WO2021158929A1

WO2021158929A1 - Ringing cancellation in nmr measurements

Info

Publication number: WO2021158929A1
Application number: PCT/US2021/016840
Authority: WO
Inventors: Shin Utsuzawa; Yi-Qiao Song; Marcus Donaldson
Original assignee: Schlumberger Technology Corporation; Schlumberger Canada Limited; Services Petroliers Schlumberger; Schlumberger Technology B.V.
Priority date: 2020-02-07
Filing date: 2021-02-05
Publication date: 2021-08-12

Abstract

An NMR method and corresponding NMR system performs a phase cycling method that cancels ringing from all pulses (including the excitation pulse) of a pulsed NMR experiment. The cycling methodology can be integrated in any NMR protocol such as relaxation (T1, T2) and diffusion (D) measurements, resulting in improved precision of porosity, permeability, and fluid type identification. The methodology can be applied to NMR well logging tools, laboratory NMR spectrometers and any other NMR spectrometer without hardware modification.

Description

RINGING CANCELLATION IN NMR MEASUREMENTS

CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present document is based on and claims priority to U.S. Provisional Application Serial No.: 62/971,468, filed February 7, 2020, which is incorporated herein by reference in its entirety.

FIELD

[0002] The present application relates generally to nuclear magnetic resonance (NMR) systems and methods.

BACKGROUND

[0003] NMR well logging tools have been in use for many years and can be used to provide analysis of subterranean formations. General background of NMR well logging tools is set forth, for example, in U.S. Pat. No. 5,023,551, the contents of which are herein incorporated by reference. Briefly, NMR well logging tools are generally configured to generate a magnetic field within a borehole (e.g., under the earth’s surface), apply a series of electromagnetic pulses to the volume around the borehole, and measure signals received in response to those pulses to determine characteristics of the volume in proximity to the borehole. Conventional characteristics of the volume measured by NMR well logging tools include longitudinal (1\) and transverse ( T₂ ) relaxation times, as well as diffusion coefficients ( D ) of the fluid inside the volume. In addition to these one-dimensional (ID) measurements of relaxation times and diffusion coefficients, NMR logs can provide two-dimensional (2D) maps showing the correlation between diffusion and relaxation times (D-T₂ or 1)-^'J\ maps) and the correlation between longitudinal and transverse relaxation times (T T₂ maps). These maps are typically used to determine rock properties such as porosity and permeability, as well as fluid properties such as the saturation of oil, water and gas. In some cases, these measurements are used to determine the viscosity of the oil. From these logs, particularly using 2D maps, the water, gas and oil signals can be distinguished, which aids in determining the saturations of the oil, gas and water. In addition, by looking at the position of the oil signal on the map, one can obtain an estimate of the viscosity of the oil, from various correlations to log mean relaxation times.

[0004] NMR involves the application of a magnetic field to an object that impacts the magnetic moment (spin) of an atom in the object. In general, the magnetic field causes the atoms in the object to align along and oscillate (precess) about the axis of the applied magnetic field. The spin of the atoms can be measured. Of interest is the return to equilibrium of this magnetization, i.e., relaxation. For example, a state of non-equilibrium occurs after the magnetic field is released and the atoms begin to relax from their forced alignment. Longitudinal relaxation due to energy exchange between the spins of the atoms and the surrounding lattice (spin-lattice relaxation) is usually denoted by a time Ti when the longitudinal magnetization has returned to a predetermined percentage (i.e., 63%) of its final value. Longitudinal relaxation involves the component of the spin parallel or anti-parallel to the direction of the magnetic field. Transverse relaxation that results from spins getting out of phase is usually denoted by time h when the transverse magnetization has lost a predetermined percentage (i.e., 63%) of its original value. The transverse relaxation involves the components of the spin oriented orthogonal to the axis of the applied magnetic field. The 72 measurement is often performed using the well-established Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence (See Carr et al., “Effects of diffusion on free precession in NMR experiments”, Physical Review , 94:630-638, 1954 and Meiboom et al., “Modified spin-echo method for measuring nuclear relaxation times”, Review of Scientific Instruments , 29:688-691, 1958) which utilizes an initial 90-degree excitation pulse followed by a series of 180-degree (pi) refocusing pulses, and the data is typically analyzed using a Laplace inversion technique or an exponential curve fit.

[0005] FIG. 1 shows the structure of a CPMG pulse sequence, which includes an initial excitation pulse followed by a train of refocusing pulses. The labels P90 and PI 80 indicate the excitation and the refocusing pulses that tip nuclear spins by 90 and 180 degrees, respectively. Note that these tip angles are independent from pulse phases denoted by +x or +y. The time spacing between adjacent refocusing pulses is called TE, and the time spacing between the excitation pulse and the first refocusing pulse is typically set to TE/2. The CPMG sequence is often repeated multiple times and the time spacing between the end of the first scan and the beginning of the second scan is called wait time WT. An echo signal is acquired after every refocusing pulses. The CPMG sequence is often used with thousands of refocusing pulses and thus thousands of echoes. This number of echoes is represented by NEcho. The phases of each pulse are adjusted to manipulate nuclear spins in a desired manner. Often times pulse/echo phases are represented by corresponding axes in an x-y plane, such as +x as 0 degree, +y as 90 degree, -x as 180 degree, and -y as 270 degree. These pulse/echo phases are independent from the spin tip angles. The tip angles represent the amount of spin rotation from the original direction, while phase indicates the phase of the applied (in case of pulses) or received (in case of echoes) RF signal with respect to the phase of the excitation pulse. CPMG scans may be repeated with different combinations of pulse phases and resulting echoes are summed or subtracted from one another by adjusting acquisition phases to select a particular coherent pathway and/or to remove effects from imperfect RF pulses (e.g., tip angle of the excitation pulse is not exactly 90 degree) and ringing. This method of data acquisition is called phase cycling.

[0006] FIG. 2 shows a typical 72 measurement that consists of a long echo train (LCPMG) and a series of short echo trains (Bursts). The LCPMG typically has hundreds to thousands of echoes. This long echo train provides the sensitivity to long Ti components, but LCPMG is often executed only once or a few times due to long execution time. Bursts, on the other hand, are often executed multiple times with short WT, such as 10 ms or 20 ms, and with fewer echoes, such as Necho = 50. Several Burst sequences can be executed after LCPMG to enhance the sensitivity to short Ti. TE may vary between LCPMG and Burst; short TE is often used in Burst to capture short Ti components that decay quickly. Such combination sequences are often used to acquire a signal from unconventional or heavy oil reservoirs. The number of the repeated burst sequences, Nr, is also a configuration parameter that can be optimized. See Hook et ah, “Improved precision magnetic resonance, acquisition: application to shale evaluation”, SPE-146883 presented at the SPE Annual Technical Conference and Exhibition, Denver, Colorado, USA, 30 October-2 November; and Kausik et ah, “Characterization of gas dynamics in kerogen nanopores by NMR”, SPE- 147198 presented at the SPE Annual Technical Conference and Exhibition, Denver, Colorado, USA, 30 October-2 November, 2011, pp. 1-16.

[0007] Other pulse sequences are also used in NMR well logging. Such as T1-T2 sequences and diffusion editing sequences as described below. T1-T2 sequence

[0008] A T1-T2 sequence measures the T1-T2 correlation spectrum (See Song et al., “ 1- 2 correlation spectra obtained using a fast-two-dimensional Laplace inversion”, Journal of Magnetic Resonance 154(2) (2002) 261-268). It is typically obtained with a series of CPMG sequences with different WTs. For example, the list of the WT can be 1 ms, 5 ms, 20 ms, 100 ms, 500 ms, and 2 s. The Necho for the different sequences could be Necho = 50, 50, 100, 500, 1000, 2500.

[0009] Such data can be used to obtain the T1-T2 correlation spectrum. Such spectrum can be useful for analysis of unconventional reservoirs. See Song et al., “NMR application in unconventional shale reservoirs - A new porous media research frontier”, Progress in Nuclear Magnetic Resonance Spectroscopy 112-113 (2019) 17-33.

Diffusion editing sequence

[0010] Diffusion of the fluid molecules can be measured with the diffusion editing sequences (See Hiirlimann et. al. "Hydrocarbon composition from NMR diffusion and relaxation data", Petrophysics 50 (02) (2009) 116-129). It typically uses modified CPMG sequences that are characterized by WT, long echo time (TEL) and short echo time (TE) and also Necho. Several echo train signals are typically acquired with a series of different values of these parameters (such as TEL and WT). Similar to other sequences, the WT is a key parameter to determine the total time it requires to complete the sequence, hence the logging speed while maintaining data quality. [0011] Note that all of these pulse sequences contain at least one CPMG portion.

[0012] NMR relaxation such as measured by T2 has been shown to be directly proportional to the surface-to-volume ratio of a porous material,

Eqn. (1) where S is the total surface area of the material, If is the pore volume, and p is the surface relaxivity (which is related to wettability), and Tro is the bulk relaxation time.

[0013] Here we ignore the diffusion contribution since it is a weak effect in most of standard NMR well-logging program using short echo spacing. The surface relaxivity p is a quantity (in micrometer/s) that defines the strength of the surface relaxation phenomenon. Because of this relationship, NMR is extensively used in petroleum exploration to obtain estimates of porosity, pore size, bound fluids, permeability, and other rock and fluid properties (i.e., “petrophysical data”). For example, it is known that the Ti distribution is closely related to the pore size distribution. Reservoir rocks often exhibit a wide range of 72 due to the difference in pore sizes, with observed Tis from several seconds down to tens of microseconds. Typically, signals at long Ti (e.g. >100 milliseconds) are from large pores and such fluids are considered to be producible. For shorter Ti signals, 3-50 milliseconds, the fluids are often considered to be bound by capillary force of the pores.

[0014] For example, in sandstone rocks, signals at Ti below 30 ms are considered bound by capillary force and will not produce. Thus, a cutoff value, Jicut, e.g., /2cm = 30 ms can be used to calculate the bound fluid volume (BFV)

Eqn. (2) where fCh) is the Ti distribution and Timm is the minimum Ti obtained in the Ti distribution. If f(Ti) is the Ti distribution for the fully saturated sample, then the porosity can be obtained by integrating (Ti) 0 according to

Eqn. (3) where Timax is the maximum Ti exhibited in the sample.

[0015] Signals with even shorter Ti, such as Ti < 3 milliseconds, are often due to clay bound water or viscous (heavy) hydrocarbon. Some rocks contain significant amount of kerogen that is solid organic matter which may exhibit lis down to tens of microseconds. Signals with such short Tis typically can’t be detected by NMR well-logging tools.

[0016] A typical pulsed NMR experiment applies strong RF pulses (often with an amplitude of hundreds to thousands of volts) to a coil and then shortly after the pulses detects the NMR signal (typically at the micro-volt level) from the same coil. Therefore, one key requirement of an NMR system is the ability to detect the signal only some micro-seconds after the RF pulses. However, the energy left in the coil at the end of an RF pulse takes time to dissipate, and during such dissipation, there can be significant level of voltage in the coil. This voltage appears as a ringing signal in the receiver, which can interfere with NMR signals to be detected. As a result, one has to wait till after this time period to detect clean NMR signals. This time period is often called dead time of the NMR system. At high field NMR systems (e.g. hydrogen Larmor frequency larger than 100 MHz), such deadtime can be as short as a few micro-seconds, however, in low field NMR such as 2 MHz or lower, the deadtime can be as long as several hundred micro-seconds.

[0017] The long deadtime is especially problematic in multi-echo NMR measurement, such as CPMG, when it is desirable to detect signals at very short inter-echo spacing TE without distortion of the echo shape. In addition, excessive ringing on the first few echoes prohibit an accurate estimation of signal amplitude, which is commonly used to determine the porosity of porous materials. Historically a combination of hardware (e.g., Q-switches, which shunt the coil with a resistor to damp the energy left in the coil after RF pulse) and pulse sequences with phase cycling have been used to mitigate this effect.

[0018] Phase Alternated Pair Stacking (PAPS) is the standard phase cycling method used on essentially all well-logging NMR tools. In a typical PAPS experiment, the phase of the excitation pulse is varied while the phase of the refocusing pulses are kept the same. However, conventional CPMG-based two-step PAPS cancels ringing only from the second and the following pulses (i.e., refocusing pulses), and thus the ringing from the first pulse (i.e., excitation pulse, which is close to the first echo that is used to compute the total porosity) is not removed. As a result, this approach achieves TE only down to 200 ps on the industry -leading NMR well logging tools in terms of TE and longer on the other Wireline/LWD NMR tools.

[0019] FIG. 3 shows an example of CPMG with two-step PAPS and FIG. 4 shows the resulting ringing cancellation effect. A person skilled in the art would recognize that there are many possible implementations of CPMG-based PAPS to achieve equivalent results, but in all cases, the following criteria will be met:

(i) The phases of the excitation pulse (i.e., the first pulse) and the refocusing pulses (i.e., the following pulses) are 90 degrees apart

(ii) One of the transmit pulse phases (i.e., either the excitation pulse or the refocusing pulses, but not both) is alternated between odd and even scans while the phases of the other pulses are unchanged (iii) Receiver phase is matched to that of the excitation pulse, regardless of the presence of phase alternation in the excitation pulse

[0020] For example, if excitation pulse phase is alternated between 0 degree and 180 degrees (i.e., +/-x) as shown in FIG. 3, receiver phase is also alternated between 0 degree and 180 degrees, resulting in even scans being subtracted from odd scans (represented by the operation A - B). In another implementation, the phases of the refocusing pulses are alternated while that of the excitation pulse (hence the receiver phase, according to the criteria (iii) mentioned above) is fixed. Then even scans will be added to odd scans to get equivalent results with the first example. In either case, phases of the refocusing pulses, hence that of associated ringing, in even scans are opposite from the receiver phase, thus removed by step (iii). However, ringing from the excitation pulse has the same phase as the receiver phase and thus persists after PAPS.

[0021] As we described earlier, essentially all pulse sequences used in NMR well logging contain a CPMG portion. And PAPS is the de-facto standard to remove ringing from a CPMG measurement on essentially all NMR well logging tools. It achieves TE down to 200 ps on the industry-leading NMR well logging tools in terms of TE, and longer on the other Wireline/LWD NMR tools. However, as described above, conventional two-step PAPS cancels ringing only from the second and the following pulses (i.e., refocusing pulses), while the ringing from the first pulse (i.e., excitation pulse) is not removed. This prohibits an accurate estimation of initial signal amplitude, which is commonly used to determine porosity of porous materials. Thus, improvement of the ringing performance of NMR instrument is necessary for all NMR well-logging.

SUMMARY

[0022] This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

[0023] In embodiments of the subject disclosure, an NMR method and corresponding system perform a phase cycling methodology that cancels ringing from all pulses (including the excitation pulse) of a pulsed NMR experiment, allowing accurate first echo measurement with short TE. The NMR method and corresponding system can be configured to work with or without a Q-switch. In an embodiment where the NMR system is equipped with a Q-switch, the phase cycling methodology can further reduce the echo spacing TE by canceling residual ringing. In another embodiment where the NMR system is not equipped with a Q-switch, the phase cycling methodology can allow for accurate first echo measurement with a short TE without using a Q- s witch.

[0024] The phase cycling methodology can be integrated in all NMR protocols such as relaxation (7i, Ti) and diffusion (I)) measurements or combinations of them (i.e., 2D measurements mentioned above), resulting in improved precision of porosity, permeability, and fluid type identification. The methodology can be applied to NMR well logging tools, laboratory NMR spectrometers and any other NMR spectrometer without hardware modification.

[0025] Advantageously, the phase cycling methodology can cancel ringing to achieve short echo spacing (TE). Such short echo spacing (TE) can be necessary to characterize unconventional samples not only in well logging applications but also in laboratory NMR systems as well as for well-site NMR analysis of drill cuttings. Short echo spacing TE can also be necessary for compact NMR sensors to measure the properties of drilling fluid and/or other nuclei that has short Ti (e.g., ²³Na). The phase cycling methodology also has benefits in general industrial/scientific mobile NMR applications for which removal of a Q-switch (i.e., a conventional mitigation for ringing suppression) simplifies the hardware.

[0026] Further features and advantages of the subject application will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present application is further described in the detailed description which follows, and in reference to the noted plurality of drawings by way of non-limiting examples of embodiments of the present application, in which like reference numerals represent similar parts throughout the several views of the drawings.

[0028] FIG. l is a schematic diagram of a typical CPMG pulse sequence;

[0029] FIG. 2 is a schematic diagram of typical h measurement that consists of multiple CPMGs; [0030] FIG. 3 is a schematic diagram of CPMG-based PAPS. The phase of the pulses and resulting NMR signals (i.e., echoes) are indicated by color and the offset from the black line; [0031] FIG. 4 is a plot of a simulation result for the CPMG-based PAPS shown in FIG. 3. R and X correspond to the real (i.e., in-phase) and imaginary (i.e., out-of-phase) components of the respective signal. The amplitude of the second echo is normalized to 4 after four scans. Ringing from the first and the second refocusing pulses (green lines) are canceled, while that from the excitation pulse (red markers) persist. As a result, when TE is short, excitation pulse ringing distorts the amplitude/shape of the first echoes and NMR measurement is compromised;

[0032] FIG. 5 is a schematic diagram of a first embodiment of a four-phase cycling methodology according to the present disclosure, which involves a combination of CPMG-based PAPS and Carr- Purcell (CP)-based PAPS to remove ringing from both the excitation and the refocusing pulses. CPMG and CP generate NMR signals in opposite phases with respect to corresponding excitation pulse phases. Therefore, the combination of CPMG and CP cancels excitation pulse ringing while maintaining NMR signal;

[0033] FIG. 6A are plots showing experimental results for conventional CPMG-based PAPS; and FIG. 6B are plots showing experimental results for the first embodiment of four-phase cycling methodology of FIG. 5. With the CPMG-based PAPS of the FIG. 6A, ringing from the excitation pulse distorts the first few echoes even with long TE. With the four-phase cycling methodology of FIG. 6B, ringing is completely removed, and echo shape and amplitude are consistent across the range of TEs, at a cost of signal reduction in the second and the following echoes;

[0034] FIG. 7 depicts plots of simulation results for the first embodiment of four-phase cycling methodology of FIG. 5, showing the first six echoes. The (A - B) plot on the top right is the same as the CPMG-based PAPS, while the (C - D) plot on the bottom right is added to obtain excitation pulse ringing in different phases. However, in the (C- D) plot, the second and the following echoes have suppressed peaks with quickly decaying amplitudes;

[0035] FIG. 8 is a plot of the simulation results of the sum (A - B) + (C - D) for the first embodiment of four-phase cycling methodology of FIG. 5, showing the first six echoes. The sum (A - B) + (C - D) cancels ringing, but the NMR signal is mostly from (A - B) due to suppressed echoes in (C - D), resulting in ~50 % smaller signal than that from a four-scan CPMG;

[0036] FIG. 9 is a schematic diagram of a second embodiment of a four-phase cycling methodology according to the present disclosure, which combines long CPMGs (i.e., A - B) with short CPs (i.e., C - D) to significantly reduce the measurement time needed to cancel excitation pulse ringing; [0037] FIG. 10 is a plot that depicts experimental results for standard CPMG-based PAPS and the first and second embodiments. The total scan count is the same for each case;

[0038] FIG. 11 is a plot that depicts additional experimental results for standard CPMG-based PAPS and the first and second embodiments. The total scan count is the same for each case;

[0039] FIG. 12 is a plot that depicts additional experimental results for standard CPMG-based PAPS, the first embodiment, and the second embodiment employing a combination of long CPMG with long WT and short CP with short WT. Total measurement time is the same for each case. The second embodiment with reduced WT, labeled (c), removes excitation pulse ringing while providing equivalent signal amplitude with the standard CPMG-based PAPS (i.e., no penalty in terms of signal-to-noise ratio (SNR));

[0040] FIG. 13 is a schematic diagram of a third embodiment of a four-phase cycling methodology according to the present disclosure, which combines CPMG (A and C) and modified CP/CPMG (B and D) to remove ringing from both the excitation and the refocusing pulses. In this third embodiment, the modified pulse sequence CP/CPMG of B and D utilize two different types of refocusing pulses;

[0041] FIG. 14 depicts plots of simulation results for the third embodiment of FIG. 13, showing the first six echoes. In this experiment, A and C are CPMG, while B and D are modified CP/CPMG. Unlike the CP part in the first embodiment, echo amplitudes in B and D are stable;

[0042] FIG. 15 is a plot of simulation results for the sum (A - B) + (C - D) according to the third embodiment of FIG. 13;

[0043] FIG. 16A are plots showing experimental results for conventional CPMG-based PAPS; and FIG. 16B are plots showing experimental results for the third embodiment of four-phase cycling methodology of FIG. 13;

[0044] FIGS. 17A - 17C depict plots of simulation results for different implementations of the third embodiment of four-phase cycling methodology of FIG. 13. The three examples start with the same phases that are combined with different sets of phases but all satisfying specified criteria of the third embodiment;

[0045] FIG. 18 is a plot of simulation results for the sum (A - B) + (C - D) according to the third embodiments of FIGS. 17A - 17C; and

[0046] FIG. 19 is a schematic diagram of a well logging system. DETAILED DESCRIPTION

[0047] The particulars shown herein are by way of example and for purposes of illustrative discussion of the embodiments of the subject disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details in more detail than is necessary for the fundamental understanding of the subject disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Furthermore, like reference numbers and designations in the various drawings indicate like elements.

[0048] FIG. 5 depicts a first embodiment of a four-step phase cycling method according to the present disclosure, which operates to remove ringing from both the excitation and the refocusing pulses of an NMR experiment. The first two steps are conventional CPMG-based PAPS with 90 degrees phase shift between excitation and refocusing pulses to remove ringing from the refocusing pulses. The extra two steps are CP -based PAPS with 0/180 degrees phase shift between excitation and refocusing pulses. This provides excitation pulse ringing and NMR signal in opposite phases, and when combined with the CPMG-based PAPS, ringing from the excitation pulse is canceled while NMR signal is accumulated.

[0049] Note that we assumed the acquisition phase of [+x, -x, +x, -x] (represented by the operation (A - B + C - D)) to maintain consistency and this is kept throughout this disclosure. A person skilled in the art would recognize that an equivalent result may be obtained by a different combination of pulse/acquisition phases as long as relative phases are maintained between them. For example, in this example, the CPMG part is implemented as [+x, +y, +y, ... ] - [-x, +y, +y, ... ], but it can also be implemented as [+x, +y, +y, ... ] + [+x, -y, -y, ... ] as indicated in the previous section. Likewise, the CP part would become [-x, +x, +x, ... ] + [-x, -x, -x, ... ].

[0050] FIGS. 6A and 6B show experimental results. In FIG. 6B, the first echo is clearly visible even at the shortest TE = 100 ps. And reproducibility of echo shape and amplitude across the range of TEs ensures that the ringing was completely removed by the first embodiment of four- step phase cycling (FIG. 6B). However, the second and the following echoes are ~50 % smaller than that from the CPMG-based PAPS with the same number of scans (FIG. 6A). Note that no additional hardware, such as a Q-switch, was used to achieve this result. [0051] FIG. 7 shows the reason behind this; in the C - D plot, the second and the following echoes have suppressed peak with decaying amplitudes. As a result, in the sum (A-B) + (C-D) shown in FIG. 8, the contributions from those echoes become negligible and observed signal is only from A - B and thus ~50 % smaller than that of the standard CPMG of the same number of scans.

[0052] In the first embodiment of FIG. 5, both the excitation pulse ringing and the CP signal acquired in the (C - D) part can decay quickly. Therefore, it is possible to obtain an equivalent result even if (C - D) is significantly shortened.

[0053] FIG. 9 depicts a second embodiment of a four-step phase cycling method according to the present disclosure, which operates to remove ringing from both the excitation and the refocusing pulses of an NMR experiment. In the second embodiment, long CPMGs (i.e., A-B) are combined with short CPs (i.e., C - D) to significantly reduce the measurement time needed to cancel excitation pulse ringing. All or substantial number of refocusing pulses are removed from C and D to suppress CP signals and only ringing from the excitation pulse is acquired.

[0054] FIG. 10 is a plot that depicts experimental results for standard CPMG-based PAPS and the first and second embodiments. The total scan count is the same for each case. The results for the second embodiment with reduced acquisition, labeled (c) is nearly identical to that for the first embodiment with full acquisition, labeled (b), indicating the validity of the second embodiment. [0055] Since we are interested in ringing, it is not necessary to generate CP signals in C and D, which do not contribute to the overall signal anyway. In the second embodiment, this can be achieved by eliminating all refocusing pulses from C and D; i.e., transmit only the excitation pulse and measure the ringing from the excitation pulse at the same timings as in the first embodiment. [0056] FIG. 11 is a plot that depicts experimental results for standard CPMG-based PAPS and the first and second embodiments. The total scan count is the same for each case. Experimental results of the second embodiment, where long CPMG is combined with short CP with no refocusing pulses and labeled (c), is identical with that for the first embodiment, with full acquisition and labeled (b), except for the first few echoes in which there are contributions from the fast decaying CP echoes in (b).

[0057] In other embodiments, reducing WT in C and D suppresses CP signal, while allowing more CPMG signals to be accumulated for a given amount of time. For example, assuming that the first embodiment is performed in conjunction with the following parameters:

WT = 500 ms TE = 0.1 ms

Necho = 4000

In this case, four-step phase cycling will take 4 x (500 + 4000 x 0.1) = 3600 ms, while providing SNR equivalent to CPMG of two scans (i.e., 1800 ms). If we reduce WT and Necho of C and D (i.e., two steps out of the four-step phase cycling) to 20 ms and 20, respectively, then it only takes 2 x (500 + 4000 x 0.1) + 2 x (20 + 20 x 0.1) = 1844 ms; i.e., only 44 ms more than the original CPMG (or less than 3 % increase from the original CPMG).

[0058] FIG. 12 is a plot that depicts experimental results for standard CPMG-based PAPS, the first embodiment with full acquisition, and the second embodiment employing a combination of long CPMG with long WT and short CP with short WT. The total measurement time is the same for each case. It recovers signal amplitude of the original CPMG while still retaining the ringing cancellation capability as that of the first embodiment.

[0059] Note that FIGS. 10 and 11 validate the ringing cancellation performance of the second embodiment, but do not show benefits from the “significantly reduced” measurement time. FIG. 12, which shows an extreme case where wait time WT was significantly reduced, packed more measurements into a given amount of time, hence provided better signal-to-noise ratio than FIGS. 10 and 11 (i.e., benefitted from the “significantly reduced” measurement time).

[0060] FIG. 13 depicts a third embodiment of a four-step phase cycling method according to the present disclosure, which operates to remove ringing from both the excitation and the refocusing pulses of an NMR experiment. This third embodiment can remove ringing from all pulses but with less signal attenuation compared to the first embodiment. In this embodiment, the pulse sequences A and C are conventional CPMG, while the pulse sequences B and D are modified CP/CPMG in which the refocusing pulses have two distinct phases; the first refocusing pulse is in parallel/anti parallel with the excitation pulse, while the second and the following refocusing pulses are 90 degrees phase shifted from the excitation pulse as well as the first refocusing pulse. Thus, in B and D, CP echoes are generated by the first two pulses, which are then refocused by the following pulses as in CPMG. Therefore, it may be considered that in B and D, the excitation pulse and the first refocusing pulse act as a composite pulse to generate an echo with desired property.

[0061] FIGS. 14 and 15 show simulation results for the third embodiment in which several echoes are displayed to demonstrate stable signal amplitude. FIG. 15 shows that the sum (A-B) + (C-D) cancels ringing while maintaining the first echo amplitude, resulting in ~50 % larger signal than the first embodiment. As will be understood, the first embodiment provides a signal amplitude of 2 out of four scans, while the third embodiment provides an amplitude of 3, which is 50% larger. Note that the quadrature component (signal (R) in this example) decays quickly as in the CP sequence.

[0062] FIGS. 16A and 16B show experimental results. For the third embodiment four-step phase cycling (FIG. 16B), signal amplitude is only 25 % less than conventional CPMG-based PAPS (i.e., 50 % greater than the first embodiment ) while the ability to cancel ringing from all pulses is maintained. Note that the transient in the X (i.e., out-of-phase) channel is not ringing but a spin dynamics effect predicted by simulation (FIG. 15).

[0063] A person skilled in the art would recognize that there are other combinations of pulse/acquisition phases that will provide equivalent results. For example, A and C (CPMG part) and B and D (modified CP/CPMG part) of the third embodiment four-step phase cycling are interchangeable with each other. Also, all pulses in all four steps may be phase shifted by the same amount (e.g., if everything is +90 degrees phase shifted, then A becomes +y +x +x +x ..., B becomes +y -y +x +x ..., and so on). The key is to combine two CPMG steps with two modified CP/CPMG steps, the latter of which has the first refocusing pulse 0/180 degrees phase shifted from the excitation pulse and +/-90 degrees phase shifted from the following refocusing pulses. As long as these criteria are satisfied, one can obtain equivalent results with different combinations of pulse phases.

[0064] For example, FIGS. 17A - 17C show three implementations of the third embodiment four- step phase cycling, all starting from the same first step [-x, -x, -y, ... ] but followed by different steps that satisfy the above criteria and achieve the same objective (i.e., to remove ringing from all pulses). Note that CPMG is assigned to B and D in FIG. 17A, B and C in FIG. 17B, and C and D in FIG. 17C, but the resulting sum (A - B) + (C - D) is the same among three examples as shown in FIG. 18.

[0065] The first, second or third embodiment of four-step phase cycling to remove ringing as described above can be used with any sequence that contains at least one CPMG. If CPMG is repeated four or more times, half the scans are executed with the conventional PAPS while the other half are executed with the phase cycling steps that are unique to the proposed methods. Or instead, extra scans, the duration of which depends on the ringdown time (typically in the order of milliseconds to a few tens of milliseconds), can be added at the end of the original CPMG to remove ringing as shown in FIGS. 9 and 10. In either case, ringing from the refocusing pulses shall be canceled by the conventional CPMG-based PAPS, which is part of the proposed four-step phase cycling.

[0066] In embodiments, the first, second or third embodiment of four-step phase cycling can be used as part of a typical 72 measurement sequence that consists of long CPMG (LCPMG) and short CPMGs (Bursts) (FIG. 2). The Bursts are often repeated 10 - 50 times or more with short WT to enhance the sensitivity to short Ti and short Ti. The proposed four-step phase cycling may be incorporated within the Bursts and/or added at the end of the Bursts as an extra. Because often times Bursts use short TE to capture short 72 (i.e., fast decaying) signal, ringing removal is crucial. [0067] In other embodiments, the first, second or third embodiment of four-step phase cycling can be used as part of a T1-T2 sequence that consists of a series of CPMGs with different Nechoes and WTs.

[0068] In yet another embodiment, the first, second or third embodiment of four-step phase cycling can be used as part of a diffusion editing sequence that uses modified CPMG sequences that are characterized by WT, long echo time (TEL) and short echo time (TE) and also Necho.

Well Loggings System

[0069] FIG. 19 shows a well logging system that can be configured to perform NMR experiments that employ one or more embodiments of four-step phase cycling as described herein in order to investigate subsurface formations 34 traversed by a borehole 12. A magnetic resonance logging tool 20 is suspended in the borehole 12 on an armored cable 10, the length of which substantially determines the relative depth of the tool 20. The length of cable 10 is controlled by suitable means at the surface such as a drum and winch mechanism. Surface equipment, represented as 40, can be of conventional type, and can include a processor subsystem that communicates with the downhole equipment, including the tool 20. It will be understood that some of the processing can be performed downhole and that, in some cases, some of the processing may be performed at a remote location.

[0070] The tool 20 also has mechanism 22 (such as a bowspring or retractable arm) that can be configured to press the body of the tool 20 against the borehole wall via standoff spacers 28 during logging. The spacers 28 and mechanism 22 help compensate for the rugosity of the borehole 12 while keeping the tool positioned closely to the side of the borehole under investigation. Although tool 20 shown in the embodiment of FIG. 19 has a single body, the tool 20 may obviously comprise separate components such as a cartridge, sonde or skid, and the tool 20 may be combinable with other logging tools as would be obvious to those skilled in the art. Similarly, although the wireline cable 10 is the form of physical support and communicating link shown in FIG. 19, alternatives are clearly possible, and the invention can be incorporated in a drill stem, for example, using forms of telemetry which may not require a wireline.

[0071] The tool 20 also includes a sensor that includes one or more magnets 24 and an array of RF antenna elements 26. The magnet(s) 24 generate a static magnetic field Bo (depicted as arrows 30) having a static field direction substantially perpendicular (90°) to the longitudinal axis of the tool 20. Each RF antenna element of the array 26 generates an oscillating RF magnetic field Bi (depicted as ovals 32) in the region under investigation (or sensitive zone) that is substantially perpendicular to both the longitudinal axis of the tool 20 and to the primary static field direction. It will be understood that the present invention may be applicable to other tool configurations. [0072] The tool 20 also includes an electronics cartridge or electronics that is operably coupled to the RF antenna elements of the array 26 and configured to cooperate with the antenna elements of the array 26 to make a measurement in the region of investigation (sensitive zone). Such measurements involve magnetically reorienting the nuclear spins of particles in the formation 34 with pulses of the oscillating magnetic field Bi transmitted by the RF antenna elements of the array 26 and then detecting the NMR signals received by the RF antenna elements of the array 26 which result from the precession of the tipped particles in the static magnetic field Bo within the region of investigation over a period of time. The electronics cartridge or electronics can be configured to perform NMR experiments that employ one or more embodiments of four-step phase cycling as described herein. The methodology can be applied to the NMR well logging by programming without hardware modification.

[0073] In other embodiments, other NMR logging tools, laboratory NMR spectrometers and any other NMR spectrometer can be configured to perform NMR experiments that employ one or more embodiments of four-step phase cycling as described herein.

[0074] Some of the methods and processes described above, can be performed by a processor. The term “processor” should not be construed to limit the embodiments disclosed herein to any particular device type or system. The processor may include a computer system. The computer system may also include a computer processor (e.g., a microprocessor, microcontroller, digital signal processor, or general purpose computer) for executing any of the methods and processes described above.

[0075] The computer system may further include a memory such as a semiconductor memory device (e.g., a RAM, ROM, PROM, EEPROM, or Flash-Programmable RAM), a magnetic memory device (e.g., a diskette or fixed disk), an optical memory device (e.g., a CD-ROM), a PC card (e.g., PCMCIA card), or other memory device.

[0076] Some of the methods and processes described above, can be implemented as computer program logic for use with the computer processor. The computer program logic may be embodied in various forms, including a source code form or a computer executable form. Source code may include a series of computer program instructions in a variety of programming languages (e.g., an object code, an assembly language, or a high-level language such as C, C++, or JAVA). Such computer instructions can be stored in a non-transitory computer readable medium (e.g., memory) and executed by the computer processor. The computer instructions may be distributed in any form as a removable storage medium with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over a communication system (e.g., the Internet or World Wide Web).

[0077] Alternatively or additionally, the processor may include discrete electronic components coupled to a printed circuit board, integrated circuitry (e.g., Application Specific Integrated Circuits (ASIC)), and/or programmable logic devices (e.g., a Field Programmable Gate Arrays (FPGA)). Any of the methods and processes described above can be implemented using such logic devices.

[0078] The components, steps, features, objects, benefits and advantages that have been disclosed are merely illustrative. None of them, nor the discussions relating to them, are intended to limit the scope of protection in any way. Numerous other embodiments are also contemplated, including embodiments that have fewer, additional, and/or different components, steps, features, objects, benefits and advantages.

[0079] Nothing that has been stated or illustrated is intended to cause a dedication of any component, step, feature, object, benefit, advantage, or equivalent to the public. While the specification describes particular embodiments of the present disclosure, those of ordinary skill can devise variations of the present disclosure without departing from the inventive concepts disclosed in the disclosure.

[0080] In the present application, reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” All structural and functional equivalents to the elements of the various embodiments described throughout this disclosure, known or later come to be known to those of ordinary skill in the art, are expressly incorporated herein by reference.

[0081] Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this invention. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. § 112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.

Claims

What is claimed is:

1. A nuclear magnetic resonance method comprising: generating a magnetic field within a sample volume; applying a series of electromagnetic resonant frequency pulses to the sample volume, wherein the series of resonant frequency pulses includes an excitation pulse and a set of refocusing pulses wherein each each pulse of the set of refocusing pluses has a phase; measuring signals received in response to the resonant frequency pulses; processing the received signals to generate a resultant signal that removes ringing from both the excitation pulse and the refocusing pulses of the set of phases; and processing the resultant signal to determine characteristics of the sample volume.

2. The nuclear magnetic resonance method according to claim 1, wherein: the method comprises four steps of phase cycling with distinct sets of pulse and acquisition phases in each step.

3. The nuclear magnetic resonance method according to claim 2, wherein: the four steps of phase cycling comprise a phase-alternated Carr-Purcell-Meiboom-Gill pair and a phase-alternated Carr-Purcell pair.

4. The nuclear magnetic resonance method according to claim 3, wherein: all or a substantial number of refocusing pulses are removed from the phase-alternated Carr-Purcell pair, but acquisition of the signal is left to last until the end of the excitation pulse ringing.

5. The nuclear magnetic resonance method according to claim 2, wherein: the four steps of phase cycling comprise a phase-alternated Carr-Purcell-Meiboom-Gill pair and another phase-alternated pair that has refocusing pulses with two distinct phases in each step.

6. The nuclear magnetic resonance method according to claim 5, wherein: the first refocusing pulses are 0 and 180 degrees phase shifted from the preceding excitation pulse and the following refocusing pulses are +/-90 degrees phase shifted from the first refocusing pulse within each step.

7. The nuclear magnetic resonance method according to claim 1, which is part of an nuclear magnetic resonance experiment to measure relaxation ( 7i and/or Ti) or diffusion (79) of the sample volume.

8. An nuclear magnetic resonance well logging tool configured to carry out the nuclear magnetic resonance method of any of claims 1-7.

9. A laboratory nuclear magnetic resonance spectrometer or other nuclear magnetic resonance spectrometer configured to carry out the nuclear magnetic resonance method of any of claims 1-7.