FR2761478A1 - RADIAL NUCLEAR MAGNETIC RESONANCE DRILLING DIAGRAPHY METHOD AND INSTRUMENT - Google Patents

Abstract

The invention relates to an instrument 42 for detection by nuclear magnetic resonance, comprising a magnet 60, 61, 62 for inducing a static magnetic field within the materials 26 subjected to analysis along a wellbore. The static magnetic field is substantially coaxial with a longitudinal axis 78 of the instrument. The magnetic field is polarized substantially perpendicular to the longitudinal axis and is symmetrical around the axis. The static magnetic field has a maximum longitudinal gradient which is inversely related to a speed of movement of the instrument along the longitudinal axis through the materials to be analyzed. The instrument includes a transmitter 93, 74, 45, 67 to generate a radio frequency magnetic field in the materials to excite the nuclei in the materials. The radiofrequency magnetic field is substantially orthogonal to the static magnetic field. The instrument includes a receiver 67, 45, 73, 89 to detect nuclear magnetic resonance signals from excited nuclei in the materials. In a preferred embodiment of the invention, the magnet comprises magnetized cylinders stacked along the longitudinal axis. The magnetization of each of these cylinders is proportional to its distance from a central plane of the magnet. The cylinders are magnetized parallel to the longitudinal axis and in the direction of the central plane. The preferred embodiment of the magnet has an end magnet disposed at each longitudinal end of the stacked cylinders. The end magnets are each magnetized parallel to the longitudinal axis and in a direction opposite to the magnetization of an adjacent cylinder.

Description

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  The present invention relates to measuring instruments and detection techniques.

  by Nuclear Magnetic Resonance (NMR). More specifically, the present invention

  This relates to NMR logging instruments and measurement techniques for surveying the geological photobiology traversed by a borehole. The invention also relates to methods of using NMR measurements to determine the properties of geological formations.

  NMR tools for well logging can be used to determine

  the properties of geological formations, including the fractional volume of

  pores (porosity), the fractional volume of the mobile fluid filling the pores of the

  geological and other petrophysical parameters. RMIN measurement methods and techniques for determining fractional pore volume, fractional volume of the mobile fluid, and other petrophysical parameters are described, for example, in Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination, MN Miller, and al. Society of Petroleum Engineers Article No. 20561, Richardson, TX, (1990) and Field Test of an Experimental Pulsed Nuclear Magnetism Tool. C.E. Morriss et al,

  SPWLA Logging Transactions Symposium, paper GGG (1993).

  NMR logging instruments typically include a permanent magnet.

  to induce a static magnetic field in geological formations and a

  transmitting antenna positioned near the magnet and shaped so that an im-

  Frequency (RF) power pulse routed through the antenna induces a

  RF magnetic field in geological formations. The RF magnetic field is large

  only orthogonal to the static magnetic field. After an RF pulse,

  are induced in a receiving antenna of the logging instrumnent, by rotation

  processional hydrogen spin axes, or other nuclei, around the magnetic field

  2s than static. The receiving antenna is typically connected to a receiver circuit of the instrument.

  which detects and measures induced voltages. In a typical NMR measurement set

  that an RF pulse sequence is applied to the transmitting antenna and a sequence of voltages is measured by the receiving antenna (note that some instruments use the same antenna for transmission and reception). The amplitude of the voltages detected and the speeds at which the voltages detected vary with time are related to

  certain petrophysical properties of the geological formation.

  One type of logging NMR instrument is described, for example, in U.S. Patent 3,597,681 (Huckbay et al.). The instrument described in the '681 patent of Huckbay et al has several disadvantages, one of which is that a unidirectional static magnetic field zone is not homogeneous along the axis of the wellbore. Conveniently, well logging instruments typically need to be able to move axially along the borehole while performing measurements. During the time required to make a typical NMRIN measurement the "sensible volume" (part of the formation in which nuclear magnetic resonance is excited) generated by the logging instrument will have moved along the wellbore such that the measuring set can not be completed. Another disadvantage to the instrument described in the 'Luckbay et al.' Patent is that a significant portion of the NMR signals come from the fluid, referred to as "tillage sludge," filling the well. Still another disadvantage of the instrument described in the '681 patent (Huckbay et al.) is that its antenna is directed from only one side of the instrument and therefore

  only a small fraction of the total volume of the unidirectional static magnetic field

  nel. This results in inefficient use of the permanent magnet of the instrument.

  Yet another disadvantage of the instrument described in the '681 patent is that

  the antenna is subjected to a high static magnetic field strength and,

  sequent, may have an unacceptable amount of magnetoacoustic resonance.

  Another disadvantage of the '681 patent instrument (Huckbay et al.) Is that the RF magnetic field generated by the antenna drops in amplitude with the power of the distance between the instrument and the sensitive volume being three. this instrument is the equivalent of a three-dimensional dipole. Such an antenna is coupled in its vicinity only to a small portion of the unidirectional static magnetic field. This has for

  result an extremely low signal-to-noise ratio.

  Another type of well logging NMR instrument is described in US Pat.

  US-4,350,955 (Jackson et al). The instrument described in the '955 Jackson et al. com-

  door permanent magnets configured to induce in geological formations a

  magnetic field which has a magnetic field strength in a toric volume

  than substantially uniform. A particular disadvantage of the instrument described in the

  Jackson et al. is that the thickness of the toric volume is very small compared to the typical speeds of axial displacement of the logging instruments. Instruments

  logging, in order to be marketable, must typically be able to

  place axially along the wellbore at speeds not less than about ten feet (3.04 m) per minute. The time required to perform a typical NMR spin echo measurement set can be several seconds. The NMR logging instrument is therefore likely to move a substantial distance during a measurement cycle. Measurements made by the instrument described in the '955 patent (Jackson et al). are therefore subject to error depending on how the instrument is moved during the

  logging operations since the antenna would not be positioned long enough

  time to be sensitive to the same toric volume that was magnetized at the beginning of a cycle

any measures.

  Another disadvantage of the instrument described in the '955 patent of Jackson et al. is

  it eliminates no NMR signal from the fluid filling the wellbore.

  Yet another disadvantage of the instrument described in the '955 patent of Jackson et al. is that the static magnetic field of toric form is subjected to amplitude changes at the same time that the instrument is subjected to variations of the ambient temperature and to variations of the terrestrial magnetic field. The antenna of the '955 patent instrument of Jaçkson et al. is granted on a single frequency. If the intensity of the field

  If the static magnet of the toroidal volume varies, the antenna may no longer be sensitive to

  NMR signals from within the toric volume. Using the instrument of the 955 patent of Jackson et al. it is difficult to compensate for the frequency of the RF magnetic field

  because of changes in the intensity of the static magnetic field inside the vo-

lume toric.

  There are additional disadvantages to the instrument described in Jackson et al. First magnetic pole pieces are opposed to each other. This results in a significant demagnetization effect that requires the permanent magnetic material to have a high coercive force. This requirement is directly opposed to the requirement of the strong residual magnetization and the high temperature stability of the permanent magnet. Secondly, the magnetized pole pieces are spaced from each other and away from the toric area, which makes the use of the magnet

  permanent less effective. Third, the antenna used in the instrument '955 Jack-

  its efficiency is lower due to weak electromagnetic coupling between the

  and resonance frequency geology for NMR investigation. QUA

  trially the antenna is located in a relatively large static magnetic field

  which stimulates magnetoacoustic resonance in the antenna. Fifth, for an NMR measurement technique, which uses a homogeneous static magnetic field, the variations of the relative position of the instrument with respect to the Earth's magnetic field

  can cause a significant disturbance in the homogeneity of the toric zone.

  Another type of borehole NMR logging instrument is described in U.S.-4,717,876 (Masi et al.). The instrument described in the Masi et al. 876 has improved the homogeneity of the toric zone compared to the instrument described in the Jackson et al patent 955, but has basically the same disadvantages as the

brevet'955.

  Another type of borehole logging NMR instrument is described in U.S. Patent No. 4,629,986 (Clow et al.). This instrument provides an improved signal-to-noise ratio over the Jackson et al 955 instrument through the incorporation into the antenna of ferrite

  high magnetic permeability. An increase in stability is achieved by accom-

  plucking NMR measurements in a static magnetic field that presents a gradient

  amplitude. However, the instrument described in the Clow et al.

  vantages. Since the magnetic properties of most permanent magnet materials are temperature dependent, the sensitive volume is stable neither in shape nor in magnetic field strength. The sensitive volume of this instrument is only a few inches (2.5 cm) long in the longitudinal direction, which means that this instrument is substantially stationary during an NMR measurement cycle. The magnetized pole pieces are substantially spaced from each other and are spaced apart from the sensitive area, making the use of the permanent magnetic material inefficient

  The antenna is located in a relatively strong magnetic field which stimulates the reso-

  Magnetoacoustic distribution of the antenna. The ferrite of high magnetic permeability of the antenna is located in a relatively strong magnetic field, which can saturate the ferrite and reduce its efficiency. A soft ferrite disposed in a static magnetic field is also a strong source of magnetostrictive resonance following an RF pulse across the antenna. In the arrangement of the magnet of the Clow et al.

  the demagnetization field is relatively large, which requires a material of great

  gnetic with high coercive force. This requirement is opposed to the strong magnetism

  and the high temperature stability of the magnetic properties also required of the permanent magnetic material. Finally, the static magnetic field in the geological formations of the sensitive volume is only about 10 Gauss and rotates 360 in a plane perpendicular to the axis of the wellbore. For this amplitude of static magnetic field, the magnitude of the Earth's magnetic field of about 0.5

  Gauss has a significant disturbance in the overall strength of the field.

  Another type of borehole logging NMR instrument described in U.S.

  4,717,878 (Taicher et al.) Provides for azimuthal resolution with respect to the axis of the well of

  drilling and a reduction of spurious signals from the wellbore fluid. Toute-

  The instrument described in the Taicher et al. 878 patent has several disadvantages.

  Since the magnetic properties of the permanent magnetic material used in the instrument are temperature dependent, the sensitive area is stable neither in shape nor in magnetic field strength. The antenna is located inside a relatively strong magnetic field, which stimulates the magnetoacoustic resonance of the antenna. In the arrangement of the magnet of the instrument described in the Taicher et al. 878 patent, the demagnetization field is very strong, which requires a magnetic material

  having a high coercive force. This requirement is directly opposed to a magnetism

  residual temperature and high temperature stability of the magnetic properties of

  the permanent magnet of a well logging instrument.

  Due to the disadvantages of previous designs of the NMR instruments of

  well logging none of them are generally well-logging instruments.

  accepted commercially. Well logging instruments accepted

  commercially include one which is described in U.S.-4,710. 713 (Taicher and

  have.). The instrument described in the '713 Taicher et al. includes an assembly of

  permanent mantle of cylindrical general shape which induces a static magnetic field having a substantially uniform magnetic field intensity within a volume

  cylindrical annular geological formations. The instrument described in the Tai-

  dear et al. 713 has several disadvantages, however. First, the antenna induces an RF magnetic field within the geological formations surrounding the instrument which decreases in intensity with the square of the radial distance from the magnet. Because the signal-to-noise ratio of the RMIN measurements made in a magnetic field gradient is typically related to the intensity of the RF magnetic field, the instrument described in the Taicher et al. 713 patent has very high power requirements. important, and

  may present difficulties in obtaining measurements with a sufficient signal-to-noise ratio.

  10 at substantial radial distances from the instrument.

  Another disadvantage of the Taicher et al. 713 is that the con-

  optimal reception of the magnet and the RF antenna, in order to optimize the signal-to-noise ratio

  requires that nuclear magnetic resonance conditions be met at a frequency of

  relatively important. Since the RF energy losses of the electroconductive fluid

  are in the wellbore are proportional to the square of the frequency, the setting

  The work of Taicher et al. 713 is limited to use in fluids of relative conductivity.

  low in the wellbore.

  Yet another disadvantage of the Taicher et al '713 patent instrument is that

  the antenna is located in a relatively strong static magnetic field that is perpendicular

  a direction of RF passage in the transmitting antenna and, therefore,

  mule the magnetoacoustic resonance of the transmitting antenna. Another RMIN logging instrument is described in U.S. Patent 5,055,787 (Kleinberg et al.) This instrument

  logging system includes permanent magnets arranged to induce a magnetic field.

  only in the geological formation having a substantially zero field gradient within 2s of a predetermined sensible volume. The magnets are arranged in a part of the housing

  instrument that is typically placed in contact with the wall of the wellbore. The an-

  This instrument is positioned in a recess on the outside of the instrument housing, which allows the instrument housing to be constructed of a very strong material such as steel. A disadvantage of the logging instrument of the '787 patent Kleinberg et al. is that its sensitive volume is only 0.8 cm away from the surface of

  the instrument and extends only 2.5 cm radially outward from the surface of the instrument.

  strument. Measurements made by this instrument are therefore subject to significant error.

  aunt caused by, among other things, a roughness of the wall of the sinking well, by deposits of the solid phase of the drilling mud (called "mud cake") on the wall of the borehole in any thickness and by the contents of formation fluid in the

invaded area.

  Another disadvantage described in the Kleinberg patent et al. 787 relates to the permanent magnetic material. Since magnet pole pieces are opposed to each other, there is an important demagnetization effect which requires a permanent magnetic material having a high coercive force. This requirement is opposed to the strong

  residual magnetization and the important stability of the temperature of the magnetic properties.

  required of the permanent magnet.

  Another NMR measuring instrument which may have application in the diagra-

  Wells are described in U.S.-5,572. 132 (Pulyer et al.). This instrument

  takes a permanent magnet to induce a polarized magnetic field along the longitudinal axis of the instrument, and antenna coils disposed around the outside of the

  the magnet. The instrument described in the Pulyer et al. 132, like many instruments

  Prior art NMR logging elements have a common drawback which is explained, for example, in US Pat. No. 5,332,967 (Shporer). This disadvantage relates to a significant phase shift of the NMR signal, which leads to a significant distortion of the height of the NMR signal and may even lead to a complete disappearance of the NMR signal, when the logging instrument moves in a direction along an amplitude gradient of the static magnetic field. In the current well logging practice, the phase shift and signal reduction may even be worse than what is suggested by the '967 Shporer patent because the logging speed can be variable, as it can be.

  is understood by those skilled in the art of well logging.

  The invention relates to a nuclear magnetic resonance detection instrument,

  comprising a magnet for inducing a symmetric static magnetic field

  radially within the materials being analyzed. The static magnetic field is

  substantially coaxial with a longitudinal axis of the instrument and is substantially biased

  pendicular to the longitudinal axis. The static magnetic field has a gradient of am-

  maximum longitudinal extent which is inversely related to a movement speed of 2s the instrument along the longitudinal axis through the materials to be analyzed. The instrument comprises a transmitter for generating a radio frequency magnetic field in the

  materials for exciting nuclei in materials. The magnetic field in radiofrequency

  quence is substantially orthogonal to the static magnetic field. The instrument comprises a receiver for detecting the nuclear magnetic resonance signals emitted by the excited nuclei in the materials

  In a preferred embodiment of the invention, the magnet comprises cylinders

  magnets stacked along the longitudinal axis. The magnetization of each of these cylinders

  dres is proportional to its distance from a central plane of the magnet. The cylinders are magnetized parallel to the longitudinal axis and in the direction of the central plane. The preferred embodiment of the magnet includes an end magnet disposed at each

  longitudinal end of the stacked cylinders. The end magnets are each ai-

  mantées parallel to the longitudinal axis and in a direction opposite to the magnetization of a cylinder of the other end. The preferred embodiment of the magnet includes a recess in the center of the stack of cylinders. An antenna that can be connected to the transmitter and / or receiver can be located in the recess to reduce resonance

magnetostrictive antenna.

  The invention will be better understood, and details of this will emerge,

  description will be made of preferred embodiments, in connection with the figures of the attached plates in which:

  Figure I illustrates an Acoustic Magnetic Resonance Logging instrument.

  tick (NMR) located in a borehole crossing geological formations.

  Figure 2 illustrates in more detail the NMR probe of the instrument of the

figure 1.

  FIG. 3 illustrates a functional block diagram of the NMR instrument of the present invention.

this invention.

  Figure 4 illustrates the main magnet assembly of the NMR instrument more

tail.

  Figure 5 illustrates the dimensions of the magnet.

  FIG. 6 is a graphical representation of the induced static magnetic field

  by the magnet in the sensitive volume.

  Figure 7 is a graph of the longitudinal component of the magnetic field

  statically induced by the magnet inside the transmitting antenna with and without

end mantle.

  Figure 8 illustrates the static magnetic field of Figure 6 in more detail.

  I. Instrument Configuration Figure I shows a nuclear magnetic resonance (NMR) well logging instrument disposed in a borehole 22 traversing geological formations 23, 24, 26, 28 to take measurements of the properties of the formations 23, 24, 26, 28. The wellbore 22 of Figure 1 is typically filled with a known fluid 34

  in the art under the name of "drilling mud". A "sensitive volume", designated in its general

  58 and having a generally cylindrical shape, is arranged in one of the formations

  defined by 26. The sensitive volume 58 is a predetermined part of the

  geological measurements 26 in which nuclear magnetic resonance (NMR) measurements are made by the logging instrument as will be explained later. A logging instrument train 32, ("instrument train") which may include

  the NMR instrument according to the invention is typically lowered into the

  rage 22 by means of a reinforced electric cable 30. The cable 30 may be introduced and removed from the

  well 22 by means of a winch or drum 48 as is known in the art. The train of

  32 can be electrically connected to a surface equipment 54 by means of a

  isolated electrical conductor (not shown in FIG. 1) forming part of the cable 30. The

  surface 54 may comprise part of a telemetry system 38 for

  control signals and data to the instrument train 32 and to a computer

  40. The computer 40 may also include a data logger 52 for recording.

  the measurements made by the instrument and transmitted to the surface equipment 54

  s over the logging cable 30.

  An NMR probe 42 according to the invention can be included in the train of instruments.

  32. The instrument train 32 is preferably centered within the wellbore

  22 by means of an upper centralizer 56 and a lower centralizer 57 attached to the train of inserts.

  trurnents 32 at axially spaced locations. The centralizers 56, 57 can be of

  any known type of art such as arc springs or operating weapons.

  other than manual or similar.

  Circuits for implementing the NMR probe 42 may be located inside

  of an NMR electronic cartridge 44. The circuits (not shown in FIG.

  The NMR probe 42 is typically located within a protective casing 43 which is adapted to shelter from the drilling mud 34. inside the probe 42. The functions of the probe 42 will be

explained later.

  Other types of logging sensors (not shown for the sake of clarity in the illustration of FIG. 1) can be part of the instrument train 32.

  is shown in FIG. 1, an additional logging sensor 47 can be placed

  above the NMR electronic cartridge 44. Additional logging sensors such as those marked 41 and 46 may be placed at or below the lower centraliser 57. The other sensors 41, 46, 47 may be of known types of the same. Those skilled in the art may include, but are not limited to, gamma ray detectors, global formation density sensors, or neutron porosity detectors. In a variant

  parts of the NMR electronics can be placed inside the electronic cartridges.

  which are part of other logging sensors. The locations of the other sensors

  41, 46, 47 shown in FIG. 1 is a matter of convenience for the system designer.

  and should not be considered as a limitation to the invention.

  Figure 2 shows the NMR probe 42 in more detail. The NMR probe 42

  preferably comprises a set of generally cylindrical permanent magnets 60.

  The magnet assembly 60 includes at least one permanent magnet 62, which extends from

  generally along a magnet axis 80 and preferably has a cross-sectional direction

  circular axis perpendicular to the magnet axis 80. The magnet axis 80 is positioned substantially coaxially with a longitudinal axis 76 of the well bore (22 in FIG.

  1), which location is provided by the upper and lower centralizers (56 and 57).

  gure 1). The preferred construction of the magnet assembly 60 will be explained in more detail.

  For the sake of clarity, the magnet or permanent magnets 62 will be considered

  together and referenced as the permanent magnet 62, and their common axis 80 as well as

  the coaxial axis 76 of the shading well will be jointly identied as the longitudinal axis.

nal represented by 78.

  In a preferred embodiment of the invention, the permanent magnet 62 comprises

  take a magnet. 61 and two inserts, an upper insert magnet 63 and a lower insert magnet 64. The main magnet 61, the upper insert magnet 63 and the lower insert magnet 64 have directions of magnetization substantially parallel to the longitudinal axis

  78. The main permanent magnet 61 is in the form of an annular cylinder having an

  cylindrical element 83 substantially through its center, in which are housed the upper insert magnet 63 and the lower insert magnet 64. The main magnet 61 has a load

  substantially homogeneous magnet along the longitudinal axis 78. To have this

  feature, the main magnet 61 can be made of thin magnetic rings having

  each a different residual magnetization to approach a distribution of the

  substantially linear mantle from one end of the main magnet 61 to

i5 the other.

  The construction of the main magnet 61 is shown in more detail in FIG. 4, which is a side view of the main magnet 61. The main magnet 61 may be composed of a

  series of cylinders magnetized axially, generally represented by 61A to 61F. The directive

  magnetization of each cylinder 61A to 61F is indicated by an arrow on each

  cylinders 61A to 61F. A particular characteristic of the axial magnetized cylinders

  61A to 61F is that the magnetization of each cylinder 61A to 61F is proportional in amplitude at its axial distance from a central plane 61P of the magnet 61, and the magnetization

  is directed to the central plane 61P. The central plane 61P is perpendicular to the longitudinal axis

  dinal 78 and divides the main magnet 61 into two substantially equal length sections.

  For example, the uppermost cylinder 61A is shown to have a strong magnetism.

  directed downwards towards the central plane 61P. In opposite correspondence is the lowest cylinder 61F which has a magnetization of sensible intensity equal to that of the highest cylinder 61A but its magnetization is directed upwards in the direction of the

  central plan 61P. Successively, smaller pairs of magnetized counter-cylinders

  such as 61B / 61E and 61C / 61D are arranged successively in a more

central plan 61P.

  The upper magnet insert 63 and the lower magnet insert 64 have directions

  parallel to the longitudinal axis 78 and are used in combination with the

  main mantle 61 for the synthesis of the preferred form of a static magnetic field.

  Fig. 5 shows a drawing including preferred dimensions of the main magnet 61

  and magnetic inserts 63, 64 for generating the preferred static magnetic field. The di-

  magnetization rections of the main magnet 61, the vertex magnet insert 63 and the magnetic bottom insert 64 are indicated by arrows in FIG. 5. These particular magnet dimensions are a matter of convenience for the designer of the system and should not be construed as a limitation to the invention. The essential characteristics of the static magnetic field realized by the preceding dimensions will be explained

later in detail.

  Referring now to FIG. 6, the resulting static magnetic field

  by the magnet 62 in the sensitive zone 58 is directed substantially radially outwards.

  from the longitudinal axis 78. The static magnetic field generated by the magnet 62 is also substantially symmetrical with respect to the longitudinal axis 78, and is substantially perpendicular to the longitudinal axis 78, and at lower radial distances at the axial length of the magnet 62, and decreases in amplitude only as the inverse of the distance

radial of the magnet 62.

  The permanent magnetic material of the permanent magnet 62 should be substantially

  radiofrequency transparent, so that an antenna used to generate a radiofrequency magnetic field can be housed inside the recess 83 in the main magnet 61, as will be explained later A type of transparent magnet In radio frequency (RF) can be realized from a ferrite magnet material such as that sold under the brand name "Spinalor" and manufactured by Ugimag, 405 Elm St., Valparaiso, IN, or other material of Ferrite magnet sold under the tradename "Permadure" and manufactured by Philips, 230 Duffy Ave., Nicksville, NY. These materials are provided by way of example only and are not intended to limit the choice of the materials of the magnet 62.

  It is sufficient that the magnet 62 is only substantially transparent to the magnetic field.

  RF genetics at the chosen frequency.

  Referring again to FIG. 2, the NMR probe 42 further comprises one year

  Transmitter head 67, which may comprise one or more windings of coils 66

  preferably disposed within the recess 83 in the main magnet 61.

  Coil bearings 66 are preferably arranged so that each coil winding is in a plane substantially perpendicular to the longitudinal axis 78.

  radiofrequency alternating current passing through the windings of coils 66 genera-

  generates an RF magnetic field in the geological formation (26 in Figure 1). The RF magnetic field generated by the current in the coil windings 66 has field directions substantially parallel to the longitudinal axis 78 within the sensitive volume 58. The coil windings 66 should have an overall length along the length of the coil. longitudinal axis 78 which is approximately equal to the diameter of the sensitive volume 58. The overall length of the windings of coils 66 parallel to the longitudinal axis 78 should also be substantially shorter than the overall length of the permanent magnet 62 along the 'axis

  longitudinal 78, as will be explained later.

  Preferably, the coil windings 66 are formed around a soft ferrite rod 68. The soft ferrite rod 68 may be formed of a material such as that sold under the designation "F6" and manufactured by MMG North America, 126 Pennsylvania Avee., Paterson, NJ, or another material sold under the trademark "3C2" and manufactured by Philips, 230 Duffy Ave., Nicksville, NY Ferrite rod 68 is preferably positioned parallel a, the longitudinal axis 78. The overall length of the ferrite rod 68 along the longitudinal axis 78 should be substantially less than the length of the permanent magnet 62 along the longitudinal axis 78. Alternatively

  a plurality of coils and a plurality of ferrite rods may be employed. The in-

  Coil windings 66 and soft ferrite rod 68 will be referred to herein as transmit antenna 67. Ferrite rod 68 has the particular function of increasing the intensity of the RF magnetic field generated by the transmitting antenna 67. 'the use of

  the ferrite rod 68'-particularly allows the transmitting antenna 67 to have a diameter of

  outer relatively small so that it can be housed inside the recess 83.

  Having a small outer diameter particularly allows the transmitting antenna 67 of the invention to be of a size such that the instrument of the present invention can be

  used in smaller diameter boreholes.

  The permanent magnet 62, the transmitting antenna 67 and the receiving antenna 70 are preferably housed inside a non-ferromagnetic protective housing 43 transparent to RF radio frequency. Such boxes and additional components (not shown) to shelter from drilling mud under high hydrostatic pressure, are familiar

to the men of the art.

  2. Functional block diagram of the NMR dia.

  FIG. 3 illustrates, in general form, the NMR probe 42 and a dia-

  functional gram of the NMR well logging instrument. An adapter circuit

  The transmitter / receiver T / R 45 may be disposed inside the housing 43. The adaptive circuit

  T / R 45 typically comprises a series of resonance capacitors (not shown).

  sent separately), a transmitter / receiver switch (not shown separately) and both the "to transmitter" and "to receiver" adapter circuit. The T / R adapter circuit may be coupled to both the radio frequency (RF) power amplifier and a receiver preamplifier 73. Although shown as being located within the housing 43, the T / R adapter circuit 45 , the RF power amplifier 74 and the preamplifier

receiver 73 may alternatively be located outside the housing 43 in the

  bottom of the upper centralizer (56 in Figure 1) or inside the NMR electronic cartridge (44 in Figure 1). The locations of the 45 T / R adapter circuit, the RF power amplifier 74 and the receiver pre-amplifier 73 should not be interpreted

  as a limitation to the invention.

  Part of the control assembly of the NMR logging instrument is

  takes a background computer 92, which among other functions delivers to a programmer

  pulses 91 of the control signals. The computer 92 and the pulse programmer

  The pulse generator 91 controls the setting and operation of the frequency-variable RF signal source 93. The RF driver 94 receives an input from

  source 93 of frequency-variable RF signal and delivers an output to the power amplifier.

  74. The RF power amplifier 74 delivers a high power signal

  to control the transmitting antenna 67 to generate an RF magnetic field in the

  o0 sensible lume (58 in Figure 1). The RF power amplifier 74 can be electric-

  connected (typically by the switch in the T / R adapter circuit 45) to the antenna

  transmitter 67 during the transmission of RF power pulses.

  During the reception of the NMR signals, the transmitting antenna 67 may be electrically

  connected to the receiver pre-amplifier 73 by means of the switch of the S5 adapter T / R circuit 45. The output of the receiver pre-amplifier 73 is delivered to a receiver

  RF 89. The RF receiver 89 also receives a phase reference input of a phase shifter 98.

  The phase shifter 98 receives a primary phase reference input from the source 93 of frequency.

  RF variable. The RF receiver 89 may include quadrature detection. The RF receiver 89 provides an output to an AID converter. The output of the A / D converter 96 may be stored in a buffer 97 until it is required for use by the bottom computer 92. Alternatively, the contents of the buffer 97 may be driven

  directly to a bottom portion of the telemetry unit 99 for transmission to the

surface area (54 in Figure 1).

  The background computer 92 typically preprocesses the data from the buffer 97 and transfers the preprocessed data to the bottom portion of the telemetry system, shown

  in its generality by 99. The bottom part of the telemetry system 99 transmits the

  pre-processed at the telemetry unit (38 in FIG. 1) to the surface equipment (54 in FIG. 1). The telemetry unit 38 transfers the data to the surface computer (40 in FIG. 1) for calculation and presentation of the desired logging output data for

  subsequent use and analysis as understood by those skilled in the art.

  All the elements described here, with the exception of the transmitting antenna 67 and the

  seems to magnet 60, can at the convenience of the system designer be willing to

  inside the housing 43, the upper centering device (56 in FIG. 1) or the electronic cartridge

  NMR (44 in Figure I1). These same elements can alternatively be located on the surface of the ground, for example in the surface equipment 54 using the cable (30 at the

  Figure 1) for transmission of electrical power and signals to the trans-

meter 67.

  Figure 3 also illustrates the static magnetic field and the magnetic field.

  RF created by the NMR logging instrument of the present invention. The direction of the magnetization of the magnet 62 is preferably parallel to the longitudinal axis 78 The direction

  of the static magnetic field inside the sensitive volume 58 generated by the magnet

  Fig. 62 is substantially perpendicular to the longitudinal axis 78 as represented by arrows 110. The nuclear magnetic moments of the material to be analyzed (the geological formation located within the sensitive volume 58) are substantially aligned with the

  direction of the static magnetic field. In the preferred embodiment of the invention,

  the direction of a linearly polarized RF magnetic field, identified by the

  120 within the sensitive volume 58 is substantially perpendicular to the static magnetic field at any point within the sensitive volume 58.

  Magnetic field layout is conventional in NMR investigations.

  The direction of the static magnetic field is symmetrical around the longitudinal axis

  dinal 78, the amplitude of the static magnetic field is therefore also symmetric

  amplitude around the longitudinal axis 78. The static magnetic field has a

  amplitude gradient within the sensitive volume 58 which is also symmetrical to

  turn of the longitudinal axis 78 and is directed substantially radially inwards in di-

  of the longitudinal axis 78. As a result of these characteristics of the magnetic field,

  static, this is usually only a substantially cylindrical surface external to

  the permanent magnet 62 which comprises a particular amplitude of a static magnetic field

  (ignoring any end effect of the magnet 62) It follows from this characteristic that

  that particular of the static magnetic field that resonance signals are scattering

  from various materials such as drilling mud (34 in Figure 1) that are born

  outside the sensitive volume 58 do not seriously affect the RMNI measurements if

  2s appropriate RF frequencies are chosen.

  Undesired end effects of the static magnetic field may be sub-

  stantially eliminated by making the transmitting antenna 67 a little shorter along

  the axis 78 that the permanent magnet 62, so as not to excite the materials at the ends.

  longitudinal mits of the static magnetic field.

  When the RF power pulses are sent into the transmitting antenna 67, the antenna 67 generates an RF equivalent magnetic dipole 87 directed parallel to the longitudinal axis 78. The equivalent magnetic dipole 87 generates a linearly polarized RF magnetic field 120 of an amplitude substantially equal to the interior of the sensitive volume 58. Since the direction of the RF magnetic field is parallel to the longitudinal axis 78, 3s the global nuclear magnetization, indicated in FIG. 3 by arrows 130, in any point of the sensitive volume 58, turns in planes perpendicular to the longitudinal axis 78. The free precession of the nuclear magnetic moments, however is created around the direction of the static magnetic field at any point inside the volume

  sensitive 58, and the free precession is always located in planes parallel to the long axis.

  78. The free precession will therefore induce an RF signal in the trans-

  67. The magnetic moment induced in the transmitting antenna 67 is shown in FIG.

Figure 3 by the arrow 140.

  3. Design Parameters of the Preferred Embodiment

  In the preferred embodiment of the invention, it is sought to optimize the ratio

  signal / noise port of the NMR measurement method. The following discussion aims to explain how some key parameters affect the signal-to-noise ratio. The main parameters o0 typically comprise the geometry of the permanent magnet (62, FIG. 2) and the transmitting antenna (67, FIG. 2), the power of the radio frequency (RF) pulses.

  used to excite the transmitting antenna 67, and the quality factor of the transmitting antenna

meter 67.

  Using the transmitting antenna 67 constructed in the manner previously described,

  In the embodiment of the invention, the amplitude of an NMR signal, S, induced

  in the transmitting antenna 67 is typically related to the amplitude of an electromagnetic field.

  RF genetics, BI, by the Reciprocity Theorem and can be expressed by the following expression: S = o mAn, (BM /) 1 in which m and Asv respectively represent the nuclear magnetization and the transversal surface of the sensitive volume (58, FIG. 1), I represents the amplitude of the current flowing in the transmitting antenna 67, the oscillation frequency of the current is represented by A,

  and 1 represent the effective length of the transmitting antenna 67. To simplify the dis-

  cussion, m and BI are assumed to be substantially homogeneous within the sensory volume.

58.

  By replacing m = XBo / uo oc represents the nuclear magnetic susceptibility of the hydrogen nuclei within the sensitive volume 58, co = Bo o Bo represents the static magnetic field generated by the permanent magnet (62, Figure 2) and given by equation (1), it is thus possible to obtain for S the following expression: S = (Y {v XI 16 (E4 A.ó l (2) The NMR signal thus acquired is therefore directly proportional at the sensitive volume

  58 of the geological formation (26, Figure 1). The geometry of the sensitive volume 58 is

  terminated by the existence of a resonance condition. In pulsed NMR the condition of regeneration

  is typically satisfied when the deviation of the amplitude of the static magnetic field Bo (R) from its value Bo (Rv), corresponding to the center frequency of

  current exciting transmitting antenna 67 (Bo (R) = caj /), is less than half of the am-

  amplitude of the magnetic field RF B1 induced by current flow in the trans-

  methane 67, as expressed by equation (3):

(3)

B / (R) - B # (R) <B / 2 (3)

  The static m-agnetic field Bo (R) at the excitation radius Rsv can also be de-

  written in the form of a Taylor development such that: B, (R) = B (R1) (<Bo / R) (R - R) (4) o (eBo / cR) represents the static magnetic field gradient at the radius R = Rsv. From equation (3): BA (R) - Bo (R) <B (5) where Ro and Ri represent, respectively, the external and internal rays of the sensitive volume

  58. From a practical point of view Ro - Ri << Rexc.

  (6) A = 2, rR, B / (8B / dR) By substituting equation (6) in (2) we obtain S = 2z (rzX /) B, (B / I) RgIB 2 / (C8B / dR) (As understood by those skilled in the art, RMS may be expressed by the expression: (s) N, = (4 TAfr) 1/2

  o ff represents the receiver bandwidth. Bandwidth is typically from

  viron B1 / z2nz for a suitable receiver; k represents the Boltzmann constant; and T represents

feel the absolute temperature.

  Hence for S / N: S / N = 12 y (rx / JB, (B / I) R, (kT fr) -1x 1B92 / (B / oR) (9) The first expression in square brackets in equation (9), for a given proton spin density and absolute temperature, depends only on the parameters of the transmitting antenna 67. The second expression in square brackets of equation (9) incorporates

  parameters used in the design of the permanent magnet (62, Figure 2) as

will post later.

  Another parameter affecting the design of the permanent magnet is the degree of homogeneity of the static field in the direction of movement of the NMR instrument

  as will be explained later.

  4. Synthesis of a static magnetic field It flows from (10) for a length! opening of the antenna, and a radius RSv of the volume

  sensitivities determined by the need for vertical resolution and deep penetration

  that the B0o2 report (Bo, 'R) should be maximized in order to provide the proper

  noise / maximum noise. For the elongated magnet with a distributed homogeneous magnetic charge (linear magnetization distribution inside the main magnet 61, Fig. 4) B. can be calculated according to: (10) Bo = q, / 2 rR oq is the magnetic charge per unit length of the magnet. By definition the magnetic charge density is given by the equation p - -pl0hvAN. Therefore, in our case we have q = pffRm = R (21rm / 7rRm, where Rm is the magnet radius, Brm is the maximum remanence of the magnetic material

o used ..

  The transfer of (11) in (10) and Bo and (DBO / DR) in (9) gives for the part of S / N linked to the permanent magnet, the expression: SIN o BmR, 21I (12)

  It is clear from (12) that for any vertical selective resolution value

  For the instrument, the S / N ratio is inversely proportional to lm. For now

  Since the length of the magnet 62 is as small as possible, it is important that the static magnetic field be substantially perpendicular and homogeneous over a fraction as small as possible.

  as much as possible of the axial length of the magnet 62. The length of the magnetic field

  perpendicular homogeneous static should exceed the opening length of the

  to ensure a stable state of the nuclear magnetization measurement even when the NMR probe 42 is moved along the wellbore (22, Fig. 1). In addition to these requirements, the static magnetic field should be minimized in the area where the transmitting antenna 67 is placed. A preferred embodiment of the permanent magnet (62 in

  FIG. 2) is shown in FIG. 5. The main magnet 61 consists of magnetic rings

  61A to 61F each having a different residual magnetization so as to approach a substantially linear magnetization distribution from one end to the other of the magnet 61. In addition, the upper magnet insert 63 and the lower magnet insert 64 serve to optimize the static field according to the criteria above, namely an important

  degree of homogeneity of the static field in the direction of the longitudinal axis 78 to

  performing NMR measurements while moving along the wellbore (22 in Figure 1), and a residual low static magnetic field at the transmitting antenna 67 so as to

  that the ferrite rod 68 remains substantially unsaturated.

  Figure 6 shows a graphical representation of the static magnetic field at

  inside the investigation volume (58, figure 2). Figure 7 shows the magnetic field

  residual longitudinal inside the recess (83, Figure 2) at the location of

the transmitting antenna 67.

  The dimensions represented by the magnet 62 in FIG. 5 are the following: L0o = 1l.2

  m; Lh = 0.64 m; Do = 0.07 m; Dh = 0.03 m. The magnet shown in FIG. 5 is particularly

  adapted to a transmitter antenna 67 with a length of 30 cm and at a sensing volume

  diameter of 24 cm (58 in Figure 2). The main magnet 61 should be transparent to the RF magnetic field emitted by the transmitting antenna 67. Since the main magnet 61 lo only needs a relatively low residual magnetization, this portion of the magnet 62 can be formed from ferritic permanent magnet material or the like, which is substantially non-conductive and radiofrequency-transparent. The end magnets 63, 64 are preferably made of a magnetic material with a high remanence such as sintered oriented cobalt-samarium or neodymium-iron-boron having a remanent magnetization of

  I T or more. The magnetic field shown in FIG. 6 assumes that the magnets of

  mity have a remanent magnetization of 0.7 T for the 63, 64 and 0.42 end magnets

T for the main magnet 61.

  The static magnetic field within the sensitive volume 58 has a substantially equal amplitude during NMR excitation. As explained for equation (3),

  the amplitude of the static magnetic field inside the sensitive volume 58 only varies

  in a narrow range from Bo-B1 / 2 to Bo + B1 / 2. It is of great importance to know how the spatial variation of this field takes place in the direction of movement of the logging instrument. The speed of this variation corresponds to the static magnetic field amplitude gradient in the direction of displacement. The gradient distribution

  2s of static magnetic field amplitude inside the sensitive volume 58 is explained.

  schematically in FIG. 8. Two lines 58L and 58M represent lines of equal amplitude of the static magnetic field, with respectively a first and a second amplitude. These amplitudes are of the order of Bo-BI / 2 to Bo + B / 2. Gradients

  amplitude of the static magnetic field at points 58A and 58D of FIG.

  proportional to the distance between two points along a direction of movement.

  81 for example, the gradient component along the direction 81 at point 58A in the central portion of the sensitive volume 58 is inversely proportional to the longitudinal axis 78, one through line 58L and the other through line 58M. at the distance between points 58A and 58C. The gradient component along direction 81 at point 58D at the top of

  Sensitive volume 58 is inversely proportional to the distance between points 58E and 58D.

  It should be apparent from Figure 8 that the gradient at the central portion of the volume

  sensitive 58 is much smaller than the gradient at its ends. The grading component

  in the direction perpendicular to the longitudinal axis 78 at point 58A in the

  center of the sensitive volume 58 is inversely proportional to the distance between the

  points 58B and 58A. The largest component of the amplitude gradient of the magnetic field

  static genetics, is in the radial direction.

  The sensitive volume 58 is determined by the RF magnetic field. In order to obtain undistorted NMR signals, no point within the sensitive volume should leave sensible volume during the duration of a measurement sequence (a complete CPMG echo train). If the movement of the instrument is such that no point can leave the

  volume during a measurement sequence, subsequent pulses of repetition

  at 180 in a Carr-Purcell echo train ("CPMG") can be applied to o parts of the geological formation that previously had not been polarized

  transverse by the initial pulse at 90. The distance, As, along a direction of movement

  N, within the sensitive volume 58 at the edge of the sensitive volume 58 can be expressed by the expression: s (N) = [B (n) B, (B) G (13) o Bo (N) represents the static magnetic field amplitude at point N inside, from

  sensitive volume 58, Bo (B) represents the static magnetic field amplitude at

  Sense of the sensitive volume 58 and G represents the static magnetic field gradient in the direction of displacement. The total movement, or displacement during a time interval, t, of the well logging instrument should be less than AS (N). More specifically: v xt <lsX (14) v x t <As (V) (4 in which v represents the speed of movement of the instrument of

logging. Of the-

  The total placement of the instrument should not represent a substantial part of the

  total lume. The inequality that should therefore be satisfied can be written as follows

B35) - 0 (B) - B ()

  IBo (N) - / M- "<< A reasonable estimate of the maximum gradient in the direction of travel can be calculated by: G <(01 BO./(v xl) (l) For practical values of B1 of the order of 2 X 10-4 and v of about 0.05 m / sec, for a

  200 milliseconds for a measurement sequence, G should be less than

  1O ron 2 X 10'3 T / m (equivalent to about 0.2 Gauss / cm). This value was used as

  a constraint in the procedure of optimizing the shape of the static magnetic field.

  It is usual for a logging tool velocity perpendicular to the well

  of drilling to be about 50 times smaller than the logging speed. This requires

  the static magnetic field in the direction perpendicular to the longitudinal axis 78 is less than 0.1 T / m. The preferable geometry of the magnet shown in FIG. 5

  corresponds to a radial static magnetic field gradient of 0.05 T / m (5 Gauss / cm).

  Other values of the radial amplitude gradient can be chosen, depending on the frequency

selected NMR excitation.

  The requirements for the radial static magnetic field gradient are also

  fected by the presence of the Earth's magnetic field He. The Earth's magnetic field is substantially homogeneous and has an amplitude of approximately 0.5 x 10-4 T. The orientation of the logging instrument with respect to the direction of the Earth's magnetic field depends on the geographic location of the well and the deviation of the drilling. This variation of the field should not substantially change the radius Rsv of the sensitive volume (58, in Figure 1). The amplitude of any change in RsV can be expressed by the ratio

  He / G. Therefore, the radial static magnetic field amplitude gradient neces-

  G should satisfy the inequality He / G << Rsv; or else G >> HeRsv 'For Rsv- = 0. I m G should be much larger than about 5 x 10 -4 T / m. In practice the radial gradient

  (2 x 10-2 T / m) of the magnet shown in Figure 6 exceeds this requirement.

  Those skilled in the art will readily realize that the scope of the invention is not limited

  the described forms of realization, but that it can be limited only by the claims

following statements.

Claims (14)

  A method for measuring the nuclear magnetic resonance properties of the geological information traversed by a wellbore, comprising the steps of:   polarize the nuclei in the so-called geological formations along a magnetic field   static geometry substantially perpendicular to and symmetrical about an axis of said well, said static magnetic field having an amplitude gradient parallel to said axis, connected to the inverse of the speed of movement of said logging tool along the well; polarizing said cores by applying a radiofrequency magnetic field substantially orthogonal to said static magnetic field; and   to capture nuclear magnetic resonance signals from said nuclei cross-referenced.   2. Method according to claim 1, wherein said gradient parallel to said axis is less than 2 x 10-3 T / m corresponding to a displacement speed of about 10 feet (3.04 m per minute.   3. The method of claim 1, wherein said static magnetic field is   that has a radial amplitude gradient connected to the inverse of an expected velocity of motion   radial of said logging instrument.   4. Method according to claim 3 wherein said radial gradient is lower at O.l T / m.   The method of claim 3, wherein said gradient is greater than about 2 x 10-2 T / m in order to minimize the effects of the Earth's magnetic field on   measurements of said nuclear magnetic resonance properties.   6. Nuclear magnetic resonance detection instrument for implementing a method according to claim 1, comprising: a magnet for inducing a static magnetic field in analyzed materials, said magnetic field being substantially perpendicularly polarized to a longitudinal axis of said magnet and substantially symmetrically about said axis   said magnetic field having a maximum longitudinal gradient connected to the   a speed of movement of said instrument along said longitudinal axis   to the said materials to be analyzed.   a transmitter for generating a radiofrequency magnetic field in the   said materials for exciting nuclei thereof, said magnetic field in radiofrequency   quence being substantially orthogonal to said static magnetic field; and a receiver for detecting the nuclear magnetic resonance signals from said excited nuclei in said materials. The instrument of claim 6, wherein said longitudinal gradient is less than 2 X 10-3 T / m corresponding to a movement speed of about 10 feet. (3.04 m).   The instrument of claim 6, wherein said static magnetic field is   radial gradient connected to the inverse of an expected velocity of the radial motion.   dial of said instrument through said materials.   The instrument of claim 8, wherein said radial gradient is less than around 0.1 T / m.   The instrument of claim 9, wherein said radial gradient is greater than   2 X 10-2 T / m in order to minimize the effects of the Earth's magnetic field on   measures made by the said instrument.   An instrument according to claim 6 wherein said transmitter comprises an antenna having a longitudinal opening shorter than a length of said magnet.   along the said direction of movement of the said instrument, whereby the nuclei are ex-   cited by said radiofrequency magnetic field while said nuclei are sensi-   in equilibrium with the said static magnetic field.   The instrument of claim 6, wherein said static magnetic field   that is substantially parallel to said longitudinal axis for a maximum fraction of one   length of said magnet along said longitudinal axis.   13. The instrument of claim 12, wherein said magnet comprises: magnetized cylinders stacked along said longitudinal axis, a magnetization of each of said cylinders proportional to its distance from a central plane of said magnet, said cylinders being magnetized. parallel to said longitudinal axis and in the direction of said central plane; and an end magnet disposed at each longitudinal end of said stack of cylinders, said end magnets each being magnetized parallel to said axis;   longitudinal direction and in a direction opposite to said magnetization of one of said cylinders ad-   said static magnetic field has a maximum longitudinal gradient and is substantially parallel to said longitudinal axis for said maximum fraction of the said length of said magnet.   The instrument of claim 13, wherein said cylinders comprise   rings, being formed in this way a central recess within said magnet, and wherein at least one of said transmitter and said receiver comprises a   antenna disposed in the recess.