Electricity: measuring and testing – Of geophysical surface or subsurface in situ – With radiant energy or nonconductive-type transmitter
Reexamination Certificate
2002-05-20
2004-09-21
Le, N. (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
With radiant energy or nonconductive-type transmitter
C324S335000, C324S339000, C702S007000
Reexamination Certificate
active
06794875
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
FIELD OF THE INVENTION
This invention relates to induction well logging, and more particularly to a new transmitter and receiver coil structure, and a new method for collecting and processing data from an induction tool.
BACKGROUND OF THE INVENTION
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods. Oil well logging has been known in the industry for many years as a technique for providing information to a petrophysicist regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or “sonde” is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde may include one or more sensors to measure parameters downhole and typically is constructed as a hermetically sealed cylinder for housing the sensors, which hangs at the end of a long cable or “wireline.” The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
The sensors used in a wireline sonde usually include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. Various sensors have been used to determine particular characteristics of the formation, including nuclear sensors, acoustic sensors, and electrical sensors.
If the formation properties are needed while drilling, sensors can also be deployed near the end of a drilling string. Measurements of formation properties can be measured and stored in memory for later retrieval and correlation with depth. Measurements can also be transmitted to the surface by pulses of mud pressure or other means. This process is referred to as “logging while drilling” (LWD).
For a formation to contain petroleum, and for the formation to permit the petroleum to flow through it, the rock comprising the formation must have certain well-known physical characteristics. Hydrocarbons are a poor conductor of electricity while most formation water conducts much better. If the porosity of an earth formation is known from other sensors, its electrical resistivity can assist the petrophysicist in determining the volume fraction of hydrocarbons in the formation. This electrical resistivity can be measured by two classes of sensors—those that use electrodes to force current to flow through the formation and to measure potential differences, and those that use coils to induce currents to flow magnetically. The particular type with coils, called induction devices, determine electrical resistivity by inducing an alternating electromagnetic field into the formation with a transmitter coil arrangement. The electromagnetic field induces alternating electric (or eddy) currents in the formation in paths that are substantially coaxial with the transmitter. These currents in turn create a secondary electromagnetic field in the medium, inducing an alternating voltage at the receiver coil. If the current in the transmitter coil is kept constant, the eddy current intensity is proportional to the conductivity of the formation. Consequently, the conductivity of the formation determines the intensity of the secondary electromagnetic field, and thus, the amplitude of the voltage at the receiver coil. As will be apparent to one skilled in the art, the propagating electromagnetic wave suffers both attenuation and phase shift as it traverses the formation.
An exemplary induction tool is shown in the prior art drawing of
FIG. 1
, in which one or more transmitters (T) and a plurality of receivers (R
i
) are shown in a logging sonde. Each transmitter or receiver is a set of coils, with modern array induction tools having several receivers of increasing transmitter-to-receiver spacing to measure progressively deeper into the formation.
In a conventional induction tool such as that shown in
FIG. 1
, the coils are wound coaxially around a cylindrical mandrel. Both transmitter coils and receiver coils are solenoidal, and are wound coaxial with the mandrel. Such coils would therefore be aligned with the principal axis of the logging tool, which is normally also the central axis of the borehole and is usually referred to as the z axis. That is, the magnetic moments of the coils are aligned with the axis of the mandrel on which they are wound. The number, position, and numbers of turns of the coils are arranged to null the signal in a vacuum due to the mutual inductance of transmitters and receivers.
During operation, an oscillator supplies alternating current to the transmitter coils, thereby inducing voltage in the receiver coils. The voltage induced in the receiver coils results from the sum of all eddy currents induced in the surrounding formations by all transmitters. Phase sensitive electronics measure the receiver voltage that is in-phase with the transmitter current divided by magnitude of the transmitter current. When normalized with the proper scale factor, this gives the apparent conductivity of the formation. The out-of-phase component can also be useful because of its sensitivity to skin effect although it is less stable and is adversely affected by contrasts in the magnetic permeability.
As noted, the induced eddy currents tend to flow in circular paths that are coaxial with the transmitter coil. As shown in
FIG. 1
, for a vertical borehole traversing horizontal formations, there is a general symmetry for the induced current around the logging tool. In this ideal situation, each line of current flow remains in the same formation along its entire flow path, and never crosses a bed boundary.
In many situations, as shown for example in
FIG. 2
, the wellbore is not vertical and/or the bed boundaries are not horizontal. The well bore in
FIG. 2
is shown with an inclination angle &thgr; measured relative to true vertical. A bed boundary between formations is shown with a dip angle &agr; relative to the axis of the borehole. The inclined wellbore strikes the dipping bed at an azimuth angle &bgr;. As a result, the induced eddy currents flow through more than one medium, encountering formations with different resistive properties. The resulting logs tend to be relatively inaccurate, especially as the dip angle &agr; of the bed boundaries become more severe. If the logging tool traverses a thin bed, the problem becomes even more exaggerated.
As shown in the graph of
FIG. 3A
, an induction sonde traversing a dipping bed produces a log with “horns.” The more severe the dip angle, the less accurate is the measurement.
FIG. 3A
represents a computer simulation of a log that would be generated during logging of a ten foot thick bed (in actual depth), with different plots for different dip angles.
FIG. 3B
shows a computer simulation of a log which would be generated if the thickness of the bed were true vertical depth, with different plots for different dip angles. As is evident from these simulated logs, as the dip angle increases, the accuracy and meaningfulness of the log decreases. In instances of high di
Conley & Rose, P.C.
Halliburton Energy Service,s Inc.
Kinder Darrell
Le N.
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