Analysis Of Magnetic Field Distribution Around Casing String Considering Influence Of Remanence

In recent years,more and more cluster wells and infill wells are being drilled,and the borehole spacing is getting smaller and smaller.How to avoid borehole collision has become a very important drilling technical problem.At present,a variety of magnetic ranging anti-collision warning technologies have been developed abroad.Among them,the active magnetic ranging method needs to put the signal generator or receiver into the adjacent well,which is affected by the production status of the adjacent well,and the working conditions are limited.The passive magnetic ranging method relies on the magnetic field of the casing of adjacent wells and is not restricted by the production condition of adjacent wells.Clarifying the magnetic field distribution around the casing string is the primary condition for the passive magnetic ranging method.However,there is a limited understanding of the magnetic field distribution around casing strings in China,so it is urgent to carry out relevant research work.In this paper,the remanence distribution around the single casing,the distribution of magnetic field around the casing string,and the passive magnetic measurement method based on the distribution of magnetic field around the casing string have been discussed.Firstly,based on the measured results of residual magnetic field around a single full-size casing,the distribution model of residual magnetic field around a single non-uniformly magnetized casing,the distribution model of induced magnetic field around the casing string and the distribution model of magnetic field around the casing string are established.The distribution characteristics of magnetic field around the casing string under different magnetic pole combinations are solved by the established model.Secondly,by means of outdoor experiments,the variation laws of different magnetic field parameters around casing string along the casing axis,radial direction and different directions are studied,and the results are compared with those of theoretical model.Finally,based on the above theoretical and experimental results,the passive magnetic ranging method based on the magnetic field of the casing string is preliminarily discussed,and the ranging method is verified by theoretical analysis and actual measurement.The results show that the residual magnetic field around a single casing mainly concentrates at two ends,and the influence range is larger than the induced magnetization field around the casing;the influence range of the induction magnetic field in the middle of the casing has the same influence range in all directions,and the influence range of the induction magnetic field near the casing coupling is larger;when different combinations of residual magnetic poles are used at the coupling,the range and distribution law of the magnetic field around the casing string are different.The influence range of remanence magnetic field in the same pole connection mode is larger than that in the opposite pole connection mode,which can increase the detection range of passive magnetic ranging.Based on the theoretical analysis of magnetic anomalies around casing strings and the actual measurement law,the passive magnetic ranging method based on the magnetic field of casing is discussed,and the corresponding error correction method is provided.The results have shown that the method can primely retrieve the relative position of casing and provide the basis for the research and development of passive magnetic ranging wellbore anti-collision technology.