Nuclear magnetic resonance logging tool antenna and probe for measurements taken close to the wellbore.

Abstract

This application provides a nuclear magnetic resonance (NMR) logging tool antenna and probe for measurements taken against the wellbore. The antenna transmits radio frequency pulses and receives NMR signals from the formation. The antenna includes an antenna winding and an antenna core. The antenna winding is located on one side of the antenna core, and the other side is used to bond with the magnet of the target NMR logging tool. The antenna winding is used to connect to the logging tool circuitry of the target NMR logging tool. This application can effectively reduce the logging tool echo interval, achieve nanopore measurement, improve the identification ability of fluids and mud-bound fluids within nanopores, and thus improve the accuracy of oil and gas reservoir evaluation.

Description

Technical Field

[0001] This application relates to the field of oil well logging technology, and in particular to a nuclear magnetic resonance logging instrument antenna and probe for measurements taken against the well wall. Background Technology

[0002] In recent years, with the continuous development of unconventional oil and gas accumulation theory research and exploration and development technologies, tight oil and gas has experienced rapid growth globally. Horizontal well technology is one of the key technologies for unconventional oil and gas exploration and development. Logging methods that are close to the wellbore can adapt to the logging needs of highly deviated wells and horizontal wells. To ensure more accurate formation parameters are obtained during nuclear magnetic resonance (NMR) logging operations, NMR logging tools are required to have shorter echo intervals (TE) and higher instrument signal-to-noise ratios.

[0003] In existing technology, a radio frequency (RF) antenna for measurements attached to the wellbore is disclosed. The RF antenna surface is attached to the wellbore and generates an RF magnetic field B1 in the formation. This RF magnetic field is focused within a certain area centered 25 mm from the antenna surface (wellbore) and is orthogonal to the static magnetic field B0. Because the focal point of the RF magnetic field B1 emitted by this RF antenna is relatively shallow, this nuclear magnetic resonance (NMR) logging tool is highly sensitive to wellbore conditions. For example, when deep pits appear in the wellbore, some or all of the NMR signals acquired by the NMR logging tool come from the mud, thus losing this portion of formation information during logging. Alternatively, due to the presence of a mud cake between the instrument and the wellbore, the fluid content in the mud cake is much higher than the fluid content in the formation. When the mud cake is thick, the focal point of the RF antenna moves towards the wellbore, and the instrument will measure a portion of the NMR signal from the mud cake. However, because the mud cake contains a large amount of fluid, the NMR signal from the mud cake will annihilate the NMR signal from the formation.

[0004] Another type of radio frequency antenna for measurements attached to the wellbore is disclosed in the prior art. This radio frequency antenna consists of two parts: a main antenna and a spoiler antenna. The main antenna transmits radio frequency pulses into the formation and generates a radio frequency magnetic field B in the formation that is orthogonal to the B0 magnetic field. 10 B 10 A magnetic field excites atomic nuclei to produce nuclear magnetic resonance; the spoiler antenna is connected in series or parallel with the main antenna. When the instrument emits, the electromagnetic field B generated by the spoiler antenna in the mud is determined according to the superposition principle. 11 With B 10 After being superimposed, B in the mud 10The energy level drops to near zero, thus disrupting the conditions for nuclear magnetic resonance (NMR) generation and preventing hydrogen nuclei in the mud from producing NMR signals. Although the instrument handles mud NMR signal interference relatively well, the main antenna requires a long time to cool to zero after transmission, and the instrument's shortest echo interval (TE) is relatively long. This results in the loss of some fluid information within smaller pores during unconventional oil and gas reservoir logging. Therefore, it has certain limitations in unconventional oil and gas logging. Summary of the Invention

[0005] To address at least one problem in the prior art, this application proposes a nuclear magnetic resonance logging tool antenna and probe for measurements close to the wellbore, which can effectively reduce the logging tool echo interval, realize the measurement of micropores, and thus improve the accuracy of oil and gas reservoir evaluation.

[0006] To address the aforementioned technical problems, this application provides the following technical solution:

[0007] In a first aspect, this application provides a nuclear magnetic resonance logging tool antenna for measurements attached to a wellbore, the antenna being used to transmit radio frequency pulses and receive nuclear magnetic resonance signals from the formation, including:

[0008] Antenna windings and antenna core;

[0009] The antenna winding is provided on one side of the antenna core, and the other side is used to bond with the magnet of the target nuclear magnetic resonance logging tool.

[0010] The antenna winding is used to connect to the logging tool circuit of the target nuclear magnetic resonance logging tool.

[0011] Furthermore, the nuclear magnetic resonance logging tool antenna includes: multiple sets of antenna windings;

[0012] The antenna winding is located on the side of the probe that is in contact with the well wall, and is evenly laid at equal intervals on the upper part of the antenna core along the axis of the logging instrument probe.

[0013] Each antenna winding has the same inductance, impedance, and quality factor, and the input current direction is consistent.

[0014] Each group of antenna windings is connected to the logging instrument circuit in parallel.

[0015] Furthermore, each antenna winding transmits radio frequency pulses and simultaneously receives nuclear magnetic resonance signals from the fluid in the formation.

[0016] The intensity of the nuclear magnetic resonance signal received by the receiving circuit is the same as the intensity of the nuclear magnetic resonance signal received by a single antenna with the same sum of the lengths of each antenna winding along the probe axis.

[0017] Furthermore, the radio frequency pulses emitted by each group of antenna windings have no phase difference, and the B1 magnetic field generated at the same position at a distance from the antenna surface is the same.

[0018] Furthermore, each group of antenna windings is connected to the antenna residual energy discharge circuit in parallel, and the residual energy discharge time of the antenna is lower than that of a single antenna whose windings are connected in series or whose lengths in the probe axis direction are the same.

[0019] Furthermore, at least one set of antenna windings is connected to other antenna windings by a switch.

[0020] Furthermore, each antenna winding consists of at least 16 wires connected in parallel.

[0021] Furthermore, the antenna core is made of iron powder core material, and the relative permeability of the antenna core is greater than or equal to 20.

[0022] Furthermore, the magnetic saturation intensity of the antenna core is greater than or equal to 5000 Gauss.

[0023] Secondly, this application provides a probe for a nuclear magnetic resonance logging tool, comprising: a magnet and the aforementioned nuclear magnetic resonance logging tool antenna;

[0024] The magnet is bonded to the antenna core, and the antenna core extends along the length of the magnet.

[0025] Furthermore, the outer surface of the magnet is provided with a radio frequency shielding layer.

[0026] As can be seen from the above technical solution, this application provides a nuclear magnetic resonance logging tool antenna and probe for measurements close to the wellbore. The antenna is used to transmit radio frequency pulses and receive nuclear magnetic resonance signals from the formation. The antenna includes: an antenna winding and an antenna core; the antenna winding is located on one side of the antenna core, and the other side is used to bond with the magnet of the target nuclear magnetic resonance logging tool; the antenna winding is used to connect with the logging tool circuit of the target nuclear magnetic resonance logging tool, which can effectively reduce the logging tool echo interval, realize the measurement of micropores, and thus improve the accuracy of oil and gas reservoir evaluation; specifically, It can achieve nanopore measurement, improve the identification of fluids within nanopores and mud-bound fluids; it eliminates the need to consider active shielding of radio frequency magnetic fields, maximally suppressing mud signal generation, improving the instrument's signal-to-noise ratio, and ensuring the accuracy of formation fluid information and rock physical parameter measurements; it can improve the antenna's residual energy dissipation rate, significantly reducing probe dead time; it can also improve the instrument's longitudinal resolution, which is beneficial for measuring thin oil layers; and it can improve the accuracy of obtaining formation mud-bound water porosity, capillary water porosity, effective porosity, and total porosity. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 A schematic diagram of the CPMG radio frequency pulses used in nuclear magnetic resonance logging tools;

[0029] Figure 2 This is a schematic diagram of the probe of the nuclear magnetic resonance logging tool in the embodiments of this application;

[0030] Figure 3 This is a schematic cross-sectional view of the probe of the nuclear magnetic resonance logging tool in the embodiments of this application;

[0031] Figure 4 This is a schematic diagram showing the connection relationship between the magnet and the antenna core in an application example of this application;

[0032] Figure 5 This is a block diagram showing the connection between the antenna and the logging tool circuit described in the embodiments of this application;

[0033] Figure 6 This is a magnetic field distribution diagram of the B1 magnetic field generated by the antenna in the stratum in the application example of this application;

[0034] Figure 7 This is a diagram showing the superimposed magnetic field lines of the static magnetic field B0 and the radio frequency magnetic field B1 in the application example of this application.

[0035] Figure 8 This is a cloud map showing the distribution of the static magnetic field B0 of the magnet in the tested stratum, as described in the application example of this application.

[0036] Symbol explanation:

[0037] 1, 1A, 1B, 1C, 1D: Antenna windings;

[0038] 2: Antenna magnetic core;

[0039] 2A: First antenna core;

[0040] 2B: Second antenna core;

[0041] 3: Magnet;

[0042] 3A: First permanent magnet;

[0043] 3B: Second permanent magnet;

[0044] 3C: Third permanent magnet;

[0045] 3D: The fourth permanent magnet;

[0046] 4: Radio frequency shielding layer;

[0047] 5: Probe antenna housing;

[0048] 6: Well wall;

[0049] 7: Magnet frame;

[0050] 8A, 8B, 8C, 8D, 8E: Tuning capacitors;

[0051] 9A, 9B, 9C, 9D, 9E: Relays;

[0052] 10: Energy-consuming resistor;

[0053] 20: Antenna;

[0054] 30: Antenna selection switch;

[0055] 40: Antenna residual energy discharge circuit;

[0056] 50: Antenna tuning circuit;

[0057] 60: Duplexer;

[0058] 70: Transmitter;

[0059] 80: Receiver. Detailed Implementation

[0060] To enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0061] In recent years, due to the widespread application of horizontal well technology, the center-type logging method used in wireline logging is no longer adequate for the logging needs of highly deviated wells and horizontal wells. During logging operations, the center-type NMR logging tool transmits radio frequency pulses into the formation being tested via its NMR radio frequency antenna. The radio frequency pulse propagation path requires passing through the mud between the instrument and the wellbore, and some of the radio frequency energy is consumed in the mud. The lower the mud resistivity, the greater the radio frequency energy consumption in the mud. Therefore, it is necessary to increase the radio frequency transmission power of the NMR logging tool to ensure that the magnitude of the radio frequency magnetic field in the tested formation remains constant. Furthermore, the extensive use of saline mud in recent years to adapt to increased drilling speeds significantly reduces mud resistivity, making the center-type NMR logging tool no longer fully suitable for logging needs in low-resistivity mud wells.

[0062] The wall-mounted logging method involves using a pusher to attach the nuclear magnetic resonance logging tool to the well wall. The radio frequency antenna is in direct contact with the well wall, and the radio frequency pulses reach the formation directly without passing through the mud. The resistivity of the mud has virtually no impact on the radio frequency transmission. In addition, by adding a crawler to the instrument, the wall-mounted logging method can meet the logging needs of highly deviated wells and horizontal wells.

[0063] Nuclear magnetic resonance (NMR) technology can measure a series of parameters in formation rocks, such as porosity, permeability, oil (gas) content, and water saturation, and estimate the oil and gas content and recoverable reserves in the formation. Among these, measuring the pore structure and size of formation rocks is one of the important indicators for evaluating oil and gas reservoirs. Due to the presence of fractures and microfractures and the significant differences in pore size within the formation, the time variation of hydrogen nucleus polarization in the fluid within the pores is substantial. Therefore, using a long echo interval (TE)... L Acquiring nuclear magnetic resonance signals inevitably results in the loss of some information from the pinholes and microholes; if a short echo interval TE is used... S Acquiring echo signals significantly increases the hardware requirements of nuclear magnetic resonance logging tools and affects the overall logging speed. To obtain measurements that accurately reflect the actual formation porosity, the internationally accepted method for nuclear magnetic resonance logging involves using long waiting times and long echo intervals (TW). L TE L ) and long waiting time, long echo interval (TW) L TE L The two methods are combined to complete the formation nuclear magnetic resonance porosity measurement.

[0064] Echo interval (TE) is a technical parameter of nuclear magnetic resonance (NMR) logging tools. Its length determines the range of formation rock pore sizes that the logging tool can measure. The smaller the echo interval (TE), the smaller the fluid information within the pores that the logging tool can detect. Unconventional oil and gas reservoirs are characterized by low porosity and low permeability, with a main reservoir porosity of less than 10%, permeability of less than 0.1 mD, and pore radii reaching the nanometer level, with some reservoirs even having pore radii of several nanometers.

[0065] The nuclear magnetic resonance (NMR) logging tool probe consists of a magnet and a radio frequency (RF) antenna. The magnet generates a static magnetic field B0 with a specific distribution in the formation being measured, which magnetizes the hydrogen nuclei within the formation. The RF antenna performs two functions during measurement: First, during the transmission period of an echo measurement cycle, the RF antenna connects to the transmitter and disconnects from the receiver. The transmitter feeds a modulated RF pulse with a frequency of f = γB0 into the RF antenna, generating a radio frequency magnetic field B1 orthogonal to the B0 magnetic field in the formation, with a frequency of f = γB0. The hydrogen nuclei in the fluid magnetized by the static magnetic field B0 in the formation undergo rotation under the influence of the RF magnetic field B1. After the RF magnetic field B1 ends, the spin axis of the hydrogen nuclei precesses along the vector direction of the B0 magnetic field. Second, after the transmission cycle ends, the RF antenna quickly disconnects from the transmitter and connects to the receiver. At this point, the precession of hydrogen nuclei induces an electromotive force E in the radio frequency antenna, generating a nuclear magnetic resonance (NMR) signal. This NMR signal is transmitted to a receiver via the radio frequency antenna. The receiver processes and amplifies the signal before uploading it to the ground system. Ground processing software then performs inversion calculations to obtain the longitudinal relaxation T1 or transverse relaxation T2 of the tested stratum. Inversion of T1 or T2 yields the rock physical properties and fluid information of the stratum, such as porosity, permeability, oil (gas) saturation, and mobile fluid parameters.

[0066] Although the static magnetic field B0 is mostly distributed within the measurement area of ​​the formation, a portion of the magnetic field still exists within the wellbore. The drilling mud within the wellbore, under the influence of both the B0 and B1 magnetic fields, will also generate nuclear magnetic resonance (NMR). Since the hydrogen nucleus content in the drilling mud is much higher than that in the formation, the NMR signal in the drilling mud will be significantly stronger than the NMR signal in the formation. This means the formation NMR signal will be annihilated by the NMR signal from the drilling mud. Even if the formation NMR signal is not annihilated, the NMR signal from the drilling mud will still interfere with the accuracy of formation measurements. Therefore, it is necessary to add an antenna to the antenna array to disrupt the conditions for generating NMR signals in the drilling mud.

[0067] like Figure 1As shown, the nuclear magnetic resonance logging tool uses a CMPG pulse sequence, which includes an excitation pulse P90 and several refocusing pulses P180. After the hydrogen nuclei in the sample are magnetized by the B0 magnetic field and reach equilibrium, the macroscopic magnetization vector in Cartesian coordinates is M = M... Z =M0, and the magnetization vector in the xy plane is M. xy =0. When an excitation pulse P90 is applied on the x-axis, the macroscopic magnetization vector M is rotated to the xy-plane around the z-axis. After a time interval 2τ, a refocusing pulse P180 is applied again, refocusing the magnetization vector components of each phase onto the y-axis. At a certain moment after P180 ends, a spin echo is generated, which is the NMR signal to be acquired. After a time interval TE(2τ) after the previous P180 ends, another refocusing pulse P180 is applied, generating a spin echo with a slightly attenuated amplitude compared to the previous spin echo at the same time position. This process of applying refocusing pulses P180 continues until the echo amplitude decays to zero, resulting in an echo train with exponential decay characteristics. The NMR characteristics of the measured stratum, such as the T2 relaxation time spectrum, are obtained by processing the echo train.

[0068] The echo train contains rich stratigraphic information. Inversion processing of the echo train can yield a series of rock properties and fluid parameters, such as porosity, permeability, and fluid saturation. Obtaining these stratigraphic parameters through nuclear magnetic resonance (NMR) measurements requires that the duration τ from the end of P180 to the appearance of the echo be less than the fluid decay time t within the pores. a , that is, τ <t a This condition. The decay time t here. a Related to pore size, the larger the pore size, the more t a The larger the value, the greater the value; conversely, the larger the value, the greater the value. a The smaller the pore size, the better. For small pores (such as nanopores), t a Very small, when τ>t a At this time, the logging tool can only collect some fluid information in the pores or cannot collect any fluid information in the pores at all.

[0069] In order for the logging tool to acquire fluid information within small pores during well logging operations, it is necessary to reduce τ or the echo interval TE to meet the above conditions (where τ and TE are related by the equation TE = 2τ), but τ or the echo interval TE cannot be reduced indefinitely. Figure 1 As shown, after pulse P90 or P180 is emitted, there is a dead time t related to the logging tool. dead Damn time t dead From the probe dead time t dead1 and the settling time t of the electronic circuit filterdead2 Composition, time t dead The duration is mainly determined by the probe dead time t dead1 The duration is determined by the time. Among them, t dead1 Related to the rate at which the remaining energy of the radio frequency antenna dissipates, t dead2 This is related to the settling time of the filter in the receiving circuit. When setting the instrument's measurement parameters, τ > t must be satisfied. dead The parameter settings for logging tools are designed to ensure the purity of the acquired echo signals and prevent the introduction of noise during echo acquisition, which could severely impact the instrument's signal-to-noise ratio (S / N). A significant portion of this instrument noise originates from probe ringing, i.e., the unattenuated residual transmitted energy oscillating within the radio frequency antenna. Therefore, reducing the logging tool's dead time t is crucial. dead Only then can the echo interval TE of the instrument be adjusted to a more ideal state, thereby meeting the requirements for the measurement of micropores.

[0070] Log tool dead time t dead =t dead1 +t dead2 , where t dead2 The settling time of the logging tool's electronic filter circuit is not described in this application; the probe dead time t will be discussed in detail below. dead1 And give a reduction in t dead1 The technical methods and solutions adopted.

[0071] probe dead time t dead1 It consists of two parts: probe ringing and antenna residual energy decay time.

[0072] 1) Probe ringing

[0073] Probe ringing occurs when the RF antenna transmits an RF magnetic field B1 to the target stratum. A portion of the RF energy enters the soft magnetic core and permanent magnet surrounding the antenna. Due to the magnetostrictive properties of the materials, ultrasonic standing waves are excited within these components. The frequency of these ultrasonic standing waves is the same as the antenna's transmission frequency. If the receiver is turned on before the ultrasonic standing waves have completely attenuated, this portion of the ultrasonic standing waves will be received by the antenna and superimposed on the echo signal, introducing noise. Furthermore, because the angle between the static magnetic field B0 and the RF magnetic field B1 is not zero, a rotational torque is applied to the antenna when it transmits. This torque varies with the number of RF pulses applied by the antenna (e.g., P90 and P180). This varying torque causes the antenna to vibrate. The vibration of the RF antenna cuts the magnetic lines of force of the static magnetic field B0, inducing an electromotive force (EMF) on the RF antenna. This induced EMF is also superimposed on the echo signal, introducing noise. This method effectively solves the probe ringing problem, controlling the ringing attenuation time to within a few microseconds.

[0074] 2) Antenna remaining energy decay time

[0075] When the RF pulse transmission ends and the transmitter is turned off, some RF energy still remains in the RF antenna coil. This energy oscillates in the LC circuit composed of the antenna coil and capacitor, and is ultimately dissipated as heat in the antenna's internal resistance. The RF antenna coil current attenuation can be expressed by the following formula:

[0076]

[0077] Where i is the antenna coil current, I is the coil current when the transmitter is off, t is the current decay time, T = L / R, L is the coil inductance, and R is the internal resistance of the RF antenna.

[0078] Based on this, in order to effectively reduce the echo interval of the logging tool, realize the measurement of micropores, and thus improve the accuracy of oil and gas reservoir evaluation, this application provides a nuclear magnetic resonance logging tool antenna with a measurement method that is close to the well wall. This antenna is used to transmit radio frequency pulses and receive nuclear magnetic resonance signals from the formation. See [link to relevant documentation]. Figure 2 and Figure 3 The antenna specifically includes:

[0079] The antenna includes an antenna winding 1 and an antenna core 2. The antenna core has the antenna winding 1 on one side and is used to bond to the magnet 3 of the target nuclear magnetic resonance logging tool on the other side. The antenna winding 1 is used to connect to the logging tool circuit of the target nuclear magnetic resonance logging tool. The antenna can be a radio frequency antenna, and the antenna winding 1 can be a coil.

[0080] Specifically, the nuclear magnetic resonance logging tool antenna may include: multiple sets of antenna windings; the antenna windings are located on the side of the probe that is in contact with the well wall, and are evenly spaced on the upper part of the antenna core along the axis of the logging tool probe; the inductance, impedance, and quality factor (Q value) of each set of antenna windings are the same, and the input current direction is consistent; each set of antenna windings is connected to the logging tool circuit in parallel. This ensures that the radio frequency magnetic field B1 generated by the antenna in the formation is evenly distributed during transmission, ensuring that there is no phase difference in the radio frequency magnetic field B1 generated by each antenna winding, and also ensuring that the received signals have the same phase, facilitating the combination of signals collected by each set of antenna windings into a single signal; setting multiple sets of antenna windings can reduce the time required to release residual energy, thereby significantly reducing the probe dead time. Figure 2 As shown, the nuclear magnetic resonance logging tool antenna may include four sets of antenna windings: antenna windings 1A, 1B, 1C, and 1D. For example... Figure 3 As shown, the nuclear magnetic resonance logging tool antenna may include: antenna core 2A and antenna core 2B.

[0081] In order to ensure that the nuclear magnetic resonance signals received by each antenna winding have the same phase, and thus ensure the final accumulation of nuclear magnetic resonance signals, the radio frequency pulses emitted by each antenna winding have no phase difference, and the B1 magnetic field generated at the same position at a distance from the antenna surface is the same.

[0082] To significantly reduce the probe dead time t without affecting the NMR signal intensity dead1 This reduces the echo interval of the nuclear magnetic resonance logging tool. Each antenna winding is connected in parallel to the antenna's residual energy discharge circuit. The residual energy discharge time of the antenna is lower than that of a single antenna with windings connected in series or with the same length as the antennas along the probe axis. The time required to release residual energy can be reduced to one-sixteenth of the time required for a single coil.

[0083] To switch the number of antenna windings connected to the logging tool circuitry, thereby improving the longitudinal resolution of the NMR logging tool and facilitating the measurement of thinner oil layers, at least one set of antenna windings is connected to other antenna windings via a switch. This switch can be an antenna selection switch, and its opening and closing can be controlled by a main control circuit. The number of antenna windings connected to the NMR logging tool circuitry can be controlled by the closing of the antenna selection switch. For example, ... Figure 5 As shown, when the antenna selection switch 30 is open, only antenna winding 1A is connected to the logging tool circuit. When the antenna selection switch is closed, all antenna windings are connected to the logging tool circuit, and each group of antenna windings can be considered as one antenna winding to complete the logging tool's transmission and reception. In this case, the longitudinal resolution of the logging tool's positioning is equal to the total length of all antenna windings. When the antenna selection switch is open, only at least one group of antenna windings is connected to the logging tool circuit, and only at least one group of antenna windings completes the logging tool's transmission and reception. In this case, the longitudinal resolution of the logging tool's positioning is equal to the length of at least one group of antenna windings. This setting can improve the instrument's longitudinal resolution, which is beneficial for measuring thinner oil layers. However, when only at least one group of antenna windings is connected to the logging tool circuit, it will cause a decrease in the signal-to-noise ratio (S / N) of the nuclear magnetic resonance logging tool. Therefore, the mode with the antenna selection switch closed is used for logging operations. Only when it is determined that a thin oil layer exists in a certain formation should the logging be performed in the mode of alternating closing of the antenna selection switch. This operation can ensure the signal-to-noise ratio (S / N) of the nuclear magnetic resonance logging tool, while also increasing the longitudinal resolution of the nuclear magnetic resonance logging tool.

[0084] To address the current attraction effect on the antenna conductors and reduce their internal resistance, each antenna winding can consist of 16 wires connected in parallel, ensuring optimal antenna efficiency. Each antenna winding can be made of copper.

[0085] In order to improve antenna performance and RF pulse switching rate and reduce hysteresis loss of antenna core, the antenna core can be made of iron powder core material, and the relative permeability of the antenna core is greater than or equal to 20.

[0086] Furthermore, the coercivity of the antenna core can be less than the coercivity threshold, which can be set according to actual needs, and this application does not impose any restrictions on it.

[0087] To prevent the antenna core from being saturated by a strong B0 magnetic field and to ensure that the antenna core functions, the magnetic saturation intensity of the antenna core is greater than or equal to 5000 Gauss.

[0088] See Figure 2 This application provides a probe for a nuclear magnetic resonance logging tool, comprising: a magnet 3 and an antenna; the magnet 3 is bonded to the antenna core 2, and the antenna core 2 extends along the length of the magnet. The antenna core is located between the antenna winding and the magnet to improve the sensitivity of the radio frequency antenna.

[0089] See Figure 3 To prevent eddy currents caused by radio frequency (RF) energy entering the magnet, which could lead to RF energy loss and probe ringing, an RF shielding layer 4 is provided on the outer surface of the magnet. The RF shielding layer 4 can be made of a non-magnetic material with good electrical conductivity, such as copper. The RF shielding layer 4 effectively shields the RF magnetic field, preventing it from entering the magnet and probe frame and causing eddy currents, thus reducing eddy current loss in the RF antenna. The magnet with the RF shielding layer can hinder the distribution of the RF magnetic field B1 in the mud, suppressing mud signal generation without requiring an additional disruptive antenna. In other words, the external shielding layer of the magnet prevents the RF magnetic field from entering the magnet and metal frame, thus avoiding eddy current generation. Simultaneously, the magnet structure with the external shielding layer can block the RF magnetic field B1 from entering the mud, preventing mud signal generation.

[0090] To effectively reduce mud signal generation and improve the accuracy of formation fluid information and rock physical parameter measurements, see [reference needed]. Figure 4In one application example of this application, the magnet includes: a magnet frame 7, and a first permanent magnet 3A, a second permanent magnet 3B, and a third permanent magnet 3C respectively bonded to the magnet frame and extending along the length direction of the magnet frame; the antenna core includes: a first antenna core 2A and a second antenna core 2B; the first and second permanent magnets are symmetrically arranged on two opposite sides along the length direction of the magnet frame; the first antenna core is bonded to one end of the first permanent magnet, and the second antenna core is bonded to one end of the second permanent magnet; the other ends of the first and second permanent magnets, as well as the magnet frame, are all bonded to the connecting end of the third permanent magnet; wherein, the magnetization direction of the third permanent magnet is parallel to the connecting end, and the magnetization directions of the first and second permanent magnets are opposite and both perpendicular to the connecting end; at the bonding point between the third and first permanent magnets, the magnetic poles of the third permanent magnet and the first permanent magnet are different.

[0091] Further, see Figure 4 The magnet further includes a fourth permanent magnet 3D; the fourth permanent magnet is disposed in a groove at one end of the magnet frame away from the third permanent magnet and is symmetrical about the central axis of the magnet frame; the third permanent magnet and the fourth permanent magnet have the same magnetization direction.

[0092] This application provides a logging device, including: a logging instrument circuit and an antenna 20; see also Figure 5 The logging tool circuit includes: an antenna selection switch 30, an antenna residual energy discharge circuit 40, an antenna tuning circuit 50, a duplexer 60, a transmitter 70, and a receiver 80. The antenna residual energy discharge circuit 40 is connected to the antenna selection switch 30, the transmitter 70, and the receiver 80. The antenna is connected to both the antenna selection switch 30 and the antenna residual energy discharge circuit 40. A duplexer 60 may be provided between the receiver 80 and the antenna residual energy discharge circuit 40. The antenna residual energy discharge circuit 40 includes a power dissipation resistor 10. 8A, 8B, 8C, 8D, and 8E represent tuning capacitors, and 9A, 9B, 9C, 9D, and 9E represent relays. The logging equipment can be used to measure formation information.

[0093] In this configuration, antenna windings 1B, 1C, and 1D of antenna 20 are connected to transmitter 70 and receiver 80 in parallel. Antenna winding 1A of antenna 20 is connected to transmitter 70 and receiver 80 via antenna selection switch 30 and is connected in parallel with antenna windings B, 1C, and 1D. When antenna selection switch 30 is open, only winding A is connected to transmitter 70 and receiver 80, which improves the longitudinal resolution of the NMR logging tool. Antenna 10 is switched between transmitter 70 and receiver 80 via duplexer 60. A Q-Switch 40 is provided between the antennas to quickly discharge residual energy in the antenna windings and reduce the logging tool echo interval TE. An antenna tuning circuit 50 is provided between the antennas to adjust the antenna resonant frequency. The rapid discharge circuit for residual antenna energy is located between the antenna and the tuning circuit and its position cannot be arbitrarily changed.

[0094] To address the problems in the existing technology, this application proposes an application example of a radio frequency (RF) antenna that can effectively reduce the echo interval (TE) of a logging tool. This RF antenna can be used on a nuclear magnetic resonance (NMR) logging tool that is used in close contact with the wellbore. The function achieved by this RF antenna is equivalent to that of the aforementioned NMR logging tool antenna. The specific description of this RF antenna is as follows:

[0095] The radio frequency antenna consists of two parts: an antenna winding and an antenna core.

[0096] The antenna winding consists of four coils: antenna winding 1A, antenna winding 1B, antenna winding 1C, and antenna winding 1D. These coils are located on the side of the probe that is in contact with the well wall and are evenly spaced along the axis of the logging tool probe. The four coils are connected in parallel to the tuning circuit. Each coil must have identical dimensions and inductance, and the current direction must be the same in each coil. This configuration ensures that the radio frequency magnetic field B1 generated by the RF antenna during transmission is evenly distributed in the formation and that there is no phase difference in the radio frequency magnetic field B1 generated by each coil. It also ensures that the received signals have the same phase, facilitating the combination of signals collected by each coil into a single signal.

[0097] Furthermore, the four sets of coils are connected in parallel with the antenna residual energy discharge circuit Q-Switch 40. When the RF pulse transmission ends and the transmitter 70 is turned off, the main control circuit controls Q-Switch 40 to open. The four sets of coils are connected in parallel to the energy dissipation resistor 10 in Q-Switch 40 and quickly release the residual energy in the coils. When the residual energy stored in the four sets of coils is the same as that stored in a single coil, the time required to release the residual energy can be reduced to one-sixteenth of the time required for a single coil, significantly reducing the probe dead time t. dead1 .

[0098] Furthermore, at least one of the four coil groups has a separate tuning circuit and antenna selection switch 30, and the transmission and reception of this coil group can be controlled by the main control circuit. Specifically, by controlling the other three coil groups 1B, 1C, and 1D to be shut down, allowing only coil 1A to be in transmission and reception mode, this setting can improve the longitudinal resolution of the instrument, which is beneficial for the measurement of thin oil layers.

[0099] Furthermore, only when the current is evenly distributed across each coil group can the coil's transmission achieve optimal efficiency. However, due to the attraction effect, the current can only be distributed on the surface of the coil conductor, which significantly increases the antenna's internal resistance. Therefore, using at least 16 wires connected in parallel to form a coil can effectively solve the current attraction effect and help reduce the coil's internal resistance, wherein each coil group is made of copper.

[0100] The antenna core 2 is located near the radio frequency antenna coil on the side of the probe that is in contact with the well wall, and is arranged along the axis of the logging tool probe. The cross-section of the antenna core is similar to a quarter circle, with a thickness of approximately 25-50 mm, and is made of a non-conductive soft magnetic material, such as ferrite or iron powder core. The function of the antenna core is to increase the sensitivity of the radio frequency antenna, improve the antenna efficiency, and isolate the radio frequency magnetic field, preventing the radio frequency magnetic field from entering the magnet and probe frame and causing radio frequency loss.

[0101] Furthermore, the antenna core should have a high relative permeability μ (with a value of not less than 20) and low coercivity, which is beneficial to improving antenna performance and RF pulse switching rate, thereby reducing the hysteresis loss of the antenna core.

[0102] Furthermore, the antenna core should have a high magnetic saturation intensity Bs to prevent it from being saturated by a strong B0 magnetic field. If the antenna core is saturated by the B0 magnetic field, its relative permeability will drop sharply, eventually decreasing to 1. At this point, the relative permeability μ of the antenna core is the same as that of air, and it will not function as an antenna core.

[0103] The radio frequency (RF) antenna is located on the side of the probe that is in contact with the well wall, with the magnet and probe metal frame behind it. The magnet and probe metal frame are encased in highly conductive copper, thus preventing the RF magnetic field from penetrating the magnet and probe metal frame. Furthermore, this probe structure provides excellent shielding against the RF magnetic field, preventing it from distributing to the back of the probe, i.e., the wellbore mud. Therefore, active shielding of the RF magnetic field does not need to be considered when designing the RF antenna.

[0104] To further illustrate this solution, this application also provides application examples of nuclear magnetic resonance logging tool antennas, which are described in detail below:

[0105] like Figure 2 and Figure 5 As shown, the RF antenna consists of four coils: antenna winding 1A, antenna winding 1B, antenna winding 1C, and antenna winding 1D, which are connected in parallel to the tuning module. An antenna selection switch 30 is located between antenna winding 1A and antenna windings 1B, 1C, and 1D. The closing of this antenna selection switch is controlled by the main control circuit, which controls the number of antenna windings connected to the nuclear magnetic resonance logging tool circuit. When the antenna selection switch is closed, all four antenna windings are connected to the logging tool circuit, and the four antenna windings can be considered as one antenna winding to complete the logging tool's transmission and reception. In this case, the longitudinal resolution of the logging tool's positioning is equal to the length of the four antenna windings. When the antenna selection switch is open, only antenna winding 1A is connected to the logging tool circuit, and only antenna winding 1A completes the logging tool's transmission and reception. In this case, the longitudinal resolution of the logging tool's positioning is equal to the length of antenna winding 1A. This configuration improves the instrument's longitudinal resolution, which is beneficial for measuring thinner oil layers. However, when only antenna winding 1A is connected to the logging tool circuit, the signal-to-noise ratio (S / N) of the NMR logging tool decreases. Therefore, during logging operations, the antenna selection switch is always closed. Only when a thin oil layer is determined to exist in a certain formation is logging performed using an alternating closed antenna selection switch mode. This operation ensures both the S / N of the NMR logging tool and increases the instrument's vertical resolution.

[0106] like Figure 1 As shown, after pulse P90 or P180 is emitted, there is a dead time t related to the logging tool. dead Damn time t dead From the probe dead time t dead1 and the settling time t of the electronic circuit filter dead2 Composition, time t dead The duration is mainly determined by the probe dead time t dead1 The duration of the signal is determined by the time required. Therefore, a residual energy discharge circuit Q-Switch40 is installed between the RF antenna and the transmitter. After the RF pulse transmission ends, the main control circuit controls Q-Switch40 to open, and the four antenna windings are connected in parallel to the energy dissipation resistor in Q-Switch40. The residual energy in the RF antenna is quickly dissipated as heat on the energy dissipation resistor. The effect is that, with the same amount of residual energy stored in the four coils as in a single coil, the time required to release the residual energy can be reduced to one-sixteenth of the time required for a single coil, significantly reducing the probe dead time t. dead1 .

[0107] like Figure 2As shown, four sets of saddle-shaped antenna windings are arranged at equal intervals along the axis of the nuclear magnetic resonance logging tool probe. The four sets of antenna windings are connected in parallel to the antenna tuning circuit, and the inductance of the four antenna windings is equal. To increase antenna sensitivity, such as... Figure 3 As shown, a soft magnetic antenna core 2 is filled at the bottom of the antenna winding. A shielding layer 4 is provided on the outside of the magnet assembly 3 (excluding the antenna). The shielding layer 4 is made of a non-magnetic material with good electrical conductivity, such as copper. The shielding layer 4 can effectively shield the radio frequency magnetic field, preventing the radio frequency magnetic field from entering the magnet and probe frame and causing eddy currents, thus reducing the eddy current loss of the radio frequency antenna.

[0108] Four sets of antenna windings generate in the strata as... Figure 6 The radio frequency magnetic field distribution shown is designed to ensure that the radio frequency magnetic field B1 generated by each antenna winding has the same distribution and phase in the stratum, and also to ensure that the nuclear magnetic resonance signal received by each antenna winding has the same phase. Therefore, it is required that the inductance of each antenna winding is equal and the current direction of each antenna winding is the same.

[0109] To ensure optimal efficiency of the radio frequency antenna, the current in each winding must be evenly distributed across the winding conductors. Therefore, the winding conductors consist of at least 16 wires connected in parallel. This arrangement not only effectively addresses the current attraction effect but also helps reduce the coil's internal resistance, ensuring optimal efficiency for the radio frequency antenna. Each coil is made of copper.

[0110] like Figure 7 and Figure 8 As shown, the antenna winding is located on the side of the probe that is in contact with the well wall. Behind it is the magnet and shielding layer; the magnet is the magnet assembly. This probe structure provides excellent shielding against radio frequency magnetic fields, preventing these fields from distributing to the back of the probe, i.e., the wellbore mud. Additionally, a radio frequency magnetic field B1 exists within the gap between the probe antenna housing 5 and the well wall 6 at the probe's contact point with the well wall. However, the magnetic field strength of the static magnetic field B0 at this location is approximately 1500 Gauss, while the magnetic field strength in the measured area is approximately 117–500 Gauss. Figure 8 The numbers in the figure represent magnetic flux density in Tesla (T), and the spatial rectangular coordinate system corresponds to the geodetic coordinate system. Since the measurement frequency used by the logging tool is much lower than the frequency in this region, no nuclear magnetic resonance signal will be generated within this gap. Therefore, active shielding of the radio frequency magnetic field does not need to be considered when designing the radio frequency antenna.

[0111] The permanent magnet of the nuclear magnetic resonance logging tool, used for contact with the wellbore, consists of at least four sub-magnets, soft magnetic material, and a magnet frame. This permanent magnet functions similarly to the aforementioned magnet and antenna core. Sub-magnets a1 and a4 have the same polarization direction; sub-magnets a2 and a3 have opposite polarization directions and are perpendicular to the polarization directions of sub-magnets a1 and a4. The magnetic system composed of sub-magnets a1, a3, and the soft magnetic material can generate a B0 magnetic field with a magnetic field strength of approximately 117–235 Gs within a formation region centered on the sensor's central axis and with an opening angle of 120° on one side of the sensor's radial direction. This B0 magnetic field is a set of approximately fan-shaped gradient magnetic fields with a gradient of approximately 20 Gs / cm. Figure 1 As shown, the magnetic field strength in the mud on the other side of the sensor is less than 26 Gs. Therefore, this magnetic field distribution design can effectively reduce the generation of mud signals. The permanent magnet of the logging tool attached to the well wall consists of at least four sub-magnets. The magnetization direction of sub-magnet a2 is SN, the magnetization direction of sub-magnet a3 is NS, and the magnetization direction of sub-magnet a4 is NS. The magnets are arranged in the order of sub-magnets a3-1-a2 to form an SNSN arrangement. Sub-magnet a4 is located between antenna cores 2A and 2B in the soft magnetic material, and its magnetization direction is the same as that of sub-magnet a1. The functions of sub-magnets a1, a2, a3, and a4 can be respectively equivalent to the third, first, second, and fourth permanent magnets mentioned above.

[0112] As described above, the RF antenna provided in this application example can directionally generate a B1 RF magnetic field with a specific distribution pattern within a 120° range on the side of the NMR logging tool probe that is in contact with the wellbore. This magnetic field is orthogonal to the static magnetic field B0. Simultaneously, the probe's structural characteristics shield the distribution of the RF magnetic field B1 within the wellbore. This directional B1 RF magnetic field distribution characteristic can maximally suppress the generation of mud signals, ensuring the accuracy of formation fluid information and rock physical parameter measurements. The four coils of the RF antenna are simultaneously connected in parallel with the Q-Switch. This connection method accelerates the release of residual energy from the RF antenna, reducing the time required to release residual energy to one-sixteenth of the time required for a single coil, significantly reducing the probe dead time t. dead1 Meanwhile, at least one of the four coils has its own tuning circuit and switch. The main control circuit controls the other three coils to shut down, allowing only one coil to be in both transmitting and receiving states. This setup can improve the longitudinal resolution of the instrument and is beneficial for measuring thinner oil layers.

[0113] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on its differences from other embodiments. In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. The terms "upper," "lower," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are used only for the convenience of describing this application and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application. Unless otherwise expressly specified and limited, the terms "installation," "connection," and "linkage" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal connection between two components. Those skilled in the art can understand the specific meaning of the above terms in this application based on the specific circumstances.

[0114] Numerous specific details are set forth in the specification of this application. However, it will be understood that embodiments of this application may be practiced without these specific details. In some instances, well-known methods, structures, and techniques have not been shown in detail so as not to obscure the understanding of this specification. Similarly, it should be understood that, in order to streamline the disclosure of this application and aid in the understanding of one or more of the various inventive aspects, various features of this application are sometimes grouped together in a single embodiment, figure, or description thereof in the foregoing description of exemplary embodiments of this application. More precisely, as reflected in the claims, inventive aspects consist of fewer than all features of a single foregoing disclosed embodiment. Therefore, the claims following the detailed description are hereby expressly incorporated into the detailed description, wherein each claim itself is a separate embodiment of this application. It should be noted that, without conflict, the embodiments and features in the embodiments of this application may be combined with each other. This application is not limited to any single aspect, nor to any single embodiment, nor to any combination and / or substitution of these aspects and / or embodiments. Moreover, each aspect and / or embodiment of this application may be used alone or in combination with one or more other aspects and / or embodiments thereof.

[0115] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application.

Claims

1. A nuclear magnetic resonance logging tool antenna for measurements attached to a wellbore, the antenna being used to transmit radio frequency pulses and receive nuclear magnetic resonance signals from the formation, characterized in that, The antenna includes: The antenna winding and the antenna core are provided. The antenna winding is composed of multiple sets of antenna windings. Each set of antenna windings is connected in parallel to the antenna residual energy discharge circuit. At least one set of antenna windings is provided with an antenna selection switch between it and other antenna windings. The number of antenna windings connected to the antenna residual energy discharge circuit is controlled by the antenna selection switch. The antenna winding is provided on one side of the antenna core, and the other side is used to bond with the magnet of the target nuclear magnetic resonance logging tool; the antenna winding is used to connect with the logging tool circuit of the target nuclear magnetic resonance logging tool; the antenna winding is located on the side of the probe that is close to the well wall, and is evenly laid on the upper part of the antenna core at equal intervals along the axis of the logging tool probe. The inductance, impedance and quality factor of each antenna winding are the same, and the direction of the input current is consistent. There is no phase difference between the radio frequency pulses emitted by each antenna winding. When the antenna selection switch is closed, all antenna windings are connected to the logging tool circuit, and each antenna winding is considered as one antenna winding to complete the logging tool's transmission and reception. At this time, the longitudinal resolution of the logging tool at a fixed point is equal to the total length of each antenna winding. When the antenna selection switch is open, at least one of the antenna windings is connected to the logging tool circuit, and the at least one antenna winding completes the logging tool's transmission and reception. At this time, the longitudinal resolution of the logging tool at a fixed point is equal to the length of the at least one antenna winding. In thin oil layers, logging is performed using an alternating closed and closed antenna selection switch mode to ensure the signal-to-noise ratio of the nuclear magnetic resonance logging tool and increase its longitudinal resolution. Each antenna winding transmits radio frequency pulses and simultaneously receives nuclear magnetic resonance signals from the fluid in the formation; the nuclear magnetic resonance signal intensity of the receiving circuit is the same as the nuclear magnetic resonance signal intensity received by a single antenna with the same sum of the lengths of each antenna winding along the probe axis. Each group of antenna windings is connected to the antenna residual energy discharge circuit in parallel. The residual energy discharge time of the antenna is lower than that of a single antenna whose windings are connected in series or whose lengths in the probe axis direction are the same.

2. The nuclear magnetic resonance logging instrument antenna according to claim 1, characterized in that, The B1 magnetic field generated by each group of antenna windings at the same distance from the antenna surface is the same.

3. The nuclear magnetic resonance logging tool antenna according to claim 1, characterized in that, Each antenna winding consists of at least 16 wires connected in parallel.

4. The nuclear magnetic resonance logging tool antenna according to claim 1, characterized in that, The antenna core is made of iron powder core material, and the relative permeability of the antenna core is greater than or equal to 20.

5. The nuclear magnetic resonance logging tool antenna according to claim 1, characterized in that, The magnetic saturation intensity of the antenna core is greater than or equal to 5000 Gauss.

6. A probe for a nuclear magnetic resonance logging tool, characterized in that, include: The magnet and the nuclear magnetic resonance logging instrument antenna according to any one of claims 1 to 5; The magnet is bonded to the antenna core, and the antenna core extends along the length of the magnet.

7. The probe of the nuclear magnetic resonance logging tool according to claim 6, characterized in that, The outer surface of the magnet is provided with a radio frequency shielding layer.

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