Note: Descriptions are shown in the official language in which they were submitted.
CA 02230902 1998-02-26
RADIAL 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")
sensing apparatus and measuring techniques. More specifically, the invention is related to
NMR well logging app~lus and measuring techniques for sensing within earth formations
10 penetrated by a wellbore. The invention also relates to methods for using NMR measurements to determine properties of the earth formations.
Description of the Related Art
NMR well logging instruments can be used for d~le.,l~ g properties of earth
15 formations, including the fractional volume of pore space ("porosity"), the fractional
volume of mobile fluid filling the pore spaces of the earth formations and otherpetrophysical parameters. Examples of methods and measurement techniques for using
NMR measurements for d~l~"l~ ing the fractional volume of pore space, the fractional
volume of mobile fluid and other petrophysical parameters are described in, Spin Echo
20 Magnetic Resonance Logging: Porosity and Free Fluid Index D~l~""il~tion, M. N. Miller
et al, Society of Petroleum Engineers paper no. 20561, Richardson, TX (1990) and in, Field
Test of an Experiment~l Pulsed Nuclear Magnetism Tool, C. E. Morriss et al, SPWLA
Logging Symposium Transactions, paper GGG (1993).
NMR well logging instruments typically include a permanent magnet to induce a
25 static magnetic field within the earth formations and include a tr~n~mitting ~nt~nn:~
positioned near the magnet and shaped so that a pulse of radio frequency ("RF") power
con(lucted through the ~nt~ nn~ induces an RF magnetic field in the earth formations. The
RF magnetic field is generally orthogonal to the static magnetic field. After an RF pulse,
voltages are inl1uced in a receiving ~nt~nn~ on the logging instrument by precessional
30 rotation of spin axes of hydrogen or other nuclei about the static magnetic field. The
receiving ~ntPnn~ is typically connected to a receiver circuit in the instrument which detects
and measures the in(luced voltages. In a typical NMR measurement set a sequence of RF
pulses is applied to the transmitting ~ntenn~ and a sequence of voltages is measured by the
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.
receiving ~nt~nn~ (note that some h~ ~llents use the same ~nt~nn~ for transmitting and
receiving). The magnitude of the detected voltages and the rates at which the detected
voltages vary with time are related to certain petrophysical properties of the earth
formation.
One type of NMR well logging appal~lus is described, for example, in U. S. patent
no. 3,597,681 issued to Huckbay et al. The appal~lus disclosed in the Huckbay et al '681
patent has several drawbacks, one of which is that a region of unidirectional static magnetic
field is not homogeneous along the wellbore axis. As a practical matter, well logging
instruments typically must be able to move axially through the wellbore while making
10 measurements. During the time needed to make a typical NMR measurement, the
"sensitive volume" (that part of the formation in which nuclear magnetic resonance is
excited) generated by the logging instrument will be moved through the wellbore so that
the measurement set carmot be completed. Another drawback to the a~p~lus disclosed in
the Huckbay et al '681 patent is that a significant part of the NMR signals originate from
15 within the fluid filling the wellbore (called "drilling mud").
Yet another drawback to the appal~lus disclosed in the Huckbay et al '681 patent is
that its ~nt~nn~ is directed to one side of the apparatus and therefore uses only a small
fraction of the total volume of unidirectional static magnetic field. This results in an
inefficient use of the perrn~nent magnet in the instrument.
Still another drawback to the app~lus disclosed in the Huckbay et al '681 patent is
that the ~nt~nn~ is subject to a high static magnetic field strength and, therefore, can have
an unacceptably high amount of magnetoacoustic ringing.
Another drawback to the apparatus disclosed in the Huckbay et al '681 patent is that
the RF magnetic field generated by the :mt~nn~ drops in magnitude as the third power of
25 the distance from the instrument to the sensitive volume since the antenna in this
instrument is the equivalent of a three dimensional magnetic dipole. Such an ~nt~nn~ is
proximally coupled only to a small part of the unidirectional static magnetic field. This
results in an extremely low signal-to-noise ratio.
Another type of NMR well logging instrument is described in U. S. patent no.
30 4,350,955 issued to Jackson et al. The instrument disclosed in the Jackson et al '955 patent
includes permanent magnets configured to induce a magnetic field in the earth formations
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which has a toroidal volume of substantially uniform magnetic field strength. A particular
drawback to the appaldlus 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 during measurement operations. Well logging instruments, in order to be
5 commercially useful, typically must be able to move axially through the wellbore at rates
not less than about ten feet per minute. The length of time needed to make a typical NMR
spin-echo measurement set can be as long as several seconds. The NMR logging
instrument is therefore likely to move a substantial distance during a measurement cycle.
Measurements made by the in~ll.ullent disclosed in the Jackson et al '955 patent are
10 therefore subject to error as the instrument is moved during logging operations, because the
~nt~nn~ would no longer be positioned so as to be sensitive to the same toroidal volume
which was magnetized at the beginning of any measurement set.
Another drawback to the ~paldlus instrument in the Jackson et al '955 patent is that
it does not elimin~te NMR signals ori~in~ting within the fluid filling the wellbore.
A still further drawback to the appaldlus disclosed in the Jackson et al '955 patent is
that the toroidally shaped static magnetic field is can change in amplitude as the instrument
is subjected to changes in ambient temperature and variances in the earth's magnetic field.
The ~nt~nn~ in the Jackson et al '955 appaldlus is tuned to a single frequency. If the field
strength of the static magnetic field in the toroidal volume changes, the antenna may no
20 longer be sensitive to NMR signals ori~in~ting within the toroidal volume. Using the
~paldlus in Jackson et al '955, it is impractical to colllpellsdle the frequency of the RF
magnetic field for changes in the static magnetic field strength within the toroidal volume.
Additional drawbacks to the appaldlus disclosed in the Jackson et al '955 patentinclude the magnet pole pieces being opposed each other. This results in a significant
25 dem~gnetizing effect which requires magnet m~tçri~l having a high coercive force. This
requirement is in directly opposed to the requirement for strong residual magnetization and
high ~elllpt;ldlUle stability of the perm~n~nt magnet. Second, the magnet pole pieces are
spaced apart and are far away from the toroidal region, which makes the use of the
p~rm~nent magnet m~t~ri~l less efficient. Third, the ~nt~nn~ used in the Jackson '955
30 apparatus has low eff1ciency as a result of low electro-magnetic coupling between the
~ntçnn~ and the earth formation at the resonant frequency for NMR experimentation.
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Fourth, the ~ntenn~ is located in a relatively strong static magnetic field, which stimulates
magnetoacoustic ringing in the antenna. Fifth, for an NMR mea~ul~l~lent technique which
uses a homogeneous static magnetic field, changes in the relative position of the instrument
with respect to the earth's magnetic field can cause a significant disturbance to the
5 homogeneity of the toroidal region.
Another type of NMR well logging apparatus is described in U. S. patent no.
4,717,876 issued to Masi et al. The apl)aldlus disclosed in the Masi et al '876 patent has
improved homogeneity in the toroidal region as compared to the appaldlus described in the
Jackson et al '955 patent, but has basically the same drawbacks as the Jacskon et al '955
10 apparatus.
Another type of NMR well logging app~dlus is described in U. S. patent no.
4,629,986 issued to Clow et al. This a~?p~dlus provides improved signal-to-noise ratio
compared with the appaldlus of Jackson et al '955 by including a high magnetic
permeability ferrite in the ~nf~nn~ Increased stability is achieved by p~;lrOllllillg the NMR
15 measurements in a static magnetic field which includes an amplitude gradient. However,
the apparatus disclosed in the Clow et al '986 patent has several drawbacks. Since the
magnetic properties of most perrn~nent magnet m~ter~ are telll~eldlul~ dependent, the
sensitive volume is not stable in shape and magnetic field intensity. The sensitive volume
of this instrument is only a couple of inches long in the longitll~in~l direction, which
20 requires that this instrument be practically stationary during an NMR measurement cycle.
The magnet pole pieces are ~u~llially spaced apart and are far from the sensitive region,
which makes the use of the p~rm~nPnt magnet m~teri~l inefficient. The ~nt~nn~ is located
in a relatively strong magnetic field, which stimul~tes magnetoacoustic ringing in the
~ntenn~ The high magnetic permeability ferrite in the slntl nn~ is located in a relatively
25 strong magnetic field, which may saturate the ferrite and reduce its eff1ciency. Soft ferrite
disposed in a static magnetic field is also a strong source of magnetostrictive ringing
following any RF pulse through the ~nt~nn~ In the magnet arrangement of the Clow et al
'986 patent, the ~lem~gnetizing field is relatively strong, which requires a magnet m~t~ri~l
having high coercive force. This requirement is opposite to the strong residual
30 m~gnt-ti7~tion and high t~ eldlule stability of the magnetic properties also required of the
permanent magnet material. Finally, the static magnetic field in the earth formations in the
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sensitive volume is only about 10 Gauss and rotates 360~ in a plane perpendicular to the
wellbore axis. For this amplitude of static magnetic field, the earth's magnetic field
amplitude of about 0.5 Gauss presents a significant disturbance to the overall field strength.
Another type of NMR well logging a~pa dlus 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 signals from the wellbore fluid. However, the d~paldlUS
disclosed in the Taicher et al '878 patent has several drawbacks.
Since the magnetic properties of the pçrm~nent magnet m~t~n~l used in this a~dldlus are
dlul~ dependent, the sensitive region does not have a stable in shape or stable
10 magnetic field intensity. The ~nt~nn~ is located within a relatively strong magnetic field,
which stimulates magnetoacoustic ringing in the ~nt~nn~ In the arrangement of the magnet
in the appaldlus disclosed in the Taicher et al '878 patent, the dem~gnetizing field is very
strong, which requires a magnet m~t~ri~l having high coercive force. This requirement is
directly opposite to the strong residual magnetization and high temperature stability of
15 magnetic plol.~llies required of the perm~nent magnet for a well logging apparatus.
Due to the disadvantages of the foregoing NMR well logging instrument designs,
none of them are generally commercially accepted well logging instruments.
Commercially accepted well logging instruments include one which is described in U. S.
patent no. 4,710,713 issued to Taicher et al. The instrument disclosed in the Taicher et al
20 '713 patent includes a generally cylindrical perm~nent magnet assembly which induces a
static magnetic field having substantially uniform magnetic field strength within an annular
cylindrical volume in the earth formations. The instrument disclosed in the Taicher et al
'713 patent has several drawbacks, however. First, the ~nt~nn~ induces an RF magnetic
field within the earth formations surrounding the tool which decreases in strength as the
25 square of the radial distance from the magnet. Because the signal-to-noise ratio of NMR
measurements made within a gradient magnetic field is typically related to the strength of
the RF magnetic field, the d~aldlus disclosed in the Taicher et al '713 has very high power
requirements, and can have difficulty obtaining measurements having sufficient signal-to-
noise ratio at substantial radial distances from the instrument.
Another drawback to the instrument of the Taicher et al '713 patent is that the
optimum design of the magnet and the RF ~nt~nn~, for purposes of optimi7.ing the signal-
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to-noise ratio, requires that nuclear magnetic resonance conditions be met at a relatively
high frequency. Since the RF energy losses in the electrically conductive fluid in the
wellbore are proportional to the square of the frequency, the operation of the Taicher et al
'713 patent is restricted to use in relatively low conductivity fluids in the wellbore.
Yet another drawback to the appal~us of the Taicher et al '713 patent is that the
~nt~nn~ is located within a relatively strong static magnetic field which is perpendicular to
a direction of RF current flow in the tr~n~mitting :~nt~nn~ and, therefore, stimulates
magnetoacoustic ringing in the transmitting ~nt~nn~
Another NMR logging instrument is described in U. S. patent no. 5,055,787 issued10 to Kleinberg et al. This logging instrument includes perm~nent magnets arranged to induce
a magnetic field in the earth formation having substantially zero 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 ~ntPnn~ in this
instrument is positioned in a recess located ~xt~rn~l to the tool housing, enabling the tool
15 housing to be constructed of high strength material such as steel. A drawback to the
logging instrument 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, roughn~s~ in the wall of the wellbore, by deposits of
20 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.
Another drawback to the instrument disclosed in the Kleinberg et al '787 patent
relates to the perm~nent magnet material. Since the magnet pole pieces are opposed each
other, there is a strong denn~gnetizing effect which requires a perm~nent magnet m~teri~l
25 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.
Another NMR measurement appal~lus which may have application for well
logging is disclosed in U. S. patent no. 5,572,132 issued to Pulyer et al. This apparatus
30 includes a p~rm~n~nt magnet for inducing a magnetic field polarized along the longitll-lin~l
axis of the a~ lus, and ~nt~nn~ coils disposed about the exterior of the magnet. The
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a~d~us described in the Pulyer et al '132 patent, as do most prior art NMR well logging
instruments, has a common drawback which is 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 NMR signal height and may even
5 lead to a complete disa~peal~lce of the NMR signal, when the logging apparatus is moving
in a direction along a static magnetic field amplitude gradient. In actual well logging
practice, the phase shift and signal reduction may be even worse than is suggested by the
Shporer '967 patent because the logging speed can be variable, as is understood by those
skilled in the art of well logging.
SUMMARY OF THE INVENTION
The invention is a nuclear m~gn~tic resonance sensing a~?pald~us, including a
magnet for inducing a substantially radially symmetrical static magnetic field within
15 m~ten~l~ to be analyzed. The static magnetic field is substantially coaxial with a
longitudinal axis of the appald~us and is polarized substantially perpendicularly to the
longitudinal axis. The static magnetic field has a m~xhllulll longitudinal amplitude
gradient which is inversely related to a speed of motion of the apparatus along the
longitudinal axis through the materials to be analyzed. The apparatus includes a transmitter
20 for generating a radio frequency magnetic field in the m~tPri~ for exciting nuclei in the
m~teri~l~. The radio frequency magnetic field is substantially orthogonal to the static
magnetic field. The al~p~dtu~ includes a receiver for detecting nuclear magnetic resonance
signals from the excited nuclei in the materials.
In a preferred embodiment of the invention, the magnet includes magnetized
25 cylinders stacked along the longit~l~lin~l axis. The magnetization of each one of the
cylinders is proportional to its distance from a center plane of the magnet. The cylinders
are magnetized parallel to the longitudinal axis and towards the center plane. The pl~r~ d
embodiment of the magnet includes an end magnet disposed at each longitudinal end of the
stacked cylinders. The end magnets are each magnetized parallel to the longitu-lin~l axis,
30 and in a direction opposite to the magnetization of an adjacent one of the cylinders. The
pl~f~ d embodiment of the magnet includes a hole in the center of the stack of cylinders.
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-
An ~nttqnn~ which can be connected to the transmitter andlor receiver can be located in the
hole to reduce magnetostrictive ringing in the ~nt~nns~
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a nuclear magnetic resonance-(NMR) well logging appaldlus
disposed in a wellbore penetrating earth formations.
Figure 2 shows the NMR probe of the appaldlus of Figure l in more detail.
Figure 3 shows a functional block diagram of the NMR appaldlus of the present
invention.
Figure 4 shows the main magnet assembly of the NMR appaldlus in more detail.
Figure 5 shows a the magnct dimensions.
Figure 6 shows a graphic representation of the static magnetic field in~luced by the
magnet within the sensitive volume.
Figure 7 shows a graph of the longitu~lin~l component of the static magnetic field
in(luced by the magnet within the transceiver ~nt~nn~, both with and without end-magnet
inserts.
Figure 8 shows the static magnetic field of Figure 6 in more detail.
DESCRIPTION OF THE PREFERRED EMBODIMENT
1. Configuration of the Appaldlu~
Figure 1 shows a nuclear magnetic resonance ("NMR") well logging instrument
disposed in a wellbore 22 penetrating earth formations 23, 24, 26, 28 for makingmeasurements 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 cylindrical shape, is
disposed in one of the earth formations, shown at 26. The sensitive volume 58 is a
predetermined portion of the earth formations 26 in which NMR measurements are made
by the logging instrument, as will be further explained.
A string of logging tools 32 ("tool string"), which can include the NMR a~paldlus
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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 as is known in the art. The tool string 32
can be electrically connected to surface equipment 54 by an insulated electricalS conductor (not shown separately in Figure 1) forming part of the eleckical cable 30. The
surface equipment 54 can include one part of a telemetry system 38 for com~llullicating
control signals and data between the tool string 32 and a colll~u~el 40. The computer 40
can also include a data recorder 52 for recording measurements made by the instrument
and transmitted to the surface equipment 54 over the logging cable 30.
An NMR probe 42 according to the invention can be included in the tool string
32. The tool string 32 is preferably centered within the wellbore 22 by means of a top
centralizer 56 and a bottom centralizer 57 attached to the tool string 32 at axially spaced
apart locations. The centralizers 56, 57 can be of any type known in the art such as
bowsprings or power operated arms or the like.
Circuitry for operating the NMR probe 42 can be located within an NMR
electronics cartridge 44. The cir~;uilly (not shown in Figure 1) 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 functions of the probe 42 will be further explained.
Other types of well logging sensors (not shown separately for clarity of the
illustration in Figure 1) may form part of the tool string 32. As shown in Figure 1, one
additional logging sensor 47 may be located above the NMR electronics cartridge 44.
Additional logging sensors, such as shown at 41 and 46 may be located within or below
the bottom centralizer 57. The other sensors 41, 46, 47 can be of types f~mili~lr to those
25 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. The locations 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
30 invention.
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Figure 2 shows the NMR probe 42 in more detail. The NMR probe 42 preferably
comprises a generally cylindrical, permanent magnet assembly 60. The magnet assembly
60 includes at least one permanent magnet 62, which is generally elongated along a
magnet axis 80 and preferably has a substantially circular cross section perpendicular to
5 the magnet axis 80. The magnet axis 80 is preferably substantially coaxial with an axis
76 of the wellbore (22 in Figure 1), which location is provided by the upper and lower
centralizers (56 and 57 in Figure 1). The preferred construction of the magnet assembly
60 will be explained in more detail. For clarity of the description, the one or more
permanent magnets 62 will be considered together and referred to as the permanent
magnet 62, and their common axis 80 and the collocated axis of the wellbore 76 will be
jointly identified as a the longitudinal axis, shown at 78.
In a pleJ~lled embodiment of the invention the permanent magnet 62 comprises a
main magnet 61 and two inserts, a top insert magnet 63 and a bottom insert magnet 64.
The main magnet 61, the top insert magnet 63 and the bottom insert 64 have
magnetization directions substantially parallel to the longitudinal axis 78. The main
magnet 61 is in a form of a annular cylinder having a cylindrical hole 83 substantially
through its center, wherein the top insert magnet 63 and the bottom insert 64 are located.
The main magnet 61 has substantially homogeneous magnetic charge along the
longitudinal axis 78. To have this characteristic, the main magnet 61 can be made up of
thin magnetic rings each having a different residual magnetization, in order to
approximate substantially linear magnetization distribution from one end of the main
magnet 61 to another.
The construction of the main magnet 61 is shown in more detail in Figure 4,
which is a side view of the main magnet 61. The main magnet 61 can be composed of a
series of axially magnetized cylinders, shown generally as 61A-61F. The magnetization
direction of each cylinder 61A-61F is indicated by an arrow on each cylinder 61A-61F.
A particular feature of the axially magnetized cylinders 61A-61F is that the
magnetization of each cylinder 61A-61F is proportional in magnitude to its axial distance
from a center plane 61P of the magnet 61, and the magnetization is directed toward the
center plane 61P. The center plane 61P is perpendicular to the longitudinal axis 78 and
bisects the main magnet 61 into two substantially equal length sections. For example,
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uppermost cylinder 61A is shown as having a large magnetization directed downwardly
towards the center plain 61P. Correspondingly opposite is lowermost cylinder 61Fwhich has substantially equal strength magnetization as does the uppermost cylinder 61A
but its magnetization is directed upwardly towards the center plane 61P. Successively
more weakly magnetized opposing pairs of cylinders, such as 61B/61E and 61C/61D are
disposed successively closer to the center plane 61P.
The top insert magnet 63 and the bottom insert 64 have magnetization directions
parallel to the longitudinal axis 78 and are used in combination with the main magnet 61
for synthesis of the preferred form of a static magnetic field. Figure 5 shows a drawing
10 including preferred dimensions for the main magnet 61 and insert magnets 63, 64 to
generate the preferred static magnetic field. Magnetization directions of the main magnet
61, the top insert magnet 63 and the bottom insert 64 are indicated by arrows in Figure 5.
These particular magnet dimensions are a matter of convenience for the system designer
and are not to be construed as a limitation on the invention. The essential features of the
15 static magnetic field achieved by the foregoing dimensions will be further explained in
detail.
Referring now to Figure 6, the resulting static magnetic field generated by the
magnet 62 in the sensitive region 58 is directed substantially radially outward from the
longitudinal axis 78. The static magnetic field generated by the magnet 62 is also
20 substantially symmetric about the longitudinal axis 78, is substantially perpendicular to
the longitudinal axis 78, and at radial distances less than the axial length of the magnet
62, decreases in amplitude only as the inverse of the radial distance from the magnet 62.
The permanent magnet material of the perm:lnent magnet 62 should be
substantially radio frequency l d~ .ll7 so that an ~nt~nn~ used to generate a radio
25 frequency magnetic field can be located inside the hole 83 in the main magnet 61, as will
be further explained. One type of the radio frequency ("RF") transparent magnet can be
made from a ferrite magnet material such as that sold under the trade name "Spinalor"
and manufactured by Ugimag, 405 Elm St., Valparaiso, IN, or another ferrite magnet
m:~tçri~l sold under the trade name "Permadure" and manufactured by Philips, 230 Duffy
30 Ave., Nicksville, NY. These materials are only provided as examples and are not
intended to limit the choice of materials for the magnet 62. The magnet 62 only need be
CA 02230902 1998-02-26
substantially transparent to the RF magnetic field at the frequency selected.
Referring once again to Figure 2, the NMR probe 42 further includes a
transceiver antenna 67, which can comprise one or more coil windings 66 preferably
arranged inside the hole 83 in the main magnet 61. The coil windings 66 are preferably
arranged so that each coil winding lies in a plane substantially perpendicular to the
longitudinal axis 78. Radio frequency altçrn~ting current passing through the coil
windings 66 generates an RF magnetic field in the earth formation (26 in Figure 1). The
RF magnetic field generated by the current flow in the coil windings 66 has field
directions substantially parallel to the longitudinal axis 78 within the sensitive volume
10 58.
The coil windings 66 should have an overall length along the longitudinal axis 78
which is about equal to the diameter of the sensitive volume 58. The overall length of
the coil windings 66 parallel to the longitudinal axis 78 should also be substantially
shorter than the overall length of the permanent magnet 62 along the longitudinal axis 78,
15 as will be further explained.
Preferably, the coil windings 66 are formed around a soft ferrite rod 68.
The soft ferrite rod 68 can be formed from a material such as one sold under trade
design~tion "F6" and m~nuf~ctured by MMG-North America, 126 Pennsylvania Ave.,
Paterson, N. J., or another material sold under trade designation "3C2" and manufactured
20 by Philips, 230 Duffy Ave., Nicksville, NY. The ferrite rod 68 preferably is positioned
parallel to the longitudinal axis 78. The overall length of the ferrite rod 68 along the
longitudinal axis 78 should be substantially less than the length of the permanent magnet
62 along the longitudinal axis 78. Alternatively, a plurality of coils and a plurality of
ferrite rods may be employed. The assembly of coil windings 66 and soft ferrite rod 68
25 will be referred to hereinafter as the transceiver antenna 67. The ferrite rod 68 has the
particular function of increasing the field strength of the RF magnetic field generated by
the transceiver ~nt~nn~ 67. Using the ferrite rod 68 particularly enables the transceiver
antenna 67 to have a relatively small external diameter so that it can be located within the
hole 83. Having a small external diameter particularly enables the transceiver antenna 67
30 of the invention to be sized so that the appald~us of the present invention can be used in
smaller diameter wellbores.
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The permanent magnet 62, the transceiver ~n~nn~ 67 and the receiver ~ntenn~ 70
are preferably housed within an RF transparent, non-ferromagnetic protective housing
43. Such housings and additional components (not shown) for excluding the drilling
mud under high hydrostatic pressure, are f:~mili~r to those skilled in the art.
5 2. Functional Block Dia~ram of the NMR Lo~in~ Apparatus
Figure 3 shows, in general form, the NMR probe 42 and a functional block
diagram of the NMR well logging apparatus. A transmitter/receiver (T/R) matchingcircuit 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
10 switch (not shown separately) and both "to-transmitter" and "to-receiver" matching
Cil1Ui~ /. The T/R m~tching 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 m:~tçlling circuit 45, the RF power amplifier 74 and the receiver
preamplifier 73 alternatively may be located outside the housing 43 within the top
centralizer (56 in Figure 1) or within the NMR electronics cartridge (44 in Figure 1).
The locations of the T/R 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 appa~d~lls includes a down-hole
computer 92, which among other functions provides control signals to a pulse
20 programmer 91. The computer 92 and the pulse programmer 91 may also be located
within the top centralizer 56 or in 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
25 provides a high power signal to drive the transceiver ~ntenn~ 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
transceiver antenna 67 during tr~n~mi~ion of RF power pulses.
During reception of NMR signals, the transceiver antenna 67 can be electrically
30 connected to the receiver preamplifier 73 by means of the switch in the T/R matching
circuit 45. The output of the RF receiver preamplifier 73 is provided to an RF receiver
CA 02230902 1998-02-26
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 may include quadrature detection. The RF
receiver 89 provides an output to an A/D converter 96. The A/D converter 96 output can
5 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 tr~n~mi~.cion to the surface equipment (54 in Figure 1).
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
10 generally at 99. The downhole portion of the telemetry system 99 transmits the
preprocessed data to the telemetry unit (38 in Figure 1) in the surface equipment (54 in
Figure 1). The telemetry unit 38 transfers the data to the surface co~ ulel (40 in Figure
1) for calculating and presenting desired well logging output data for further use and
analysis as is understood by those skilled in the art.
All of the elements described herein, except the transceiver antenna 67 and the
magnet assembly 60, can at the convenience of the system designer be disposed within
the housing 43, the top centralizer (56 in Figure 1) or 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 tr:~n~mi.~sion of
20 electrical power and signals to the transceiver ~ntcnn~ 67.
Figure 3 also illustrates the static magnetic field and the RF magnetic field
created by the NMR well logging a~l)aldlus of the present invention. The direction of
magnetization of the magnet 62 is preferably parallel to the longitudinal axis 78. The
direction of the static magnetic field within the sensitive volume 58 generated by the
25 permanent magnet 62 is substantially perpendicular to the longitudinal axis 78 as shown
by arrows 110. Nuclear magnetic moments in the material to be analyzed (the earth
formation located within the sensitive volume 58) are substantially aligned in the
direction of the static magnetic field. In the preferred embodiment of the invention, the
direction of a linearly polarized RF magnetic field, denoted by arrows 120, within the
30 sensitive volume 58 is substantially perpendicular to the static magnetic field at any point
within the sensitive volume 58. Such a magnetic field arrangement is conventional for
CA 02230902 1998-02-26
NMR experiments.
The static magnetic field direction is symmetrical about the longitudinal axis 78,
the static magnetic field magnitude is, therefore, also symmetric in amplitude about the
longitudinal axis 78. The static magnetic field has an amplitude gradient within the
5 sensitive volume 58 which is also symmetrical about the longitudinal axis 78 and is
directed substantially radially- inwardly towards the longitudinal axis 78. As a result of
these features of the static magnetic field there is generally only one substantially
cylindrical surface external to the permanent magnet 62 which has a particular static
magnetic field amplitude (ignoring end effects of the magnet 62). It follows from this
10 particular feature of the static magnetic field 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 seriously affect the NMR measurements if apll,opliate RF
frequencies are selected.
Undesired static magnetic field end effects may be substantially elimin~ted by
15 making the transceiver antenna 67 somewhat shorter along the longitudinal axis 78 than
the perm~nent magnet 62, so as not to excite materials at the extreme longitudinal ends
of the static magnetic field.
When RF power pulses are conducted through the transceiver antenna 67, the
~n~nn~ 67 generates an RF equivalent magnetic dipole 87 directed parallel to the20 longitudinal axis 78. The equivalent magnetic dipole 87 generates a linearly polarized
RF magnetic field 120 of substantially equal magnitude within the sensitive volume 58.
Since the RF magnetic field direction is parallel to the longitudinal axis 78, the bulk
nuclear magnetization, denoted in Figure 3 by arrows 130, at any point in the sensitive
volume 58 rotates in planes perpendicular to the longitudinal axis 78. The free
25 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 parallel to the longitudinal axis 78. The free precession will therefore induce an
RF signal in the transceiver antenna 67. The ind~lce~l magnetic moment in the
transceiver antenna 67 is shown in Figure 3 as arrows 140.
CA 02230902 1998-02-26
16
3. Design parameters for the plefelled embodiment
In the preferred embodiment of the invention, the signal-to-noise ratio for the
NMR measuring process is sought to be optimized. The following discussion is intended
to explain how certain principal parameters affect the signal-to-noise ratio. The principal
S parameters typically include the geometry of the permanent magnet (62 in Figure 2) and
the transceiver antenna (67 in Figure 2), the power of radio frequency pulses used to
energize the transceiver ~nt.onn~ 67 and the quality factor of the transceiver antenna 67.
Using the transceiver antenna 67 constructed as previously described in the
present embodiment of the invention, the magnitude of an NMR signal, S, induced in the
10 transceiver ~nt~nn~ 67 is typically related to the magnitude of an RF electromagnetic
field, B" by the Reciprocity Theorem and can be described as in the following
expression:
S = ~)ffl As~ (Bl/ll) I (1)
where ffl and AsV~ respectively, represent the nuclear magnetization and the cross
sectional area of the sensitive volume (58 in Figure 1), I, represents the magnitude of the
current flowing in the transceiver antenna 67, the oscillating frequency of the current is
represented by ~ and I represents the effective length of the transceiver ~ntçnn~ 67. For
20 simplicity of the discussion, ffl and B, are assumed to be substantially homogeneous
within the sensitive volume 58.
By substituting ffl = X BJ,~o; where c represents the nuclear magnetic
susceptibility of hydrogen nuclei within the sensitive volume 58, ~ = y Bo~ where Bo
represents the static magnetic field generated by the perm~nent magnet (62 in Figure 2)
25 and described in equation (1), it is therefore possible to derive the following expression
for S:
5 = (y */~ Bo2 (B~/IJ Asv I (2)
The NMR signal thus acquired is therefore directly proportional to the sensitivevolume 58 in the earth formation (26 in Figure 1). The geometry of the sensitive volume
CA 02230902 1998-02-26
58 is determined by the existence of a resonance condition. In pulsed NMR, the
resonance condition is typically met when the deviation of the static magnetic field
magnitude Bo(R) from its value Bo(Rs,~, corresponding to the central frequency of the
current energizing the transceiver ~nt~nn~ 67 (Bo(R)=~ / y), is no greater then half the
5 magnitude of the RF magnetic field Bl in(lllce~l by passing current through the
transceiver antenna 67, expressed as shown in equation (3):
Bo(R) - Bo(RS.~ < B~/2 (3)
10 The static magnetic field Bo(R) at the excitation radius Rs~ may also be described in the
form of a Taylor expansion as:
Bo (R) =Bo(R~) - (dBo/dR)(R-R~ (4)
15 where (dBo/dR) represents the static magnetic field gradient at radius R = R
From equation (3):
Bo(R~ - Bo(R~ < Bl (5)
20 where Ro and Ri represent, respectively, the outer and inner radii of the sensitive volume
58. As a practical matter Ro~ Ri << Rexc
As~ = 2~TRs, B,/(dBo/dR) (6)
25 Substitution of the equation (6) into (2) yields:
5 =2~( ~X/~ Bl(B,/II) RsVlBo2/(dBo/dR) (7)
As is understood by those skilled in the art, the root-mean-square (RMS) thermal noise
30 can be described by the expression:
CA 02230902 1998-02-26
N""~ = (4 k T ~f r) 1~2 (8)
where ~f represents the receiver bandwidth. The bandwidth is typically about
yB~/2~r for a matched receiver; k represents Boltzmann's constant; and T represents the
absolute temperature.
Then for SIN we have:
s~ 2~(rx/~ B~(B~J RsV(kT~f r)~l'21x[lBO2/(dBJdR)] (g
The first bracketed expression in equation (9), for a given proton spin density and
absolute temperature, depends only on the transceiver ant~nn~ 67 parameters. The second
bracketed expression in equation (9) describes parameters used in the design of the
permanent magnet (62 in Figure 2), as will be further explained.
Another parameter affecting the design of the permanent magnet is the degree of
the static field homogeneity in the direction of NMR tool motion as will be further
explained.
4. Synthesis of the static ma~netic field
It follows from (lO) that given the antenna aperture length I and the radius Rs~ of
the sensitive volume are determined by the vertical resolution and penetration depth
requirements, the ratio Bo2/(dBJdR) should be m~ximi7Pd to provide maximum signal-
to-noise ratio. For the elongated magnet with homogeneous distributed magnetic charge
(linear distribution of magnetization within the main magnet 61 in Figure 4) Bo can be
25 calculated as:
BO=q/21rR (10)
where q is the magnetic charge per unit length of the magnet. By the definition the
30 magnetic charge density is given by the equation ~ odivM. Therefore, in our case we
have:
CA 02230902 1998-02-26
19
q=p7rRm2 =(2B,m/lm) ~TRm (11)
where Rm is the magnet radius; Brm is the m~imllm remanence of the magnet material
5 used.
Substitution (11) to (10) and then Bo and (dBo/~R) to (9) gives for the permanent magnet
related part of the SIN as follows:
S,qV ~XBrmRm2 I/lm (12)
It is clear from (12) that for any value of vertical resolution selected for the instrument,
the S/N is inversely proportional to Im. To keep the length lm of the magnet 62 as short as
is practical, it is important for the static magnetic field to be substantially perpendicular
and homogeneous for as great a fractional amount as possible of the axial length of the
magnet 62. The length of the perpendicular, homogeneous static magnetic field should
also exceed the :~nt~nn~ ap~ e length I to ensure a steady state nuclear magnetization
measurement even while the NMR probe 42 is moving through the wellbore (22 in
Figure 1). In addition to these requirements, the static magnetic field should be
minimi7f~d in the region where the transceiver ~nt~nn~ 67 is placed. A plere.ledembodiment of the perm~nent magnet (62 in Figure 2) is shown in Figure 5. The main
magnet 61 is made of up of annular magnetic rings 61A-61F each having a dirr~ellt
residual magnetization so as to approximate substantially linear magnetization
distribution from one end of the magnet 61 to the other. Additionally, the top insert
magnet 63 and the bottom insert magnet 64 serve to optimize the static field according to
the criteria stated above, namely, a high degree of static field homogeneity in the
direction of longitudinal axis 78 for performing NMR measurements while moving along
the wellbore (22 in Figure 1), and a low residual static magnetic field at the transceiver
antçnn~ 67 so the ferrite rod 68 will remain substantially unsaturated.
Figure 6 shows a graphic representation of the static magnetic field within the
volume of investigation (58 in Figure 2). Figure 7 shows the residual longitudinal
CA 02230902 1998-02-26
magnetic field within the hole (83 in Figure 2) at the location of the transceiver antenna
67.
The dimen.~ions shown for the magnet 62 in Figure 5 are as follows: L0=1.2m;
Lh=0.64 m; Do =0 07 m; Dh=0.03 m. The magnet presented in Figure 5 is especiallysuitable for a 30 cm length transceiver antenna 67 and a 24 cm diameter sensitive volume
(58 in Figure 2). The main magnet 61 should be transparent to the RF magnetic field
emitted by the transceiver antenna 67. Since the main magnet 61 need only have
relatively low remanence magnetization this part of the magnet 62 can be formed from
ferrite petm~nent magnet material or the like which is substantially nonconductive and
10 radio frequency transparent. The end magnets 63, 64 preferably are made from a high
remanence magnetization material such as sintered oriented Samarium-Cobalt or
Neodyllliulll-Iron-Boron having a remanence magnetization of 1 T or more. The
magnetic field shown in Figure 6 assumes the end magnets have a remanence
magnetization of 0.7 T for the end magnets 63, 64 and 0.42 T for the main magnet 61.
The static magnetic field within the sensitive volume 58 has substantially equalamplitude for NMR excitation. As was explained for equation (3), within the sensitive
volume 58 the static magnetic field amplitude varies only within a narrow range from 1~0
-Bl/2 to Bo +BI/2. It is of great importance how fast the spatial variation of this field is in
a direction of motion of the logging instrument. The rate of this variation corresponds 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 8. Two lines 58L and 58M l~plesell~ lines of equal magnitude of
the static magnetic field, with a first magnitude and a second magnitude, respectively.
These magnitudes are in the range from Bo -B,/2 to Bo +Bl/2. Static magnetic field
magnitude gradients at location 58A and location 58D in Figure 8 are inversely
proportional to a distance between two points along a direction of movement 81 parallel
to the longitudinal axis 78, one on line 58L and other on line 58M. For example, the
gradient component in the direction 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 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
CA 02230902 1998-02-26
should be apparent from Figure 8 that-the gradient at the central part of the sensitive
volume 58 is much smaller than the gradient at its ends. The gradient component in the
direction perpendicular to the longitudinal axis 78 at location 58A in the center part of
the sensitive volume 58is inversely proportional to the distance between points 58B and
58A. The strongest component of the static magnetic field amplitude gradient is in the
radial direction.
The sensitive volume 58 is determined by the RF magnetic field. 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
10 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, ~s, 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:
as(N) = [BO(n) - Bo(B)I / G (13)
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 ~S(N). More specifically:
v xt< ~fi~ (14)
where v represents the speed of motion of the well logging instrument. The totaldisplacement of the instrument should not represent a substantial portion of the total
volume. The inequality which should thus be satisfied can be written as:
CA 02230902 1998-02-26
22
lBo(2V) ~ Bo(B)l - << Bl (15)
A reasonable estimate of the maxi~llulll gradient in the direction of motion can be
calculated as:
G < (0.1 B,) / (v x t) (16)
For practical values of Bl in the range of 2 x 10~ T, and v of about 0.05 m/sec, for a time
of 200 milliseconds for a measurement sequence 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 optimi~ing the shape of the static magnetic field.
It is common for a logging instrument velocity perpendicular to the wellbore to
be about 50 times smaller than the logging speed. This requires the static magnetic field
gradient in the direction perpendicular to the longitudinal axis 78 to be less than about
0.1 T/m. The preferable geometry of the magnet shown in Figure 5 has a radial static
magnetic field gradient of 0.05 T/m (5 Gauss/cm). Other values of the radial amplitude
gradient may be selected, depending on the NMR excitation frequency selected.
The requirements for the radial static magnetic field gradient are also affected by
the presence of earth magnetic field He. Earth's magnetic field is substantiallyhomogeneous and is about 0.5 x 104 T in magnitude. The logging tool orientation with
respect to the earth's magnetic field direction depends on the wellbore geographical
location and drilling deviation. This field variation should not substantially change the
radius Rsv of the sensitive volume (58 in Figure 1). The magnitude of any such change in
Rs~ may be expressed as the ratio He /G. Therefore, the required radial static magnetic
field magnitude gradient G should satisfy the inequality He IG << RsV; or G >> He IRsV
For Rs~ = 0.1 m G should be much greater than about 5 x104 T/m. As a practical matter
the radial gradient (2 x 1 o-2 T/m) of the magnet shown in Figure 6 more than meets this
requirement.
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 present invention should be limited only by the claims which follow.
