Note: Descriptions are shown in the official language in which they were submitted.
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AN APPARATUS AND METHOD FOR WELLBORE RESISTIVITI'
DETERMINATION AND IMAGING USING CAPACITIVE COUPLING
Martin Townley Evans, Andrew Richard Burt, Albert Alexy, Leonty Abraham
Tabarovsky
BACKGROUND OF THE INVENTION
1. Field of the Invention
j0001] This invention generally relates to explorations for hydrocarbons
involving
electrical investigations of a borehole penetrating an earth formation. More
specifically, this invention relates to highly localized borehole
investigations
employing the introduction and measuring of individual survey currents
injected into
the wall of a borehole by capacitive coupling of electrodes on a tool moved
along the
borehole within the earth formation.
2. Background of the Art
[0002] Electrical earth borehole logging is well known and various devices and
various techniques have been described for this purpose. Broadly speaking,
there are
two categories of devices used in electrical logging devices. In the first
category, a
measure electrode (current source or sink) id used in conjunction with a
diffuse return
electrode (such as the tool body). A measure current flows in a circuit that
connects a
current source to the measure electrode, through the earth formation to the
return
electrode and back to the current source in the tool. In inductive measuring
tools, an
antenna within the measuring instrument induces a current flow within the
earth
formation. The magnitude of the induced current is detected using either the.
same
antenna or a separate receiver antenna. The present invention belongs to the
first
category.
[0003] There are several modes of operation that may be used: in one, the
current at
the measuring electrode is maintained constant and a voltage is measured while
in the
second mode, the voltage of the electrode is fixed and the current flowing
from the
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electrode is measured. Ideally, it is desirable that if the current is varied
to maintain
constant the voltage measured at a monitor electrode, the current is inversely
proportional to the resistivity of the earth formation being investigated.
Conversely, it
is desirable that if this current is maintained constant, the voltage measured
at a
monitor electrode is proportional to the resistivity of the earth formation
being
investigated. Ohm's law teaches that if both current and voltage vary, the
resistivity
of the earth formation is proportional to the ratio of the voltage to the
current.
[0004] Birdwell (US Patent 3,365,658) teaches the use of a focused electrode
for
determination of the resistivity of subsurface formations. A survey current is
emitted
from a central survey electrode into adjacent earth formations. This survey
current is
focused into a relatively narrow beam of current outwardly from the borehole
by use
of a focusing current emitted from nearby focusing electrodes located adjacent
the
survey electrode and on either side thereof. Ajam et al (US Patent 4,122,387)
discloses an apparatus wherein simultaneous logs may be made at different
lateral
distances through a formation from a borehole by guard electrode systems
located on
a sonde which is lowered into the borehole by a logging cable. A single
oscillator
controls the frequency of two formation currents flowing through the formation
at the
desired different lateral depths from the borehole. The armor of the logging
cable
acts as the current return for one of the guard electrode systems, and a cable
electrode
in a cable electrode assembly immediately above the logging sonde acts as the
current
return for the second guard electrode system. Two embodiments are also
disclosed
for measuring reference voltages between electrodes in the cable electrode
assembly
and the guard electrode systems
[0005] Techniques for investigating the earth formation with arrays of
measuring
electrodes have been proposed. See, for example, the U.S. Pat. No. 2,930,969
to
Baker, Canadian Pat. No. 685,727 to Mann et al. U.S. Patent No. 4,468,623 to
Giahzero, and U.S. Patent No. 5,502,686 to Dory et al.. The Baker patent
proposed
a plurality of electrodes, each of which was formed of buttons which are
electrically
joined by flexible wires with buttons and wires embedded in the surface of a
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collapsible tube. The Mann patent proposes an array of small electrode buttons
either
mounted on a tool or a pad and each of which introduces in sequence a
separately
measurable survey current for an electrical investigation of the earth
formation. The
electrode buttons are placed in a horizontal plane with circumferential
spacings
between electrodes and a device for sequentially exciting and measuring a
survey
current from the electrodes is described.
[0006] The Gianzero patent discloses tool mounted pads, each with a plurality
of
small measure electrodes from which individually measurable survey currents
are
injected toward the wall of the borehole. The measure electrodes are arranged
in an
array in which the measure electrodes are so placed at intervals along at
least a
circumferential direction (about the borehole axis) as to inject survey
currents into the
borehole wall segments which overlap with each other to a predetermined extent
as
the tool is moved along the borehole. The measure electrodes are made small to
enable a detailed electrical investigation over a circumferentially contiguous
segment
of the borehole so as to obtain indications of the stratigraphy of the
formation near the
borehole wall as well as fractures and their orientations. In one technique, a
spatially
closed loop array of measure electrodes is provided around a central electrode
with
the array used to detect the spatial pattern of electrical energy injected by
the central
electrode. In another embodiment, a linear array of measure electrodes is
provided to
inj ect a flow of current into the formation over a circumferentially
effectively
contiguous segment of the borehole. Discrete portions of the flow of current
are
separably measurable so as to obtain a plurality of survey signals
representative of the
current density from the array and from which a detailed electrical picture of
a
cixcumferentially continuous segment of the borehole wall can be derived as
the tool
is moved along the borehole. In another form of an array of measure
electrodes, they
are arranged in a closed loop, such as a circle, to enable direct measurements
of
orientations of resistivity of anomalies
[0007] The Dory patent discloses the use of an acoustic sensor in combination
with
pad mounted electrodes, the use of the acoustic sensors making it possible to
fill in
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the gaps in the image obtained by using pad mounted electrodes due to the fact
that in
large diameter boreholes, the pads will necessarily not provide a complete
coverage
of the borehole.
[0008] The prior art devices, being contact devices, are sensitive to the
effects of
borehole rugosity: the currents flowing from the electrodes depend upon good
contact
between the electrode and the borehole wall. If the borehole wall is
irregular, the
contact and the current from the electrodes is irregular, resulting in
inaccurate
imaging of the borehole. A second drawback is the relatively shallow depth of
investigation caused by the use of measure electrodes at the same potential as
the pad
and the resulting divergence of the measure currents.
[0010] Yet another drawback with the use of contact devices injecting
electrical
currents into a wellbore arises when oil-based muds are used in drilling. Oil-
based
muds must be used when drilling through water soluble formations. An
increasing
number of present day exploration prospects lie beneath salt layers. Besides
reducing
the electrical contact between the logging tool and the formation, invasion of
porous
formations by a resistive, oil-based mud greatly reduces the effectiveness of
prior art
resistiviiy imaging devices and conduction-based devices for determination of
formation resistivities. This problem is not alleviated by the use of focusing
electrodes.
[0011] Tt would be desirable to have an apparatus and method of determination
of
formation resistivity that is relatively insensitive to borehole rugosity and
can be used
with either water based or with oil-based muds. The present invention
satisfies this
need.
SUMMARY OF THE INVENTION
[0012] The present invention is an apparatus and method for use in a borehole
having
a substantially nonconducting fluid therein for obtaining a resistivity
parameter of an
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earth formation penetrated by the borehole. At least one measure electrode is
capacitively coupled to the earth formation through the nonconducting fluid. A
measure current is conveyed into the formation, and by measurements of the
current
in the electrode and its potential, the resistivity may be determined. A
plurality of
measure electrodes may be used. With an array of measure electrodes, a
resistivity
image of the formation may be obtained.
j0013] In one embodiment of the invention, the measure current is a modulated
electrical current having a carrier frequency selected to have a low impedance
due to
the capacitive coupling. An isolator is provided on the logging tool to
minimize the
cross-talk between the current from a current source and the measure signals.
Focusing and guard electrodes may be used with the measure electrodes. The
logging
tool may be conveyed on a wireline or form part of a bottom hole assembly
conveyed
on a drilling tubular.
[0014] In an MWD embodiment of the invention, numerous options are available
for
the disposition of the measure electrodes. They may be on a stabilizer, a non-
rotating
sleeve, or on a pad. An extension device may be provided to maintain the
measure
electrode at a specified distance from the borehole wall. When direction
sensors are
provided, a downhole processor can provide resistivity images without the
necessity
of having arrays of electrodes with a large number of electrodes.
[0015] In an alternate embodiment of the invention, a modification is made to
enable
the device to be used with water based muds. The measure electrode is
capacitively
coupled to the source of modulated electrical current.
(0016] The invention also makes a provision for making multifrequency
measurements. When such multifrequency measurements are made, frequency
focusing may be used to determine formation resistivity.
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BRIEF DESCRIPTION OF THE FIGURES
[0017] The present invention is best understood by reference to the following
figures
wherein like numbers refer to like components, and wherein:
Fig. l is a circuit diagram representing a formation resistivity device
according to the
present invention.
Fig. 2 shows a comparison of signals representative of the measure current and
the
voltage for the circuit of Fig. 1 for a 1 kHz sinusoidal excitation signal.
Fig. 3 shows a comparison of signals representative of the measure current and
the
voltage for the circuit of Fig. 1 for a 10 kHz sinusoidal excitation signal.
Fig. 4 shows a comparison of signals representative of the measure current and
the
voltage for the circuit of Fig. 1 for a 10 kHz square wave excitation.
Fig. 5 (Prior Art) shows a schematic illustration of a prior art imaging tool
in a
borehole.
Fig. 6 illustrates a model used for deriving the impedance of an imaging tool.
Figs. 7a-7f illustrate the impedance of a measure electrode at a frequency of
1 kHz.
Figs. 8a-8f illustrate the impedance of a measure electrode at a frequency of
10 kHz.
Fig. 9 shows the imaging tool of this invention suspended in a borehole.
Fig.10 is a mechanical schematic view of the imaging tool.
Fig. 10A is a detail view of an electrode pad.
Fig. 1l is a schematic circuit diagram showing the principles of operation of
the tool.
Figs.12a and 12b shows a comparison between a prior art modulated signal and a
reverse modulated signal according to the present invention.
Fig.13 is a schematic circuit diagram of the tool when used with a conducting
borehole fluid.
Fig.14 illustrates an alternate embodiment of an electrode pad.
Fig.15 (Prior art) is a schematic illustration of a drilling system.
Fig.16 is a schematic illustration of the invention in which resistivity
measurements
are made at various azimuths
Fig.17 illustrates the pads on a non-rotating sleeve used for resistivity
measurements.
DETAILED DESCRIPTION OF THE INVENTION
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[0018]In order to gain a proper understanding of the present invention,
reference is
made to Figs 1-17. The description of the present invention starts first with
a
discussion of the concepts of capa~itive coupling used in the present
invention. A
multifrequency tool is then discussed, followed by a discussion of embodiments
of
the invention in which resistivity images of the borehole may be obtained.
[0019] Fig. 1 is a circuit diagram illustrating the methodology of formation
resistivity measuring devices. A measure electrode depicted by 3 injects a
measure
current into a formation denoted by 7 having a resistivity Rt. This current is
supplied
by a current source 1. The current from the formation returns (not shown)
through a
return electrode (ground) denoted by 7. Typically, a voltage drop 11 across a
resistor
10 in the circuit is used as an indication of the measure current. By
measuring the
voltage drop 13 between the measure electrode and the return electrode,
information
is derived about the impedance encountered by the current between the measure
electrode and the ground.
[0020] This impedance, as noted above, includes the desired formation
resistivity Rt.
In addition, there is also an impedance 5 between the measure electrode 3 and
the
formation 7. In water based (conductive) muds, this impedance is almost
entirely
resistive and is caused by the mud cake and any invasion of the borehole fluid
into the
formation. However, in oil-based (non conductive) muds, the impedance between
the
measure electrode 3 and the formation 7 is primarily capacitive, denoted by a
capacitance M~ . This capacitance manifests itself in a phase shift between
the
measure current signal and the voltage drop from the measure electrode to
ground.
This is seen in Fig. 2 which shows a phase shift between the signals 11' and
13' for a
sinusoidal current of 1 kHz. This frequency is typical of prior art formation
resistivity
measurement devices. The curves in Fig. 2 are normalized independently to
emphasize the phase shift: in reality, there could be differences of several
orders of
magnitude between the two signals.
'30
[0021] Turning riow to Fig. 3, the signals 11" and 13" for a sinusoidal
current of 10
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kHz are shown. The phase shift between the two signals is seen to be much
smaller.
This is due to the fact that at the higher frequency of 10 kHz, the effect of
the
capacitance is less than at 1 kHz. This suggests that by using higher
frequencies, it
would be possible to get signals indicative of the formation resistivity. This
is
confirmed in Fig. 4 which shows the signals 11"' and 13"' for a square wave
excitation at 10 kHz. As can be seen, both the signals rise and fall almost
instantaneously: this is due to the fact that a square wave contains a lot of
high
frequencies that are essentially unimpeded by the capacitance of the mud. The
use of
higher frequencies forms the basis for the pxesent invention as described
next.
[0022] Fig. 5 is a schematic illustration of a portion of a prior art imaging
tool
suitable for use with the method of the present invention. Shown is a borehole
51 that
is filled with a borehole fluid (drilling mud). A mud-cake 53 is formed
between the
borehole fluid and the formation 55. The tool comprises one or more measure
electrodes 59 carried on a conducting pad 57. In the illustration, only two
electrodes
are shown. As discussed below, the actual number of electrodes may be much
larger
and they may be arranged in an array. The electrodes 59 are separated from
each
other by insulator 61. For simplifying the illustration, additional insulation
between
the electrodes 59 and the pad 51 is not shown.
[0023] In prior art imaging tools, the pad functions as a guard electrode and
is
maintained at a potential related to the potential of the measure electrodes.
As would
be known to those versed in the art, due to the presence of the guard
electrode and the
current flowing into the formation therefrom, the current from the measure
electrodes
flows in current paths such as that shown by I and is prevented from diverging
due to
the focusing current F from the guard electrode. Optionally, additional
focusing
electrodes may be used (not shown). Focusing electrodes are known in the prior
art
but a specific embodiment using focused electrodes for a resistivity imaging
tool is
discussed below. The current flowing from the measure electrode is related to
the
potential V and the impedance of the electrical circuit in which the measure
currents
flow.
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[0024] When a device such as that shown in Fig. 5 is used with a water-based
drilling
mud, the impedance of the mud and the mudcake is relatively small compared to
the
impedance of the formation. As would be known to those versed in the art, at
the
frequencies used in prior art devices, the formation impedance is primarily
resistive
and from a knowledge ~of the potential V and the measure current I, the
formation
resistivity can be derived.
[0025] On the other hand, in oil-base mud, the measured impedance of
individual
measure electrodes severely depends on the mud cake parameters. In addition,
an oil
film on the pad surface may completely eliminate the electrical contact
between pad
and formation.
[0026] The size of a measure electrode is associated with the tool spatial
resolution.
Usually, the measure electrode radius is in the range of 1 to 2 mm that
creates a very
large ground resistance. For example, a 2 mm measure electrode on a typical
pad
device has the ground resistance of 10,000 S2 in a 1 S2-m formation or 10 M SZ
in a
1,000 S2-m formation. This illustrates the technical challenge of producing a
high
definition image in a resistive environment
[0027] There are several possible ways to overcome the physical limitation of
DC
imaging in oil-base mud. One approach that has been used is to change
composition
of oil-base mud to increase the mud calve conductivity. The present invention
relies
on increasing the frequency to produce capacitive coupling between pad and
formation.
[0028] Turning now to Fig. 6, the impedance of the measure electrode is
derived. We
consider a model consisting of two conductive layers 103,105 enclosed between
.
insulating half space at the top 101 and a perfect conductor at the bottom
107. From
the upper boundary, a uniform current is injected with the surface density,
Js. A
measure electrode of any shape may be studied by cutting out an appropriate
area 109
from the injection plan. The upper half space 101 represents a borehole filled
with
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oil-base mud. The conductor 107 at the bottom is a current sink. In reality,
at a
certain distance, depending on the focusing conditions, current lines diverge.
This
provides a finite value for the measure electrode's K-factor. To simplify
modeling,
we introduce a parallel current flow. We can change the I~-factor by placing
the
current return (perfect conductor) at different distances from the borehole.
It is well
known that the K-factor of a cylindrical volume with a cross section, S, and
length, L,
is defined by the following equation:
dl
K - ~ S~l) (1)
where S(l) is the cross-sectional area at a distance l along the current path.
[0029] The mud cake 103 is characterized by a conductivity o',, permittivity
~', and
thickness h,. Similarly, the formation 105 is characterized by a conductivity
o'a,
permittivity EZ and thickness h2. The complex conductivities of the mudcake
and
formation are given by
v1 = a-1 + i~~l (2)
and
va = 62 + irvsa (3)
respectively, where tv = 2 ~z'f (f being the frequency).
[0030] Denoting by E, and E2 the electric field in the mud cake and the
formation and
by V the potential difference between the measure electrode and the current
return
(ground on Fig. 1), the following equations result:
JB = v1 E, S (current injected through the electrode)
v,E,=vZE2 (continuity of current) and
EI h, +E2 h2 =V (overall voltage).
This gives
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Y =_ Jb h' + ha (4)
V1 ya
[0031] Introducing the electrode impedance, we finally obtain
z__~ _ z h, +~a ~_z hl + ha ( )
s
Jb ~ S v, y2 y S ~~ + iwsl 6a + ir~~2
The first term on the right hand side in eq. (S) represents the impedance of
the mud
s cake while the second term represents the impedance of the formation. At low
frequencies (c~ ~ 0), the measured impedance depends primarily on the mud cake
conductivity axzd the formation conductivity, i.e., it does nat depend upon
the
dielectric constant of the mud cake and the formation. However, if the mud is
oil
based ( mud cake is resistive), then the measured impedance may become so
large
that it would be virtually impossible to inject any current into the
formation.
[0032 Eq. (5) indicates that we can reduce the mud cake impedance by
increasing the
frequency cv, This can be done by selecting the frequency such that
~E, > > o',
is
While reducing the mud cake impedance, we must also maintain the frequency
such
that the second term in eq. (5) depends mostly oni the formation conductivity
a2 .
This leads to the condition
« 6a (~).
Combining eqs. (6) and (7) gives the results
c~J « 6a (8).
~1 ~2
In an oil-based mud, both inequalities in eq. (6) must be satisfied because
cf't < < a'a
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Under these conditions, eq. (5) may be written in the form
1 h~ 1- 0-1 + h2 ~ _ iwsa
Z~ - _
,S' iwEl iCV~I o-2 a'a
(9)
_1 h2 h, ~-, h, h2 w s2
-+ a - i +
(~81 ) X81 62
Eq. (9) can be written in the form
Z = ~ (Z) + s (Z) (10)
where ~t(Z) and ~ (Z) are the real and imaginary (inphase and quadrature)
parts of the
impedance given by
~(Z) = 1 h2 + 61h12 (11) and
s ~-2 (~~, ~
1 h h w~
:5(Z) _ - 1 + 2 2 2 (12).
S w s,
[0033] The following points may be noted about eq. (11) ( the real part of the
impedance):
1. The first term depends on formation conductivity and does not include
dielectric permittivity.1t exactly represents the resistivity reading in the
absence of
mud cake.
2. The second term contains only mud cake properties. Importantly, it is
inversely proportional to the second power of the frequency.
3. The second term may be eliminated in two different ways. The first way is
to
use a high frequency. The second way to eliminate the second term is by
combining
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measurements are two different frequencies. This is given by the following
equation:
w i ~tZ(w 1 ) - w 2 ~i.Z(w ~ ) 1 h2
Chi -frl2 = S ~2
(13).
[0034] Turning now to eq. (12), the quadrature (out of phase) component of the
impedance, the following points may be noted.
1. With the frequency increase, the formation contribution (the second term)
becomes more significant.
2. While dominating, the formation signal retains dependence on the formation
dielectric constant. This introduces undesirable uncertainty in the process of
interpretation.
3. Due to eq. (8) the out of phase component is typically small compared to
the
in phase component.
[0035] The points noted above are brought out in Figs. 7 - 8 which show exact
relationships derived from eq. (5). Calculations were done for an electrode
radius of
2 mm, K factor of 12,OOOrri' , and a relative dielectric constant of 10 for
both the mud
and the formation. The relative dielectric constant is the ratio of the
permittivity of a
medium to that of free space.
[0036] Referring now to Fig. 7a, the abscissa is the formation resistivity in
SZm and
the ordinate is the ~Jt(Z). Values are plotted for a frequency of 1 kHz. Three
curves
are shown for mud cake resistivities of 10 kSZm, 100 kS2m and 1000 kS2m and a
mud
cake thickness of 0.1 mm.. As can be seen, the J~(Z) depends not only on the
formation resistivity but also on the resistivity of the mud cake.
[0037] Fig. 7b is similar to Fig. 7a except that the mud cake thickness is 0.5
mm.
Differences between Fig. 7b and Fig. 7a show that the ~t(Z) is also dependent
upon
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the mud cake thickness. Fig. 7c is a plot of the absolute value of the
electrode
impedance for a mud cake thickness of 0.1 mm.
[0038] Turning now to Fig. 7d, a plot of the dual frequency impedance
determined by
eq. (13) for a mud cake thickness of 0.1 mm is shown. The dual frequency
values
were obtained using measurements at 1 kHz and 2 kHz respectively. Fig. 7e
shows
the results of dual frequency measurements for a mud cake thickness of 0.2 mm.
Finally, Fig. 7f shows a plot of the ratio of ~J2(Z) to ~ (Z).
[0039] In summary, Figs. 7a - 7f explain why measurements made by conventional
resistivity imaging tools do not work with oil based muds: the measured
impedance
for audio frequency signals depends on many factors other than the formation
resistivity.
[0040] Turning now to Figs. 8a - 8f, a completely different picture emerges.
The
figures are similar to Figs. 7a - 7f with the significant difference that the
operating
frequency is now 1 MHz (compared to 1 kHz in Figs. 7a - 7f). For a relatively
thin
mud cake (Fig. 8a), the ~t(Z) is primarily dependent upon the formation
resistivity
except for extremely conductive formations where some dependence upon the mud
cake resistivity is noted. The effect is more noticeable for a thicker mud
cake (0.5
mm in Fig. 8b). The amplitude of the impedance (Fig. 8c) shows little
variation with
mud cake resistivity but does exhibit a nonlinear dependence upon the
formation
resistivity. The dual frequency measurements (Fig. 8d, 8e) show that the
measured
impedance is substantially independent of mud cake thickness and resistivity
and
further exhibits the desirable property of being linearly related to the
formation
resistivity. The presem invention takes advantage of the fact that at high
frequencies
(>_ IMHz or so), the effect of the mud cake and the mud impedance may be
ignored
for all practical purposes.
[0041] The dual frequency solution given by eq. (13) is a special case of
multifrequency focusing. In an alternate embodiment of the invention,
measurements
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are made at a plurality of frequencies cv, , wz , w3 . . . . cv"~ . As
disclosed in United
States Patent 5,703,773 to Tabarovsky et al., the contents of which are fully
incorporated herein by reference, the response at multiple frequencies may be
approximated by a Taylor series expansion of the form:
U CO 1 ~ I/2 ~ 3/2 , . a
a( 1) ~ n/2 0
1 1 I
1/2 3/2 n/1
~a(~2) 1 ~2 ~I "' ~2 '~1/2
i/2 3/2 n/2
6a (ant-I 1 Win:-1 Win:-1 ... 'S(n-I)/2
) C~,u-I
2/2 3l2 n/2
6a(~m) ~ f.~n: C!)m ... COm snl2
(14).
In a preferred embodiment of the invention of the number m of frequencies tv
is ten.
Using the measuremen;.s at the m frequencies, the quantities so , s"~ a s3;2
are
determined. In eq.(12), h is the number of terms in the Taylor series
expansion. This
can be any number less than or equal to m. The coefficient s3/a of the tv3~a
term (cv
being the square of k, the wave number) is generated by the primary field and
is
relatively unaffected by any inhomogeneities in the medium surround the
logging
instrument, i.e., it is responsive primarily to the formation parameters and
not to the
borehole and invasion zone. In fact, the coefficient s3/z of the tv3'2 term is
responsive
to the formation parameters as though there were no borehole in the formation.
This
frequency focusing method has been shown to give reliably consistent results
even
when there is a significant invasion of the formation by borehole fluids. In
one
embodiment of the invention, a processor controls the signal generator to
provide a
measure current at a plurality of frequencies. The processor then performs a
frequency focusing of the apparent conductivity at the plurality of
frequencies to
obtain the coefficients sj,2. This is then used as an estimate of the
formation
conductivity.
[0042]Turning now to embodiments of the present invention suitable for
resistivity
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imaging, Fig. 9 shows an imaging tool 110 suspended in a borehole 112, that
penetrates earth formations such as 113, from a suitable cable 114 that passes
over a
sheave 116 mounted on drilling rig 118. By industry standard, the cable 114
includes
a stress member and seven conductors for transmitting commands to the tool and
for
receiving data back from the tool as well as power for the tool. The tool 110
is raised
and lowered by draw works 120. Electronic module 122; on the surface 123,
transmits the required operating commands downhole and in return, receives
data
back which may be recorded on an archival storage medium of any desired type
for
concurrent or later processing. The data may be transmitted in analog or
digital form.
Data processors such as a suitable computer 124, may be provided for
performing
data analysis in the field in real time or the recorded data may be sent to a
processing
center or both for post processing of the data.
[0043]Figs. 10a and lOb are schematic external views of a borehole sidewall
imager
system. The tool 110 comprising the imager system includes resistivity arrays
126
and, optionally, a mud cell 130 and a circumferential acoustic televiewer 132.
Electronics modules 128 and 138 may be located at suitable locations in the
system
and not necessarily in the locations indicated. The components may be mounted
on a
mandrel 134 in a conventional well known manner. The outer diameter of the
assembly is about 5 inches and about fifteen feet long. An orientation module
136
including a magnetometer and an accelerometer or inertial guidance system may
be
mounted above the imaging assemblies 126 and 132. The upper portion 138 of the
tool 110 contains a telemetry module for sampling, digitizing and transmission
of the
data samples from the various components uphole to surface electronics 122 in
a
conventional manner. If acoustic data are acquired, they are preferably
digitized,
although in an alternate arrangement, the data may be retained in analog form
for
transmission to the surface where it is later digitized by surface electronics
122.
[0044]Also shown in Fig. 10a are three resistivity arrays 126 (a fourth array
is hidden
in this view Referring to Figs. 10a and 10b, each array includes measure
electrodes
141a,141b, . . . 141n for injecting electrical currents into the formation,
focusing
16
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electrodes 143a,143b for horizontal focusing of the electrical currents from
the
measure electrodes and focusing electrodes 145a,145b for vertical focusing of
the
electrical currents from the measure electrodes. By convention, "vertical"
refers to
the direction along the axis of the borehole and "horizontal" refers to a
plane'
perpendicular to the vertical.
[0045] In a preferred embodiment of the invention, the measure electrodes are
rectangular in shape and oriented with the long dimension of the rectangle
parallel to
the tool axis. Other electrode configurations are discussed below with
reference to
Fig.14. For the purpose of simplifying the illustration, insulation around the
measure
electrodes and focusing electrodes for electrically isolating them from the
body of the
tool are not shown.
(0046] Other embodiments of the invention may be used in measurement-while-
drilling (MWI~), logging-while-drilling (LWD) or logging-while-tripping (LWT)
operations. The sensor assembly may be used on a substantially non-rotating
pad as
taught in U.S. Patent 6,173,793 to Thompson et al., having the same assignee
as the
present application and the contents of which are fully incorporated herein by
reference. ~'he sensor assembly of the present invention may also be used with
rotating sensors as described in Thompso~. These embodiments are discussed
below
with reference to Figs. 15 -17. The sensor assembly may also be used on a non-
rotating sleeve such as that disclosed in United States Patent 6,247,542 to
Kruspe et
al, the contents of which are fully incorporated here by reference.
[0047] For a 5" diameter assembly, each pad can be no more than about 4.0
inches
wide. The pads are secured to extendable arms such as 142. Hydraulic or spring-
loaded caliper-arm actuators (not shown) of any well-known type extend the
pads and
their electrodes against the borehole sidewall for resistivity measurements.
In
addition, the extendable caliper arms 142 provide the actual measurement of
the
borehole diameter as is well known in the art. Using time-division
multiplexing, the
voltage drop and current flow is measured between a common electrode on the
tool
17
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and the respective electrodes on each array to fiunish a measure of the
resistivity of
the sidewall (or its inverse, conductivity) as a function of azimuth.
[0048] Turning now to Fig. 11, a circuit diagram showing the principles of
operation
of the tool is given. A Jource of electrical power 151 produces an electrical
current
that is provided to the measure electrodes. In one embodiment of the
invention, the
apparatus is intended for use with oil based drilling mud and the capacitor
157 depicts
the capacitive coupling between a measure electrode such as 141a in Fig. lOb
and the
formation 113 in Fig. 9. The electrical current flows through the formation
that has
an equivalent impedance of Z f and returns to the current source 151 through
an
equivalent capacitor 159 representing the coupling between the formation and
the
diffuse return electrode, typically the body of the tool. The measurement of
the
voltage drop across a resistor 153 is used as an indication of the current
flowing to a
measure electrode. Other methods for measurement of the current in the measure
electrode may also be used. Such methods would be known to those versed in the
art
and are not discussed here. In a preferred embodiment of the invention, the
value of
the resistor 153 is 1k SZ. The impedance of the rest of the return path in the
body of
the tool can be ignored.
[0049] Still referring to Fig.11, a voltage detector 161 measures the voltage
difference between the measure electrode and the diffuse return electrode and
controls
the current at the current generator to maintain a constant voltage. In this
case, the
output of the current measuring circuit serves as a measure signal.
Alternatively (not
shown), the output of the current measuring circuit 155 is used to maintain a
constant
current and the output of the voltage detector is used as a measure signal. As
still
another alternative, both the voltage detected by the voltage detector 161 and
the
current measured by the current measuring circuit 155 are used as measure
signals.
(0050]Selection of the size of the measure electrode and the operating
frequency is
based upon several considerations. One important consideration is that the
impedance
of the formation must be substantially resistive at the operating frequency so
that the
18
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currents in the measure electrode are indicative of the formation resistivity
and
substantially unaffected by its dielectric constant, used upon typical values
of
formation dielectric constant such as that disclosed in United States Patent
5,811,973
issued to Meyer et al, the operating frequency should be less than 4 MHz. As
S mentioned above, a prcferred embodiment of the present invention uses a
measuring
current at a frequency of lMHz. A second consideration is that the impedance
(i.e.,
resistance). of the formation be greater than the impedance of the rest of the
circuit of
Fig. 11. Another consideration is the desired resolution of the tool. A
reasonable
resolution for a useful imaging tool is approximately 3 mm. in the horizontal
and
vertical directions.
[0051] The impedance of the equivalent capacitance 159 and the body of the
tool may
be ignored at 1 MHz since the equivalent capacitor has an enormous area
comparable
to the size of the tool. The capacitance of 157 is a function of the
dielectric constant
of the borehole fluid, the area of the electrode, and the stand-off between
the electrode
and the borehole wall. Formation resistivities encountered in practice may
range
between 0.2 S2-m and 20,000 SZ-m. As noted above and discussed below, the
present invention makes use of focusing electrodes so that, in general, the
effective
dimensions of the formation that are sampled by an electrode are less than the
actual
physical size of the electrodes. used upon these considerations, and the
requirement
that a plurality of electrodes must fit on a single pad, in a preferred
embodiment of the
invention as shown in Figs 10a, 10b, the individual measure electrodes
141a,141b . .
. 141n have a width of 8 mm. and a length of between 20 - 30 mm. This makes it
possible to have eight electrodes on a single pad. The corresponding value of
the
capacitance 157 is then typically between 1pF and 100 pF. At the lower value,
the
impedance of the capacitance 107 at 1 MHz is approximately 160 kS2 and at the
higher
value approximately l.6kSZ.
[0052] In the present device, the focusing electrodes 145a,145b are of some
importance as they perform a significant amount of focusing. Denoting by Vthe
potential of the measure electrodes 141a,141b. . . the electrodes 145a,145b
are
19
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WO 02/086459 PCT/US02/11727
maintained at a potential of V + a The body of the pad is maintained at a
voltage V +
E The pad functions as a guard electrode and prevents divergence of the
measure
current until the current has penetrated some distance into the formation.
This makes
it possible to get deeper readings. A typical value of the voltage V is 5
volts while
typical value of 8 and r, are 500 ~,V and 100 ~,V, with E being less than ~
Since
little focusing is needed in the horizontal direction, the side focusing
electrodes 143a9
143b are maintained at substantially V volts. Those versed in the art would
recognize
that the device could also function if all the voltages were reversed, in
which case, the
voltages mentioned above as typical values would be magnitudes of voltages.
[0053] With the potentials of the measure electrodes, the focusing electrodes
and the
pads as discussed above, the current from the current source 151 in Fig. 11
will be
focused down to square blocks approximately 8 mm. on the side. The operating
frequency of the present device is typically 1 MHz, compared to an operating
frequency of 1.1 kHz for the device of the '431 application.
[0054]Those versed in the art would recognize that a considerable amount of
cross-
talk would normally be generated between the current flowing to the measure
electrodes from the electronics module 138 and the measure signals) returning
from
the measure electrodes carrying information about the voltages and/or currents
of the
electrodes. The measuring electrodes are preferably isolated from the
electronics
module by an isolator section such as 137 that is preferably between 2'6" and
15' long.
Cross-talk between conductors (not shown) over such distances would be quite
large
at an operating frequency of 1 MHz would overwhelm the measure signals)
indicative
of the formation resistivity.
[0055]This problem is addressed in the present invention by modulating the
current
output of the generator at l.lkHz. The result is that the current traveling
down
conductors in the isolator section and into the formation is a lMHz current
modulated
at l.lkHz. A demodulator (not shown) is provided in the voltage measuring
circuit so
that the return signal to the electronics module 138 is a 1.lkHz signal. This
makes it
CA 02444942 2003-10-20
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possible to use substantially the same hardware configuration as in prior art
devices
designed to substantially attenuate the cross-talk.
[0056]To further reduce the effects of cross-talk, instead of conventional
amplitude
modulation of the currents, an inverse modulation is used. Conventional
amplitude
modulation is given by a current i(t)
Z(t) = COS(COmZ')COS(CO~t) (1)
where tv", is the modulating signal frequency (l.lkHz) and cv~ is the carrier
frequency
(lMHz). The inverse modulation of the present invention uses a modulation of
the
form
i(t) _ (1- aCos(tr~",t))Cos(r~~t) (2)
where a is small compared to 1. The result is that the current output of the
generator
151 is substantially at : MHz with an amplitude close to unity at all times.
This
makes the cross-talk substantially independent of the magnitude of the measure
current. Substantially the same result may be obtained in alternate
embodiments of
the invention by using frequency or phase modulation of the 1 MHz carrier
signal.
[0057] Figs. 12a and 12b show a comparison between a prior art modulated
signal
and a reverse modulated signal according to the present invention. A carrier
signal
161 having a carrier frequency has its amplitude modulated by a lower
frequency
modulating signal 143. As can be seen, the level of amplitude of the modulated
signal goes to zero whenever the modulating signal goes to zero at times such
as 165.
A reverse modulated signal is shown in Fig. 12b with a carrier signal 161' and
a
modulating signal 163'. This modulated signal always has a significant current
flowing. The advantage of using such a reverse modulated signal is that the
cross talk
is substantially unaffected by the level of the modulating signal.
[0058] In an alternate embodiment of the invention, the measure signals) is
sent
through an optical fiber. When an optical fiber is used for the purpose, there
will not
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WO 02/086459 PCT/US02/11727
be any cross talk between the current conveyed through the isolator section
and the
measure signal. Modulation of the current is then not necessary.
[0059] In an alternate embodiment of the invention, the principles described
above
are used when the measure electrodes are not part of an array of electrodes.
~7Vith a
single electrode, measurements indicative of the resistivity of the formation
may be
obtained. With a plurality of azimuthally distributed electrodes, such output
measurements may be processed using prior art methods, such as those used in
dipmeters, to obtain information relating to the dip of formations relative to
the
borehole. When combined with measurements of the borehole orientation and tool
,,
face orientation, such relative dip information may be further processed to
give
estimates of absolute dip of the formations.
[0060]Another embodiment of the present invention may be used with water based
muds. The equivalent circuit for this embodiment is shown in Fig. 13. It is
identical
to Fig. 11 except that the gap between the measure electrode and the formation
is a
conductive gap denoted by the points 209 - 211 and a return gap denoted by 219-
221.
An additional capacitor 207 may be incorporated into the circuit. The
operation of
the device is substantially unchanged from that used for non-conducting muds.
The
conductive paths through the mud shunts any effect of the capacitance of the
tool
stand-off.
[0061] Such an arrangement has been used in the past with contact electrodes
for
resistivity measurements or resistivity imagers. The function of an internal
capacitor
in such prior art circuits has been solely for the purpose of blocking any
extraneous
currents emanating from sources external to the measure circuit from entering
the
amplifiers and distorting the operation of such prior art apparatus. Other
methods
have also been used for compensating for such extraneous currents. However,
the
particular embodiment utilizing an external capacitor constructed from
instrument
electrode plate, conductive earth formation plate and drilling mud dielectric,
with
high frequency, modulated measure currents such as are used in the present
invention
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WO 02/086459 PCT/US02/11727
and depicted in Fig.13 have not previously been used.
[0062]The resolution of the devices disclosed above is substantially equal to
the
dimensions of the focused current at a depth where the current from the
measure
electrode has the smallest dimensions. Those versed in the art would recognize
that if
lower resolution is acceptable, the focusing electrodes may be eliminated. In
such a
device; the ;beam of measure current is only guarded or constrained to flow
substantially outward from the surface of the measure electrode, as in prior
art non-
focused conductive mud devices, by the pad (or guard electrode) being
maintained at
substantially the same voltage as the measure electrode.
[0063]Alternatively, other configurations of the electrodes on a measuring pad
may
also be used. Fig.14 shows an arrangement in which five circular measure
electrodes
303a, 303b . . . 303e are located on a pad 301. Each measure electrode is
surrounded
by an associated focusing electrode 305a, 305b . . . 305e with insulation
307a, 307b. .
. 307e therebetween. For simplifying the illustration, the insulation between
the
guard electrodes and the pad 301 is not shown.
[0064]The invention has further been described by reference to logging tools
that are
intended to be conveyed on a wireline. I-iowever, the method of the present
invention
may also be used with measurement-while-drilling (MWD) tools, or logging while
drilling (LWD) tools, either of which may be conveyed on a drillstring or on
coiled
tubing.
[0065] FIG.15 shows a schematic diagram of a drilling system 410 having a
drilling
assembly 490 shown conveyed in a borehole 426 for drilling the wellbore. The
drilling system 410 includes a conventional derrick 411 erected on a floor 412
which
supports a rotary table 414 that is rotated by a prime mover such as an
electric motor
(not shown) at a desired rotational speed. The drill string 420 includes a
drill pipe
422 extending downward from the rotary table 414 into the borehole 426. . The
drill
bit 450 attached to the end of the drill string breaks up the geological
formations
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when it is rotated to drill the borehole 426. The drill string 420 is coupled
to a
drawworks 430 via a Kelly joint 421, swivel, 428 and line 429 through a pulley
423.
During drilling operations, the drawworks 430 is operated to control the
weight on
bit, which is an important parameter that affects the rate of penetration.
'The operation
of the drawworks is well known in the art and is thus not described in detail
herein.
[0066] During drilling operations, a suitable drilling fluid 431 from a mud
pit (source)
432 is circulated under pressure through the drill string by a mud pump 434.
The
drilling fluid passes from the mud pump 434 into the drill string 420 via a
desurger
436, fluid line 328 and Kelly joint 421. The drilling fluid 431 is discharged
at the
borehole bottom 451 through an opening in the drill bit 450. The drilling
fluid 431
circulates uphole through the annular space 427 between the drill string 420
and the
borehole 426 and returns to the mud pit 432 via a return line 435. A sensor S,
preferably placed in the line 438 provides information about the fluid flow
rate. A
surface torque, sensor SZ and a sensor S3 associated with the drill string 420
respectively provide information about the torque and rotational speed of the
drill
string. Additionally, a sensor (not shown) associated with line 429 is used to
provide
the hook load of the drill string 420.
[0067] In one embodiment of the invention, the drill bit 450 is rotated by
only rotating
the drill pipe 452. In another embodiment of the invention, a downhole motor
455
(mud motor) is disposed in the drilling assembly 490 to rotate the drill bit
450 and the
drill pipe 422 is rotated usually to supplement the rotational power, if
required, and to
effect changes in the drilling direction.
[0068] In the embodiment of FIG. 15, the mud motor 455 is coupled to the drill
bit
450 via a drive shaft (not shown) disposed in a bearing assembly 457. The mud
motor rotates the drill bit 450 when the drilling fluid 431 passes through the
mud
motor 455 under pressure. The bearing assembly 457 supports the radial and
axial
forces of the drill bit. A stabilizer 458 coupled to the bearing assembly 457
acts as a
centralizer for the lowermost portion of the mud motor assembly.
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[0069] In one embodiment of the invention, a drilling sensor module 459 is
placed
near the drillwbit 450. The drilling sensor module contains sensors, circuitry
and
processing software and algorithms relating to the dynamic drilling
parameters. Such
parameters preferably include bit bounce, stick-slip of the drilling assembly,
backward rotation, torque, shocks, borehole and annulus pressure, acceleration
measurements and other measurements of the drill bit condition. The drilling
sensor
module professes the sensor information and transmits it to the surface
control unit
440 via a suitable telemetry system 472.
~ [0070] Fig:. 16 shows an embodiment of the invention in which sensors
mounted on
stabilizers of a drilling assembly are used to determine the resistivity of
the formation.
~ne or more of the stabilizers 1033 is provided with a recess 1035 into which
a sensor
module 1054 is set. Each sensor module 1054 has one or more measure electrodes
1056 for injecting measure currents into the formation as described above. As
discussed above, the body of the sensor module is maintained at approximately
the
same potential as the measure electrode to operate as a guard electrode.
Optionally,
focusing, electrodes may be provided as discussed above.
[0071] In a measurements while drilling environment, there is usually a small
gap
between the stabilizer and the borehole wall (not shown): the diameter of the
drill bit
(not shown) conveyed on the drilling tubular 1040 is greater than the outer
diameter
as defined by the stabilizers. The operation of the stabilizers would be known
to
those versed in the art and is not described f1u-ther here. When used with a
nonconducting fluid in the borehole, the gap defines the capacitance 107
discussed
above: If necessary, extendable arms (not shown) may be provided to keep the
gap
within acceptable limits. When used with a conducting borehole fluid, the size
of the
gap is~ not critical. An electronics module 1052 at a suitable location is
provided for
processing the data acquired by the sensors 1056.
[0072] FIG. 17 illustrates the arrangement of the sensor pads on a non-
rotating
sleeve. This is similar to an arrangement of sensors taught by Thompson though
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other configurations could also be used. Shown are the drilling tubular 1260
with a
non-rotating sleeve 1262 mounted thereon. Pads 1264 with one or more measure
7..
electrodes 1301 are attached to sleeve 1262. The mechanism for moving the pads
out
to contact th~,borehole, whether it be hydraulic, a spring mechanism or
another
mechanism is not shown. The shaft 1260 is provided with stabilizer ribs 1303
for
controlling.the direction of drilling.
[0073] Data may be acquired using the configuration of either Fig. 16 or
Fig.17
while the well is being drilled and the drillstring and the measure electrodes
thereon
are rotating. In a MWD environment, telemetry capability is extremely limited
and
accordingly, much of the processing is done downhole. Processing of the data
in the
present invention is accomplished using the methodology taught in Thompson et
al.
The resistivity measurements are made concurrently with measurements made by
an
orientation:~sensor (not shown) on the drilling assembly. As the resistivity
sensor
rotates in the borehole while it is moved along with the drill bit, it traces
out a spiral
path with known depths and azimuths. The depths are determined either from
data
telemetered from the surface or by using at least two axially space apart
measure
electrodes to give a rate of penetration. In one embodiment of the invention,
the
downhole'~irocessor uses the depth information from downhole telemetry and
sums
all the data within a specified depth and azimuth sampling interval to improve
the S/N
ratio and to reduce the amount of data to be stored. A typical depth sampling
interval
would be one inch and a typical azimuthal sampling interval is 15° .
Another method
of reducing the amount of data stored would be to discaxd redundant samples
within
the deptlf=.and azimuth sampling interval. Further details of the processing
method
may be found in the teachings of Thompson et al.
[0074]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.
26
