A probe for a nuclear magnetic logging tool with separated transmitting and receiving coils

Abstract

The present invention belongs to the technical field of nuclear magnetic resonance logging, and provides a nuclear magnetic resonance logging instrument probe with separated transmitting and receiving coils, comprising: a probe frame, a first permanent magnet and a second permanent magnet, a transmitting coil, a first receiving coil and a second receiving coil. The probe frame is in the shape of an elongated cylinder, and a mud through hole is provided in the center of the probe frame. The first and second permanent magnets are symmetrically arranged in the upper and lower positions in the space area between the outer wall of the probe frame and the mud through hole. The magnetization directions of the first and second annular permanent magnets coincide with the axial direction of the probe frame, and the magnetization polarities are opposite. The transmitting coil is wound in the middle position of the probe frame, and the first and second receiving coils are symmetrically arranged between the transmitting coil and the first annular permanent magnet and between the transmitting coil and the second annular permanent magnet, with the same number of turns and opposite polarities, connected in series and output. The present invention makes the parameter design of the transmitting and receiving coils independent, does not require a transmitting and receiving multiplexing circuit with isolation and switching functions, simplifies the instrument design, improves reliability, improves the signal-to-noise ratio, and improves measurement accuracy.

Description

Technical Field

[0001] The invention belongs to the technical field of nuclear magnetic resonance logging, and in particular relates to a nuclear magnetic resonance logging while drilling probe with separated transmitting and receiving coils. Background Art

[0002] The NMR logging while drilling instrument uses the principle of nuclear magnetic resonance to measure the formation conditions around the oil well, thereby detecting relevant information about the hydrogen-containing fluid in the formation. Specifically, the NMR logging while drilling instrument uses a probe to form the static magnetic field and excitation field required for measurement, generates a nuclear magnetic resonance signal by exciting the hydrogen-containing fluid around the oil well, and then receives and analyzes the nuclear magnetic resonance signal to obtain relevant parameters of the hydrogen nuclei in the formation, and then measures relevant information about the hydrogen-containing fluid in the formation, such as the volume percentage of the hydrogen-containing fluid in the formation, the flow characteristics of the hydrogen-containing fluid, and the conductivity characteristics.

[0003] Among them, the probe is a key component of the NMR logging instrument that establishes a static magnetic field around the oil well and generates an excitation field to excite the hydrogen-containing fluid in the layer to produce NMR phenomenon and receive NMR signals. The structural design of the probe determines the key performance such as the range of the detection sensitive area, the measurement method of the NMR logging instrument, the intensity of the NMR signal generated, and the original signal-to-noise ratio of the received NMR signal.

[0004] The structure of the probe of the existing drilling nuclear magnetic resonance logging tool is as follows: Figure 1 As shown, the probe is in the shape of a long cylinder, and mainly includes: a probe frame 10, two permanent magnets 13 and 14 symmetrically arranged up and down, and a coil 15 for multiplexing in the middle position. The probe frame 10 includes a central mud through hole 11 and a probe housing 12; the two permanent magnets 13 and 14 symmetrically arranged up and down have their magnetic poles arranged opposite to each other, and such an arrangement will form a relatively uniform static magnetic field B0 in the sensitive area M. The coil 15 is multiplexed for multiplexing. The multiplexing coil 15 in the transmitting state generates an alternating excitation field B1 under the drive of the transmitting circuit of the while-drilling nuclear magnetic resonance logging instrument, and the direction of the alternating excitation field B1 is perpendicular to the direction of the static magnetic field B0. The hydrogen nuclei contained in the fluid in the sensitive area M absorb energy and enter a high-energy state under the action of the excitation field B1. After the excitation stops, the multiplexing coil 15 switches to the receiving state, and the hydrogen nuclei in the sensitive area are detected to produce nuclear magnetic resonance phenomenon in the process of gradually recovering from the high-energy state to the thermal steady state. The process of nuclear magnetic resonance is called the relaxation process, and the time used is called the relaxation time. During this relaxation time, the magnetic resonance phenomenon of the hydrogen fluid in the sensitive area M will cause the magnetic field around the receiving coil to change periodically, thus forming an induced voltage signal in the receiving coil, which is the nuclear magnetic resonance signal received by the receiving coil. During the relaxation time, the nuclear magnetic resonance signal output by the receiving coil changes from strong to weak and finally disappears.

[0005] The coil in the existing NMR logging while drilling tool probe is multiplexed for transmission and reception. Therefore, the coil needs to meet the constraints of both the transmitting and receiving circuits, such as coil wire diameter, number of turns, structure, installation position, etc. Figure 1 As shown, due to these constraints, the transceiver multiplexing coil 15 can only be installed at the center of the probe 10 and has a thicker wire diameter and fewer turns, which results in a smaller equivalent receiving area, poorer anti-interference ability, and a lower signal-to-noise ratio of the measurement result when the multiplexing coil 15 is used as a receiving coil. In addition, the multiplexing coil 15 requires a transceiver multiplexing circuit, which is used to achieve high-voltage isolation and full-speed switching functions. The transceiver multiplexing circuit increases the complexity of the while-drilling nuclear magnetic resonance logging instrument and reduces the overall reliability of the while-drilling nuclear magnetic resonance logging instrument. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a nuclear magnetic logging instrument probe with separated transmitting and receiving coils, which adopts separate transmitting coils and receiving coils, eliminates the common constraints on coils in the process of transmitting and receiving multiplexing of the prior art system, makes the transmitting coil wire diameter thicker, has fewer turns, and is installed in the center, and makes the receiving coil consist of two coils with thinner wire diameters and more turns, which are symmetrically installed at the upper and lower parts of the transmitting coil axially, and the two coils have the same number of turns and are connected in series with opposite polarities to output signals. The present invention achieves the purpose of increasing the equivalent receiving area of ​​the receiving coil and improving the amplitude of the received signal; the symmetrical structure eliminates the influence of environmental interference and improves the signal-to-noise ratio; no need for a transmitting and receiving multiplexing circuit, reduces system complexity, and improves reliability.

[0007] The present invention is achieved in this way.

[0008] A nuclear magnetic logging while drilling probe with separated transmitting and receiving coils, the probe comprising: a probe frame, a first annular permanent magnet and a second annular permanent magnet arranged at both ends of the probe frame and coaxially arranged with the probe frame, a transmitting coil arranged in the middle of the probe frame, and a first receiving coil and a second receiving coil symmetrically arranged at the upper and lower ends of the transmitting coil.

[0009] Furthermore: the probe skeleton is in the shape of an elongated cylinder, and a mud through hole is provided at the center of the probe skeleton; a first annular permanent magnet and a second annular permanent magnet are symmetrically arranged in upper and lower positions in the space area between the outer wall of the probe skeleton and the mud through hole, and the first annular permanent magnet and the second annular permanent magnet are magnetized along the axial direction of the probe skeleton, and the ends with the same magnetization polarity are opposite to each other; the first receiving coil and the second receiving coil are respectively arranged between the transmitting coil and the first annular magnet and between the transmitting coil and the second annular magnet.

[0010] Furthermore: the first receiving coil and the second receiving coil have the same number of turns and opposite polarities, and the first receiving coil and the second receiving coil are connected in series for output.

[0011] Furthermore: the transmitting coil and the receiving coil use copper cables or silver cables, which are spirally wound on the probe frame to form spiral conductive cables.

[0012] Furthermore: the cross-sectional shape, cable diameter, and number of coil turns of the conductive cable used to wind the transmitting coil and the receiving coil are optimized and designed according to the needs of transmitting and receiving, respectively, and the transmitting coil wire diameter is larger than the receiving coil wire diameter, and the number of turns is smaller than the receiving coil number.

[0013] Furthermore: the receiving coil is always directly connected to the receiving amplification circuit during the entire measurement process.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] (1) Separating the transmitting and receiving coils, the wire diameter, number of turns, and installation position can be designed according to the needs of the transmitting coil and the receiving coil, making the parameter design of the transmitting and receiving coils independent, thereby optimizing their respective application effects. It is recommended that the transmitting coil use a coil with a thick wire diameter and a small number of turns and be installed in the center, and the receiving coil use a coil with a thinner wire diameter and a large number of turns and be installed coaxially and symmetrically with the transmitting coil.

[0016] (2) Since the two receiving coils with the same number of turns and opposite polarity are connected in series, they are symmetrically located above and below the transmitting coil. As a result, the induced electromotive forces generated by the two receiving coils cancel each other out during the process of the transmitting coil generating the alternating excitation field. That is, the receiving coil will not sense the transmitting signal during the transmission process. Therefore, the receiving coil can be directly connected to the receiving amplifier circuit throughout the entire measurement process, without the need for a transceiver multiplexing circuit with isolation and switching functions. Eliminating the transceiver multiplexing circuit will bring two benefits:

[0017] The first is to simplify the whole circuit of the while-drilling nuclear magnetic resonance logging tool, reduce system complexity and improve reliability;

[0018] Secondly, the influence of the isolation switching process on the receiving circuit is eliminated, the detection speed is increased, and the signal quality is improved.

[0019] (3) The detection sensitive area is in the shape of a ring, located outside the probe, and is radially horizontal to the transmitting coil (note: originally coplanar), and the center of the ring coincides with the center of the probe. After excitation, the hydrogen nuclei contained in the fluid in the sensitive area produce nuclear magnetic resonance, that is, an alternating magnetic field is generated in the sensitive area. Since the axes of the two receiving coils also coincide with the axes of the probe and are symmetrically distributed at the upper and lower parts of the transmitting coil, the polarity of the alternating magnetic field generated by the magnetic resonance phenomenon in the two receiving coils is exactly opposite, that is, the magnetic flux of the upper receiving coil is enhanced, while that of the lower one is weakened, and vice versa. Since the two receiving coils adopt an anti-polarity series structure, the induced electromotive force generated by the alternating magnetic field in the sensitive area in the two receiving coils is superimposed, that is, the two receiving coils have a superimposed enhancement effect on the nuclear magnetic resonance signal.

[0020] For more distant interference, the distances between the two receiving coils and the interference source are basically the same. Therefore, the induced electromotive forces generated by the interference source in the two receiving coils are basically the same in magnitude but opposite in polarity, and will cancel each other out in the receiving coils without generating any output signal. That is, the two receiving coils with the same number of turns and opposite in polarity have a cancelling effect on more distant interference, with the beneficial effect of improving the signal-to-noise ratio and the measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a structural schematic diagram of a probe of a nuclear magnetic logging tool while drilling in the prior art;

[0022] Figure 2 A schematic diagram of the structure of a NMR logging tool probe with separated transmitting and receiving coils;

[0023] Figure 3 The schematic diagram of the principle of signal reception of the probe of the NMR logging tool while drilling with separated transmitting and receiving coils;

[0024] Figure 4 Schematic diagram of the principle of anti-interference of the probe of the while-drilling nuclear magnetic resonance logging tool with separated transmitting and receiving coils. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0026] The NMR logging while drilling tool can be used in vertical wells, inclined wells, and even horizontal wells. The probe posture is different in different applications, so the "axial direction" of the probe does not necessarily refer to the vertical direction; but for the convenience of explanation, this embodiment is described by taking the vertical well as an example, that is, in the following embodiments, the "axial direction" is set to the vertical direction, but this setting is only for the convenience of the embodiment description and does not constitute a limitation of the present invention.

[0027] This embodiment provides a probe of a nuclear magnetic resonance logging tool while drilling with separated transmitting and receiving coils, such as Figure 2 As shown, it includes: a long cylindrical probe skeleton 20, wherein the probe skeleton 20 includes a vertically centered mud through hole 21 and a probe shell 22 coaxial with the mud through hole 21, and a through hole structure is realized by inserting a cylinder with an inner diameter consistent with the mud through hole 21 into the probe shell 22; a first permanent magnet 23 and a second permanent magnet 24 symmetrically arranged up and down, the first permanent magnet 23 and the second permanent magnet 24 are annular, and are sleeved on the cylinder, wherein the first permanent magnet 23 and the second permanent magnet 24 are of the same size and the magnetic poles are arranged oppositely, and the upper and lower permanent magnets (23, 24) placed in this way will form a relatively uniform static magnetic field B0 in the sensitive area M. A transmitting coil 25 is arranged in the middle of the probe, and generates a magnetic field at the Larmor frequency (f0) under the drive of the transmitting circuit of the while drilling nuclear magnetic resonance logging instrument. L ) an alternating excitation field B1, the direction of the alternating excitation field B1 is perpendicular to the direction of the static magnetic field B0. The hydrogen nuclei contained in the fluid in the sensitive area M absorb energy and enter a high energy state under the action of the excitation field B1. The receiving coil is composed of a first receiving coil 26 and a second receiving coil 27 with the same number of turns and opposite polarities (winding in opposite directions) symmetrically installed at the upper and lower parts of the transmitting coil. The first receiving coil 26 and the second receiving coil 27 are both coaxial with the transmitting coil, and the output of the receiving coil is the output of the first receiving coil 26 and the second receiving coil 27 connected in series. Since the transmitting coil 25 is located between the first receiving coil 26 and the second receiving coil 27, the first receiving coil 26 and the second receiving coil 27 are symmetrically distributed above and below the transmitting coil 25. Therefore, the alternating magnetic field B1 generated by the transmitting coil 25 under the drive of the transmitting circuit of the while drilling nuclear magnetic resonance logging instrument generates magnetic flux changes in the first receiving coil 26 and the second receiving coil 27 of the same magnitude and direction. Since the two receiving coils have the same number of turns and are connected in series with opposite polarities, during the process of the transmitting coil 25 transmitting a signal, the induced electromotive forces generated by the change in the magnetic flux in the two receiving coils are of the same magnitude and opposite polarity, and the induced electromotive forces exactly cancel each other out, that is, the receiving coil will not sense the transmitting signal during the transmitting process.

[0028] The excitation of nuclear magnetic resonance generates an electromagnetic field of a specific frequency, which is the Larmor frequency f L . Larmor frequency f L The calculation formula is as follows:

[0029]

[0030] Where γ is the gyromagnetic ratio. For hydrogen nuclei, is a constant, so the Larmor frequency f of the hydrogen nucleus is L It is determined by the static magnetic field strength B0 at the location of the hydrogen nucleus.

[0031] The process of receiving NMR signals by the probe of the NMR logging tool while drilling based on the separation of the transmitting and receiving coils is as follows: Figure 3 As shown, the excited hydrogen nuclei in the sensitive area M absorb electromagnetic energy of a specific frequency and enter a high-energy state. The hydrogen nuclei in the low-energy state are the hydrogen nuclei in the paramagnetic state, and the hydrogen nuclei in the high-energy state are the hydrogen nuclei in the diamagnetic state. The magnetic poles of the hydrogen nuclei in the paramagnetic state are in the same direction as the static magnetic field B0, and the magnetic poles of the hydrogen nuclei in the diamagnetic state are in the opposite direction to the static magnetic field B0. From a macroscopic level, the excitation process is the process of generating high-energy hydrogen nuclei. Due to the appearance of high-energy hydrogen nuclei, a magnetic field B2 is formed in the opposite direction to the static magnetic field B0. B2 makes regular up and down swings at a specific frequency during the relaxation time and gradually decays. This up and down swing and gradual attenuation includes two aspects. One is that the intensity of B2 gradually decreases to zero, and the other is that the angle between the direction of B2 and B0 swings up and down at a certain frequency. Figure 3 The middle curve G reflects the process of up and down swinging and gradually attenuating, which is also called "precession". Since the polarity of the change in magnetic flux caused by the precession of B2 in the two receiving coils (26, 27) is exactly opposite, that is, when the magnetic flux of one receiving coil is enhanced, the magnetic flux of the other is weakened, and vice versa. Since the two receiving coils (26, 27) adopt a reverse polarity series structure, the induced electromotive force generated by the precession of B2 in the two receiving coils (26, 27) is superimposed, that is, the voltage output by the receiving coil is the result of the sum of the induced electromotive forces of the two series coils.

[0032] The remote anti-interference principle of the probe of the while-drilling nuclear magnetic logging tool with separated transmitting and receiving coils is as follows Figure 4 As shown, assuming that the alternating interference magnetic field generated by the interference source at a relatively long distance (generally not less than several meters) is B3, the polarity of the magnetic flux change caused by B3 in the two receiving coils (26, 27) is the same. Since the length of the probe of the drilling nuclear magnetic logging instrument with separated transmitting and receiving coils is about 1 meter, the distance between the two receiving coils (26, 27) is very close (generally at the decimeter level), so the distance from B3 to the receiving coils (26, 27) is basically the same, so it can be considered that the magnitude of the magnetic flux change caused by B3 in the two receiving coils (26, 27) is basically the same. In addition, because the receiving coils (26, 27) are connected in series with opposite polarity, the induced electromotive force generated by B3 in the two receiving coils is basically the same in magnitude and opposite in direction, and the outputs after the series connection basically cancel each other out.

[0033] The NMR logging probe with separated transmitting and receiving coils provided in this embodiment realizes the separation of transmitting and receiving coils. In engineering, parameters such as wire diameter, number of turns and installation position can be designed respectively according to the needs of the transmitting coil and the receiving coil, so as to optimize their respective application effects. The two receiving coils with the same number of turns and reverse polarity series connection are located at symmetrical positions on the axis of the transmitting coil. During the transmission process, the receiving coil will not sense the transmitting signal, so that the receiving coil can be directly connected to the receiving amplification circuit throughout the entire measurement process, and there is no need for a receiving and transmitting multiplexing circuit with isolation and switching functions. This not only simplifies the overall circuit of the NMR logging instrument, reduces system complexity and improves reliability, but also eliminates the influence of the isolation switching process on the receiving circuit, improves the detection speed and improves the signal quality. Since the two receiving coils adopt a reverse polarity series structure, the alternating magnetic field generated by the NMR phenomenon in the sensitive area induces electromotive forces in the two receiving coils that are superimposed, thereby enhancing the receiving effect of the NMR signal. The two receiving coils with the same number of turns and reverse polarity series connection have a canceling effect on distant interference, which can improve the signal-to-noise ratio and measurement accuracy.

[0034] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A nuclear magnetic logging while drilling probe with separate transmitting and receiving coils, characterized in that: The probe comprises: a probe frame, a first annular permanent magnet and a second annular permanent magnet arranged at both ends of the probe frame and coaxially arranged with the probe frame, a transmitting coil arranged in the middle of the probe frame, and a first receiving coil and a second receiving coil symmetrically arranged at the upper and lower ends of the transmitting coil; The probe skeleton is in the shape of an elongated cylinder, and a mud through hole is provided at the center of the probe skeleton; a first annular permanent magnet and a second annular permanent magnet are symmetrically arranged in the upper and lower positions in the space area between the outer wall of the probe skeleton and the mud through hole, and the first annular permanent magnet and the second annular permanent magnet are magnetized along the axial direction of the probe skeleton, and the ends with the same magnetization polarity are opposite; a first receiving coil and a second receiving coil are respectively arranged between the transmitting coil and the first annular permanent magnet and between the transmitting coil and the second annular permanent magnet; The first receiving coil and the second receiving coil have the same number of turns and opposite polarities, and are connected in series to output.

2. The probe of a nuclear magnetic logging while drilling tool with separated transmitting and receiving coils according to claim 1, characterized in that: The transmitting coil and the receiving coil use copper cables or silver cables, which are spirally wound on the probe frame to form spiral conductive cables.

3. The probe of a nuclear magnetic logging while drilling tool with separated transmitting and receiving coils according to claim 2, characterized in that: The cross-sectional shape, cable diameter, and number of coil turns of the conductive cable used to wind the transmitting coil and the receiving coil are optimized and designed according to the needs of transmitting and receiving, respectively. The transmitting coil has a larger wire diameter than the receiving coil, and the number of turns is smaller than the receiving coil.

4. The probe of the nuclear magnetic logging while drilling tool with separated transmitting and receiving coils according to claim 1, characterized in that: The receiving coil is always directly connected to the receiving amplification circuit during the entire measurement process.

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