Page 5 - Oil and Gas Simulations
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INDUCTION LOGGING

FIGURE 2. Imaginary magnetic fields, all three direct components, for frequency 20 kHz  of detecting, measuring, and visualizing
when the tool is centered in the borehole. The lower frequency corresponds to a larger  bed boundaries and oil/water contact
region of investigation. The gray shaded regions indicate the oil-bearing layers.       zones hours before they can be “seen”
                                                                                        with conventional sensors.

                                                                                        èSIMULATING THE TOOLS

                                                                                        Dutta and his colleague, Dr. Fei Le, are
                                                                                        using COMSOL Multiphysics® software
                                                                                        to model induction logging tools, like
                                                                                        AziTrak™, in a variety of situations.
                                                                                        	 “Over the course of their continuous
                                                                                        development, induction logging tools
                                                                                        have become increasingly difficult
                                                                                        to characterize using simple models.
                                                                                        Similarly, with the advent of directional
                                                                                        drilling, it has become imperative to
                                                                                        model fully 3D formations,” says Dutta.
                                                                                        	 And although Baker Hughes does
                                                                                        possess in-house codes for true 3D models,
                                                                                        there are limitations. “The lack of user
                                                                                        interface and visualization capabilities
                                                                                        in our in-house tools motivated us
                                                                                        to look for commercial packages.
                                                                                        COMSOL Multiphysics is one of a suite of
                                                                                        commercial FEM-based packages that we
                                                                                        have been using for some time now.”
                                                                                        	 To illustrate his use of COMSOL®
                                                                                        software, Dutta described a simulation
                                                                                        of a full 3D formation model with a
                                                                                        simplified induction tool in a borehole:
                                                                                        The formation (see Figure 1) consists of
                                                                                        five horizontal layers with anisotropic
                                                                                        oil-bearing layers invaded by the

INDUCTION LOGGING EXPLAINED

                        mandrel  secondary       Consider a pipe mandrel with two multi-turn coils wound over it,
                                 magnetic field  surrounded by an earth formation. The transmitter coil T is fed an
         receiver coil                           alternating current with a suitable frequency. The voltage (both
                R                induced         amplitude and phase) across the receiver coil R is measured.
                                 currents        	 This is a simple representation of an induction logging
formation                                        tool with coaxial transmitter/receivers. The current through
                                                 the transmitter coil T generates an axial primary magnetic
transmitter coil                                 field (shown in blue). The alternating primary magnetic field
        T                                        induces electric current loops (shown in red) in the formation.
                                                 These induced currents generate a secondary magnetic
                                                 field (shown in green), which also crosses the receiver coil R
                                                 in addition to the primary magnetic field. Thus, the signal
                                                 generated in the receiver is in response to both the primary
                                                 and secondary magnetic fields. The currents induced in the
                                                 formation are a function of formation resistivity. Therefore,
                                                 the receiver signal is an indicator of formation resistivity. Since
                                                 the primary magnetic field is several orders of magnitude
                                                 stronger than the secondary magnetic field, typical induction
                                                 logging tools use multiple coil systems with hardware/software
                                                 schemes to cancel the signal due to the primary magnetic field.

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