MXPA99001993A - Nuclear magnetic resonance apparatus and method for generating an axisimetric field with right hipsometric curves in the resonan region - Google Patents

Abstract

The present invention is directed towards a nuclear magnetic resonance apparatus and a method for generating an axisymmetric magnetic field having long and straight hypsometric curves in the resonance region. A magnetically permeable member is used to form an electrostatic magnetic field generated by a set of permanent magnets. The magnetically permeable member minimizes the variations of the electrostatic magnetic field in the formation, due to the vertical displacement of the apparatus while obtaining a measurement by nuclear magnetic resonance. In addition, the magnetically permeable member can minimize the variations of the electrostatic magnetic field of the formation, due to the lateral displacement of the apparatus while obtaining a measurement by magnetic resonance

Description

NUCLEAR MAGNETIC RESONANCE APPARATUS AND METHOD FOR GENERATING AN AXISIMETRIC FIELD WITH RIGHT HIPSOMETRIC CURVES IN THE REGION OF RESONANCE DATA OF THE INVENTION The present invention is generally related to an apparatus and method for measuring properties, by nuclear magnetic resonance, of a geological formation traversed by a borehole and, more specifically, with an apparatus and method for generating a highly axisymmetric electrostatic magnetic field with curves. hypsometric long and straight in the resonance region. It is a widely accepted concept that the particles of a geological formation with a magnetic moment of the non-zero nuclear spin, for example protons, tend to align with an electrostatic magnetic field imposed in the formation. Such a magnetic field can be generated naturally, as in the case of the earth's magnetic field, BE. After a radiofrequency (RF) pulse applies a second oscillating magnetic field Bi, transverse to BE, the protons tend to precess around the vector BE with a characteristic resonance or Larmol ?L frequency that depends on the strength of the electrostatic magnetic field and the gyromagnetic coefficient of the particle. The hydrogen nuclei (protons) that precess around a magnetic field BE of 0.5 gaussians, for example, have a characteristic frequency of approximately 2kHz. If a population of hydrogen nuclei were forced to effect a precession in phase, the combined magnetic fields of the protons could generate a detectable oscillating voltage, known to those skilled in the art as a free induction degradation or a spin echo, in a receiver coil. Hydrogen cores in water and hydrocarbons that occur in the pores of rocks produce nuclear magnetic resonance (NMR) signals other than signals from other solids. U.S. Pat. Numbers 4,717,878, issued to Taicher et al., And 5,055,787, issued to Kleinberg et al., Describe NMR instruments that use permanent magnets to polarize hydrogen nuclei and generate an electrostatic magnetic field, B0, and RF antennas. to excite and detect nuclear magnetic resonance to determine the porosity, proportion of free fluid and permeability of a formation. The atomic nuclei are aligned with the applied field, B0, with a time constant of T_. After a period of polarization, the angle between the nuclear magnetization and the applied field can be changed by applying an RF field, Bl? perpendicular to the static field B0, in the Larmor frequency fL =? B0 / 2p, where? is the gyromagnetic coefficient of the proton and B0 desgna the strength of the electrostatic magnetic field. After the termination of the RF pulse, the protons begin to precess in the plane perpendicular to B0. A sequence of refocusing RF pulses generates a sequence of spin echoes that produce a detectable NMR signal on the antenna. U.S. Pat. Number 5,557,201 describes a pulsed nuclear magnetism instrument for evaluating a formation during drilling. The instrument includes a drill bit, a sounding column and a pulsed nuclear magnetic resonance device, housed inside a drill collar, made of a non-magnetic alloy. The instrument includes a channel, within the sounding column and the pulsed NMR device, through which the drilling mud is pumped into the borehole. The pulsed NMR device comprises two tubular magnets, mounted with similar poles, facing one another, surrounding the channel, and an antenna coil mounted on an external surface of the bore column between the magnets. This instrument has been designed to resonate nuclei in a measurement region known to those skilled in the art as the equilibrium point. The Great Britain Patent with the Application Number 2,310,500, published on August 27, 1997, discloses a measurement instrument during drilling that includes a detector apparatus to make measurements by nuclear magnetic resonance of the geological formation. The NMR detector apparatus is mounted in an annular recess formed in the outer surface of the drill collar. In a physical representation, a flow plug is inserted into the recess. A magnet is installed on the external radial surface of the flow plug. The magnet is constructed with a plurality of radial segments that are magnetized radially outwardly from the longitudinal axis of the instrument. The flow shutter is required to provide an adequate directional orientation of the magnetic field. The instruments developed in previous projects have disadvantages that limit their usefulness in applications of nuclear magnetic resonance imaging. The designs of the magnets of the previous instruments do not simultaneously produce a highly axisymmetric electrostatic magnetic field with long and straight hypsometric curves in the resonance region of the formation being evaluated. These factors adversely affect NMR measurement due to the vertical displacement of an instrument in the drill string and the vertical and lateral displacement of a graphing instrument during drilling.

COMPENDIUM OF THE INVENTION The disadvantages of the previous instruments mentioned above are overcome by means of the present invention, which consists of an apparatus and method for generating an electrostatic magnetic field, considerably axisymmetric, with long and straight hypsometric curves in the resonance region. A drilling cable or logging apparatus during drilling, inside a borehole that traverses a geological formation, determines a characteristic of the formation obtaining a measurement by nuclear magnetic resonance. The apparatus produces such an electrostatic magnetic field, B0, in the formation, that the hypsometric curves generated by the electrostatic magnetic field are considerably straight in the axial direction, at the depth of investigation where the measurement by nuclear magnetic resonance is obtained. To obtain the NMR measurement, an oscillating field, Bl r, is produced in the same region of the formation as the electrostatic magnetic field. The apparatus includes at least one magnetically permeable member to focus the electrostatic magnetic field. The magnetically permeable member minimizes the variations of the electrostatic magnetic field in the formation, due to the vertical displacement of the apparatus while obtaining the measurement by nuclear magnetic resonance. Likewise, the magnetically permeable member can minimize the variations of the electrostatic magnetic field in the formation due to the lateral displacement of the apparatus while obtaining measurements by nuclear magnetic resonance. In addition, the magnetically permeable member can add a significant pre-polarization causing the B0 field to have a considerable magnitude well before the actual research region, which may allow a higher recording speed. The electrostatic magnetic field is produced using a magnet with an axial, radial or coil design. For the axial design, the electrostatic magnetic field is produced by an upper magnet around the transport medium and a lower magnet around the transport medium, separated axially from the upper magnet by a distance such that the hypsometric curves, generated by the electrostatic magnetic field , they are considerably straight in the axial direction, to the depth of investigation where the measurement by nuclear magnetic resonance is obtained. The magnets are axially magnetized producing a radially polarized field B0 in the research region. Between the lower magnet and the upper magnet there is at least one magnetically permeable member to form the electrostatic magnetic field. The electrostatic magnetic field has both a low gradient and a high gradient, depending on the separation of the magnets, at the depth of investigation where the measurement by nuclear magnetic resonance is obtained. For radial design, the electrostatic magnetic field is produced by a set of annular cylindrical magnets around the transport medium. The set of magnets comprises a plurality of segments, each segment being magnetized in a radially outward direction from, and perpendicular to, the longitudinal axis of the apparatus. The magnetically permeable member comprises a section of the conveying means, a chassis around a section of the conveying means or a combination of the chassis and the section of the conveying means. For the coil design, the electrostatic magnetic field is produced by a plurality of magnetic rings located geometrically and axisymétrically around the transport means. The plurality of rings comprises an upper ring, a plurality of inner rings and a lower ring. The radius of the upper and lower rings is greater than the radius of each inner ring. Each plurality of rings is axisymmetrically polarized and the direction of polarization for each ring differs progressively along the ring of magnets. The polarization direction of the upper ring is radially opposite to the polarization direction of the lower ring. The polarization of each inner ring changes progressively so that an angle between the polarization and a transverse vector radius varies linearly for each inner ring.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages of the present invention will be apparent in the following description of the accompanying drawings. It is understood that the illustrations should be used for a merely illustrative purpose and not as a definition of the invention. In the illustrations: Figure 1 illustrates a graphing instrument during nuclear magnetic resonance drilling; Figure 2 represents the design of a low gradient magnet; The Figures 2a-2d illustrate the hypsometric curves I Bol corresponding to four configurations of low gradient magnets; Figures 3a-3d represent the hypsometric curves of the I VBJ gradient corresponding to four configurations of low gradient magnets; Figure 4 represents the design of a high gradient magnet; Figure 4a represents the hypsometric curves | Bol corresponding to the configuration of a high gradient magnet; Figure 4b represents the hypsometric curves of the gradient I VBol corresponding to the configuration of a high gradient magnet; Figure 5 represents the design of a coil magnet; Figure 5a represents the hypsometric curves I Bol corresponding to the configuration of a coil magnet with a non-magnetically permeable member; Figure b represents the hypsometric curves I Bol corresponding to the configuration of a coil magnet with a magnetically permeable member; Figure 6 represents the design of a radial magnet; Figure 6a represents the hypsometric curves I Bol corresponding to the configuration of a radial magnet with a non-magnetically permeable member; Y Figure 6b represents the hypsometric curves I Bol corresponding to the configuration of a radial magnet with a magnetically permeable member; Figure 7 depicts a combined magnet assembly using three magnets; Y, Figure 7a represents the hypsometric curves 1 Bol corresponding to a set of combined magnets of low gradient-low gradient.

DETAILED DESCRIPTION OF PREFERRED PHYSICAL REPRESENTATION Referring to Figure 1, a graphing instrument is illustrated during nuclear magnetic resonance (NMR) drilling 10. The instrument 10 includes a drill 12, a probe column 14, a set of magnets 16, an RF antenna 18 and electronic circuits 20 housed inside the drill collar 22. A means for drilling a borehole 24 in the formation comprises a drill 12 and a drill collar 22. The sludge flow sleeve 28 defines a channel 30 for carrying the fluid perforation through the sounding column 14. A driving mechanism 26 rotates the drill 12 and the sounding column 14. This driving mechanism is suitably described in U.S. Patent Number 4,949,045 issued to Clark et al. ., the description of which is incorporated by reference in this specification. However, the current invention also projects the use of a mud motor, at the bottom of the bore, installed in the column being drilled, as drive mechanism 26.

The magnetic field generated by the set of magnets 16 is focused, as a minimum, by a magnetically permeable member 36 installed inside the drill collar. With this arrangement, the member 36 can extend a considerable distance, in the axial direction, without reducing the force of the mechanism of the drill collar 22. Furthermore, if the member 36 consists of a mechanically weak material, a mud flow sleeve 28, separate, provides a degree of protection against pressure, cuts and abrasion of drilling mud.

The placement of member 36 outside the drill collar 22 would significantly weaken the mechanical integrity of the instrument, since that arrangement would involve cutting a recessed area from the outside of the drill collar to accommodate the member 36 thereby weakening the collar 22 because the section of the drill collar, between the channel 30 and the recess, has a smaller thickness compared to other sections of the drill collar.

In this invention it is projected that the magnetically permeable member 36 comprises a segment 38 of the sleeve 28. In this case, an additional layer of space for the member 36 inside the drill collar is not required and the available space is sufficient to accommodate a set of magnets of greater volume.

LOW GRADIENT DESIGN Referring to Figure 2 in a preferred physical representation of the invention, hereinafter referred to as a low gradient design, the set of magnets 16 comprises an upper magnet 32 axially spaced from a lower magnet 34. The area between the magnets 32, 34 It is suitable for housing elements such as electronic components, an RF antenna and other similar elements. Both magnets 32, 34 surround the sleeve 28. A magnetically permeable member 36 is located within the piercing collar 22 between the magnets 32, 34. The member 36 may consist of a single piece or a plurality of combined sections between the magnets. The member 36 is made of a magnetically permeable material, such as ferrite, permeable steel or other alloy of iron and nickel, permeable stainless steel or permeable steel, which has a structural function in the design of the member, such as stainless steel. -5 Ph. The magnetically permeable member 36 focuses the magnetic field and can both transport the drilling fluid through the bore column as well as provide structural support to the drill collar. In addition, the member 36 improves the shape of the electrostatic magnetic field generated by the magnets 32, 34 and minimizes the variations of the electrostatic magnetic field due to the vertical and lateral displacement of the instrument during the period of acquisition of the NMR signal. The segment 38 of the sleeve 28 between the magnets 32, 34 may comprise a magnetically permeable member 36. In this case, the segments 40, 42 of the sleeve 28 under the magnets 32, 34 will consist of a non-magnetic member. Alternatively, a magnetically permeable chassis 44 around the segment 38 defines the member 36. In this case, the segment 38 may consist of a magnetic or non-magnetic material. This invention is intended to integrate the chassis 44 and the segment 38 to form the member 36.

The magnets 32, 34 are biased in a direction parallel to the longitudinal axis of the instrument 10 with similar magnetic poles, facing each other. For each magnet 32, 34, the magnetic induction lines move outward, from one end of the magnet 32, 34, into the formation, to create an electrostatic field parallel to the axis of the instrument 10 and move inwardly, another end of the magnet 32, 34. In the region between the upper magnet 32 and the lower magnet 34, the magnetic induction lines move from the center outward, into the formation, creating an electrostatic field in the direction perpendicular to the axis of the instrument 10. The induction magnetic lines then move inwardly, symmetrically above the upper magnet 32 and below the lower magnet 34 and converge in the longitudinal direction inside the sleeve 28. Due to the separation, the magnitude of the field Electrostatic magnet in the central region, between the upper magnet 32 and the lower magnet 34, is relatively homogeneous. The amount of separation between the magnets 32, 34 is determined by selecting the required strength and homogeneity characteristics of the magnetic field. As the separation between the magnets 32, 34 decreases, the magnetic field becomes stronger and less homogeneous. Conversely, as the separation between the magnets 32, 34 increases, the magnetic field becomes weaker and more homogeneous. Figures 2a-2d illustrate the hypsometric curves I Bol corresponding to four configurations of upper magnets 32 and lower 34. The configuration corresponding to Figure 2a comprises a non-magnetically permeable member separating an upper magnet 32 and a lower magnet 34 a distance of 25 inches (63, 5cm). The configuration corresponding to Figure 2b comprises a non-magnetically permeable member that separates an upper magnet 32 and a lower magnet 34 a distance of 18 inches (45.7cm). The configuration corresponding to Figure 2c comprises a non-magnetically permeable member separating an upper magnet 32 and a lower magnet 34 a distance of eight inches (20.3cm). The low gradient design, corresponding to Figure 2d, comprises a magnetically permeable member 36 that separates an upper magnet 32 and a lower magnet 34 a distance of 25 inches (63.5cm). Figures 3a-3d represent hyposometric curves of the I VBJ gradient corresponding respectively to the configurations illustrated in Figures 2a-2d.

In the low gradient design, a significant portion of the magnetic flux is deflected by the magnetically permeable member 36 to the center of the instrument 10. To illustrate, the magnitude of the B0 field, shown in Figure 2d at a distance of about seven inches (17 , 8cm), radially from the longitudinal axis of the one shown in Figure 2a, which was generated by the same configuration of magnets separated by a non-magnetically permeable member. In addition, the low gradient design produces a longer and more uniform extension of the electrostatic magnetic field in the axial direction. The NMR signal measured in this physical representation is considerably less sensitive to the vertical displacement of the instrument. Referring to Figure 3d, with the low gradient design, a relatively small gradient, approximately 3 Gaussians / cm, is measured at a distance of approximately seven inches (17.8 cm) radially from the longitudinal axis of the instrument. This low gradient results in a measurement of the NMR signal that is considerably less sensitive to the lateral displacement of the instrument 10. Further, with the low gradient design, the region of the probe well, rich in protons, around the instrument. , it will resonate only at frequencies higher than those applied to the research volume, that is, there is no signal from the sounding well. This is a characteristic of all the physical representations of this invention. Other NMR-sensitive nuclei found in drilling mud, such as sodium-23, resonate to electrostatic magnetic field forces much higher than hydrogen forces when excited at the same RF frequency. These higher field strengths are not produced in the borehole region around the instrument or near the antenna where such unwanted signals can be detected. This is a characteristic of the designs of the axial magnets of this invention, including the high gradient design.

HIGH GRADIENT DESIGN As described above, with the low gradient design, a significant portion of magnetic flux is deflected by the magnetically permeable member 36 to the center of the instrument 10. Without the deflection of the magnetically permeable member 36, a high gradient design is achieved by separating the upper magnet 32 and lower 34 to obtain the same I BJ illustrated in Figure 2d. As shown in Figure 2b, a magnetic field strength of 60 Gaussians is achieved, at a distance of approximately seven inches (17.8cm) radially from the longitudinal axis of the instrument 10, by a non-magnetic permeable member separating the magnets. 32, 34 a distance of 18 inches (45.7cm). Without embedding, the shape of the research volume where the strength of the electrostatic magnetic field is in resonance with the RF frequency remains curved, and the hypsometric curves of the field are relatively short in the axial direction. In addition, the receiver that detects the NMR signal is sensitive to the sounding well signal, as indicated by the two separate regions of the magnetic field shown in Figure 2b. For a high gradient design using a non-magnetically permeable member, the curved shape of the research volume and the sounding hole signal are remedied by decreasing the separation between the magnets 32 and 34. As illustrated in Figure 2c, if it is discharged the separation of the magnets to eight inches (20.3cm) approximately, the hypsometric curves of the strength of the electrostatic magnetic field become straighter and the force of I Bol increases. However, the gradient I VB * ol becomes larger, as illustrated in Figure 3c, at a distance of approximately seven inches (17.8 cm) radially from the longitudinal axis of the instrument. The hypsometric curves of I VBol are curved, denoting the variation of the gradient in the axial direction. With reference to Figure 4, in the high gradient design is improved by inserting a magnetically permeable member 36 between the magnets 32, 34. Figure 4a represents hypsometric curves of I Bol corresponding to a configuration where the magnetically permeable member 36 separates the upper 32 and lower magnets 34 a distance of eight inches (20.3cm). The hypsometric curves of Figure 4a show less curvature in the axial direction than the hypsometric curves of Figure 2c. Also, as illustrated in Figure 4b, the magnetically permeable member 36 produces a more constant gradient I VBol in the axial direction.

COIL DESIGN With reference to Figure 5, in a second physical representation of the invention, hereinafter referred to as a coil design, the set of magnets 16 comprises a set of magnetically and magnetically axisymmetric magnets 40 around the sleeve 28. Preferably, the sleeve 28 is made of a suitable material, magnetically permeable, such as ferrite, permeable steel or other alloy of iron and nickel, permeable stainless steel or permeable steel, which has a structural function in the design of the member, such as in stainless steel 15- Ph. However, in the present invention it is projected to have a non-magnetically permeable sleeve. The set of magnets 40 comprises a ring of magnets 43, 44, 45, 46, 47 and 48. The radius of the highest ring 47 and that of the lower ring 48 is greater than that of the plurality of rings 43, 44, 45, 46 which defines a central assembly 42. The area between the rings 47 and 48 can accommodate a deep RF antenna installed in the drill collar 22. With the coil design, each ring of the assembly 40 is axisymmetrically polarized, but the polarization directions differ progressively along the assembly 40. The polarizations of the Higher ring 47 and lower ring 48 are oriented such that their respective extension lines intersect in the NMR area of the formation. Accordingly, the magnetization orientations of the rings 47 and 48 are radially opposed to each other. For example, Figure 5 illustrates the orientation of the ring 47 directed outward, into the formation, and the orientation of the ring 48 directed inwardly. Progressing from the highest ring 47, the polarization of each ring 43, 44, 45, 46 is tilted and changes progressively such that the angle between the polarization and the transverse vector radius varies linearly for each ring in the central assembly 42 .

With the coil design, the passage of the magnetic induction lines is shifted outward, away from the upper ring 47, into the formation, to create an electrostatic magnetic field parallel to the axis of the borehole in the center of the instrument. and moves inward toward the lower ring 48. Referring to Figure 5b, the magnet configuration shown in Figure 5, used in conjunction with a magnetically permeable sleeve 28, produces a more uniform and longer electrostatic field in the direction axial. The hypsometric curves I Bol, represented in Figure 5b, are straighter in the center of the instrument 10 than the hypsometric curves I BJ illustrated in Figure 5a. Also, the present magnetically permeable sleeve of the invention allows the set of magnets 34 to generate a stronger field at the same location of the array compared to the set of magnets 34 around a nonmagnetically permeable sleeve. The greater strength of the electrostatic field significantly improves the signal-to-noise ratio and increases the depth of investigation.

RADIAL DESIGN With reference to Figure 6, in a third embodiment of the invention, hereinafter referred to as a radial design, the set of magnets 16 comprises a set of annular cylindrical magnets 50 around a segment 38 of the sleeve 28. The set of magnets 50 it is formed of a plurality of segments, each segment being magnetized radially, i.e., outwardly, from the longitudinal axis of the instrument 10. Said set of magnets is described in U.S. Pat. Number 4,717,876 to Masi et al., For example. In an outer recess 54 of the drill collar 22, an antenna 52 is installed. The recess 54 is filled with a layer of a non-conductive, magnetically permeable material 56, such as ferrite. The antenna 52 also surrounds the sleeve 28. The RF magnetic field, Bi, generated by the current flowing through the antenna 52, has field directions substantially parallel to the longitudinal axis of the instrument 10. Alternatively, the RF magnetic field, Bi, is generated by a set of antennas and Bi extends azimuthally about the longitudinal axis of the instrument 10. With reference still to the Figure 6, the magnetically permeable member 36 is composed of the segment 38. Similar to the low gradient design, a chassis around the segment 38 can define the permeable member 36. For illustrative purposes only, the radial design described herein refers to a permeable member magnetically 36 consisting of segment 38, made of a suitable material, magnetically permeable, such as ferrite, permeable steel or other alloy of iron and nickel, permeable stainless steel or permeable steel, which has a structural function in the design of the sleeve, such as stainless steel 15-5 Ph. The use of a magnetically permeable material for The segment 38 improves the shape of the electrostatic magnetic field generated by the set of magnets 50 and minimizes the variations of the electrostatic magnetic field due to the vertical displacement of the instrument during the acquisition period of the NMR signal. The direction of the electrostatic field is illustrated with vectors. The path of the magnetic induction lines goes from the central section of the magnet assembly 50 outwards, to the interior of the formation, creating an electrostatic magnetic field in a direction perpendicular to the axis of the borehole, symmetrically going inwards above and through under the magnet assembly 50, through the segment 38 and then converges in the longitudinal direction inside the sleeve 28, returning to the center section of the magnet assembly 50. The magnetically permeable material causes the magnetic induction return lines to be more orthogonal to the axial direction when crossing the outer surface of the segment 38. Figures 6a and 6b compare field strength of the set of magnets 50, around a non-magnetically permeable segment 38, with the field strength of the set of magnets 50, around of a magnetically permeable segment 38. With reference to Figure 6a, with a non-magnetically permeable segment 38, the Magnetic energy is concentrated mainly at the ends of the cylindrical set of magnets 50. This feature of heterogeneity of B0 extends to the surrounding formation. The portions of the electrostatic field near the ends of the assembly 50 are larger than the field located in the center of the instrument 10. The shape of the volume of the formation in which the electrostatic magnetic field strength is in resonance with the RF frequency is curved , and the hypsometric curves of the field are relatively short in the axial direction. Referring to Figure 6b, with a magnetically permeable sleeve 28, a longer and more uniform electrostatic field is generated in the axial direction. The hypsometric curves of I Bol, represented in Figure 6b, are straighter in the center of the instrument 10 than the hypsometric curves of I BJ illustrated in Figure 6a. The magnetically permeable sleeve 28 has the dual purpose of focusing the magnetic field and transporting the drilling fluid through the bore column. Also, the magnetically permeable sleeve of the present invention allows the set of magnets 50 to generate a stronger field at the same location of the array as compared to the set of magnets 50 surrounding a nonmagnetically permeable sleeve. For example, as illustrated in Figure 6a, the strength of the magnetic field is 50 Gaussians, where r = 6 inches. (15.2cm) and z = 5in (12.7cm). In contrast, as illustrated in Figure 6b with a magnetically permeable sleeve, the strength of the magnetic field increases to 200 Gaussians, where r = 6 in. (15.2cm) and z = 5in. (12.7cm). The greater strength of the electrostatic field significantly improves the signal-to-noise ratio of the NMR measurement and increases the depth of measurement investigation. In the present invention, it is projected to generate an electrostatic magnetic field combining sets N + l of magnets 16 to obtain, as a minimum, N regions of research in training. The combinations projected in this invention include, without being limited thereto, a combination of sets 16 of low gradient-low gradient, high gradient-high gradient, high gradient-low gradient or low gradient-high gradient. For example, Figure 7 illustrates a first set of low gradient magnets combined with a second set of low gradient magnets. In the region between the upper magnet 60 and the central magnet 62, the induction magnetic lines move from the center, outward, inward, outward, inward of the formation, creating a first electrostatic field in a direction perpendicular to the axis of the instrument 10. In the region between the central magnet 62 and the lower magnet 64, the induction magnetic lines move from the center, outwards, inwards, outwards, inwards of the formation, creating a second electrostatic field in the direction perpendicular to the axis of the instrument 10. Figure 7a illustrates the hypsometric curves of I BJ corresponding to a configuration where a first magnetically permeable member separates the upper magnet 60 and the central magnet 62 a distance of approximately 25 inches (63.5cm) and a second magnetically permeable member separates the central magnet 62 and the lower central magnet 64 a distance of approximately 25 inches (63.5c) m). The designs of the low gradient, high gradient, coil and radial magnets of the present invention are also useful in an application of a logging instrument on a drill string. The sleeve 28 will define a tubular member within the drill string instrument that provides structural strength for the instrument. Where the sleeve 28 is the magnetically permeable member, the sleeve is designed to withstand considerably axial forces exerted on the instrument during fishing operations. If the sleeve 28 is the magnetically permeable member, the sleeve can be used to magnetically shield the electronic components, such as the electromagnetic relays, which must be within the region of the highly magnetic field produced by the surrounding magnets. Moreover, member 36 can be used for magnetic shielding. The above description of the preferred and alternative physical representations of the present invention has been presented for illustrative and descriptive purposes. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Logically, those skilled in the art will observe many modifications and variants. The physical representations were chosen and described in order to explain, in the best possible way, the principles of the invention and its practical application, thus allowing other experts in the art to understand the invention for various physical representations and with various modifications according to the invention. suitable for the particular use projected. Our intention is that the scope of the invention be defined by the appended claims and their equivalents.

Claims (22)

R E I V I ND I C A C I O N S

1. - An apparatus for generating a magnetic field comprising the following: a) a housing; b) a measuring means, located within the housing, for measurements by nuclear magnetic resonance, and wherein the measurement means comprises the following: i) a means for producing a substantially axisymmetric electrostatic magnetic field, through the housing and within of the formation, in such a way that the hypsometric curves generated by. the electrostatic magnetic field are considerably straight in the axial direction, at the depth of investigation where the measurement by nuclear magnetic resonance is obtained; ii) a means for producing an oscillating field in the formation; and, a member that is at least magnetically permeable to form the electrostatic magnetic field.

2. - The apparatus of claim 1, wherein the means for producing the electrostatic magnetic field comprises a plurality of segments around the permeable member, each segment being magnetized in a radially outward direction from, and perpendicular to, the longitudinal axis of the apparatus.

3. - The apparatus of claim 1, wherein the means for producing an electrostatic magnetic field comprises a plurality of geometrically and axisymmetric rings around the permeable member, and wherein the plurality of rings further comprises the following: a set of central rings, an upper ring, located above the set of central rings, and a lower ring, located below the set of central rings, where each ring is axisymmetrically polarized and the direction of polarization for each ring differs progressively along the plurality of rings

4. - The apparatus of claim 1, wherein the means for producing the electrostatic magnetic field comprises the following: a) an axially magnetized upper magnet: and b) an axially magnetized lower magnet, axially separated from the upper magnet by a distance such that the curves hypsometric generated by the electrostatic magnetic field are considerably straight in the axial direction.

5. - The apparatus of claim 4, wherein an electrostatic magnetic field is generated, with a low gradient, at the depth of investigation where the nuclear magnetic resonance measurement is obtained.

6. - The apparatus of claim 4, wherein an electrostatic magnetic field with a high gradient is generated at the depth of investigation where the measurement by nuclear magnetic resonance is obtained.

7. - an apparatus for generating a magnetic field comprising the following: a) a drilling means for drilling a borehole into the formation; b) means for transporting the drilling fluid through the drilling means; c) a measuring means, connected to the drilling means, for performing measurements by nuclear magnetic resonance while the drilling well is drilled, and where the measuring means comprises the following: i) a means for producing a plurality of electrostatic magnetic fields , considerably axysymmetric, through a drilling medium and within the formation, in a plurality of research regions where the measurement by nuclear magnetic resonance is obtained, and in such a way that hypsometric curves generated by an electrostatic magnetic field, such as minimum, they are considerably straight in the axial direction; ii) a means for producing an oscillating field in the formation; and, d) a magnetically permeable member, at least, installed within the piercing means to form the electrostatic magnetic field.

8. - The apparatus of claim 7, wherein the means for producing a plurality of electrostatic magnetic fields, substantially axisymmetric, further comprises a means for producing a first electrostatic magnetic field comprising an upper magnet, axially magnetized, around the transport means and a central magnet, axially magnetized, around the conveying means, and axially spaced from the upper magnet by a first distance.

9. - The apparatus of claim 7, wherein the means for producing a plurality of electrostatic magnetic fields, substantially axisymmetric, further comprises a means for producing a second electrostatic magnetic field, comprising the central magnet, magnetized axially around the conveying means , and a lower magnet, axially magnetized, around the transport means, and axially separated from the central magnet by a second distance.

10. - The apparatus of claim 9, wherein the means for producing the first electrostatic magnetic field generates an electrostatic magnetic field, with a low gradient, in a first research region where the measurement by nuclear magnetic resonance is obtained.

11. - The apparatus of claim 10, wherein the means for producing the second electrostatic magnetic field generates an electrostatic magnetic field, with a low gradient, in a second research region where the nuclear magnetic resonance measurement is obtained.

12. - The apparatus of claim 10, wherein the means for producing the second electrostatic magnetic field generates an electrostatic magnetic field, with a high gradient, in a second research region where the measurement by nuclear magnetic resonance is obtained.

13. - The apparatus of claim 9, wherein the means for producing the first electrostatic magnetic field generates an electrostatic magnetic field, with a high gradient, in a first research region where the measurement by nuclear magnetic resonance is obtained.

14. - The apparatus of claim 13, wherein the means for producing the second electrostatic magnetic field generates an electrostatic magnetic field, with a high gradient, in a second research region where the measurement by nuclear magnetic resonance is obtained.

15. - an apparatus for generating a magnetic field comprising the following: a) a housing; b) a measuring means, located within the housing, for measurements by nuclear magnetic resonance, and wherein the measurement means comprises the following: i) a means for producing a plurality of substantially axisymmetric electrostatic magnetic fields through the housing and within the formation, in a plurality of research regions where the measurement by nuclear magnetic resonance is obtained, and in such a way that the hypsometric curves generated by an electrostatic magnetic field, at least, are considerably straight in the axial direction; ii) a means for producing an oscillating field in the formation; and, c) a magnetically permeable member, at least, to form the electrostatic magnetic field.

16. - The apparatus of claim 15, wherein the means for producing a plurality of electrostatic magnetic fields, substantially axisymmetric, further comprises a means for producing a first electrostatic magnetic field comprising an upper magnet, magnetized axially around the permeable member, and a central magnet, axially magnetized, around the permeable member and axially separated from the upper magnet by a first distance.

17. - The apparatus of claim 16, wherein the means for producing a plurality of electrostatic magnetic fields, substantially axisymmetric, further comprises a means for producing a second electrostatic magnetic field comprising the central magnet, axially magnetized, around the permeable member and a lower magnet, axially magnetized, around the permeable member, and axially separated from the central magnet by a second distance.

18. - The apparatus of claim 17, wherein the means for producing the first electrostatic magnetic field generates an electrostatic magnetic field, with a low gradient, in a first research region.

19. - The apparatus of claim 18, wherein the means for producing the second electrostatic magnetic field generates an electrostatic magnetic field, with a low gradient, in a second research region where the measurement by nuclear magnetic resonance is obtained.

20. - The apparatus of claim 18, wherein the means for producing the second electrostatic magnetic field generates an electrostatic magnetic field, with a high gradient, in a second research region where the measurement by nuclear magnetic resonance is obtained.

21. - The apparatus of claim 17, wherein the means for producing the first electrostatic magnetic field generates an electrostatic magnetic field with a high gradient, in a first research region where the measurement by nuclear magnetic resonance is obtained.

22. - The apparatus of claim 21, wherein the means for producing the second electrostatic magnetic field, generates an electrostatic magnetic field with a high gradient, in a second research region where the measurement by nuclear magnetic resonance is obtained.