GB2366390A

GB2366390A - NMR well-logging apparatus

Info

Publication number: GB2366390A
Application number: GB0122001A
Authority: GB
Inventors: Jian-Qun Wu; Macmillan Morgan Wisler
Original assignee: Baker Hughes Inc
Current assignee: Baker Hughes Holdings LLC
Priority date: 1997-04-21
Filing date: 1998-04-21
Publication date: 2002-03-06
Anticipated expiration: 2018-04-21
Also published as: GB2366390B; GB0122001D0; GB2366389A; GB0121999D0; GB2366389B

Abstract

An NMR well-logging tool 30 comprises an RF transmitting / receiving antenna formed by a wire or conducting sheath 48 arranged longitudinally within the tool, so that there is a linear electrical conduction path. The frequency may be swept during operation of the tool. The static magnetic field may be produced by various permanent magnet arrangements (see figures 3A, 3B) so that it is parallel to the tool axis 36.

Description

2366390 NUCLEAR MAGNETIC RESONANCE APPARATUS AND METHOD FOR GEOLOGICAL

APPLICATIONS This invention relates generally to determining geological properties of subsurface formations and more particularly to Nuclear Magnetic Resonance ("NTNM") apparatus and methods for logging wellbores, particularly for obtaining infori-nation relating to the presence of hydrocarbons in such formations.

A variety of techniques; are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling of the wellbores, which is referred to as measurement-whiledrilling "MWD" or logging-while-drilling ("LWD").

One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the of the liquids in the geological formations 1 414-9392-GB surrounding the wellbore so that certain parameters such as spin density (generally referred to in the art as '71" and relaxation! (generally referred to as '72") of the geological formations can be measured. From such measurements, porosity, permeability and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.

The NNR tools generate a constant or near constant static magnetic field in a region of interest surrounding the wellbore. A radio frequency (RF) signal is induced in the formation to generate an alternating magnetic field which is perpendicular or near perpendicular to the static magnetic field in the region of interest to perturb the nuclei. The perturbation is measured to determine the parameters of interest.

The NMR logging tools typically utilize a magnetic source such as specially configured permanent magnets to generate the desired static magnetic field and one or more coils to generate the corresponding perpendicular RF magnetic field in the region of interest around the borehole.

Examples of prior art NMR logging tools and methods can be found in the following United States Patent Nos.: 3,508,438 (Alger et al.), 3,617,867 (Herzog), 3,667,035 (Slichter), 4,350,955 (Jackson), 4,467,642 (Givens), 4,528,508 (Vail), 4,560,663 (Nicksic et al,), 4,629,986 (Clow et al.), 4, 710,713 (Strikman), 4,713,881 (Givens), 4,933,638 (Kenyon et al.), 4,717, 876 (Masi et al.), 5,212,447 (Paltiel), 5,280,243 (Miller), 5,309098 (Coates, et al.) and 5,517,115 (Prammer).

2 414-9392-GB Most of the above-noted and other prior art logging tools utilize relatively complex permanent magnet and RF signal coils combinations. Furthermore, most of these tools are designed for wireline logging, te, for logging boreholes after they have been drilled. The formation evaluation MVW tools must operate in much harsher environments compared to wireline logging tools. This is primarily because the M" tools are disposed in the drill string above the drill bit and, thus, must withstand high radial and axial vibrations. Furthermore, M" tools rotate as the drill string rotates, which in prior art NMR tools tends to cause the static magnetic field to vary in the zone of interest, thereby decreasing signal to noise ratio and thus, the quality of the measurements. The Miller patent discloses an NMR apparatus that is designed for MWD applications. However, the disclosed apparatus still utilizes relatively complex magnet and coil structures.

All of the prior art logging tools tend to utilize magnets to impart magnetic field that extends radially outward from the longitudinal axis of the tool and the borehole, and loop antennas for generating RF magnetic field which is perpendicular (orthogonal) to the static magnetic field. These types of structures tend to lead to relatively complex designs whose static magnetic field is affected due to the rotation of the tool during MWD applications.

The present invention addresses some of the above-noted problems with the prior art NMR logging tool and provides NMR logging tools wherein the RF antennas are linear, i.e., along the tool longitudinal direction, and one or more permanent

3 magnets disposed along the tool longitudinal axis, primarily with their north and south poles along the tool longitudinal direction.

The present invention provides a nuclear magnetic resonance ("NNM") tool for determining geological properties of formations surrounding a borehole when said tool is traversed in the borehole. The NMR tool includes at least one permanent magnet that is disposed substantially parallel to a longitudinal axis of the tool to generate a static magnetic field at a known distance in the formation. The static magnetic field is azimuthally symmetrical and has components in the z and p directions of a cylindrical coordinate system. An R-F magnetic field in the zone of interest is generated by forcing a sheet of current in the longitudinal direction (linear electrical conduction path along the longitudinal axis of the tool) at a known frequency. The frequency may be swept during operation of the tool. A receiver detects signals from the formation that are responsive to the induced radio frequency magnetic field. These signals are processed to determine one or more formation parameters such as porosity, type of fluid, hydrocarbon saturation and permeability of the zone of interest. The permanent magnet may include a plurality of permanent magnets syrnmetrically arranged around the tool.

The present invention provides a method for determining geological properties of interest of formations surrounding a borehole by inducing nuclear magnetic resonance in the formation by a logging tool whose longitudinal axis is along the borehole. The method comprises- (a) inducing a substantially constant static magnetic 4 field in the direction of the longitudinal axis or in a radial direction; (b) inducing an RF magnetic field by passing RF current through a conductor disposed substantially linearly to the longitudinal axis; (c) measuring the response of the formation to the induced RF magnetic field; and (d) processing the measured response to determine the properties of the geological properties of interest of the formation.

Examples of the more important features of the invention have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.

For a detailed understanding of the present invention, references should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein:

FIG. 1 shows a schematic illustration of an elevational view of the relationship of the static or d.c. and RF magnetic fields relative to the borehole and tool longitudinal axes utilized in the NMR logging tool of the present invention.

FIG. 2 shows a schematic elevational illustration of an NMR logging tool according to the present invention.

FIG. 3A shows an elevational view of an arrangement of permanent magnets for use in the NMR tool of the present invention.

FIG. 3B shows an elevational view of an alternative arrangement of permanent magnets for use in the NMR tool of the present invention.

FIG. 3C shows a view of an alternative arrangement of permanent magnets for use in the NMR tool of the present invention that produces a radial static field.

FIG. 4 shows a manner of placing permanent magnets in a tubular arrangement for use in the NMR logging tool of the present invention.

FIG. I shows a schematic illustration of an elevational view of the relationship of the static or d.c. and RF magnetic fields relative to the borehole and tool longitudinal axes utilized in one embodiment of the NMR logging tool of the present invention. FIG. I shows a borehole 10 with a longitudinal axis or z -axis 12 and a zone of interest (formation) 14 surrounding the borehole 10. During operations, the logging tools traverse the borehole along the z -axis 12. NNM principles require the generation of a constant or near constant static or d.c. magnetic field in the zone of interest and a corresponding RF magnetic field perpendicular to the static magnetic field in the zone of interest. The RF signal frequency is selected based upon the strength of the static magnetic field in the zone of interest and the desired depth of investigation.

6 414-9392-GB Still re ferring to FIG. 1, in the present invention, a constant or substantially constant static magnetic field HO is induced which is azimuthally symmetrical and has direction in the z and p directions of a cylindrical coordinate system, A corresponding RF magnetic field H, which is perpendicular to the z direction (static magnetic field Ho) is generated by forcing a sheet of current 16 in the longitudinal direction 12 (also referred herein as the "linear RF current"). The linear RF current 16 passes along the entire length of the zone of interest, thereby creating an RF magnetic field H, that is perpendicular to the z axis throughout the zone of interest. The static magnetic field Ho substantially symmetrical around the borehole 10 and is preferably generated by suitably placed permanent magnets in the tool, as described later in reference to FIGS 2-4.

FIG. 2 shows a schematic elevational illustration of an NMR logging tool 30 according to the present invention along with the relationship of the static and RF magnetic fields. The NMR tool 30 is shown as an MAD tool. The concepts and the tool described herein are equally applicable to other NNM applications, including wireline logging applications. The NNM tool 30 is disposed in a drill string 32, which has a drill bit at its downhole end (not shown). The NMR tool 30 preferably is a part of a bottomhole assembly, which may contain any number of other fon-nation evaluation devices, such as resistivity devices, acoustic devices and nuclear devices, and any number of other devices, such as a mud motor, stabilizers, direction control devices and steering devices. The use of such other devices and sensors is known in the art. For simplicity and not as limitation, such devices are not shown or described 7

414-9392-GB herein.

Still referring to FIG. 2, the NMR logging tool 30 includes a magnet 34 arrangement that is linearly disposed in the tool 30 so as to induce a constant static magnetic field Ho in the zone of interest 40 at a predetermined median distance "d' from the tool axis 36. The static magnetic field so generated is azimuthally symmetrical and has components in the z and p directions of a cylindrical coordinate system as shown in FIG 2. The magnet arrangement 34 preferably comprises one or more permanent magnets as more fully described later in reference to FIGS. 3A, 3B and 3C. Any suitable permanent magnet, including ones made from high mu-metal may be utilized for the purposes of the NMR logging tool 30 of the present invention. An RF magnetic field H, is generated by passing or forcing linear RF current 46 via a conducting sheet (which may be a wire or conducting sheet around the drill string 32) disposed along the z axis. The conducting sheet is connected to the drill string at a suitable point 56. One or more non-conducting sections, such as section 54 are provided in the drill string. A ground connection 44 is provided outside the nonconducting section on the drill string 32. The RF current is induced into the conducting sheet on one side of the non-conducting section while the return (ground) 44 is maintained on the other side of the non-conducting section. The linear conductor is generally denoted herein by numeral 48. A pulse generator 50 may be utilized to pass the required current at a desired frequency through the linear conductor 48. The RF frequency is selected based on the static magnetic field strength in the zone of interest 40. A range of frequencies may be selected for various depths of investigation. The pulse generator 50 may be used to sweep the frequencies in one or more ranges 8

414-9392-GB of frequencies.

During operations, the NNM tool 30, whether in the wireline mode or in the N4WD environment travels along the borehole 10. The magnet 34 induces a constant static magnetic field in the zone of interest 40 mainly in the zdirection. The RF source 50 passes electric current through the linear conductor 48 at a known frequency, which in turn induces a corresponding RF magnetic field Hi perpendicular to the static magnetic field HO. The RF magnetic field III perturbs the nuclei of the fluids in the zone of interest 40, which generates a response signal. A receiver 52 detects the response signals, which are processed by methods known in the art to determine the parameters of interest, including porosity, hydrocarbon saturation and permeability of the rock matrix in the zone of interest 40. k switch 54 may be utilized to switch between the transmitter of the RF signals and the receiver 52.

The NMR tool or system 30 of the present invention preferably utilizes a downhole computer (n-dcroprocessor) and other digital and analog circuits for signal processing. Such circuits are known in the art and are, thus, not described herein. It will, however, be understood that any suitable circuits, devices and programs may be utilized for processing the signals in the tool 30.

FIG. 3A shows an elevational view of an arrangement of permanent magnets for use in the NMR tool 30 (FIG. 2) of the present invention for generating substantially constant static magnetic field in a zone of interest around a borehole. In one embodiment, the magnet arrangement 80 may include a number of individual 9 permanent magnets 82 symmetrically disposed axially (parallel to the z - axis) to fonn a cylinder 84. Similar poles of all such magnets are all oriented in the same direction. Thus, the north pole of all magnets 82 are in one direction and the south pole of all magnets are on the opposite direction. The individual magnets 82 may be embedded in a non- magnetic material or otherwise disposed around a suitable material. The individual magnets 82 may be equally spaced apart. In an alternative embodiment as shown in FIG. 3B, more that one cylindrical configurations of the magnets 82 such as shown by cylinders 90a and 90b may be disposed along the axial direction, with the opposite poles facing adjacent cylinder ends. For example, the north pole of the cylinder 90a faces uphole while the south pole faces downhole. The north pole of tile cylinder 90b faces the south pole of the cylinder 90a. A suitable gap 92 may be provided between adjacent cylinders. Any number of cylinders or segments may be utilized, with or without any gaps, such as gaps 92, depending upon the zone or zones of interest and the depth of investigation desired. Gaps can also be used to substantially reduce the conductance of the magnets. High mu materials may be used in the gaps. In the region of interest 40, the static magnetic field is substantially longitudinal in direction and orthogonal to the RF magnetic field.

In all alternate embodiment of the invention, the permanent magnet assembly is used to produce a static field Ho that has a region of constant field strength 40 in which the field direction is radial, i.e., in the p - direction. As discussed above, the static and RF fields are still orthogonal to each other, a necessary condition for making NMR measurements. This radial magnetic field is produced by a magnet arrangement as shown in FIG. 3C, Shown are two sets of outer cylindrical magnets 94a, 94b. However, unlike the arrangement shown in FIG. 3B, these magnets have their directions of magnetization opposed, For example, the magnets in 94a have north poles to the top and the magnets in 94b have their north poles to the bottom. As would be known to those versed in the art, this arrangement of matched magnets produces a toroidal region of substantially uniform magnetic field strength n-ddway between the magnets 94a and the magnets 94b centered on the longitudinal axis of the assembly. The direction of the magnetic field in this region is radial, i.e., orthogonal to the longitudinal axis of the assembly. This radial static field is still orthogonal to the RF magnetic field, a prerequisite for making NMR measurements.

FIG. 3C also shows a second set of inner magnets 96 interposed between the outer magnets 94a, 94b. The inner magnets are radially magnetized, and, in the example shown, have north poles on the inside (close to the axis) and south poles on the outside. This arrangement helps to shape the zone of substantially constant field strength 40. Additionally, high rnu materials may be used in the gaps between the sets of magnets to further shape the region 40.

FIG- 4 shows a method of placing the permanent magnets 82 shown in FIGS. 3A and 3B in a tool suitable for M-WD applications. The illustration shows the arrangement of magnets 82 but the same arrangement may be used with the magnets 94 and 96 in FIG. 3C. Permanent magnets 82 may be embedded or placed in a nonmetallic (such as fiber glass) casing 102 disposed in a non-magnetic steel housing 104. A suitable insulator 106 may be placed at a desired distance from the permanent 11 1 magnets 82. The required circuitry, including the RF circuitry 110 may be disposed in the steel housing 104. The placement of such circuits in MWD tools is known in the art and is, thus, not described in detail here. Instead of multiple individual permanent magnets 82, a single cylindrical permanent magnet may be disposed in the manner described above.

While the foregoing disclosure is directed to the preferred embodiments of the invention, various modifications will be apparent to those skilled in the art. It is intended that all variations within the scope of the appended claims be embraced by the foregoing disclosure.

12 - 13 68176001.357

Claims

Claims

5 1. A nuclear magnetic resonance tool for determining a property of interest in a zone of interest of a formation surrounding a borehole when said tool is traversed in the borehole, said tool having a longitudinal axis along the borehole, said tool 10 comprising:

(a) a permanent magnet assembly to generate a static magnetic field in the zone of interest substantially parallel to the longitudinal direction; and 15 (b) a device for generating radio frequency (RF) magnetic field in the zone of interest and for detecting echos, said device comprising:

(i) a linear electrical conductor which is substantially parallel to the longitudinal 20 axis so that when electrical current at a radio frequency is passed through the linear electrical conductor, an alternating magnetic field is generated that is substantially perpendicular to the static magnetic field 25 generated by the permanent magnet; and (ii) a source for inducing electrical current into the linear electrical conductor.
2. The nuclear magnetic resonance tool according to 30 claim 1, wherein the permanent magnet assembly comprises a plurality of permanent magnets disposed symmetrically around a periphery of the tool and substantially parallel to the longitudinal axis of the apparatus.

35
3. The nuclear magnetic resonance tool according to claim 1 or 2, wherein the permanent magnet assembly comprises at least two sets of permanent magnets disposed linearly along the longitudinal axis of the tool, wherein each set includes a plurality of individual permanent magnets disposed symmetrically around the periphery of the tool. 5
4. The nuclear magnetic resonance tool according to claim 1, 2, or 3, wherein the device for generating the RF magnetic field is adapted to sweep frequencies within a range of frequencies. 10
5. The apparatus according to claim 3, wherein the at least two sets of permanent magnets are disposed within a non- magnetic material placed along a non-magnetic housing.