Research On The Influence Factors On Cross-well Electromagnetic Logging

The cross-well logging technique of measuring electric resistivity of stratum through metal casing has wide foreground, but it is still in the primary stage of investigation and exploiture. During the logging of measuring electric resistivity of stratum through metal casing, the receiving coil receives the coupled signals of metal casing and cement sheath and stratum. The influence to the signal of stratum is complicated, because of the coupled function of metal casing and cement sheath and stratum. In order to get more accurate logging data, it is very important to do research on the influence rule of metal casing and cement sheath to cross-well electromagnetic logging.Based on the theory of electromagnetic field, assuming the subsurface inhomogeneous medium is axially symmetrical in respect of the bole axis where the transmitter is located and the transmitter source is the n-loop winding with indirect current in it which is vertical to the bole axis, full considered the casing and the cement wreath to the influence factor of the signal of stratum, built up the method of low-frequency electromagnetic fields to resolve four layers medium model with drilling fluid, casing, cement wreath and stratum geologic strata. In this paper, according to the principle of cross-well electromagnetic logging through metal casing, the theoretical formula is derived when the transmitting hole is metal casing of concretion cement wreath and the receiving well is naked hole. With this theory, this paper numerically simulates the relationship of the transmitting frequency and the well-to-well distance with the receiving signal. And the thickness, the magnetic conductance, and the electrical conductance of metal casing, analyzed the influencing rules of the casing and cement wreath, transmitting frequency andwell-to-well distance on cross-well electromagnetic logging by the numbers. The analysis and study shows the principle and method of cross-well electromagnetic logging established in this paper is feasible and valid. The method can not only research low-frequency current and its character of logging through metal casing but also confirm the method of measuring electric resistivity of stratum through metal casing.