The Study Of Finite Element Numerical Simulation For Magnetism-stress Detecting In Oil Casing Based On ANSYS

The casing damage has been a big problem in oilfield production, theaccumulation of stress is the main reason for the casing damage. The present detectionmethods mostly are used for detecting the geometric conditions after casing damageand could not been used for predicting the casing damage, which are not veryeffective. That detecting the stress of casing accurately and effectively to predict thecasing damage is the key problem for detecting casing damage. Magnetic anisotropytechnique based on counter magnetostriction effect is suitable for detecting the stressof oil casing. However, the technique is in the development stage. The former mostlygot the relationship between stress and magnetic signals by physical experiment, thestudy of physical mechanisms on relative magnetic permeability which connects thestress and magnetic signals is rarely reported. In order to apply this technique intodetecting the stress of oil casing accurately, knowing that how the relative magneticpermeability effect the stress and magnetic signals clearly is strongly required. It willbe of great fundamental importance to develop magnetism-stress detecting machineand software for imaging the stress of oil casing. It will be of great economic andsocial benefit for developing our country`s oil and gas resources.The counter magnetostriction effect is that the changes of ferromagneticmaterial`s relative magnetic permeability under load will change the magnetic field.The changes of the load can be got by the changes of the magnetic field. Magneticanisotropy technique based on this effect contains two relationships. One is therelationship between load and relative magnetic permeability. This research has beenrelatively mature which has both theoretical derivation and physical experiment, somerelevant results can be referred. The other is the relationship between relativemagnetic permeability and magnetic signals. For the oil casing model, when somepart of the oil casing`s relative magnetic permeability are uneven distribution, whatthe distribution of the magnetic signals around the oil casing will be. The analyticalsolution of this problem is not existed and the finite element numerical simulation cansolve this problem effectively.The large general-purpose finite element analysis software ANSYS is moreconvenient and superior than the common methods to use finite element numericalsimulation in follow aspects, such as meshing model, loading initial value,constraining boundary conditions, solving equation, presenting results and so on.More and more researchers have applied ANSYS into solving all kinds of problems ingeophysical prospecting, ANSYS has great application prospect. The paper use theelectromagnetics module of ANSYS and program APDL to do the finite elementnumerical simulation for the oil casing model and reach the conclusion that themagnetic flux density near the casing wall is inversely proportional to the relativemagnetic permeability and the variable quantity of them have a good linearrelationship. Referring to the variable quantity relationship between the stress and relative magnetic permeability which is got by the former`s physical experiment, thepaper come to the conclusion that the quantitative relationship between the stress’svariation and magnetic flux density`s variation is: K*B, K=8.04*109.In order to evaluate the above result, on the one hand, the physical experimenthas been done. The conclusion of physical experiment is that the load and magneticflux have quantitative relation. Combining this result with the quantitative relationbetween load and stress which is given by using the structural module of ANSYS, thequantitative linear scale relation between the stress`s variation and magnetic flux`svariation can prove that the numerical simulation`s result is true adequately. On theother hand, the former`s theoretical derivation also prove the numerical simulation`sresult true. However, the K in the theoretical derivation can not be determined but itcan be determined by the method proposed in this paper. Thus, the stress`s variationof oil casing can be got by the magnetic flux density`s variation. So, this techniquecan be applied into detecting the stress of oil casing accurately in real work.