Patent ID: 8756017
Filing Date: 2014-06-17
Classification: G01V

Abstract:
1. A method for detecting formation resistivity outside a metal casing using borehole time-domain electromagnetic pulse, the method comprising the steps of: a) in a target well section comprising a metal casing, supplying a pulse current to a transmitting coil of a large power pulse transmitting source of a downhole instrument to generate an instantaneous high power pulsed magnetic field excitation; recording an induced electromotive force ε using a receiving and recording system of the downhole instrument through a receiving coil with certain distance from the transmitting source, the induced electromotive force ε changing with time and being produced by a vertical component Bz of a magnetic field; and full time digital recording a transmitted waveform and a received signal; b) conducting transmission-reception at the same depth point for multiple times and superimposing received signals to improve signal to noise ratio; c) calculating corresponding casing response according to known casing parameters and recorded current waveform of the transmitting source to obtain a relative induced electromotive force Δε d) correcting relative induced electromotive force value according to the information of casing collars and perforating positions; e) carrying out one-dimensional inversion and converting the observation signal, changing with time, into radial variation information of the formation resistivity surrounding a well; f) obtaining a two dimensional image of the longitudinal and radial resistivity distribution of outer cased formation resistivity for measured well sections according to the inversion resistivity of different measuring points; and g) according to electrical logging data and interpretation results after the well completed, determining residual oil distribution in a reservoir from outer cased reservoir resistivity distribution.