Through-casing resistivity logging (TCRL) is a new kind of logging method for formation evaluation, reservoir monitoring and saturation changes tracking. In this paper, the basic principle of TCRL is detailed, and the modified transmission line equation method (MTLEM) is first proposed in China. The MTLEM has been employed to simulate the responses of TCRL, on the basis of which, the effects of formation resistivity, cement and casing on the responses of TCRL are analyzed, and the signals induced in different formations are also calculated, which can help the design of TCRL tool and data processing and interpretation in China.through-casing resistivity logging (TCRL), modified transmission line equation method (MTLEM), casing, cement sheath

Because difference of formation model affects the potential distribution on the metal casing, the resistivity logging response can be obtained through casing by measuring the potentials on the metal casing. In fact, the logging responses are the comprehension effects of complex formation and metal casing, so that in the coefficients of transmission line equation (TLE), we should consider the effects of formation model in radialdirection. In the following, for the formation taking on multi-layer in axial-direction, and step in radial-direction, the resistivity logging responses through casing are researched and discussed, and given the recurrence formulae to compute potential. Using the potential distributions which are obtained by resolving TLE we realize the calculation of conductivity of practice formation model through casing, and give numerical computing examples for difference formation models. The computing results show that: this method has a strong capability to distinguish layer-formations in axial-direction; while, in the radial-direction, for the uniform formation, the conductivity of formation is computed is very approximating to that of formation model, which is able to reveal characteristic of practical formation; for the high conductive formation and the low conductive cement layer, the method has an obvious logging response; for the formation taking on steps in radial-direction, the computing conductivity is comprehension reflect for every layer in radial. The computing examples also show that this method has a fast computing speed (the run time of computing example is less than 1s). For analysis the logging response of multi-layers and block formations, and the design of logging instruments, the research is important significance and useful.

Because the potential distribution on the metal casing contains the information of metal casing and formations, the change of resistivity of metal casing has an important effect on through‐casing resistivity logging responses. The effect due to the change of casing resistivity on the coefficients of transmission line equation (TLE) is considered, and the calculation method of potential distribution on the heterogeneous casing is also given. On the basis of the potential distributions obtained from TLE, through‐casing resistivity logging responses of heterogeneous casing are calculated, and the effects of the change of metal casing resistivity on the resistivity logging response are analyzed. The computation results show that the casing change whether thinner or thicker has an obvious effect on the logging responses, and the abnormality of logging response appears where the casing resistance suddenly decreases or increases. The computation results also show that the spacing of electrodes has an important effect on through‐casing resistivity logging responses.

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