Patent EA-001862-B1 (EA)
Title: Electrical logging of a laminated earth formation
Inventor: BEAR DAVID RONALD
IPC: G01V3/28
Filed: 1997-06-30
Abstract: 1. A method of determining an electric conductivity of an earth formation of different earth layers, which earth formation is penetrated by a wellbore containing a wellbore fluid, the method comprising: - lowering an induction logging tool into the wellbore to a location surrounded by a selected one of the earth layers, the tool comprising magnetic field transmitter means for inducing magnetic fields of different frequencies in said earth formation, and magnetic field receiver means for receiving response magnetic fields and for providing a signal representative of each response magnetic field; - selecting at least two said different frequencies; - for each selected frequency, operating the transmitter means so as to induce a magnetic field in the earth formation and operating the receiver means so as to provide a signal representing a response magnetic field; and - combining the signals in a manner so as to create a combined signal having a reduced dependency on the electric conductivity in the wellbore region, and determining from the combined signal the formation resistivity, characterized in that at least one of the transmitter means and the receiver means has a plurality of magnetic dipole moments in mutually orthogonal directions, in that the at least one of the transmitter and receiver means is operated in the mutually orthogonal directions, in that the combined signal comprises a plurality of combined signals, and in that the method further comprises determining from the plurality of combined signals the relative orientation of the logging tool with respect to the formation layering. 2. The method of claim 1, wherein the receiver means is suitable to receive response magnetic fields at different spacings from the transmitter means, at least two of said different spacings being selected, the transmitter means and the receiver means being operated for different combinations of the selected frequencies and the selected spacings, and the signals are combined in a manner so that said combined signal has a reduced dependency on the electric conductivity of an earth layer adjacent the selected earth layer. 3. The method of claim 1 or 2, wherein each of the transmitter means and the receiver means has a plurality of magnetic dipole moments in mutually orthogonal directions, and wherein each of the transmitter and receiver means is operated in said mutually orthogonal directions. 4. The method of any of claims 1-3, wherein each of the transmitter means and the receiver means has three magnetic dipole moments in three orthogonal directions, and wherein each of the transmitter and receiver, means is operated in said three mutually orthogonal directions. 5. The method of any of claims 2-4, wherein each signal includes a component which is a linear function of the frequency of the response magnetic field and is substantially dependent on the conductivity in the wellbore region, and a component which is a non-linear function of the frequency of the response magnetic field and is substantially dependent on the conductivity of said adjacent earth layer, and wherein said linear component is eliminated from the combined signal prior to eliminating the non-linear component from the combined signal. 6. A system for determining an electric conductivity of an earth formation formed of different earth layers, which earth formation is penetrated by a wellbore containing a wellbore fluid, the system comprising: - means for lowering an induction logging tool into the wellbore to a location surrounded by a selected one of the earth layers, the tool comprising magnetic field transmitter means for inducing magnetic fields of different frequencies in said earth formation, and magnetic field receiver means for receiving response magnetic fields and for providing a signal representative of each response magnetic field; - means for selecting at least two of said different frequencies; - means for operating the transmitter means so as to induce a magnetic field in the earth formation and operating the receiver means so as to provide a signal representing a response magnetic field, for each selected frequency; and - means for combining the signals in a manner so as to create a combined signal having a reduced dependency on the electric conductivity in the wellbore region, and for determining from the combined signal the formation resistivity, characterized in that at least one of the transmitter means and the receiver means has a plurality of magnetic dipole moments in mutually orthogonal directions, wherein the at least one of the transmitter and receiver means is operated in the mutually orthogonal directions, and in that the system further comprises means for determining the relative orientation of the logging tool with respect to the formation layering from the combined signal, which comprises a plurality of combined signals.