,instrument eccentricity,coal seam borehole collapse and coal seam thickness on the measurement of resistivity through finite element numerical simulation.The resistivity response law of the amplitude ratio and phase difference calculation under the condition of high-resistivity coal rock formation was analyzed.On this basis,a three-layer geological mathematical model was established to simulate the difference between analytical and numerical solutions of the amplitude ratio and phase difference resistivity when the electromagnetic wave instrument while drilling was drilling in the coal seam at different emission frequencies,as well as the influence of formation relative dielectric constant on the amplitude ratio and phase difference calculation.The simulation results show that the analytical and numerical solutions of resistivity calculated by the amplitude ratio and phase difference are in good agreement,but when the resistivity is greater than 100Ω·m,the amplitude ratio resistivity could not reflect the real resistivity of the coal seam,so it is better to use phase difference resistivity in the actual processing and interpretation.At different emission frequencies of high-resistivity coal seams,the resistivity data are mainly sensitive to the formation resistivity and insensitive to the dielectric constant.Only at ultrahigh frequencies,the dielectric constant will have a great impact on the propagation of the electromagnetic wave.