In-situ high-signal-to-noise-ratio small-diameter nuclear magnetic logging system and design method

Abstract

The invention belongs to the field of nuclear magnetic resonance logging, and particularly relates to an in-situ high-signal-to-noise-ratio small-diameter nuclear magnetic logging system and a design method, and the method comprises the steps: forming a magnetic resonance signal-to-noise-ratio environment item according to environment factors in a detection area; the required magnetic resonance signal-to-noise ratio is set, system parameter items are obtained according to a magnetic resonance signal-to-noise ratio model, and the magnetic resonance signal-to-noise ratio model is the magnetic resonance signal-to-noise ratio, the magnetic resonance signal-to-noise ratio environment item and the system parameter items; selecting system working frequency, system receiving bandwidth and system transmitting power according to the system parameter items; and designing the nuclear magnetic logging system according to the system working frequency, the system receiving bandwidth and the system transmitting power, so that the structure of the nuclear magnetic logging system is matched with the system working frequency, the system receiving bandwidth and the system transmitting power. The nuclear magnetic logging system designed by the design method can obtain a high signal-to-noise ratio under the condition of realizing miniaturization. And the in-situ application effect of the small-diameter magnetic resonance logging equipment is expanded.

Description

Technical Field

[0001] The present invention belongs to the field of nuclear magnetic resonance logging, and in particular relates to an in-situ high signal-to-noise ratio small-diameter nuclear magnetic resonance logging system and a design method thereof. Background Art

[0002] Groundwater is a vital component of water resources for human life and industrial production. However, with rapid population growth and increasing urbanization, groundwater resources in many regions are threatened by a variety of emerging pollutants, including pesticides, detergents, and industrial chemicals. Hydrocarbons are widely present in these sources. Nuclear magnetic resonance logging is an effective solution for automated in-situ characterization, quantification, and localization of hydrocarbon contaminants in shallow groundwater resources. It combines the advantages of non-destructive detection and high detection efficiency of electromagnetic systems with the centimeter-level high resolution that traditional electromagnetic geophysical equipment lacks.

[0003] Nuclear magnetic resonance logging equipment used to monitor groundwater hydrocarbon contamination requires the use of small-diameter hydrological monitoring wells. By controlling the position of the logging probe within the well, effective samples can be detected in the geological environment surrounding the target measurement point. However, to accommodate the spatial limitations of small-diameter hydrological monitoring wells, traditional nuclear magnetic resonance logging equipment must be miniaturized. This results in an extremely low signal-to-noise ratio, limiting its applicability for practical hydrogeological surveys. Summary of the Invention

[0004] On the one hand, an embodiment of the present invention provides an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system to solve the technical problems of the adaptability of in-situ nuclear magnetic logging instruments used in hydrogeological exploration in confined working spaces and low detection signal-to-noise ratio.

[0005] Another aspect of the present invention provides a method for designing an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system.

[0006] A method for designing an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system according to an embodiment of the first aspect of the present invention includes: The magnetic resonance signal-to-noise ratio environmental term is formed according to the environmental factors in the detection area; The required MRI signal-to-noise ratio is set, and the system parameter items are obtained according to the MRI signal-to-noise ratio model. The MRI signal-to-noise ratio model is: ,in is the magnetic resonance signal-to-noise ratio, is the magnetic resonance signal-to-noise ratio environmental term, is the system parameter item; Select the system operating frequency, system receiving bandwidth and system transmitting power according to the system parameter items; The nuclear magnetic logging system is designed according to the system operating frequency, system receiving bandwidth and system transmitting power, so that the structure of the nuclear magnetic logging system matches the system operating frequency, system receiving bandwidth and system transmitting power.

[0007] Furthermore, the magnetic resonance signal-to-noise ratio environment term is: ,in represents the number of spins in the resonant state per unit volume, represents the gyromagnetic ratio, represents the reduced Planck constant, represents the spin quantum number, represents the Boltzmann constant, Indicates the ambient temperature, is the magnetic permeability of vacuum.

[0008] Furthermore, the system parameter items are: , Indicates the coil current intensity, Indicates the system receiving bandwidth, Indicates the system operating frequency, represents the coil resistance, Indicates the height of the magnetic field in the sensitive area, Represents the residual magnetization of the magnetic material, represents the gyromagnetic ratio.

[0009] Furthermore, the nuclear magnetic logging system includes: The nuclear magnetic logging electronic sub is connected to the logging probe through a connector and is lowered into the well along with the logging probe. The nuclear magnetic logging ground station is used to connect to the host computer to realize the transmission of upper control instructions, and convert the host computer transmission instructions from USB protocol to RS485 protocol through the instruction protocol conversion module, and then transmit them to the nuclear magnetic logging electronic short section through the interface, and collect the full-wave signal returned by the downhole nuclear magnetic logging electronic short section through the acquisition card.

[0010] Furthermore, the nuclear magnetic logging electronic sub includes: a lower main control module located in the well, a radio frequency digital power amplifier, an isolation switching switch and a differential narrowband signal receiver. The radio frequency digital power amplifier amplifies the digital signal generated by the lower main control module located in the well into a radio frequency signal, which acts on the integrated radio frequency coil for transmission and reception in the logging probe. The isolation switching switch is responsible for realizing the rapid switching of the connection relationship between the integrated radio frequency coil for transmission and reception and the radio frequency digital power amplifier and the differential narrowband signal receiver at different detection timings.

[0011] Furthermore, the nuclear magnetic logging system is designed based on the system operating frequency, system receiving bandwidth, and system transmitting power, and the circuit of the nuclear magnetic logging electronic sub is designed, including: A radio frequency digital power amplifier adopts a full-bridge class-D topology, wherein the transistors in the full-bridge class-D topology adopt gallium nitride metal oxide semiconductor field effect transistors; Using gallium nitride metal oxide semiconductor field effect transistor as the switching element of the isolation switch; A differential narrowband signal receiver adopts a cascaded operational amplifier structure, and the operational amplifier used in the first stage adopts a differential input structure.

[0012] According to an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system according to an embodiment of the second aspect of the present invention, the structure of the nuclear magnetic logging system satisfies the magnetic resonance signal-to-noise ratio represented by a magnetic resonance signal-to-noise ratio model, wherein the magnetic resonance signal-to-noise ratio model is: ,in is the magnetic resonance signal-to-noise ratio, is the magnetic resonance signal-to-noise ratio environmental term, is the system parameter item; The magnetic resonance signal-to-noise ratio environment term is: ,in represents the number of spins in the resonant state per unit volume, represents the gyromagnetic ratio, represents the reduced Planck constant, represents the spin quantum number, represents the Boltzmann constant, Indicates the ambient temperature, is the vacuum permeability; The system parameter items are: , Indicates the coil current intensity, Indicates the system receiving bandwidth, Indicates the system operating frequency, represents the coil resistance, Indicates the height of the magnetic field in the sensitive area, Represents the residual magnetization of the magnetic material, represents the gyromagnetic ratio.

[0013] Furthermore, the nuclear magnetic logging system includes: The nuclear magnetic logging electronic sub is connected to the logging probe through a connector and is lowered into the well along with the logging probe. The nuclear magnetic logging ground station is used to connect to the host computer to realize the transmission of upper control instructions, and convert the host computer transmission instructions from USB protocol to RS485 protocol through the instruction protocol conversion module, and then transmit them to the nuclear magnetic logging electronic short section through the interface, and collect the full-wave signal returned by the downhole nuclear magnetic logging electronic short section through the acquisition card.

[0014] Furthermore, the nuclear magnetic logging electronic sub includes: a lower main control module located in the well, a radio frequency digital power amplifier, an isolation switch and a differential narrowband signal receiver. The radio frequency digital power amplifier amplifies the digital signal generated by the lower main control module located in the well into a radio frequency signal, which acts on the integrated radio frequency coil in the logging probe. The isolation switch switches the connection between the integrated radio frequency coil in different positions and the differential narrowband signal receiver.

[0015] Furthermore, the radio frequency digital power amplifier adopts a full-bridge class D topology structure, and the transistors in the full-bridge class D topology structure are replaced by gallium nitride metal oxide semiconductor field effect transistors; The isolation switch uses a gallium nitride metal oxide semiconductor field effect transistor as a switching element; The differential narrowband signal receiver adopts a cascade operational amplifier structure, and the operational amplifier adopted in the first stage adopts a differential input structure.

[0016] The embodiments of the present invention have at least the following technical effects: Based on a magnetic resonance signal-to-noise ratio model, this paper derives system parameters suitable for in-situ high-SNR small-diameter nuclear magnetic logging. These parameters are used to design and optimize nuclear magnetic logging systems. This miniaturization enables high SNR, expanding the in-situ application potential of small-diameter magnetic resonance logging equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A structural block diagram of an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system provided by an embodiment of the present invention; Figure 2 A simplified circuit diagram of the radio frequency digital power amplifier portion of an electronic sub in an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system provided by an embodiment of the present invention; Figure 3 A simplified circuit diagram of one of the isolation switches in an electronic sub for an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system provided by an embodiment of the present invention; Figure 4 A circuit block diagram of a differential narrowband signal receiver in an electronic sub for an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system provided by an embodiment of the present invention; Figure 5 A schematic diagram of a power supply strategy for an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system provided by an embodiment of the present invention; Figure 6 A schematic diagram of the first echo signal obtained by testing a 15% gasoline sample using an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system provided by an embodiment of the present invention; Figure 7 A schematic diagram of system noise collected by a receiving window of an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system provided by an embodiment of the present invention; Figure 8 Schematic diagram of a theoretical modeling model for the signal-to-noise ratio of a probe in a wellbore of an in-situ high signal-to-noise ratio small-diameter nuclear magnetic well logging method provided by an embodiment of the present invention; Figure 9 A schematic diagram of establishing a sensitive area of ​​a probe in a wellbore of an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging method provided by an embodiment of the present invention; DETAILED DESCRIPTION

[0018] In order to make the purpose, technical solutions 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 intended to limit the present invention.

[0019] See also Figure 1 As shown, in one embodiment of the present invention, an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system is provided, comprising: The nuclear magnetic logging electronic sub is connected to the logging probe through a connector and is lowered into the well along with the logging probe. The nuclear magnetic logging ground station is used to connect to the host computer to realize the transmission of upper control instructions. It converts the host computer instructions from USB protocol to RS485 protocol through the instruction protocol conversion module, and then transmits them to the nuclear magnetic logging electronic short section through the interface. It also collects the full-wave signal returned by the downhole nuclear magnetic logging electronic short section through the acquisition card.

[0020] The logging probe is carried up and down within the wellbore by integrating a fixed pulley and steel cable on a hoisting bracket. The bracket can include universal wheels mounted at the bottom and a bottom support structure with adjustable fork angles. A height-adjustable middle support link is mounted on the bottom support structure.

[0021] The well logging probe is wound with multiple turns of parallel transceiver radio frequency coils. For example, five turns of transceiver radio frequency coils are arranged at equal intervals.

[0022] The NMR logging electronic sub is connected to the logging probe via a connector and is lowered into the well along with the probe. This short-distance connection between the NMR logging electronic sub and the logging probe minimizes the transmission distance of the transmitted power signal and the original echo signal, ensuring the transmission quality of key signals in the system.

[0023] The logging probe contains an integrated radio frequency coil for transmission and reception, and adopts a small-diameter neodymium iron boron Strikman magnet (a radially magnetized dipole magnet) and an epoxy resin shell to protect the magnet. The integrated radio frequency coil for transmission and reception is used to convert high-power radio frequency excitation pulses into electromagnetic waves to excite the sample to generate magnetic resonance signals during the excitation sequence, and serves as a sensing structure to sense the magnetic resonance signals generated by hydrogen protons during the relaxation process during the reception sequence. The neodymium iron boron Strikman magnet is used to provide a gradient polarization background magnetic field.

[0024] The nuclear magnetic logging ground station includes an acquisition card, a signal acquisition card voltage-stabilized power supply module, some command protocol conversion modules, and a connection interface with the downhole nuclear magnetic logging electronic short section; the nuclear magnetic logging ground station is connected to the host computer through a USB transmission line to realize the transmission of upper control commands, and after a series of command protocol conversions, it continues to transmit them to the downstream nuclear magnetic logging electronic short section through the interface; the acquisition card in the nuclear magnetic logging ground station is responsible for collecting the pre-processed full-wave signal returned by the downhole nuclear magnetic logging electronic short section, and also transmits it to the host computer through the USB interface and stores it in the host computer.

[0025] The nuclear magnetic logging electronic sub includes: a lower main control module located in the well, a radio frequency digital power amplifier, an isolation switch, and a differential narrowband signal receiver. The radio frequency digital power amplifier amplifies the digital signal generated by the lower main control module located in the well into a radio frequency power signal, which acts on the transceiver radio frequency coil in the logging probe. The isolation switch is responsible for quickly switching the connection between the transceiver radio frequency coil, the radio frequency digital power amplifier, and the differential narrowband signal receiver at different detection timings.

[0026] The RF digital power amplifier adopts a full-bridge Class D topology, and the full-bridge Class D topology includes a bridge circuit consisting of four transistors, and the transistors are gallium nitride metal oxide semiconductor field effect transistors; The isolation switch uses a gallium nitride metal oxide semiconductor field effect transistor as a switching element; The differential narrowband signal receiver adopts a cascade operational amplifier structure, and the operational amplifier adopted in the first stage adopts a differential input structure.

[0027] The NMR logging electronic sub can cover all functions related to RF operation during the NMR logging detection process, and is designed in combination with miniaturization requirements and high signal-to-noise ratio design points. Specifically, the RF digital power amplifier has high frequency, high efficiency, and high power output capabilities, the isolation switch has a high power tolerance level and extremely short state switching dead time, and the differential narrowband signal receiver has an ultra-narrow filtering bandwidth, ultra-low background noise, and high weak signal gain. These can achieve stronger magnetic resonance echo signal excitation capabilities and echo signal reception preprocessing quality.

[0028] Specifically, the lower master control module includes an RS485 to TTL protocol conversion module, an STM32 master control module and an FPGA controller. The STM32 master control module is responsible for establishing two-way communication with the host computer and triggering the FPGA controller to work in different experimental sequences. The FPGA controller implements specific and precise timing control in different experimental sequences under the control of the STM32 master control module.

[0029] The RF digital power amplifier uses a full-bridge Class D topology. To achieve stable, high-power RF output at higher operating frequencies and meet high signal-to-noise ratio design requirements, the transistors in the full-bridge Class D topology are Gallium Nitride Metal Oxide Semiconductor Field-Effect Transistors. At the same power level, this results in a smaller package size due to lower total gate charge and higher voltage withstand capability. According to the formula: , in is the GaN MOSFET driving state transition time, is the total gate charge, The lower total gate charge reduces the time required for state conversion of GaN MOSFET under certain driving conditions, while the smaller package size makes it suitable for the design requirements of small-diameter nuclear magnetic logging systems. Figure 2 As shown in the figure, the designed RF digital power amplifier operates at 2 MHz and the supply voltage is =30 V, at 50 Up to 625 W of peak power can be developed at the load.

[0030] For details, see Figure 2 The schematic diagram of the full-bridge output circuit in the RF digital power amplifier shown in the figure shows a full-bridge Class D topology structure using four gallium nitride metal oxide semiconductor field-effect transistors to form a bridge circuit. Specifically, the bridge circuit includes a first field-effect transistor G1, a second field-effect transistor G2, a third field-effect transistor G3, and a fourth field-effect transistor G4. The field-effect transistors are turned on and off by a pulse width modulation signal, thereby generating a differential PWM signal across the load. It is understood that the RF digital power amplifier also includes a digital input and processing circuit and a pulse width modulation module for generating a PWM signal, as well as a drive circuit for driving the bridge Class D topology structure according to the PWM signal. The digital input and processing circuit, the pulse width modulation module, and the drive circuit all adopt a conventional design structure. Detailed description is not required here.

[0031] See also Figure 3The single-channel isolation switch shown in the figure requires fast switching of coil connections under different timing conditions and weak current structure protection in the transceiver multiplexing coil system. It needs to have high state switching speed, high isolation, and high RF power tolerance. To achieve these design requirements within the limited design space, a gallium nitride metal oxide semiconductor field effect transistor is used as the switch element. It also includes three RF switches with the same operating conditions and design requirements. It has a rated design power level of 2kW. Figure 3 As shown in the figure, when testing one of the channels, it has a matching working bandwidth of up to 5MHz when turned on (S 11 < -13 dB, S 21 > -0.6 dB), with -50 dB bandwidth isolation when off, and 80 ns controlled turn-on dead time and 200 ns controlled turn-off dead time during controlled state switching. The shorter dead time helps ensure the acquisition integrity of the echo signal within the receiving timing.

[0032] Taking one circuit of the isolation switch as an example, it specifically includes a fifth field-effect transistor (FET) G5 and a sixth field-effect transistor (FET) G6, both of which utilize gallium nitride metal oxide semiconductor (GaN) field-effect transistors (FETs). The drain of the fifth FET G5 is connected to the signal input port via a resistor, with the gate serving as the control terminal. The source of the fifth FET G5 is connected to the drain of the sixth FET G6, and then connected to a broadband impedance matching network as the output terminal. The source of the sixth FET G6 is grounded. It is understood that the device also includes a PWM signal generation circuit and a driver module for driving the conduction of the fifth and sixth FETs G5 and G6.

[0033] The differential narrowband signal receiver is used to achieve the gain and filtering preprocessing of weak echo signals. V level) nuclear magnetic resonance signals are amplified to the dynamic range allowed by the acquisition card to achieve limited acquisition. The differential narrowband signal receiver has a built-in four-stage operational amplifier, and the target gain is achieved through negative feedback control. In addition, in order to achieve the best possible filtering of the echo signal to provide a good signal foundation for further digital signal post-processing, a sixth-order Butterworth bandpass filter is integrated in the differential narrowband signal receiver. In order to reduce the receiver's background noise as much as possible, the differential narrowband signal receiver is constructed using an ultra-low background noise operational amplifier and performs input noise source resistance matching. For example, the AD8428 ultra-low background noise operational amplifier, the operational amplifier used in its first stage has a differential input structure that can also achieve common-mode noise suppression. The noise coefficient of the cascaded operational amplifier structure mainly depends on the first-stage operational amplifier. The voltage noise density of the first-stage operational amplifier of the differential narrowband signal receiver at 1 kHz is only 1.3 , and as low as 1.5 The current noise density is so low that the noise amplitude introduced by the noise source matching resistor is negligible. The preprocessed full-wave signal is collected by an acquisition card with a high sampling rate of 25MHz and stored in the host computer. This supports the further use of advanced digital signal processing methods to compress the equivalent receiving bandwidth and further improve the signal-to-noise ratio. This nuclear magnetic logging echo signal acquisition and processing method based on full-wave acquisition and combining software and hardware features has higher degrees of freedom, a more ideal processing environment, and lower design difficulty compared to traditional hardware-based LIA extraction solutions. Figure 4 As shown, the differential narrowband signal receiver has a signal voltage gain of 80.9 dB at 2 MHz, a 2 MHz center frequency, and a 200 kHz (quality factor =10) ultra-narrow bandwidth.

[0034] In one embodiment, see Figure 4 As shown, the differential narrowband signal receiver specifically includes: a first operational amplifier, a first sixth-order Butterworth bandpass filter, a second operational amplifier, a second sixth-order Butterworth bandpass filter, a third sixth-order Butterworth bandpass filter, a third operational amplifier and a fourth operational amplifier connected in sequence, and negative feedback is set for the second operational amplifier, the third operational amplifier and the fourth operational amplifier.

[0035] In one embodiment, see Figure 5 As shown, the embodiment of the present application also includes a transmitting circuit voltage-stabilized power supply module, which uses a 5VDC-DC voltage-stabilized module (DC to DC) to convert the 12 V power supply to a 5 V regulated power supply voltage, and further uses the lower master control module LDO voltage-stabilized unit (low voltage difference linear regulator) to realize the conversion of the regulated power supply voltage from 5 V to 3.3 V to meet the power supply requirements of the lower master control module.

[0036] The ultra-low-noise, bidirectional, regulated power supply module for the receive circuit provides a bidirectional, regulated power supply for the rail-to-rail op amps in the differential narrowband signal receiver, reducing power supply noise interference with sensitive receivers. It includes +5V and -5V LDO voltage regulators and uses digital isolators to isolate the receive circuit reference ground plane from the transmit circuit reference ground plane. By using an isolated ground to establish a reference between the positive and negative battery potentials and balancing the resulting bidirectional voltages with a symmetrical voltage divider network, a single battery can be used to provide the bidirectional voltages required for the differential narrowband signal receiver and the +5V voltage for the isolated switching switch. The 12V power supply is supplied to the capture card via a 12V DC-DC regulator module. With the exception of the RF digital power amplifier, which requires a separate 30V lithium battery, the system can be powered by a single 12V power supply, enhancing portability.

[0037] The structure of the above-mentioned nuclear magnetic logging system satisfies the magnetic resonance signal-to-noise ratio represented by the magnetic resonance signal-to-noise ratio model, and the magnetic resonance signal-to-noise ratio model is: ,in is the magnetic resonance signal-to-noise ratio, is the magnetic resonance signal-to-noise ratio environmental term, is the system parameter item; The magnetic resonance signal-to-noise ratio environment term is: ,in represents the number of spins in the resonant state per unit volume, represents the gyromagnetic ratio, represents the reduced Planck constant, represents the spin quantum number, represents the Boltzmann constant, Indicates the ambient temperature, is the magnetic permeability of vacuum.

[0038] The system parameter items are: , Indicates the coil current intensity, Indicates the system receiving bandwidth, Indicates the system operating frequency, represents the coil resistance, Indicates the height of the magnetic field in the sensitive area, Represents the residual magnetization of the magnetic material, represents the gyromagnetic ratio.

[0039] In one embodiment, the diameter of the logging probe and the NMR logging electronic sub are both 70 mm, and the logging probe length is 450 mm. A magnetic field layer with a background field strength of 47 mT (corresponding to a hydrogen proton Larmor frequency of 2 MHz) is selected as the sensitive area. At this point, the radial detection range outside the logging probe can reach 55 mm, and the vertical resolution is 190 mm. Compared to the hundreds of kHz operating frequencies of existing mainstream commercial small-diameter NMR logging instruments, this embodiment significantly improves the theoretical signal-to-noise ratio of small-diameter magnetic resonance systems.

[0040] See also Figure 6 As shown in FIG, an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system provided in one embodiment of the present invention performed 64 stacking detection experiments on a simulated hydrological monitoring well sample of uniform fine sand with a porosity of 0.462 filled with 15% gasoline samples, 10% gasoline samples, and 5% gasoline samples. The peak amplitude of the first echo signal obtained was 87.78 mV. Figure 7As shown in the figure, when detecting an invalid sample using an in-situ high signal-to-noise ratio (SNR) small-diameter NMR logging system provided in one embodiment of the present invention, the noise amplitude collected by the receiving window was 3.5 mV. Therefore, the SNR of the high SNR system for detecting the target sample can reach as high as 25.08 after 64 stacking experiments.

[0041] On the other hand, an embodiment of the present invention provides a method for designing an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system, comprising: The magnetic resonance signal-to-noise ratio environmental term is formed according to the environmental factors in the detection area; The required MRI signal-to-noise ratio is set, and the system parameter items are obtained according to the MRI signal-to-noise ratio model. The MRI signal-to-noise ratio model is: ,in is the magnetic resonance signal-to-noise ratio, is the magnetic resonance signal-to-noise ratio environmental term, is the system parameter item; Select the system operating frequency, system receiving bandwidth and system transmitting power according to the system parameter items; The nuclear magnetic logging system is designed according to the system operating frequency, system receiving bandwidth and system transmitting power, so that the structure of the nuclear magnetic logging system matches the system operating frequency, system receiving bandwidth and system transmitting power.

[0042] The magnetic resonance signal-to-noise ratio environment term is: ,in represents the number of spins in the resonant state per unit volume, represents the gyromagnetic ratio, represents the reduced Planck constant, represents the spin quantum number, represents the Boltzmann constant, Indicates the ambient temperature, is the magnetic permeability of vacuum.

[0043] The system parameter items are: , Indicates the coil current intensity, Indicates the system receiving bandwidth, Indicates the system operating frequency, represents the coil resistance, Indicates the height of the magnetic field in the sensitive area, Represents the residual magnetization of the magnetic material, represents the gyromagnetic ratio.

[0044] In one embodiment, the radio frequency digital power amplifier adopts a full-bridge class D topology structure, and the transistors in the full-bridge class D topology structure adopt gallium nitride metal oxide semiconductor field effect transistors; The isolation switch uses a gallium nitride metal oxide semiconductor field effect transistor as a switching element; The differential narrowband signal receiver adopts a cascade operational amplifier structure, and the operational amplifier used in the first stage adopts a differential input structure.

[0045] In this embodiment, based on the existing theoretical model of the signal-to-noise ratio of magnetic resonance equipment detection, the existing theoretical model is improved by combining the actual spatial position and magnetic field characteristics of the magnetic resonance logging probe in a small-diameter hydrological monitoring well, so that an explicit connection is established between the theoretical model and the system parameters of the nuclear magnetic logging system; Specifically, the original magnetic resonance signal-to-noise ratio model is as follows: , in represents the number of spins in the resonant state per unit volume, represents the gyromagnetic ratio, represents the reduced Planck constant, represents the spin quantum number, represents the Boltzmann constant, Indicates the ambient temperature, Indicates the static magnetic field strength in the detection area, Indicates perpendicular to The excitation radio frequency magnetic field strength, Indicates the receiving bandwidth, represents the excitation pulse bandwidth, represents the coil resistance, Indicates the coil current intensity, Indicates the distance between the detection area and the center of the magnet, Indicates the position of any point in the detection area, Indicates the volume of the detection area; See also Figure 8 The schematic diagram of the signal-to-noise ratio theoretical modeling model is shown in In the coordinate system, the inner dotted circle represents the probe outline, and the radio frequency coil is longitudinally wrapped around the logging probe. The dark gray dotted circle represents the sensitive area outline. Any point in the sensitive area The background magnetic field at , the exciting magnetic field is , Figure 8 in Indicates the distance between the detection area and the center of the magnet, Indicates the position of any point in the detection area, is the probe magnet radius.

[0046] Due to the strong gradient of the magnetic field of the logging probe, the width of the sensitive area is very small, so it can be considered that the sensitive area and The propagation is uniform, and the magnetic resonance signal-to-noise ratio model can be modified as follows: , Specifically, since the magnetic resonance experiment is a point frequency application, the system excitation bandwidth Much smaller than the system receiving bandwidth , taking the 90-degree excitation pulse as an example, It can be expressed as: , in, The excitation pulse width is 90 degrees.

[0047] Since the 90-degree pulse corresponds to a 90-degree proton flip angle, if the excitation process is considered to be completely effective, The exciting magnetic field at It can be further expressed as: , Specifically, the magnetic resonance signal-to-noise ratio model is modified as follows: , Since the small diameter hydrological monitoring well does not limit the length of the logging probe magnet, ideally the magnet length is assumed to be infinite to further characterize the magnet and establish the relationship between the magnetic field strength and the radial distance. The background magnetic field at can be expressed as: , in, is the residual magnetization of the magnetic material, is the radius of the probe magnet. Since the Strikman probe generates a gradient magnetic field, its gradient It can be expressed as: , Specifically, see Figure 9 The diagram shows a schematic diagram of the sensitive area established by the probe in the well for an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging method. The circle represents the cross section of the sensitive area, and its width is , the probe magnet radius, i.e. the outer diameter, is , the inner diameter is . Set the magnetic field height of the sensitive area to , its cross-sectional area It can be expressed as: , Specifically, due to Much smaller than , which can be further simplified as: , It can be expressed as: , because and Approximately equal, the MRI signal-to-noise ratio model corrected The term can be further expressed as: , Specifically, It can be further simplified as: , Specifically, the magnetic resonance signal-to-noise ratio model can be further modified as follows: , The above-mentioned modified MRI SNR model is the magnetic field transfer efficiency, which can be further expressed as: , in, , can be expressed as: , To characterize all points in the sensitive area The overall effect requires the above Integrating to reduce the angle effect, it can be further expressed as: , because It can be further expressed as: , Can be further Corrected to: , The MRI signal-to-noise ratio model can be further modified as follows: , in: , It can be concluded that for an in-situ high SNR small-diameter NMR logging system, increasing the system operating frequency can improve the system SNR by a fourth power, increasing the system transmit power can improve the system SNR linearly, and reducing the system receive bandwidth can improve the system SNR by a quadratic power. This can be directly applied to the SNR optimization design of NMR logging systems.

[0048] 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 scope of protection of the present invention.

Claims

1. A design method for an in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system, characterized in that: include: The magnetic resonance signal-to-noise ratio environmental term is formed according to the environmental factors in the detection area; The required MRI signal-to-noise ratio is set, and the system parameter items are obtained according to the MRI signal-to-noise ratio model. The MRI signal-to-noise ratio model is: ,in is the magnetic resonance signal-to-noise ratio, is the magnetic resonance signal-to-noise ratio environmental term, is the system parameter item; Select the system operating frequency, system receiving bandwidth and system transmitting power according to the system parameter items; The nuclear magnetic logging system is designed according to the system operating frequency, system receiving bandwidth and system transmitting power, so that the structure of the nuclear magnetic logging system matches the system operating frequency, system receiving bandwidth and system transmitting power.

2. The design method of the in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system according to claim 1 is characterized in that: The magnetic resonance signal-to-noise ratio environment term is: ,in represents the number of spins in the resonant state per unit volume, represents the gyromagnetic ratio, represents the reduced Planck constant, represents the spin quantum number, represents the Boltzmann constant, Indicates the ambient temperature, is the magnetic permeability of vacuum.

3. The design method of the in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system according to claim 2 is characterized in that: The system parameter items are: , Indicates the coil current intensity, Indicates the system receiving bandwidth, Indicates the system operating frequency, represents the coil resistance, Indicates the height of the magnetic field in the sensitive area, Represents the residual magnetization of the magnetic material, represents the gyromagnetic ratio.

4. The design method of the in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system according to claim 1 is characterized in that: The nuclear magnetic logging system comprises: The nuclear magnetic logging electronic sub is connected to the logging probe through a connector and is lowered into the well along with the logging probe. The nuclear magnetic logging ground station is used to connect to the host computer to realize the transmission of upper control instructions, and convert the host computer transmission instructions from USB protocol to RS485 protocol through the instruction protocol conversion module, and then transmit them to the nuclear magnetic logging electronic short section through the interface, and collect the full-wave signal returned by the downhole nuclear magnetic logging electronic short section through the acquisition card.

5. The design method of the in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system according to claim 4 is characterized in that: The nuclear magnetic logging electronic sub includes: a lower main control module located in the well, a radio frequency digital power amplifier, an isolation switch, and a differential narrowband signal receiver. The radio frequency digital power amplifier amplifies the digital signal generated by the lower main control module located in the well into a radio frequency signal, which acts on the integrated radio frequency transceiver coil in the logging probe. The isolation switch is responsible for quickly switching the connection relationship between the integrated radio frequency transceiver coil and the radio frequency digital power amplifier and the differential narrowband signal receiver at different detection time sequences.

6. The design method of the in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system according to claim 5 is characterized in that: Design the nuclear magnetic logging system based on the system operating frequency, system receiving bandwidth and system transmitting power, and design the circuit of the nuclear magnetic logging electronic sub, including: A radio frequency digital power amplifier adopts a full-bridge class-D topology, wherein the transistors in the full-bridge class-D topology adopt gallium nitride metal oxide semiconductor field effect transistors; Using gallium nitride metal oxide semiconductor field effect transistor as the switching element of the isolation switch; A differential narrowband signal receiver adopts a cascaded operational amplifier structure, and the operational amplifier used in the first stage adopts a differential input structure.

7. An in-situ high signal-to-noise ratio small diameter nuclear magnetic logging system, characterized in that: The structure of the nuclear magnetic logging system satisfies the magnetic resonance signal-to-noise ratio represented by the magnetic resonance signal-to-noise ratio model, which is: ,in is the magnetic resonance signal-to-noise ratio, is the magnetic resonance signal-to-noise ratio environmental term, is the system parameter item; The magnetic resonance signal-to-noise ratio environment term is: ,in represents the number of spins in the resonant state per unit volume, represents the gyromagnetic ratio, represents the reduced Planck constant, represents the spin quantum number, represents the Boltzmann constant, Indicates the ambient temperature, is the vacuum permeability; The system parameter items are: , Indicates the coil current intensity, Indicates the system receiving bandwidth, Indicates the system operating frequency, represents the coil resistance, Indicates the height of the magnetic field in the sensitive area, Represents the residual magnetization of the magnetic material, represents the gyromagnetic ratio.

8. The in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system according to claim 7, characterized in that: The NMR logging system includes: The nuclear magnetic logging electronic sub is connected to the logging probe through a connector and is lowered into the well along with the logging probe. The nuclear magnetic logging ground station is used to connect to the host computer to realize the transmission of upper control instructions, and convert the host computer transmission instructions from USB protocol to RS485 protocol through the instruction protocol conversion module, and then transmit them to the nuclear magnetic logging electronic short section through the interface, and collect the full-wave signal returned by the downhole nuclear magnetic logging electronic short section through the acquisition card.

9. The in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system according to claim 8, characterized in that: The nuclear magnetic logging electronic sub includes: a lower main control module located in the well, a radio frequency digital power amplifier, an isolation switch, and a differential narrowband signal receiver. The radio frequency digital power amplifier amplifies the digital signal generated by the lower main control module located in the well into a radio frequency signal, which acts on the integrated radio frequency transceiver coil in the logging probe. The isolation switch is responsible for quickly switching the connection relationship between the integrated radio frequency transceiver coil and the radio frequency digital power amplifier and the differential narrowband signal receiver at different detection time sequences.

10. The in-situ high signal-to-noise ratio small-diameter nuclear magnetic logging system according to claim 9, characterized in that: The radio frequency digital power amplifier adopts a full-bridge class D topology structure, and the transistors in the full-bridge class D topology structure are replaced by gallium nitride metal oxide semiconductor field effect transistors; The isolation switch uses a gallium nitride metal oxide semiconductor field effect transistor as a switching element; The differential narrowband signal receiver adopts a cascade operational amplifier structure, and the operational amplifier adopted in the first stage adopts a differential input structure.

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