Downhole nuclear magnetic resonance (nmr) tool with transversal-dipole antenna configuration.

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DIVISIONAL SUB-DIRECTOR OF PATENT FUND EXAMINATION OF MECHANICAL, ELECTRICAL AND INDUSTRIAL DESIGNS AND USEFUL MODELS

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PEDRO DAVID FRAGOSO LÓPEZ

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NUCLEAR MAGNETIC RESONANCE TOOL (NMR) OF THE WELL FUND WITH TRANSVERSE DIPOLO ANTENNA SETTING

FIELD OF THE INVENTION

This description refers to transverse dipole antenna configurations for nuclear magnetic resonance (NMR) tools at the bottom of the well, for example, to obtain NMR data from an underground region.

BACKGROUND OF THE INVENTION

In the field of record acquisition (for example, acquisition of records by cable, acquisition of records during drilling (LWD) and measurement during drilling (MWD)), Employed nuclear magnetic resonance (NMR) tools to explore the subsoil based on the magnetic interactions with the subsoil material. Some well bottom NMR tools include a magnet assembly that produces a static magnetic field, and a coil assembly that generates radio frequency (RF) control signals and detects the phenomenon of magnetic resonance in the subsoil material. The properties of the subsoil material can be identified from the detected phenomenon.

Ref. 263106

DESCRIPTION OF THE FIGURES

Figure 1A is a diagram of an example of a well system.

Figure IB is a diagram of an example of a well system that includes an NMR tool in a cable record acquisition environment.

Figure 1C is a diagram of an example of a well system that includes an NMR tool in a record acquisition environment during drilling (LWD).

Figure 2A is a diagram of an example of a bottomhole tool for obtaining NMR data from an underground region.

Figure 2B is a diagram of another example of a bottomhole tool for obtaining NMR data from an underground region.

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<td colspan="2">azimuthal selectivity for</td><td>an example of</td><td colspan="2">tool</td><td>of the</td>

<td>bottom of</td><td>water well.</td><td></td><td></td><td></td><td></td>

<td>The</td><td>Figure 3B is a</td><td>diagram of</td><td>other</td><td>example</td><td>from</td>

bottomhole tool to obtain NMR data from an underground region.

Figure 4A is a flow chart showing an example of a technique for obtaining NMR data from an underground region.

Figure 4B is a flow chart showing another example of a technique for obtaining NMR data from an underground region.

Similar reference symbols in the various figures indicate similar elements.

DETAILED DESCRIPTION OF THE INVENTION

In some implementations, an NMR instrument may offer practical solutions to obtain NMR data from the subsoil. In some cases, the instrument may provide a higher signal-to-noise ratio (SNR) (for example, for a given amount of available direct current), motion immunity, azimuthal selectivity of the measurements or a combination of these or other advantages. In some cases, the instrument may have high resistance to environmental factors and provide accurate or accurate information for the subsoil analysis.

Some examples of configurations of a well bottom NMR instrument include a substantially two-dimensional (2D) transverse dipole assembly for magnet assembly and antenna assembly. The magnetic fields generated by the magnet and the antennas may have axial homogeneity (that is, homogeneity along the axis of the NMR instrument) suitable for use during axial movement. In some cases, it is possible to use wider band excitation (nuclear magnetization saturation), for example, to achieve axial symmetry (roundness) with this type of instrument. In some implementations, a well bottom NMR tool is configured to generate magnetic fields with axial symmetry with a magnet assembly that generates a radial magnetic field and an antenna assembly that generates a longitudinal RF magnetic field (which also has a longitudinal sensitivity direction).

In some cases, an NMR instrument can produce a longitudinal static magnetic field in the volume of interest. In some examples, the instrument includes multiple transverse dipole antennas (for example, two identical transverse dipole antennas) that produce circular polarized excitation and provide quadrature coil detection. It is possible to use an assembly of multiple orthogonal antennas, for example, with a longitudinal dipole magnet that generates an axial static magnetic field in the volume of interest. In some examples, the instrument includes a multi-volume assembly that uses different regions of the magnet assembly to acquire the NMR signal. In some examples, a research region has a suitable way to take action by moving the drill string (that is, by moving the drill string into the well). Some examples of implementations include a combination of a response with axial symmetry with respect to the transverse dipole antenna and a response with axial symmetry with respect to the monopole antenna, which makes it possible to take unidirectional NMR measurements determined azimutically in some cases.

Figure 1A is a diagram of an example well system 100a. The example well system 100a includes an NMR record acquisition system 108 and an underground region 120 below the land surface 106. A well system may include different or additional features that are not shown in Figure 1A. For example, the well system 100a may include additional drilling system components, electrical record acquisition system components, etc.

The underground region 120 may include all or part of one or more underground zones or formations. The example underground region 120 shown in Figure 1A includes multiple subsoil layers 122 and a pit 104 penetrated through the subsoil layers 122. The subsoil layers 122 may include sedimentary layers, rock layers, sand layers, or combinations of these and other different types of subsoil layers. One or more of the subsoil layers may contain fluids, for example brine, oil, gas, etc. Although the example well 104 shown in Figure 1A is a vertical well, the NMR record acquisition system 108 can be implemented in other well orientations. For example, the NMR record acquisition system 108 may be adapted for horizontal wells, inclined wells, curved wells, vertical wells or combinations thereof.

The example NMR record acquisition system 108 includes a record acquisition tool 102, surface equipment 112 and a computer subsystem 110. In the example shown in Figure 1A, the record acquisition tool 102 is a tool for acquiring records from the bottom of the well that works while in well 104. The sample surface equipment 112 shown in Figure 1A operates at or below surface 106, for example, near the wellhead 105, to control the record acquisition tool 102 and possibly other background equipment from the well or other components of the well system 100. The computer subsystem example 110 can receive and analyze data from record acquisition of the record acquisition tool 102. An NMR record acquisition system may include additional or different features, and the features of an NMR record acquisition system may be arranged and function as depicted in Figure 1A or otherwise.

In some cases, all or part of the computer subsystem 110 may be implemented as a component, or it may be integrated with one or more components, of the surface equipment 112, of the record acquisition tool 102 or both. In some cases, the computer subsystem 110 may be implemented as one or more computer structures of the surface equipment 112 and the record acquisition tool 102.

In some implementations, the computer subsystem 110 is included in the record acquisition tool 102, and the computer subsystem 110 and the record acquisition tool 102 can operate simultaneously when they are in well 104. For example, although the computer subsystem 110 is shown on the surface 106 in the example illustrated in Figure 1A, all or part of the computer subsystem 110 can be found below the surface 106, for example, in the location of the acquisition tool of records 102 or near it.

The well system 100a may include communication or telemetry equipment that allows communication between the computer subsystem 110, the record acquisition tool 102 and other components of the RMN record acquisition system 108. For example, each of the components The NMR record acquisition system 108 may include one or more transceivers or similar devices for wired or wireless data communication between the various components. For example, the NMR record acquisition system 108 may include systems and apparatus for optical telemetry, telemetry via electrical cable, cabled telemetry, pulse transmission through the mud, acoustic telemetry, electromagnetic telemetry, or a combination thereof. and other different types of telemetry. In some cases, the record acquisition tool 102 receives commands, status signals or other types of information from the information subsystem 110 or other source. In some cases, the computer subsystem 110 receives acquisition data from records, status signals or other types of information from the record acquisition tool 102 or other source.

The operations of acquisition of NMR records can be performed in connection with various types of well bottom operations at various stages in the life of a well system. The structural components and attributes of the surface equipment 112 and the record acquisition tool 102 may be adapted for various types of NMR record acquisition operations. For example, the acquisition of NMR records can be performed during drilling operations, during record acquisition operations by electric cable or in other contexts. As such, the surface equipment 112 and the record acquisition tool 102 may include or be operated in connection with drilling equipment, record acquisition equipment by electric cable or other equipment for other types of operations.

In some implementations, the record acquisition tool 102 includes a magnet assembly that includes a central magnet and two end magnets. Examples are shown in Figures 2A, 2B, and 3B. The end magnets may be separated from the axial ends of the central magnet. The end magnets together with the central magnet can define four magnetic poles, which can be arranged to improve the static magnetic field in a volume of interest. In some cases, the central magnet defines a first magnetic field orientation, and the end magnets define a second magnetic field orientation that is orthogonal to the first magnetic field orientation. The record acquisition tool 102 may also include multiple orthogonal transverse dipole antennas. Orthogonal transverse dipole antennas can produce circular polarized excitation in an underground volume and acquire a response from the volume by quadrature coil detection.

In some implementations, the record acquisition tool 102 includes a magnet assembly that produces a magnetic field in multiple different sub-volumes in the underground region 120. An example is shown in Figure 2B. A first sub-volume may be an elongated cylindrical outer region that extends in the longitudinal direction (parallel to the axis of the well), and the magnetic field in the first sub-volume may be oriented substantially uniformly along the direction longitudinal. The second and third sub-volumes may be separated from the axial ends of the first sub-volume, and the static magnetic field in the second and third subvolumes may have a radial orientation (perpendicular to the longitudinal direction). The second and third sub-volumes can be located a different distance from the center of the tool string than the first volume. In some cases, the locations of the second and third sub-volumes allow the record acquisition tool to collect information to perform the mud filtration invasion profile. The record acquisition tool 102 may also include multiple antenna assemblies at respective locations along the longitudinal axis. Each of the antenna assemblies can detect an NMR response of a respective sub-volume of the different subvolumes.

In some cases, the record acquisition tool 102 includes a magnet assembly and a monopole and transverse dipole antenna assembly. An example is shown in Figure 3B. The monopole and transverse dipole antenna assembly can obtain an azimuthal and unidirectional selective NMR response of an underground volume around the magnet assembly. The monopole and transverse dipole antenna assembly may include orthogonal transverse dipole antennas and a monopole antenna.

In some examples, RMN record acquisition operations are performed during record acquisition operations by electric cable. Figure IB shows an example well system 100b that includes the record acquisition tool 102 in a record acquisition environment by electric cable. In some examples of record acquisition operations by electric cable, the surface equipment 112 includes a platform on the surface 106 equipped with a drilling tower 132 that supports an electric cable 134 that extends into the well 104. The operations of acquiring records by electric cable can be performed, for example, after a drill string is removed from the well 104, to allow the record acquisition tool by electric cable 102 to descend through an electric wire or of acquisition of records within well 104.

In some examples, the NMR record acquisition operations are performed during drilling operations. Figure 1C shows an example well system 100c that includes the record acquisition tool 102 in a record acquisition environment during drilling (LWD). Normally, drilling is carried out using a string of drill columns connected to each other to form a drill string 140 that is lowered using a rotary table in the well 104. In some cases, a drill rig 142 on the surface 106 it holds the drill string 140, as the drill string 14 0 is operated to drill a well that penetrates the underground region 120. The drill string 140 may include, for example, a drill rod, a drill column, a bottomhole assembly and other components. The bottom assembly of the well in the drill string may include wick holders, drill bits, record acquisition tool 102 and other components. Record acquisition tools may include measurement tools during drilling (MWD), LWD tools and others.

In some implementations, the record acquisition tool 102 includes an NMR tool for obtaining NMR measurements of the underground region 120.

As shown, for example, in Figure IB, the record acquisition tool 102 can be suspended in the well 104 through a flexible pipe, electrical cable or other structure connecting the tool to a surface control unit or other components of surface equipment 112. In some example implementations, the record acquisition tool 102 is lowered to the bottom of a region of interest and subsequently pulled up (for example, at a substantially constant rate) through the region of interest. As shown, for example, in Figure 1C, the record acquisition tool 102 can be deployed in well 104 in an articulated drill column, wired drill column or other deployment hardware. In some example implementations, the record acquisition tool 102 collects data during drilling operations as it moves down through the region of interest. In some example implementations, the record acquisition tool 102 collects data while the drill string 140 moves, for example, while entering or exiting the well 104.

In some example implementations, the record acquisition tool 102 collects data at separate record acquisition points in well 104. For example, the record acquisition tool 102 can be moved up or down incrementally toward each record point in a series of depths in pit 104. At each record acquisition point, the instruments in the record acquisition tool 102 perform measurements in the underground region 120. The measurement data can be communicated to the computer subsystem 110 for storage, processing and analysis. The data can be collected and analyzed during drilling operations (for example, during record acquisition operations during drilling (LWD)), during record acquisition operations by electric cable or during other types of activities.

The computer subsystem 110 can receive and analyze the measurement data of the record acquisition tool 102 to detect the properties of several subsoil layers 122. For example, the computer subsystem 110 can identify the density, viscosity, porosity, content of the material or other properties of the subsoil layers 122 depending on the NMR measurements acquired by the record acquisition tool 102 in well 104.

In some implementations, the record acquisition tool 102 obtains NMR signals by polarizing the nuclear spins in the underground region 120 and providing pulses to the cores with a radiofrequency (RF) magnetic field. Several pulse sequences (ie, radiofrequency pulse series, delays, and other operations) can be used to obtain NMR signals, which include the Carr Purcell Meiboom Gilí (CPMG) sequence (where the spins are first offset using a phase shift pulse followed by a series of refocusing pulses), the optimized refocusing pulse sequence (ORPS) where the refocusing pulses are less than 180 °, a saturation recovery pulse sequence and other pulse sequences.

The acquired spin echo signals (or other NMR data) can be processed (for example, inverted, transformed, etc.) to a distribution of relaxation time (for example, a distribution of transverse relaxation times T₂ or a distribution of longitudinal relaxation times T_r) , or both. The distribution of relaxation time can be used to determine various physical properties of the formation by solving one or more inverse problems. In some cases, relaxation time distributions are acquired for multiple record acquisition points and used to form a model of the underground region. In some cases, relaxation time distributions are acquired for multiple record acquisition points and used to predict the properties of the underground region.

Figure 2A is a diagram of an example NMR tool 200A. The example NMR tool 200A includes a magnet assembly that generates a static magnetic field to produce the polarization, and an antenna assembly that (a) generates a radiofrequency (RF) magnetic field to generate the excitation, and (b) Acquires NMR signals. In the example shown in Figure 2A, a magnet assembly that includes the end magnets 11A, 11B and a central magnet 12 generates the static magnetic field in the research volume 17. In the research volume 17, the The direction of the static magnetic field (shown as the dark black arrow 18) is parallel to the longitudinal axis of the well. In some examples, a magnet configuration with bipolar force can be used to increase the strength of the magnetic field (for example, up to 100-150 Gauss or greater in some cases).

In the example shown in Figure 2A, the antenna assembly 13 includes two mutually transverse orthogonal dipole antennas 15, 16. In some cases, the NMR tool 2 00A can be implemented with a single transverse dipole antenna. For example, one of the transverse dipole antennas 15, 16 can be omitted from the antenna assembly 13. The example transverse dipole antennas 15, 16 shown in Figure 2A are placed on an external surface of a soft magnetic core 14, which is used for the concentration of RF magnetic flux. The static magnetic field may be axially symmetric (or axially symmetrical substantially) and, therefore, may not require a wider band excitation associated with additional energy loss. The investigation volume can be made long enough and axially thick enough (for example, 20 cm long, and 0.5 cm thick in some environments) to provide immunity or otherwise decrease sensitivity to axial movement, movement lateral or both. A region of greater sensitivity may allow measurements during the drilling of the drill string. The sensitivity region can be shaped by sizing magnets 11A, 11B, 12 and the soft magnetic material of core 14.

In some implementations, the antenna assembly 13, additionally or alternatively, includes a set of integrated coils that performs the operations of the two transverse dipole antennas 15, 16. For example, the integrated coil can be used (for example, in instead of the two transverse dipole antennas 15, 16) to produce circular polarization and perform quadrature coil detection. Examples of integrated coil assemblies that can be adapted to perform the operations include arrangements of multiple coils or complex individual coils, such as, for example, cage coils commonly used for high field magnetic resonance imaging (MRI).

Compared to some example axially symmetrical designs, the use of the longitudinal dipole magnet and the transverse dipole antenna assembly also has an advantage of less stray current losses in the formation and drilling fluid (i.e., mud) in the well due to a path of parasitic current greater than for some longitudinal dipole antennas.

In some aspects, NMR measurements in multiple sub-volumes can increase data density and, therefore, SNR per unit of time. Multiple volume measurements can be achieved in a static magnetic field that has a radial gradient, for example, by the acquisition of NMR data at a second frequency while waiting for the recovery of nuclear magnetization (for example, after a train of CPMG pulses) at a first frequency. A variety of different frequencies can be used to perform a multi-frequency NMR acquisition that involves a number of excitation volumes with a different depth of investigation. In addition to higher SNR, multiple frequency measurements can also allow the profile of the fluid invasion profile in the well, allowing for a better evaluation of the permeability of land formations. Another way to perform multiple frequency measurements is by using different regions of the magnet assembly to acquire an NMR signal. NMR measurements from these different regions can be executed at the same time (for example, simultaneously) or at different times.

Figure 2B is a diagram of another example NMR 2 00B tool. The example NMR tool 200B also includes a magnet assembly that generates a static magnetic field to produce the polarization, and an antenna assembly that (a) generates a radiofrequency (RF) magnetic field to generate the excitation, and (b ) acquires NMR signals. In the example shown in Figure 2B, the magnet assembly produces a magnetic field that has a dominant axial component in the investigation volume 21. The directions of the RF magnetic field (produced by two transverse dipole antennas as in Figure 2A) and the static magnetic field in this region are shown in 22. In the example shown in Figure 2B, two separate investigation volumes 24A, 24B are created near the poles of the magnet (beyond the axial ends of the central magnet) where the static magnetic field has a predominantly radial component. The example NMR antennas shown in 23A and 23B can generate RF magnetic fields in investigation volumes 24A and 24B near the longitudinal dipole antenna. The longitudinal direction of the RF magnetic fields in research volumes 24A and 24B, and the radial direction of the static magnetic field in research volumes 24A and 24B, are shown in 25A and 25B.

In some aspects, a combination of monopole and transverse dipole antennas can be used to allow selective unidirectional azimuthal measurements, without substantially reducing the SNR in some cases. In some examples, NMR excitation can be axially symmetric substantially (for example, using the transverse dipole antenna or the monopole antenna) while a combination of transverse dipole antenna of axially symmetric sensitivity and the monopole antenna responses of axially symmetric sensitivity they can allow azimuthal resolved measurements.

Figures 3A and 3B illustrate aspects of an example azimuthal selective NMR tool. Figure 3A is a graph 300A showing an example of azimuthal data selected from the bottom well tool 3 00B shown in Figure 3B. The example NMR tool 300B includes a magnet assembly that generates a static magnetic field to produce the polarization, and an antenna assembly that (a) generates a radiofrequency (RF) magnetic field to generate the excitation, and (b) Acquires NMR signals. The antenna assembly 31 shown in Figure 3B includes a monopole antenna and two orthogonal transverse dipole antennas 35 and 36. The example monopole antenna includes two coils 37A and 37B connected in reverse polarity to generate a substantially radial RF magnetic field in the investigation volume 34. Due to reciprocity, the same coil configuration may have a radial sensitivity direction. The example RF Brf magnetic fields presented in 32 and 33 may reflect the total sensitivity direction when the response of the monopole antenna is combined with one of the responses of the transverse dipole antenna.

The example monopole antenna shown in Figure 3B includes an arrangement of coils that locally generate a substantially radially directed magnetic field, that is, the field that would be produced by an individual magnetic load or magnetic pole. Here, the term monopole is used to distinguish this type of magnetic field from a dipole magnetic field (transverse or longitudinal). In some cases, the monopole antenna assembly generates quasi-stationary magnetic fields (relatively low frequency). In the example shown, coils 37A and 3 7B, which are connected in reverse polarity, are two parts of a monopole antenna assembly. Each coil itself can be implemented as a standard longitudinal antenna. A monopole antenna can be implemented in another way.

The polar graph in Figure 3A shows an example of antenna sensitivity, demonstrating unidirectional azimuthal selectivity. A combination of the responses of each of the transverse orthogonal dipole antennas with the response of the monopole antenna can provide any of four possible directions covering all quadrants of the transverse plane. Rotation of the drill string during drilling can cause a modulation of the amplitude of the selective response azimuthally and, therefore, a modulation of the amplitude of the NMR relaxation signal (e.g., an echo train of CPMG). The amplitude modulation parameters may indicate the azimuthal variations of the NMR properties (for example, the variations of the NMR porosity).

The coils 37A and 37B of the example monopole antenna shown in Figure 3B can be used in combination with the transverse dipole antennas 35 and 36, for example, to achieve azimuthal selectivity. Any of the coils 37A and 37B can also be used as a separate antenna (in addition to the transverse dipole antennas 35, 36 or without them), for example, to gain SNR. In some cases, an NMR tool is implemented with a monopole antenna and a longitudinal magnet, without other antennas. For example, transverse dipole antennas 35 and 36 can be omitted from antenna assembly 31, in some cases.

Figure 4A is a flow chart showing an example process 400 for obtaining NMR data from an underground region; and Figure 4B is a flow chart showing another example process 420 for obtaining NMR data from an underground region. Each of the processes 400 and 420 can be performed independently of the other, or the processes 400 and 420 can be performed simultaneously or together. For example, processes 400 and 420 can be performed in series or in parallel, or one of the processes can be performed without performing the other.

The processes 400 and 420 can be performed by bottomhole NMR tools such as the example 200A, 200B or 300B NMR tools shown in Figures 2A, 2B and 3B, or by another type of NMR tool. Processes 400 and 420 can be performed by an NMR tool at the bottom of the well while the tool is inside the well during well system operations. For example, the NMR tool at the bottom of the well can be suspended in the well for record acquisition by electric cable (for example, as shown in Figure IB), or the NMR tool at the bottom of the well can be attached to a drill string for NMR LWD (for example, as shown in Figure 1C).

Each of the processes 400 and 420 may include the operations shown in Figures 4A and 4B (respectively), or any of the processes may include additional or different operations. The operations can be performed in the order shown in the respective figures or in another order. In some cases, one or more of the operations can be performed in series or in parallel, for periods of time that overlap or not. In some cases, one or more of the operations may be iterated or repeated, for example: for a specified number of iterations, for a specified length of time, or until a termination condition is achieved.

In 402 in the example process 400 shown in Figure 4A, the NMR tool is positioned in a well. In some cases, the NMR tool includes an assembly of magnets to produce a magnetic field in one volume in the underground region around the well. The volume may include, for example, all or part of any of the research volumes 17, 21, 24A, 24B, 34 shown in Figures 2A, 2B or 3B, or other volume of interest. Generally, the NMR tool includes an assembly of magnets to polarize the nuclear spins in the volume of interest, and an antenna assembly to excite the nuclear spins and to acquire an NMR signal based on the excitation.

In 404, polarization is generated in a volume around the well. The polarization is generated by a static magnetic field, which is produced by the assembly of magnets of the NMR tool in the well. Polarization refers to the magnetic polarization of nuclear spins in volume. In other words, a part of the nuclear spins aligns with the static magnetic field, and the volume develops a volumetric magnetic moment. In some cases, the static magnetic field is configured (for example, by the shape and position of the magnet assembly) to produce longitudinal polarization (for example, parallel to the long axis of the well) or polarization that has another orientation.

In some examples, the magnet assembly includes a central magnet (for example, the central magnet 12 shown in Figures 2A, 2B, 3B, or other type of central magnet) and two end magnets (for example, the magnets of the ends 11A, 11B shown in Figures 2A, 2B, 3B or other type of end magnet). In some cases, the magnets in the magnet assembly are permanent magnets. As shown, for example, in Figure 2A, the central magnet may be an elongated permanent magnet having a first axial end and a second opposite axial end, with the magnet of the first end separated from the first axial end of the central magnet, and with the magnet of the second end separated from the second axial end of the central magnet. In some cases, the two end magnets have a common magnetic field orientation, and the central magnet has the opposite magnetic field orientation (for example, so that both end magnets have a magnetic field orientation that is orthogonal with respect to the magnetic field orientation of the central magnet).

In 406, circular polarized excitation is generated in the volume around the well. Circular polarized excitation occurs in volume through an antenna assembly. For example, power can be supplied to the antenna assembly by means of a radio frequency current, which produces a radio frequency magnetic field (RF) in the volume around the well. The RF magnetic field generated by the antenna assembly manipulates the nuclear spins to produce an excited spin state that exhibits circular polarization. In other words, the resulting spin polarization has a circular (or circumferential) orientation in the volume around the well.

In some examples, the antenna assembly includes orthogonal transverse dipole antennas. The antenna assembly 13 shown in Figures 2Ά and 2B and the antenna assembly 31 shown in Figure 3B are examples of antenna assemblies that include two orthogonal transverse dipole antennas. Each antenna 15, 16 in the example antenna assembly 13 can independently produce a transverse dipole magnetic field, for example, when conducting radiofrequency current. In the examples shown, each transverse dipole magnetic field has a transverse orientation with respect to the longitudinal axis of the NMR tool. In other words, the transverse dipole magnetic field is oriented orthogonally to the long axis of the well.

In the example shown, the transverse dipole magnetic field produced by the antenna 15 is orthogonal with respect to the transverse dipole magnetic field produced by the other antenna 16. For example, in a Cartesian coordinate system of three mutually orthogonal directions, the longitudinal axis of the NMR tool can be considered the z-direction, and the transverse dipole magnetic fields (produced by the antennas 15, 16) are oriented along the x and y directions, respectively.

In some implementations, the NMR tool produces other types of excitation. For example, in some cases, circular polarized excitation occurs in a first sub-volume (for example, the investigation volume in Figure 2B) by transverse orthogonal dipole antennas, and excitation with another orientation occurs in the second and third sub-volumes (for example, research volumes 24A, 24B in Figure 2B) that are separated from the axial ends of the first sub-volume. The excitation in the second and third subvolumes can be produced, for example, by a longitudinal dipole RF field generated by other antenna assemblies (for example, by antennas 23A and 23B in Figure 2B). Different sub-volumes may be useful for different purposes. For example, the first sub-volume may be elongated (parallel to the long axis of the well), to acquire NMR data of the first sub-volume while the NMR tool moves along the well (for example, during the movement of a drill string). In some cases, the other sub-volumes can be positioned to acquire NMR data to perform the invasion profile of the sludge filtering or other applications.

In 408, an NMR signal is acquired by quadrature coil detection. The NMR signal is based on the excitation generated in 406. The NMR signal can be, for example, an echo train, a free induction emission (EIL) or other type of NMR signal. In some cases, the NMR data acquired includes IT relaxation data, T2 relaxation data or other data. The NMR signal can be acquired by the antenna assembly that produced the excitation or by another antenna assembly. In some cases, an NMR signal can be acquired in multiple subvolumes.

Orthogonal transverse dipole antennas can perform quadrature coil detection. Quadrature coil detection can be performed by using two orthogonal coils, where each one picks up the signal induced by circular polarization nuclear magnetization (the signal in the coils has a phase difference of 90 degrees). Even if only one coil is used during transmission (for example, which produces a linear polarized RF magnetic field), the same nuclear magnetization can be circular polarized. Quadrature coil transmission (two orthogonal coils driven by RF currents that have a phase difference of 90 degrees) can allow circular polarized excitation, which can help reduce energy consumption compared to linear polarized excitation in some cases. Quadrature coil detection can be used, for example, to increase the signal-to-noise ratio (SNR) when only one coil is energized (without using circular polarized excitation to simplify hardware), or circular polarization can be used to save power while detecting signals with a coil. In some cases, both circular polarization and quadrature coil detection can be used to save energy and increase SNR. In some cases, the use of circular polarization or quadrature coil detection (or both) is effective when the mutually orthogonal antennas are substantially identical. This is possible in the example magnet / antenna configuration having a longitudinal dipole magnet and two transverse antennas. Other configurations where one of the two antennas is less effective than the other, although they allow mutually orthogonal antennas, may not provide the same advantages in some cases.

In 410, the NMR data is processed. NMR data can be processed to identify physical properties of the underground region or to extract other types of information. For example, NMR data can be processed to identify the density, viscosity, porosity, material content, or other properties of the underground region around the well.

In 422 in the example process 420 shown in Figure 4B, the NMR tool is positioned in a well, and in 424 the polarization is generated in a volume around the well. Operations 422 and 424 in Figure 4B are similar to operations 402 and 404 shown in Figure 4Ά. For example, the NMR tool includes an assembly of magnets to polarize the nuclear spins in the volume of interest, and an antenna assembly to excite the nuclear spins and to acquire an NMR signal based on the excitation. Polarization can occur at 424 in the manner described with respect to operation 404 of Figure 4A and by the same type of magnet assembly; or the polarization can occur 424 in another way or by another type of magnet assembly.

In 426, excitation is generated in a volume around the well. The excitation is produced in the volume by means of an antenna assembly. For example, power can be supplied to the antenna assembly by means of a radio frequency current, which produces a radio frequency magnetic field (RF) in the volume around the well. The RF magnetic field generated by the antenna assembly manipulates the nuclear spins to produce an excited spin state. In some cases, the spin state has a higher excitation in a selected azimuthal direction, so that the spin excitation level varies along a circular (or circumferential) direction around the well, for example, due to a magnetic field of selective azimuthal RF.

In some examples, the antenna assembly includes a monopole and transverse dipole antenna assembly. The antenna assembly 31 shown in Figure 3B is an example of an antenna assembly that includes a monopole and transverse dipole antenna assembly. In the example shown in Figure 3B, the monopole and transverse dipole antenna assembly includes two orthogonal transverse dipole antennas 3 5 and 36 in a central region, and a monopole antenna that includes a first coil 37A at a first axial end of the transverse dipole antennas 35 and 36 and a second coil 37B at a second opposite axial end of the transverse dipole antennas 35 and 36; coils 37A and 37B of the monopole antenna are arranged with opposite polarity.

In 428, a selective NMR signal is acquired azimutically. The NMR signal is based on the excitation generated in 426. The NMR signal can be, for example, an echo train, a free induction emission (EIL) or other type of NMR signal. In some cases, the NMR data acquired includes IT relaxation data, T2 relaxation data or other data. The NMR signal can be acquired by the antenna assembly that produced the excitation or by another antenna assembly. In some cases, the NMR signal is acquired by an antenna assembly that has azimuthal selective sensitivity, such as a monopole and transverse dipole antenna assembly.

In some implementations, the azimuthal selective NMR signal is acquired as a combination of multiple NMR signal acquisitions. Signal acquisitions may include, for example, acquisitions by one or more transverse dipole antennas and one or more monopole antennas. The signals can be combined to allow azimuthal resolved measurements of the volume around the well. For example, in some cases, an appropriate combination of the responses of each of the orthogonal transverse dipole antennas with the response of the monopole antenna can provide any of four possible directions covering all quadrants of the transverse plane.

At 43 0, the NMR data is processed. NMR data can be processed to identify physical properties of the underground region or to extract other types of information. For example, NMR data can be processed to identify the density, viscosity, porosity, material content, or other properties of the underground region around the well. In some cases, NMR data is processed to identify azimuthal variations in the underground region around the well. For example, the rotation of the NMR tool can cause a modulation of the amplitude of the selective response azimuthal. The amplitude modulation parameters may indicate the azimuthal variations of the properties that affect the NMR signal (eg, porosity, density, viscosity, material content, etc.).

While this description contains many details, these should not be construed as limitations on the scope of what can be claimed, but as descriptions of the specific features of particular examples. Certain features that are described in the present description can also be combined in the context of separate implementations. On the contrary, several features described in the context of a single implementation can also be implemented in multiple modalities separately or in any suitable sub-combination.

A number of examples have been described. However, it will be understood that different modifications can be made. Accordingly, other implementations are within the scope of the following claims.

It is noted that in relation to this date, the best method known by the applicant to implement said invention is that which is clear from the present description of the invention.