瞬变电磁井间勘探的全空间几何因子

井间是剩余油的主要分布区域,为探测井间剩余油,提高采收率,提出了基于全空间几何因子的瞬变电磁井间勘探方法。在本井使用线圈发射、邻井使用线圈接收,根据瞬变电磁场理论,在阶跃信号的激励下发射线圈在地层中激发出沿圆周方向的闭合瞬变电场,该电场在导电地层中产生与地层电导率呈正比的涡流。由Doll地层环模型可知,地层中的涡流在空间任意点激发得到与地层电导率成正比的二次场响应信号(有用信号),并可表示为空间各点电导率的加权平均值,其权重即为井间瞬变电磁勘探的全空间几何因子;全空间几何因子集中分布在发射线圈和接收线圈附近,其它区域分布较少,在发射线圈和接收线圈两侧呈现不同的极性;对瞬变电磁响应与地层电导率、井间距和源距的变化规律研究可知,瞬变电磁井间勘探有用信号随着地层电导率的增大而增大,随着井间距的增加单调减小,在发射线圈和接收线圈处于同一深度时该响应信号幅度最大。

Full-space geometric factors for cross-well transient electromagnetic exploration

Most of the remaining oil is confined in the cross-well region.To identify its location and therefore improve the oil recovery ratio,a transient electromagnetic cross-well exploration method based on full-space geometric factors was proposed.In the field experiments,coils were used as the transmitting source in this well and as receiving probes in the adjacent well.According to the basic theory of transient electromagnetic fields,the transmitting coil,when excited by a steep voltage signal,generates a closed transient electric field along a circumference,thereby producing eddy currents within the formation.This signal is proportional to the electric conductivity of the formation.Using Doll's electromagnetic induction current loop model,eddy currents can generate a quadratic response signal.This signal is proportional to the electric conductivity of the formation and expressed as the weighted average of the electrical conductivity,whose weight is the full-space geometric factor.The full-space geometric factors demonstrated stronger geometric factors near the transmitting and receiving coils,with different polarities.The study on transient electromagnetic response,formation conductivity,well spacing,and source spacing demonstrated that the strength of useful signals for transient electromagnetic cross-well exploration increased with formation conductivity,whereas it decreased monotonically with an increase of well spacing;the maximum amplitude of the signals can be achieved by placing the transmitting and receiving coils at the same depth.