井中激发、接收的瞬变电磁深层精细勘探方法

在钻孔底部使用线圈或电极激发瞬变电磁场,采用阵列接收线圈或电极的响应波形对深部地层的电导率分布进行精细探测。瞬变电磁场满足的微分方程是对空间坐标的二阶导数,有两个复特征根,其实部描述衰减,虚部刻画相移,瞬变电磁能量在地层中既衰减又相移。相移描述瞬变电磁能量在空间的扩散及在电导率界面的反射和透射,反射和透射场均携带地层电导率差异信息及界面信息。由一维模型测试结果可知:不同于地震勘探的反射波与入射波在时间域分开,瞬变电磁的反射场存在于整个响应波形中。相移引起时间延迟,使不同源距的接收波形起始点、上升沿和峰值出现的时间差异明显,响应波形差异大。地层中各个位置的响应波形均有变化最快的时刻,此刻该地层位置的涡流再次激发的响应最大,且与该位置地层的电导率成正比,该响应叠加在所有源距的接收波形中,是瞬变电磁响应的有用信号。套管井瞬变电磁测井波形在多个时刻的幅度随深度变化,所形成的曲线与裸眼井电导率测井曲线一致,对比实验的结果验证了井中激发、接收的瞬变电磁深层精细勘探方法的有效性。此外,井中阵列接收波形的幅度谱和相位谱均可用于计算地层的电导率。

in University,Tianjin 300072,China; 2.Beijing Huahui Shengshi Energy Technongy Co.Ltd,Beijing 101300,China)Abstract: In this research,transmitting and receiving arrays were placed at the bottom of a borehole,a transient electromagnetic (TEM) field was generated by a coil or an electrode,and the TEM response waveform was recorded to evaluate the distribution of conductivity in a deep formation.The differential equation describing the TEM field is a second-order derivative of the spatial coordinates and has two complex eigenvalues.The real part describes the phase shift,and the imaginary part describes the attenuation.Transient electromagnetic is both attenuated and phase shifted in the formation.The phase shift causes time delay,and the time difference of the initial point,rising edge and peak of the received waveform with different source distances is obvious,and the transient electromagnetic energy diffuses outward.The two roots of the equation indicate a reflection and transmission at the conductivity interface.The reflection field carries information on the interface and on the difference in conductivity between the formations.A one-dimensional model demonstrates the presence of the reflection field in the entire response waveform,while the reflection of seismic exploration is separated from the incident wave in time.The response waveform at each depth position has the moment when it changes the fastest.At this moment,the response of the eddy current in the deep formation is the largest.It is directly proportional to the conductivity of the formation at that depth and is superimposed on the received waveforms from all source distances.The amplitude curve of the TEM log in the cased hole is consistent with that of the conductivity log in the open hole.The results of the comparison confirm the above conclusions.In addition,the conductivity of the formation can be calculated using the amplitude and phase spectra of the waveform captured by the receiving array in the well.

“十三五”国家重点研发计划(2016YFC0802008)资助。