Note: Descriptions are shown in the official language in which they were submitted.
CA 02232654 1998-03-18
NMR WELL LOGGING APPARATUS AND METHOD
BACKGROUND OF THE INVENTION
Field of the Invention
The invention is related to the field of nuclear magnetic resonance ("NMR")
apparatus and measuring techniques. More specifically, the invention is
related to well
logging apparatus and measuring techniques for NMR measurement within earth
formations penetrated by a wellbore.
Description of the Related Art
NMR well logging instruments typically include a permanent magnet to induce a
static magnetic field in the earth formations and a transmitting antenna,
positioned near the
magnet and shaped so that a pulse of radio frequency (RF) power conducted
through the
antenna induces an RF magnetic field in the earth formation. The RF magnetic
field is
generally orthogonal to the static magnetic field. After an RF pulse, voltages
are induced in
a receiving antenna positioned near the magnet by precessional rotation of
nuclear spin
axes of hydrogen or other nuclei about the static magnetic field. The
receiving antenna is
typically connected to a receiver which detects and measures the induced
voltages.
In a typical NMR measurement set a sequence of RF pulses is applied and a
sequence of voltages is measured. The magnitudes of the voltages and the rates
at which
the voltages vary are related to certain petrophysical properties of the earth
formation.
These properties can include the fractional volume of pore space, the
fractional volume of
mobile fluid filling the pore spaces of the earth formations and other
petrophysical
parameters. Methods and measurement techniques for using NMR measurements to
determine the fractional volume of pore space, the fractional volume of mobile
fluid and
other petrophysical parameters are described, for example, in Shin Echo Ma,_
ng etic
Resonance Logging: Porosity and Free Fluid Index Determination, M. N. Miller
et al,
Society of Petroleum Engineers paper no. 20561, Richardson, TX, 1990 and in
Field Test
of an Experimental Pulsed Nuclear Magnetism Tool, C. E. Morriss et al, SPWLA
Transactions 1993, paper GGG.
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One NMR well logging instrument is described, for example, in U. S. patent no.
3,597,681 issued to Huckbay et al. A drawback to the apparatus disclosed in
the Huckbay
et al '681 patent is that a region of unidirectional magnetic field (the
"sensitive region")
induced in the formation by the magnet is not homogeneous along the wellbore
axis.
Logging tools typically must be able to move axially through the wellbore
while
performing measurements. During the time needed to make a typical NMR
measurement
set the sensitive region will change its position before the measurement set
can be
completed, leading to error in the measurements. Another drawback to the
apparatus
disclosed in the Huckbay et al '681 patent is that a significant part of the
NMR signal can
originate within a fluid ("drilling mud") filling the wellbore. Another
drawback to the
apparatus disclosed in the Huckbay et al '681 patent is that the RF magnetic
field decreases
in amplitude with respect to the third power of the distance between the
antenna and the
sensitive region, as the antenna can be modeled as the equivalent of a three
dimensional
magnetic dipole. Such an antenna is proximally coupled to only a small part of
the
unidirectional static magnetic field. This results in an extremely low signal-
to-noise ratio.
Another drawback to the apparatus disclosed in the Huckbay et al '681 patent
is that the
antenna is subjected to a very high static magnetic field strength and will
have an
unacceptably large amount of magnetoacoustic ringing as a result.
Another type of NMR well logging apparatus is described in U. S. patent
no.4,350,955 issued to Jackson et al. The apparatus disclosed in the Jackson
et al '955
patent includes permanent magnets configured to induce a static magnetic field
in the earth
formations which has a toroidal volume of substantially uniform magnetic field
strength. A
particular drawback to the apparatus disclosed in the Jackson et al '955
patent is that the
thickness of the toroidal volume is very small relative to typical rates of
axial motion of
well logging tools. Well logging tools, in order to be commercially useful,
typically must
be able to be moved axially through the wellbore at rates not less than about
ten feet per
minute. The length of time needed to make a typical NMR measurement set can be
as long
as several seconds. The NMR logging tool is therefore likely to move a
substantial
distance during a measurement cycle. Measurements made by the apparatus
disclosed in
the Jackson et al '955 patent are therefore subject to error.
Another drawback to the apparatus disclosed in the Jackson et al '955 patent
is that
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it does not eliminate NMR signal originating within the fluid filling the
wellbore. A still
further drawback to the apparatus disclosed in the Jackson et al '955 patent
is that the
toroidally shaped static magnetic field is subject to changes in field
strength as the
instrument is subjected to changes in ambient temperature and variances in the
earth's
magnetic field. The antenna in the Jackson et al '955 apparatus is tuned to a
single
frequency. If the field strength of the static magnetic field in the toroidal
volume changes,
the antenna may no longer be sensitive to NMR signals originating within the
toroidal
volume. Using the apparatus in Jackson et al '955, it is impractical to
compensate the
frequency of the RF magnetic field for changes in the static magnetic field
strength within
the toroidal volume.
Additional drawbacks to the apparatus disclosed in the Jackson et al '955
patent are
as follows. Since the magnet pole pieces are in opposed polarity to each
other, there is a
significant demagnetizing effect which requires a permanent magnet material
having high
coercive force. This requirement is opposite to the strong residual
magnetization and high
temperature stability of magnetic properties required of the permanent magnet.
The
magnet pole pieces are significantly spaced apart and far from the magnetic
field's
homogeneous region, which makes the usage of the permanent magnet material
less cost-
effective. Low antenna efficiency is a result of low electro-magnetic coupling
between the
antenna and the earth formation at resonance. The antenna is located in a
relatively strong
magnetic field, which stimulates strong magnetoacoustic ringing in the
antenna. Because it
uses a homogeneous magnetic field, any changes in the orientation of the
apparatus with
respect to the earth magnetic's field can result in a significant disturbance
to the
homogeneity of the static magnetic field. Furthermore, some techniques for
diffusion
measurement require a substantial magnetic field gradient in the static
magnetic field,
which are made impossible by the homogeneous static magnetic field of the
Jackson et al
'955 apparatus.
Another type of NMR well logging apparatus is described in U. S. patent
no.4,717,876 issued to Masi et al. 'The apparatus disclosed in the Masi et al
'876 patent has
a "toroidal" static magnetic field providing improved homogeneity in the
toroidal region as
compared to apparatus in the Jackson et al '955 patent, but the Masi et al
apparatus has
basically the same drawbacks as the Jackson et al apparatus.
Another type of NMR well logging apparatus is described in U. S. patent no.
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4,629,986 issued to Clow et al. This apparatus provides improved signal-to-
noise ratio,
when compared to the apparatus of the Jackson et al '955 patent, by including
a high
magnetic permeability fernte in the antenna. Increased stability is achieved
by having a
static magnetic field gradient as part of the static magnetic field in the
sensitive region.
However, the apparatus disclosed in the Clow et al '986 patent has the
following
drawbacks. Since the magnetic properties of the permanent magnet material are
temperature dependent, the sensitive region is not stable in shape and field
intensity.
The sensitive region is only a couple of inches long in the longitudinal
direction, which
requires this tool to be practically motionless during an NMR measurement
cycle. Magnet
pole pieces are significantly spaced apart and far from the homogeneous field
region, which
makes the usage of permanent magnet material not cost-effective. The antenna
is located in
a relatively strong static magnetic field, which stimulates magnetoacoustic
ringing in the
antenna. 'The high magnetic permeability fernte in the antenna is located in a
relatively
strong magnetic field which may saturate the ferrite and reduce its
efficiency. Soft fernte
in a static magnetic field is also a strong source of magnetostrictive ringing
following each
RF pulse. In the magnet arrangement disclosed by Clow et al the demagnetizing
field is
substantially strong, which requires magnet material having a high coercive
force. This
requirement is opposite to the strong residual magnetization and high
temperature stability
of magnetic properties required of the permanent magnet. The static magnetic
field in the
earth formation at resonance is only about 10 Gauss and rotates 360° in
a plane
perpendicular to the wellbore axis. For this level of static magnetic field,
the earth's
magnetic field of about 0.5 Gauss presents a significant disturbance to the
static field
induced by the magnet.
Another type of NMR well logging apparatus is described in U. S. patent no.
4,717,878 issued to Taicher et al provides azimuthal resolution with respect
to the wellbore
axis and reduction of spurious signal from wellbore fluid. However, the
apparatus
disclosed in the Taicher et al '878 patent has the following drawbacks. Since
the magnetic
properties of the permanent magnet material are temperature dependent, the
sensitive
region is not stable in shape and field intensity. The antenna is located in a
relatively strong
magnetic field, which stimulates magnetoacoustic ringing in the antenna. In
the magnet
arrangement disclosed by Taicher et al, the demagnetizing field is very
strong, which
requires a magnet material having high coercive force. This requirement is
opposite to the
CA 02232654 1998-03-18
strong residual magnetization and high temperature stability of magnetic
properties
required of the permanent magnet. Due to the disadvantages of the foregoing
NMR well
logging instruments, none of them generally has become commercially accepted.
One NMR well logging apparatus which has become commercially accepted is
5 described in U. S. patent no. 4,710,713 issued to Taicher et al. The
apparatus disclosed in
the Taicher et al '713 patent includes a substantially cylindrical permanent
magnet
assembly which induces a static magnetic field having substantially uniform
field strength
within an annular cylindrical volume. The apparatus disclosed in the Taicher
et al '713
patent, however, has several drawbacks. First, the antenna induces an RF
magnetic field in
the formations surrounding the tool which decreases in strength as the square
of the radial
distance from the axis of the magnet. Moreover, a significant portion of the
RF energy can
be lost in an electrically conductive fluid in the wellbore. Because the
signal-to-noise ratio
of NMR measurements made in a gradient magnetic field is typically related to
the strength
of the RF magnetic field, the apparatus disclosed in the Taicher et al '713
has large
electrical power requirements and can have difficulty obtaining measurements
having
sufficient signal-to-noise ratio at substantial radial distances from the axis
of the
instrument.
Another drawback to the instrument described in the Taicher et al '713 patent
is that
the optimum design of the magnet and the RF antenna requires the resonance
conditions to
be met at a relatively high frequency to obtain a suitable signal-to-noise
ratio. Since the RF
energy losses in the fluid in the wellbore (if it is conductive) are
proportional to the square
of the frequency, the operation of the Taicher et al '713 patent is generally
restricted to
operating in a low electrical conductivity wellbore fluid.
Another drawback to the apparatus described in the Taicher et al '713 patent
is that
the optimum design of the magnet and the RF antenna requires the sensitive
volume to be
at about 12 inches to 15 inches in diameter in order to provide acceptable
signal-to-noise
ratio. Many wellbores are inclined from vertical and logging tools cannot be
ideally
centralized in such wellbores. Moreover, the wellbore can sometimes have a
very large
internal diameter as a result of "washouts" or similar effects known in the
art. For
wellbores having a nominal diameter of larger than about 10 inches, and
particularly those
highly inclined form vertical, the sensitive volume of this apparatus may be
positioned at
least partially within the wellbore itself rather than wholly within the earth
formation
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leading to errors in the measurement.
Yet another drawback to the apparatus described in the Taicher et al '713
patent is
that the antenna is located in a relatively strong magnetic field, which is
perpendicular to a
direction of RF current flow in the transmitting antenna and, therefore,
stimulates
S magnetoacoustic ringing in the transmitting antenna.
Another commercially accepted NMR logging apparatus is described in U. S.
patent no. 5,055,787 issued to Kleinberg et al. This logging instrument
includes permanent
magnets arranged to induce a magnetic field in the earth formation having
substantially
zero static magnetic field gradient within a predetermined sensitive volume.
The magnets
are arranged in a portion of the tool housing which is typically placed in
contact with the
wall of the wellbore. The antenna in this logging instrument is positioned in
a recess
located external to the tool housing, enabling the tool housing to be made
from a high
strength material such as steel. A drawback to the instrument described in the
Kleinberg et
al '787 patent is that its sensitive volume is only about 0.8 cm away from the
tool surface
and extends only to about 2.5 cm radially outward from the tool surface.
Measurements
made by this instrument are therefore subject to large error caused by, among
other things,
roughness in the wall of the wellbore, by deposits of the solid phase of the
drilling mud
(called "mudcake") onto the wall of the wellbore in any substantial thickness,
and by the
fluid content of the formation in the "invaded zone" (typically defined as the
zone in which
the liquid phase of the drilling mud infiltrates the pore spaces of the
formation proximal to
the wellbore).
Another drawback to the apparatus disclosed in the Kleinberg et al '787 patent
relates to the magnet material used. Since the magnet pole pieces are opposed
each other,
there is a strong demagnetizing effect which requires a magnet material having
a high
coercive force. This requirement is opposite to the strong residual
magnetization and high
temperature stability of magnetic properties required of the permanent magnet.
All of the prior art NMR well logging instruments described herein typically
have
antennas for generating the RF magnetic field and for receiving the NMR
signals which are
substantially the same length as the axial extent of the static magnetic
field. A drawback to
prior art NMR apparatus having such antenna dimensions is that measurements
made in
which the instrument is moving are subject to significant error. The first
source of error is
that the RF magnetic field may be generated in a region different from that
which is
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completely "prepolarized" by the static magnetic field. A second source of
error is that the
receiving antenna may be sensitive to an axial region which is different from
the axial
region in which the NMR signal is likely to originate, as the instrument is
axially moved
during measurement.
Prior art NMR well logging instruments have a common drawback as explained,
for
example, in U. S. patent no. 5,332,967 issued to Shporer. This drawback is
related to a
significant phase shift of the NMR signal, which leads to significant
distortion of the signal
height and may even lead to a complete disappearance of the signal, when the
logging
apparatus is moving in a direction along a static magnetic field magnitude
gradient. This
signal reduction may become even more pronounced when the speed of motion of
the
instrument is variable and uncontrolled. Causes of variation in the speed of
motion of a
logging instrument are well known in the art.
SUMMARY OF THE INVENTION
The invention is a nuclear magnetic resonance well logging apparatus including
a
magnet for inducing a static magnetic field in a substantially cylindrically-
shaped sector.
The sector is located substantially entirely within earth formations
penetrated by a
wellbore. The sector subtends an angle araund the axis of the cylinder of
about 60
degrees, and the sector is located only on one side of the wellbore. The
longitudinal axis
of the sector is substantially parallel to the wellbore. The apparatus also
includes a
transmitter for generating a radio frequency magnetic field in the sector for
exciting
nuclei of the earth formations. The radio frequency magnetic field and the
static
magnetic field satisfy nuclear magnetic resonance excitation conditions
substantially
exclusively within the sector. The apparatus further includes a receiver for
detecting
nuclear magnetic resonance signals from the excited nuclei. In one embodiment
of the
invention, the static magnetic field has a longer dimension in the sector
along the
longitudinal axis in the direction of motion of the apparatus than the active
length of the
transmitter, and the active length of the transmitter is greater than the
active length of the
receiver in the direction of motion so that nuclear magnetic resonance is
excited only in
substantially fully polarized nuclei, and NMR signals are detected only from
substantially fully radio frequency- excited nuclei.
In a particular embodiment of the invention, the transmitter includes two
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g
substantially identical antenna coils having sensitive axes rotated 90 degrees
with respect
to each other, and the coils are energized by RF power sources having a 90
degree phase
shift between them to induce a circularly polarized RF magnetic field in the
sector. In the
particular embodiment, the receiver includes two substantially identical coils
having
sensitive axes rotated 90 degrees with respect to each other, and the receiver
includes
circuits for phase-sensitive quadrature detection of the NMR signals.
The preferred embodiment of the invention comprises a permanent magnet
assembly having two opposed polarity permanent magnets polarized along the
longitudinal axis of the instrument. The magnets have dimensions selected to
induce a
static magnetic field within the sector which has radial and longitudinal
amplitude
gradients specifically related to a speed of motion of the logging instrument
along the
wellbore and an expected speed of radial motion of the logging instrument
within the
wellbore.
In accordance with one aspect of the present invention there is provided a
nuclear
magnetic resonance well logging apparatus, comprising: a magnet for inducing a
static
magnetic field in earth formations penetrated by a wellbore, said magnet
polarized along
a longitudinal axis of said apparatus, said static magnetic field having a
maximum
longitudinal amplitude gradient related to an expected speed of motion of said
apparatus
along said wellbore; a transmitter for inducing a radio frequency magnetic
field in a
substantially cylindrical sector, on one side of said wellbore, said radio
frequency
magnetic field polarized substantially orthogonally to said static magnetic
field, said radio
frequency magnetic field for exciting nuclei in said earth formation, said
static magnetic
field and said radio frequency magnetic field particularly shaped to excite
nuclei within
said sector, said sector having a longitudinal axis substantially parallel to
said
longitudinal axis of said apparatus; a receiver for detecting nuclear magnetic
resonance
signals from said sector; said transmitter comprising: two substantially
identical antennas
each having a sensitive axis rotated 90 degrees with respect to the other,
said antennas
each energized by a radio frequency power source having a 90 degree phase
shift with
respect to the other said power source to induce a circularly polarized radio
frequency
magnetic field in said sector; and said receiver comprises two substantially
identical
antennas each having a sensitive axis rotated 90 degrees with respect to the
other, said
receiver including circuits for phase-sensitive quadrature detection of the
NMR signals
connected to said receiver antennas.
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8a
In accordance with another aspect of the present invention there is provided a
nuclear magnetic resonance well logging apparatus, comprising: a magnet for
inducing a
static magnetic field in earth formations penetrated by a wellbore, said
magnet polarized
along a longitudinal axis of said apparatus, said static magnetic field having
a maximum
longitudinal amplitude gradient related to an expected speed of motion of said
apparatus
along said wellbore; a transmitter for inducing a radio frequency magnetic
field in a
substantially cylindrical sector on one side of said wellbore, said radio
frequency
magnetic field polarized substantially orthogonally to said static magnetic
field, said radio
frequency magnetic field for exciting nuclei in said earth formation, said
static magnetic
field and said radio frequency magnetic field particularly shaped to excite
within said
sector, said sector having a longitudinal axis substantially parallel to said
longitudinal
axis of said apparatus; a receiver for detecting nuclear magnetic resonance
signals from
said sector; said transmitter comprising: an antenna having a sensitive axis
orthogonal to
that of an antenna forming part of said receiver, whereby substantially no
signal is
induced directly into said receiver antenna by radiation from said transmitter
antenna.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure Z shows a nuclear magnetic resonance well logging apparatus disposed in
a
wellbare penetrating earth formations.
Figure 2 shows an NMR probe portion of the apparatus of Figure 1 in more
detail.
Figure 3A shows a detailed drawing of a transmitter antenna for generating a
circularly polarized RF magnetic field.
Figure 3B shows a detailed drawing of a receiving antenna for quadrature
two-channel phase-sensitive detection of NMR signals.
Figure 4 shows a functional block diagram of the NMR apparatus of the present
invention.
Figure 5 shows a graphic representation of a static magnetic field and a radio
frequency (RF) magnetic field.
Figures 6A and 6B show a detailed drawing of a magnet for the invention.
Figure 7A shows a graph of the static magnetic field in the X-Y plane as
induced
by the magnet of Figure 6 in a sensitive volume in the earth formations
surrounding the
logging instrument.
Figure 7B shows a graph of the static magnetic field in the X-Z plane induced
by
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9
the magnet of Figure 6 in the sensitive volume.
Figure 8 shows a graph of the sensitive volume as a cross-plot of the static
magnetic field induced by the magnet of Figure 6 and the RF magnetic field.
Figure 9 shows a detailed graph of the static magnetic field induced by the
magnet of Figure 6 within the sensitive volume.
DESCRIPTION OF THE PREFERRED EMBODIMENT
1. Overall Confi;~uration of the Well Logging-AApparatus
Figure 1 shows a string of logging tools 32 disposed in a wellbore 22 drilled
through earth formations 23, 24, 26, 28 for the purpose of making measurements
of
properties of the earth formations 23, 24, 26, 28. The wellbore 22 in Figure 1
is typically
filled with a fluid 34 known in the art as "drilling mud". A "sensitive
volume", shown
generally at 58 and having generally the shape of a cylindrical sector, is
disposed within
one of the earth formations, shown at 26. The sensitive volume 58 is located
on one side
of the wellbore 22 and is a predetermined portion of the earth formations 26
in which
nuclear magnetic resonance ("NMR") measurements are made by the logging
instrument,
as will be further explained in detail.
A string of logging tools 32, which can include an NMR logging instrument
designed according to the invention, is typically lowered into the wellbore 22
by a means
of an armored electrical cable 30. The cable 30 can be extended into and
withdrawn
from the wellbore 22 by means of a winch or drum 48 or similar device known in
the art.
The tool string 32 can be electrically connected to surface equipment 54 by an
insulated
electrical conductor (not shown separately in Figure 1) forming part of the
cable 30. The
surface equipment 54 can include one part of a telemetry system 38 for
communicating
control signals and data between the tool string 32 and a computer 40. The
computer can
also include a data recorder 52 for recording measurements made by the logging
apparatus and transmitted to the surface equipment 54 over the cable 30.
An NMR probe 42 according to the invention can be included in the string of
logging tools 32. The NMR probe 42 preferably has a face 21 placed in contact
with the
wellbore wall, and having an appropriate curvature so that only a very small
gap
generally exists between the face 21 and the wellbore 22 wall. The probe 42
can also
CA 02232654 1998-03-18
have a selectably extensible arm 48, or similar means for urging the probe 42,
which can
be activated to press the probe 42 in the direction of the wellbore 22 wall,
so that the face
21 is firmly pressed against the wellbore 22 wall during measuring operations.
Circuitry for operating the NMR probe 42 can be located within an NMR
5 electronics cartridge 44. The circuitry can be connected to the NMR probe 42
through a
connector 50. The NMR probe 42 is typically located within a protective
housing 43
which is designed to exclude the drilling mud 34 from the interior of the
probe 42. The
function of the probe 42 and the circuitry in the cartridge 44 will be further
explained.
Other well logging sensors may form part of the tool string 32. As shown in
10 Figure l, one of the additional logging sensors, shown at 47, may be
located above the
NMR electronics cartridge 44. Other logging sensors, such as shown at 41 and
46 may
be located below the NMR probe 42. The other sensors 41, 46, 47 can be of
types
familiar to those skilled in the art and can include, but are not limited to,
gamma ray
detectors, formation bulk density sensors or neutron porosity detectors.
Alternatively,
parts of the NMR electronics may be located within electronic cartridges which
form part
of other logging sensors 41, 46, 47. The locations of, and types of, the other
sensors 41,
46, 47 shown in Figure 1 are a matter of convenience for the system designer
and are not
to be construed as a limitation on the invention.
Figure 2 shows the NMR probe 42 in more detail. The NMR probe 42 preferably
includes a permanent magnet assembly 60. The magnet assembly 60 can include a
permanent magnet 62, which is generally elongated along a magnet axis 80 and
preferably has a generally circular cross section perpendicular to the
magnet's axis 80.
The magnet axis 80 is should be substantially parallel to the axis 76 of the
wellbore (22
in Figure 1). Alternatively, a plurality of smaller permanent magnets (not
shown
separately) may be assembled together to make up the magnet assembly 60. For
clarity
of the description of the invention, the one or more permanent magnets will be
considered together and referred to as the permanent magnet 62, and their
common axis
80 will be jointly identified as the "longitudinal axis", as shown at 78.
The magnetization direction of the permanent magnet 62 is preferably parallel
to
the longitudinal axis 78. The dimensions of the permanent magnet 62 in cross-
section
along the magnet axis 80 affect the characteristics of the static magnetic
field which is
induced by the permanent magnet 62 within the sensitive volume 58. The
dimensions of
CA 02232654 1998-03-18
11
the magnet 62 which will provide the desired characteristics in the sensitive
volume 58
will be described in more detail later. An important feature of the magnet 62
is that the
static magnetic field as it is induced within the sensitive volume 58 is
generally in the
shape of a cylindrical sector having a "sensitive volume longitudinal axis" 75
substantially parallel to the longitudinal axis 78. The direction of the
static magnetic
field within the sensitive volume 58 induced by the magnet 62 is substantially
parallel to
both the sensitive volume longitudinal axis 75 and to the longitudinal axis
78.
Permanent magnet materials from which the permanent magnet 62 should be
made are substantially transparent to RF energy at the frequency used for NMR
measurement by the probe 42, so that an antenna used to generate a radio
frequency
magnetic field can be located on the outer surface of the permanent magnet 62,
as will be
further explained. One type of material suitable for the magnet 62 is a
ferrite magnet
material such as that sold under the trade name "Spinalor" and manufactured by
Ugimag,
405 Elm St., Valparaiso, IN. Another suitable ferrite magnet material is sold
under the
trade name "Permadure" and is manufactured by Philips, 230 Duffy Ave.,
Nicksville,
NY. These materials are only presented here as examples of suitable magnet
materials
and are not meant to limit the magnet materials which may be used in the
invention.
The NMR probe 42 also includes a transmitter antenna 67, which can comprise
one or more coil windings, as will be further explained in more detail. The
transmitter
antenna 67 is preferably arranged along the exterior surface of the magnet 62
adjacent to
the probe face 21. Radio frequency (RF) alternating current passing through
the
transmitter antenna generates an RF magnetic field in the earth formation (26
in Figure
1). The RF magnetic field should have field directions substantially
perpendicular to the
sensitive volume longitudinal axis 75 where the RF field passes through the
sensitive
volume 58.
The overall length of the transmitter antenna 67 parallel to the longitudinal
axis
78 should be substantially longer than the length of the antenna 67
perpendicular to the
longitudinal axis 78, so that the transmitter antenna 67 will function
essentially as a two-
dimensional magnetic dipole. Such an antenna generates substantially equal RF
magnetic field amplitudes at any location within the sensitive volume 58. In
addition,
the overall length of the transmitter antenna 67 parallel to the longitudinal
axis 78 should
be substantially shorter along a direction of movement of the NMR probe 42, as
denoted
CA 02232654 1998-03-18
12
by arrow 81, than the overall length of the permanent magnet 62 along the
longitudinal
axis 78, as will be further explained.
The NMR probe 42, can also include a receiver antenna 70, which can include
one or more coil windings, preferably arranged along the exterior surface of
the
permanent magnet 62 adjacent to the probe face (21 in Figure 1). Preferably
the receiver
antenna 70 has an overall length parallel to the longitudinal axis 78 which is
less than the
overall length of the transmitter antenna 67 in the direction of movement 81.
As a
consequence, the overall length of the receiver antenna 70 parallel to the
longitudinal
axis 78 should be substantially shorter in the direction of movement 81 than
the length of
the permanent magnet 62 along the longitudinal axis 78. A particular property
of the
receiver antenna 70 arrangement as described herein is that it is
substantially orthogonal
to, and consequently substantially insensitive to, the direct RF magnetic
field generated
by the transmitter antenna 67. This insensitivity to the direct RF field
enables the
receiver coil 70 to provide the logging apparatus with a very short "dead
time", while the
current flowing through the transmitter antenna 67 is decaying to zero after
application of
each RF power pulse. An alternative arrangement of the transmitter antenna 67,
which
will be described later in more detail, may be used to generate a circularly
polarized RF
magnetic field in the sensitive volume 58 which utilizes only half the RF
power as
compared to that used for a linearly polarized RF magnetic field.
Additionally, as will be
further explained, two signals in quadrature may be induced by the nuclear
magnetic
resonance signals and into a particular embodiment of the receiver antenna 70
and be
detected in quadrature with respect to each other. Details of the static
magnetic field in
the sensitive volume 58 and details of generating the radio frequency magnetic
field in
the sensitive volume 58 for exciting nuclei of the earth formations using the
transmitter
antenna 67 and detecting an induced NMR signal using the receiver antenna 70
will be
further explained.
The permanent magnet 62, the transmitter antenna 67 and the receiver antenna
70
are preferably housed within an RF transparent protective housing 43. Such
housings
and additional components (not shown) for excluding the drilling mud under
high
hydrostatic pressure, are familiar to those skilled in the art.
CA 02232654 1998-03-18
13
2. Transmittin~~ and Receiving Antenna Arran ement
Figure 3A shows an embodiment of the transmitter antenna (67 in Figure 2)
which further improves the performance of the apparatus of the invention.
Transmitting
an NMR experiment requires a "rotating" or circularly polarized RF magnetic
field. One
way to transmit such a field is by applying a linearly polarized RF magnetic
field, which
can be represented as two counter-rotating components. Only one component is
useful in
the NMR experiment; the second one is redundant. This wastes half of the RF
power
applied to the antenna (67 in Figure 2). In well logging applications, the
amount of
power available is limited since it must be transmitted along the cable (30 in
Figure 1 )
and, therefore, its conservation is important. Alternatively, the RF power can
be traded
for signal-to-noise improvement. The transmitter antenna 67 shown in Figure 3A
includes a first transmitter coil 85 and a second transmitter coil 86. Both
transmitter
coils are preferably arranged along the exterior surface of the permanent
magnet 62
adjacent to the probe face (21 in Figure 1). These coils 85, 86 are
substantially identical
in form, but each coil is oriented so as to have its sensitive direction 90
° rotated respect
to the other coil about its longitudinal axis. The longitudinal axis of each
coil 85, 86 is
parallel to the longitudinal axis 78.
Figure 3B shows an embodiment of the receiver antenna (70 in Figure 2) which
comprises a first receiving coil 71 and a second receiving coil 72. Both
receiver coils 71,
72 are preferably arranged along the exterior surface of the permanent magnet
62
adjacent to the probe face (21 in Figure 1). These coils 71, 72 are
substantially identical
in form, but each coil is rotated 90 ° with respect to the other about
its longitudinal axis.
The longitudinal axis of each receiver coil 71, 71 is substantially parallel
to the
longitudinal axis 78.
The advantages of having two separate RF coils orthogonal to each other, where
both coils may generate an RF magnetic field orthogonal to the static magnetic
field
within the sensitive volume (58 in Figure 2), include the fact that separate
orthogonal
transmitting and receiving coils may be individually optimized. There is also
the
potential to improve the signal-to-noise ratio by a factor 2 for any
particular amount of
RF power used to generate the RF magnetic field. This mprovement in the signal-
to-
noise ratio may be achieved by a technique known in the art of NMR measurement
as
CA 02232654 1998-03-18
14
two-channel quadrature phase-sensitive detection. Detailed design and
necessary
circuitry for orthogonal transmitting and two-channel quadrature phase-
sensitive
receiving by a phase-splitting network is explained, for example, in C-N Chen
et al,
Biomedical Ma~~netic Resonance Technology, Adam Hilger, p. 149 (1989).
Reference is now made to Figure 4. In addition to saving half of the RF power
and/or improving the signal-to-noise ratio, a significant dead time reduction
may be
achieved by orthogonal transmitting and receiving. Thus, during transmission
of the RF
magnetic field by conducting RF pulses through the transmitter antenna 67 as
shown in
Figure 4, there is substantially zero signal directly induced in the receiver
coil 70. As a
result, the dead time of the whole receiving system may be reduced
significantly with
respect to prior art NMR logging instruments having only a single transceiver
antenna.
Another aspect of the preferred embodiment of the transmitter antenna 67 and
the
receiver antenna 70 is their relative sizes with respect to movement along the
longitudinal axis 78. As previously explained, the receiver antenna 70
preferably has an
overall length parallel to the longitudinal axis 78 less than an overall
length of the
transceiver antenna 67 along the direction of movement 81 as can be seen in
Figure 4.
These antenna dimensions can be included to provide the logging apparatus with
the
ability to RF-excite nuclei within a region completely polarized by the static
magnetic
field, and to receive NMR signals from completely RF-excited nuclei even while
the
logging apparatus is moving along the wellbore (22 in Figure 1 ).
3. Functional Block Diagram
Figure 4 also shows, in general form, the NMR probe 42 and a functional block
diagram of the circuitry of the NMR well logging apparatus. A
transmitter/receiver
(T/R) matching circuit 45 can be disposed within the housing 43. The T/R
matching
circuit 45 typically includes a series of resonance capacitors (not shown
separately), a
transmitter/receiver switch (not shown separately) and both "to-transmitter"
and "to-
receiver" matching circuitry. The T/R matching circuit 45 can be coupled both
to a radio
frequency (RF) power amplifier 74 and to a receiver preamplifier 73. While
shown as
located inside the housing 43, the T/R matching circuit 45, the RF power
amplifier 74
and the receiver preamplifier 73 alternatively may be located outside the
housing 43
within the NMR electronics cartridge (44 in Figure 1 ). The locations of the
T/R
CA 02232654 1998-03-18
matching circuit 45, the RF power amplifier 74 and the receiver preamplifier
73 are not
to be construed as a limitation on the invention.
Part of the control circuitry for the NMR logging apparatus includes a down-
hole
computer 92, which among other functions provides control signals to a pulse
5 programmer 91. The computer 92 and the pulse programmer 91 may also be
located
within the NMR electronics cartridge 44. The pulse programmer 91 controls the
timing
and operation of the variable frequency RF signal source 93. The RF driver 94
receives
an input from the variable frequency RF source 93 and provides an output to
the RF
power amplifier 74. The RF power amplifier 74 provides a high power signal to
drive
10 the transmitter antenna 67 for generating an RF magnetic field in the
sensitive volume
(58 in Figure 1). The RF power amplifier 74 can be electrically connected
(typically by
the switch in the T/R matching circuit 45) to the transmitting antenna 67
during
transmission of RF power pulses.
For transmitting a circularly polarized RF magnetic field the T/R matching
circuit
15 45 can include a two-way sputter (not shown separately) to split the RF
power from
amplifier 74 into two equal separate channels. One channel can be directly
connected to
the first transmitting coil (85 in Figure 3A). The second channel can be
connected to the
second transmitting coil (86 in Figure 3A) through a power-capable 90 °
phase shifter
(not shown separately) which can be located in the T/R matching circuit 45. .
During reception of the induced NMR signal, the receiver antenna 70 can be
electrically connected to the receiver preamplifier 73 by means of the switch
(not
shown)in the T/R matching circuit 45. The output of the RF receiver
preamplifier 73 is
provided to an RF receiver 89. The RF receiver 89 also receives a phase
reference input
from a phase shifter 98. The phase shifter 98 receives a primary phase
reference input
from the variable frequency RF source 93. The RF receiver 89, as previously
explained,
may include the capability for phase-sensitive quadrature detection. The RF
receiver 89
provides an output to an A/D converter 96. The A/D converter 96 output can be
stored in
a buffer 97 until required for use by the down-hole computer 92.
Alternatively, the
buffer 97 contents can be conducted directly to a downhole part of the
telemetry unit 99
for transmission to the surface equipment (54 in Figure 1).
For quadrature two-channel phase-sensitive detection the receiver antenna (70
in
Figure 2) may be constructed in accordance with Figure 3B and can include the
first
CA 02232654 1998-03-18
16
receiving coil (71 in Figure 3B) and the second receiving coil (72 in Figure
3B). These
coils, as previously explained, are substantially identical in form, but are
offset 90 ° with
respect each other about their longitudinal axes. NMR events induce a first
signal into
the first receiving coil (71 in Figure 3B) and induce a second signal, which
has a 90°
phase difference with respect to the first signal, into the second receiving
coil (72 in
Figure 2). Coils 71 and 72 can be electrically connected to the receiver
preamplifier 73
by means of the switch (not shown) in the T/R matching circuit 45. As is
understood by
those skilled in the art of NMR systems, for quadrature two-channel phase-
sensitive
detection two substantially equal and independent channels should be provided
in the
T/R matching circuit 45, the receiver preamplifier 73, the RF receiver 89, the
A/D
converter 96 and the buffer 97.
The downhole computer 92 typically preprocesses the data from the buffer 97
and
transfers the preprocessed data to the downhole portion of the telemetry
system, shown
generally at 99. The downhole portion of the telemetry system 99 can transmit
the
preprocessed data to the telemetry unit (38 in Figure 1) in the surface
equipment (54 in
Figure 1). The telemetry unit 38 can transfer the data to the surface computer
(40 in
Figure 1) for calculating and presenting desired well logging output data for
further use
and analysis.
All of the elements described herein and shown in Figure 4, except the
transmitter
antenna 67, the magnet assembly 60 and the receiver antenna 70, as a matter of
convenience for the system designer may be disposed within the housing 43 or
in the
NMR electronics cartridge (44 in Figure 1). These same elements may
alternatively be
located at the earth's surface, for example in the surface equipment 54 using
the cable (30
in Figure 1) for transmission of electrical power and signals to the
transmitter antenna 67
and the receiver antenna 70. The location of these elements should therefore
not be
construed as a limitation on the invention.
4. Static and RF Magnetic Field Geometry
Figure 5 shows graphically the geometry of the static magnetic field and the
RF
magnetic field induced by the NMR well logging apparatus of the invention. The
magnet
62, as previously explained, preferably has a magnetization direction
substantially
parallel to the longitudinal axis 78. The direction of the static magnetic
field within the
CA 02232654 1998-03-18
17
sensitive volume 58, as shown by arrows 110, is also substantially parallel to
the
longitudinal axis 78. Nuclear magnetic moments in the earth formation located
within
the sensitive volume 58 become substantially aligned in the direction of the
static
magnetic field, resulting in a bulk nuclear magnetization in a direction
denoted by arrows
130. In the preferred embodiment of the invention, the direction of an
equivalent linearly
polarized RF magnetic field, as generated by the transmitter antenna 67 and
denoted by
arrows 120, would be substantially perpendicular to the static magnetic field
at any point
within the sensitive volume 58. Such a magnetic field arrangement is
conventional for
NMR experiments.
The static magnetic field has an amplitude gradient within the sensitive
volume
58 which is directed substantially radially inwardly towards the longitudinal
axis 78. As
a result of the structure of the static magnetic field there is generally only
one
substantially cylindrically-shaped volume external to the permanent magnet 62
which
has a single static magnetic field amplitude (ignoring the end effects of the
magnet 62).
This structure of the static magnetic field provides that stray resonance
signals from
diverse materials such as the drilling mud (34 in Figure 1), which originate
outside of the
sensitive volume 58 do not substantially affect the NMR measurements if
appropriate RF
magnetic field frequencies are selected.
Undesired static magnetic field end effects may be substantially eliminated by
making the transmitting antenna 67 somewhat shorter along the longitudinal
axis 78 than
the magnet 62, so as not to excite materials at the extreme longitudinal ends
of the static
magnetic field. Additionally, since the transmitting antenna 67 is offset to
one side of
the longitudinal axis 78 in the direction of the face 21, the sensitive volume
58 will
subtend only about a 60 ° angular sector around the longitudinal axis
78 wherein the RF
magnetic field strength is substantially uniform.
When RF power pulses are conducted through the transmitter antenna 67, the
antenna 67 generates an RF equivalent magnetic dipole 87 located at the
transmitter
antenna 67 center and directed perpendicularly to the longitudinal axis 78.
This
equivalent magnetic dipole 87 generates an equivalent linearly polarized RF
magnetic
field 120 of substantially equal amplitude within the sensitive volume 58.
Since the RF
magnetic field direction is perpendicular to the sensitive volume longitudinal
axis 75, the
bulk nuclear magnetization, denoted in Figure 5 by arrows 130, at any point in
the
CA 02232654 1998-03-18
18
sensitive volume 58 rotates in planes parallel to the sensitive volume
longitudinal axis
75. The free precession of the nuclear magnetic moments, however, is around
the static
magnetic field direction at any point within the sensitive volume 58, and the
free
precession is always in planes perpendicular to the sensitive volume
longitudinal axis 75.
The free precession will, therefore, induce a first RF signal in the receiver
antenna 70.
An induced magnetic moment in the receiver coil 70 is shown in Figure 5 by
arrow 140.
For generation of a circularly polarized RF magnetic field, as previously
explained, the transmitter antenna 67 can include a first transmitter coil (85
in Figure 3A)
and a second transmitter coil (86 in Figure 3A) (not shown separately in
Figure 5).
When RF power pulses separated in phase by 90°are conducted separately
through these
two orthogonal transmitting coils, then two orthogonal, substantially equal
amplitude,
linearly polarized RF magnetic fields, denoted in Figure 5 by arrows 120 and
160, are
generated within the sensitive volume 58. Since the orthogonal transmitting
coils are fed
by RF currents 90 ° out of phase with each other, the resulting RF
magnetic field within
the sensitive volume 58 will be circularly polarized or "rotating" in planes
perpendicular
to the sensitive volume longitudinal axis 75.
Those skilled in the art of nuclear magnetic resonance measurements will
readily
comprehend that the free precession of the bulk nuclear magnetization about
the static
magnetic field will also induce a second RF signal in the transmitter coil 67.
An induced
magnetic moment in the receiver coil 70 is shown in Figure 5 as an arrow 150.
The
signal induced in the transmitter coil 67 will have a 90 ° phase shift
with respect to the
signal which is induced in the receiver coil 70.
5. Design Parameters for the Preferred Embodiment
In the preferred embodiment of the invention, the signal-to-noise ratio for
the
NMR measuring process is optimized, while keeping the vertical resolution of
the
instrument to an acceptable value and while keeping the geometry of the
sensitive
volume 58 constrained to avoid exciting nuclei in spurious locations such as
the wellbore
(22 in Figure 1). The following discussion is intended to explain how certain
principal
design parameters affect the signal-to-noise ratio so that the selection of
the design
parameters can be explained.
The principal design parameters typically include: the overall geometry of the
CA 02232654 1998-03-18
19
permanent magnet (62 in Figure 2) and the transmitter antenna (67 in Figure
2); the
power of radio frequency pulses used to energize the transmitter antenna 67;
and the
quality factor, Q, of the receiver antenna 70. For simplicity, this discussion
assumes that
the transmitter antenna 67 and the receiver antenna 70 have the same length
along the
longitudinal axis 78, and that the transmitter antenna 67 is rotated
90° with respect to the
receiver antenna 70 around its own longitudinal axis, which is parallel to the
longitudinal
axis 78.
'The magnitude of an NMR signal, S, induced in the receiver antenna 70 can be
described by using the principle of reciprocity, as shown in the following
expression:
S = ~ m A~, (B,/1~ I (1)
where m and A~" respectively, represent the nuclear magnetization amplitude
and the
cross-sectional area of the sensitive volume (58 in Figure 1). Bl, represents
the magnetic
field produced by a hypothetical unit current, l~, flowing in the receiver
antenna 70, the
oscillating frequency of the current is represented by ~, and l represents the
effective
lengths of both the transmitter antenna 67 and the receiver antenna 70. m and
Bl, are
assumed to be substantially homogeneous within the sensitive volume 58. By
substituting m = x B%o , where x represents the nuclear magnetic
susceptibility of
hydrogen nuclei within the sensitive volume 58, ~ = yBo, where Bo represents
the static
magnetic field generated by the permanent magnet (62 in Figure 2), the
following
expression for S can be derived:
S = (y,'~,uu~ Bol (Bill,) A~ 1 (2)
The magnitude of the NMR signal thus acquired is directly proportional to the
physical
volume of the sensitive volume 58 within the earth formation (26 in Figure 1).
The
geometry of the sensitive volume 58 is determined by where nuclear magnetic
resonance
conditions exist. In pulsed NMR, the resonance conditions typically exist
where the
deviation of the static magnetic field magnitude, Bo(R), from its average
value in the
sensitive volume, Bo(R~, (corresponding to the central frequency of the
current
CA 02232654 1998-03-18
energizing the transmitter antenna 67 (Bo(R)=~ l ~) is no greater then half
the magnitude
of the RF magnetic field, B,~, induced by passing RF current through the
transmitter
antenna 67. An expression for this condition is shown in equation (3):
5 Bo(R) - Bo(R.,.,) c B,~2 (3)
The static magnetic field amplitude, Bo(R), at the radius of the sensitive
volume, R~" may
also be described in the form of a Taylor expansion as:
10 Bo (R) = Bo (R~ -(~ol~) (R - R.s,)
where (~80 /aR) represents the static magnetic field amplitude gradient at
radius R = R~,.
From equation (3):
15 Bo(R~ - Bo(R~ s B,~ (5)
where Ro and R; represent, respectively, the outer and inner radii of the
sensitive volume
58. As a practical matter Ro- R; « R~" therefore:
20 A."~ = 2~a R."~ Bar l (~o%~) (6)
where a represents the angle subtended by the "sector" (the sensitive volume
58) which
is excited by the RF magnetic field. Substitution of equations (6) and (5)
into equation
(2) yields the following expression:
l2~ (y,'L~,uo! R~ Bit B~. W »a 1 Bo1 / (~o~d~)~ (7)
As is understood by those skilled in the art, the root-mean-square (RMS)
thermal noise
can be described by the expression:
Nr",s=(4kTdf'r)'n (8)
CA 02232654 1998-03-18
21
where d/'represents the receiver bandwidth. The receiver bandwidth is
typically about
~t12~ for a matched receiver; k represents Boltzmann's constant; and T
represents the
absolute temperature. Then substituting for equations (7) and (8) yields the
following
expression for signal-to-noise ratio (S/l~:
SlN =4~ R~, l~y,'~,uo~ t' k T df r)'~Bn Bm W »a l Bo ~ ~o~o~r~~~ (9)
The first bracketed expression in equation (9), for a given proton spin
density and
absolute temperature, depends only on the transmitter antenna 67 and the
receiver
antenna 70 parameters.
The second bracketed expression in equation (9) describes parameters used in
the
design of the permanent magnet (62 in Figure Z), as will be further explained.
It should
be noted that the second bracketed expression includes terms related to the
subtended
angle of the sensitive volume sector, a, the aperture of the antennas, l, and
the amplitude
gradient of the static magnetic field, ~Bd~Z. As suggested by equation (9),
the signal-
to-noise ratio ("SNR")could be improved by increasing the subtended angle of
the
sensitive volume sector, but increasing this angle would create an
unacceptably high
likelihood of having some of the NMR signal originate in the wellbore (22 in
Figure 1)
or in mudcake which may be deposited on the wellbore wall. The SNR could also
be
improved by increasing the length of the magnet 62 and correspondingly the
aperture of
the transmitting and receiving antennas, but to do so would compromise the
vertical
resolution of the logging instrument. Similarly, the static field amplitude
gradient within
the sensitive volume must be limited to avoid undesirable effects on the NMR
measurement. The invention seeks to optimize the overall SNR of a side-looking
well
logging instrument within the constraints of vertical resolution and depth of
investigation
which are likely to be commercially accepted.
6. Characteristics of the Static Magnetic Field
Reference is now made to Figure 6 which shows the magnet assembly (also
shown as 60 in Figure 2) of the preferred embodiment in more detail. The
magnet (62 in
Figure 2) can include a first permanent magnet cylinder 122 having an outside
diameter,
CA 02232654 1998-03-18
22
D0, and a length, L0. The first magnet cylinder 122 can have two substantially
identical
holes formed coaxially with a first magnet longitudinal axis 132 of the
cylinder 122,
these holes being located at the top and the bottom of the magnet cylinder
122. As
shown in Figure 6, each of the two holes can have two cylindrical sections of
different
diameter, a first top hole section 126A and a first bottom hole section 1268
each having
diameter Dhl; and a second top hole section 126C and a second bottom hole
section
126D each having diameter Dh2. The first top hole section 126A and the first
bottom
hole section 1268 are separated by distance Lhl, and the second top hole
section 126C
and the second bottom hole section 126D are separated by distance Lh2 as shown
in
Figure 6.
The first permanent magnet cylinder 122 can have a cylindrical recess 128. The
recess 128 should have its geometrical center located substantially in the
center plane
perpendicular to the axis 132 of the first magnet cylinder 122, and offset to
one side of
the longitudinal axis 132 adjacent to the probe face 21. The recess 128 can
have a
diameter of Dh3 and a length of Lh3. A second permanent magnet cylinder 124
can be
substantially centered within the recess 128. As shown in Figure 6, the second
permanent magnet cylinder 124 can be made from five individual sections. These
include a center section 124A having an outside diameter of D3 and length of
L3; a
substantially identical top middle section 124C and bottom middle section 124B
having
outside diameters of D2 and lengths of (L2-L3)/2; and a substantially
identical top end
section 124E and bottom end section 124D having outside diameters of D1 and
lengths
of (L1-L2)/2. The top middle section 124C and the bottom middle section 1248
are
adjacent to the center section 124A at the top and at the bottom of it,
respectively. The
top end section 124E is adjacent to the top of the top middle section 124C and
the
bottom end section 124D is adjacent to the bottom of the bottom middle section
1248.
The first permanent magnet cylinder 122 and the second permanent magnet
cylinder 124
are magnetized uniformly, parallel to the longitudinal axis 78, and in
opposite directions
to each other.
The specific geometry and arrangements of the first permanent magnet cylinder
122 and the second permanent magnet cylinder 124 shown in Figure 6 are a
matter of
convenience for the system designer and are not to be construed as absolute
limitations
on the invention. The geometry selected for the permanent magnet cylinders is
related to
CA 02232654 1998-03-18
23
three basic characteristics of the static magnetic field which are to be
optimized. The
first of these characteristics is the static magnetic field magnitude, the
second is the static
magnetic field homogeneity within the sensitive volume, and the third
characteristic is
the angle, a, with respect to the longitudinal axis 78, subtended by the
sector forming the
sensitive volume 58.
As will be further explained, the static magnetic field amplitude gradient in
the
direction of motion of the logging instrument should be inversely related to
the speed of
motion of the instrument. For a logging speed of about 10 feet per minute, the
longitudinal gradient should be less than about 0.2 Gauss/cm. For expected
speeds of
radial motion of the logging instrument the gradient in the transverse
direction should be
less than about 2 Gauss/cm. It should be specifically emphasized that the
proposed
magnet assembly could theoretically produce a larger homogeneous region in the
sensitive volume 58 by having a substantially zero static magnetic field
amplitude
gradient within the sensitive region 58. However, as has also been previously
explained,
using a static magnetic field having zero amplitude gradient can result in
significant
instabilities of the homogeneous volume and its associated static magnetic
field
magnitude, because of changes in temperature and pressure as well as the
relative
orientation of the earth's magnetic field with respect to the logging
instrument. The static
magnetic field strength in the sensitive volume 58 was selected to be about
9.4 mT (94
Gauss) as a compromise between signal-to-noise ratio, vertical resolution of
the logging
instrument and the overall external diameter of the logging instrument. The
azimuthal
selectivity was selected to be about 60 ° which represents an overlay
of two annular
cylinders having different radii. One cylinder is located between cylindrical
surfaces
having the static magnetic field magnitude of Bo-B,12 and Bo+B,/2, the second
cylinder
has a radio frequency magnetic field strength of about B,.
To keep the length of the magnet 62 as short as is practical and also to
ensure a
steady state nuclear magnetization measurement even while the NMR probe 42 is
moving through the wellbore (22 in Figure 1), it is important to provide a
homogeneous
static field amplitude length exceeding the antenna 67 aperture length, l, as
previously
explained.
It follows from equation (9) that for any given a radius R~, of the sensitive
volume, vertical resolution and penetration depth requirements, the ratio alBo
l(~Bol~)
CA 02232654 1998-03-18
24
should be maximized to provide maximum signal-to-noise ratio. In the preferred
configuration the first permanent magnet cylinder (122 in Figure 6) and the
second
permanent magnet cylinder (124 in Figure 6) are magnetized in opposite
directions
parallel to the longitudinal axis 78. Figure 7A shows a graphic representation
of the
static magnetic field 110 which has a field strength of 9.4 mT at the center
of the
sensitive volume 58. The two lines in Figure 7A are contour plots for static
magnetic
field amplitudes of 9.3 mT and 9.5 mT, which determine the shape of the
sensitive
volume 58 in the X-Y plane (as shown in Figure 5). Figure 7B shows a contour
plot of
the static magnetic field amplitude in the X-Z plane.
The selected static magnetic field strength of 9.4 mT corresponds to an RF
magnetic field frequency of 0.4 MHz. The sensitive volume 58 is determined by
the
overlap of the two areas: one is defined according to equation (5) (shown in
Figure 8);
and the other represents the region of appropriate RF magnetic field. Overlap
of the two
regions is shown in Figure 8. According to Figure 8 the subtended angle a of
the
sensitive volume 58 can be estimated to be about 60°. The subtended
angle should be as
great as possible to maximize the area of the sensitive volume 58, and as a
result
maximize the SNR, but this maximization must be consistent with limiting the
lateral
extent of the sensitive volume 58 to avoid its contacting any portion of the
wellbore (22
in Figure 1) or any portion of "mudcake" which may become deposited on the
wellbore
wall in permeable earth formations.
Figure 6 shows the geometry of the preferred shape of the magnet 62. The
dimensions shown for the magnet in Figure 6 are preferably as follows: DO=
l2cm;
Dhl= 5.6cm; Dh2= 4cm; Dh3= 2.6cm; LO= 80cm; Lhl= 32cm ; Lh2=l3.Scm; Lh3=
40cm; D1=l.4cm; D2= 2.1m; D3= 2.6cm; L1= 40cm; L2=l9cm; L3= 7.Scm. The
magnet 62 presented in Figure 6 is particularly suitable for a 20 cm length
for the
transmitter antenna 67 and a 7 cm distance between the sensitive volume (58 in
Figure 2)
and the face (21 in Figure 2). These parameters are a matter of convenience
for the
system designer and are not to be construed as a limitation on the invention.
Other
dimensions of the magnet 62 can be selected if the logging instrument has
different
specifications for vertical resolution and radial depth of penetration into
the earth
formations.
The permanent magnet 62 can be formed from ferrite permanent magnet material,
CA 02232654 1998-03-18
as previously explained. These materials have remanence magnetization between
about
0.38 T and 0.42 T. Another magnet material can be bonded rare-earth Samarium
Cobalt
permanent magnet having remanence magnetization of about 0.7 T and a particle
size of
a rare-earth Samarium Cobalt magnet material powders of generally less than
about 0.1
5 mm. The choice of maximum particle size of 0.1 mm is determined by the
requirement
for the permanent magnet 62 to be radio frequency transparent at an RF
magnetic field
frequency of about 0.4 MHz. The field distributions shown in Figure 7A and
Figure 7B
are calculated for a magnet material having remanence magnetization of 0.7 T.
As explained earlier, another parameter affecting the design of the permanent
10 magnet 62 is the degree of the static field homogeneity in the direction of
the wellbore
axis (76 in Figure 2). The static magnetic field in the sensitive volume 58
has
substantially equal magnitude in sense of NMR excitation. As was explained
earlier (see
equation (3)), within the sensitive volume 58 the static magnetic field
amplitude should
vary only in a narrow range: from Bo-Bl/2 to Bo+B~/2. It is of great
importance how fast
15 is the spatial variation of this field amplitude along a direction of
motion of the logging
instrument. The rate of this variation is directly related to the static
magnetic field
amplitude gradient in the direction of motion. The static magnetic field
amplitude
gradient distribution inside the sensitive volume 58 is explained
schematically in Figure
9. Two lines 58L and 58M represent lines of equal amplitude of the static
magnetic
20 field, with a first magnitude and a second magnitude, respectively, which
are in the range
from Bo-B,/2 to Bo+B,/2. Static magnetic field amplitude gradients at a
location 58A
and a location 58D in Figure 9 are inversely proportional to a distance
between two
points along the direction of movement 81 parallel to the wellbore axis 78,
one point is
on line 58L and the other point is on line 58M. For example, the amplitude
gradient
25 component in the direction of motion 81 at location 58A in the central part
of the
sensitive volume 58 is inversely proportional to the distance between points
58A and
58C. The gradient component in the direction of motion 81 at location 58D (in
the top
end of the sensitive volume 58) is inversely proportional to the distance
between points
58E and 58D. It should be apparent from Figure 9 that the amplitude gradient
in the
central part of the sensitive volume 58 is much smaller than the gradient at
the ends of
the sensitive volume 58. The amplitude gradient component in a direction of
motion
perpendicular to the longitudinal axis 78 at location 58A in the center part
of the
CA 02232654 1998-03-18
26
sensitive volume 58 is inversely proportional to the distance between points
58B and
58A. 'The strongest component of the amplitude gradient is in the radial
direction.
The following discussion is to explain a limitation on the static magnetic
field
amplitude gradient in the direction of motion 81. The sensitive volume 58 is
determined
by the RF magnetic field amplitude. To obtain undistorted NMR signals, any
point
within the sensitive volume should not leave the sensitive volume during the
time span of
a measurement sequence (a full CPMG echo train). If tool motion is such that
any point
may leave the sensitive volume during a measurement sequence, subsequent
180°
rephasing pulses in a Carr-Purcell ("CPMG") echo train may be applied to parts
of the
earth formation which had not previously been transversely polarized by the
initial 90 °
pulse. The distance, Os, along a direction of motion from a point, N, inside
the sensitive
volume 58 to the boundary of the sensitive volume 58 can be estimated by the
expression:
~~~ = IBo<n~ - Bo~B~~ ~ G (10)
where Bo(N) represents the static magnetic field amplitude at point N inside
the sensitive
volume 58, Bo(B) represents the static magnetic field amplitude at the
boundary of the
sensitive volume 58 and G represents the static magnetic field gradient in the
direction of
motion. The total movement, or displacement during a time interval, t, of the
well
logging instrument should be less than OS(N). More specifically:
vx~<ds~~
where v represents the speed of motion of the well logging instrument. The
total
displacement of the instrument should not represent a substantial portion of
the total
volume. The inequality which should thus be satisfied can be written as:
~Bo~~ - Bo~B~I - « Bi (12)
A reasonable estimate of the maximum gradient in the direction of motion can
be
calculated as:
CA 02232654 1998-03-18
27
G<(0.1 B~l(v Xt) (13)
For practical values of RF magnetic field amplitude, B,, in the range of 2 X
10~' T, and
tool speed, v, of about 0.05 m/sec, during a time of 200 milliseconds for a
typical
measurement sequence the gradient, G, should be less than about 2 X 10-3 T/m
(equivalent to about 0.2 Gauss/cm). This value was used as a constraint in the
procedure
for optimizing the shape of the static magnetic field.
It is common for the logging instrument velocity perpendicular to the
longitudinal
axis 78 to be about 50 times less than the logging speed. This requires the
static
magnetic field amplitude gradient perpendicular to the longitudinal axis 78
(the radial
component of the gradient) to be less than 0.1 T/m. The preferable geometry of
the
magnet as shown in Figure 6 has a radial static magnetic field amplitude
gradient of 0.02
T/m (2 Gauss/cm). Other values of the radial gradient may be selected by
changing the
frequency of the RF magnetic field.
The design requirements for the radial static magnetic field gradient are also
affected by the earth's magnetic field He, which may vary in value and
direction as a
function of geographical location of the wellbore and the geographic direction
that the
wellbore is drilled. Earth's magnetic field is substantially homogeneous and
is about
0.5.10 T in magnitude. Any static magnetic field variation resulting from
variation in
the logging instrument orientation with respect to the earth's magnetic field
should not
significantly change the radius, RS~, of the sensitive volume 58. The shift
may be
expressed as the ratio He /G. Therefore, the required radial static magnetic
field
amplitude gradient, G, should satisfy the inequality He IG « R5~ or G » He
/RS~. For
RS~ = 0.1 m, then G should be greater than about 5 X 10~ T/m. As a practical
matter the
existing radial gradient of the static magnetic field of the preferred
embodiment of the
magnet (2 X 10-Z T/m, or 2 Gauss/cm) more than meets this requirement. These
gradient
requirements are a function of the operating specifications of the NMR well
logging
instrument such as logging speed, depth of investigation, vertical resolution,
and signal-
to-noise ratio. Therefore, the specific embodiment and apparatus parameters
are a matter
of convenience for the system designer and are not to be construed as
limitations on the
invention.
CA 02232654 1998-03-18
28
It will be readily appreciated by persons skilled in the art that the
invention is not
limited to what has been particularly shown and described herein. Rather the
scope of
the invention should be limited only by the claims which follow.
