水平井瞬变电磁边界远探测数值模拟与响应特征分析

Numerical Simulation and Response Characteristics Analysis Transient Electromagnetic Remote Boundary Detection in Horizontal Wells

为克服频率域电磁感应远探仪器过长的缺点,提出采用瞬变电磁远探测方法进行边界探测,给出了均匀地层中响应解析计算方法,基于COMSOL Multiphysics有限元数值软件,建立了层状地层水平井瞬变电磁远探测响应计算模型,并对其在地层界面识别时的响应特征进行了分析。结果显示:双对数坐标系下,均匀地层中瞬态电磁远探测响应随采集时间呈线性衰减规律,且地层电阻率越大,响应衰减越快,同时刻采集的响应值越小;在两层地层中,定义背景地层响应与层状地层响应差异为边界场响应,岩层边界会导致边界场产生局部极值,该极值点对应的时间与岩层电阻率对比度、探边距离以及仪器倾斜角度密切相关,增大收发线圈间距可导致边界场响应极值向晚期时刻移动;在三层地层中,瞬变电磁远探测响应同时包含岩层上边界和下边界的信息,在边界场响应中形成两个局部极值点。研究成果为瞬变电磁远探测打下良好基础。

In order to overcome the shortcoming of great length of frequency domain electromagnetic induction romote detector,it is proposed that transient electromagnetic remote detection method is used for the detection of rock boundary,and an analytical calculation method for the detection response in uniform formation is provided.Based on COMSOL Multiphysics finite element software,a transient electromagnetic remote detection response calculation model for horizontal wells layered formation was established,and its response characteristics in formation interface identification were analyzed.The results show that in the double logarithmic coordinate system,there is a linear attenuation relationship between transient electromagnetic remote detection response and acquisition time in a uniform formation,and the higher the formation resistivity,the faster the response attenuation,and the smaller the collected response value at the same time.In a two-layer formation,the difference between the response of the background strata and the response of the layered strata is defined as the boundary field response.The boundary of the strata will cause local extremum of the boundary field,and the time corresponding to this extremum point is closely related to the contrast of the rock resistivity,the distance to the detected boundary and the inclination angle of the instrument.Increasing the distance between the transmitting and receiving coils can cause the extreme value of the boundary field response to move towards the later time.In a three-layer formation,the transient electromagnetic remote detection response includes both the upper and lower boundaries of the rock layers,forming two local extremum points in the boundary field response.The research results have laid a solid foundation for transient electromagnetic remote detection.