Note: Descriptions are shown in the official language in which they were submitted.
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SIDE-LOOKING NMR PROSE FOR OIL WELL LOGGING
FIELD OF THE INVENTION
This invention relates to apparatus and techniques for making nuclear
magnetic resonance (NMR) measurements in boreholes, and to methods for
determining magnetic characteristics of formations traversed by a borehole.
Specifically, the invention relates to a side-looking NMR tool that attenuates
NMR
signals from the borehole while maintaining a large region of investigation
within the
formation.
BACKGROUND OF THE INVENTION
A variety of techniques have been used in determining the presence and in
estimating quantities of hydrocarbons (oil and gas) in earth formations. These
methods are designed to determine formation parameters, including among other
things, porosity, fluid content, and permeability of the rock formation
surrounding the
wellbore drilled for recovering hydrocarbons. Typically, the tools designed to
provide
the desired information are used to log the wellbore. Much of the logging is
done
after the well bores have been drilled. More recently, wellbores have been
logged
while drilling of the wellbores, which is referred to as measurement-while-
drilling
("MWD") or logging-while-drilling ("LWD"). Measurements have also been made
when tripping a drillstring out of a wellbore: this is called measurement-
while-
tripping ("MWT").
One recently evolving technique involves utilizing Nuclear Magnetic
Resonance (NMR) logging tools and methods for determining, among other things
porosity, hydrocarbon saturation and permeability of the rock formations. The
NMR
logging tools are utilized to excite the nuclei of the fluids in the
geological formations
in the vicinity of the wellbore so that certain parameters such as spin
density,
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longitudinal relaxation time (generally referred to in the art as "Tl"), and
transverse
relaxation time (generally referred to as "TZ") of the geological formations
can be
estimated. From such measurements, porosity, permeability, and hydrocarbon
saturation are determined, which provides valuable information about the make-
up of
the geological formations and the amount of extractable hydrocarbons.
A typical NMR tool generates a static magnetic field B° in the vicinity
of the
wellbore, and an oscillating field Bl in a direction perpendicular to
B°. This
oscillating field is usually applied in the form of short duration pulses. The
purpose
of the B° field is to polarize the magnetic moments of nuclei parallel
to the static field
and the purpose of the Bl field is to rotate the magnetic moments by an angle
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controlled by the width tp and the amplitude B1 of the oscillating pulse. For
NMR
logging, the most common sequence is the Carr-Purcell-Meiboom-Gill ("CPMG")
sequence that can be expressed as
TW-90-(t-180-t-echo)" (1)
where TW is a wait time, 90 is a 90 degree tipping pulse, 180 and is a 180
degree
refocusing pulse.
After being tipped by 90°, the magnetic moment precesses around the
static
field at a particular frequency known as the Larmor frequency c.~° ,
given by c~° _ 'y B°
where B° is the field strength of the static magnetic field and y is
the gyromagnetic
ratio. At the same time, the magnetic moments return to the equilibrium
direction (i.e.,
aligned with the static field) according to a decay time known as the "spin-
lattice
relaxation time" or Tl . Inhomogeneities of the B° field result in
dephasing of the
magnetic moments and to remedy this, a 180° pulse is included in the
sequence to
refocus the magnetic moments. This gives a sequence of h echo signals. These
echo
sequences are then processed to provide information about the relaxation
times.
United States Patent 4,350,955 to .Iacksoh et al discloses a pair of permanent
magnets arranged axially within the borehole so their fields oppose, producing
a
region near the plane perpendicular to the axis, midway between the sources,
where
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the radial component of the field goes through a maximum. Near the maximum,
the
field is homogeneous over a toroidal zone centered around the borehole. With
the
Jackson arrangement, the axial extent of the region of examination is quite
limited.
As a result of this, the device can only be operated at relatively low logging
speeds:
otherwise, because of the tool motion during logging, the magnitude of the
static field
changes significantly within a fixed region of the formation with an
accompanying
degradation of NMR signals.
There are three approaches that may be taken in the design of an eccentric
logging tool. One approach is to have a static field defining a region of
examination
that is primarily on one side of the tool. A second approach is to have a RF
antenna
that is primarily sensitive to signals from one side of the tool. The third
approach is to
have both the static field and the RF antenna with directional sensitivity.
United States Patent 5,488,342, to Hanley, discloses a variation of the
Jackson
device wherein a shaping magnet is positioned adjacent the space between the
pair of
opposed magnets with its magnetic axis transverse to the borehole axis. The
arrangement in the Hanley '342 patent has a region of uniform static field
that is
limited to one side of the magnet arrangement. United States Patent 5,646,528
also to
Hanley, discloses another variation of the Jackson device in which a shield of
electrically conductive material is positioned adjacent to and laterally
offset from the
set of electrical coils whereby the magnetic field generated by the RF antenna
is
asymmetrically offset from the axis of the first magnets. The region of
uniform static
field remains a toroid, as. in the Jackson device. The Hanley '528 device may
be
operated eccentrically within a large borehole with a reduction in the
borehole signal.
Both of the Hanley devices suffer from the drawback that the axial extent of
the
region of examination is small, so that they cannot be operated at high
logging speeds.
There are several devices in which the problem of limited axial extent of the
basic Jackson configuration of permanent magnets is addressed. United States
Patent
4,717,877 to Taicher et al teaches the use of elongated cylindrical permanent
magnets
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in which the poles are on opposite curved faces of the magnet. The static
field from
such a magnet is like that of a dipole centered on the geometric axis of the
elongated
magnets and provides a region of examination that is elongated parallel to the
borehole axis. The RF coil in the Taicher device is also a dipole antenna with
its
center coincident with the geometric axis of the magnet, thereby providing
orthogonality of the static and magnetic field over a full 360° azimuth
around the
borehole.
United States Patent 6,023,164 to Prammer discloses a variation of the
Taiclae~ patent in which the tool is operated eccentrically within the
borehole. In the
Pramme~ device, NMR logging probe is provided with a sleeve having a
semi-circular RF shield covering one of the poles of the magnet: the shield
blocks
signals from one side of the probe. The probe is provided with elements that
press the
uncovered side of the probe to the sidewall of the borehole so that signals
from the
uncovered side arise primarily from the formation.
For both the Pramme~ and the Ha~cley '528 devices, in order to get the best
attenuation in the field behind the probe while maintaining sensitivity in
front of the
probe, the shield should be positioned as far away from the front region as
possible.
The effectiveness of the shield is limited by the diameter of the tool. In the
absence of
a shield, the P~ammer and Haley '52~ tools have a circular sensitive region,
so that
use of either device in an eccentric manner would result in a large signal
from the
borehole fluid.
United States Patent 5,055,787 to Kleihberg et al combines the RF shield
concept taught in Prammer with a shaping of the static field and with actually
separating the effective center of the RF dipole antenna and the center of the
magnet
arrangement. Three magnets with parallel magnetization are used to produce the
static field, the center magnet being opposed in polarity to the magnets on
either side.
The device has a region in front of the tool with a zero gradient while the
region
behind the tool has a large gradient. Consequently, the volume of the
sensitive region
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in front of the tool is much larger than the sensitive region behind the tool,
so that
borehole signals are greatly reduced. One drawback of the Kleinberg
arrangement is
that the region of examination is very close to the tool. This makes it
difficult to
make measurement deeper into the formation, a serious drawback if there is
significant invasion of the formation by borehole fluids. The Kleihberg device
is also
what is known as a zero gradient tool, i.e., the static field has
substantially zero
gradient in the region of examination. This is a disadvantage in N1VIR logging
because many interpretation techniques for deriving petrophysical information
about
fluid diffusion in the formation from NMR data depend upon having a'known and
finite gradient.
SUMMARY OF THE INVENTION
The present invention is a side-looking NMR probe for well logging
applications. It incorporates a number of design features, some of which are
novel in
themselves, in a novel combination that greatly improves the effectiveness of
the tool.
The starting point is a configuration that includes a static field similar to
that of a
dipole, and a dipole-like RF field substantially orthogonal to the static
field, with the
centers of the equivalent static and RF dipoles laterally displaced to provide
a match
on a side defined as the front of the tool and a mismatch on the back of the
tool. The
basic static field may be produced by a main magnet. The static field in the
region of
investigation has a field strength within predetermined limits and a
substantially
uniform gradient. Those versed in the art would recognize that in a so-called
zero
gradient logging tool, in contrast, the static field in the region of
investigation has a
substantially uniform field strength around a saddle point. A second, shaping
magnet
is used to shape the static field to conform to the RF field over a larger
azimuthal
sector around the tool. The RF field is also shaped to increase the effective
radius in
the front of the tool, giving a greater depth of penetration, and making it
conform to
the static field over a larger azimuthal sector. The antenna dipole is
configured with
as large a dipole as possible, thereby increasing its efficiency. A static
shield may be
used to reduce the RF field behind the tool. The static and RF dipoles are
rotated 90°
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relative to prior art, so that the static dipole points to the side of the
tool and the RF
dipole to the front of the tool. With this arrangement, eddy currents in the
shield are
substantially increased, increasing its effectiveness. The RF antenna includes
a core
made of a soft magnetic material to increase its efficiency. The shaping
magnet also
acts as a bucking magnet to allow use of the core material: in its absence,
the static
field would be shorted out due to the rotated field orientation. This greatly
reduces
the field in the core and hence also reduces magnetostrictive ringing. In
addition, the
ringing in the core is reduced by using a soft magnetic material comprising
particles
of powdered material small enough to be transparent to the RF magnetic field
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 depicts diagrammatically an eccentric NMR logging tool in a borehole.
FIGURE 2 shows the field distributions for an eccentric logging tool with
separated
RF and static dipoles..
FIGURE 2A illustrates a desired field configuration of static and RF fields
for a side-
looking NMR device.
FIGURE 3, 3A and 3B show configurations of magnets, antenna and shield of the
present invention for achieving the desired field configuration.
FIGURE 4 illustrates the static and RF field isolines for the present
invention.
FIGURE 5 shows the azimuthal variation of fields and sensitivity in the
present
invention.
FIGURE 6 shows the effect of varying the size of the second magnet in the
present
invention.
FIGURE 7a, 7b and 7c shows the static and RF field distribution for the tool
configuration of FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Fig. 1 depicts a borehole 10 which has been drilled in a typical fashion into
a
subsurface geological formation 12 to be investigated for potential
hydrocarbon
producing reservoirs. An NMR logging tool 14 has been lowered into the hole 10
by
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means of a cable 16 and appropriate surface equipment represented
diagrammatically
by a reel 18 and is being raised through the formation 12 comprising a
plurality of
layers 12a through 12g of differing composition, to log one or more of the
formation's
characteristics. The NMR logging tool is provided with bowsprings 22 to
maintain
the tool in an eccentric position within the borehole with one side of the
tool in
proximity to the borehole wall. The permanent magnets used for providing the
static
magnetic field is indicated by 23 and the magnet configuration is that of a
line dipole.
Signals generated by the tool 14 are passed to the, surface through the cable
16 and
from the cable 16 through another line 19 to appropriate surface equipment 20
for
processing, recording and/or display or for transmission to another site for
processing,
recording and/or display.
Turning now to Fig. 2, the RF and static fields isolines are illustrated for a
configuration comprising separated RF and static 2D dipoles placed inside a
NMR
probe 14 eccentrically located within a borehole 114. For simplifying the
illustration,
the locations of the centers of the RF and static 2D dipoles are not shown. As
shown
in Fig. 2, the difference in curvature of the static 112 and RF 113 magnetic
field
isolines creates a mismatch which rapidly increases with the departure from
the
central point of the region of examination 120. The region of examination 120
subtends only a small angle 8 at the center 122 of the tool 114.
The maximum volume possible for acquiring a NMR signal from the
formation in the side-looking tool may be extended up to the boundary of the
borehole
114. Preferably, the arc of the working region 120 extends so that the end
120a lies
between points 115a and 116a in Fig. 2 and the end 120b lies between the
points
115b and 116b. This extension takes into account the largest boreholes and
even
possible washouts. When the working region extends to points 115a, 115b, and
when
the radius of the tool equals the depth of the working region into the
formation, the
angle 8 would be 120°. For most applications, the angle 8 lies between
90° and 135° .
Fig. 2A shows desired field configurations for the static and RF fields in a
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side-looking NMR device. Shown are isolines in cross section for the static
161 and
RF 151 fields corresponding to offset dipoles. The desired field configuration
has the
elongate oval or pear shaped isoline 163 for the static field and the
flattened oval
shaped isoline 153 for she RF field. where the RF and the static isolines
match over
the arcuate segment 171a -171b. This defines the region of investigation that
may be
used in NMR investigations.
Fig. 3 schematically illustrates a preferred embodiment of the present
invention wherein this shaping of the static and RF fields is accomplished.
The tool
cross-sectional view in Fig. 3 illustrates a main magnet 217, a second magnet
218,
and a transceiver antenna, comprising wires 219 and core material 210. The
arrows
'221 and 223 depict the polarization (e.g., from the South pole to the North
pole) of
the main magnet 217 and the secondary magnet 218. A noteworthy feature of the
arrangement shown in Fig. 3 is that the polarization of the magnets providing
the
static field is towards the side of the tool, rather than towards the front of
the tool (the
right side of Fig. 3) as in prior art devices. The importance of this rotated
configuration is discussed below.
The second magnet 218 is positioned to augment the shape of the static
magnetic field by adding a second magnetic dipole in close proximity to the RF
dipole
defined by the wires 219 and the soft magnetic core 210. This moves the center
of the
effective static dipole closer to the RF dipole, thereby increasing the
azimuthal extent
of the region of examination, the desirability of which has been discussed
above. The
second magnet 218 also reduces the shunting effect of the high permeability
magnetic
core 210 on the main magnet 217: in the absence of the second magnet, the DC
field
would be effectively shorted by the core 210. Thus, the second magnet, besides
acting
as a shaping magnet for shaping the static field to the front of the tool (the
side of the
main magnet) also acts as a bucking magnet with respect to the static field in
the core
210. Those versed in the art would recognize that the bucking function and a
limited
shaping could be accomplished simply by having a gap in the core; however,
since
some kind of field shaping is required on the front side of the tool, in a
preferred
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embodiment of the invention, the second magnet serves both for field shaping
and for
bucking. If the static field in the core 210 is close to zero, then the
magnetostrictive
ringing from the core is substantially eliminated.
As noted above, within the region of investigation, the static field gradient
is
substantially uniform and the static field strength lies within predetermined
limits to
give a substantially uniform Larmor frequency. Those versed in the art would
recognize that the combination of field shaping and bucking could be
accomplished by
. other magnet configurations than those shown in Fig. 3. For example, Fig. 3A
shows a single magnet 227 and magnetic core 230 that produces substantially
the
same static field as that produced by the combination of magnets 217 and 218
in Fig.
3. A substantially similar field configuration results from the arrangement in
Fig. 3B
with the magnet 237 and the core 240. What is being accomplished by the magnet
arrangements in Figs. 3, 3A and 3B is an asymmetry in the static magnetic
field in a
direction orthogonal to the direction of magnetization. In an optional
embodiment of
the invention (not shown) the second magnet is omitted.
Returning to Fig. 3, the transceiver wires 219 and core pieces 210 should
preferably be separated as far as possible towards the sides of the tool. This
separation increases the transceiver antenna efficiency by increasing the
effective RF
dipole of the antenna and augments the shape of the RF magnetic field isolines
so that
they better conform to the static magnetic field isolines. This separation is
not
possible in the Klei~cbe~g design. The secondary magnet is preferably made of
nonconducting material to minimize eddy currents induced by the RF field,
thereby
increasing the RF antenna efficiency.
The core is preferably made of a powdered soft magnetic material, other than
ferrite. It preferably has a high saturation flux density and comprises
particles of
powdered material small enough to be transparent to the RF magnetic field.
Such a
material has been described in a co-pending application entitled "A Method and
Apparatus of Using Soft Magnetic Material in a Nuclear Magnetic Resonance
Probe"
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filed on June 28, 2000, under Attorney Docket No. 584-13268, the contents of
which
are fully incorporated herein by reference.
Turning now to Fig. 4, results of a finite element modeling of the device
illustrated in Fig. 3 are shown. The tool 14 having a diameter of 5 inches is
shown
within a borehole 301 with a diameter of 10 inches. The contour 303 denotes a
static
field strength B ° between 184 Gauss and 186 Gauss. Within the region
305, the RF
field varies by less than 10%, a suitable value for performing gradient NMR
measurements. As can be seen the region of investigation characterized by the
arc
from 321a to 321b is much larger than in the arrangement shown in Fig. 2.
The effective axc length is illustrated in Fig. 5. The abscissa is the
azimuthal
angle measured from the front of the receiver. For simplifying the
illustration, only
one half of the azimuthal distribution is shown. The line 423 is the azimuthal
distribution of the received signal. The effective arc length 421 is the width
of the
rectangle having the same area as the received signal integrated over
180°. Also
shown in Fig. 4 is the effective RF field 422 that is the product of the RF
field
magnitude and the cosine of the angle between the RF field and the static
field. The
gradient of the static magnetic field is denoted by 424 and indicates that the
static
magnetic field gradient is substantially constant in the region of
investigation.
The selection of the size of the second magnet is based upon a finite element
model of the magnet and antenna configuration as well as well as the NMR
signal and
noise calculations. This is illustrated in Fig. 6 that shows the SNR of the
tool 550 and
the signal 551 from the borehole as a function of the normalized second magnet
size
(abscissa). The second magnet size is normalized with respect to the gap
length
between the core segments 210 and a residual flux density of 1 Tesla is
assumed. It is
desirable to keep the borehole signal below a threshold such as 1.5%, giving a
normalized second magnet size of approximately 0.37 (555) and a SNR of 1.63
(556).
The main contribution to the improved SNR comes from the increase in arc
length,
with less of an contribution due to the increased field strength that results
from a
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larger magnet size.
Turning now to Fig. 7a, the static field for one half of the magnet
arrangement
of Fig. 3 is shown. The contours 601a, 601b, . . , indicate the field strength
and the
arrow 605 indicates the direction of the static field at the origin. It may be
seen in Fig.
7a that the static field is close to zero near the antenna wire position 610.
This has the
beneficial effect of reducing magnetostrictive ringing of the antenna. Fig. 7b
shows
the RF field isolines when the permanent magnet 617 is non-conducting. Even
with a
nonconducting permanent magnet, the RF field in the magnet is small, as
indicated by
the sparseness of the isofield lines 612x, 612b. . . near the permanent
magnet. Fig. 7c
shows the RF isofield lines when the permanent magnet is conducting, as in the
preferred embodiment of the present invention.
The permanent magnets of the present invention are made of a conductive
material such as Samarium-Cobalt. Conductive magnets, besides being able to
provide a stronger static field, also act as a shield since the RF field has
to go to zero
in their vicinity. Ringing of the magnet is also less than for one made of non-
conducting material.
Another beneficial effect is that the RF field near the shield is
substantially
perpendicular to the shield (not shown in Figs. 7b and 7c). This means that
the eddy
currents induced in the shield 611by the RF field would be large compared to
prior art
devices such as Prammer, increasing the shielding effect with respect to
borehole
signals. As noted above, the RF shield is optional since the magnet itself
provides
considerable shielding of signals from the borehole.
While the foregoing disclosure is directed to the preferred embodiments of the
invention, various modifications will be apparent to those skilled in the art.
It is
intended that all variations within the scope and spirit of the appended
claims be
embraced by the foregoing disclosure.
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