Abstract:
Nuclear magnetic resonance properties of a sample containing fast relaxation components are determined using direct detection of the longitudinal component of the nuclear magnetization. Excitation and detection can be performed in different frequency ranges, which enables short dead time of measurements. In some implementations a nuclear magnetic resonance apparatus can be configured for use in oil well logging.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to a provisional patent application claiming the benefit 35 USC 119(e). The provisional patent application number is 61/830,136; filing date is Jun. 2, 2013. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates generally to the field of measuring nuclear magnetic resonance properties of porous media or biological tissues. More particularly, the invention presents a method of low frequency NMR relaxometry to acquire total amount of hydrogen in a sample containing a constituent with fast spin-spin NMR relaxation. The fast relaxation constituent could be, for example, kerogen in a core or drill cuttings samples of earth formations or protein in biological samples. 
     Background Art 
     NMR relaxation measurements use a static magnetic field to align nuclei in a sample with the direction of the static magnetic field to achieve a thermal equilibrium state characterized by a bulk nuclear magnetization. The rate at which the bulk magnetization is established is described by a spin-lattice relaxation (also called longitudinal relaxation) characterized by a time constant T 1 . A RF magnetic field orthogonal to the static magnetic field is typically used to disturb the equilibrium state to produce precession of the nuclear magnetization about the static magnetic field. The RF magnetic field is typically applied in a form of short pulses that produce free induction decay signals in an NMR antenna. The decay of the nuclear magnetization in the plane perpendicular to the static magnetic field is associated with a spin-spin relaxation (also called transversal relaxation) characterized by a time constant T 2 . If the static magnetic field is in Z-direction of Cartesian coordinates, then the transversal component of the nuclear magnetization is in X-Y plane (rotating due to precessional motion of the nuclear magnetization). The spin precession induces in an induction coil—a typical NMR antenna—a sinusoidal signal due to precession of the bulk nuclear magnetization about the static magnetic field with characteristic resonance or Larmor frequency corresponding to the static magnetic field strength. In order for an NMR signal to be induced in the induction coil the coil is adapted to have its sensitivity direction in the X-Y plane. The signal in the NMR antenna is proportional to the density of protons present in the sample. The bulk nuclear magnetization in X-Y plane decays due to reversible (caused by an inhomogeneity of the static magnetic field) and irreversible (true transversal relaxation) processes of de-phasing. The reversibly de-phased spins can be re-phased using refocusing RF pulses, in particular in a form of a standard CPMG sequence. 
     Acquiring fast spin-spin relaxation components of the NMR signal in the NMR relaxometry is typically limited by the “dead-time” of the measurements. The “dead-time” is typically determined by the RF pulse width and the after-pulse ringing time. Both limiting factors cause the “dead-time” to be inversely proportional to the NMR frequency. Typically, for low field NMR the “dead-time” can be made as short as 0.05 ms. This make the low field NMR measurements well suitable for acquiring NMR relaxation signals from the hydrogen nuclei in liquid constituents of a sample. The liquid constituents typically do not have NMR relaxation times shorter than 0.2 ms. Low field (low Larmor frequency) NMR relaxomentry has been successfully used to characterize porous space and fluids in the earth formations (e.g., U.S. Pat. No. 4,717,878, U.S. Pat. No. 5,055,787, and U.S. Pat. No. 6,452,388) as well as other samples including porous samples and biological tissues (e.g., U.S. Pat. No. 6,882,147 and U.S. Pat. No. 7,366,559). It has not been used for analyzing substances containing constituents with spin-spin relaxation times in the microsecond range, for example, for acquiring signature and the total amount of hydrogen in kerogen, or in protein molecules. 
     Thus known in the art low field NMR relaxometry is not suitable for NMR measurements when the measurement samples contain extremely fast relaxation components, for example, rock samples containing kerogen or biological tissues containing protein. Therefore it is an objective of the present invention to provide a solution for NMR characterization of samples having fast transversal NMR relaxation using low field NMR relaxometry. 
     BRIEF SUMMARY OF THE INVENTION 
     One aspect of the present invention is a low frequency NMR relaxometer and measuring techniques for conducting measurements on rock samples (cores, drill cuttings) and biological samples (for in-vivo or in-vitro measurements), the sample containing fast spin-spin relaxation components of NMR relaxation spectrum. The relaxometer comprises a magnet to generate static magnetic field, a RF antenna to generate RF magnetic field in the sample. The RF antenna sensitivity direction (in X-Y plane) is perpendicular to the static magnetic field direction (Z-direction) to generate precession and nutation of the nuclear magnetization about the static magnetic field. The relaxometer also comprises a magnetic field sensor to directly observe a longitudinal component (Z-component) of the nuclear magnetization. In a preferred embodiment the magnetic sensor is an induction coil. The coil has a sensitivity direction parallel to the direction of the static magnetic field to sense variations of the longitudinal component (Z-component) of the nuclear magnetization modified by the RF magnetic field. The signal produced in the induction coil is proportional to time derivative of the Z-component of the nuclear magnetization. RF magnetic field has a carrier frequency equal or close to the Larmor frequency (corresponding to the static magnetic field), which is typically much higher than the main part of frequency spectrum of the Z-component of the nuclear magnetization. Therefore the RF pulse as well as an after-pulse ringing that could interfere with measurements of the X-Y component of the nuclear magnetization does not affect the Z-component measurement. The Z-component signal in the induction coil is used to determine NMR characteristics, in particular, a total amount of hydrogen in a sample containing a constituent with fast spin-spin NMR relaxation. 
     Another aspect of the present invention is to use an excitation regime that increases the total duration of the Z-component magnetization signal induced in the induction coil and therefore increases signal-to-noise ratio per unit time. In one embodiment the excitation regime described in Nuclear-Magnetic-Resonance Line Narrowing by Rotating RF Field, by M. Lee an W. I. Goldburg is implemented. This excitation regime gives an example of longer lasting Z-component signal variations. 
     Yet another aspect of the present invention is a side-looking NMR sensor assembly for directly acquiring Z-component of the nuclear magnetization. The sensor is suitable for NMR oil well logging and other NMR measurements requiring an “inside-out” NMR sensor. 
     Other aspects and advantages of the invention will be apparent from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This invention is best understood with reference to the accompanying figures in which like numerals refer to like elements. 
         FIG. 1  shows an exemplary embodiment of a standard low field NMR relaxometer of prior art. 
         FIG. 2A  and  FIG. 2B , collectively referred to as  FIG. 2 , show typical RF pulse sequences used by prior art and illustrate main problems associated with acquiring fast relaxation components of the spin-spin NMR relaxation. 
         FIG. 3A  and  FIG. 3B , collectively referred to as  FIG. 3 , present exemplary RF excitation regimes and the corresponding variations of the Z-component of the nuclear magnetization. 
         FIG. 4  depicts an exemplary embodiment of NMR relaxometer of present invention. 
         FIG. 5A ,  FIG. 5B , and  FIG. 5C , collectively referred to as  FIG. 5 , illustrates an exemplary embodiment of magnet and antenna assemblies (side-looking NMR sensors) for high resolution measurement of fast components of NMR relaxation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an exemplary NMR relaxometer of prior art. It comprises a permanent magnet  12  (shown are the north and the south poles of the magnet) generating a substantially homogeneous static magnetic field  14  in a sample  10 , a RF coil  16  generating a RF magnetic field  20  in the sample  10 . The arrows  18  show direction of RF current in the antenna wire. In the exemplary embodiment of  FIG. 1  the RF coil  16  is used for both generating the RF magnetic field in the sample and receiving NMR signal from the sample. In the Cartesian coordinate system shown at  22  the static magnetic field is in Z-direction, the RF magnetic field and the sensitivity direction of the RF antenna is in Y-direction. The bulk nuclear magnetization (not shown) of the sample undergoes a precessional motion about the direction of the static magnetic field (Z) and therefore has both Y and X components. The NMR relaxometer may have two RF coils with mutually orthogonal sensitivity directions in the X-Y plane. 
     Turning now to  FIG. 2 , where typical RF pulse sequences used by prior art to measure the amount of hydrogen (or other nuclei) in a sample and the relaxation properties of the nuclei in the sample are presented. A RF pulse flips the nuclear magnetization away from its equilibrium state (Z-direction as shown in  FIG. 1 ). This results in X-Y plane component of the magnetization that directly measured by NMR receiver. The Z-component of the magnetization also changes but this change is not observed by the antenna. Shown on the  FIG. 2  are envelopes of RF pulses and the envelopes of the RF nuclear magnetization component in X-Y plane.  FIG. 2A  depicts CPMG pulse sequence shown at  30  and  32  used to generate a plurality of spin echoes  36 , the amplitudes of the echoes as a function of time represent true transversal relaxation curve (not distorted by reversible de-phasing caused by inhomogeneity of the static magnetic field). Also shown in  FIG. 2A  is a free induction decay signal (FID) after the first 90-degree pulse  30  that flips the nuclear magnetization into the X-Y plane. As shown in the  FIG. 2A  the FID illustrates the main problem associated with acquiring fast relaxation components of the transversal NMR relaxation. The FID indicates a fast transversal relaxation component  34 A and a slow relaxation component  34 B. The latter is typically determined by the reversible de-phasing of the nuclear spins. The fast relaxation typically has a characteristic relaxation time in the range 0.01-0.05 ms while the typical time to the first echo (limited by the “dead-time”) is more than 0.1 ms. Thus the fast relaxation components irreversibly decay before the first echo is formed. In this case the spin echoes and the transversal relaxation curve do not contain the fast relaxation components. Using FID or a sequence of FIDs shown in  FIG. 2B  at  38  is generally not practical due to an after-pulse ringing that typically lasts longer than the fast relaxation components to be measured. 
       FIG. 3  gives examples of simple RF pulse sequences suitable for directly acquiring Z-component of the nuclear magnetization. The RF pulses  40  in  FIG. 3A  (shown at  40  are the envelopes of the RF pulses) are the 180-degree pulses that periodically flip the magnetization from its equilibrium state in +Z direction to the −Z direction and back, thus producing maximum possible changes in the Z magnetization component. The Z-component of nuclear magnetization is shown at  42 . Since during this process the magnetization experiences some spin-spin relaxation the change of the Z-component of magnetization gradually decreases. It can be recovered by allowing some waiting time after each 360-degree rotation cycle. It is typical for a constituent with a short spin-spin relaxation time T 2  to have a spin-lattice relaxation time T 1  that is much greater than T 2 . Shown in  FIG. 3A  is the Z-component of the nuclear magnetization corresponding to the case when the distance between the pulses  40  is shorter than the shortest spin-lattice relaxation time of the substance in the sample.  FIG. 3B  represents response of the Z-component of the nuclear magnetization  46  to a long RF pulse  44  (dashed line  48  demonstrates the envelope of the Z-component of the nuclear magnetization). The  FIG. 3B  illustrates the excitation regime described in the article Nuclear-Magnetic-Resonance Line Narrowing by Rotating RF Field by M. Lee an W. I. Goldburg, Physical Review volume 140, 1965. As described in the article a certain relationship between the Larmor frequency, the amplitude and the carrier frequency of the RF pulse must be held in order for the Z-component of the nuclear magnetization variations to last almost as long as the spin-lattice relaxation time. It is suggested in the article to measure the Z-component of magnetization existing after the first (long) RF pulse by using a second pulse that flips the Z-component of the magnetization into the X-Y plane and detecting the FID. In a preferred embodiment of the method of present invention the Z-component of the magnetization (reflecting nutation of the nuclear magnetization) is measured directly during the long RF pulse. The excitation regimes presented in  FIG. 3  increase the total duration of the Z-component magnetization signal induced in the induction coil and therefore increases signal-to-noise ratio per unit time 
       FIG. 4  depicts an exemplary embodiment of NMR relaxometer of the present invention. It comprises a permanent magnet  12  (shown are the north and the south poles of the magnet) generating a substantially homogeneous static magnetic field  14  in a sample  10 , a RF coil  16  generating a substantially homogeneous RF magnetic field  20  in the sample  10 . In the exemplary embodiment the RF coil  16  is used for generating the RF magnetic field in the sample. In the Cartesian coordinate system shown at  22  the static magnetic field is in Z-direction, the RF magnetic field and the sensitivity direction of the RF antenna is in Y-direction. The bulk nuclear magnetization (not shown) of the sample undergoes a precessional motion about the direction of the static magnetic field (Z) and therefore has both Y and X components. In order to directly measure the Z-component of the nuclear magnetization M Z  an induction coil  50  is used having the sensitivity direction  52  parallel to the direction of the static magnetic field  14 . The voltage induced in the coil  50  is proportional to the time derivative of the Z-component of the nuclear magnetization dM Z /dt. The RF magnetic field generated by the coil  16  has a carrier frequency equal or close to the Larmor frequency, which is typically much higher than the main components in the frequency spectrum of the Z-component of the nuclear magnetization therefore any parasitic signals at Larmor frequency can be filtered out without distorting the main signal of the Z-component. Also the parasitic signals are small because the sensitivity direction of the induction coil  50  is substantially orthogonal to the RF magnetic field generated by the coil  16 . Thus the RF pulse as well as the after-pulse ringing that would interfere with measurements of the X-Y component of the nuclear magnetization (prior art) do not affect the Z-component measurement of the present invention. Thus the method of present invention enables acquiring nuclear magnetization data (Z-component of the magnetization) during and immediately after the RF excitation pulses ( 40  and  44  in  FIG. 3 ) and therefore obtaining NMR magnetization signal corresponding to fast spin-spin relaxation components. The total amount of hydrogen in the sample can be, for example, determined by integrating the voltage induced in the coil  50  and extrapolating the integrated signal to zero time. A narrow band (low noise) reception is preferably implemented to acquire the Z-component signal. For example, if the measurement regime presented in  FIG. 3B  is implemented, then a narrow band receiver with a central frequency equal to the frequency of nutation of the nuclear magnetization (frequency of oscillation of the Z-component illustrated in  FIG. 3B ) can be used. Since the coil  50  is used to acquire signal having much lower frequency than the Larmor frequency of the NMR excitation the coil  50  should preferably have larger number of turns than the RF coil  16  in order to provide a required noise matching with a preamplifier used for the Z-component signal reception. It is to be clearly understood that the coil  16  or other coil having sensitivity direction in the X-Y plane can be used to acquire signal proportional to the X-Y components of the nuclear magnetization (for example acquiring CPMG echo train as shown in  FIG. 2A ) in order to measure NMR relaxation properties of a sample. Combination of the Z-component measurement and the X-Y component measurement enables differentiation between constituents of the sample (e.g. solid or solid-like constituents and liquids). The Z-component measurements to acquire NMR signal that includes fast spin-spin relaxation constituents (e.g. a solid matter) and the X-Y component measurements to acquire relatively slow spin-spin relaxation constituents (liquids) can be run sequentially or during the same CPMG pulse sequence. In case of using the same CPMG sequence the Z-component of the nuclear magnetization is measured during the excitation RF pulse (shown at  30  in  FIG. 2A ). 
     It would be readily understood by those skilled in the art that other than the induction coil  50  magnetic sensors can be used to acquire Z-component of nuclear magnetization. For example, a high sensitive atomic magnetometer could be used. In case of using a magnetometer as the magnetic sensor of the Z-component of nuclear magnetization the NMR magnet/antenna assembly (NMR sensor unit) can be placed inside a magnetic screen in order to shield the magnetic sensor from the Earth&#39;s magnetic field. It should be understood that only high sensitivity magnetic sensors can be used to acquire Z-component of the nuclear magnetization in low frequency (low field) NMR relaxomentry. For example a magnetic sensor described in the patent EP 2 515 131 A1 would not have sufficient sensitivity as applied to the measurements described in the present invention. 
       FIG. 5A ,  FIG. 5B , and  FIG. 5C  represent another aspect of the present invention: a side-looking NMR sensor, that can be used for NMR well logging. In one embodiment of the sensor shown in  FIG. 5A  the sensor comprises a source of local static magnetic field represented by a magnet  56  and a soft magnetic core  58 . The magnetic flux of the magnet and the static magnetic flux in the core is presented at  60 . Magnetic field  62  in the volume of investigation  61  in the earth formations is perpendicular to the axis of the borehole (the borehole axis is perpendicular to the plane of the drawing). The tool axis is parallel to the borehole axis. A radio-frequency magnetic flux in the core is generated by the a RF coil, the two parts of which are shown at  64 A and  64 B. The radio-frequency magnetic flux direction in the core is shown at  66 . The radio-frequency magnetic field  67  at the volume of investigation  61  is perpendicular to the direction of the static magnetic field and also perpendicular to the borehole axis. An induction coil made of two parts  68 A and  68 B is used to directly acquire signal produced by the Z-component of the nuclear magnetization (Z-component of nuclear magnetization is the component parallel to the static magnetic field  62 ). The sensitivity direction of the induction coil is shown at  69 . In another embodiment of the side-looking sensor shown in  FIG. 5B  the source of the static magnetic field is formed by a coil  80  and a magnetic core  81 . The static magnetic flux direction in the magnetic core is shown at  82 . The static magnetic field direction in the volume of investigation  71  is shown at  72 . A radio-frequency magnetic flux in the core is generated by a RF coil, the two parts of which are shown at  74 A and  74 B. The radio-frequency magnetic flux direction in the core is shown at  76 . The radio-frequency magnetic field  77  at the volume of investigation  71  is perpendicular to the direction of the static magnetic field and also perpendicular to the borehole axis (the latter is perpendicular to the plane of the drawing). An induction coil made of two parts  78 A and  78 B is used to directly acquire signal due to Z-component of the nuclear magnetization. The sensitivity direction of the coil is shown at  79 . 
     In both embodiments of the side-looking sensor presented in  FIG. 5  the soft magnetic core is made of a magnetically permeable material which is preferably macroscopically non-conductive (e. g. ferrite or stack of thin soft magnetic metal ribbons or tapes separated by insulating layers). The core is used as part of the static magnetic field generation, the radio-frequency magnetic field generation and the nuclear magnetization signal reception subsystems of the sensor. In both embodiments of the side-looking sensor presented in  FIG. 5  the RF coils  64 A,B and  74 A,B can be used for generating the radio-frequency magnetic field in the volume of investigation and also to receive signals produced by X-Y components of the nuclear magnetization (the X-Y components of the nuclear magnetization are the orthogonal components in the plane perpendicular to the static magnetic field). The coils  68  A,B and  78  A,B should preferably have larger number of turns than the RF coils  64 A,B and  74 A,B in order to provide a required noise matching with a preamplifier used for the Z-component signal reception. 
       FIG. 5C  shows a side view of the side-looking sensors representing an exemplary positioning of the sensors. Shown at  83  is a part of the logging tool in the borehole  82 . The side-looking sensor  84  is attached to the tool using a retractable arm  85 . 
     The sensors presented in  FIG. 5  are configured as a magnetic head-type device with substantially no parasitic NMR excitation in the borehole. 
     A plurality of sensors presented in  FIG. 5  can be used to enable azimuthally selective NMR measurements. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefits of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of invention as disclosed herein.