Study On Downhole Casing Inspection Technology Of Multi-pipe Strings With Transient Electromagnetic Method

The oil production wells in China have entered the middle and late stages.The corrosion,the bottom movement and the large increase in production technology make more and more casing damage.Meanwhile,many methods of casing damage detection have developed.However,with the strengthening of the government control and the improvement of the oil field safety requirements,the multilayer pipe string is widely used in the underground to protect the oil and gas well,which has brought great difficulties to the detection,especially with the recent decline of the oil price strategy.Effective logging requires that the oil pipe damage detection can be realized without pulling out oil pipe,which puts forward higher requirements for casing detection technology,and many casing damage detection methods can not realize multi-layer pipe column detection,only electromagnetic detection method can be realized.This paper first introduces the principle of transient electromagnetics and derives the principle of transient electromagnetic logging.On this basis,the detection model of underground multilayer pipe string is established,and the primary field signal generated by the electromagnetic probe under the bipolar step current excitation and the secondary response signal generated in each layer of medium are analyzed.The inductive electromotive force formula of electromagnetic probe is derived,and the transient electromagnetic response of multilayer pipe string is simulated.Then the structure parameters of the iron core material,length,diameter,coil number,other physical parameters such as the duration,period and current of the transmitting signal,and the influence of the spatial distribution of the probe on the detection performance are studied.The parameters of the transmitting and receiving system are optimized,and the transverse multicomponent electromagnetic probe and the longitudinal array electromagnetic probe are designed.In addition,aiming at the problem of the power consumption of probe detection mechanism increasing,the processing circuit of each module and the power supply mode of each probe in the downhole detection system are optimized.In this way,the power consumption of the entire detection system is reduced without affecting the detection performance.Finally,based on the successful development of the prototype,indoor and outdoor experiments are carried out to calibrate the casing damage,and the correctness of the theory is verified.The experimental results show that the method proposed in this paper can effectively improve the horizontal resolution of the inner tube column and the columnar delamination of the multi-layer pipe column,and effectively improve the SNR.