Systems and methods for NMR logging
First Claim
1. A system for nuclear magnetic resonance (NMR) logging of an underground formation traversed by a borehole, the system having a longitudinal axis and comprising:
- two magnets polarized along the longitudinal axis, the magnets having like poles facing each other to generate in the underground formation a static magnetic field azimuthally symmetric with respect to the longitudinal axis, at least one of the facing magnetic poles being extended by one or more pole pieces with high permeability; and
an antenna assembly interposed between the two facing magnetic poles, the antenna assembly capable of operating at multiple frequencies and generating in a portion of the formation RF fields substantially perpendicular to the static magnetic field.
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Abstract
An NMR logging tool for conducting NMR measurements in a plurality of sensitive volumes ranging up to a meter from the tool. The tool comprises a magnetic assembly using one or more permanent magnets and at least one pole piece for extending a magnet pole and shaping the magnetic field to simulate a magnetic monopole in a sensitive volume within the formation. Different embodiments of a segmented antenna enable directional NMR logging. The tool embodiments and methods of their use are suitable for wireline or LWD logging, and can be used for directional drilling.
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Citations
65 Claims
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1. A system for nuclear magnetic resonance (NMR) logging of an underground formation traversed by a borehole, the system having a longitudinal axis and comprising:
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two magnets polarized along the longitudinal axis, the magnets having like poles facing each other to generate in the underground formation a static magnetic field azimuthally symmetric with respect to the longitudinal axis, at least one of the facing magnetic poles being extended by one or more pole pieces with high permeability; and
an antenna assembly interposed between the two facing magnetic poles, the antenna assembly capable of operating at multiple frequencies and generating in a portion of the formation RF fields substantially perpendicular to the static magnetic field. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
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16. A system for NMR logging of an underground formation traversed by a borehole, the system having a longitudinal axis and comprising:
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means for imparting a static magnetic field in the formation surrounding the borehole, the static magnetic field being substantially radially-oriented with respect to the longitudinal axis in a portion of the formation;
means for applying a first oscillating magnetic field having magnetic field lines substantially orthogonal to the static magnetic field in substantially all azimuthal directions;
means for applying one or more directional oscillating magnetic fields being substantially orthogonal to the static magnetic field in corresponding one or more angular sectors of a volume in the formation surrounding the system; and
means for directionally sensitive detection of NMR signals from the formation surrounding the system. - View Dependent Claims (17, 18, 19, 20, 21, 22, 23, 24)
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25. A method for NMR logging a borehole in a geologic formation by means of a tool having longitudinal axis, the method comprising the steps of:
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imparting a static magnetic field having substantially radial orientation with respect to the tool axis in a volume of the formation surrounding the borehole;
imparting a plurality of oscillating magnetic fields substantially orthogonal to the static magnetic field in the volume of the formation in which the static magnetic field has substantially radial orientation, the plurality of oscillating magnetic fields being azimuthally directional;
detecting NMR signals from the formation in a plurality of directionally sensitive antenna segments; and
processing of the detected NMR signals to obtain directionally sensitive information about at least one property of the formation. - View Dependent Claims (26)
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27. An antenna assembly for directional NMR logging of a borehole in a underground formation by means of a logging tool having a longitudinal axis, the assembly comprising:
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a soft magnetic core longitudinally disposed around the axis;
at least one solenoid coil wound around the core for producing an axially-oriented and azimuthally symmetric oscillating magnetic fields in the formation surrounding the borehole; and
at least two saddle coils disposed substantially orthogonally with respect to each other around the core and capable of receiving azimuthally asymmetric NMR signals from a sensitive region in the formation.
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28. A magnet configuration for use in NMR logging of a geologic formation surrounding a borehole, the magnet configuration comprising:
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a single permanent magnet polarized along a longitudinal axis substantially parallel to the axis of the borehole;
at least one pole piece made of highly permeable material(s) extending from one or both poles of the magnet to shape its magnetic field in a manner as to simulate in a portion of the formation a substantially radial magnetic field. - View Dependent Claims (29, 30, 31)
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32. A method for determining an NMR property of formation surrounding a borehole with the aid of a logging device, comprising the steps of:
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providing a magnetic configuration producing a substantially radial static magnetic field into a portion of the formation;
providing a plurality of directionally sensitive antenna segments disposed along a longitudinal axis of the logging device generating RF pulsed magnetic fields in the formation, the RF fields being substantially orthogonal to the static magnetic field;
receiving NMR signals from a sensitive region of the formation at two adjacent antenna segments; and
determining a property of the formation in a direction based on orientation of both the adjacent antenna segments. - View Dependent Claims (33, 34, 35, 36, 37, 38, 39, 40, 41, 42)
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43. An apparatus for directionally sensitive NMR logging in a borehole for collecting directionally sensitive formation data comprising:
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a segmented antenna comprising a plurality of independently addressable antenna segments;
a magnet simulating a monopole static magnetic field in a volume surrounding the borehole; and
means for transmitting signals at a plurality of frequencies from a plurality of segments in the segmented antenna for obtaining NMR signals from a plurality of volumes such that the location of each volume corresponds to a particular frequency and the plurality of segments in the segmented antenna. - View Dependent Claims (44, 45, 46, 47, 48)
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49. A method for obtaining a directionally-sensitive NMR log in a borehole of a geologic formation, comprising:
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producing a static magnetic field and a plurality of RF magnetic fields having different resonance frequencies at an NMR logging device, the static and the plurality of RF magnetic fields having mutually orthogonal components in an investigation region in the formation surrounding the logging device;
receiving NMR signals from the formation at a plurality of directionally sensitive antenna segments of the NMR logging device;
processing the received NMR signals to determine at least one formation property value associated with a particular spatial direction with respect to the logging device, wherein said at least one formation property is determined based on NMR signals received in two or more adjacent antenna segments. - View Dependent Claims (50)
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51. A method for obtaining an azimuthally-sensitive log in a borehole traversing a formation comprising
placing a standoff for separating a segmented antenna section in a wireline tool from a wall of the borehole; -
providing a plurality of segments in an antenna, wherein at least one segment is capable of transmitting directionally sensitive pulsed magnetic field signals into the formation;
providing a magnetic field simulating a magnetic monopole in a volume of the formation surrounding the borehole; and
switching frequencies such that different volumes in the formation are investigated from the same antenna segment by receiving an NMR echo train in response to a corresponding transmission of a train of pulses.
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52. A method of drilling a borehole in a desired direction in a geologic formation, the method comprising:
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(i) receiving NMR signals from the formation at one or more directionally sensitive antenna segments of an NMR logging tool attached to a drill string;
(ii) computing at least one formation property value associated with a predetermined portion of the geologic formation based on NMR signals received at the directionally sensitive antenna segment(s); and
(iii) repeating step (ii) at least once and directing a drill bit on the drill string in a direction determined by one or more of;
an increase in the value of the property, a decrease in the property, and maintaining a value of the property within a set of bounds. - View Dependent Claims (53, 54, 55, 56, 57)
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58. A method for NMR logging a borehole in a geologic formation, comprising the steps of:
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lowering a magnet assembly to a location in the borehole;
using the lowered magnet assembly for conducting NMR measurements in;
(i) at least one shallow region of the formation surrounding the borehole, which shallow region may be subject to borehole fluid invasion; and
(ii) at least one deep region of the formation surrounding the borehole, the deep region being substantially free of borehole fluid invasion; and
combining NMR measurements from the at least one shallow region of the formation and NMR measurements from the at least one deep region of the formation to determine petrophysical properties of the formation. - View Dependent Claims (59, 60, 61, 62, 63, 64, 65)
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Specification