瞬变电磁测井的过套管地层电导率探测

Measuring formation conductivity by transient electromagnetic logging through casing

剩余油探测需要套管井条件下的地层电导率,而利用瞬变电磁激发,在套管井测量的响应波形中包含了地层电导率信息。因此,需要研究该测井波形以及电导率信息在波形中的分布形式和地层电导率曲线的计算方法。首先研究了套管节箍处的响应,并借助该响应获得了套管井的几何因子。采用几何因子分析了地层电导率曲线的计算方法:将两个相邻深度测量的波形相减去掉波形中幅度很大的无用信号和井眼影响;再从其中任取一个时刻的值得到一条实测曲线,其特征与普通电导率曲线差异很大,其几何因子是两个相同的套管井几何因子错开一个深度间隔之后相减得到的差分几何因子。将套管井的差分几何因子与实测曲线进行反褶积可得到地层的电导率曲线。实际测井数据的处理结果表明,处理后的电导率曲线与裸眼井的电导率测井曲线具有一致性,表明瞬变电磁测井响应可以用几何因子描述,且能进行有用信号和无用信号的分离。最终利用有用信号实现套管井地层电导率测量。源距越大,得到的套管井条件下的曲线和裸眼井的曲线越一致,而目前最大源距的套管井曲线与裸眼井曲线依然存在差异,说明目前的仪器探测深度浅,下一步的研究应该增加仪器的源距。

To quantify the remaining oil,the conductivity of formation has to be measured through the metal casing of the production wells.Using transient electromagnetic excitation,the response waveform measured through casing is informative of the conductivity of the formation.Therefore,it is essential to study the logging waveform,the distribution of conductivity information in this waveform,and the method for calculating the formation conductivity curve.The response of a casing collar was investigated to determine the geometric factor of the well casing.A method for computing the formation conductivity curve from the geometric factor was proposed.Firstly,the uninformative signal with large-amplitude and the wellbore effect was removed by subtracting two responses measured at adjacent depths in the well.A measured curve can be obtained by taking the value of the signal of at any time;the measured curves are quite different from the ordinary conductivity curve,because the geometric factor associated with the curve is a differential geometric factor,which is obtained by subtracting geometric factors from two identical casing wells after staggering of a depth interval.Finally,the formation conductivity logging curve was obtained by deconvolution of the cased-hole differential geometric factor and the measured curve.By processing results of actual logging data,it was showed that this curve was consistent with the open-hole well logging curve,thereby verifying that the response of transient electromagnetic logging can be described by geometric factors,and informative and uninformative signals can be separated.The formation conductivity curve in cased-hole condition can be obtained from the useful signals.With the spacing between the source and the receiver increasing,the radial investigation depth increases,and the conductivity curve approaches that evaluated in open-hole condition.This demonstrates that this method can detect shallow depth and needs further improvement.