套管井瞬变电磁测井在节箍处的响应与几何因子分析

Response of casing hoop and geometry factor to transient electromagnetic logging in cased wells

剩余油的探测需要利用套管井条件下的电阻率测井曲线.为此,用线圈电流的导通和关断激发电磁场,这些电磁场以低频为主,集肤深度大,能够穿过套管进入地层,并在地层中激发涡流,涡流激发的磁场能够再次穿过套管在接收线圈上产生响应.测量不同源距的瞬态电磁响应波形,将同一源距相邻两个深度点测量的波形相减去掉无用信号,获得的波形反映了地层的电导率.取其峰值得到测井曲线,该曲线在套管节箍处幅度很大,随深度变化的形状与同源距的Doll几何因子在半径比较小时随深度变化的形状相似.将同源距的Doll几何因子在径向上压缩、纵向上拉伸到与实际测量的曲线形状一致,压缩拉伸后的Doll几何因子即为套管井的几何因子,套管的高电导率和磁导率对响应的影响被归并到几何因子中.这为分析套管井瞬变电磁测井的纵向和径向特征,获得不同纵向分辨率和探测深度的测井曲线提供了基础.

The detection of residual oil requires the evaluation of resistivity curves in cased wells.An electromagnetic field is produced in the well by the passage of an alternate current through a coil.This electromagnetic field is mainly made up of low frequencies,and its skin depth is large.The field can pass through the casing (tube) into the formation,and excite the eddy currents.Consequently,the magnetic field produced by the eddy currents of the formation can enter the tube and reach the receiving coil in its center.Transient waveforms of different ranges are measured using appropriate tools,and the unwanted information is subtracted using the measured waveforms of the same range at two adjacent depths.The waveform that remains contains information on the conductivity of the formation that has to be measured.The first peak from this data is used to obtain a conductivity curve of the formation.The values of the curve at the casing hoop are high;the shape of the curve varies with depth and is similar to that of the Doll geometry factor at the same range when the radius is relatively small.If the Doll geometry factor is compressed in the radial direction,stretched in the longitudinal direction,and kept consistent with the shape of the curve actually measured,the geometric factor of the casing well is obtained.The effects of the high conductivity and permeability on the casing well are incorporated into the geometric factor.The geometric factor of the casing well can be used to analyze the longitudinal and radial characteristics of the transient electromagnetic logging in the casing well.This provides the basis for establishing logging curve processing methods with different vertical resolutions and detection depths.