MXPA99010270A - Pre-conditioning of espines near a region of magnetic resonance nucl - Google Patents

Abstract

A method and an apparatus for obtaining measurements by nuclear magnetic resonance are disclosed. A measuring device by nuclear magnetic resonance, the measured sample, or both elements are subject to movement during the measurement. The envelope of a radio frequency carrier signal is modulated according to an envelope to generate a first sequence of radio frequency pulses. The envelope, the phase of the radio frequency signal, and / or a static magnetic field can be varied during the radiation of the first sequence by substantially saturating a first region of the sample. The first sequence may include reconcentration radio frequency pulses which, when SOPL coupled with the movement of the nuclear magnetic resonance or sample measuring apparatus, may also be used to substantially saturate the first legion. A second sequence of radio frequency pulses is radiated to establish a resonance region within the first region and measure an attribute of the sample

Description

PRE-CONNECTION OF ESPINES NEAR A REGION OF NUCLEAR MAGNETIC RESONANCE BACKGROUND In general terms, this invention relates to the preconditioning of spins near a region of nuclear magnetic resonance (NMR). Nuclear magnetic resonance (NMR) measurements can be used to investigate properties of a sample, for example, of a body tissue (for medical imaging purposes), or of an underground formation (for well-drilling purposes). For example, for underground formation, nuclear magnetic resonance can be used to determine and locate the porosity, formation type, permeability and oil content of the formation. Referring to Figure 1, as an example, nuclear magnetic resonance can be used in a simultaneous gravimetric operation with the perforation to raise planes the properties of an underground formation 10. In this way, an axisymmetric nuclear magnetic resonance tool 6 it can be part of a drilling rod 5 which is used to form a borehole 3 in the formation 10. The tool 6 can be, as examples, one of the tools described in Sezginer et. al., United States Patent No. 5,705,927, entitled "Pulse Nuclear Magnetism Tool for Simultaneous Training Evaluation with Drilling Including a Shortened or Truncated CPMG Sequence", granted on January 6, 1998; Miller, U.S. Patent No. 5,280,243, entitled - - "System to Perform Diagram to a Well During the Drilling of the same", granted on January 18, 1994. The measurement process with nuclear magnetic resonance is separated by two distinguishing features of most other bottomhole formation measurements.

First, the nuclear magnetic resonance signal of the array comes from a small resonance volume, as is generally the case with the thin resonance volume 20a (see FIG. 2), and the thin resonance volume 20a may have a radial thickness which is proportional to the magnitude of a magnetic field ^ i (which is not shown). Depending on the shape of the resonance zones, the volume can be understood, for example, from as little as 1 millimeter (mm) in one direction and as long as several inches in another direction. Second, measurement by nuclear magnetic resonance may not be instantaneous. These two combined factors make measurements by nuclear magnetic resonance prone to movements of the tool, such as nuclear magnetic resonance tool 6 that moves around the periphery of the borehole 3, as described in detail below. To perform nuclear magnetic resonance measurements, the nuclear magnetic resonance tool 6 may include permanent magnets to establish a static magnetic field called ^ ° (not shown); a radio frequency (RF) coil, or antenna, to radiate the magnetic field variable in time ß? which is perpendicular to the field - °°; and a radio frequency (RF) coil, or antenna, - to receive spin echoes from the array in response to a nuclear magnetic resonance measurement, as described below. These two coils can be combined into a single transmitter / receiver antenna. As an example, the nuclear magnetic resonance tool 6 can measure spin-spin relaxation times T2 of hydrogen nuclei of the formation 10 by the radiation of nuclear magnetic resonance detection sequences to cause the nuclei to produce spin echoes. The spin echoes, in turn, can be analyzed to produce a distribution of times T2, and from this distribution the properties of the formation can be obtained. For example, a nuclear magnetic resonance detection frequency of this type is a Carr-Purcell-Meiboom-Gill (CPMG) sequence 15 which is described in Figure 4. By applying sequence 15, a time distribution T2 can be obtained. , and this distribution can be used to determine and raise the properties of formation 10. A technique that uses CPMG 15 sequences to measure times T2 can include the following steps: In the first step, the nuclear magnetic resonance tool - >; 6 transmits the field "i" during an appropriate time interval to apply an excitation pulse of 90 ° 14a to rotate the spins of hydrogen nuclei by 90 ° (which are initially aligned in the direction of the field ^ °).

Although not shown, each pulse is effectively an envelope, or burst, of a carrier radio frequency signal. After the spins have been rotated 90 ° from the direction of the field °, immediately the spins ~ begin to make precession in the plane perpendicular to the field ¿> or, at the beginning in unison, and then losing synchronization gradually. For step two, at a fixed time T following the nuclear magnetic resonance pulse 14a, the nuclear magnetic resonance tool pulses the field "i for a longer period of time (than the nuclear magnetic resonance pulse 14a) to apply a reconcentration pulse 14b to rotate the precession spins through an additional angle of 180 ° with its carrier phase changed by 90. The nuclear magnetic resonance pulse 14b causes the spins to re-synchronize and radiate an associated spin echo 16 (see Figure 5) having a peak at a time approximately equal to T, after reconcentrating the nuclear magnetic resonance pulse 14b at 180 ° C. Step two can be repeated "k" times (where "k" is called number of echoes and can assume any value from several hundred to as high as several thousand, as an example) in the interval of te (approximately 2 * T). For step 3, after completing the eco-esp sequence ín, a waiting period is required (normally called waiting time) to allow the spins to recover the equilibrium along the °° field before beginning the next CPMG sequence 15 to collect another set of spin echoes. The decrease of each set of spin echoes is observed and used to derive the distribution T2. The time T2 * characterizes a time for the spins to cease precessing in unison after the application of the excitation pulse 14a at 90 °. In this way, - - at the end of the excitation pulse 14a at 90 °, all spins point in a common direction perpendicular to the static field Bo, and the spins perform precession at a resonance frequency called the Larmor frequency for a field perfectly homogeneous. The Larmor frequency can be described by the formula =, where? it is the gyromagnetic radius, a nuclear constant. However, the field - ^ B is typically non-homogeneous, and after excitation, the spins are offset by T2 * due to inhomogeneity in the field * > or. This decrease is reversible and is reversed by the reconcentration pulses 14b caused by the echoes. Likewise, an irreversible lag occurs (spin-spin relaxation) that is described by the time constant T2. This results in the decay of successive echo amplitudes in the CPMG sequence according to the time constant T2. Typically, with "inside-out" nuclear magnetic resonance, spins are measured with T2 »T2 *. As already indicated, the distribution of times T2 can be used to determine the properties of the formation. For example, with reference to Figure 6, the formation may include small pores containing bound fluid and large pores containing free, producible fluid. A separation limit time T2 (called the cut-off time in Figure 6) can be used to separate the T2 distribution into two parts: a part that includes times shorter than the Tcorte time indicated by the bound fluids, and a part that includes times greater than Tcorte time than free, producible fluids.

- - Typically, each time T2 is calculated by observing the decomposition of the spin echoes 16 that are produced by a particular CPMG sequence. Unfortunately, the drilling rod 5 (see Figure 1) can experience severe lateral movement. However, time T2 is roughly proportional to another time constant called a spin-reticle time TI. Time TI characterizes the time for the spins to return to the direction of equilibrium along the field., and in this way, considering both the TI and T2 times, one can think of each spin as returning to the equilibrium position in a very narrow pitch spiral during the TI recovery. Fortunately, times TI and T2 are approximately proportional. As a result, the T2 distribution can be derived from the measured TI times. In effect, the original work in establishing linked fluid cuts was done using TI times. These results were then expressed and used commercially in terms of T2. See .E. Enyon, J.J. Howard, A. Sezginer, C. Straley, A. Matteson, K. Horkowitz, and R. Ehrlich, Pore-Size Distribution and NMR in Microporous Cherty Sandstones (Distribution of Pore Size and Nuclear Magnetic Resonance in Cherty Sandstones), Paper LL (paper presented at the 30th Annual Symposium of Diagraphy, SWPLA, June 11-14, 1989). Polarization-based measurements can use either inversion recovery sequences or saturation recovery sequences. With the saturation recovery sequences, the spin system becomes saturated, - - for example, with several 90 ° pulses that reduce the magnetization to zero. Then the spin system is allowed to recover for a variable length of time before applying a monitor pulse or a pulse sequence, such as the CPMG sequence. The inversion recovery technique suggests that after the cores have aligned along the static magnetic field, a 180 ° pulse is applied to reverse the direction of the spins. Over time, the spins decay towards their equilibrium direction in accordance with TI, but no measurement is yet made as long as the 180 ° pulse does not induce a signal in the detector. However, before the decomposition is completed, it is interrupted by a monitor pulse or a pulse sequence, such as the CPMG sequence, which spins the spins within the measurement plane (ie, induces a signal in the detector). The information of interest is the amplitude of the signal immediately after the initial "read" pulse of 90 °. This amplitude clearly depends on the recovery time between the initial pulse of 180 ° and the pulse of 90 °. Following an amplitude determination, the spin system is allowed to completely relax to equilibrium again, and then the pulse sequence is repeated. An example of the use of recovery sequences by inversion in a deep well is described in Kleinberg et. US Pat. No. 5,023,551, entitled "Nuclear Magnetic Resonance Pulse Sequences For Use With Sounding Well Diagram Tools", issued June 11, 1991. However, the sequences of - The investment recovery described in US Patent 5,023,551 does not use adiabatic pulses and therefore results in a narrow research region. Likewise, in "inside-out" conditions along with movement, it may be easier to saturate a region than to completely reverse it. Consequently, it may be preferable to saturate a region.

Referring again to Figure 2, TI times are typically measured using measurements based on polarization instead of the decay-based measurements described above. In this manner, each measurement based on polarization may first include the application of a saturation sequence to saturate the spins in a resonance region (such as the cylindrical resonance volume 20a as described in Figure 2, for example). Next, a period of polarization passes to allow the polarization of the resonance volume 20a to the static magnetic field B °. Next, a detection sequence, such as the CPMG sequence, is used to produce spin echoes from formation 10. The amplitudes of the first spin echoes are then analyzed to determine a weighted polarization integral F (tespere) of the porosity distribution F (T1). Because it is only necessary to observe the first echoes to determine the amplitude of the signal, the TI measurement can be performed in a shorter time duration than the T2 measurement based on the decay, and in this way is less prone to the movement of the tool nuclear magnetic resonance 6. The detection sequence can be repeated successively (after the appropriate sequence of saturation) several times with different waiting times to obtain a porosity distribution F (T1). As an example, a measurement based on polarization can be used to measure the TI times for hydrogen nuclei in the resonance volume 20a located within the saturated volume 20b (see Figure 2). In this way, the nuclear magnetic resonance tool 6 can saturate first spins within the saturated volume 20b. However, the polarization period may be long enough to allow the nuclear magnetic resonance tool 6 to move significantly into the borehole. In that case, the movement of the tool 6 causes the resonance volume 20a to change and causes the nuclear magnetic resonance tool 6 to receive spin echoes from a changed resonance volume 20a '(see FIG. 3) that falls partially off of the original saturated volume 20b. As a result, the changed resonance volume 20a 'can encompass a region without saturated spins (an effect typically called "fresh spiked input") and a region of the original saturated volume 20b with saturated spins. Unfortunately, saturation-based nuclear magnetic resonance techniques may not be able to tolerate the entry of "fresh spins" during the polarization period, since fresh spins may introduce measurement errors. For example, the measurements may erroneously indicate a volume of bound fluid greater than that actually present in the formation. One way to saturate a larger region is described in the PCT Serial Application No. PCT / US97 / 23975, entitled "Method for the Evaluation of a Drilling Formation", consigned on December 29, 1997. This patent application discloses , at the beginning of a measurement, the transmission of one or more pulses of radio frequencies covering a relatively wide frequency range and / or additional bandwidths, or the use of one or more pulses that are swept out of frequency to saturate a cylindrical volume around a nuclear magnetic resonance tool. The patent application further describes the use of peak acceleration values to determine when to invalidate measurements due to the movement of the tool beyond the extent of the saturated region, the patent application further describes the suitability of the tool with spacer supports to prevent the movement of the tool beyond the extent of the saturated region. In this way, there is a continuing need to minimize the errors introduced by the relative movement between a nuclear magnetic resonance measuring device and a sample being investigated.

COMPENDIUM OF THE INVENTION. Disclosed is a method for use with a nuclear magnetic resonance measuring apparatus that is subject to relative movement between the apparatus and a sample. The - - apparatus, the sample or both elements may be subject to movement. In one embodiment of the invention, the method comprises radiating a first sequence of radio frequency pulses. The first sequence has an envelope. The envelope is varied during the radiation of the first sequence to substantially saturate a first region of the sample. A second sequence of radio frequency pulses is radiated to establish a resonance region within the first region and measures an attribute of the sample. In another embodiment, a method for use with a nuclear magnetic resonance measuring apparatus that is subject to relative movement between the apparatus and a sample, comprises using a radio frequency carrier signal to radiate a first sequence of radio frequency pulses. . The carrier signal has a phase. The phase is varied during the radiation of the sequence in order to substantially saturate a first region of the sample. A second sequence of radio frequency pulses is radiated to establish a resonance region within the first region and measure an attribute of the sample. In still another embodiment, a method for use with a nuclear magnetic resonance measuring apparatus that is subject to relative movement between the apparatus and a sample, comprises at least one magnet for establishing a static magnetic field, a first coil, a second coil and a pulse generator. The pulse generator is coupled to the first and second coils and is adapted to use the first coil to radiate a first sequence of radio frequency pulses in order to create a time-varying magnetic field. The first sequence includes at least one reconcentration pulse to produce at least one echo from a resonance region of the sample. The pulse generator is further adapted to momentarily use the second coil to modify the static magnetic field at least once during the radiation of the first sequence in order to cause saturation of a region greater than the resonance region.

In still another embodiment, a method for use with a nuclear magnetic resonance measuring apparatus that is subject to relative movement between the apparatus and a sample, includes using an inversion recovery sequence comprising at least one or more adiabatic pulses. Other embodiments of the invention will become apparent from the description, the drawings and the claims.

- - BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of an underground well. Figure 2 is a cross-sectional view of the well taken along line 2-2 of Figure 1. Figure 2 is another cross-sectional view of the well after a movement of the nuclear magnetic resonance tool. Figures 4 and 5 are waveforms illustrating a pulse sequence CPMG. Figure 6 is an example of distribution of relaxation times T2. Figure 7 is a flow chart illustrating a polarization-based measurement according to an embodiment of the invention. Figures 8, 9 and 10 are schematic diagrams of nuclear magnetic resonance tools according to different embodiments of the invention. Figure 11 is a cross-sectional view of a nuclear magnetic resonance tool taken along line 11-11 of Figure 10. Figure 12 is a waveform illustrating a sequence of nuclear magnetic resonance pulses. Figures 13, 16, 18 and 20 are contour plots showing saturation in a resonance region.

Figures 14, 15, 17, 19 and 21 are graphs of relative signal amplitudes received from a region surrounding the nuclear magnetic resonance tool, which illustrates saturation. Figures 22 and 23 are contour plots illustrating saturation in a resonance region for different numbers of pulses with free periods of evolution interspersed and without them. Figures 24 and 25 are contour plots illustrating saturation in a resonance region for different numbers of pulses with free periods of evolution interspersed and without them.

DETAILED DESCRIPTION OF A PREFERRED INCORPORATION Referring to Figure 7, an incorporation 50 of a process for obtaining a TI measurement based on polarization according to the invention can be used by a nuclear magnetic resonance measuring device (a tool for nuclear magnetic resonance, as an example) that is prone to movement. Because the measured sample can be subject to movement, this process can be used when the sample, the measuring device or both elements experience movement. The process 50 includes saturating (block 52) spins in a region of a sample whose characteristics are to be measured. Then, a predetermined time interval (block 54) is allowed to pass to allow at least partial polarization of spins to occur in the region. Next, the process 50 includes applying (block 56) a detection sequence (a sequence based on CPMG, for example) to produce spin echoes from a resonance region in the sample. As described in detail below, techniques are used to maximize the boundaries and saturation density of the saturated region to maintain the resonance region substantially within the saturated region as the measuring apparatus is moved by nuclear magnetic resonance. As a result of these techniques, measurement errors can be reduced and stabilizers for the nuclear magnetic resonance measuring apparatus may not be necessary, for example, if it is used in a low gradient geometry.

As examples, the process 50, as described in more detail below, can be used for the purpose of drawing planes of the properties of underground formations and can also be used in other applications (other applications of nuclear magnetic resonance "inside-out", for example). example) in which relative movement occurs between a sample and a nuclear magnetic resonance measuring device. In some embodiments, the nuclear magnetic resonance measuring apparatus can include electromagnetic field generation units (a coil, an electromagnet and a permanent magnet, as examples) to generate at least two magnetic fields: a magnetic field called a magnetic field. > (not shown) and a magnetic field called (not shown) that is - substantially perpendicular to the magnetic field ß °. As an example, referring to Figure 8, in some embodiments the nuclear magnetic resonance measuring apparatus can be a tool for performing a graph while drilling 60 which includes, as an example, annular permanent magnets 32 and 34 for establishing the magnetic field °° and a coil 39 to establish the magnetic field variable in time. In - - some additions, the magnetic field can (when pressed) have a radio frequency conveyor component called ^. - > - Generally the carrier frequency of the field & can be represented by * > . The transmission of the field creates a resonance region that has a radial thickness, in terms of frequency, which is determined by the gradients of * ^ and *? in the excited region, where ^ i is the? - »projection of yB over the Bo field, In some additions, field" can also be generated (at least partially) by gradient coils 40 and 42 to make the field or have a component that varies with a low frequency, such as described below, the nuclear magnetic resonance measurement tool 60 may also include processing circuitry which may include a pulse generator 65, for example, which is coupled to the coil or coils (such as coils 39, 40 and 42, for example) and is adapted to radiate the fields and B in a manner described below In principle, each measurement based on polarization includes the three building blocks 52, 54 and 56 (see Figure 7), and one or more measurements can be used to perform saturation (i.e., execute the functions of block 56) and thus eliminate block 52 if the requirements are met: the measurements are they repeat successively (called "stacked" experiments), and the signal detection sequence 68 completely destroys the magnetization for the next measurement. If this technique is used, the results of the first measurement are discarded, since the first measurement is made with an incorrect polarization time. Alternatively, the excitation may be performed adiabatically by applying a rapid adiabatic passage pulse in the resonance zone just prior to the application of the detection sequence.

- - Other variations of the three basic blocks 52, 54 and 56 are also possible. As another example, block sequence 54 - block 56 - block 52 can also be used to perform each of the measurements, and this variation can be advantageous from a point of programming view.

When the second variation is used, the first measurement is discarded.

Other variations of the process 50 are also possible as long as the functions of blocks 52, 54 and 56 are performed.

The objective of saturation, regardless of whether the saturation is performed by an explicit saturation sequence or by a detection sequence, is to saturate a large region or volume with radio frequency radiation. As described below in more detail and illustrated by simulations, depending on the particular embodiment, the saturation can be created by applying a sequence of radio frequency pulses, such as the CPMG detection sequence, which is made to the measure to achieve the desired saturation using the movement of the nuclear magnetic resonance measuring tool 60; a sequence characteristic with or without movement of the measuring tool by nuclear magnetic resonance 60 is slowly varied over time; stochastically varying the characteristics of the sequence, with or without movement of the measuring tool by nuclear magnetic resonance 60; or using a combination of these techniques.

A simple CPMG sequence that has constant parameters develops strongly saturated regions, called "holes", in the spins distribution. The burning of holes is long-range, but only leads to a weak saturation since the holes are widely separated from each other. Moreover, once the magnetization in the hole positions is destroyed, the sequence may continue to not increase saturation further. The movement of the nuclear magnetic resonance measuring tool 60 can increase the saturation density by "sweeping" these voids over the saturation volume, as described in detail below. The CPMG detection sequence can be modified to increase the number of reconcentration pulses above the typical number (10, for example) of reconcentration pulses that are necessary to measure the initial amplitude of each echo tail. This method works well if the movement of the nuclear magnetic resonance measuring tool 60 during the polarization time is always coupled with the movement of the nuclear magnetic resonance measuring tool 60 during the detection sequence. However, and unfortunately, an unsatisfactory situation can occur if the nuclear magnetic resonance measuring tool 60 is stationary during the detection sequence 68 but moves during the polarization time. The simulations (discussed later) show that this problem can be avoided by slowly changing the sequence characteristics - - to expand the saturated region, even in the absence of tool movement, as described in detail below. . In this context, the phrase "sequence characteristic" may refer generally to a sequence envelope or to a phase of the radio frequency carrier frequency, as examples. As examples of possible ways of varying the envelope, the envelope may include pulses 120 (see Figure 12) each of which has a duration (called tp) and pulses 120 may be spatially separated (from center to center) by time intervals called te. In this manner, the duration tP and / or the time interval te (as examples) may be varied to expand the saturated region, as described in detail below. The characteristic of the detection sequence (ie, the sequence used for saturation purposes) can be varied not only slowly, but also in an uncorrelated, or stochastic, way from pulse to pulse, as described in detail below. The stochastic end is the irradiation of incoherent noise. The stochastic variation of the characteristics must be contrasted with the slow variation of the characteristics in which the effects of saturation are far-reaching because they dominate the coherent, non-stochastic characteristics. As a result, the slow variation of the characteristics may result in increased burning of resonance voids far apart by consecutive pulses. The points where saturation is created for a short time interval are well separated from each other. However, stochastic variations cause consecutive pulses of the sequence that do not contribute to the same gap and the creation of saturation propagates more evenly for short time intervals. As a result, the stochastic variation of the pulses generally provides a more consistent saturation density. As described below (and illustrated by simulations), these two techniques can be combined to improve the execution of the sequence. As also described below, if a movement is present that is fast enough to sweep gaps over the distance separating adjacent gaps for only a few pulses, the coherent element of the sequence is destroyed, and a sequence with varied characteristics can slowly be destroyed. function similarly to a sequence with stochastically varied characteristics. As described below, it may not be necessary for the launch angles of the reconcentration pulses in the CPMG sequence to be large to create saturation out of resonance if they are coupled with some other variation (phase variation of the carrier frequency , for example). Therefore, by trimming the radio frequency pulses, the power needed for saturation may decrease. For sufficiently short pulses, the influence of hole burning is negligible. This being the case, the period of free evolution between pulses can be shortened, and saturation can be achieved in a much shorter time. At the very short pulse limit, this technique results in incoherent noise radiation whose structure can be designed to suit the needs. In practice, the finite times of rise and fall of the pulses set the lower limit of the duration of the pulses. There may be a trade-off between the time and power required to achieve saturation and saturation bandwidth, as described below. SATURATION USING A CPMG SEQUENCE In the following, an example of saturation is discussed in detail using a CPMG sequence with and without changes in * &or induced by slow movement. Although this description specifically refers to a CPMG sequence, as an example, the above-described hole burning can be achieved by all the multiple pulse frequencies that characterize a large number of repetitions of a pulse building block. The coherent pulse repeated during a sequence CPMG excites selected spins with? »« "»?, Where «" > ? is approximately equal to the radial thickness of the resonance volume, and? (the distance in frequency space) can be defined specifically by the following equation:? = yB - *, where * is the radio frequency of field B? for the first CPMG sequence. The arousal steps become smaller and smaller with the increase of? , but excitations are added from pulse to pulse in the gaps for significant quantities. Because the transverse magnetization is decomposed according to T2, the selected spins become "saturated". The separation (called k) of these holes is determined by the periodicity of the frequency.

The non-negligible pulse duration and out-of-resonance effects cause some deviation, so that the separation? ^ The gaps are roughly described by the following equation:? *** =, where tß is the echo separation from the beginning of a reconcentration pulse until the beginning of the next reconcentration pulse. Coupled with relaxation, the simple CPMG sequence technique results in burning of voids at certain frequencies outside of resonance. It may not be possible to measure in the range of burned holes, by the width? < In the measurement region, it extends over? ^ * 2 ^ ?, which for reconcentration pulses of 180 ° of duration tp becomes? ^ «2p / tp. Since it is always greater than tp, «> ^, and there may be several burned holes within a resonance region. To calculate the extent of signal loss, field geometries, relaxation times and detection bandwidths should be taken into account. To illustrate the hole distribution, Figure 13 is a two-dimensional contour plot 80 (derived from a simulation) showing a calculated contour plot of the distribution of burnt holes in a longitudinal magnetization of Mz = 1 with linear variation in < "> on the horizontal axis and tp on the vertical axis, the white areas represent total conservation of the magnetization, and the black areas represent a saturation reduction of 100%, or inverted magnetization.

- - The first CPMG sequence is applied in? - O and the effect on magnetization outside resonance Mz is shown immediately after the end of this CPMG sequence. The parameters of the pulse sequence CPMG are te = 500 μs; floor = 125 μs; k = 1000; where k is the number of reconcentration pulses. Relaxation times are selected to be long, but a fraction of the duration of the echo queue. In this simulation, perfectly rectangular pulses were used. However, embodiments of the invention can use substantially rectangular pulses and can use substantially non-rectangular pulses. In Figure 13, the effect of the first excitation pulse was not simulated. Figure 14 shows for various relaxation times, the resulting simulated relative signal amplitudes 82 (ie, MZ / MQO) that are available for a second measurement at the frequency changed by the abscissa? , which is reduced by saturation from a first measurement (as described above) for < "?? - tP = p, when averaged? = 0.75? This means that the frequency <2 ° of the carrier frequency has been changed by? Between measurements, each of the relative signal amplitudes 82 is associated with a time IT different (equal to 2 * T2, as an example). The parameters for the second measurement were the same as for the first measurement and the pulse release angle was selected to be 180 °. In the figures (and in the simulation), it was assumed that = 0, that is, the change in the field? it is negligible in the vicinity of the resonance region. For an axisymmetric gradient geometry, the horizontal scale (? Is proportional to the radius difference (of the resonance region) between the first and second measurements.The assumption above that ^? Is a constant is a valid approach when the radius difference is much smaller than the radius, a fact that justifies the selection of a constant throw angle in the graph.

As can be seen from Figure 14, the saturated region basically extends no further than 2 * which is twice the thickness of the radius of the resonance region. In this way, the next measurement begins only with full saturation, if the resonance region is changed radially by less than 1 * / < "&Fig. 15 shows the relative signal amplitudes 84, each of which is associated with a number of reconcentration pulses at the first frequency As can be seen, most of the saturation at? small occurs within of the first 10 echoes Here and in the following examples, TI = 2 «T2 = 100 msg was selected The movement of the tool during the first CPMG sequence can result in an increase in loss in nearby resonance regions. For example, Figure 16 shows an outline graph 86 of the development of the magnetization out of resonance Mz during the first sequence for a tool translation speed of -20 ^ / s.The horizontal axis denotes the frequency radius outside of resonance? ^ i on < ^ >? (pulse amplitude) of the first CPMG sequence The contour describes the relative longitudinal magnetization that remains after the first CPMG sequence. amplitude of the pulses is constant. The pulse parameters and relaxation times are the same as those of the previous example. The vertical axis indicates how many reconcentration pulses were applied in the first CPMG sequence with the RF carrier frequency, which is approximately proportional to the duration of this sequence. The number k of reconcentration pulses varies from a reconcentration pulse (i.e., a block spanning approximately 500 μs) for the upper graph to 100 reconcentration pulses (i.e., a block spanning approximately 50 ms) for the lower graph . In this example, for 50 ms, the nuclear magnetic resonance measurement tool 60 travels the distance of + l «™ ?, which is about half the width of a cover. At the beginning, the carrier frequency "? F corresponds to? = O, and at the end, the carrier frequency w corresponds to? = + L" < "> ? As shown, with an increasing number of echoes, the translation of the nuclear magnetic resonance measuring tool 60"sweeps" the gaps over the spins distribution, and thus increases the density of the saturation. The resulting relative signal amplitudes 88 (ie, Mz / Moo) »when averaging a wrap (rectangular, for simulation purposes) of width ± 0.75 ?? are shown in figure 17. From the top to the foot, the amplitudes 88 represent the result for k = 1, 11, 21, 31, 41, 51, 61, 71, 81, 91. Note that the loss increases with the number of echoes and for more than 10 echoes becomes much stronger than the non-moving saturation effect of the nuclear magnetic resonance measurement tool 60, as shown in Figures 14 and 17. The saturated region now has a width of more than 5 «< ^ ?. The loss increases for a time comparable to the spin relaxation times and can even lead to a negative signal for? little. The exact profile depends on the movement and the relaxation times of the set of spins. The profile becomes narrower for smaller relaxation times. In the above description it is assumed that the pulses of the CPMG sequences are perfectly rectangular pulses. However, real "rectangular" pulses may never reach this ideal but may be subject to finite times of rise and fall. This limits the width of the frequency spectrum contained in the pulses. In very remote resonance, the width of the burned holes and the speed of their burning becomes proportional to the amplitude of the frequency component in the hole position. Therefore, in some embodiments, the burning of voids by remote resonance may be less effective than in the simulations described above. For the pulses discussed in this application, a wide frequency distribution is beneficial. Therefore, in some embodiments, rectangular pulses with the shortest possible rise and decay time constants may be preferable. Moreover, the saturation region can be optimized by varying the pulse-envelope shape for the pulse frequency content. In general, long-range saturation can be created in the absence of motion by irradiating a sequence of repetitive multiple pulses with variable parameters and broadband pulses. If the pulse sequence parameters are slowly varied while the sequence is applied, the position of the burned holes moves slowly over the spins distribution and saturation increases. The varied pulse parameters include: * variation of pulse separation, te, * variations of tp, * variations of «" »? In, as examples, pulse amplitude, field direction and carrier frequency <; * w, * variation of < ">, and * variation of the pulse phase Variations of combinations of these parameters and variations of other parameters are also possible Variations of ffi? and ^ can be caused by effective variations of the fields Bo and B? (eg , variation of magnet and antenna separation or orientation, and / or radio frequency power) or by the relative movement of the sample and the nuclear magnetic resonance measurement tool 60. In this way, the relative movement of the sample with respect to the nuclear magnetic resonance measurement tool 60 may originate from the movement of the sample (eg, flow or diffusion of fluid) or from the movement of the tool.

- - Another way to vary * > or it is to vary the static magnetic field with the help of an electromagnet, or "gradient coil". For example, referring to Figure 8, in some embodiments the nuclear magnetic resonance measurement tool 60 may include the upper permanent magnet 32 and the lower permanent magnet 34 circumscribing a sleeve 28 of the nuclear magnetic resonance measuring tool 60 and producing an axisymmetric radial field B, The magnets 32 and 34 are biased in a direction parallel to the longitudinal axis of the nuclear magnetic resonance tool 35 to cooperate with each other in order to supply a low gradient Bo field. As an example, the north poles of the magnets 32 and 34 may be facing each other to provide a field or have field lines that extend radially outward from the longitudinal axis of the nuclear magnetic resonance measuring tool 60. In some Incorporations, a magnetically permeable member 36 can circumscribe the sleeve 28 and can be positioned between the upper magnet 32 and the lower magnet 34. As a result of this arrangement, the magnetically permeable element 36 concentrates the Bo field to minimize the gradient of the Bot field and produce in this way a more uniform field in the region of interest. The nuclear magnetic resonance measuring tool 60 may or may not include the sleeve 36. More detailed descriptions of these arrangements may be found in Serial US Patent Application No. 09,033,965, entitled "Apparatus and Resonance Method. Nuclear Magnetic to Generate an Axisymmetric Magnetic Field that Has Straight Contour Lines in the Resonance Region ", consigned March 3, 1998; and U.S. Patent No. 4,350,955, entitled "Nuclear Magnetic Resonance Apparatus", issued September 21, 1982, which are given here as reproduced as a reference. To vary the Bo field, the nuclear magnetic resonance tool 35 can include gradient coils, such as coils 40 and 42, which also circumscribe the sleeve 28. The coils 40 and 42 can be pulsed with a direct current (by a generator pulse, as the generator -> 50 pulse), to produce an additional component, 2j a? field ", B2 is essentially radial if the currents in the coils 40 and 42 flow in opposite directions The coils 40 and 42 can be placed between the magnets 32 and 34 in such a way that the two coils 40 and 42 contribute with one component positive to field B which may or may not be • jr * substantially aligned field B in the region of interest, depending on the incorporation In some embodiments, coils 40 and 42 may be well formed by a pair of simple current loops or by multiple loop current loops with equal magnitude currents and opposite flow directions For example, coils 40 and 42 can form a saddle coil Other embodiments using gradient coils 40 and 42 are also possible. in conjunction with a radial and axisymmetric design Bo, for example, referring to Figure 9, in another nuclear magnetic resonance tool - - 61, the permanent magnets 32 and 34 can be replaced by a permanent magnet 62 which circumscribes the sleeve 36, for example, and is positioned between the coils 40 and 42. The magnet 62 - * produces field lines B0 that extend axially parallel to the axis of the tool 61. To make B2 - * substantially parallel to, the currents in the coils 40 and 42 must flow in the same direction. As an example, the upper part of the magnet 62 can form the north pole of the magnet 62, and the lower part of the magnet 62 can form the south pole. Arrangements other than the radial and axisymmetric B designs described above are also possible.

For example, gradient coils can be used with rr >designs; Two-Dimensional Bipolar (2-D) An example of a bipolar 2-D design can be found in U.S. Patent No. 5,280,243, entitled "System for Performing Well Diagnosis During Perforation of the same ", granted on January 18, 1994, issued to Melvin Miller In this way, a nuclear magnetic resonance tool 68 using a B 2-D bipolar design can include an annular magnet 72 that establishes a pattern bipolar for the field - as shown in figures 10 and 11. Unlike their counterparts in tools 60 and 61, the radio frequency coils 73 and 74 are not concentric with the longitudinal axis of the tool 68, but rather the coils of radio frequencies 73 and 74 are arranged to produce a bipolar pattern in the field such that - * the contour lines of the field B \ are substantially rr * perpendicular to the boundary lines of the Bo field in the - - region of resonance. Tool 68 may include gradient coils 76 and 77, each of which may include one or more rectangular loops to produce a gradient field that is aligned with the Bo field in the region of interest that is established by the magnet 72. In this way, as a result of the arrangements described above, the spins precess around ^ + • adíente. The greatest effect occurs if both vectors are parallel. In this way, like. result of this technique,? it can be varied without changing < *. This is advantageous for varying «* because the bandwidth of an antenna with high quality factor limits the range of variation possible for« * (without tuning the antenna again, which is not practical during a saturation sequence, so less if it is done changing capacitors through the use of mechanical switches). In some embodiments, a disadvantage of this method may be the relatively large amount of energy needed to direct the electromagnet (compared to the use of an imaging device) if it should be switched on with varying amplitudes along the saturation sequence. . There are several ways to use the gradient coil (or coils): * A substantially constant current is established in the - »gradient coil through a pulse (from the field ° i) to effectively change the radius of the resonance region for this pulse. The current in the gradient coil is varied through a pulse (from the ßt field to create a "sweep" pulse without changing the frequency of the radio frequency pulse.) Depending on the actual parameters, the sweep pulse can reverse, excite or saturate a particular region This technique can be used in an inversion recovery sequence (instead of a saturation sequence) to invert a large region around the nuclear magnetic resonance tool.

* The gradient coil is lit between the pulses (from the? Field to destroy the transverse magnetization that may have been preserved.) If the duration of the gradient pulse (called tgrad) is short enough so that the variation of a = ° ^ tgri Over the saturated region is negligible, this is similar to stochastically varying the phase - * of the field pulse \, * The current in the gradient coil can be pressed -> concurrently with each pulse of the B field, Current in the gradient coil can be used to create the stochastic or continuous variations described above.

* Other uses of the current in the gradient coil are possible. CPMG SEQUENCE WITH STATIC VARIATIONS The pulse queue characteristics of the CPMG sequence can also be varied stochastically. For example, the pulse phase carrying the radio frequency can be randomly varied to randomly create pulse phases of 0 °, 90 °, 180 ° and 270 ° (at least these pulse phases are available in typical spectrometers by magnetic resonance nuclear), as examples. Referring to Figure 18 (showing a contour plot 90 of relative signal losses for different numbers of echoes) and Figure 19 (showing a contour plot 92 of relative signal losses for different numbers of echoes when averaged on a volume thickness of ± 0.75 ??), an example is shown where the pulses are generated randomly, and the tool 60 does not move. Except for this random character of the pulse phases, all the spin and pulse parameters are the same as in the examples described above. As can be seen, saturation burns wide and well spaced fringes in the spin distribution. The width of the saturated region is smaller than the width of the region created by the movement influenced by the CPMG sequence. This indicates a tradeoff between the extension of the resonance region (using coherent characteristics) and the reliable quantitative saturation profile (using stochastic characteristics). It should be noted that the profiles created by a CPMG sequence will also give a milder form for the spins with T?, 2 (here 100 ms) «tm (here 50 ms), where tm is the duration of the CPMG sequence. The occurrence of movement during the application of the random phase sequence slightly increases its performance, but the profile remains smooth. Incomplete saturation bands occur because not all holes are burned with the same "speed" - - .Depending on the position of? , some gaps can even be completely suppressed, as can be seen, as an example, in figure 20, where each hollow room is missing. The position of these insufficiently saturated points depends on the duration of the reconcentration pulse: Out of resonance, a pulse of duration tp rotates a spin through the angle a (?) Its "axis of effective rotation" that points in the direction ^? +). Unsaturated "nodes" appear where a is a multiple of 2p. Therefore, by varying **! • tp, these points can also be saturated. The effect is illustrated in Figure 20 (which shows a contour plot 94 of relative signal losses for different number of echoes, and in Figure 21 (which shows contour plots 96 of relative signal losses for different numbers of echoes when are averaged over a volume thickness of ± 0.75 ??), for the example of slowly increasing the pulse length (denoted "tp" in Fig. 12) In this simulation, the pulse length was linearly increased from 125 μs (a 180 ° pulse) for the first reconcentration pulse up to 250 μs (one 360 ° pulse) for the centennial reconcentration pulse while the pulse (the distance between pulses, as described in Figure 12) remained fixed All the other parameters are the same as in the previous example.The resulting saturation profile is smoother and slightly wider without variation of the pulse length.

- - Again, in general the saturation effect of the pulse sequence can be optimized for a particular range of motion by varying the different parameters of the sequence, such as te, which is approximately inversely proportional to the separation of the burned holes, te, pulse phases, etc., and compensating the characteristics consistent with stochastics. The above examples of saturation sequences used the effect of gap burning by remote resonance to create saturation. As indicated above, a pulse of duration tp rotates a spin that is out of resonance through the angle a (?) Which is always greater than the nominal throw angle a (0). Therefore, for reconcentration pulses with a (0) = 180 ° (that is, "180 ° pulses"), always hold at (0) > 180 ° for pulses out of resonance. On the other hand, the optimal excitation, and in this way, the optimal excitation out of resonance occurs if a (?) = (2n + l) «180 °. Then the effective throw angle through which a spin is returned from the longitudinal axis is T =? MAX with? Mix = 2 arctan [- ^ being the maximum effective throw angle for a? dice. Therefore, using 180 ° pulses to create saturation by remote resonance can be a waste of energy. Figures 22 and 23 illustrate the dependence of the saturation profile (averaged over a resonance envelope thickness) at (0) on the reconcentration pulses used in the sequence. FIG. 22 shows relative losses of signal 98 for free taper evolution time (i.e., - - time interval between reconcentration pulses, as illustrated in FIG. 12), set at 375 μs, and on the FIG. 23 shows relative losses of signal 100 for a time of free evolution tare set to zero. In both Fig. 22 and Fig. 23, the relative losses of signal 98 and 100 are illustrated for 1 to 100 pulses for the launch angles of 9, 20, 30, 45, 90, and 180 as a function of ? . The different launch angles are created by varying the pulse duration tp. As can be seen, the signal loss distributions are almost identical for different tabar times, and thus, under stochastic phase variation, the saturation pattern is determined mainly by the pulse duration and not by the duration of the evolution period. free. The minimum pulse duration that can be used with a given device is determined by the time constant of elevation (called tr) of the pulse. Yes t <; 3tr, then the pulse does not reach 8i before being disconnected and quickly becomes less effective when tp is further reduced. For a nuclear magnetic resonance well logging apparatus, tr = 5 ... 30 μs is a good estimate. When tp decreases, the saturated region becomes wider. Of practical interest is mainly the region with V v »í» > ! , that is, the region with a (?) < 2p inside the two unsaturated internal nodes. The maximum throw angle? Max decreases with the increase of? . Therefore, the wider the saturation region, the more pulses are needed to create saturation in the outer parts of the region. If the time constant for saturation is Tß, then only the spins with Ti > Ts can be completely saturated. Therefore, compensation can be made between the saturation bandwidth and the lower Ti that can still be saturated. This also shows that, in some embodiments, it is advantageous to keep the sequence as short as possible by minimizing the lowest possible value that can be obtained with the available equipment (here equipment problems can include phase change time, lift times and pulse drop, and overload of the radio frequency circuitry with long and continuous radio frequency pulses). Figures 24 and 25 illustrate the losses 102 and 104 for sequences with (figure 24) and without (figure 25), times tare, respectively. Losses 102 and 104 are shown for different relaxation times. At t = 375 μs, the frequency of 100 reconcentration pulses lasts 40 ms, and without a period of free evolution, the frequency lasts only 2.4 ms. For a nominal launch angle of a (0) = 35 °, both sequences are able to saturate spins with free fluid relaxation times (Tl> 50 ms), but the sequence without period of free evolution is able to saturate spins with Ti 20 times shorter, which is necessary if one wants to solve spin distributions within the bound fluid. In both cases, the energy needed to create saturation is 100 * iso «20 times the energy for a simple 180 ° reconcentration pulse that should not present serious problems for nuclear magnetic resonance spectrometers for depth wells that are generally capable of create queues of hundreds of 180 ° reconcentration pulses of the energy stored in capacitors during tw. In some additions, profiles burned with sequences that include a period of free evolution are - somewhat softer than patterns burned by continuous irradiation. This could be the product of additional lags that occur during the period of free evolution that is lacking in the second case, but it is not critical. Also, if a tool with axisymmetric field geometries moves by the distance ^, any spin, depending on its position in the azimuth, experiences a different displacement in frequency space? = d% $ or fis < s§n. This leads to an additional effective softness of the actual saturation profile. In the simulations, the fourth pulse phases were selected using a random generator. Therefore, the realization of a sequence varies slightly from one simulation to another. In some embodiments, a predetermined sequence could be used to optimize saturation performance. In some embodiments, an optimal variation of parameters may be one without periodicity. In summary, the foregoing describes examples of techniques for preconditioning spins in the vicinity of the resonance region of nuclear magnetic resonance. These techniques allow polarization-based TI measurements even when the magnetic resonance - nuclear measuring device (the nuclear magnetic resonance measuring tool 60 or 35, for example) is moving with respect to the sample, and these techniques allow measurements based on polarization while drilling unstabilized ground, at least together with a low gradient as described in the Serial US Patent Application No. 09,033,965, cited above. The fact that it can operate without a stabilizer, makes the tool more "driller friendly", and consequently greatly increases the ease of use of a tool to perform a log while drilling. While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations thereof. It is intended that the appended claims cover all such modifications and variations insofar as they fall within the true spirit and scope of the invention.

Claims (18)

1. - - REIVINDI CAC IONS: 1. A method for use with a nuclear magnetic resonance measuring device subject to relative movement between the apparatus and a sample; The method comprises: a) producing a static magnetic field; b) radiate a first sequence of radio frequency pulses, where the first sequence has a parameter; c) varying the parameter during the radiation of the first sequence in order to substantially saturate a first region of the sample; and d) radiating a second sequence of radio frequency pulses to establish a resonance region within the first region; and e) measure an attribute of the sample. The method of claim 1, wherein the parameter is an envelope and the step of varying the parameter comprises the step of varying the envelope. The method of claim 2, wherein the step of varying the envelope further comprises the step of varying the amplitudes of the pulses. The method of claim 2, wherein the step of varying the envelope further comprises the step of varying the spacings between the pulses. The method of claim 2, wherein the step of varying the envelope further comprises the step of varying the durations of the pulses. The method of claim 1, further comprising the step of using a radio frequency transport signal to radiate the first sequence of radio frequency pulses, wherein the parameter is a phase of the transport signal, and the step of varying the parameter comprises the step of varying the phase. 7. A method for use with a nuclear magnetic resonance measuring apparatus subject to relative movement between the apparatus and a sample; The method comprises: a) producing a static magnetic field; b) radiating a sequence of radio frequency pulses, wherein the sequence includes at least one echo of a resonance region of the sample; c) saturating a region greater than the resonance region; and d) measure an attribute of the sample. The method of claim 7, wherein the saturation step comprises modifying the static magnetic field at least once during the radiation of the sequence to saturate a region greater than the resonance region. The method of claim 8, wherein the step of modifying the static magnetic field further comprises the step of radiating at least one more pulse. The method of claim 9, further comprising the step of synchronizing the radiation of at least one more pulse to occur when the radio frequency pulses of the sequence are not being radiated. The method of claim 9, further comprising the step of synchronizing the radiation of at least one more pulse to occur during radiation of at least one radio frequency pulse. The method of claim 7, wherein the saturation step comprises radiating pulses of additional radio frequency to saturate a region greater than the resonance region. The method of any of the preceding claims, wherein the relative movement occurs due to the movement of the nuclear magnetic resonance apparatus. 14. The method of claims 1 to 12, wherein the relative movement occurs due to the movement of the sample. 15. The method of any of the preceding claims, further comprising the step of producing an axisymmetric static magnetic field having contour lines in the resonance region, and the contour lines being substantially straight in a direction that is substantially aligned with a longitudinal axis of the nuclear magnetic resonance apparatus. The method of any of the preceding claims, further comprising the step of measuring the attribute of the sample during the drilling of a borehole in a ground formation. 17. A nuclear magnetic resonance measuring device subject to relative movement between the apparatus and a sample; the apparatus comprises: a) at least one magnet; b) at least one coil; and c) a generator pulse coupled to the coil, the generator pulse adapted to: i) radiate a first sequence of radio frequency pulses with the coil, and the first sequence has a parameter; ii) varying the parameter during the radiation of the first frequency to substantially saturate a first region of the sample; iii) radiating a second sequence of radio frequency pulses with the coil to establish a resonance region within the first region; and iv) measure an attribute of the sample. 18. The method of claim 17, wherein the parameter is an envelope and the generator pulse varies the envelope. The method of claims 17 and 18, wherein the relative movement occurs due to the movement of the nuclear magnetic resonance apparatus. The method of claims 17 and 18, wherein the relative movement occurs due to the movement of the sample.