保护电极(又称三电极系侧向测井)系统采用与井眼相垂直的电流薄盘测量地层电阻率。利用位于测量电极上下较长的等电位电极,以强制方式达到对此电流盘的控制。用此系统取得的测井曲线没有“滞后”、“平绥”、“屏蔽”、“反射”以及其他普通测井曲线的失真迹象。当泥浆和泥浆滤液电阻率较小时,井眼尺寸和侵入带电阻率对视电阻率读数的影响也相应减少。因此,保护电极系统最适用于导电泥浆。附录给出了该原理的数学推导。导出了对泥浆电阻率、井眼尺寸、侵入带电阻率和侵入深度以及地层真电阻率的影响作用进行评价的方程式。复制了几个地区的试验测井记录并与普通电测井作了比较,说明保护电极系统能表示出更为详细的岩性情况。 Protective electrode (also known as three-electrode lateral logging) system using well-current perpendicular to the thin plate to measure the formation resistivity. With the electrode located above and below the measuring electrode, the control of the current plate is achieved in a forced manner. Logs acquired with this system do not show signs of distortion such as “hysteresis,” “flat-snapped,” “shielded,” “reflected,” and other common well logs. When mud and mud filtrate resistivities are small, the effect of borehole size and intrusion zone resistivity on apparent resistivity readings is also reduced. Therefore, the protective electrode system is most suitable for conductive mud. The appendix gives a mathematical derivation of this principle. The equation for evaluating the effect of mud resistivity, borehole size, intrusion zone resistivity and intrusion depth, and true resistivity of the formation is derived. Experimental well logs were duplicated in several areas and compared with normal electrical logs showing that the guard electrode system can show more detailed lithology.