Study On Distribution Of Magnetic Field Around Casing String

At present,many onshore oilfields have been in the later period of production and in order to increase the production and efficiency,more and more infill wells and adjustment wells are drilled;in order to use the space reasonably and reduce the cost,distance between wellhead of cluster wells is short,and the result is that distance between vertical sections is short.Therefore,anti-collision has become a very important issue.Regular anti-collision methods depend too much on accuracy of inclinometer data,and measurement of inclinometer is easily affected by other magnetic field source,of which magnetic field of adjacent wells is most common.The position of drilling well relative to adjacent well can be judged based on magnetic field around casing.Study on magnetic field rules around casing can enhance the understanding of magnetic interference of adjacent wells and is helpful to establish the phonetic relationships between magnetic susceptibility factors and magnetic domain,and will help to establish an anti-collision method with wide accommodation and low cost.In this paper,the factors that affect magnetic field around casing are analyzed and result shows that induced magnetic field,residual magnetism and casing coupling are main factors that affect magnetic field around casing,and magnetic field distribution around casing is studied theoretically and experimentally.Main research contents and achievements are as follows: separation variable method is used to solve Laplace equation to establish magnetic field model around infinite casing,and the model is analyzed and solved,then magnetic field model around casing in any attitude is obtained through using the vector projection method;3-dimention integral equation method is used to establish magnetic field model around finite casing;full size casing is experimentally tested to analyze the abnormal value and gradient of total magnetic field,horizontal magnetic field,vertical magnetic field and dip angle of field points in different position,and changing rules of different magnetic field parameters along the radial and tangential are obtained,which laid a foundation for space inversion in the future;small size ferromagnetic pipes are used for field test,and space magnetic changing rules of the pipes with and without residual magnetism and coupling are discussed;theoretical model and experimental results are compared and the correctional inductive magnetic field model is obtained at last.