井中激发接收的瞬变电磁深层精细勘探方法——涡流差

In-hole excitation and receiving transient electromagnetic method for deep fine exploration:eddy current difference

套管井中瞬变电磁勘探得到的瞬态响应中直接耦合响应占据总响应很大幅度且与地层电导率无关,为了提取包含地层电导率信息的涡流再次激发响应,需要研究去除直接耦合响应的方法。根据Doll电流环模型和瞬变电磁响应精确解的级数展开给出了直接耦合响应与地层涡流再次激发响应之间的耦合关系——频谱的实部与虚部,确定了瞬变电磁响应携带地层电导率信息的方式——响应的相位移动。结合瞬变电磁响应的扩散过程,给出了地层电导率计算方法:用不同地层电导率时的响应相减消除直接耦合信号,响应差正比于地层电导率差。利用点源激发的推迟势可将上述方法推广到电极激发响应,其涡流再次激发响应与地层电导率的开平方成反比关系。以此为基础,设计了井内发射、接收系统同时移动跨越浅层测量深部地层电阻率的精细勘探方法。该方法从套管井测量资料中得到验证:瞬变电磁能量可穿过套管和近地层测量深部地层的电导率。

In transient electromagnetic exploration,the direct coupling response accounts for a large proportion of the total response and is independent of the formation conductivity.To extract the re-excitation response of the eddy current,which contains information on formation conductivity,a method to remove the direct coupling response needs to be developed.Using Doll’s direct coupling response and the series expansion of the exact solution of the transient electromagnetic field,the coupling relation between the direct coupling response and the re-excitation response of the formation eddy current(i.e.,the real and imaginary parts of the spectrum)was investigated.The results helped to determine that transient electromagnetic responses carry formation conductivity information through the phase shift of the response.A method for calculating the formation conductivity was then developed,by incorporating information on the diffusion process of the transient electromagnetic response.The direct coupling signal was eliminated by subtracting the responses relative to different formation conductivities and it was found that differences in response were proportional to differences in conductivity.The method was also used to study the electrode excitation response using the delay potential of the point source excitation.It was evaluated that the response of the eddy current re-excitation was inversely proportional to the square of the formation conductivity.Based on the above,a refined exploration method was designed to measure the resistivity of deep strata by simultaneously moving the transmitting and receiving systems across the shallow strata.The method was verified by cased hole measurements,demonstrating that transient electromagnetic energy can be used to measure the conductivity of deep formations using casing and shallow formation data.