阅读说明：本技术 一种核磁共振测井用复合射频脉冲方法 (Composite radio frequency pulse method for nuclear magnetic resonance logging ) 是由 朱万里 陈涛 王炜 贺飞 侯学理 陈江浩 钟剑 方璐 孙佩 王雷 郭庆明 彭宇 于 2021-01-04 设计创作，主要内容包括：本发明公开了一种核磁共振测井用复合射频脉冲方法,采用变幅度的射频脉冲作为复合射频脉冲,脉冲幅度在持续时间T内变化,实现核磁共振测井CPMG脉冲序列,完成自旋回波信号测量。能够提高核磁共振回波信号带宽,从而提高核磁共振测量的信噪比。(The invention discloses a composite radio frequency pulse method for nuclear magnetic resonance logging, which adopts variable-amplitude radio frequency pulses as composite radio frequency pulses, wherein the pulse amplitude is changed within duration time T, so that a nuclear magnetic resonance logging CPMG pulse sequence is realized, and spin echo signal measurement is completed. The bandwidth of the nuclear magnetic resonance echo signal can be improved, and therefore the signal-to-noise ratio of nuclear magnetic resonance measurement is improved.)

1. A composite radio frequency pulse method for nuclear magnetic resonance well logging is characterized in that a radio frequency pulse with variable amplitude is adopted as a composite radio frequency pulse, the pulse amplitude changes within duration T to realize a CPMG pulse sequence for nuclear magnetic resonance well logging, and spin echo signal measurement is completed.

2. Method according to claim 1, characterized in that the variation of the pulse amplitude of the radiofrequency pulses within the duration T is in particular: the Sinc function shape is first raised, then the constant amplitude pulse shape, and finally the Sinc function shape is lowered to zero.

3. The method of claim 2, wherein the duration of the Sinc function shape rise is T/4.

4. The method of claim 2, wherein the shape of the Sinc function rises to a pulse amplitude peak A.

5. The method of claim 2, wherein the duration of the constant amplitude pulse is T/2.

6. The method of claim 2, wherein the Sinc function shape is dropped to zero for a duration of T/4.

7. The method as defined in claim 1, wherein the amplitude comprises 90 ° and 180 °, and the frequency, pulse amplitude peak and pulse amplitude trend of the 90 ° rf pulse are consistent with those of the 180 ° rf pulse.

8. The method of claim 7, wherein the 180 ° rf pulse is twice as long in duration as the 90 ° rf pulse.

9. The method of claim 1, wherein the pulse frequency is a nuclear magnetic resonance larmor frequency f.

Technical Field

The invention belongs to the technical field of petroleum and natural gas exploration and development, and particularly relates to a composite radio frequency pulse method for nuclear magnetic resonance well logging.

Background

For nmr logging, the signal-to-noise ratio is the most prominent issue. Due to the low magnetic field intensity, small sample volume and the like, the nuclear magnetic resonance logging result directly influences the accuracy of reservoir physical property parameters (such as porosity, permeability, pore size distribution and the like) and a T2 cut-off value which are calculated subsequently. The nuclear magnetic resonance signal generation mechanism is analyzed, and the radio frequency pulse emission method influences the amplitude of a nuclear magnetic resonance echo signal, so that the signal-to-noise ratio of nuclear magnetic resonance measurement is influenced. Because the RF pulses effect a reversal of the magnetization vector, the magnetization reversal angle is proportional to the RF pulse energy (i.e., the amplitude, duration, and shape of the RF pulses). The amplitude of the rf pulse is limited by the instrument hardware and cannot be increased indefinitely, and the pulse duration and shape can be set by parameters. For example, if the shape is changed, the duration of the rf pulse is changed with the fixed pulse amplitude.

The pulse duration and shape determine the size of the signal bandwidth. First, the longer the duration, the smaller the bandwidth. Secondly, after the hard pulse which is in the shape of square wave in the time domain is subjected to Fourier transform, the hard pulse is a function in the frequency domain; and a soft pulse having a functional shape in the time domain is a square in the frequency domain after fourier transform. Considering that the energy of the hard pulse near the zero point of the main lobe in the frequency domain is small, the corresponding frequency range in the amplitude of the main lobe is used as the effective bandwidth of the hard pulse, and at this time, the signal component in the bandwidth occupies the total energy of the signal. Therefore, the soft pulses have a larger effective bandwidth than the hard pulses, all other parameters being equal.

At present, nuclear magnetic resonance logging instruments at home and abroad adopt a hard pulse transmitting mode, hard pulses are constant-amplitude radio frequency pulses with fixed frequency, are square-wave-shaped in a time domain, but are Sinc function-shaped in a frequency domain, and are unevenly distributed in a very wide frequency range, so that the effective signal bandwidth is small, and the measured nuclear magnetic resonance echo signal intensity is small. The novel composite radio frequency pulse transmitting method can be adopted, so that the transmitting efficiency is high, the position and the thickness of a sample in a stratum are clear, the volume of the sample to be detected is increased, and the signal intensity measured by a nuclear magnetic resonance logging instrument is increased.

Disclosure of Invention

The technical problem to be solved by the present invention is to provide a composite rf pulse method for nuclear magnetic resonance well logging, which is used for implementing a CPMG pulse sequence for nuclear magnetic resonance well logging and completing spin echo signal measurement.

The invention adopts the following technical scheme:

a composite radio frequency pulse method for nuclear magnetic resonance well logging adopts a variable-amplitude radio frequency pulse as a composite radio frequency pulse, the pulse amplitude changes within duration time T, a CPMG pulse sequence of nuclear magnetic resonance well logging is realized, and spin echo signal measurement is completed.

Specifically, the change of the pulse amplitude of the radio frequency pulse within the duration T is specifically: the Sinc function shape is first raised, then the constant amplitude pulse shape, and finally the Sinc function shape is lowered to zero.

Further, the duration of the rise of the shape of the Sinc function is T/4.

Further, the shape of the Sinc function rises to the pulse amplitude peak a.

Further, the duration of the constant amplitude pulse is T/2.

Further, the duration of the Sinc function shape dropping to zero is T/4.

Specifically, the amplitude includes 90 degrees and 180 degrees, and the frequency, the pulse amplitude peak value and the pulse amplitude variation trend of the 90-degree radio frequency pulse are consistent with those of the 180-degree radio frequency pulse.

Further, the 180 ° rf pulse has twice the duration of the 90 ° rf pulse.

Specifically, the pulse frequency is a nuclear magnetic resonance larmor frequency f.

Compared with the prior art, the invention has at least the following beneficial effects:

according to the composite radio frequency pulse method for nuclear magnetic resonance logging, disclosed by the invention, the amplitude of the pulse changes along with time, so that the effective signal bandwidth of the nuclear magnetic resonance logging can be improved, the amplitude of a nuclear magnetic echo signal is improved, and the signal-to-noise ratio of an instrument is improved; through the setting of the pulse shape parameters and the amplitude parameters, the amplitude of the radio frequency pulse generated by the transmitting circuit of the nuclear magnetic resonance logging instrument is increased to A (the instrument can bear the maximum amplitude of the radio frequency pulse) in a Sinc function shape, the duration is T/4, then the constant amplitude pulse with the duration of T/2 is generated, then the Sinc function shape is decreased until the Sinc function shape is zero, the duration is T/4, the bandwidth of a nuclear magnetic resonance echo signal is increased to the maximum extent, and therefore the signal-to-noise ratio of nuclear magnetic resonance measurement is increased.

The technical solution of the present invention is further described in detail by the accompanying drawings and embodiments.

Drawings

FIG. 1 is a schematic diagram of RF pulses, wherein (a) is a conventional hard RF pulse and (b) is a composite RF pulse;

FIG. 2 is a diagram showing the response of a hard RF pulse in the time domain and the frequency domain, wherein (a) is the response of a rectangular RF pulse in the time domain, and (b) is the response of a rectangular RF pulse in the frequency domain;

FIG. 3 is a diagram illustrating the time domain and frequency domain responses of the composite RF pulse, wherein (a) is the time domain response of the composite RF pulse and (b) is the frequency domain response of the composite RF pulse;

fig. 4 is a diagram of a 90 ° rf pulse and a first 180 ° rf pulse in a CPMG pulse sequence using a composite rf pulse method.

FIG. 5 is a comparison graph of nuclear magnetic resonance echo signals using a composite RF pulse and a hard RF pulse technique, respectively.

Detailed Description

The invention relates to a composite radio frequency pulse method for nuclear magnetic resonance well logging, which adopts a variable-amplitude radio frequency pulse as a composite radio frequency pulse, wherein the pulse frequency is a nuclear magnetic resonance Larmor frequency f, the duration time is T, the peak value of the pulse amplitude is A, and the pulse amplitude changes within the duration time T, so that a nuclear magnetic resonance well logging CPMG pulse sequence is realized, and the measurement of a spin echo signal is completed.

The pulse amplitude variation trend is that the Sinc function shape rises firstly, the duration is T/4, then the constant amplitude pulse with the duration of T/2 is carried out, and then the Sinc function shape falls to zero, and the duration is T/4.

The frequency, the pulse amplitude peak value and the pulse amplitude variation trend of the 90-degree radio frequency pulse are consistent with those of the 180-degree radio frequency pulse, and the duration of the 180-degree radio frequency pulse is twice of that of the 90-degree radio frequency pulse.

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. The components of the embodiments of the present invention generally described and illustrated in the figures herein may be arranged and designed in a wide variety of different configurations. Thus, the following detailed description of the embodiments of the present invention, presented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

According to the characteristics of nuclear magnetic resonance radio frequency signals, the Larmor frequency is set to be 500kHz, the magnetization turning angle is set to be 90 degrees, the pulse amplitude is set to be 5Gauss, the pulse duration is 100us, about 50 radio frequency signal periods are obtained, and the distribution of the signal intensity in a frequency domain is obtained through Fourier transform. The response in the frequency domain shows that the rectangular pulse has multiple harmonic components, the frequency spectrum is distributed in a sinc function, the corresponding frequency range is taken as the effective bandwidth when the amplitude of the main lobe is-3 dB, and the effective bandwidth is about 8 kHz.

The composite radio frequency pulse adopts a combination mode of sinc function pulse and rectangular pulse, the Larmor frequency is still set to be 500kHz, a piecewise function is required to be used for representing, and the distribution of the signal intensity in a frequency domain is obtained through Fourier transform.

The response in the frequency domain can be obtained, the composite radio frequency pulse is similar to the rectangular pulse, the frequency spectrum is in sinc function distribution, but the harmonic component is less, the energy is relatively concentrated, and therefore, the corresponding frequency range at the amplitude of-3 dB is also required to be used as the effective bandwidth, and the effective bandwidth is about 10.4 kHz.

The nuclear magnetic resonance logging instrument is tested and verified, and a composite radio frequency pulse technology and a hard radio frequency pulse technology are respectively adopted, so that the amplitude of a nuclear magnetic resonance echo signal obtained by the composite radio frequency pulse technology is 22% higher than that of the nuclear magnetic resonance echo signal obtained by the hard radio frequency pulse technology, and the signal-to-noise ratio is improved by 28%.

Referring to FIG. 1, RF pulses are illustrated, where (a) is a conventional hard RF pulse and (b) is a composite RF pulse. To obtain the same radio frequency energy, the duration of the latter is 1.2 times that of the former.

Referring to fig. 2, the response of the hard rf pulse in the time domain and the frequency domain is shown, with a pulse duration of 100 us. Through numerical calculation, the response in a frequency domain can be obtained, the rectangular pulse has multiple harmonic components, the frequency spectrum is distributed in a sinc function, the corresponding frequency range is taken as an effective bandwidth when the amplitude of the main lobe is-3 dB, and the effective bandwidth is about 8 kHz.

Referring to fig. 3, a schematic diagram of the response of the composite rf pulse in the time domain and the frequency domain, and the pulse duration 120us. are calculated numerically, the response in the frequency domain can be obtained, the composite rf pulse is similar to the rectangular pulse, the frequency spectrum is distributed as a sinc function, but the harmonic components are few, the energy is relatively concentrated, and therefore, the corresponding frequency range at the amplitude of-3 dB is also required as the effective bandwidth, and the effective bandwidth is about 10.4 kHz. It can be seen that the hard pulses have the same energy as in fig. 2, but the signal bandwidth is increased by 26%.

Referring to FIG. 4, NMR logging uses a CPMG pulse sequence to perform spin echo signal measurements, where the pulse sequence is a 90 RF pulse followed by a long train of 180 RF pulses. The figure shows a 90 DEG radio frequency pulse and a first 180 DEG radio frequency pulse signal in a CPMG pulse sequence, and adopts a composite radio frequency pulse technology.

Referring to fig. 5, a comparison graph of the nmr echo signals using the composite rf pulse and the hard rf pulse techniques, respectively. The nuclear magnetic resonance logging instrument is tested and verified, and a composite radio frequency pulse technology and a hard radio frequency pulse technology are respectively adopted, so that the amplitude of a nuclear magnetic resonance echo signal obtained by the composite radio frequency pulse technology is 22% higher than that of the nuclear magnetic resonance echo signal obtained by the hard radio frequency pulse technology, and the signal-to-noise ratio is improved by 28%.

In summary, the composite radio frequency pulse method for nuclear magnetic resonance logging is obviously different from the traditional hard pulse or soft pulse, and the purpose of the method is to obviously improve the bandwidth of nuclear magnetic resonance echo signals, so that the signal-to-noise ratio of nuclear magnetic resonance measurement is improved.

The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.