Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FAST T1 MEASUREMENT OF AN EARTH FORMATION BY USING DRIVEN EQUILIBRIUM
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention is related to methods of geological exploration
in
wellbores. In particular, the present invention is a method of improving
nuclear
magnetic resonance pulse techniques.
2. Description of the Related Art
[0002] A variety of techniques are currently utilized in determining the
presence and estimation of quantities of hydrocarbons (oil and gas) in earth
formations. These methods are designed to determine formation parameters,
including among other things, the resistivity, porosity and permeability of
the rock
formation surrounding the wellbore drilled for recovering the hydrocarbons.
Typically, the tools designed to provide the desired information are used to
log the
wellbore. Much of the logging is done after the wellbores have been drilled.
More
recently, wellbores have been logged while drilling, which is referred to as
measurement-while-drilling (MWD) or logging-while-drilling (LWD). One
advantage of MWD techniques is the reduced amount of time necessary to obtain
information about the rock formation. Whereas there is a huge cost associated
with
the amount of time spent in oil exploration, reducing this amount of time is
an
iinportant factor to consider when designing related testing methods and
tools.
[0003] 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 liquids in the
geological
formations surrounding the wellbore so that certain parameters such as spin
density,
longitudinal relaxation time (generally referred to in the art as TI) and
transverse
relaxation time (generally referred to as T2) of the geological formations can
be
measured. From such measurements, porosity, permeability and hydrocarbon
saturation are determined, which provides valuable information about the malce-
up of
the geological formations and the amount of extractable hydrocarbons.
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[0004] The NMR tools generate a static magnetic field in a region of interest
surrounding the wellbore. NMR is based on the fact that the nuclei of many
elements
have angular momenttun (spin) and a magnetic moment. The nuclei have a
characteristic Lannor resonant frequency related to the magnitude of the
magnetic
field in their locality. Over time the nuclear spins align themselves along an
externally applied magnetic field. This equilibrium situation can be disturbed
by a
pulse of an oscillating magnetic field, which tips the spins with resonant
frequency
within the bandwidth of the oscillating magnetic field away from the static
field
direction. The angle 0 through which the spins exactly on resonance are tipped
is
given by the equation:
B = yB,tn (1)
where y is the gyromagnetic ratio, B, is the effective field strength of the
active
rotating field component and tP is the duration of the RF pulse.
[0005] After tipping, the spins precess around the static field at a
particular
frequency known as the Larmor frequency coo, given by
w = yBp (2)
where Bo is the static field intensity. At the same time, the spins return to
the
equilibrium direction (i.e., aligned with the static field) according to an
exponential
decay time known as the spin-lattice relaxation time or TI. For hydrogen
nuclei, y/2'R
= 4258 Hz/Gauss, so that a static field of 235 Gauss would produce a
precession
frequency of 1 MHz. The TI of fluid in pores is controlled totally by the
molecular
environment and is typically ten to several thousand milliseconds in rocks.
[0006] At the end of a 6= 90 tipping pulse, spins on resonance are pointed in
a common direction perpendicular to the static field, and they precess at the
Larmor
frequency. However, because of inhomogeneity in the static field due to the
constraints on tool shape, imperfect instrumentation, or microscopic material
heterogeneities, each nuclear spin precesses at a slightly different rate.
Hence, after a
time long compared to the precession period, but shorter than TI, the spins
will no
longer be precessing in phase. This de-phasing occurs with a time constant
that is
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commonly referred to as T, * if it is predominantly due to the static field
inhomogeneity of the apparatus and as T, if it is due to properties of the
material.
[0007] One method to create a series of spin echoes uses the so-called Carr-
Purcell sequence. This method is discussed, for example, in Fukusima, E., and
Roeder, B., "Experimental Pulse NMR: A Nuts and Bolts Approach", 1981, as well
as
Slichter, C. P., "Principles of Magnetic Resonance", 1990. The pulse sequence
starts
with a delay of several T, to allow spins to align along an applied static
magnetic field
axis. Then a 90 tipping pulse is applied to rotate the spins into the
transverse plane,
where they precess with angular frequency determined by local magnetic field
strength. The spin system loses coherence in accordance with time constant, T,
After a short time (tCP) a 180 tipping pulse is applied which continues to
rotate the
spins, inverting their position in the transverse plane. The spins continue to
precess,
but now their phases converge until they momentarily align a further time tCP
after
application of the 180 pulse. The realigned spins induce a voltage in a
nearby
receiving coil, indicating a spin echo. Another 180 pulse is applied after a
further
time tcp, and the process is repeated many times, thereby forming a series of
spin
echoes with spacing 2 tcp between them.
[0008] While the Carr-Purcell sequence would appear to provide a solution to
eliminating apparatus-induced inhomogeneities, it was found by Meiboom and
Gill
that if the duration of the 180 pulses in the Carr-Purcell sequence were even
slightly
erroneous so that focusing is incomplete, the transverse magnetization would
steadily
be rotated out of the transverse plane. As a result, substantial errors would
enter the
T~ determination. Thus, Meiboom and Gill devised a modification to the Carr-
Purcell
pulse sequence (known as the CPMG sequence) such that after the spins are
tipped by
90 and start to de-phase, the carrier of the 180 pulses is phase shifted by
7U/2 radians
relative to the carrier of the 90 pulse. This phase change causes the spins
to rotate
about an axis perpendicular to both the static magnetic field axis and the
axis of the
tipping pulse. If the phase shift between tipping and refocusing pulses
deviates
slightly from 7r/2 then the rotation axis will not be perfectly oi-thogonal to
the static
and RF fields, but this has negligible effect. As a result any error that
occurs during
an even numbered pulse of the CPMG sequence is cancelled out by an opposing
error
in the odd numbered pulse. The CPMG sequence is therefore tolerant of
imperfect
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spin tip angles. This is especially useful in a well logging tool which has
inhomogeneous and imperfectly orthogonal static and pulse-oscillating (RF)
magnetic
fields. For an explanation, the reader is referred to a detailed account of
spin-echo
NMR techniques, such as in Fi{kushima aracd Roeder, "Experimental Pulse NMR: A
Nuts and Bolts Approach".
[0009] Other pulses sequences are known in the prior art. U.S. Patent No.
6,466,013, to Hawkes et al., for example, discusses a method, referred to as
the
Optimized Rephasing Pulse Sequence (ORPS), which optimizes the timings for
inhomogeneous Bo and B1 fields to obtain maximum NMR signal or, alternatively,
to
save radio frequency power. A pulsed RF field is applied which tips the spins
on
resonance by the desired tip angle for maximum signal, typically 90 tipping
pulse. A
refocusing pulse having a spin tip angle substantially less than 180 is
applied with
carrier phase shifted by typically 7r/2 radians with respect to the 90
tipping pulse.
Although the refocusing pulses result in spin tip angles less than 180
through the
sensitive volume, their RF bandwidth is closer to that of the original 90
pulse. Hence
more of the nuclei originally tipped by 90 are refocused, resulting in larger
echoes
than would be obtained with a conventional 90 refocusing pulse. ORPS is not a
CPMG sequence, since the timing and duration of RF pulses are altered from
conventional CPMG to maximize signal and minimize RF power consumption.
Nevertheless ORPS also possesses the characteristic that the tipping pulse is
phase
shifted by 7/2 with respect to the refocusing pulses. An additional forced
recovery
pulse at the end of an echo train may be used to speed up the acquisition
and/or
provide a signal for canceling the ringing artifact. The forced recovery pulse
occurs
at the same time as the formation of an echo and acts about the same axis as
the
original 90 tipping pulse. The final pulse rotates the nuclear spins (that
are in the
process of forming the echo) away from the transverse (XY) plane and back into
substantial alignment with the magnetic field. Since the final magnetization
is in
equilibrium with the static magnetic field, such a pulse sequence is often
referred to as
a Driven Equilibrium pulse sequence. It is shown by Edzes ("An analysis of the
Use
of Pulse Multiplets in the Single Scan Determination of Spin-Lattice
Relaxation
Rates", J. Mag. Res., 17, 301-313 (1975)) that errors arising from
inhomogeneities in
the static and RF magnetic fields, from improper RF phases or from resonance
offset,
can largely be compensated by using a proper pulse multiplet, i.e. a driven
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equilibrium pulse sequence. Edzes also discusses a method of obtaining a spin-
lattice
relaxation constant using pulse multiplets evenly spaced in time after an
inversion
pulse.
[0010] The use of a driven equilibrium pulse sequence is discussed, for
example, in U.S. Patent No. 6,597,171, to Hurlimann et al. A sequence of
magnetic
pulses is applied to a fluid in a rock, the sequence including a first part
that is
designed to prepare a system of nuclear spins in the fluid in a driven
equilibrium
followed by a second part that is designed to generate a series of magnetic
resonance
signals. The first part can be a driven equilibrium pulse sequence. Repeated
use of a
driven equilibrium block results in an equilibrium magnetization that is
dependent on
T, and T,. Combining such driven equilibrium blocks with the usual CPMG
sequence
gives the T, and T, of the sample. While Hurlimann 171 uses driven
equilibrium
pulses for preparation of a sample for Ti or T, measurements, there is no
discussion of
using echo signals within the driven equilibrium sequence for directly
determining
fonnation and/or fluid properties.
[0011] U.S. Pat. Nos. 6,531,868, to Pranzmer, and 6,717,404, to Pramiiier-,
discusses a method of detennining longitudinal relaxation times TI based on
NMR
relaxation time measurements using pulsed NMR tools with magnetic fields that
are
rotationally symmetric about the longitudinal axis of the borehole. At least
one radio
frequency pulse is generated covering a relatively wide range of frequencies
to
saturate the nuclear magnetization in a cylindrical volume around the tool;
transmitting a readout pulse at a frequency near the center of the range of
covered
frequencies, the readout pulse following a predetermined wait time; applying
at least
one refocusing pulse following the readout pulse; receiving at least one NMR
echo
corresponding to the readout pulse; repeating the above steps for a different
wait time
to produce a plurality of data points on a T, relaxation curve; and processing
the
produced T, relaxation curve to derive petrophysical properties of the
formation.
[0012] There is a general need for improving the speed at which one can
obtain nuclear magnetic resonance data from a wellbore. The use of driven
equilibrium pulses can address this need. The present invention satisfies that
need.
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SUMMARY OF THE INVENTION
[0013] One embodiment of the present invention is a method of evaluating an
earth formation. A driven-equilibrium (DE) pulse group is applied to the earth
fonnation to generate at least one echo signal. A longitudinal relaxation time
T, of
the earth formation is estimated using an amplitude of the at least one echo
signal.
The at least one echo signal may be a plurality of echo signals. A plurality
of DE
groups may be applied after a saturation sequence to get a T, distribution. A
Tj
distribution may also be obtained by applying a plurality of DE groups after
an
inversion sequence. After the sequence of DE groups a CPMG or ORPS sequence
may follow to gather T~7 relaxation decay data from which a T2 distribution
can be
estimated. The signals may be further processed to determine, porosity, clay
bound
water, bound water irreducible, bound water moveable, diffusivity and/or
permeability.
[00141 Another embodiment of the invention is an apparatus for evaluating an
earth formation. The apparatus includes a nuclear magnetic resonance (NMR)
tool
which applies at least one driven-equilibrium (DE) pulse group to the earth
formation
to generate at least one echo signal. A processor estimates a longitudinal
relaxation
time T, of the earth formation using an amplitude of the at least one echo
signal. The
at least one echo signal may comprises a plurality of echo signals. The at
least one
DE pulse group may have a plurality of DE pulse groups, and when the plurality
of
DE groups are applied subsequent to a saturation sequence a T, distribution
may be
estimated. A T, distribution may be obtained also be estimated by applying a
plurality of DE groups following an inversion sequence. After the sequence of
DE
groups a CPMG or ORPS sequence may follow to gather T1? relaxation decay data
from which a T2 distribution can be estimated. The processor may further
estimate
porosity, clay bound water, bound water irreducible, bound water moveable,
diffusivity, and/or permeability. The NMR tool may be a zero gradient tool or
one in
which a static field gradient is present in a region of examination. The NMR
tool may
be on a bottomhole assembly (BHA) for drilling operations, or may be part of a
downhole logging assembly conveyed on a wireline
[00151 Another embodiment of the invention is a machine readable medium
having instnictions of evaluation of an earth formation, the medium includes
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instructions for estimating a longitudinal relaxation time T, of the earth
formation
using an amplitude of at least one echo signal produced by applying at least
one
driven-equilibrium (DE) pulse group to the earth formation. With a plurality
of echo
signals, the medium further includes instructions for estimating a
distribution of
values of TI. The medium may fiirther include instructions for estimating
porosity,
clay bound water, bound water irreducible, bound water moveable, diffusivity
and/or
permeability. The medium may also include instructions for applying one or
more
DE pulse groups to the earth formation. The medium may also include
instntctions
for applying pulse sequences including a plurality of DE groups, and
processing the
resulting signals to determine a T2 distribution.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The present invention is best understood with reference to the
accompanying figures in which like numerals refer to like elements and in
which:
FIG. 1(Prior Art) shows a measurement-while-drilling tool suitable for use
with the
present invention;
FIG. lA (Prior Art) shows the antenna and magnet configuration of an exemplary
NMR device suitable for use with the present invention;
FIG. 2 shows a typical driven equilibrium (DE) group;
Figs. 3A-B show spin echo responses to a series of DE pulse groups
Figs. 4A-B show spin echo responses to a series of DE pulse groups, each
designed to
give rise to tliree spin echoes;
Fig. 5A (prior art) shows a pulse sequence usable for a conventional
saturation
recovery T, method;
FIG. 5B (prior art) shows a pulse sequence for a conventional inversion
recovery TI
method;
Fig. 6A shows a pulse sequence for a fast saturation recovery TI method;
Fig. 6B shows a pulse sequence usable for a fast inversion recovery T, method;
Fig. 7A shows a pulse sequence that combines a fast saturation recovery TI
method
with a CPMG or ORPS sequence to also measure T., or a T, distribution; and
Fig. 7B shows a pulse sequence that combines a fast inversion recovery T,
method
with a CPMG or ORPS sequence to also measure T, or a T, distribution.
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DETAILED DESCRIPTION OF THE INVENTION
[0017] Fig. 1 shows a schematic diagram of a drilling system 10 with a
drillstring 20 carrying a drilling assembly 90 (also referred to as the bottom
hole
assembly, or "BHA") conveyed in a"wellbore" or "borehole" 26 for drilling the
wellbore. The drilling system 10 includes a conventional derrick 11 erected on
a floor
12 which supports a rotary table 14 that is rotated by a prime mover such as
an
electric motor (not shown) at a desired rotational speed. The drillstring 20
includes a
tubing such as a drill pipe 22 or a coiled-tubing extending downward from the
surface
into the borehole 26. The drillstring 20 is pushed into the wellbore 26 when a
drill
pipe 22 is used as the tubing. For coiled-tubing applications, a tubing
injector, such as
an injector (not shown), however, is used to move the tubing from a source
thereof,
such as a reel (not shown), to the wellbore 26. The drill bit 50 attached to
the end of
the drillstring breaks up the geological formations when it is rotated to
drill the
borehole 26. If a drill pipe 22 is used, the drillstring 20 is coupled to a
drawworks 30
via a Kelly joint 21, swive128, and line 29 through a pulley 23. During
drilling
operations, the drawworks 30 is operated to control the weight on bit, which
is an
iinportant parameter that affects the rate of penetration. The operation of
the
drawworks is well known in the art and is thus not described in detail herein.
[0018] During drilling operations, a suitable drilling fluid 31 from a mud pit
(source) 32 is circulated under pressure through a channel in the drillstring
20 by a
mud pump 34. The drilling fluid passes from the mud pump 34 into the
drillstring 20
via a desurger (not shown), fluid line 38 and Kelly joint 21. The drilling
fluid 31 is
discharged at the borehole bottom 51 through an opening in the drill bit 50.
The
drilling fluid 31 circulates uphole through the annular space 27 between the
drillstring
20 and the borehole 26 and returns to the mud pit 32 via a return line 35. The
drilling
fluid acts to lubricate the drill bit 50 and to carry borehole cutting or
chips away from
the drill bit 50. A sensor Sl typically placed in the line 38 provides
information about
the fluid flow rate. A surface torque sensor S2 and a sensor S3 associated
with the
drillstring 20 respectively provide information about the torque and
rotational speed
of the drillstring. Additionally, a sensor (not shown) associated with line 29
is used to
provide the hook load of the drillstring 20.
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[0019] In one embodiment of the invention, the drill bit 50 is rotated by only
rotating the drill pipe 22. In another embodiment of the invention, a downhole
motor
55 (mud motor) is disposed in the drilling assembly 90 to rotate the drill bit
50 and the
drill pipe 22 is rotated usually to supplement the rotational power, if
required, and to
effect changes in the drilling direction.
[0020] In an exemplary embodiment of Fig. 1, the mud motor 55 is coupled to
the drill bit 50 via a drive shaft (not shown) disposed in a bearing assembly
57. The
mud motor rotates the drill bit 50 when the drilling fluid 31 passes through
the mud
motor 55 under pressure. The bearing assembly 57 supports the radial and axial
forces of the drill bit. A stabilizer 58 coupled to the bearing assembly 57
acts as a
centralizer for the lowermost portion of the mud motor assembly.
[0021] In one embodiment of the invention, a drilling sensor module 59 is
placed near the drill bit 50. The drilling sensor module contains sensors,
circuitry and
processing software and algorithms relating to the dynamic drilling
parameters. Such
parameters typically include bit bounce, stick-slip of the drilling assembly,
backward
rotation, torque, shocks, borehole and annulus pressure, acceleration
measurements
and other measurements of the drill bit condition. A suitable telemetry or
communication sub 72 using, for example, two-way telemetry, is also provided
as
illustrated in the drilling assembly 90. The drilling sensor module processes
the
sensor information and transmits it to the surface control unit 40 via the
telemetry
system 72.
[0022] The communication sub 72, a power unit 78 and an MWD tool 79 are
all connected in tandem with the drillstring 20. Flex subs, for example, are
used in
connecting the MWD too179 in the drilling assembly 90. Such subs and tools
form
the bottom hole drilling assembly 90 between the drillstring 20 and the drill
bit 50.
The drilling assembly 90 makes various measurements including the pulsed
nuclear
magnetic resonance measurements while the borehole 26 is being drilled. The
communication sub 72 obtains the signals and measurements and transfers the
signals,
using two-way telemetry, for example, to be processed on the surface.
Alternatively,
the signals can be processed using a downhole processor in the drilling
assembly 90.
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[0023] The surface control unit or processor 40 also receives signals from
other downhole sensors and devices and signals from sensors SI-S3 and other
sensors
used in the system 10 and processes such signals according to programmed
instructions provided to the surface control unit 40. The surface control unit
40
displays desired drilling parameters and other information on a
display/monitor 42
utilized by an operator to control the drilling operations. The surface
control unit 40
typically includes a computer or a microprocessor-based processing system,
memory
for storing programs or models and data, a recorder for recording data, and
other
peripherals. The control unit 40 is typically adapted to activate alarms 44
when
certain unsafe or undesirable operating conditions occur.
[0024] The magnet and antenna configuration of an exemplaiy NMR device
suitable for use with the present invention is shown in Fig. 1A. Magnets 132
and 134
are permanently magnetized, for example, in the axial direction and, in one
embodiment, are positioned in opposing directions. Like magnetic poles, for
example, the north magnetic poles of the two magnets 132 and 134 face one
another
for producing a toroidal region of substantially hoinogeneous radial magnetic
field
140 perpendicular to the pair of axially aligned magnets 132 and 134. A radio
frequency (RF) transmitting antenna or coil 136 is located, for example,
between the
two spaced-apart magnets 132 and 134. The RF coil 136 is connected to a
suitable
RF pulse transmitter for providing power at selected frequencies and a
processor
which determines a pulse sequence timing. The RF coil 136 is pulsed and
creates a
high frequency RF field orthogonal to the static magnetic field. The pulsed RF
coil
136 creates the pulsed RF field 142 illustrated by dashed lines. The distance
of the
toroidal region 140 of homogeneous radial magnetic field from the axis of the
magnets 132 and 134 is dependent upon the distance between like poles of the
magnets 132 and 134. Rock pores (not shown) in the earth formations are filled
with
fluid, typically water or hydrocarbon. The hydrogen nuclei in the fluid are
aligned in
the region of homogeneous magnetic field 140, generated by the magnets 132 and
134. The hydrogen nuclei are then "flipped" away from the homogeneous magnetic
field 140 by the pulsed RF field 142 that must fulfill the resonance condition
(2) and
is produced by RF coil 136. At the termination of the pulsed RF field from
coil 136,
the hydrogen nuclei revolve or precess at high frequency around the magnetic
field
140 inducing an NMR signal in the RF coi1136. The induced NMR signals are sent
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to the surface for processing or can be processed by a downhole processor (not
shown). Other variations for conducting NMR experiments would be known to
those
versed in the art, and any of these could be used in the application of the
present
invention. This basic structure is used, for example, in U.S. Patent 6,215,304
to
Slade, the contents of which are fiilly incorporated herein by reference.
[0025] The tool of Slade is what is called a "zero gradient" tool in which the
static
magnetic field gradient in the region of examination is close to zero.
However, the
method of the present invention may also be used with NMR tools that have
field
gradients. An example of such a device is shown in U.S Patent 6,348,792 to
Beard et
al., having the same assignee as the present invention and the contents of
which are
fully incorporated herein by reference.
[0026] Equilibrium in an NMR system is the state in which the affected
nuclear spins are in equilibrium with the surrounding magnetic field and
temperature.
(i.e., parallel to the static external field, generally referred to as the z-
axis). The actual
magnetization can be manipulated by RF pulses to point along the z-axis using
a
method of "Driven Equilibrium".
[0027] One example of a driven equilibrium (DE) group used in accordance
with the present invention is shown in Fig. 2. A group of driven equilibrium
RF
pulses can be used to probe the z-magnetization by tipping it into the xy
plane,
generating an echo and tipping it back into the z direction. The DE group of
Fig. 2
comprises the sequence:
90X - i- 180y - i(echo) i- 180y - i- 90_,,. (3)
Following an initial long delay, that may be set in a typical application to 6
sec., a
combination of 90x tipping pulse 201 and 180y refocusing pulse 203 (applied at
a time
i after the tipping pulse) trigger a spin echo 220 in the acquisition window
between
203 and 205. The dephasing spins are refocused using a second 180y refocusing
pulse
205. A time i after the latter refocus pulse 205 the spins refocus again and
at this time
a 90-,, recovery pulse 207 performs the opposite function of the initial 90,,
tipping
pulse 201 by flipping the magnetization back along the z direction.
Approximately
80%-90% of the initial magnetization can be recovered in practice. It will be
appreciated that other pulse sequences can be used in practice, such as
replacing a
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signal acquisition window with a 90_,, pulse following the second or
subsequent
echoes. It will be apparent to a person of skill in the art that different
timing can be
used in various practical applications as well.
[0028] The DE group, like the CPMG pulse sequence, corrects for cumulative
pulse errors. In CPMG, cumulative pulse errors are compensated from the second
echo of the primary echo train onward. In a DE group, the error from the 90,
pulse is
also compensated. Consequently, the echoes of successive DE groups have the
same
amplitude. Minor differences in the amplitudes of echoes from successive DE
groups
are generally attributable to the presence of stimulated echoes. The use of
the driven
equilibrium concept in a "fast" saturation recovery sequence followed by a
CPMG or
ORPS enable one to obtain a T, decay (and by inversion a T, distribution) plus
a T~,
decay (and by inversion a T, distribution) in the same amount of time in which
a T,
distribution alone is presently obtained. In the same manner, one can perform
a "fast"
inversion recovery sequence for T, measurements plus a T2 measurement. This
variant, however, needs an extra recovery wait time at the beginning of the
sequence.
[0029] Figs. 3-4 show simulations using a series of DE pulse groups obtained
using an NMR simulation program. For the refocusing pulses, pulse lengths
corresponding to 180 are not used. Instead, shorter pulse lengtlls, such as
those used
in an ORPS sequence, are implemented.
[0030] Figs. 3A-B show spin echo responses to a series of DE pulse groups.
Each DE pulse group is designed to give rise to one spin echo (e.g. the DE
group of
Eq. (3)). Fig. 3A shows an echo sequence with 10 such DE pulse groups 300. In
Fig
3A time is shown along the abscissa in seconds, and amplitude is shown along
the
ordinate in arbitrary units. The resultant spin echo magnetization vah.ies
along the x-
axis 305 are also shown. Fig. 3B shows echo amplitudes corresponding to each
of the
10 DE pulse groups of Fig. 3A. As can be seen from Fig. 3B, the echo
amplitudes,
after an initial transition period, hardly vary at all.
[0031] Alternatively, it is possible to use DE pulse groups which give rise to
more than one spin echo. Typically, in a CPMG sequence, the first spin echo
does not
achieve the full amplitude whereas the second spin echo is generally more
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representative of the maximum possible echo amplitude. Figs. 4A-B show spin
echo
responses to a series of DE pulse groups, each designed to give rise to three
spin
echoes. At the time when the 4th echo would appear, a 90_, pulse aligns the
magnetization back along the z-axis. As expected, the second echo of each
pulse
sequence achieves a greater amplitude. It can be useful to obtain more than
one echo
by increasing the length of the driven equilibrium block.
[0032] Fig. 4A shows an echo sequence with 10 DE pulse groups 400
comprising 3 spin echoes each. In Fig. 4A time is shown along the abscissa in
seconds and amplitude along the ordinate in arbitrary units as in Fig. 3A.
Spin echo
magnetization values along the x-axis 405 are shown. Fig. 4B shows echo
amplitudes
obtained with the DE pulse groups of Fig. 4A. The DE group index is shown
along
the abscissa. Letters a to c denote the first to third echoes of each DE
group. The
continuity of echo amplitudes can be seen. The arbitrary amplitude units of
all the
four figs. 3A/B and 4A/B are the same for the echo amplitudes. Comparing Fig.
4B
with Fig. 3B we see that for the DE groups with 3 echoes all the echo
amplitudes are
greater than the average echo ainplitude of Fig. 3B that produced one echo per
DE
group.
[0033] Figs. 5-6 illustrate how the driven equilibrium sequence can be used to
speed up T, measurements. Fig. 5A shows a pulse sequence usable for a
conventional
saturation recovery T, method. Blocks marked S (501) indicate a saturation
sequence,
e.g. aperiodic sequence (APS), and blocks marked D (503) denote a detection
sequence, e.g. short CPMG or ORPS. iWi2, i3 etc. are delay times. By plotting
the
detected signal amplitudes versus i, one can obtain a Tj saturation recovery
curve
from which can be derived a T, distribution. By way of example three different
ti; are
shown in Fig. 5A but less or more are possible. Alternatively, Fig. 5B shows a
pulse
sequence usable for a conventional inversion recovery T1 method. Blocks marked
I
(507) indicate an inversion sequence (e.g. 180 pulse or fast adiabatic
sweep), and
blocks marked D (509) denote a detection sequence, e.g. short CPMG or ORPS.
il,
i2, etc. are delay times, and Tw is a wait time of sufficient length to
achieve
equilibrium magnetization. By way of example two i; are shown in Fig. 5A but
less or
more are possible. Typically Tw is about 3 to 5 times the longest expected Ti.
By
plotting the detected signal amplitudes versus i, one can obtain the T,
inversion
13
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WO 2006/058005 PCT/US2005/042328
recovery curve, from which one can derive a T, distribution. The inversion
recovery
method for obtaining T, gives higher quality data than the saturation sequence
(Fig.
5A) because the detected magnetizations span a range of two 1L1o while the
magnetizations using the saturation recovery span only one MO,where Mo is the
equilibrium magnetization in the applied static magnetic field. However, the
incorporation of wait times Tffi in the inversion recovery method cause it to
take much
longer than the saturation recovery method.
[0034] Both the conventional saturation and inversion recovery methods have
in common that after each sampling of the NMR signal on the recovery curve the
recovery has to start from the beginning again. As the number of different tii
increases, these methods therefore become time-consuming.
[0035] Using driven equilibrium blocks enables one to obtain the NMR signal
in less time. Fig. 6A shows a pulse sequence usable for a fast saturation
recovery T,
method. The block marked S (601) indicates a saturation sequence, e.g.
aperiodic
sequence (APS). Blocks marked DE (603) denote a driven equilibrium block. Each
DE block detects one or more echoes and ends with magnetization in z
direction. il,
ti2, i3 etc. are the times at which the recovering magnetization is sampled.
By plotting
the detected signal amplitudes versus i, one can obtain the T, saturation
recovery
curve, from which one can derive a T, distribution.
[0036] Fig. 6B shows a pulse sequence usable for a "fast" inversion recovery
T, method. T, indicates the wait time to reach equilibrium magnetization. The
block
marked I(607) indicates an inversion sequence (e.g. 180 pulse or fast
adiabatic
sweep), and blocks marked DE (609) denote a driven equilibrium block,
detecting one
or more echoes and ending with magnetization in z direction. tii, i2, i3 etc.
are times
at which the recovering magnetization is sampled. By plotting the detected
signal
amplitudes versus i, one obtains a T, inversion recovery curve from which one
can
derive a T, distribution. A comparison of Fig. 6A to Fig. 5A shows a reduced
time
necessary for obtaining the measurement. Even more drastic is the comparison
between Fig. 6B and Fig. 5B. We see that the use of DE blocks saves
substantial
measurement time for both, saturation recovery and inversion recovery.
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WO 2006/058005 PCT/US2005/042328
[0037] In many standard NMR measurements the sequence starts optionally
with a saturation sequence followed by a long magnetization recovery wait time
of
several seconds followed by the ORPS (or CPMG) sequence to detect the signal
and
determine a T, decay. By inserting driven equilibrium blocks into the
magnetization
recovery wait time, it is possible to use the present magnetization recovery
wait time
to measure T, recovery (from which a T, distribution can be estimated) without
extra
time penalty. This is shown in Figs. 7a and 7b.
[0038] Fig. 7a shows the fast saturation recovery sequence of Fig. 6a followed
by a
CPMG or ORPS sequence. 710 is an excitation pulse (typically 90 ), 711 are the
refocusing pulses (180 for CPMG, less than 180 for ORPS) and 712 are spin
echoes. Fig. 7b shows the inversion recovery sequence of Fig. 6b followed by
CPMG or ORPS sequence. 710' is an excitation pulse (typically 90 ), 711' are
the
refocusing pulses (180 for CPMG, less than 180 for ORPS) and 712' are spin
echoes. The nuinber of DE groups in figures 7A and 7B may be more or less than
those shown.
[0039] Once the T, and T, distributions have been obtained, they can be
processed using prior art methods to determine parameters of interest of the
earth
formation and fluids in the earth formation. These parameters include
porosity, clay
bound water, bound water irreducible, bound water moveable, diffusivity and
penneability
[0040] For a determination of a full TI and T2 distribution the wait time with
the DE blocks before the CPMG needs to be at least 3 to 5 times the longest
expected
TI time. It is worth mentioning that the recovery sampled by the DE groups is
strictly
not governed by TI relaxation alone but contains some contribution of T2
relaxation
within each DE block In the same way the T2 ineasurement in the following CPMG
is
governed by a contribution of TI relaxation too due to the inhomogeneous
magnetic
field and hence a stimulated echo contribution.
[0041] If the measurement sequence of [0038] is repeated several times with
varying wait times (with or without) DE blocks this is called Tl editing (M.D.
Hurlimann and L. Venkataramanan, J. Magn. Reson. 157, 31-42 (2002)). NMR data
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WO 2006/058005 PCT/US2005/042328
sampled in this way can be graphed tliree-dimensionally to show a 77 -T2
distribution
of the earth formation.
[0042] All the RF pulse sequences stated so far can be phase cycled to create
a
phase alternated pair (PAP) to remove acoustic and electronic ringing as well
as
signal offset. This technique is well known for the CPMG or ORPS sequence and
is
~
equally applicable to DE groups.
[0043] The NMR signals obtained from the echoes within the DE groups may
be affected by motion of the NMR tool. Where the motion is known the signals
may
be corrected very similar to the method disclosed in US patent application
Ser. No.
10/918,965 filed on August 16, 2004
[0044] The processing of the data may be accomplished by a downhole processor.
Alternatively, measurements may be stored on a suitable memory device and
processed upon retrieval of the memory device. Implicit in the control and
processing
of the data is the use of a computer program on a suitable machine readable
medium
that enables the processor to perform the control and processing. The machine
readable medium may include ROMs, EPROMs, EAROMs, Flash Memories and
Optical disks.
[0045] The invention has been described with an example of a MWD tool.
The method is equally applicable to wireline applications in which the NMR
tool is
conveyed on a wireline. For wireline applications, all or part of the
processing may
be done at the surface or at a remote location. For wireline applications, the
NMR
tool is typically part of a downhole string of logging instruments.
[0046] While the foregoing disclosure is directed to the specific einbodiments
of the invention, various modifications will be apparent to those skilled in
the art. It is
intended that all such variations within the scope of the appended claims be
embraced
by the foregoing disclosure.
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