Method for detecting formation resistivity outside of metal casing using time-domain electromagnetic pulse in well

摘要

A method for detecting formation resistivity outside of metal casing using time-domain electromagnetic pulse in a borehole, the method including steps of a) providing a borehole large power pulse transmitting source, recording an induced electromotive force ε, and full time digital recording a transmitted waveform and a received signal; b) conducting transmission-reception 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 Δεf; d) correcting relative induced electromotive force value; e) carrying out one-dimensional inversion and converting the observation signal into radial variation information of the formation resistivity; f) obtaining a two-dimensional image of the longitudinal and radial resistivity distribution of outer cased formation resistivity for measured well sections; and g) determining residual oil distribution in a reservoir from outer cased reservoir resistivity distribution.

主权项

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 Δεf of an outer cased formation:Δɛf=ɛo-ɛcɛc wherein, εO represents an observed induced electromotive force and εC represents a calculated casing response; 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.