CA2331904C - Detecting tool motion effects on spin echoes obtained with nuclear magnetic resonance measurements - Google Patents
Detecting tool motion effects on spin echoes obtained with nuclear magnetic resonance measurements Download PDFInfo
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- CA2331904C CA2331904C CA002331904A CA2331904A CA2331904C CA 2331904 C CA2331904 C CA 2331904C CA 002331904 A CA002331904 A CA 002331904A CA 2331904 A CA2331904 A CA 2331904A CA 2331904 C CA2331904 C CA 2331904C
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
- G01V3/00—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation
- G01V3/18—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging
- G01V3/32—Electric or magnetic prospecting or detecting; Measuring magnetic field characteristics of the earth, e.g. declination, deviation specially adapted for well-logging operating with electron or nuclear magnetic resonance
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N24/00—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects
- G01N24/08—Investigating or analyzing materials by the use of nuclear magnetic resonance, electron paramagnetic resonance or other spin effects by using nuclear magnetic resonance
- G01N24/081—Making measurements of geologic samples, e.g. measurements of moisture, pH, porosity, permeability, tortuosity or viscosity
Abstract
An NMR measurement obtained with a logging tool is potentially subject to relative motion between the apparatus and a sample. An echo train is produced having a plurality of spin echo signals. At least two of the spin echo signals are analyzed to determine a effect of tool motion on the NMR measurement.
Description
DETECTING TOOL MOTION EFFECTS ON SPIN ECHOES OBTAINED WITH
NUCLEAR MAGNETIC RESONANCE MEASUREMENTS
Background of tha Invention The invention genera.:l.~l.y rel~~.tes to ins:ide--out nuclear magnetic resonance (NMR) measurements, and more particularly, the invent;iron reml.<~.tes t:c~ detecting total motion effects an NMR measurements of formation properties surrounding a borehc:>le, such as measurements of the hydrogen content of the forrriatian, foz~ example., Referring to Fig. :L, as an example, nuclear magnetic resonance NMR) measurements may be obtained in a logging while dri.ll~_ng (LWD) c:~pe:r_a2:ior. t;o rnap t:he properties of a subterranean .formati.on ~.tl. In tris manner, an axisymmetric NMR tool 6 may be pax-t of a Grill string 5 that is used to drill a bore~uol.e 3 :ire t~tue forcriat::4on In. Th.e tool 6 may be, as examples, one of the tools described in Se~giner et. al. , U.S. Pat.ent Nc>~ _'~, "7G >, 92'7, ent~~t;led, "Pulsed Nuclear Magnetism Tool Fc~~: For_mat:ian Eva)_uation While Drilling Including a Shortened or Truncated CPMG
Sequence°', granted January 6, 1.9!33; Nfil.:Le:r, U.;~. Patient No. 5,280,243, entitled, "Sy:~tem For Logging a Well During the Drill ing Thereof " , grant~~~c.~ ,.7~<~nuary 18 , 1994 ; Taicher et. al., U.S. Patent No. 5,75';,186, entitled, "Nuclear Magnetic Resonance Well Logg.:~_ng Appax:atus and Met;hod Adapted for Measurement-While-Drilling", granted May 26, 1998;
Jackson et. al., U.S. Patent No. 4,350,95!x, entitled, "Magnetic Resonance Apparatu:~;" , ~.~r,-~n.te:;i September 21 , 1982 ;
U.S. Patent No. 6,24b,236, entitled, "Apparatus and Method for L ...
Obtaining a Nuclear Magnei~ic~ Re~~on.anc::~::. M~>asurement While Drilling", filed on November 5, 1998~ or Prammer et. al., W099/36801 entit~Led "Method arzc:~ Apparatu.a for ~"~uclear Magnetic Resonance Measur:i.ng Wha.l.e i~r:;..l.ling" published on July 22, 1999.
The NMR measuring pz~oc~ess :i.~.~~ se:parated by two distinct features from most other downhole formation measurements . First: , the=. NMR :~a.c~na::l f:roni the forrnaq~ ion comes from a small resonance volume, such as a generally thin resonance shel.:i. 20a see F~'i.c:~. 2) , a~~d the resonzarrce volume 20a has a radial thickness that: is proportional to the magnitude of an oscil.lati.n.g magrmt,~.c field and :inversely proportional to the gradient of a static magnetic field.
Depending on the shape c~~ t:hr~ :rvar~on~~rac:~e zones, the volume extends, as an example, from as little as 1 millimeter (mm.) in one direction anc~ as long as several inches in another.
Secondly, the NMR measurement may not. be instantaneous.
Both of these facts combined rr~akr~ true. NMR measurements prone to tool motions, such as the mot.inn that is attributable to the movement of the NMR tool 6 a.ror.znd the periphery of the borehole 3, as fu.rthe.r described be:~.ow, The NMR tool 6 measures T2 spin-spin relaxation times of hydrogen nuclei of t:.he ~;orwation 10 by rad~.ating NMR detection sequences to cause the nuclei to produce spin echoes. The spin echoes, in t. lar-n, nray be analyzed t:o produce a distribution of T2 times, _2_ :~T7OR\'L1' DO('KG'r S().: 24.11832 and the properties of the formation may be obtained ti-om this distribution.
For example, one such NMR detection sequence is a Carr-Purcell-Meiboom-Gill (CPMG) sequence that is depicted in Fig. 3. By applying the sectuence 15, a distribution of T2 times may be obtained, and this distribution may be used to determine and map the properties of the formation 10.
A technique that uses C'PMG sequences 15 to measure the T? times may include the following steps. The NMR tool 6 pulses the Bi field for an appropriate time interval to apply a 90° excitation pulse 14a to rotate the spins of hydrogen nuclei that are initially aligned along the direction of the Bn field. Although not shown in detail, each pulse is 1 ~ effectively an envelope, or burst, of a radio frequency RF carrier signal.
When the spins are rotated around Bi away from the direction of the B" field, the spins immediately begin to process around Bu. The pulse is stopped when the spins are rotated by 90° into the plane perpendicular to the B" field. They continue to process in this plane tirst in unison, then gradually losing synchronization. At a fixed timr Tc n following the excitation pulse 14a, the NMR tool 6 pulses the Bi field for a longer period of time (than the excitation pulse 14a) to apply an NMR refocusing pulse 14b to rotate the processing spins through an angle of 18l)° with the carrier phase shifted by ~ ~)0°. This step may be repeated "k"
times (where "k" is called the number of echoes and may assume a value anywhere from several to as many as several thousand, as an example) at the interval of 2vl'o,~. The NMR
'?0 pulse 14b causes the spins to resynchronize and radiate an associated spin echo signal 16 (see Fig. 4) that peaks at a time called Tc n after the l80° refocusing NMR pulse 14b.
After completing the spin-echo sequence, a waiting period (usually called a wait time) is _3_ required to allow the spins to return too equilibrium along the Bo field before starting the neact ~:PMO sequence 15 to collect another set of spin echc~> signals. '.Che deravs~ of each set of spin echoes is observed and used to derive the T2 distribution.
One way to i.dent:ify pc.~tenta.~~.1 ~;~ro:blems caused by motion effects requ~.res the use of a motion detection device, such as a straiaa gauge, an ult~z~asonic range finder, an accelerometer or a magnet~:~~rreter . ;~:n this manner. , t:he motion detection device is used to establish a threshold for evaluating the qual a.ty «f thc~ NNf:R mE:.a~~ur~ement . :such an arrangement is desG~:-ibed i.n PCT .Publi.c~ation Number WO 98/29639, entitled, "Method for E~cpzmation Evaluation While Drilling", that was filed on :C)e~c°errsbc~r 29Y 199'7.
However, conventional motion detection devices may not specifically indicate desired c;c~:.rrF~c;t~.c~ns tc7 the measurement data to compensate for tool, cnc~ti..on.
Thus, there is a continuing need for a method to more precisely detect tool m~~ticw effects c;n NMR
measurements. There is also s~ c~c:ardt~~..r~:ui.ng need for a method to adapt NMR measurement analysis in response to the detected tool motion effects.
Summary of the Invention In one aspect of tlrxe pz°esent in~rentian, traere is provided a method far detecting motion effects on a nuclear magnetic resonance (NMR) measi.xx~emez~t. obta:i.ned from a tool positioned within a :borehole tra.versi.ng an earth formation, comprising the steps of: producing as-~ echo train having a plurality of spin echo signer=t s; :~el.ec:t z.ng at least. t.wo spin echo signals wherein each spin echo si~gna:L is influenced differently by tool motion; ariri ~an~~.l.yz~rig the selected signals to determine motion effer.:t:s ~~z; true spin echo by using the NMR spin echo measurement. it:.sel.f.
In a second aspect, there i.s provided a method for detecting motion ef:f:ects can a zn.acleaa:° magn.et:ic resonance measurement obtaa_ned from a tool. positioned within w borehole traversing an earth f'e~rmatic»u, c:ornprising the steps of : producing an echo tn~.ai.n fzav:ing ~.~ p:Lurality of ;spi.n echo signals; separating the spin echo signals o.f the echo train into a plurality of subsets, c~oznpr_~isirEC~ the steps of :
selecting a first subset c~f the spin echo signals wherein each spin echo ir:~ the subset :is :Lnfl.u.enc~ed :.in substantially the same manner by tool mot:ic:>t~; and, ~:~electing a sec:;~orad subset of the spin echo signals wherein each spin echo in the second subset. is influenc:~ec~. irz ~su.k~st:ant:ial_I_y then same manner by tool motion, the separating step is performed so that tool motion inf luencE~s :~:igraals ir; the first subset differently from signals in the second subset; selecting at least one spin echo signal from each subset;.; and an<alyzing the selected signal> to determ.i.ne mot:i.on effects on th.e spin echo by using the NMR spin er_ho measurement itself.
The above disadvantages of the prior art are overcome by means of embodiments the subject: ir~venti..on comprising a method for detecting motion effects on a nuclear magnetic resonance m~~,asurement obtained fronn a tool positioned within a borehole traversing an earth formation.
An echo train is produced ha~ri_r;.g a p~.ural ity of spin echo signals. At least two spin ~~c:.h.o si.gn.als acre selected such that each spin echo signal is influenced differentl~.r _4a..
~1TTOR'VGY DO( KG'1' VO.: 24.IIR32 by tool motion. The selected spin echo signals may comprise adjacent spin echoes in the echo train. Alternatively, the selected spin echo signals may comprise odd and even spin echoes. The selected signals are analyzed to determine motion effects on the spin echo.
Brief nescrintinn of the OrawinuT
The advantages of the present invention will become apparent from the following description of the accompanying drawings. It is to be understood that the drawings arc to be used for the purpose of illustration only, and not as a definition of the invention.
In the drawings:
Fig. 1 is a schematic diagram of a subterranean well;
Fig. 2 is a cross-sectional view of the well taken along line ?-2 of Fig. 1;
Figs. 3 and ~ are waveforms illustrating a C'PMG pulse sequence;
Fig. 5 is a tlow chart illustrating an algorithm to determine motion effects based on shapes of spin echo signals;
Fig. f~ is a block diagram of a system that is used to determine motion effects from spin echo signal shapes:
Fig. 7 illustrates filter output signals of the system of Fig. f> for the case of motion;
Fig. H illustrates filter output signals of the system of Fig. f> for the case of no motion;
Fig. q is a block diagram of a system that is used to determine motion effects by comparing different spin echo signals in an echo train;
_5_ rTOR\EY DCI( KL'T \'O.: 2.(1832 Fig. 10 shows simulated an ~plitudes of the first two echoes for an axisynmnetric gradient geometry undergoing lateral motion; and Fig. 1 1 shows the ratio of the amplitudes of the first two echoes of Fig. 10.
A method in accordance with the invention detects tool motion effects during an NMR measurement by using the NMR measurement itself. In this manner, the method may include detection, characterization and/or quantification of the tool motion effects.
Thus, the method may be used for quality control of the recorded data, such as determining whether a measured porosity is accurate, determining the maximum echo number at which the echo amplitudes are accurate, determining whether the entire T2 spectrum is valid, and/or determining whether a bound fluid measurement is accurate, as just a few examples. If the accuracy of the motion measurement is high enough to allow accurate quantification of the effects of the motion, the measured data may be modified to compensate for tool motion. Where the indications of motion effects are available in real time, the measurement process may be modified to suppress motion effects.
In the context of this application, the phrases "motion" and "tool motion generally refer to a relative motion that occurs between the sample and the fields that arc created by an NMR measurement tool. Therefore, the n notion may be attributable to movement of the tool, movement of the sample (where the sample is a flowing fluid, for ?0 example) or movement of both the sample and the tool.
Referring to Figure s, a process l20 to characterize tool motion effects uses the observation that the ti-equency contents of the spin echo signals change when the too( is _(,_ 17°roR\~Y DO( KG'r \o.: 2:7.11832 moving during detection of the echo train. Thus, the process 120 includes radiating (block 122) a sequence of NMR pulses to produce spin echo signals. The spin echo signals are then analyzed (block 124) to determine motion effects.
When the tool is not moving ctrtring detection of the echo train, the shape and amplitude of the spin echo signals vary initially due to magnetization that is not aligned along the etfiective rotation axis from echo signal (a characteristic of each pulse sequence) to echo signal. These variations, which ~rr~ predictable froth known measurement parameters, die down within a few echo signals. For the rest of the sequence, the echo amplitudes decay while the spins relax, but the echo signal shape stays the same.
Figure fi depicts a system 126 that may be used to indicate the effect of tool motion. The system 126 includes at least two different types of filters 128 and 130 that, as described below, may be used to detect motion of the tool. As an example, in some embodiments, the system 126 may be part of the electronic circuitry of the NMR
tool.
However, in other embodiments, the system 126 may be used to process logged data that is provided by the NMR tool.
Figure 7 illustrates an echo train, recorded with two different echo detection tilters, in the absence of motion. Train 140 is detected with a broadband titter (filter 128, for example) while train 142 is detected with a thatched, thus band-restricted filter (filter 130, for example). A simple example of a broadband filter would be zero everywhere except at the echo maximum. As depicted in Figure 7. if no motion of the tool occurs, the decays represented by the two graphs 140 and 142 are proportional to each other.
However, if motion occurs, the decays are no longer proportional to each other, as _7_ depicted by the graphs 140 and 142 of= Figure 8: both curves decay faster due t.o motiaz~, hut. at c3_i~fea:erzt rates. The band restricted signal decays faster. An 7.ndication of motion, as depicted by block 132 in Fgux:e 6, may be derived by comparing the x°atMios of the signal.~~ px:~ovided :by the matched and broadband fi.lt.erc (i::~~ze fia texs 128 and .130, as examples). Or more generally, an indication of motion is provided by compari.r~rg t:he ra~.:i.<::~s of ,~~.gnsls detected with different filters trrat ~lnave diff:ereznt: rrtotion dependcsnc:ies.
Referring to Figure 43, anotrr.er process 150 to characterize tool motion effects uses the observation that the echo train becomes modulated from echo to echo during movement of the t.oo1 . Thu s, t~~re process 150 inc:lude~s radiating (block 152) a sequence of NMR pulses to produce an echo train having a plurality of spur echo signals. The spin echo signals are there an<:~:~.y<'e~ (k:~~ock 1541 t:.o determine motion effects.
For a CPMG echo train under Laboratory conditions, tool motion has been evidenced as different motion camping of odd and even spin. echoes in t:he f::cho t x°ai r~z a In t:he presence of motion along a field gradient., the amplitudes of odd spin echoes were more attenuated than the amplitudes of even spin echoes. The origin of the ef f:ec:t~ i..s a speed dependent phase shift of the transverse spin components that. are present at the time of the odd echoes, but not compensated at the time of the even echoes . ,See H . '~" . ~"~~rx~ arar.~ E . M . Purcel.l , Effects of Diffusion on Free Precessaa,n .in Nuclear D~agnetic Resonance Experiment s, 94 PHys . RE~Y~ . 6.3 U , ~3 7 ( 1954 ) .
Still referring to Figure 9, in a preferred embodiment of the .:iz~~,~ent_i.on, t:: he spin ec:hc::a si.gnal.s are analyzed (block 154) by selec~~ta..z~rg at l.c=ast~ t;wo spin echoes that are _. 8 ~7°1 OR.\Gl' U(1('KET \O.: 24.IIRB2 influenced differently by tool motion. For example, the two spin echoes may comprise adjacent echoes 160 and 162 (see Figure LO) of an echo train and the tool motion detection comprises comparing the amplitude or ratio of the selected spin echoes (see Figure 1 I ). The ratio may be used to determine an amplification factor to correct tile motion induced attenuation of the spin echoes. Specifically, in the presence of low motion velocity, the amplitude of the first spin echo 160 is more attenuated than the amplitude of the second spin echo 162; however, with increasing motion velocity, the first spin echo 160 becomes more influenced by tool motion than the second spin echo 162. For a tool having a nonaxisymmetric gradient geometry, it is within contemplation of the subject invention to use the echo phases, instead of the echo amplitudes, for the comparison.
In an alternate embodiment, the two slain echoes may comprise odd and even spin echoes of an echo train. It is within contemplation of the subject invention to negate the effect of motion on the ~fMR measurement by modifying the spin echo train to eliminate the spin echoes that are strongly influenced by motion. The remaining spin echoes may be analyzed to produce a distribution of T2 times, however, the modified spin echo train tnay result in reduced sensitivity for decaying signal components.
The foregoing description of the preferred and alternate embodiments of the present invention has been presented for purposes of illustration and description. tt is not intended to be exhaustive nor to limit the invention to the precise form disclosed.
Obviously, many modifications and variations will be apparent to those skilled in the art.
The embodiments were chosen and described in order to best explain the principles of the _y_ 1'I~ I~OR\L 1' DO( IW:'1 \().: 24.11X32 invention and its practical application thereby enabling other s skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intende(i that the scope of the invention be defined by the accompanying claims and their equivalents.
v
NUCLEAR MAGNETIC RESONANCE MEASUREMENTS
Background of tha Invention The invention genera.:l.~l.y rel~~.tes to ins:ide--out nuclear magnetic resonance (NMR) measurements, and more particularly, the invent;iron reml.<~.tes t:c~ detecting total motion effects an NMR measurements of formation properties surrounding a borehc:>le, such as measurements of the hydrogen content of the forrriatian, foz~ example., Referring to Fig. :L, as an example, nuclear magnetic resonance NMR) measurements may be obtained in a logging while dri.ll~_ng (LWD) c:~pe:r_a2:ior. t;o rnap t:he properties of a subterranean .formati.on ~.tl. In tris manner, an axisymmetric NMR tool 6 may be pax-t of a Grill string 5 that is used to drill a bore~uol.e 3 :ire t~tue forcriat::4on In. Th.e tool 6 may be, as examples, one of the tools described in Se~giner et. al. , U.S. Pat.ent Nc>~ _'~, "7G >, 92'7, ent~~t;led, "Pulsed Nuclear Magnetism Tool Fc~~: For_mat:ian Eva)_uation While Drilling Including a Shortened or Truncated CPMG
Sequence°', granted January 6, 1.9!33; Nfil.:Le:r, U.;~. Patient No. 5,280,243, entitled, "Sy:~tem For Logging a Well During the Drill ing Thereof " , grant~~~c.~ ,.7~<~nuary 18 , 1994 ; Taicher et. al., U.S. Patent No. 5,75';,186, entitled, "Nuclear Magnetic Resonance Well Logg.:~_ng Appax:atus and Met;hod Adapted for Measurement-While-Drilling", granted May 26, 1998;
Jackson et. al., U.S. Patent No. 4,350,95!x, entitled, "Magnetic Resonance Apparatu:~;" , ~.~r,-~n.te:;i September 21 , 1982 ;
U.S. Patent No. 6,24b,236, entitled, "Apparatus and Method for L ...
Obtaining a Nuclear Magnei~ic~ Re~~on.anc::~::. M~>asurement While Drilling", filed on November 5, 1998~ or Prammer et. al., W099/36801 entit~Led "Method arzc:~ Apparatu.a for ~"~uclear Magnetic Resonance Measur:i.ng Wha.l.e i~r:;..l.ling" published on July 22, 1999.
The NMR measuring pz~oc~ess :i.~.~~ se:parated by two distinct features from most other downhole formation measurements . First: , the=. NMR :~a.c~na::l f:roni the forrnaq~ ion comes from a small resonance volume, such as a generally thin resonance shel.:i. 20a see F~'i.c:~. 2) , a~~d the resonzarrce volume 20a has a radial thickness that: is proportional to the magnitude of an oscil.lati.n.g magrmt,~.c field and :inversely proportional to the gradient of a static magnetic field.
Depending on the shape c~~ t:hr~ :rvar~on~~rac:~e zones, the volume extends, as an example, from as little as 1 millimeter (mm.) in one direction anc~ as long as several inches in another.
Secondly, the NMR measurement may not. be instantaneous.
Both of these facts combined rr~akr~ true. NMR measurements prone to tool motions, such as the mot.inn that is attributable to the movement of the NMR tool 6 a.ror.znd the periphery of the borehole 3, as fu.rthe.r described be:~.ow, The NMR tool 6 measures T2 spin-spin relaxation times of hydrogen nuclei of t:.he ~;orwation 10 by rad~.ating NMR detection sequences to cause the nuclei to produce spin echoes. The spin echoes, in t. lar-n, nray be analyzed t:o produce a distribution of T2 times, _2_ :~T7OR\'L1' DO('KG'r S().: 24.11832 and the properties of the formation may be obtained ti-om this distribution.
For example, one such NMR detection sequence is a Carr-Purcell-Meiboom-Gill (CPMG) sequence that is depicted in Fig. 3. By applying the sectuence 15, a distribution of T2 times may be obtained, and this distribution may be used to determine and map the properties of the formation 10.
A technique that uses C'PMG sequences 15 to measure the T? times may include the following steps. The NMR tool 6 pulses the Bi field for an appropriate time interval to apply a 90° excitation pulse 14a to rotate the spins of hydrogen nuclei that are initially aligned along the direction of the Bn field. Although not shown in detail, each pulse is 1 ~ effectively an envelope, or burst, of a radio frequency RF carrier signal.
When the spins are rotated around Bi away from the direction of the B" field, the spins immediately begin to process around Bu. The pulse is stopped when the spins are rotated by 90° into the plane perpendicular to the B" field. They continue to process in this plane tirst in unison, then gradually losing synchronization. At a fixed timr Tc n following the excitation pulse 14a, the NMR tool 6 pulses the Bi field for a longer period of time (than the excitation pulse 14a) to apply an NMR refocusing pulse 14b to rotate the processing spins through an angle of 18l)° with the carrier phase shifted by ~ ~)0°. This step may be repeated "k"
times (where "k" is called the number of echoes and may assume a value anywhere from several to as many as several thousand, as an example) at the interval of 2vl'o,~. The NMR
'?0 pulse 14b causes the spins to resynchronize and radiate an associated spin echo signal 16 (see Fig. 4) that peaks at a time called Tc n after the l80° refocusing NMR pulse 14b.
After completing the spin-echo sequence, a waiting period (usually called a wait time) is _3_ required to allow the spins to return too equilibrium along the Bo field before starting the neact ~:PMO sequence 15 to collect another set of spin echc~> signals. '.Che deravs~ of each set of spin echoes is observed and used to derive the T2 distribution.
One way to i.dent:ify pc.~tenta.~~.1 ~;~ro:blems caused by motion effects requ~.res the use of a motion detection device, such as a straiaa gauge, an ult~z~asonic range finder, an accelerometer or a magnet~:~~rreter . ;~:n this manner. , t:he motion detection device is used to establish a threshold for evaluating the qual a.ty «f thc~ NNf:R mE:.a~~ur~ement . :such an arrangement is desG~:-ibed i.n PCT .Publi.c~ation Number WO 98/29639, entitled, "Method for E~cpzmation Evaluation While Drilling", that was filed on :C)e~c°errsbc~r 29Y 199'7.
However, conventional motion detection devices may not specifically indicate desired c;c~:.rrF~c;t~.c~ns tc7 the measurement data to compensate for tool, cnc~ti..on.
Thus, there is a continuing need for a method to more precisely detect tool m~~ticw effects c;n NMR
measurements. There is also s~ c~c:ardt~~..r~:ui.ng need for a method to adapt NMR measurement analysis in response to the detected tool motion effects.
Summary of the Invention In one aspect of tlrxe pz°esent in~rentian, traere is provided a method far detecting motion effects on a nuclear magnetic resonance (NMR) measi.xx~emez~t. obta:i.ned from a tool positioned within a :borehole tra.versi.ng an earth formation, comprising the steps of: producing as-~ echo train having a plurality of spin echo signer=t s; :~el.ec:t z.ng at least. t.wo spin echo signals wherein each spin echo si~gna:L is influenced differently by tool motion; ariri ~an~~.l.yz~rig the selected signals to determine motion effer.:t:s ~~z; true spin echo by using the NMR spin echo measurement. it:.sel.f.
In a second aspect, there i.s provided a method for detecting motion ef:f:ects can a zn.acleaa:° magn.et:ic resonance measurement obtaa_ned from a tool. positioned within w borehole traversing an earth f'e~rmatic»u, c:ornprising the steps of : producing an echo tn~.ai.n fzav:ing ~.~ p:Lurality of ;spi.n echo signals; separating the spin echo signals o.f the echo train into a plurality of subsets, c~oznpr_~isirEC~ the steps of :
selecting a first subset c~f the spin echo signals wherein each spin echo ir:~ the subset :is :Lnfl.u.enc~ed :.in substantially the same manner by tool mot:ic:>t~; and, ~:~electing a sec:;~orad subset of the spin echo signals wherein each spin echo in the second subset. is influenc:~ec~. irz ~su.k~st:ant:ial_I_y then same manner by tool motion, the separating step is performed so that tool motion inf luencE~s :~:igraals ir; the first subset differently from signals in the second subset; selecting at least one spin echo signal from each subset;.; and an<alyzing the selected signal> to determ.i.ne mot:i.on effects on th.e spin echo by using the NMR spin er_ho measurement itself.
The above disadvantages of the prior art are overcome by means of embodiments the subject: ir~venti..on comprising a method for detecting motion effects on a nuclear magnetic resonance m~~,asurement obtained fronn a tool positioned within a borehole traversing an earth formation.
An echo train is produced ha~ri_r;.g a p~.ural ity of spin echo signals. At least two spin ~~c:.h.o si.gn.als acre selected such that each spin echo signal is influenced differentl~.r _4a..
~1TTOR'VGY DO( KG'1' VO.: 24.IIR32 by tool motion. The selected spin echo signals may comprise adjacent spin echoes in the echo train. Alternatively, the selected spin echo signals may comprise odd and even spin echoes. The selected signals are analyzed to determine motion effects on the spin echo.
Brief nescrintinn of the OrawinuT
The advantages of the present invention will become apparent from the following description of the accompanying drawings. It is to be understood that the drawings arc to be used for the purpose of illustration only, and not as a definition of the invention.
In the drawings:
Fig. 1 is a schematic diagram of a subterranean well;
Fig. 2 is a cross-sectional view of the well taken along line ?-2 of Fig. 1;
Figs. 3 and ~ are waveforms illustrating a C'PMG pulse sequence;
Fig. 5 is a tlow chart illustrating an algorithm to determine motion effects based on shapes of spin echo signals;
Fig. f~ is a block diagram of a system that is used to determine motion effects from spin echo signal shapes:
Fig. 7 illustrates filter output signals of the system of Fig. f> for the case of motion;
Fig. H illustrates filter output signals of the system of Fig. f> for the case of no motion;
Fig. q is a block diagram of a system that is used to determine motion effects by comparing different spin echo signals in an echo train;
_5_ rTOR\EY DCI( KL'T \'O.: 2.(1832 Fig. 10 shows simulated an ~plitudes of the first two echoes for an axisynmnetric gradient geometry undergoing lateral motion; and Fig. 1 1 shows the ratio of the amplitudes of the first two echoes of Fig. 10.
A method in accordance with the invention detects tool motion effects during an NMR measurement by using the NMR measurement itself. In this manner, the method may include detection, characterization and/or quantification of the tool motion effects.
Thus, the method may be used for quality control of the recorded data, such as determining whether a measured porosity is accurate, determining the maximum echo number at which the echo amplitudes are accurate, determining whether the entire T2 spectrum is valid, and/or determining whether a bound fluid measurement is accurate, as just a few examples. If the accuracy of the motion measurement is high enough to allow accurate quantification of the effects of the motion, the measured data may be modified to compensate for tool motion. Where the indications of motion effects are available in real time, the measurement process may be modified to suppress motion effects.
In the context of this application, the phrases "motion" and "tool motion generally refer to a relative motion that occurs between the sample and the fields that arc created by an NMR measurement tool. Therefore, the n notion may be attributable to movement of the tool, movement of the sample (where the sample is a flowing fluid, for ?0 example) or movement of both the sample and the tool.
Referring to Figure s, a process l20 to characterize tool motion effects uses the observation that the ti-equency contents of the spin echo signals change when the too( is _(,_ 17°roR\~Y DO( KG'r \o.: 2:7.11832 moving during detection of the echo train. Thus, the process 120 includes radiating (block 122) a sequence of NMR pulses to produce spin echo signals. The spin echo signals are then analyzed (block 124) to determine motion effects.
When the tool is not moving ctrtring detection of the echo train, the shape and amplitude of the spin echo signals vary initially due to magnetization that is not aligned along the etfiective rotation axis from echo signal (a characteristic of each pulse sequence) to echo signal. These variations, which ~rr~ predictable froth known measurement parameters, die down within a few echo signals. For the rest of the sequence, the echo amplitudes decay while the spins relax, but the echo signal shape stays the same.
Figure fi depicts a system 126 that may be used to indicate the effect of tool motion. The system 126 includes at least two different types of filters 128 and 130 that, as described below, may be used to detect motion of the tool. As an example, in some embodiments, the system 126 may be part of the electronic circuitry of the NMR
tool.
However, in other embodiments, the system 126 may be used to process logged data that is provided by the NMR tool.
Figure 7 illustrates an echo train, recorded with two different echo detection tilters, in the absence of motion. Train 140 is detected with a broadband titter (filter 128, for example) while train 142 is detected with a thatched, thus band-restricted filter (filter 130, for example). A simple example of a broadband filter would be zero everywhere except at the echo maximum. As depicted in Figure 7. if no motion of the tool occurs, the decays represented by the two graphs 140 and 142 are proportional to each other.
However, if motion occurs, the decays are no longer proportional to each other, as _7_ depicted by the graphs 140 and 142 of= Figure 8: both curves decay faster due t.o motiaz~, hut. at c3_i~fea:erzt rates. The band restricted signal decays faster. An 7.ndication of motion, as depicted by block 132 in Fgux:e 6, may be derived by comparing the x°atMios of the signal.~~ px:~ovided :by the matched and broadband fi.lt.erc (i::~~ze fia texs 128 and .130, as examples). Or more generally, an indication of motion is provided by compari.r~rg t:he ra~.:i.<::~s of ,~~.gnsls detected with different filters trrat ~lnave diff:ereznt: rrtotion dependcsnc:ies.
Referring to Figure 43, anotrr.er process 150 to characterize tool motion effects uses the observation that the echo train becomes modulated from echo to echo during movement of the t.oo1 . Thu s, t~~re process 150 inc:lude~s radiating (block 152) a sequence of NMR pulses to produce an echo train having a plurality of spur echo signals. The spin echo signals are there an<:~:~.y<'e~ (k:~~ock 1541 t:.o determine motion effects.
For a CPMG echo train under Laboratory conditions, tool motion has been evidenced as different motion camping of odd and even spin. echoes in t:he f::cho t x°ai r~z a In t:he presence of motion along a field gradient., the amplitudes of odd spin echoes were more attenuated than the amplitudes of even spin echoes. The origin of the ef f:ec:t~ i..s a speed dependent phase shift of the transverse spin components that. are present at the time of the odd echoes, but not compensated at the time of the even echoes . ,See H . '~" . ~"~~rx~ arar.~ E . M . Purcel.l , Effects of Diffusion on Free Precessaa,n .in Nuclear D~agnetic Resonance Experiment s, 94 PHys . RE~Y~ . 6.3 U , ~3 7 ( 1954 ) .
Still referring to Figure 9, in a preferred embodiment of the .:iz~~,~ent_i.on, t:: he spin ec:hc::a si.gnal.s are analyzed (block 154) by selec~~ta..z~rg at l.c=ast~ t;wo spin echoes that are _. 8 ~7°1 OR.\Gl' U(1('KET \O.: 24.IIRB2 influenced differently by tool motion. For example, the two spin echoes may comprise adjacent echoes 160 and 162 (see Figure LO) of an echo train and the tool motion detection comprises comparing the amplitude or ratio of the selected spin echoes (see Figure 1 I ). The ratio may be used to determine an amplification factor to correct tile motion induced attenuation of the spin echoes. Specifically, in the presence of low motion velocity, the amplitude of the first spin echo 160 is more attenuated than the amplitude of the second spin echo 162; however, with increasing motion velocity, the first spin echo 160 becomes more influenced by tool motion than the second spin echo 162. For a tool having a nonaxisymmetric gradient geometry, it is within contemplation of the subject invention to use the echo phases, instead of the echo amplitudes, for the comparison.
In an alternate embodiment, the two slain echoes may comprise odd and even spin echoes of an echo train. It is within contemplation of the subject invention to negate the effect of motion on the ~fMR measurement by modifying the spin echo train to eliminate the spin echoes that are strongly influenced by motion. The remaining spin echoes may be analyzed to produce a distribution of T2 times, however, the modified spin echo train tnay result in reduced sensitivity for decaying signal components.
The foregoing description of the preferred and alternate embodiments of the present invention has been presented for purposes of illustration and description. tt is not intended to be exhaustive nor to limit the invention to the precise form disclosed.
Obviously, many modifications and variations will be apparent to those skilled in the art.
The embodiments were chosen and described in order to best explain the principles of the _y_ 1'I~ I~OR\L 1' DO( IW:'1 \().: 24.11X32 invention and its practical application thereby enabling other s skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intende(i that the scope of the invention be defined by the accompanying claims and their equivalents.
v
Claims (14)
1. A method for detecting motion effects on a nuclear magnetic resonance (NMR) measurement obtained from a tool positioned within a borehole traversing an earth formation, comprising the steps of:
producing an echo train having a plurality of spin echo signals;
selecting at least two span echo signals wherein each spin echo signal is influenced differently by tool motion; and analyzing the selected signals to determine motion effects on the spin echo by using the NMR measurement itself.
producing an echo train having a plurality of spin echo signals;
selecting at least two span echo signals wherein each spin echo signal is influenced differently by tool motion; and analyzing the selected signals to determine motion effects on the spin echo by using the NMR measurement itself.
2. The method of claim 1 wherein the selecting step further comprises the step of selecting adjacent spin echoes in the echo train.
3. The method of claim 2 wherein the analyzing step further comprises the step of comparing the adjacent spin echoes.
4. The method of claim 3 wherein the comparing step further comprises the step obtaining a ratio of the adjacent spin echoes.
5. The method of claim 1 wherein the analyzing step further comprises the step of comparing the selected echo signals.
6. The method of claim 5 wherein the comparing step further comprises the step of comparing amplitudes of the selected echo signals.
7. The method of claim 5 wherein the comparing step further comprises the step of comparing phases of the selected echo signals.
8. The method of claim 1 wherein the analyzing step further comprises the step of obtaining a ratio of the selected echo signals.
9. The method of claim 1 wherein the echo train comprises a plurality of odd and even spin echo signals, the method further comprising the step of separately analyzing the odd and even spin echo signals to determine motion effect on the spin echo signals.
10. A method for detecting motion effects on a nuclear magnetic resonance measurement obtained from a tool positioned within a borehole traversing an earth formation, comprising the steps of:
producing an echo train having a plurality of spin echo signals;
separating the spin echo signals of the echo train into a plurality of subsets, comprising the steps of:
selecting a first subset of the spin echo signals wherein each spin echo in the subset is influenced in substantially the same manner by tool motion; and, selecting a second subset of the spin echo signals wherein each spin echo in the second subset is influenced in substantially the same manner by tool motion, the separating step is performed so that tool motion influences signals in the first subset differently from signals in the second subset;
selecting at least one spin echo signal from each subset; and -12-~
analyzing the selected signals to determine motion effects on the spin echo lay using the NMR measurement itself.
producing an echo train having a plurality of spin echo signals;
separating the spin echo signals of the echo train into a plurality of subsets, comprising the steps of:
selecting a first subset of the spin echo signals wherein each spin echo in the subset is influenced in substantially the same manner by tool motion; and, selecting a second subset of the spin echo signals wherein each spin echo in the second subset is influenced in substantially the same manner by tool motion, the separating step is performed so that tool motion influences signals in the first subset differently from signals in the second subset;
selecting at least one spin echo signal from each subset; and -12-~
analyzing the selected signals to determine motion effects on the spin echo lay using the NMR measurement itself.
11. The method of claim 10 wherein the analyzing step further comprises the step of comparing the selected signals.
12. The method of claim 11 wherein the comparing step further comprises the step of comparing amplitudes of the selected signals.
13. The method of claim 11 wherein the comparing step further comprises the step of comparing phases of the selected signals.
14. The method of claim 10 wherein the analyzing step further comprises the step of obtaining a ratio of the selected echo signals.
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| US09/495,606 US6297632B1 (en) | 1999-07-19 | 2000-02-01 | Detecting tool motion effects on spin echoes obtained with nuclear magnetic resonance measurements |
| US09/495,606 | 2000-02-01 |
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| CA002331904A Expired - Fee Related CA2331904C (en) | 2000-02-01 | 2001-01-22 | Detecting tool motion effects on spin echoes obtained with nuclear magnetic resonance measurements |
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| CA (1) | CA2331904C (en) |
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| US6492809B1 (en) * | 1998-12-04 | 2002-12-10 | Schlumberger Technology Corporation | Preconditioning spins near a nuclear magnetic resonance region |
| US6566874B1 (en) * | 1998-07-30 | 2003-05-20 | Schlumberger Technology Corporation | Detecting tool motion effects on nuclear magnetic resonance measurements |
| US6518756B1 (en) * | 2001-06-14 | 2003-02-11 | Halliburton Energy Services, Inc. | Systems and methods for determining motion tool parameters in borehole logging |
| US6528995B1 (en) * | 2001-09-10 | 2003-03-04 | Schlumberger Technology Corporation | Methods and apparatus for measuring flow velocity in a wellbore using NMR and applications using same |
| US6774628B2 (en) * | 2002-01-18 | 2004-08-10 | Schlumberger Technology Corporation | Nuclear magnetic resonance imaging using phase encoding with non-linear gradient fields |
| US6714009B2 (en) * | 2002-05-16 | 2004-03-30 | Schlumberger Technology Corporation | Method for the inversion of CPMG measurements enhanced by often repeated short wait time measurements |
| US6765380B2 (en) * | 2002-05-23 | 2004-07-20 | Schlumberger Technology Corporation | Determining wettability of an oil reservoir using borehole NMR measurements |
| US6947852B2 (en) * | 2002-06-04 | 2005-09-20 | Kla-Tencor Technologies Corporation | Monitoring and correcting for non-translational motion in a resonance measurement apparatus |
| US6937013B2 (en) * | 2002-06-19 | 2005-08-30 | Schlumberger Technology Corporation | NMR tool for making formation evaluation measurements using gradient echoes |
| US6956370B2 (en) * | 2002-10-04 | 2005-10-18 | Schlumberger Technology Corporation | Method for reducing ringing in NMR measurements by combining NMR signals having a spin echo and spurious signal component |
| US6808028B2 (en) * | 2002-12-03 | 2004-10-26 | Schlumberger Technology Corporation | Method and apparatus utilizing NMR measurements to gather information on a property of the earth formation surrounding a wellbore |
| MXPA03010645A (en) * | 2002-12-03 | 2004-06-14 | Schlumberger Technology Bv | Method and apparatus utilizing nmr measurements to gather information on a property of the earth formation surrounding a wellbore. |
| WO2004099817A2 (en) | 2003-05-02 | 2004-11-18 | Halliburton Energy Services, Inc. | Systems and methods for nmr logging |
| US6958604B2 (en) * | 2003-06-23 | 2005-10-25 | Schlumberger Technology Corporation | Apparatus and methods for J-edit nuclear magnetic resonance measurement |
| CA2852097A1 (en) | 2003-10-03 | 2005-04-21 | Halliburton Energy Services, Inc. | System and methods for t1-based logging |
| US7268547B2 (en) * | 2003-10-07 | 2007-09-11 | Baker Hughes Incorporated | Correction of motion influences in NMR signals |
| US7180287B2 (en) * | 2004-05-05 | 2007-02-20 | Baker Hughes Incorporated | Reduction of motion artifacts in NMR |
| US7196516B2 (en) * | 2004-08-16 | 2007-03-27 | Baker Hughes Incorporated | Correction of NMR artifacts due to constant-velocity axial motion and spin-lattice relaxation |
| US8373412B2 (en) * | 2009-01-23 | 2013-02-12 | Baker Hughes Incorporated | NMR-LWD imaging tool |
| US8324895B2 (en) * | 2009-01-23 | 2012-12-04 | Baker Hughes Incorporated | MWD/LWD NMR imaging with long echo trains |
| US10088594B2 (en) | 2014-10-16 | 2018-10-02 | Schlumberger Technology Corporation | Methods and apparatuses for echo processing of nuclear magnetic resonance (NMR) data |
| US10126457B2 (en) * | 2016-03-04 | 2018-11-13 | Baker Hughes, A Ge Company, Llc | Motion detection and correction of magnetic resonance data |
| US10895659B2 (en) * | 2019-04-18 | 2021-01-19 | Halliburton Energy Services, Inc. | Detecting nuclear magnetic resonance logging tool motion |
| US11422283B1 (en) | 2021-07-14 | 2022-08-23 | Halliburton Energy Services, Inc. | Reducing motion effects on nuclear magnetic resonance relaxation data |
| US11899158B2 (en) | 2022-03-14 | 2024-02-13 | Halliburton Energy Services, Inc. | Logging tool motion error reduction for nuclear magnetic resonance logging via pulse sub-sequences |
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| US6242912B1 (en) * | 1995-10-12 | 2001-06-05 | Numar Corporation | System and method for lithology-independent gas detection using multifrequency gradient NMR logging |
| US6051973A (en) * | 1996-12-30 | 2000-04-18 | Numar Corporation | Method for formation evaluation while drilling |
| US5959453A (en) * | 1997-10-29 | 1999-09-28 | Western Atlas International, Inc. | Radial NMR well logging apparatus and method |
| US6246236B1 (en) * | 1998-03-03 | 2001-06-12 | Schlumberger Technology Corporation | Apparatus and method for obtaining a nuclear magnetic resonance measurement while drilling |
| US6566874B1 (en) * | 1998-07-30 | 2003-05-20 | Schlumberger Technology Corporation | Detecting tool motion effects on nuclear magnetic resonance measurements |
| US6229308B1 (en) * | 1998-11-19 | 2001-05-08 | Schlumberger Technology Corporation | Formation evaluation using magnetic resonance logging measurements |
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| GB0100793D0 (en) | 2001-02-21 |
| CA2331904A1 (en) | 2001-08-01 |
| GB2359632B (en) | 2002-05-15 |
| US6297632B1 (en) | 2001-10-02 |
| GB2359632A (en) | 2001-08-29 |
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