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Numerical analysis of the electromagnetic logging responses in metal casing pipe

SHEN Jin-song1,2,SUN Wen-bo1(1.Faculty of Resources and Informatics,China Petroleum University,Beijing 102249,China2.CNPC Key Lab of Geophysical Exploration,China Petroleum University,Beijing 102249,China)  
The electromagnetic responses of the finite size induction coils were evaluated, and the effects of the borehole mud,casing pipe and radius of the induction coils were analyzed,the sensitivity of the electromagnetic response to the formation resistivity was checked against the crosshole measurements.It is shown by the numerical results of the radial inhomogeneous model that,the borehole mud has little effects on the electromagnetic responses when compared with that of the casing pipe,hence we can ignore of the mud effect in casing hole environment in crosshole electromagnetic measurements.At low frequency band,the contributions of the casing pipe and the formation to the electromagnetic responses are uncoupling in crosshole electromagnetic configuration,we can separate the formation response from the comprehensive field.In case of the specified parameters of the casing pipe,the ratio of the casing response to that of the formation is almost a constant.Furthermore,the sensitivity analysis of the electromagnetic responses to the formation resistivity indicates that,in crosshole environment,the electromagnetic responses are fairly sensitive to the formation resistivity and the casing pipe attenuation is very stable.By increasing of the transmitter coils' radius,we can enhance the intensity of the electromagnetic response,and make it feasible of the large stride crosshole measurement.The numerical results of different magnetic permeability of the casing pipe show that in crosshole environments,non-magnetic or low-magnetic steel pipe is superior to magnetic iron pipe.
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