Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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NUCLEAR MAGNETIC RESONANCE LOGGING
WITH AZIMUTHAL RESOLUTION
This invention relates to the field of well logging and,
more~particularly, to a method and apparatus for determining
nuclear magnetic resonance logging characteristics of earth
formations surrounding a borehole, either during or after the
drilling of the borehole.
In the evaluation of earth boreholes drilled in earth
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formations to produce hydrocarbons, determination of the porosity
of the formations is considered essential for decision making.
Nuclear magnetic resonance ("NMR") provides a means of measuring
total and producible porosity of earth formations. In certain
conditions NMR well logging can provide important information on
the pore size of formation rock and on the type of fluid
contained therein. Measurement of nuclear resonance requires a
static magnetic field Bo and a radio frequency (RF) magnetic
field in the earth formation that is being probed. [As used
herein, an RF field generally has a frequency in the range 2 KHz
to 10 MHz.] Atomic nuclei with a nonzero nuclear magnetic moment
and spin angular momentum precess about the static field Bo with
an angular frequency coo = YBo when perturbed from their thermal
equilibrium. The constant Y is the gyromagnetic ratio of the
resonating nucleus, most commonly the hydrogen nucleus. For
hydrogen nuclei, the gyromagnetic ratio is 2.675198775 x 108
radian/second/Tesla. To manipulate the spin state of the
particles, for example, to perturb the thermal equilibrium, a
radio frequency (RF) magnetic field B1 is needed. The frequency
of the RF field B1 should be close to wo and substantially
perpendicular to the static field Bo in the region of
investigation. Magnetic resonance is observed by detecting the
oscillating magnetic field produced by the precession of the
spins. Typically, but not necessarily, the same coil that
z5 -produces the RF field B1 is used for detection. In pulsed-NMR,
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repeated pulses are applied to the coil and spin-echoes are
detected in between the transmitted pulses. Reference can be
made, for example, to U.S. Patents 5,376,884, 5,055,788,
5,055,787, 5,023,551, 4,933,638, and 4,350,955 with regard to
known nuclear magnetic resonance logging techniques.
In logging-while-drilling, the measurement apparatus is
mounted on a drill collar. Drill collars are long, tubular
pieces of a strong material, typically nonmagnetic stainless-
steel. Drill collars and drill pipes transmit the torque from
the surface apparatus to the drill bit. During drilling, the
drill collars typically rotate about their axes, which are
substantially aligned with the axis of the borehole. The rates
of rotation of the drill collars and the drill bit are the same
in rotary drilling, and can be different if a downhole mud motor
is used. In either case, the drill collar is subject to
rotation. For logging-while-drilling NMR logging, the magnitudes
of Bo, B1, and the angle between them should be substantially
invariant of the rotation angle in the region of investigation.
This does not preclude the possibility that the directions of Bo
and B1 may depend on the rotation angle. The foregoing
invariance is required because magnetic resonance measurements
take on the order of 0.01 to 1 seconds during which the drill
collar may rotate by a substantial angle. Consistent preparation
and measurement of spin states are not possible without the
?5 rotational invariance.
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Directional drilling involves the drilling of a well bore
along a deviated course in order to reach a target region at a
particular vertical and horizontal distance from the original
surface location. Directional drilling is employed, for example,
to obtain an appropriate well bore trajectory into an oil
producing formation bed (or "pay zone") and then drill
substantially within the pay zone. A horizontally drilled well
can greatly increase the borehole volume in the pay zone with
attendant increase in oil production. Recent advances in
directional drilling equipment and techniques have greatly
improved the accuracy with which drilling paths can be directed.
Nuclear magnetic resonance logging systems have previously
been proposed for logging-while-drilling applications. If an NMR
logging device of a logging-while-drilling system has an axially
symmetric response, the NMR characteristics measured by the
logging device will tend to average the signals received
circumferentially from the formations. For example, when
drilling a near-horizontal well along the boundary between two
formation beds with dissimilar producible porosities, such a
logging device would give indication of an intermediate porosity.
It would be very advantageous to be able to use NMR to better
delineate the presence, locations, and characteristics of the
formation beds in this type of a situation.
It is among the objects of the present invention to address
limitations of the prior art with regard to nuclear magnetic
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resonance logging techniques and apparatus.
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SCJMMAR1~ c~E 'Tf-i~~ 1NVF N':L' i:t~N
The inventi c>n des<.r i.l_,c>d :i.n L . r . ~'at:ent
No. 5,977,'768, provices t~l~ue c,~ipabil.i.t.y c,~fazimuthal.l5r
resolved nuclear m.agr~.et.ic:. resoruan<:~:~ l.cgg:in~. Treat i.r:vention
and the in-vention hexe>.of can 1;>c;th be used irr so--cal.lE:d
wireline logging, but the inverntz.ons are p,articu:LarlS~
advantageous in achiE:vi.ncJ azirnutt~al.J.y reso:l_ved NMR lc.~gging-
whi:Le-drilling measurements.
A form of the ~.nvent.:.i.;:_>n t~et: f:;.~x~t:n in t~.>. F?atent
No. 5, 977, 768 is directed tc> ,_rn apparatus and mE:thod for
determining a nuclear mac~neti~-~ resc..~rlarAc:~. prc>per_ty c>f
formations surrounding a bo.rvetro:l.e wh:i.Je dri:Ll:irrg t:he
borehole with a rotating c~ri..:l:l._ bi.i~ or: ,~ drill. string. An
embodiment of the met crc:~c~ of tHuat :i.rAv~..:~r~t:ion :irvc.:Ludes the
following steps: prcoid~_r1g a lc?gc~s_nc~ devvi~.~2 :ir~u the drill
string, the logging device be.:inc~ ~:vot:ata.t::abl..e with t:hE: dril:L
str:Lng or a portion c L ttuk= dr:i :1.l st r:i_rag, the locxging device
having a rotational axis; pro<:tucvrp.:, a st:.atic magnetic field
and an RF magnet.ic field at tiuf_= loggir;g de~.~i_c.e, the .>tatic
and RF magnetic fields havi ng mnat:ua:l.l y ~:.r.~t~oc~onal components
in an investigation regic>n in t::t~e f-ormat.iovs surrouncJing the
logging device, the rr~agnr..tude,.~ ~At t.hc: stati.c.~ anti Rf nvagnet:Lc
fields in 'the invest:i gat_or~ r~~g.ic;ru being substantial.? y
rotationally invariar t as the l c:;gc~z nr;; c~c~~v:i.:~e rotates around
its axis; receiving r:~ac:tear m~:rgruet:::ic~ rc w:nanc:e
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spin echoes at at least one circumferential sector on the logging
device; and determining a nuclear magnetic resonance property of
the formations, for different portions of the investigation
region, from the received nuclear magnetic resonance spin echoes.
[It will be understood that the static and RF magnetic fields are
defined as having "mutually orthogonal components" if they are
not parallel. Typically, but not necessarily, the static and RF
magnetic fields will be substantially perpendicular in the
investigation region.]
In another form of the invention set forth in said copending
Application, the receiving of nuclear magnetic resonance spin
echoes is implemented at a plurality of different circumferential
sectors on the logging device and comprises providing a plurality
of arcuate receiver segments around the logging device and
detecting nuclear magnetic resonance spin echoes in signals
received by the individual receiver segments.
In embodiments hereof, in order to make an azimuthally-
resolved NMR measurement, the receiving radio-frequency antenna
has a non-axisymmetric response pattern. The transmitting
?0 (pulsing) antenna can be either the same non-axisymmetric
antenna, or a separate non-axisymmetric antenna, or a separate
axisymmetric antenna. If the azimuthal measurement is to be
performed while the tool is rotating (as is typically the case in
MWD), the static magnetic field (Bo) should be axisymmetric, at
5 least in terms of its magnitude. In certain embodiments hereof,
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the rf antennas employ axial currents (parallel to the tool and
wellbore axis), which excite an azimuthally-oriented rf magnetic
field (B1). For this situation, the static magnetic field (Bo)
should be either radial or axial in its orientation, so as to be
approximately perpendicular to the azimuthal B1 field excited by
the rf antenna, as is desirable for efficient NMR signal
generation and reception. In another embodiment, the rf antenna
excites a radially-oriented rf magnetic field. In this case, the
static magnetic field should be either axial, transverse, or
azimuthal in its orientation. In embodiments of the invention to
be described, a region of generally uniform static field
magnitude and polarization produced in the formations is
relatively long in axial extent, and an advantage is that the rf
antenna used to obtain azimuthally resolved measurements can also
be made relatively long in the axial direction, thereby
increasing the volume of spins ultimately sensed by the antenna
and increasing signal-to-noise ratio. This increase tends to
offset the decrease in the volume of spins that are detected when
the azimuthal range of investigation is limited to a sector that
is a fraction of a full circumference.
A form of the invention is a method for determining a
nuclear magnetic resonance property of formations surrounding a
borehole during a drilling operation in the borehole with a drill
string, comprising the following steps: providing a logging
~5 . device in the drill string, the logging device having a
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longitudinal axis; producing, from said logging device, a static
magnetic field and an rf magnetic field in the formations; and
receiving nuclear magnetic resonance signals from an
investigation region of the formations at an antenna having a
response pattern that is non-axisymmetric.
In some embodiments, the response pattern having an
azimuthal polarization in the investigation region.
In accordance with an embodiment of apparatus in accordance
with the invention, there is disclosed an apparatus for
determining a nuclear magnetic resonance property of formations
surrounding a borehole which comprises. a logging device
moveable through the borehole; means in said logging device for
producing a static magnetic field in the formations; and antenna
means in said logging device for pLoducing an rf magnetic field
in the formations, and for detecting nuclear magnetic resonance
signals from the formations, the antenna means including a
plurality of spaced apart generally cylindrical arc-shaped
conductors, and means coupled across the arc-shaped conductors
for detecting signals induced in the conductors.. In a preferred
embodiment of this form of the invention, the logging device has
a longitudinal axis, and the cylindrical arcs of the conductors
are concentric with said axis. In this embodiment, the means for
producing an rf magnetic field in said formations is operative to
generate current flowing in adjacent conductors in opposing axial
directions and the means for detecting signals is operative to
detect currents flowing in adjacent conductors in opposing axial
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directions.
In another embodiment of the invention, the antenna means
includes at least one multi-turn current loop having an axis that
is substantially perpendicular to the longitudinal axis of the
logging device, and means coupled with said current loop for
detecting signals induced in said current loop. In a preferred
form of this embodiment, the at least one multi-turn current loop
is formed in a generally cylindrical arc shape, said arc being
centered on a line oriented in the direction of the longitudinal
axis.
In embodiments of the invention, an antenna is low profile
and can be formed in an outer groove in the drill collar without
necessarily reducing the inner diameter of the drill collar,
which is ordinarily done to increase strength in a region of
drill collar where the outer diameter has been recessed to
provide an antenna. [The reduction in the bore size of the drill
collar is preferably avoided, if possible, as it requires extra
machining of the drill collar and contributes to constriction of
mud flow.] In accordance with these embodiments there is
disclosed an apparatus for nuclear magnetic resonance logging
that is mountable in a drill string for logging of formations
surrounding a borehole, comprising: a tubular drill collar
having a generally cylindrical inner surface having an inner
diameter and a generally cylindrical outer surface having an
outer diameter; first means in the drill collar for producing a
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first magnetic field; seCOnd means in the drill collar for
producing a second magnetic fiehd; and means in the drill collar
for receiving nuclear magnetic resonance signals from an
W vestigar.ion region in the formations; the second means
comprising an antenna disposed in a recess spanning an axial
extent in the outer_cylindrical surface, the outer surface of
said drill collar having a diameter that is reduced from the
outer diameter over the axial extent of the recess, end the inner
surface of the drill collar having a diameter that is not reduced
from the inner diameter over the axial extent:of the recess_ Tn
a form of this embodiment, the antenna comprises at least one
generally. cylindrical arc-shaped conductor plate in the recess.
In another form of this embodiment, the antenna comprises at
~5
least one multi-turn loop formed in a generally cylindrical arc
shape in the recess_
Another broad aspect provides a method for
determining a nuclear magnetic resonance property. of
formations surrounding a~borehole, comprising the steps of:
providing a logging device moveable through said borehole;
producing, from said logging device, a static magnetic field.
and an rf magnetic field in said formations; and receiving
nuclear magnetic resonance signals from an investigation
region of said formations at an antenna having a response
2~ pattern that is non-axisymmetric.
Further features and advantages of the invention will become
more readily apparent from the following detailed description
when taken in conjunction with the accompanying drawings.
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Figure I is a diagram of a logging-while-drilling system in
which an embodiment of the invention can be utilized and which
can be used in practicing the method of the invention.
Figure 2 is a cross-sectional view of a logging device in
accordance with an embodiment of the invention as set forth in
the above-referenced U.S. Patent No. 5,977,768.
Figure 3 is a cross sectional view of the logging device of
Figure 2 as taken through a section defined by the arrows 3-3 of
Figure 2.
Figure 4 is a block diagram of circuitry used in an
15 embodiment of the invention as set forth in the above-referenced
U.S. Patent No. 5,977,768.
Figure 5 is across-sectional view of a logging device in
accordance with an embodiment of the invention as set forth in
the above-referenced U.S. Patent No. 5,977,768.
Figure 6 is a cross-sectional view of the logging device of
Figure S as taken through a section defined by the arrows 6-6 of
Figure 5.
Figure 7 is a block diagram of circuitry used in an
embodiment of the invention as set forth in the above-referenced
U.S. Patent No. 5,977,768.
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Figure 8 is a simplified diagram of directions and
orientations in the borehole that is useful in understanding an
aspect~of the invention in accordance with an embodiment of the
invention as set forth in the above-referenced U.S. Patent
No. 5,977,768.
Figure 9 is a flow diagram of a routine that can be used for
programming a processor in accordance with an embodiment of the
invention as set forth in the above-referenced U.S. Patent
No. 5,977,768.
Figure 10 is a cross-sectional partially broken away view of
a receiving antenna on a logging device, in accordance with a
further embodiment of the invention as set forth in the above-
referenced U,S. Patent No. 5,977,768.
Figure 11 is a cross-sectional view of a logging device in
accordance with another embodiment of the invention as set forth
in the above-referenced U.S. Patent No. 5,977,768.
Figure 12 is a schematic diagram of an antenna in accordance
with an embodiment of the invention and which can be used in
practicing an embodiment of the method of the invention.
Figure 13 is a cross-sectional view of the antenna of the
Figure 12 embodiment.
Figure 14 illustrates the modeled rf magnetic field (Bt)
orientation and magnitude (given approximately by the lengths of
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the arrows) for the antenna geometry of Figure 12, and a 60
degree angular extent of the smaller segment, which is pointing
upward in this axial view.
Figure 15 shows contours of equal magnitude of the rf
magnetic field (B1) for the same 60 degree antenna geometry shown
in Figure 14.
Figure 16 shows the modeled rf magnetic field (B1)
orientation and magnitude (given approximately by the lengths of
the arrows) for the antenna geometry of Figure 12, assuming in
this case only a 30 degree angular extent of the smaller segment
which is again pointing upward in this axial view.
Figure 17 shows contours of equal magnitude of the rf
magnetic field (B1) for the same 30 degree antenna geometry shown
in Figure 16.
Figure 18 show geometrically-spaced contours of equal
magnitude of the rf magnetic field (B1) for the 60 degree antenna
geometry of Figures 14 and 15, but with the drill collar or
pressure housing no longer concentric with the electrodes.
Figure 19 shows the B1 rf field amplitude on a cross-section
of the 60 degree antenna along its axis of symmetry; that is from
bottom to top in Figure 18.
Figure 20 is a schematic diagram of a type of circuit
arrangement that can be utilized for transmitting and receiving
from the rf antenna of Figure 10 and other antennas hereof.
~5 Figure 21 is cross-sectional view of an rf antenna having
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four quadrants of arc shaped conductive electrodes for obtaining
azimuthally resolved NMR signals.
Figure 22 is a schematic diagram of a type of circuitry that
can be used in conjunction with the antenna of Figure 21.
Figure 23A is a cross-sectional view of an embodiment of a
slot antenna that utilizes separated arc shaped plate electrodes,
and Figure 23B illustrates a simplified diagram of the electrodes
in perspective, showing the radially polarized rf magnetic field
that can be produced thereby.
Figure 24A illustrates an embodiment that utilizes a single
plate electrode, and Figure 24B illustrates how the plate
electrode can be configured with circuitry for transmitting and
receiving with return current path through the drill collar, if
desired.
Figures 25A and 25B illustrates conceptually the replacement
of arc-shaped plate electrodes with coils in which current flows
primarily axially.
Figure 26 illustrates a spirally wound multi-turn loop
antenna in an arc shaped configuration.
Figure 27 illustrates a twin-loop multi-turn planar loop
antenna in accordance with an embodiment of the invention and
which can be used in practicing an embodiment of the method of
the invention.
Figure 28 is a diagram showing the rf magnetic field pattern
;5 of the antenna of Figure 27, as a function of azimuth angle, as
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seen along the centerline of the antenna.
Figure 29 shows a further embodiment of a multi-turn loop
antenna, this antenna utilizing a single loop.
Figure 30 is a diagram of the rf magnetic field pattern of
the antenna of Figure 29, as a function of azimuth angle, as seen
along the centerline of the antenna.
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Referring to Figure 1, there is illustrated a
logging-while-drilling apparatus and method in which embodiments
of the invention can be practiced. A platform and derrick 10 are
positioned over a borehole 32 that is formed in the earth by
rotary drilling. A drill string 12 is suspended within the
borehole and includes a drill bit 115 at its lower end. The
drill string 12, and the drill bit 115 attached thereto, is
rotated by a rotating table 16 (energized by means not shown)
which engages a kelly 17 at the upper end of the drill string.
The drill string is suspended from a hook 18 attached to a
traveling block (not shown). The kelly is connected to the hook
through a rotary swivel 19 which permits rotation of the drill
string relative to the hook. Alternatively, the drill string may
be rotated from the surface by a "top drive" type of drilling
rig. Drilling fluid or mud 26 is contained in a pit 27 in the
earth. A pump 29 pumps the drilling fluid into the drill string
12 via a port in the swivel 19 to flow downward through the
:0 center of drill string 12. The drilling fluid exits the drill
string via ports in the drill bit 115 and then circulates upward
in the region between the outside of the drill string and the
periphery of the borehole. As is well known, the drilling fluid
thereby carries formation cuttings to the surface of the earth,
5 and the drilling fluid is returned to the pit 27 for
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recirculation. The small arrows in the Figure illustrate the
typical direction of flow of the drilling fluid.
Mounted within the drill string 12, preferably near the
drill bit 115, is a downhole sensing, processing, storing and
transmitting subsystem 100. Subsystem 100 includes a measuring
apparatus 200 in accordance with an embodiments of the invention
which are illustrated hereafter. Also provided in the downhole
subsystem is a device or tool 59, of a type known in the art, for
measuring and/or computing the direction and inclination of the
bottom hole assembly and the rotational orientation of the bottom
hole assembly ("tool face"). Reference can be made, for example,
to U.S. Patent No. 5,473,158. A communications transmitting
portion of the downhole subsystem includes an acoustic
transmitter 56, which generates an acoustic signal in the
drilling fluid that is representative of the measured downhole
conditions. One suitable type of acoustic transmitter, which is
known in the art, employs a device known as a "mud siren" which
includes a slotted stator and a slotted rotor that rotates and
repeatedly interrupts the flow of drilling fluid to establish a
desired acoustic wave signal in the drilling fluid. The
generated acoustic mud wave travels upward in the fluid through
the center of the drill string at the speed of sound in the
fluid. The acoustic wave is received at the surface of the earth
by transducers represented by reference numeral 31. The
?5 transducers, which are, for example, piezoelectric transducers,
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convert the received acoustic signals to electronic signals. The
output of the transducers 31 is coupled to the uphole receiver
subsystem 90 which is operative to demodulate the transmitted
signals, which are then coupled to processor 85 and recorder 95.
Transmitter 56 can be controlled by conventional transmitter
control and driving electronics 57 which includes
analog-to-digital (A/D) circuitry that converts (if necessary)
the signals representative of downhole conditions into digital
form. The control and driving electronics 57 may also include a
suitable modulator, such as a phase shift keying (PSK) modulator,
which conventionally produces driving signals for application to
the transmitter 56. These driving signals can be used to apply
appropriate modulation to the mud siren of transmitter 56. It
will be understood that alternative techniques can be employed
for communicating logging information to the surface of the
earth.
The downhole subsystem 100 further includes acquisition and
processor electronics 58, which can include electronics as shown
in Figure 4. The acquisition and processor electronics 58 are
coupled to the measuring apparatus 200 and obtain measurement
information therefrom. In known manner, the acquisition and
processor electronics is capable of storing data from the
measuring apparatus, processing the data and storing the results,
and coupling any desired portion of the information it contains
?5 to the transmitter control and driving electronics 57 for
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transmission to the surface by transmitter 56. A battery 53 may
provide downhole power. As known in the art, a downhole
generator (not shown) such as a so-called "mud turbine" powered
by the drilling fluid, can also.be utilized to provide power
during drilling.
If desired, the drilling equipment can be a directional
drilling equipment. Such equipment (not shown) typically
includes an offset (or "bent") sub, a mud motor that is driven by
the flowing mud. The mud motor and bent sub can alternatively be
combined in a mud motor unit upper portion of the housing and
bearings in the bottom portion of the housing, with the motor
drive in the upper portion of the housing and bearings in the
bottom portion of the housing. The bent sub or bent housing
typically has an offset or bend angle of 1/2 to 2 degrees. As is
known in the art, when the bit is driven by the mud motor only
(with the drill string not rotating), the bit will deviate in a
direction determined by the tool face direction in which the
drill string and~bent sub are oriented [so-called "sliding
mode"J. When it is desired to drill substantially straight, the
drill string containing the mud motor is rotated [so-called
"rotating mode"J.
Figure 2 illustrates a form of the downhole measuring
apparatus 200 (of Figure 1) in accordance with an embodiment of
the invention as set forth in the above-referenced U.S. Patent
o. 5,977,768. The tool 200 is
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rotationally symmetric about axis 260 of the drill collar 230 in
which the tool is constructed, and which is substantially
aligned with the axis of the borehole. The static magnetic field
Bo is produced by tubular, axially polarized, permanent magnets
240 and 242 mounted inside the drill collar 230. channel 234
located inside the tool and the magnets, permits drilling mud to
flow toward the drill bit. In the region between the permanent
magnets, the drill collar has a circumferential recess 223 which,
in the present embodiment, has an arcuate cross-section. A
segmented antenna 210 is provided in. the recess 223. A non-
conduc~'iwe material 220 is provided in the recess beneath the
antenna. The material 220 is preferably a ferrite to increase
the efficiency of the antenna. The antenna is protected from the
abrasion and impact of the drilling environment by a shield 224,
which can comprise <~ slotted m:~tallic tube and/or insulating
material.
Figure 3 is a cross-sectional view through a section of the
20 logging device that includes the antenna 210 which, in the
illustrated embodiment, as set forth in the above-referenced
U.S. Patent No. 5,977,768 has a plurality of segments, and
is used for both transmitting and receiving. In this
embodiment there are four antenna segments, labeled 210a,
25 210b, 210c, and 210d. Each segment is a circurnferential
sector that is approximately a quadrant of a one turn coil,
and has one end at ground reference potential, which
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can be coupled with the drill collar. The other end of each coil
segment passes through a respective feed-through slot (labeled
214a, 214b, 2i4c, and 2I4d), each of which runs lengthwise
through the drill collar,~and the respective wiring is coupled to
circuitry in the module 58, which is shown in further detail in
Figure 4.
In the circuit block diagram of Figure 4, as set forth in
the above-referenced U.S. Patent No. 5,977,768 a transmitter
section includes an oscillator, represented at 410, an
output of which is coupled to an rf gate 415, and then a
power amplifier 418. The rf gate is under timing
control of a timing block. The output of power amplifier 418 is
coupled via respective extender diode circuits 414a, 414b, 414c
and 414d, to the antenna segments 210a, 210b, 210c, and 210d,
'! 5
which are shown in Figure 4 conjunction with respective tuning
capacitances Ca, Cb, C~ and Cd _ Each of the extender diode
circuits 414a, 414b, 414c and 414d includes a pair of back-to-
back diodes, each designated DL. Also coupled with the
respective coil segments 210a, 210b, 210c and 2IOd are respective
Q-switch circuits 412a, 412b, 412c and 4i2d. Each Q-switch
circuit includes a critical-damping resistor R~ and semiconductor
switch S,, for example a htOSFET. The antenna segments 210x,
5 210b, 2loc and 2lod are~also coupled, via respective duplexer
circuits 411a, 411b, 4ilc and 411d, to respective receive r
circm tL-y that includes r2spactive preamplifiers 432a, 132b, 432c
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and 432d and respective phase sensitive detectors 435a, 435b,
435c and 435d. Each of the duplexer circuits (411a-d) includes a
quarter wavelength transmission line U1 and an array of pairs. of
back-to-back diodes, each designated Dz, arranged as shown. Each
of the phase sensitive detectors (435a-d) receives a reference
signal from the oscillator 410. The outputs of the phase
sensitive detectors are coupled to a downhole processor 450,
which may typically be a digital processor with associated memory
and input/output circuitry (not separately shown). Timing
control circuitry is associated with the processor, as
represented at 452, and timing control is suitably provided as
illustrated in other places in the diagram.
The processor 450 also receives an input from module 59,
which includes signals representative of the rotational
orientation of the downhole assembly. These signals can be used
to relate the NMR signals, which are measurements with respect to
the device geometry, to the formation surrounding the borehole.
Alternatively, the tool 200 can be provided with a dedicated
subsystem for determining tool rotational orientation and
performing the necessary processing, as described in U.S. Patent
5,473,158. Telemetry circuitry 57 is conventionally provided for
communicating with the earth s surface.
In operation, and as is known in the art, nuclear magnetic
resonance circuitry can operate in three modes: transmitting,
damping, and receiving. Reference can be made, for example, to
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U.S. Patent Nos. 4,933,638, 5,055,787,,5,055,788, and 5,376,844.
As described in the referenced patents, during the transmitting
mode, the transmitter section generates relatively large rf power
for a short precisely timed period, shuts off this current very
quickly, within about 10 microseconds, and then. isolates a:~y
signals or noise of the power circuits from coupling with
detection circuitry. In the embodiment of Figures 2-4, as set
forth in the above-referenced U.S. Patent No. 5,977,768
transmitting and receiving are both implemented from the
plurality of coil segments. Immediately after transmitting,
the purpose of the Q-switches (412a-d) is to damp the rf
energy in the coil as fast as possible so that the
preamplifiers 432a-d can start detecting the much smaller NI~tR
signal as soon as possible after the rf pulse. Under timing
control, the switch S~ is closed to achieve this purpose.
Duplexer circuits 411a-d protect preamplifiers 432a-d from the rf
pulses applied by power amplifier 418. It is important that
duplexers 411a-d do not load the output of power amplifier 418.
2Q This is achieved by the quarter wavelength transmission line U1
and the diodes Dz_ When high power is applied, the diodes Dz
conduct and become almost short circuits. The quarter wavelength
transmission line U, inverts the impedance of the diodes DZ.
Z5 Therefore, the combination of the diodes D2 and the quarter
wavelength transmission line is seen as an open circuit by the
power amplifier. During receiving, the signal is below the
-24-
CA 02270757 2003-03-14
' 77483-26
threshold voltage of the diodes D2, and they appear as an open
circuit. Then, the respective quarter wavelength transmission
lines U1 connect rf coils 210a-d to preamplifiers 432a-d. The
quarter wavelength lines can be implemented by any suitable
means, for example by lumped capacitors and inductors, because
the length of an actual quarter wave transmission line would be
impracticably long, for example 375 m at 200 kHz. The diodes D1
of extender circuits 414a-d conduct only during transmission.
These diodes isolate the preamplifiers 432a-d from parasitic
noise and ringing that may be produced in power amplifier 418.
As above indicated, the antenna segments 210a-d are
collectively used as a transmitter coil and then, during
~~5 reception, as receiver segments that provide reception at
different circumferential sectors on the logging device. When
all segments of the coil are energized in parallel, the resulting
B1 field is substantially invariant with respect to rotation
angle. There will be some deviation from rotational symmetry in
the vicinity of the coil segments, but further away from the
coils, such as in the investigation region, the field will have
substantially rotational symmetry.
Figures 5, 6 and 7 illustrate a further embodiment as set
forth in the above-referenced U.S. Patent No. 5,977,768,
wherein the receiver coil comprises the coil segments
previously described (and represented by reference numeral
210 in Figure 5, and reference numerals 210a, 210b, 210c
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77483-26
and 210d in Figure ?). Figure 5 shows the downhol~e measuring
apparatus, with similar components to those of Figure 2
represented by like reference numerals. This embodiment includes
a separate transmitting antenna 2o1 spaced from the segmented
coil 210. The transmitter coil 201 is axis}~rmetric in
construction and produces an axisymmetric rf magnetic field.
Figure 6 is a cross-sectional view through a section of the
logging device that includes the transmitting antenna 201. In
this embodiment, as described in the U.S. Patent No.
5,977,768 the transmitting antenna is an axially symmetric
single turn coil, although plural turns may be used. The
wiring leads 202 for energizing the coil 201 pass through
the insulating medium 220 to a feed-.through slot 204 in the
.'~ drill collar 230 that runs lengthwise along the drill
collar, parallel to the axis thereof, to the circuitry in
the module 58. The wiring in this and other feed-throughs is
insulated.
Figure 7 is a block diagram of an embodiment, as
set forth in the above-referenced U.S. Patent No. 5,977,768
of the circuitry in the module 58 for use in conjunction
with the antennas of Figure 5, 6 embodiment. Portions of the
circuitry are similar to those of the Figure 4 embodiment
and are represented by like reference numerals. In this
embodiment the transmitter circuitry again includes
oscillator 410, rf gate 415, and power amplifier 418, and the
transmitter coil 201 (shown in parallel with tuning capacitor CZ)
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77483-26
is driven via extender diode circuit 716 and has associated Q-
switch 713 that comprises switch Sz and critical damping resistor
Rz. The rest of the circuitry is similar to its counterpart in
Figure 4.
The following analysis, as set forth in the above-referenced
U.S. Patent No. 5,977,768 illustrates how the signals
received from the different circumferential sectors can be
used in conjunction with information from the direction and
Q orientation module 59 (e.g. Figures 1, 4, and 7) in
determining properties of different portions (e. g. different
circumferential portions) of the investigation region. In
the simplified diagram of Figure 8, the axis of the drill
collar is designated as the z axis. The axis z is not
-~5 necessarily vertical but it is substantially in the
direction of drilling. Assume that a key mark is scribed axially
on the outer surface of the drill collar. This mark serves as a
reference for measurement of azimuth angle around the tool (tool
rotational orientation). The axis x is perpendicular to the axis
2a
z and it points radially from the axis of the drill collar to the
key mark as shown in Figure 8. The axis y is orthogonal to the
axes x and z. The axes x, y, z define an orthogonal reference
frame that rotates together with the tool. Magnetometers Mx and
25 My are mounted in the drill collar so that their sensitive axes
are x and y, respectively. Similarly, accelerometers Ax and Ay
are mounted so that their sensitive axes are x and y,
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CA 02270757 1999-OS-04
respectively. The magnetic and gravitational fields of the Earth
define two azimuth directions that are fixed with respect to the
Earth. Let B1 be the component of the geomagnetic field that is
perpendicular to the axis z of the drill collar. Similarly, let
Gi be the component of the gravitational field that is
perpendicular to the axis z of the drill collar. B1 points South
and G1 points down. Either one of these directions can be used
as the reference for azimuth that is fixed with respect to the
earth formation. The preferred reference is the magnetic one as
the measurement of acceleration can be confused by the drilling
vibration since acceleration and gravitational attraction are
fundamentally indistinguishable. A limitation of the described
technique for determining rotational orientation is when the axis
z of the drill collar, the geomagnetic field, and the
gravitational field are all aligned with each other. In such
case, there is a lack of an azimuth reference that is fixed with
respect to the earth. This could happen, for example, in
vertical wells close to the North or South Pole. The azimuth of
the key mark with respect to Bi is ~=atan2(-My,Mx) where atan2 is
the 4-quadrant inverse tangent function defined, for example, in
standard FORTRAN or C manuals or ISO 9899. Similarly, the
azimuth of the key mark with respect to G1 is ~=atan2(-Ay,Ax).
Both of the angles ~ and ~ are time dependent when the drill
string is rotating. Assume there are N receiver coils labeled
?5 n=1,2...N centered at azimuths 2nn/N on the drill collar,
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measured with respect to the key mark. The following components
of the tool are physically aligned in azimuth: the sensitive axis
of the accelerometer Ax, the sensitive axis of the magnetometer
Mx, the key mark, and the center of the receiver coil N. All of
these rotate together. The normalized sensitivity function of
receiver n is: f(~-2lzn/N) where ~ is the azimuth angle measured
with respect to the key mark on the drill collar. This function
can by obtained by using a radially oriented sheet-like specimen
and measuring the signal amplitude as a function of the azimuth
of the radially oriented specimen. The function f is by
definition periodic with period 2n. Ideally, the sensitivity
functions of the receivers is a partition of unity of design,
i.e..
N
1 = ~ f(~-2nlT/N) (1)
n - 1
This ensures that the receivers collectively do not have blind
zones in azimuth. Since the actual sensitivity functions are
:0 smoothly varying functions of azimuth, the receivers necessarily
have overlapping sensitive zones to satisfy the partition of
unity. If equation (1) is not satisfied, a linear combination of
the outputs of the receivers can be formed so that the linear
combinations are as close as possible to a partition of unity.
5 For a spin-echo that occurs at time instant t, the output of
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receiver n can be called e(t,n). Let ~r be the azimuth angle
measured with respect to the direction B1 in the Earth. The
function E(t,~r) is the azimuthally resolved signal at time t:
N
E ( t, fir) - ~ a ( t, n) f (fir-~ ( t) -2nn/N) (2)
n=1
The formula (2) gives a sequence of echoes E(t,~r) for each
azimuth fir. For each azimuth fir, these echoes can be analyzed in
the usual fashion as described, for example, in U.S. Patents
5,363,041 and 5,389,877. In two ways, formula (2) is in
agreement with common sense: First, in the event that all
receivers have the same output, i.e., e(t,l)=e(t,2)=...=e(t,N),
by partition of unity, the azimuthally resolved output E(t,~r)
becomes independent of the azimuth angle and equal to e(t,n) for
any n. Second, suppose the sensitivity functions f were
perfectly sharp; that is, each receiver were equally sensitive
to a range of azimuth values and had no sensitivity outside this
range:
~0
f (~) - 1 if -nlN < ~ ~nlN.
- 0 otherwise
'.5
This assumption is not realistic because magnetic fields in the
formation cannot have sharp transitions. In reality, f is a
bell-shaped curve. Therefore, the azimuthal resolution is not
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77483-26
sharp. Nevertheless, the'idealization (3) above leads to a
useful thought experiment. In this case, E(t,~) would be equal
to the output of one of the receivers: E(t,~r) - e(t,n), where n
is such that ~(t)+(2n-1)n/N < ~y _< ~(t)+ (2n+1)rc/N. That is, the
part of the formation that is in the ~n/N azimuthal neighborhood
of receiver coil n at the time instant t, would be assigned the
reading e(t,n). This is the intended action. The sensitivity
function f is actually a smoothly varying, bell-shaped curve_
Therefore, the results will vary smoothly as a function of
azimuth. The azimuthal resolution will be on the order of the
width of the bell-shaped curve f(~), which is about 2n/N.
Figure 9 is a flow diagram of a routine for controlling the
processor 450,. or other processor, to implement the processing~to
'! 5
obtain azimuthally resolved NMR measurements with respect to an
earth reference, in accordance with an embodiment of the
invention as set forth in the above-referenced U.S. Patent
No. 5,977,768. The block 910 represents the determination
of the sensitivity function f(~) which, as previously
described, can be performed before logging.
The magnetometer and/or accelerometer measurements are input, as
represented by the block 920. The azimuth of tool reference,
with respect to earth reference, can then be determined as a
function of time using the formulas ~=atan2(-My, Mx) or ~=atan2(-
Ay, Ax) described above, this being represented by the block 930.
The detected receiver segment outputs e(t,l), e(t,2), ...e(t,n)
-31-
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77483-2s
are input (block 940). Then, using equation (2),~the azimuthally
resolved output, with respect to the earth reference, can be
determined, as represented by the block 950. For processing at
further depths or time references, the block 920 is re-entered.
In an above described embodiment, there are four receiver
coil segments, e.g. the coil segments 210a, 210b, 210c and 210d
of Figures 3, 4, 5 and 6. One or a plurality of such segments
can be utilized in receiving spin echo signals from a portiow of
~~ the investigation region. When the logging device rotates, the
rotating antenna segment or segments can provide azimuthally
resolved NMR properties of the full 360 degree span of the
surrounding formations. It is within contemplation of this
,~5 invention to collect fewer echoes in accordance with the methods
set forth in U.S. Pat. 110. 5,705,927. It will be understood that
there are trade-offs between resolution, signal strength,
complexity and cost, when selecting the number, dimension, and
configuration of antennas. In the embodiment of Figure 10, in
2~ accordance with an embodiment of the invention as set forth in
the above-referenced U.S. Patent No. 5,977,768 a single
receiving antenna 210a (which may be, for example, one of
the quadrant receiver antennas of the Figure 5, 6
embodiment, which has a separate transmitting antenna 201) is
25 Shown and is mounted in the previously described manner. The
Figure also shows drill collar 230, non-conductive material 220,
and shield 224 which covers antenna 210a. The circuitry of
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77483-26
Figure 7 that is coupled with transmitting antenna 201 and
receiving antenna 210a can be used in conjunction with this
embodiment.
Figure 11 illustrates an embodiment of a logging device 200
(e.g. in Figure 1) in accordance with a further form of the
invention as set forth in the above-referenced U,S. Patent
Application No.5,977,768. In this embodiment, a
plurality of receiver coil components 1002 are located in a
respective plurality of axially oriented slots 1001 machined on
the external surface of the drill collar 230. At least three
such coils are preferred, with four or more being more preferred
RF coils 1002 are wound on ferrite rods'1003_ The axes of the
ferrite rods 1003, the axes of the slots 1001, and the axis of
the drill collar are all oriented in the same direction. The rf
coils 1002 are covered by a nonmagnetic and insulating material
such as ceramic, plastic, or rubber. Each coil is connected to
the acquisition and processing electronics, which can be similar
to that previously described in conjunction with Figure 4, via a
respective feed-through (as in Figure 3). The rotationally
symmetric transmitter field is produced by driving all coils in
parallel. The transmitted field is rotationally symmetric at
radial distances that are larger than the separation of the coils
1002. This embodiment leads to a relatively sturdy mechanical
design, but generally with less receiver sensitivity than the
prior embodiments.
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77483-26
In embodiments here~:af to bc: t.reated subseqi.zen.tly,
a region c>f genera.ll~ un:i_fc~rm :~t:at.:l.c; tic:ld magnitude anal
polarization produced. in t:Yce ta:~~rrr<~t ~c:n~: i s rF~lativel.~y long
in axial extent, a.nd an advant~ac:~ea A_s tYriat the :rf ant.E:~nn.a
used to obtain azimuthally z_e;~c~lvecY r~wasurerr~ents <:an also be
made relatively long i~n i_Yne a;m.~~:1 ciir-~~c_;.iorl, thereby
increasing the volumE of spi.n~ u1. ~ i_m~:~t c :1y sensed by t: he
antenna and :inc:reasir~g s:i_c~nal--t.c>-o~oi~;c~ r~.-at..i~~;. ':'.'h::~s
_i.ncrease
tends to offset the ciecrE~aSe i_~o tlne vcl.ume r;f spins t: hat are
obtained when the azi.rnutlial. rs~=ye of irvc~stigation i:
limited to a sector t hat i :, a f r ~~c t. i~.~ , ;, f ~ f ~.~1 :x.
circumference. A magnet ccnf Lg~:r<:at:;Lc.r:.c~f ti~<~ type shown
in
Figures 2 and can );ae u.1_iz~~s.,~ :Lr~ ~~ radi_a~~ly
5 ul~ ~o ~L:~ta
polarized static:rr~agrzc~tic~fz.el..c.:~ clecmall:y un__form
~hr<.~t i~~
over an investi.gatior, regic:~n .:a r7 c.1 has t~ substant=z_al aerial
extent. 'fhe useful ~:ropert:ies c~f tn:5.:~ type-: c~f stat:i.c:
magnetic. field can b~ imY~r_ove~.a us:i_ng t: he techniques and
structures disclosed in l).>, fat:ent. ~lurriber_ a>, <?46, 23~,
'The just re~ferE~nced cc7~:e.>nd:ir,~g A,pul;Lc:at:.ic:n also
discloses techniques and st..ru~~"cures u:;~ fu3_ in conjunca=ion
herewith for producir~~g an ax:i,.~ 1l y pc~:l.~~xvi.zed static magnetic
fie:Ld that is
CA 02270757 1999-OS-04
generally uniform over an investigation region having a
substantial axial extent. In the embodiments hereof to be
described subsequently, the rf field is azimuthally polarized in
the investigation region, so the static magnetic field in the
investigation region is tailored to have a radial and/or axial
polarization in the investigation region.
Figure 12 illustrates schematically an rf antenna in
accordance with one embodiment of the invention. The drill
collar (as in Figures 2 and 5) is represented at 23a. [In a
wireline embodiment, 230 can be the sonde pressure housing.]
Electrodes 1211 and 1212 are thin plates of conductive metal, for
example copper, disposed at a small radial distance outside the
drill collar. In the illustrated embodiment the electrodes are
cylindrical arcs with the smaller electrode 1211 preferably
subtending an arc in the range 10 degrees to 120 degrees and the
larger electrode preferably subtending an arc in the range 350
degrees to 240 degrees. [The total will be less than 360
degrees, as the longitudinal gaps between electrodes subtend at
least a few degrees each.] An insulating material, such as
?0 fiberglass, or a ferrite (which can enhance the efficiency of the
antenna), is disposed between the drill collar and the electrodes
(in region 1220), or between the sonde pressure housing, if
electrically conductive, and the electrodes. The plate
electrodes are covered with a tough protective sleeve or shield
5 (such as is shown at 1280 in Figure 13) which may, for example,
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CA 02270757 1999-OS-04
be formed of fiberglass, rubber, or a ceramic. An rf energizing
source 1261 is inductively coupled (e. g. using transformer 1265)
across the electrodes 1211 and 1212 at one longitudinal end
thereof, and the electrodes are coupled at the opposing end, in
this case by capacitor 1270, which passes the rf current). The
capacitive and inductive elements can provide resonance at the
desired frequency. If ferrite is not used in the antenna,
additional inductance may need to be added externally. If the
external inductance and tuning capacitance are located within the
pressure housing, that would reduce the degree to which the
antenna's resonant frequency changes with wellbore pressure
variations. If desired, matching of the drive to the antenna
could be further optimized by means of a second tap on the
external inductor or by use of a capacitive voltage divider using
-5 the external capacitor and/or other suitable means. The large
arrows in Figure 12 shows the directions of the axial current
flow for a particular instantaneous polarity of the rf source.
Figure 13 shows a cross-section of the Figure 12 antenna and
illustrates an advantage that can accrue in this and other
0 embodiments hereof by employing low profile antennas. The
diagram of Figure 13 shows the drill collar 230, insulating
material 1220, conductive plate (1211 or 1212), and the
protective sleeve or shield 1280, all contained in a recess or
groove 1231 of the drill collar 230. [The circuitry and wiring,
5 and the feedthroughs, which can be of conventional type, are not
-36-
CA 02270757 1999-OS-04
shown in the diagram.] In this embodiment the antenna is low
profile and can be formed in an outer groove in the drill collar
without necessarily reducing the inner diameter of the drill
collar, which is ordinarily done to increase strength in a region
of drill collar where the outer diameter has been recessed to
provide an antenna. Reference can be made, for example, to
Figures 2 and 5 which illustrate the decreased inner diameter of
the drill collar at the antenna position. It will be understood
that the configurations of Figures 2 and 5 might be implemented
in a low profile configuration as shown in Figures 12 and 13
without reduction of the drill collar inner diameter, since the
thickness of the antenna conductors) is relatively small.
Furthermore, in a form of the embodiment of Figures 12, 13, the
groove depth in the drill collar outer surface can be kept
sufficiently small even with a substantial axial extent of the
antenna that a reduction of the drill collar inner diameter may
be dispensed with. The thin plates of conductive material used
for the electrodes of the Figure 12, 13 embodiment and subsequent
embodiments also employing conductive plates, as well as other
subsequent embodiments employing spirally wound coils, facilitate
low profile antenna configurations that can permit dispensing
with the reduction of the inner diameter of the drill collar at
the rf antenna location.
In an example of fabrication of the plate antenna structure,
?5 the radial gap spacing between the drill collar (or pressure
-37-
CA 02270757 1999-OS-04
housing for a wireline implementation) and the electrodes can be
built accurately, such as by wrapping high-temperature fiberglass
onto the cylindrical collar/housing and machining it to.the
desired radius. For acoustic isolation, a layer of rubber or
other elastomer is molded onto the drill collar. The layer of
rubber is then wrapped with a sheet of copper which serves as a
ground plane. The antenna structure is then loosely assembled and
wrapped with a fiberglass or ferrite material. See e.g., U. S:
Pat. No. 5,644,231. If desired, a layer of rubber or other
elastomer can be molded onto the fiberglass for purposes of
further magnetoacoustic isolation. Then, the conductive
electrodes of copper or other suitable metal can be deposited
onto the rubber or attached under pressure with epoxy. A further
molding step can cover the electrodes with a layer of rubber and
then with epoxy or fiberglass and then with a layer of rubber for
protection. The entire "sandwich" of layers can be covered with
a tough, protective covering in the form of a tube or cylindrical
shells. .This protective covering will preferably be nonmetallic,
in order to allow sufficient transmission and reception of the
NMR rf pulses and formation echo signals and to eliminate
additional nearby sources of Johnson noise.
Figure 14 shows the modeled rf magnetic field (B1)
orientation and magnitude (given approximately by the lengths of
the arrows) for the antenna geometry of Figure 12, a 60 degree
-38-
CA 02270757 1999-OS-04
angular extent of the smaller segment, which is pointing upward
in this axial view. The fields inside the borehole (i.e., inside
the empty circular spot in the center) are not shown. It can be
noted that there is a strong front-to-back dissymmetry in the
magnitude of the field. Also, there is a region of reasonable
angular extent at the top of the plot where the rf field's
orientation is nearly azimuthal at a constant radius. This
region roughly defines the volume of spins contributing to the
NMR signal. The thickness of the resulting arc-shaped shell of
spins depends on the magnitude of the rf magnetic field B1 and on
the spatial gradient of the static field Bo.
Figure 15 shows contours of equal magnitude of the rf field
(B1) for the same 60 degree antenna geometry shown in Figure 14.
The contours are geometrically spaced in terms of the magnitude
of B1, so each additional contour (moving inward toward the
center of the antenna) denotes a ,/~2 greater field magnitude than
the previous contour. Thus, moving outward by 6 contours
corresponds to an 8-fold (or 2~6~2') decrease in the field
amplitude. The outer heavy-lined circle denotes the borehole
diameter (same as in Figure 14), and the inner heavy-lined circle
denotes the diameter on which the antenna electrodes are located.
The innermost (light-lined) circle denotes the diameter of the
drill collar or (for wireline application) the sonde pressure
housing, which is here concentric with the antenna electrodes.
Figure 16 shows the modeled rf magnetic field (B1)
-39-
CA 02270757 1999-OS-04
orientation and magnitude (given approximately by the lengths of
the arrows) for the antenna geometry of Figure 12, assuming now
only a 30 degree angular extent of the smaller segment, which is
again pointing upward in this axial view. Again, the fields
inside the borehole (i.e., inside the empty circular spot in the
center) are not shown. Now the front-to-back dissymmetry in the
magnitude of the field is somewhat more pronounced, and the
region of interrogation at the top of the plot is somewhat
smaller in angular extent. Thus, it is seen that varying the
angular extent of the antenna electrode segment can provide
considerable freedom in shaping the B1 field distribution.
Figure 17 shows contours of equal magnitude of the rf field
(B1) for the same 30 degree antenna geometry shown in Figure 16.
Again, the contours are geometrically spaced in terms of the
magnitude of B1, so each additional contour (moving inward toward
the center of the antenna) denotes a d2 greater field magnitude
than the previous contour. Thus, moving outward by 6 contours
corresponds to an 8-fold (or 2~6~z') decrease in the field
amplitude. Again, the outer heavy-lined circle denotes the
?0 borehole diameter (same as in the previous Figures), and the
inner heavy-lined circle denotes the diameter on which the
antenna electrodes are located. The innermost (light-lined)
circle denotes the diameter of the drill collar or (for wireline
application) the sonde pressure housing, which is here concentric
5 with the antenna electrodes.
-40-
CA 02270757 1999-OS-04
Figure 18 shows geometrically-spaced contours of equal
magnitude of the rf field (B1) for the 60 degree antenna geometry
of Figures 14 and 15. Now, however, the collar or pressure
housing (shown by the innermost, light-lined circle) is no longer
concentric with the electrodes but is, rather, eccentered by a
substantial fraction (70%) of the radial gap spacing. Again, the
contours are geometrically spaced in terms of the magnitude of
B1, so each additional contour (moving inward toward the center
of the antenna) denotes a ,r2 greater field magnitude than the
previous contour. It can be noted that this eccentering
increases the front-to-back dissymmetry in the field amplitude
and therefore can be used as a control parameter to tailor the
field geometry to the desired results. A similar effect can be
realized by maintaining the two segments concentric with the
drill collar or (for wireline application) the pressure housing
but locating them at different radial distances from the center
by, e.g., inserting them at different layers in a wrapped
cylindrical composite structure.
On a linear scale, Figure 19 shows the B1 rf field amplitude
on a cross section of the 60 degree antenna along its axis of
symmetry, i.e, from bottom to top in Figure 18. As previously,
only the fields outside the borehole radius are plotted. On the
right side of Figure 19 ("front" of antenna), the field amplitude
is shown for the case of the drill collar/pressure housing being
centered (lower curve), being eccentered by 40°s of the radial gap
-41-
CA 02270757 1999-OS-04
distance (middle curve), and being eccentered by 70% of the
radial gap distance (upper curve). On the left side of Figure 19
("back" of antenna), the field amplitude is shown for the
centered case (upper curve), the 40% eccentered case (middle
curve), and the 70% eccentered case (lower curve).
The embodiment of Figure 12, and similar configurations, can
generally be driven with circuitry of the type described above in
conjunction with Figures 4 and 7. Figure 20 illustrates a type
of circuit arrangement that can be utilized for transmitting and
receiving from the Figure 12 type of rf antenna. The circuit
elements 414, 412, and R1 are the same as in Figure 4, to provide
perspective for the circuitry shown. The line 2001 (which caries
the energizing and the received signals in this embodiment, is
coupled with tapped inductor 2005 in impedance matching fashion,
as previously described. [As above, a transformer drive can
alternatively be utilized.] The inductor 2005 is coupled across
the cylindrical arc shaped conductive plate electrodes 1211 and
1212, and capacitor 2015 is coupled across the other ends of the
plate electrodes.
Figure 21 illustrates an embodiment that utilizes four
quadrants of arc shaped conductive plate electrodes (also shown
individually in Figure 22), for obtaining azimuthally resolved
NMR signals as described in conjunction with Figures 3 and 4
above, but with the electrodes and magnetic fields associated
with the conductive plate electrodes. The transmitting/receiving
-42-
CA 02270757 2003-03-14
77483-26
circuitry can be of the type described in conjunction with Figure
4. One form of circuitry is illustrated in Figure 22 which shows
the individual conductive plates 1221, 1222, 1223, and
1224, which are coupled with the tapped inductor 2005 via
electronic switches 1231, 1232, 1233 and 1234, respectively, and
with capacitor 2015 via electronic switches 1241, 1242 1243, and
1244. As the switches cycle through their respective four
~ positions (e. g. under control of processor 450), it is seen that
the individual plate electrodes are coupled, one at a time,
betweeti~inductor 2005 and one side of capacitor 2015, and that
the other side of capacitor 2015 is coupled to each of the other
plate electrodes for return in the opposite direction to ground
~ reference potential. The azimuthally resolved NMR properties can
then be obtained in the manner previously described.
Referring to Figure 23A, there is shown a further embodiment
wherein a substantial slot 2482 is provided between arc shaped
~ plate electrodes 2111 and 2112, which can be coupled with the
electronics as previously described, e.g. in Figure 20. In this
case, the investigation region is in front of the slot, and the
rf magnetic field polarization is radial. This is illustrated in
Figure 23B, which illustrates B, in the investigation region as
being out of (or into) the plane of the paper. Accordingly, in
this case, the static magnetic field in the investigation region
should have axial polarization (as described in the U. S.
Patent No.6,246,236), transverse, or azimuthal: polarization_
-43-
CA 02270757 2003-03-14
77483-26
It will be understood that a plurality of antennas at
different positions, as shown in Figure 23A, could also be
utilized (as in Figure 21).
In the embodiment of Figures 24A and 248, a single arc-
shaped plate electrode 2411 is utilized. Here, as above;
instantaneous current flow will be as shown by the large arrow,
and the rf magnetic field polarization in the investigation
region of the formations (generally opposing the electrode) will
be azimuthally polarized (for use in conjunction with a static
magnetic field polarization in the investigation region that is
radial and/or axial). The energizing/receiving circuitry of the
antenna can be provided floating, as above, or, as is shown in
Figure 24B, can use the drill collar 230 as a return path for the
rf currents. The components 1261, 1265 and 1270 correspond to
those of like reference numerals in Figure 12.
In embodiments of the invention to be described next, one or
more coils are utilized to obtain rf magnetic fields (BI) that
2p are azimuthally polarized in the investigation region, and which
have substantial axial extent. The arc-shaped plate electrodes)
can be envisioned as being replaced by coils) wound on an axis
that is perpendicular to the tool axis (which is, generally, the
borehole axis), and in which current flows primarily axially,
this being shown conceptually in Figures 25A and 25B. IThe coils
are wound to conform to the cylindrical arc of the drill collar
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CA 02270757 1999-OS-04
contour, and preferably have an axial extent that is
substantially greater than their circumferential extent. The
instantaneous current flow in the plate electrodes as 1211 and
1212, as previously described, is illustrated in Figure 25A, and
the analogous current flow in the coils 2501 and 2502 is shown in
Figure 25B. The coil or coils can be wound in spiral fashion
(e. g. with increasing periphery dimensions for each successive
turn, as shown conceptually in Figure 26), to minimize coil
thickness in the radial direction, and can be in the positions)
of the plate or plates as shown above in Figure 13, with
insulator at 1220 and protective covering at 1280 within the
groove 1231 in drill collar 230. This facilitates a low profile
type of structure, as first described above.
Figure 27 illustrates a twin-loop multi-turn (8 turns being
shown in this example) planar loop antenna, depicted unwrapped
from the tool body. The antenna is shown as being driven by an
rf source, V (with the sensed signals, during the receiving mode,
being detected as first shown above, for example, in Figure 4) at
a tap to match the resonant antenna's resistance to line
ZO resistance Ro. The capacitor (or capacitors), C, tune the
antenna.
Figure 28 shows the rf magnetic field pattern of the antenna
of Figure 27, as a function of azimuth angle, as seen along the
centerline of the antenna. The magnetic field, Bm, is a
:5 tangential component; i.e., as indicated above, polarized in the
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CA 02270757 1999-OS-04
azimuth direction. The side lobes add to the NMR signal
sensitivity, which is shown in the shaded regions in Figure 28.
It will be understood that a plurality of antennas can be
employed, as represented, for example, in Figure 23A (with the
coil antennas substituted for the plate electrodes), and the
array can be driven with circuitry similar to that of Figure 4.
Figure 29 shows a further embodiment of a multi-turn planar
loop antenna, unwrapped from the tool body. This antenna can be
visualized as half the antenna of Figure 27 (i.e., single loop),
with eight turns being shown in the single loop of this example.
Figure 30 shows the rf magnetic field pattern of the antenna of
Figure 29, as a function of azimuth angle, as seen along the
centerline of the antenna. Again, the magnetic field, Bm, is a
tangential component, and the two lobes are of opposite polarity.
The NMR signal sensitivity, which exhibits an approximately 90
degree pattern and good front-to-back isolation, is shown in the
shaded area. Again, a plurality of antennas can be used, as in
Figure 23A, and driven using the Figure 4 type of circuitry.
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