U.S. patent application number 11/627172 was filed with the patent office on 2008-08-21 for borehole conductivity simulator verification and transverse coil balancing.
This patent application is currently assigned to Baker Hughes Incorporated. Invention is credited to Michael S. Crosskno, Stanislav W. Forgang, Randy Gold, Luis M. Pelegri.
Application Number | 20080197851 11/627172 |
Document ID | / |
Family ID | 39645517 |
Filed Date | 2008-08-21 |
United States Patent
Application |
20080197851 |
Kind Code |
A9 |
Forgang; Stanislav W. ; et
al. |
August 21, 2008 |
Borehole Conductivity Simulator Verification and Transverse Coil
Balancing
Abstract
Calibration of the arrays of a multicomponent induction logging
tool is achieved by positioning the tool horizontally above ground.
The upper and lower housings of the tool are connected by a
borehole conductivity simulator which as a resistance comparable to
that of a borehole. Axial and radial positioning of the transmitter
coils is done by monitoring outputs at receiver coils to achieve a
minimum.
Inventors: |
Forgang; Stanislav W.;
(Houston, TX) ; Gold; Randy; (Houston, TX)
; Pelegri; Luis M.; (Humble, TX) ; Crosskno;
Michael S.; (Spring, TX) |
Correspondence
Address: |
MADAN, MOSSMAN & SRIRAM, P.C.
2603 AUGUSTA, SUITE 700
HOUSTON
TX
77057
US
|
Assignee: |
Baker Hughes Incorporated
Houston
TX
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20070170923 A1 |
July 26, 2007 |
|
|
Family ID: |
39645517 |
Appl. No.: |
11/627172 |
Filed: |
January 25, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11371052 |
Mar 8, 2006 |
7205770 |
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11627172 |
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11340785 |
Jan 26, 2006 |
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11371052 |
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Current U.S.
Class: |
324/339 |
Current CPC
Class: |
G01V 3/28 20130101 |
Class at
Publication: |
324/339 |
International
Class: |
G01V 3/18 20060101
G01V003/18 |
Claims
1. A method of preparing a multicomponent induction logging tool
having a plurality of transmitter coils and a plurality of receiver
coils, the method comprising: (a) positioning the logging tool in a
calibration area substantially free from components capable of
interfering with magnetic and electric fields produced by the said
tool; (b) coupling a first conductive housing of the tool with a
second conductive housing of the tool through a borehole
conductivity simulator (BCS) having an impedance similar to that of
a borehole environment; (c) activating a first coil of the
plurality of transmitter coils and measuring a signal in a first
coil of the plurality of receiver coils; (d) moving the first coil
of the plurality of transmitter coils relative to a conductive
feed-through pipe between the first housing and the second housing
to reduce a magnitude of the signal; and (e) moving the first coil
of the plurality of receiver coils relative to the feed-through
pipe until the magnitude of the signal is substantially equal to
zero.
2. The method of claim 1 further comprising positioning the first
coil of the plurality of receiver coils in an eccentered position
in the logging tool prior to step (d).
3. The method of claim 1 further comprising orienting the logging
tool with its longitudinal axis substantially parallel to the
ground.
4. The method of claim 1 wherein the first coil of the plurality of
transmitter coils has an axis that is one of (i) substantially
parallel to a longitudinal axis of the tool, and (ii) substantially
orthogonal to a longitudinal axis of the tool.
5. The method of claim 1 wherein the first coil of the plurality of
receiver coils has an axis that is one of (i) substantially
parallel to a longitudinal axis of the tool, and (ii) substantially
orthogonal to a longitudinal axis of the tool.
6. The method of claim 1 further comprising: (i) rotating the tool
about a longitudinal axis of the tool; (ii) activating a second
coil of the plurality of transmitter coils and measuring an
additional signal in a second coil of the plurality of receiver
coils; and (iii) moving the second coil of the plurality of
transmitter coils with respect to the feed-through pipe to reduce a
magnitude of the additional signal.
7. The method of claim 1 further comprising: (i) magnetically
coupling the tool to a calibrator; (ii) activating the first coil
of the plurality of transmitter coils; (iii) determining from a
signal received at a specific coil of the plurality of receiver
coils a transfer function between the specific coil and the first
coil of the plurality of transmitter coils.
8. The method of claim 1 wherein the tool is positioned inside the
calibrator.
9. The method of claim 1 wherein the moving is in a direction
selected from (i) substantially parallel to a longitudinal axis of
the tool, and (ii) substantially orthogonal to a longitudinal axis
of the tool.
10. An apparatus for evaluating performance of a multicomponent
induction logging tool having a plurality of transmitter coils and
a plurality of receiver coils, the tool being positioned in a
calibration area substantially free from components capable of
interfering with magnetic and electric fields produced by the said
tool, the apparatus comprising: (a) a borehole conductivity
simulator (BCS) having an impedance similar to that of a borehole
environment the BCS coupling a first housing of the tool with a
second housing of the tool; (b) a processor configured to activate
a first coil of the plurality of transmitter coils; (c) a first
coil of the plurality of receiver coils configured to provide a
signal responsive to the activation of the first coil; and (d) a
device configured to: (A) move the first coil of the plurality of
transmitter coils relative to the first coil of the plurality of
receiver coils to reduce a magnitude of the signal; and (B) move
the first coil of the plurality of receiver coils relative to the
conductive feed-through pipe until the magnitude of the signal is
substantially zero.
11. The method of claim 10 wherein the device is further configured
to position the first coil of the plurality of receiver coils in an
eccentered position in the logging tool.
12. The apparatus of claim 10 wherein the first coil of the
plurality of transmitters has an axis that is one of (i)
substantially parallel to a longitudinal axis of the tool, and (ii)
substantially orthogonal to a longitudinal axis of the tool.
13. The apparatus of claim 10 wherein the first coil of the
plurality of receivers has an axis that is one of (i) substantially
parallel to a longitudinal axis of the tool, and (ii) substantially
orthogonal to a longitudinal axis of the tool.
14. The apparatus of claim 10 further comprising a calibrator and
wherein: (i) the logging tool is magnetically coupled with the
calibrator, and (ii) the processor is further configured to
determine from the signal a transfer function between the first
coil of the plurality of transmitters and the first coil of the
plurality of receivers.
15. The apparatus of claim 10 wherein the logging tool is
positioned within the calibrator.
16. The apparatus of claim 9 wherein the device is configured to
produce movement in a direction selected from (i) substantially
parallel to a longitudinal axis of the tool, and (ii) substantially
orthogonal to a longitudinal axis of the tool.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present invention is a continuation-in-part of U.S.
patent application Ser. No. 11/371,052 filed on the 8 Mar. 2006,
and U.S. application Ser. No. 11/340,785 filed on 26 Jan. 2006.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention is related to the field of apparatus
design in the field of oil exploration. In particular, the present
invention describes a method for calibrating multicomponent logging
devices used for detecting the presence of oil in boreholes
penetrating a geological formation.
[0004] 2. Description of the Related Art
[0005] Electromagnetic induction resistivity well logging
instruments are well known in the art. Electromagnetic induction
resistivity well logging instruments are used to determine the
electrical conductivity, and its converse, resistivity, of earth
formations penetrated by a borehole. Formation conductivity has
been determined based on results of measuring the magnetic field of
eddy currents that the instrument induces in the formation
adjoining the borehole. The electrical conductivity is used for,
among other reasons, inferring the fluid content of the earth
formations. Typically, lower conductivity (higher resistivity) is
associated with hydrocarbon-bearing earth formations. The physical
principles of electromagnetic induction well logging are well
described, for example, in, J. H. Moran and K. S. Kunz, Basic
Theory of Induction Logging and Application to Study of Two-Coil
Sondes, Geophysics, vol. 27, No. 6, part 1, pp. 829-858, Society of
Exploration Geophysicists, December 1962. Many improvements and
modifications to electromagnetic induction resistivity instruments
described in the Moran and Kunz reference, supra, have been
devised, some of which are described, for example, in U.S. Pat. No.
4,837,517 to Barber, in U.S. Pat. No. 5,157,605 to Chandler et al.,
and in U.S. Pat. No. 5,600,246 to Fanini et al.
[0006] The conventional geophysical induction resistivity well
logging tool is a probe suitable for lowering into the borehole and
it comprises a sensor section containing a transmitter and receiver
and other, primarily electrical, equipment for measuring data to
infer the physical parameters that characterize the formation. The
sensor section, or mandrel, comprises induction transmitters and
receivers positioned along the instrument axis, arranged in the
order according to particular instrument or tool specifications and
oriented parallel with the borehole axis. The electrical equipment
generates an electrical voltage to be further applied to a
transmitter induction coil, conditions signals coming from receiver
induction coils, processes the acquired information, stores or by
means of telemetry sending the data to the earth surface through a
wire line cable used to lower the tool into the borehole.
[0007] In general, when using a conventional induction logging tool
with transmitters and receivers (induction coils) oriented only
along the borehole axis, the hydrocarbon-bearing zones are
difficult to detect when they occur in multi-layered or laminated
reservoirs. These reservoirs usually consist of thin alternating
layers of shale and sand and, oftentimes, the layers are so thin
that due to the insufficient resolution of the conventional logging
tool they cannot be detected individually. In this case the average
conductivity of the formation is evaluated.
[0008] Conventional induction well logging techniques employ coils
wound on an insulating mandrel. One or more transmitter coils are
energized by an alternating current. The oscillating magnetic field
produced by this arrangement results in the induction of currents
in the formations which are nearly proportional to the conductivity
of the formations. These currents, in turn, contribute to the
voltage induced in one or more receiver coils. By selecting only
the voltage component which is in phase with the transmitter
current, a signal is obtained that is approximately proportional to
the formation conductivity. In conventional induction logging
apparatus, the basic transmitter coil and receiver coil has axes
which are aligned with the longitudinal axis of the well logging
device. (For simplicity of explanation, it will be assumed that the
borehole axis is aligned with the axis of the logging device, and
that these are both in the vertical direction. Also single coils
will subsequently be referred to without regard for focusing coils
or the like.) This arrangement tends to induce secondary current
loops in the formations that are concentric with the vertically
oriented transmitting and receiving coils. The resultant
conductivity measurements are indicative of the horizontal
conductivity (or resistivity) of the surrounding formations. There
are, however, various formations encountered in well logging which
have a conductivity that is anisotropic. Anisotropy results from
the manner in which formation beds were deposited by nature. For
example, "uniaxial anisotropy" is characterized by a difference
between the horizontal conductivity, in a plane parallel to the
bedding plane, and the vertical conductivity, in a direction
perpendicular to the bedding plane. When there is no bedding dip,
horizontal resistivity can be considered to be in the plane
perpendicular to the bore hole, and the vertical resistivity in the
direction parallel to the bore hole. Conventional induction logging
devices, which tend to be sensitive only to the horizontal
conductivity of the formations, do not provide a measure of
vertical conductivity or of anisotropy. Techniques have been
developed to determine formation anisotropy. See, e.g. U.S. Pat.
No. 4,302,722 to Gianzero et al. Transverse anisotropy often occurs
such that variations in resistivity occur in the azimuthal
direction.
[0009] Thus, in a vertical borehole, a conventional induction
logging tool with transmitters and receivers (induction coils)
oriented only along the borehole axis responds to the average
horizontal conductivity that combines the conductivity of both sand
and shale. These average readings are usually dominated by the
relatively higher conductivity of the shale layers and exhibit
reduced sensitivity to the lower conductivity sand layers where
hydrocarbon reserves are produced. To address this problem, loggers
have turned to using transverse induction logging tools having
magnetic transmitters and receivers (induction coils) oriented
transversely with respect to the tool longitudinal axis. Such
instruments for transverse induction well logging has been
described in PCT Patent publication WO 98/00733 of Beard et al. and
U.S. Pat. No. 5,452,761 to Beard et al.; U.S. Pat. No. 5,999,883 to
Gupta et al.; and U.S. Pat. No. 5,781,436 to Forgang et al.
[0010] In transverse induction logging tools, the response of
transversal coil arrays is also determined by an average
conductivity; however, the relatively lower conductivity of
hydrocarbon-bearing sand layers dominates in this estimation. In
general, the volume of shale/sand in the formation can be
determined from gamma-ray or nuclear well logging measurements.
Then a combination of the conventional induction logging tool with
transmitters and receivers oriented along the well axis and the
transversal induction logging tool can be used for determining the
conductivity of individual shale and sand layers.
[0011] One, if not the main, difficulties in interpreting the data
acquired by a transversal induction logging tool is associated with
vulnerability of its response to borehole conditions. Among these
conditions is the presence of a conductive well fluid as well as
wellbore fluid invasion effects
[0012] In induction logging instruments, the acquired data quality
depends on the formation electromagnetic parameter distribution
(conductivity or resistivity) in which the tool induction receivers
operate. Thus, in the ideal case, the logging tool measures
magnetic signals induced by eddy currents flowing in the formation.
Variations in the magnitude and phase of the eddy currents
occurring in response to variations in the formation conductivity
are reflected as respective variations in the output voltage of
receivers. In the conventional induction instruments these receiver
induction coil voltages are conditioned and then processed using
analog phase sensitive detectors or digitized by digital to analog
converters and then processed with signal processing algorithms.
The processing allows for determining both receiver voltage
amplitude and phase with respect to the induction transmitter
current or magnetic field waveform. It has been found convenient
for further uphole geophysical interpretation to deliver the
processed receiver signal as a vector combination of two voltage
components: one being in-phase with transmitter waveform and
another out-of-phase, quadrature component. Theoretically, the
in-phase coil voltage component amplitude is the more sensitive and
noise-free indicator of the formation conductivity.
[0013] There are a few hardware margins and software limitations
that impact a conventional transversal induction logging tool
performance and result in errors appearing in the acquired
data.
[0014] The general hardware problem is typically associated with an
unavoidable electrical field that is irradiated by the tool
induction transmitter simultaneously with the desirable magnetic
field, and it happens in agreement with Maxwell's equations for the
time varying field. The transmitter electrical field interacts with
remaining modules of the induction logging tool and with the
formation; however, this interaction does not produce any useful
information. Indeed, due to the always-existing possibility for
this field to be coupled directly into the receiver part of the
sensor section through parasitic displacement currents, it
introduces the noise. When this coupling occurs, the electrical
field develops undesirable electrical potentials at the input of
the receiver signal conditioning, primarily across the induction
coil receiver, and this voltage becomes an additive noise component
to the signal of interest introducing a systematic error to the
measurements.
[0015] The problem could become even more severe if the induction
logging tool operates in wells containing water-based fluids. The
water-based mud has a significantly higher electrical permittivity
compared to the air or to the oil-based fluid. In the same time,
the electrical impedance to the above mentioned displacement
currents can be always considered as capacitive coupling between
the source--the induction transmitter and the point of coupling.
This circumstance apparently would result in a fact that capacitive
coupling and associated systematic errors are environment dependant
because capacitive impedance will be converse to the well mud
permittivity.
[0016] The conventional method in reducing this capacitive coupling
in the induction logging instrument lays in using special
electrical (Faraday) shields wrapped around both transmitter and
receiver induction coils. These shields are electrically attached
to the transmitter analog ground common point to fix their own
electrical potential and to provide returns of the displacement
currents back to their source--transmitter instead of coupling to
any other place in the tool. However, geometry and layout
effectiveness of Faraday shields becomes marginal and contradictory
in the high frequency applications where conventional transverse
induction tools can operate. These limitations occur due to the
attenuation these shields introduce to the magnetic field known in
the art as a shield "skin effect". The shield design limitations
are unavoidable and, therefore, the possibility for the coupling
through displacement currents remains.
[0017] Another source of hardware errors introduced into the
acquired log data is associated electrical potential difference
between different tool conductive parts and, in particular, between
transmitter and receiver pressure housings if these modules are
spaced apart or galvanically separated. These housings cover
respective electronic modules and protect them from exposure to the
harsh well environment including high pressure and drilling fluids.
Typically, the pressure housing has a solid electrical connection
to the common point of the electronic module it covers, however,
design options with "galvanically" floating housings also exist. If
for some reasons, mainly imperfections in conventional induction
tools, the common points of different electronic modules have an
electrical potential difference between them, this difference will
appear on the pressure housings. It may occur even in a design with
"galvanically" floating housings if the instrument operates at the
high frequencies and, in particular, through the capacitive
coupling that these metal parts might have to the electronic
modules encapsulated in a conductive metallic package.
[0018] Having different electrical potentials on separate pressure
housings will force the electrical current to flow between them.
This current would have a conductive nature and high magnitude if
the induction tool is immersed in a conductive well fluid and it
will be a displacement current of typically much less magnitude for
tool operations in a less conductive or oil-based mud. In both
cases this current is time-varying; therefore it produces an
associated time varying magnetic field that is environmentally
dependent and measured by the induction receiver. For those who are
skilled in the art it should be understood that the undesirable
influence of those currents on the log data would be significantly
higher in the conventional transverse induction tool compared to
the instruments having induction coils coaxial with the tool
longitudinal axis only. In particular, this is due to the commonly
accepted overall design geometry of induction logging tools where
transmitter and receiver sections are axially separated by the
mandrel. It can be noticed that employing the induction tool in the
logging string where it has mechanical and electrical connections
(including telemetry) with instruments positioned both above and
below could also result in the appearance of the above-mentioned
currents.
[0019] Another source of the housings' potential offsets is the
induction tool transmitter itself. The remaining electrical field
that this transmitter irradiates simultaneously with a magnetic
field could be different on the surface of separate pressure
housings. Severity of this error also depends on Faraday shields'
imperfections as described earlier.
[0020] There is an additional problem that the potential difference
creates in conventional tool layouts having transmitter and
receiver electronic modules spaced apart and using interconnection
wires running throughout the sensor (mandrel) section. These wires
should be electrically and magnetically shielded from induction
receiver coils in the sensor section. The entire bundle of wires is
placed inside of a highly conductive metal shield that is
electrically connected to the common points of separated
transmitter and receiver electronic modules. This shield's
thickness is selected to enable sufficient suppression of mutual
crosstalk between wires and sensor section coils within the entire
operational frequency bandwidth and, primarily, at its lower end.
In some cases, this shield is a hollow copper pipe, often called as
a feed-though pipe, with a relatively thick wall.
[0021] However, besides protecting the sensor section transmitter
and receiver coils and interconnecting wires from mutual crosstalk,
this shield simultaneously creates a galvanic path for the currents
that could be driven by pressure housings and/or electronic
potential difference, or induced by the induction transmitter (as
discussed in U.S. Pat. No. 6,586,939 to Fanini et al, having the
same assignee as the present application and the contents of which
are incorporated herein by reference). This path apparently exists
along the shield's external surface and for a given frequency its
depth and impedance has been controlled by the shield geometry,
material conductivity and magnetic permeability. The time varying
currents also generate a respective magnetic field that crosses
induction receiver coils and induces error voltages. Unfortunately,
these error voltages are also environmentally dependent and their
changes cannot be sufficiently calibrated out during tool
manufacturing. The overall analysis of the potential difference
influence demonstrates that in the conductive well fluid, galvanic
currents flowing through the fluid along external surface of the
induction tool would dominate. The superposition and magnitude of
these galvanic currents strongly depend up on the ambient
temperature that pushes the conventional induction tool performance
to further deterioration.
[0022] Another source of systematic errors introduced in the log
data is directly determined by uncertainties in mechanical
dimensions of multi-component transmitter and receiver coils in the
sensor section related both to their overall dimensions and
positions with respect to each other. Thus, to keep required signal
phase relationships, conventional tool designs have primarily
relied on the mechanical stability and electrical properties of
advanced ceramics and plastic materials to build the mandrel.
However, even slight physical assembly deviations in the coil wires
position and non-uniform coil form material temperature
dependencies might destroy a factory pre-set compensation of the
transmitter primary magnetic field coupled in the receiver coil
(bucking) during well logging, and create non-recoverable errors
due to mechanical displacement or imperfections.
[0023] U.S. Pat. No. 6,734,675 and U.S. Pat. No. 6,586,939 to
Fanini et al., both having the same assignee as the present
application and the contents of which are incorporated herein by
reference, address some of the issues present in the calibration
and use of multicomponent induction logging tools. Fanini '939
discloses a transverse induction logging tool having a transmitter
and receiver for downhole sampling of formation properties, the
tool having a symmetrical shielded split-coil transmitter coil and
a bucking coil interposed between the split transmitter coils to
reduce coupling of the transmitter time varying magnetic field into
the receiver. The tool provides symmetrical shielding of the coils
and grounding at either the transmitter or receiver end only to
reduce coupling of induced currents into the received signal. The
tool provides an insulator between receiver electronics and the
conductive receiver housing having contact with conductive wellbore
fluid, to reduce parasitic current flowing in a loop formed by the
upper housing, feed through pipe, lower housing and wellbore fluid
adjacent the probe housing or mandrel. An internal verification
loop is provided to track changes in transmitter current in the
real and quadrature component of the received data signal.
[0024] Fanini '675 discloses a transverse induction logging tool
having a transmitter and receiver for downhole sampling of
formation properties, the tool having a symmetrical shielded
split-coil transmitter coil and a bucking coil interposed between
the split transmitter coils to reduce coupling of the transmitter
time varying magnetic field into the receiver. The tool provides
symmetrical shielding of the coils and grounding at either the
transmitter or receiver end only to reduce coupling of induced
currents into the received signal. The tool provides an insulator
between receiver electronics and the conductive receiver housing
having contact with conductive wellbore fluid, to reduce parasitic
current flowing in a loop formed by the upper housing, feed through
pipe, lower housing and wellbore fluid adjacent the probe housing
or mandrel. An internal verification loop is provided to track
changes in transmitter current in the real and quadrature component
of the received data signal
SUMMARY OF THE INVENTION
[0025] One embodiment of the invention is a method of preparing a
multicomponent induction tool having a plurality of transmitter
coils and a plurality of receiver coils. The method includes
positioning the logging tool in a calibration area substantially
free from components capable of interfering with magnetic and
electric fields produced by the tool. A first conductive housing of
the tool is coupled with a second conductive housing of the tool
through a borehole conductivity simulator having an impedance
similar to that of a borehole environment. A first coil of the
plurality of transmitter coils is activated and the signal in a
first coil of the plurality of receiver coils is measured. The
first coil of the plurality of transmitter coils is moved relative
to a conductive feed-through pipe between the first housing and a
second housing to reduce the magnitude of the signal. The first
coil of the plurality of receive accordance is moved relative to
the feed-through pipe until the magnitude of the signal is
substantially equal to zero. That method may further include
positioning the first coil of the plurality of receiver coils in an
eccentered position in the logging tool prior to making the
measurement. The method may further comprise orienting the logging
tool with its longitudinal axis substantially parallel to the
ground. The first coil of the plurality of transmitter coils may
have an axis that is substantially parallel to a longitudinal axis
of the tool or substantially orthogonal to a longitudinal axis of
the tool. The first coil for the plurality of receiver coils may
have an axis that is substantially parallel to a longitudinal axis
of the tool or substantially orthogonal to a longitudinal axis of
the tool. The method may further include rotating the tool about a
longitudinal axis of the tool, activating a second coil of the
plurality of transmitter coils and measuring an additional signal
in a second coil of the plurality of receiver coils, and moving the
second coil of the plurality of transmitter coils with respect to
the feed-through pipe to reduce the magnitude of the additional
signal. The method may further include magnetically coupling the
tool to a calibrator, activating the first coil of the plurality of
transmitter coils, and determining from a signal received at a
specific coil of the plurality of receiver coils a transfer
function between the specific coil and a first coil of the gravity
of transmitter coils. The tool may be positioned inside the
calibrator.
[0026] Another embodiment of the invention is an apparatus for
evaluating performance of the multicomponent induction logging tool
having a plurality of transmitter coils and a plurality of receiver
coils. The tool is positioned in a calibration area substantially
free from components capable of interfering with magnetic and
electric fields produced by the tool. The apparatus includes a
borehole conductivity simulator having an impedance similar to that
of a borehole environment, the borehole conductivity simulator
coupling a first housing of the tool with a second housing of the
tool. The apparatus includes a processor configured to activate a
first coil of the plurality of transmitter coils. The apparatus
further includes a first coil of the plurality of receiver coils
configured to provide a signal responsive to the activation of the
first transmitter coil. The apparatus includes a device configured
to move the first coil of the plurality of transmitter coils
relative to the first coil of the plurality of receiver coils to
reduce the magnitude of signal, and move the first coil of the
plurality of receiver coils relative to the conductive feed-through
pipe until the magnitude of the signal is substantially zero. The
device may further be configured to position the first coil of the
plurality of receiver coils in an eccentered position in the
logging tool. The first coil of the plurality of transmitter coils
may have an axis that is substantially parallel to the longitudinal
axis of the tool or substantially or organelle to a longitudinal
axis of the tool. The first coil of the plurality of receiver coils
may have an axis that is substantially parallel to a longitudinal
axis of the tool or substantially orthogonal to a longitudinal axis
of the tool. The apparatus may further include a calibrator wherein
the logging tool is magnetically coupled with a calibrator and the
processor is further configured to determine from the signal a
transfer function between the first coil of the plurality of
transmitters and the first coil of the plurality of receivers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] The present invention is best understood with reference to
the accompanying figures in which like numerals refer to like
elements and in which:
[0028] FIG. 1 (prior art) shows schematically a wellbore extending
into a laminated earth formation, into which wellbore an induction
logging tool as used according to the invention has been
lowered;
[0029] FIG. 2A (prior art) illustrates a conventional resistivity
measurement in the vertical direction;
[0030] FIG. 2B (prior art) illustrates a resistivity measurement in
the horizontal direction;
[0031] FIG. 3 is an overall flow chart of the procedures of the
present invention;
[0032] FIG. 4 illustrates a borehole conductivity simulator (BCS)
used in the present invention;
[0033] FIG. 5 illustrates an assembly for calibrating of transverse
arrays in a logging tool;
[0034] FIG. 6 illustrates an assembly for calibrating longitudinal
arrays in a logging tool;
[0035] FIGS. 7-8 illustrate assemblies for calibrating XY
cross-component arrays; and
[0036] FIGS. 9-10 illustrate assemblies for calibrating XZ
cross-component arrays.
DETAILED DESCRIPTION OF THE INVENTION
[0037] The instrument structure provided by the present invention
enables increased stability and accuracy in an induction wellbore
logging tool and its operational capabilities, which, in turn,
results in better quality and utility of wellbore data acquired
during logging. The features of the present invention are
applicable to improve the structure of a majority of known
induction tools.
[0038] The invention will now be described in more detail and by
way of example with reference to the accompanying drawings. FIG. 1
schematically shows a wellbore 1 extending into a laminated earth
formation, into which wellbore an induction logging tool as used
according to the present invention has been lowered. The wellbore
in FIG. 1 extends into an earth formation which includes a
hydrocarbon-bearing sand layer 3 located between an upper shale
layer 5 and a higher conductivity than the hydrocarbon bearing sand
layer 3. An induction logging tool 9 used in the practice of the
invention has been lowered into the wellbore 1 via a wire line 11
extending through a blowout preventor 13 (shown schematically)
located at the earth surface 15. The surface equipment 22 includes
an electric power supply to provide electric power to the set of
coils 18 and a signal processor to receive and process electric
signals from the receiver coils 19. Alternatively, the power supply
and/or signal processors are located in the logging tool.
[0039] The relative orientation of the wellbore 1 and the logging
tool 9 with respect to the layers 3, 5, 7 is determined by two
angles, one of which .theta. as shown in the FIG. 1. For
determination of these angles see, for example, U.S. Pat. No.
5,999,883 to Gupta, et al. The logging tool 9 is provided with a
set of transmitter coils 18 and a set of receiver coils 19, each
set of coils 18, 19 being connected to surface equipment 22 via
suitable conductors (not shown) extending along the wire line
11.
[0040] The relative orientation of the wellbore 1 and the logging
tool 9 with respect to the layers 3, 5, 7 is determined by two
angles, one of which .theta. as shown in the FIG. 1. For
determination of these angles see, for example, U.S. Pat. No.
5,999,883 to Gupta, et al. The logging tool 9 is provided with a
set of transmitter coils 18 and a set of receiver coils 19, each
set of coils 18, 19 being connected to surface equipment 22 via
suitable conductors (not shown) extending along the wire line
11.
[0041] Each set of coils 18 and 19 includes three coils (not
shown), which are arranged such that the set has three magnetic
dipole moments in mutually orthogonal directions, that is, in x, y
and z directions. The three-coil transmitter coil set transmits
magnetic fields T.sub.x, T.sub.y and T.sub.z; the receiver coils
measure induced signal from main directions R.sub.x, R.sub.y and
R.sub.z as well as the cross components, R.sub.xy, R.sub.xz and
R.sub.zy. Thus, coil set 18 has magnetic dipole moments 26a, 26b,
26c, and coil set 19 has magnetic dipole moments 28a, 28b, 28c. In
one embodiment the transmitter coil set 18 is electrically isolated
from the receiver coil set 19. The coils with magnetic dipole
moments 26a and 28a are transverse coils, that is they are oriented
so that the magnetic dipole moments are oriented perpendicular to
the wellbore axis, whereby the direction of magnetic dipole moment
28a is opposite to the direction of magnetic dipole moment 26a.
Furthermore the sets of coils 18 and 19 are positioned
substantially along the longitudinal axis of the logging tool
9.
[0042] As shown in FIG. 2A, conventional induction logging tools
provide a single transmitter and receiver coil that measure
resistivity in the horizontal direction. In the mode shown in FIG.
2A, the resistivities of adjacent high resistivity sand and low
resistivity shale layers appear in parallel, thus the resistivity
measurement is dominated by low resistivity shale. As shown in
FIGS. 1 and 2B, in the present invention a transverse coil is added
to measure resistivity in the vertical direction. In the vertical
direction, the resistivity of the highly resistive sand and low
resistivity shale are appear in series and thus the vertical series
resistivity measurement is dominated by the resistivity of the
highly resistive sand.
[0043] For ease of reference, normal operation of the tool 9, as
shown in FIGS. 1 and 2B, will be described hereinafter only for the
coils having dipole moments in the x-direction, i.e. dipole moments
26a and 28a. During normal operation an alternating current of a
frequency f.sub.1 has been driven by the tool electronics (not
shown) connected to the coil 26 which, in turn, is supplied by the
electric power supply of surface equipment 22 to transmitter coil
set 18 so that a magnetic field with magnetic dipole moment 26a is
induced in the formation. In an alternative embodiment, the
frequency is swept through a range f.sub.1 through f.sub.2. This
magnetic field extends into the sand layer 3 and induces a number
of local eddy currents in the sand layer 3. The magnitude of the
local eddy currents is dependent upon their location relative to
the transmitter coil set 18, the conductivity of the earth
formation at each location, and the frequency at which the
transmitter coil set 18 is operating. In principle, the local eddy
currents act as a source inducing new currents, which again induce
further new currents, and so on. The currents induced into the sand
layer 3 induces a response magnetic field in the formation, which
is not in phase with the transmitted magnetic field, but which
induces a response signal in receiver coil set 19. The magnitude of
the current induced in the sand layer 3 depends on the conductivity
of the sand layer 3, the magnitude of the response current in
receiver coil set 19. The magnitude also depends on the
conductivity and thereby provides an indication of the conductivity
of the sand layer 3. However, the magnetic field generated by
transmitter coil set 18 not only extends into sand layer 3, but
also in the wellbore fluid and in the shale layers 5 and 7 so that
currents in the wellbore fluid and the shale layers 5 and 7 are
induced.
[0044] The overall procedures of the present invention used to
ensure proper functioning of a deployed multicomponent induction
logging tool is summarized in FIG. 3. Calibration of the
instrument's arrays is done, particularly estimating its transfer
coefficient 101. Subsequently, a final verification of the tuning
and calibration consistency is performed 103. This is followed by a
verification of isolator sufficiency 105 for preventing an axial
current flow between the tool's top and bottom housings/electronics
through the feed-through pipe and conductors while logging in the
boreholes filled with conductive mud.
[0045] In further detail, the fully made tool is placed in
calibration area which has a small number of external parts that
could interfere with magnetic and electric fields produced or
received by the tool and thus affect tool readings (machinery,
measurement tools, etc.). For example, positioning the tool at
approximately 15 ft (4.6 m) above the ground typically reduces the
tool environmental reading to a value less than about 10 mS/m. The
tool is positioned parallel to the Earth with the array to be
adjusted pointing normal to the ground.
[0046] FIG. 4 illustrates the BCS, comprising an assembly of
conductor 401 and resistor 410, which electrically couples top
housing 405 and bottom housing 404. A closed circuit is thus
created from bottom housing 404 through resistor 410 through top
housing 405 through a feed-through pipe running from bottom housing
to top housing through mandrel 408. The value of resistor 410 can
be configured to be approximately equal to a total conductivity (or
resistivity) value between top and bottom housings which the tool
would experience inside a borehole according to its specifications.
A resistance value of approximately 20 Ohms is typically
chosen.
[0047] In this arrangement the tool becomes very sensitive to the
axial current that could be induced by the array transmitter in the
following loop: "top housing--conductive feed-through pipe--bottom
housing--BCS". The magnitude of the current will be proportional to
the array coils displacement from their longitudinal alignment
(almost true for small displacements .about.1/d) and simulator
resistor value.
[0048] To balance the array its transmitter coil may be moved in
the plane parallel to the ground. This coil movement is performed
until an absolute minimum in the receiver reading is reached. In
one embodiment of the invention, the receiver coil is positioned
off-center relative to the tool. At this position, the receiver
signal is particularly sensitive to misalignment of the
transmitter. This makes it easier to determine the minimum. Upon
adjustment the transmitter coil frame is fixed inside the mandrel.
This could be accomplished with the sets of non-conductive screws
and/or with epoxy; however, alternative means could be applied, as
well. Shorting the isolator between the upper housing and the
mandrel is done to significantly increase the magnitude of the
axial current in this test procedure and, therefore, increase
accuracy of balancing. A similar positioning may be done in the
vertical direction. As discussed below, the tool is more sensitive
to mis-positioning in the vertical direction than in the horizontal
direction. Suitable positioning screws may be provided in the
logging tool to accomplish this movement.
[0049] Following the positioning of the transmitter coil, the
receiver coil is moved to a position where the signal and the
receiver coil is zero. When this is done, the particular
transmitter and receiver are properly balanced. The description
above has been made with respect to movement of the coils relative
to each other. It is to be understood that when these movements are
made, the coils are also being moved relative to the feed-through
pipe.
[0050] After the first horizontal array has been tuned the tool is
rotated about its axis and similar procedure has been performed
with next horizontal array. Generally, the instrument might have a
plurality of transverse and tilted arrays so that similar tuning
could be developed for each sensor. After balance of all arrays has
been completed, the tool isolation short is removed and mandrel is
covered with the non-conductive pressure sleeve.
[0051] Calibration of transfer coefficient is done after the
instrument is positioned in the low conductive calibration
environment and inserted inside the calibrator. The calibration
principle lies in introducing a certain dissipative load through
magnetic coupling for calibrating array so that its signal readings
are identical to the values to be read while logging a homogeneous
formation with finite conductivity. This is done with use of a
calibrator whose electromagnetic parameters and coupling with the
tool are precisely known. Using the calibrator, tool loading is
achieved by the connecting certain impedance to the terminal of
normally-open calibrator loop. Thus, the open loop presents an
infinitely resistive formation. Conversely, by shorting, almost
infinitely conductive formation is presented. Therefore, any value
of the formation conductivity corresponds to its unique value of
the calibration loop load.
[0052] Acquiring the calibration signal is typically done in the
mode "calibration load connected-disconnected". This difference in
the tool reading indicates on how much the tool output voltage
swings when the formation conductivity changes from 0 to the
calibrated value. To perform calibration the tool array may be
oriented normal to the ground as this leads to more consistency in
measurements and apparently make its transversal arrays less
sensitive to any residual noise currents that maybe circulating on
the Earth surface in place of measurement (machinery,
radio-stations, etc.).
[0053] After the tool transfer coefficient has been determined, the
tool readings while the calibrator loop is not loaded reflect
environmental conductivity and, in particular, ground conductivity.
This data has to be known and stored for further processing.
[0054] The last step in calibration is verification of the tool
symmetry and immunity to axial currents. The overall tool symmetry
assumes that the same array reads the same values of the "ground"
or environmental conductivity while its measurement direction
points to ground or from the ground. For these purposes the tool is
rotated around its longitudinal axis on 180.degree.. Absence of
such a "direction sensitivity" would indicate normal tool
functioning and ensure respective symmetry while operating in the
well bore.
[0055] For verification of the suppressing axial currents--a
modified BCS test may be run with the short removed in the
feed-through. Thus, connecting and disconnecting the BCS to the
tool should result in absolute minimal difference in readings that
would indicate for proper operation in the well without
formation-dependable offset in the tool data. This modified BCS
test could be run as described, or, to reduce calibration time,
performed right after the transfer coefficient is determined.
[0056] Turning now to FIG. 5, one arrangement of the alignment loop
is discussed. Shown therein is an alignment loop 501 surrounding an
array characterized by the transmitter coil 504 directed along an X
direction (T.sub.x) and the receiver coil 508 directed along the X
direction (R.sub.x). Bucking coil B.sub.x 506 is also shown. This
array is denoted as XX, using a nomenclature in which the first
letter signifies the orientation direction of the transmitter coil
and the last letter signifies the orientation direction of the
receiver coil. This nomenclature is generally used herein. The XX
and YY arrays in the multi-component tool are ideally aligned at
90.degree. from each other. When this alignment is not met, the
response of the cross components (XY, YX) are affected by part of
the reading of the related main component. The alignment measuring
method of the present invention is based on analyzing the output of
the cross-component system when the tool is rotated inside of an
alignment loop.
[0057] The alignment loop 501 is a stationary loop, lying so that
the longitudinal axis of the loop and the longitudinal axis of the
well-logging tool are substantially aligned. Its dimensions are
such as to obtain substantial inductive coupling with the
transmitter as well as with the receiver of both XX and YY arrays.
The long "box" calibrator of FIG. 4 is used to performed
calibration of the horizontal arrays. A detailed analysis of the
signals is given later in this document.
[0058] FIG. 6 illustrates a loop alignment assembly usable for
aligning ZZ arrays in a testing device. Transmitter TZ 601, bucking
coil BZ 603 and receiver RZ 605 are disposed along the feed-through
pipe 615 and have a common longitudinal axis. Alignment loop 610 is
substantially coaxial with receiver RZ 605 and substantially
centered on RZ.
[0059] Cross component array calibration is discussed next. FIG. 7
illustrates an embodiment for calibration of an XY array using a
calibration box. Transmitter 701 and bucking coil 703 are disposed
along the feed-through pipe oriented to produce a magnetic moment
in an X-direction. Receiver 705 is disposed along the same
feed-through pipe having an orientation so as to receive components
of a magnetic moment in a is disposed along the same feed-through
pipe having an orientation so as to receive components of a
magnetic moment in Y-direction. The alignment box 710 is disposed
at an angle of 45.degree. so as to be oriented halfway between the
X-direction and the Y-direction.
[0060] FIG. 8 illustrated an alternate embodiment for aligning an
XY array. Alignment box 815 is located at the TX 801, and alignment
box 810 is positioned at the RXY cross-component receiver 805. Both
alignment boxes are oriented along the same direction as their
respective transmitter/receiver. A wire 820 electrically couples
alignment box 810 and alignment box 815. (in this configuration box
815 receives signal from transmitter coil, the voltage induced
across its winding produces current flowing through winding of both
boxes and load impedance. While going though winding of 810 this
current generates filed that is picked up by cross-component
receiver)
[0061] FIG. 9 illustrates an assembly for orienting of the XZ
cross-component array. Transmitter TX 901 and bucking coil BX 903
are disposed along the feed-through pipe oriented so as to produce
a magnetic moment along an X-direction. The receiver RZ 905 is
disposed along the feed-through pipe and oriented so as to be
receptive to Z-components of magnetic moments. The alignment box
920 can be positioned centrally between main X-transmitter 901 and
Z-cross-component receiver 905 and tilted 45.degree. with respect
to the tool longitudinal axis 910. The assembly of FIG. 8 displays
small signals during XZ array calibration. This signal tends to
display a high sensitivity to the angle.
[0062] FIG. 10 illustrates an alternate embodiment for aligning the
XZ cross-component array. Transmitter TX 1001 and bucking coil BX
1003 are disposed along the feed-through pipe oriented so as to
produce a magnetic moment along an X-direction. The receiver RZ
1005 is disposed along the feed-through pipe and oriented so as to
be receptive to Z-components of magnetic moments. Alignment box
1010 is centered on transmitter TX 1001, and alignment loop 1015 is
coaxial with receiver RZ 1005. A wire 1020 electrically couples
alignment box 1010 and alignment loop 1015. In contrast to the
assembly of FIG. 0, calibration using two alignment devices
displays a large signal for the XZ array calibration.
[0063] We next discuss in detail the use of the alignment box for
establishing the coil orientation. When examining a cross-component
array, the XY or YX response obtained by rotating the tool inside
of the alignment loop has a zero-crossing each time that either a
transmitter or a receiver coil is perpendicular to the plane of the
loop. Whichever coil (transmitter or receiver) is substantially
aligned with the loop (enclosed in the same plane) experiences a
maximum coupling with the alignment loop. When the position of the
aligned coil is varied around the point of alignment with the
alignment loop, the coupling response between them undergoes a slow
change corresponding to the variation. The non-aligned coil
experiences a minimum coupling with the alignment loop. When the
position of the non-aligned coil is varied around this point of
minimal coupling, the coupling experiences an abrupt change. The
coupling becomes zero when the non-aligned coil achieves
perpendicularity with the alignment loop. A practitioner in the art
would recognize that the zero-crossings of the coupling response
are significantly affected by the coil that is at right angle to
the alignment loop, regardless of whether the perpendicular coil is
a receiver or a transmitter. The substantially aligned coil plays
little or no role in the production of a zero-crossing. The angle
between successive zero crossings thereby represents an alignment
angle between the two related coils.
[0064] Mathematically, the inductive coupling between two coils
resembles a cosine function of the angle between them. Thus, the
coupling response system of coils made by an aligned system of
cross components and an alignment loop is given by the following
expression:
R ( .phi. ) = K cos ( .phi. ) cos ( .phi. - .pi. 2 ) . ( 1 )
##EQU00001##
Applying trigonometric identities, Eq. (1) can be simplified to
R(.phi.)=Kcos(.phi.)sin(.phi.), (2).
and since
sin ( .phi. ) cos ( .phi. ) = 1 2 sin ( 2 .phi. ) , ( 3 )
##EQU00002##
it follows that
R ( .phi. ) = K 1 2 sin ( 2 .phi. ) . ( 4 ) ##EQU00003##
Eqn. (4) illustrates that there are two cycles of variation for
each cycle of tool rotation.
[0065] By considering a misalignment angle .beta. between
transmitter and receiver, the response function can now be
expressed as
R ( .phi. , .beta. ) = K cos ( .phi. ) cos ( .phi. - .pi. 2 +
.beta. ) , ( 5 ) ##EQU00004##
where each cosine function characterizes the response of the
individual cross component coils. It is easy to see that
R(.phi.,.beta.)=0 (6),
when
.phi. = n .pi. 2 or when .phi. - .pi. 2 + .beta. = n .pi. 2 with n
= .+-. 1 , 2 , 3 , Eq . ( 7 ) ##EQU00005##
According to eqn. (7), the angle between successive zero-crossings
represents the alignment angle among the cross component coils. An
intuitive graphical approach can therefore be used to measure the
misalignment angle between transmitter and receiver.
[0066] Alternatively, the misalignment angle can be obtained simply
by using a trigonometric regression function to analyze the
response of the system. Applying trigonometric identities to Eqn.
(5), the response of the misaligned system can be written as
R ( .phi. , .beta. ) = K cos ( .phi. ) sin ( .phi. ) cos ( .beta. )
+ K cos 2 ( .phi. ) sin ( .beta. ) R ( .phi. , .beta. ) = K 1 2 sin
( 2 .phi. ) cos ( .beta. ) + K cos 2 ( .phi. ) sin ( .beta. ) R (
.phi. , .beta. ) = K 2 sin ( 2 .phi. + .beta. ) + K 2 sin ( .beta.
) ( 8 ) ##EQU00006##
The last expression in eqn. (8) indicates that a graphical
representation of the coupling response of the misaligned cross
component system resembles a sinusoidal function. The period of
this sinusoid equals 180.degree. and has offsets on both the
abscissa and the ordinate. The offset on the abscissa is .beta.,
and the offset on the ordinate is (K/2)sin(.beta.). Also, the
coupling response is of the form A sin(x+B)+C, where A=K/2,
B=.beta. and C=(K/2)(sin(.beta.). The coefficient .beta. obtained
with such fitting represents the misalignment angle. The cross
component response can thus be fit to this curve.
[0067] The sensitivity to possible displacement along the tool's
longitudinal axis or vertically can be analyzed by changes in the
product M=M.sub.T-CM.sub.C-R, where M.sub.T-C is the mutual
inductance between the transmitter and the alignment coils, and
M.sub.R-C is the mutual inductance between the alignment and the
receiver coils. Table 1 illustrates mutual inductances that result
from misalignment or displacement of an alignment coil in the
horizontal direction (longitudinally). There is in general a
flexibility of 1'' without substantially affecting the induction
response.
TABLE-US-00001 TABLE 1 Calibrator Displacement M.sub.T-C M.sub.C-R
M [inch] [microHenry] [microHenry] [.mu.H].sup.2 4 13.348 16.700
222.912 2 13.409 13.580 182.094 1 13.443 13.521 181.763 3/4 13.452
13.510 181.736 1/2 13.461 13.499 181.710 0 13.479 13.479 181.683
-1/2 13.499 13.461 181.710 -3/4 13.510 13.452 181.736 -1 13.521
13.443 181.763 -2 13.580 13.409 182.094 -4 16.700 13.348
222.912
Table 2 shows the effects of misalignment in the vertical
direction. A misalignment exceeding 5/16'' produces an error
greater than 0.22%. Thus vertical misalignment has a greater effect
on induction response than horizontal misalignment.
TABLE-US-00002 TABLE 2 Calibrator Displacement M.sub.T-C M.sub.C-R
M [inch] [microHenry] [microHenry] [.mu.H].sup.2 % 0 13.479 13.479
181.683 0 3/16 13.474 13.474 181.549 0.074 5/16 13.464 13.464
181.279 0.22 7/16 13.449 13.449 180.876 0.44
[0068] To properly position the arrays, the transmitter coil of one
array is moved in the direction normal to the ground. This coil
movement is performed until an absolute minimum in the coupling
response is determined. Upon adjustment, the transmitter coil frame
is fixed inside the mandrel. After the first horizontal array has
been tuned, the tool is rotated on its axis and a similar procedure
is performed with the other horizontal array. Generally, similar
tuning can be developed for an instrument having a plurality of
transverse and tilted arrays. After balance of all arrays has been
achieved, the tool isolation short is removed and mandrel is
covered with the non-conductive pressure sleeve to protect
induction coils from being directly exposed to borehole fluids.
[0069] A final verification of the coil balancing and calibration
consistency is made. Calibration of a transfer coefficient is
performed once the instrument is inserted inside the calibrator in
the low conductive calibration environment. A magnetic load is
introduced suitable for calibrating array, so that its signal
readings are identical to the values to be read while logging a
homogeneous formation. The magnetic load is introduced using the
above-referenced calibrator using known electromagnetic parameters
and coupling parameters. The tool loading can be achieved by
connecting selected impedance to the terminal of a normally-open
calibrator loop. Thus, the open loop represents an infinitely
resistive formation. Once shorted, the closed loop represents an
almost infinitely conductive formation (limited only by internal
impedance of the wires of the calibrator loop). Therefore, a
calibration loop load can be chosen effectively representing a
given formation conductivity values.
[0070] Implicit in the control and processing of the data is the
use of a computer program on a suitable machine readable medium
that enables the processor to perform the control and processing.
The machine readable medium may include ROMs, EPROMs, EEPROMs,
Flash Memories and Optical disks.
[0071] While the foregoing disclosure is directed to the preferred
embodiments of the invention, various modifications will be
apparent to those skilled in the art. It is intended that all
variations within the scope and spirit of the appended claims be
embraced by the foregoing disclosure.
* * * * *