Method and system for evaluating shale pore connectivity based on nuclear magnetic resonance pulse sequence

Abstract

The invention relates to a method and a system for evaluating shale pore connectivity based on a nuclear magnetic resonance pulse sequence. The method comprises the following steps: editing and measuring gradient magnetic field distribution in shale based on a stimulated echo pulse sequence; measuring shale two-dimensional transverse relaxation time-transverse relaxation time distribution based on the T2 exchange experiment pulse sequence; according to the internal gradient magnetic field distribution of the shale and the two-dimensional transverse relaxation time-transverse relaxation time distribution of the shale, the exchange rate of the fluid among different apertures is analyzed; and evaluating the shale pore connectivity according to the exchange rate. The system comprises a first measurement module, a second measurement module, an analysis module and an evaluation module. According to the method, the influence of transverse relaxation time in the gradient editing process is reduced, and the interference of relaxation attenuation on signals in micro pores is reduced; a gradient magnetic field in a measurement sample is edited by using a stimulated echo pulse sequence, and the influence of nuclides of other types on sample measurement and analysis is small.

Description

Technical Field

[0001] The invention belongs to the technical field of rock physics experimental testing and analysis, and in particular relates to a method and system for evaluating shale pore connectivity based on a nuclear magnetic resonance pulse sequence. Background Art

[0002] Due to the unique generation mechanism and occurrence state of the fluid inside shale, its rock physical response mechanism is different from that of conventional sandstone and carbonate rock. At present, the use of nuclear magnetic resonance technology to detect shale is still in the initial stage of exploration, but because nuclear magnetic resonance technology can provide accurate rock physical properties and internal material magnetic characteristics for the exploration and development of shale oil and gas, it can be used to study the generation mechanism and occurrence conditions of shale oil and gas.

[0003] Shale is rich in clay minerals. The presence of these minerals greatly increases the contact surface area between the solid skeleton and the liquid / gas phase, thereby increasing the surface relaxation rate. Therefore, the transverse relaxation time T2 of shale is one to two orders of magnitude lower than that of sandstone. The internal gradient magnetic field is usually determined by the magnetic susceptibility difference between the rock solid and liquid and the pore size. For shale, there are a large number of tiny pores inside, which also leads to a relatively strong internal gradient field in these tiny pores.

[0004] How to calibrate the transverse relaxation time T2 distribution through the study of the internal gradient magnetic field distribution, effectively analyze the pore structure of shale, and evaluate the pore connectivity has become a technical problem that needs to be solved. Summary of the invention

[0005] In view of the above problems, the present invention proposes a method and system for evaluating shale pore connectivity based on nuclear magnetic resonance pulse sequence. The method measures the internal gradient magnetic field distribution of shale based on FFG-STE pulse sequence (internal magnetic field gradient-stimulated echo pulse sequence), measures the two-dimensional transverse relaxation time-transverse relaxation time distribution based on T2 Exchange pulse sequence (T2 exchange experimental pulse sequence), and studies the exchange rate of fluid between different pore sizes in combination with T2 Exchange pulse sequence experiment, and evaluates the shale pore connectivity based on the exchange rate; ultimately, it helps petroleum geologists to better understand the rock physical properties of shale.

[0006] The technical solution adopted by the present invention is as follows: a method for evaluating shale pore connectivity based on a nuclear magnetic resonance pulse sequence, the method comprising the following steps: editing and measuring the internal gradient magnetic field distribution of shale based on a stimulated echo pulse sequence; 2 The exchange experiment pulse sequence measures the two-dimensional transverse relaxation time-transverse relaxation time distribution of shale; based on the gradient magnetic field distribution inside the shale and the two-dimensional transverse relaxation time-transverse relaxation time distribution of the shale, the exchange rate of fluid between different pore sizes is analyzed; and the pore connectivity of the shale is evaluated based on the exchange rate.

[0007] Furthermore, the method of editing and measuring the internal gradient magnetic field distribution of shale based on the stimulated echo pulse sequence includes: based on the stimulated echo pulse sequence, by increasing the pulse interval time δ, the pulse sequence overall editing time T d The stimulated echo amplitude is unchanged, and the stimulated echo amplitude under different pulse intervals δ is collected in sequence; the data acquisition parameters pulse interval δ, molecular diffusion observation window △, and pulse sequence overall editing time T are established during the execution of the pulse sequence. d , longitudinal and transverse relaxation time and internal magnetic field gradient g in The one-dimensional Laplace transform relationship between them can be used to obtain the internal gradient magnetic field distribution.

[0008] Furthermore, the kernel function of the response equation of the one-dimensional inverse Laplace transform relationship is:

[0009]

[0010]

[0011] Where, k is the collected NMR signal; g in is the magnetic field gradient inside the pore space; γ is the proton gyromagnetic ratio; δ is the pulse interval time; D is the molecular diffusion coefficient; △ is the molecular diffusion observation window; T d is the overall editing time of the pulse sequence, T d Equal to the sum of δ and △, unit: ms; T 1 is the longitudinal relaxation time of the observed proton; T 2 is the transverse relaxation time of the observed proton.

[0012] Furthermore, based on T 2 The exchange experimental pulse sequence measures the two-dimensional transverse relaxation time of shale - transverse relaxation time distribution, including: T 2 In the exchange experiment pulse sequence, the number of echoes p in one dimension increases according to a certain rule, and in the other dimension, echo string data containing q echoes are collected at each editing time; then, through the two-dimensional inverse Laplace transform, the two-dimensional transverse relaxation time-transverse relaxation time distribution of shale is obtained.

[0013] Furthermore, the kernel function of the response equation of the two-dimensional inverse Laplace transform is:

[0014]

[0015] Among them, k 1 and k 2 They represent the kernel functions corresponding to the case where the number of echoes is p and the number of echoes is q respectively; T 1 is the longitudinal relaxation time of the observed proton; T 2 is the transverse relaxation time of the observed proton; tE Indicates the echo interval corresponding to when the number of echoes is p or the number of echoes is q; t m Indicates the observation time.

[0016] Furthermore, according to the internal gradient magnetic field distribution of shale and the two-dimensional transverse relaxation time-transverse relaxation time distribution of shale, the exchange rate of fluid between different pore sizes is analyzed, including: setting different observation times t m , according to T 2 The exchange experiment obtains a two-dimensional distribution spectrum; the signal of the exchange peak on the non-diagonal line of the two-dimensional distribution spectrum is integrated to establish the signal integration peak value and the observation time t m The corresponding relationship between the signal integral peak value and the observation time t m The corresponding relationship between the two aperture sizes is obtained by exponential fitting, and the exchange rate R between different aperture sizes is calculated based on the exchange time R. E .

[0017] Furthermore, the signal integration peak value and the observation time t m The corresponding relationship is:

[0018] E=E 0 *[1-exp(-t m / R)];

[0019] Where E is each observation time t m The signal integration peak value corresponding to the lower non-diagonal exchange peak; E 0 is the integrated initial peak value of the off-diagonal exchange peak; R is the apparent exchange time;

[0020] The exchange rate R E for:

[0021] R E =1 / R+1 / T 2 –Dγ 2 g in 2 t E 2 / 12;

[0022] Where D is the molecular diffusion coefficient, g in is the magnetic field gradient inside the pore space; γ is the proton gyromagnetic ratio; T 2 is the transverse relaxation time of the observed proton; t E Indicates the echo interval corresponding to when the number of echoes is p or the number of echoes is q.

[0023] In addition, the present invention also proposes a system for evaluating shale pore connectivity based on a nuclear magnetic resonance pulse sequence, the system comprising a first measurement module, a second measurement module, an analysis module and an evaluation module; the first measurement module is used to edit and measure the internal gradient magnetic field distribution of shale based on a stimulated echo pulse sequence; the second measurement module is used to edit and measure the internal gradient magnetic field distribution of shale based on a stimulated echo pulse sequence; 2 The exchange experiment pulse sequence measures the two-dimensional transverse relaxation time-transverse relaxation time distribution of shale; the analysis module is used to analyze the exchange rate of fluid between different pore sizes based on the internal gradient magnetic field distribution of shale and the two-dimensional transverse relaxation time-transverse relaxation time distribution of shale; the evaluation module is used to evaluate the pore connectivity of shale based on the exchange rate.

[0024] Furthermore, the first measurement module includes an acquisition unit and a transformation unit, wherein the acquisition unit is used to increase the pulse interval time δ based on the stimulated echo pulse sequence, and the overall editing time T of the pulse sequence is d The stimulated echo amplitude is unchanged, and the stimulated echo amplitude is collected in sequence to obtain the stimulated echo amplitude under different pulse intervals δ; the transformation unit is used to establish the data acquisition parameters pulse interval δ, molecular diffusion observation window △, pulse sequence overall editing time T during the execution of the pulse sequence d , longitudinal and transverse relaxation time and internal magnetic field gradient g in The one-dimensional Laplace transform relationship between them can be used to obtain the internal gradient magnetic field distribution.

[0025] Furthermore, the analysis module includes an acquisition unit, an integration unit, a fitting unit and a calculation unit; the acquisition unit is used to set different observation times t m , according to T 2 The exchange experiment obtains a two-dimensional distribution spectrum; the integration unit is used to perform signal integration on the exchange peak on the non-diagonal line of the two-dimensional distribution spectrum, and establish a signal integration peak value and observation time t m The fitting unit is used to fit the signal integral peak value and the observation time t m The corresponding relationship between the two aperture sizes is obtained by exponential fitting, and the visual exchange time R between the two aperture sizes is obtained; the calculation unit is used to calculate the exchange rate R between different aperture sizes according to the visual exchange time R E .

[0026] The present invention has the following beneficial effects:

[0027] (1) The influence of transverse relaxation time is reduced during the gradient editing process, which reduces the interference of relaxation attenuation on the signal in the microscopic pores and detects the correlation between the internal magnetic field gradient and pore size in different pore sizes;

[0028] (2) The stimulated echo (STE) pulse sequence is used to edit and measure the internal gradient magnetic field of the sample, which is less affected by other types of nuclides when measuring and analyzing the sample. The entire measurement only observes hydrogen protons, and there is no influence of other nuclides, such as carbon, sodium, etc., in the measurement signal;

[0029] (3) The use of pulsed magnetic field gradient technology requires more complex hardware conditions, including gradient coils and gradient amplifiers, which are more expensive and only applicable in laboratories. The present invention uses FFG (internal magnetic field gradient) technology for measurement, which is also feasible for technical promotion in field application scenarios such as well logging.

[0030] Other features and advantages of the present invention will be described in the following description, and part of them will become obvious from the description, or be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0032] Figure 1 A schematic diagram of a method for evaluating shale pore connectivity based on a nuclear magnetic resonance pulse sequence;

[0033] Figure 2 Schematic diagram of internal gradient magnetic field distribution for FFG-STE pulse sequence measurement;

[0034] Figure 3 Schematic diagram of the transverse relaxation time distribution measured by the T2Exchange pulse sequence;

[0035] Figure 4a , Figure 4b , Figure 4c , Figure 4d are different observation times t m (t m 0.1, 1, 10, 100m respectively s ) under T2Exchange association map;

[0036] Figure 5 Schematic diagram of a system for evaluating shale pore connectivity based on nuclear magnetic resonance pulse sequence. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] A method for evaluating shale pore connectivity based on NMR pulse sequence, such as Figure 1 As shown, the method comprises the following steps: S1, measuring the internal gradient magnetic field distribution of shale based on the stimulated echo pulse sequence editing; S2, measuring the internal gradient magnetic field distribution of shale based on the T 2 The exchange experiment pulse sequence measures the two-dimensional transverse relaxation time-transverse relaxation time distribution of shale; S3. According to the internal gradient magnetic field distribution of shale and the two-dimensional transverse relaxation time-transverse relaxation time distribution of shale, the exchange rate of fluid between different pore sizes is analyzed; S4. The pore connectivity of shale is evaluated based on the exchange rate.

[0039] In the present invention, the internal gradient magnetic field refers to the magnetic field gradient generated in the rock sample due to the difference in magnetic susceptibility between the rock skeleton and the fluid in the pores. The internal gradient magnetic field is related to the rock skeleton, fluid type and pore size. In dense rocks such as shale, due to the small pore size, there is a strong internal gradient magnetic field, which in turn affects the accuracy of nuclear magnetic resonance measurement of rock pore size distribution. By measuring the internal gradient magnetic field distribution of the rock, the transverse relaxation time T2 spectrum can be corrected to obtain an accurate pore size distribution curve.

[0040] In the present invention, pore connectivity reflects the permeability of rock and is a key parameter for characterizing the oil production capacity of reservoirs. The T2Exchange pulse sequence can quantitatively analyze the exchange rate of fluid between different pores by measuring the transverse relaxation characteristics of hydrogen nuclei in the fluid before and after the diffusion editing time, thereby determining the pore connectivity.

[0041] In step S1, the FFG-STE pulse sequence is selected to edit and measure the internal gradient magnetic field of the sample. By increasing the pulse interval time δ, the overall editing time T d The stimulated echo amplitude is unchanged, and the echo amplitude under different δ is collected in sequence, and then the echo amplitude under different δ is obtained; the distribution of the internal gradient magnetic field is obtained through one-dimensional inverse Laplace transform (1D ILT). The kernel function of one-dimensional inverse Laplace transform (1D ILT), or the response formula in signal acquisition, includes the data acquisition parameters (such as δ, △, etc.) required during the execution of the FFG-STE pulse sequence and the internal magnetic field gradient g of the pore space. in ; The kernel function response formula is as follows:

[0042]

[0043]

[0044] Where, k is the collected NMR signal; g in is the magnetic field gradient inside the pore space, in T / m; γ is the proton gyromagnetic ratio, in MHz / T; δ is the time interval between 90-degree pulses, in ms; D is the molecular diffusion coefficient, in m 2 / s; △ is the molecular diffusion observation window, in ms; T d The overall editing time unit for the pulse sequence is ms; T 1 is the longitudinal relaxation time of the observed proton, in ms; T 2 is the transverse relaxation time of the observed proton, in ms. Figure 2 The figure shows the pulse sequence required to execute the FFG-STE sequence, which includes the transmitted RF pulses and the receiving window; the received signal response conforms to the kernel function response formula in step S1; Figure 2 Medium T d During the experiment, T d Equal to the sum of δ and △.

[0045] Step S2 includes: the number of echoes p in one dimension of the T2Exchange pulse sequence increases according to a certain rule, and the echo string data containing q echoes at each editing time is collected in another dimension; then the two-dimensional transverse relaxation time-transverse relaxation time distribution of shale is obtained through two-dimensional inverse Laplace transform (2D ILT). The kernel function matrix of the two-dimensional inverse Laplace transform (2DILT) is composed of k 1 and k 2 The final kernel function matrix contains all the acquisition parameters of the T2Exchange pulse sequence (such as the echo interval t E , echo number p and q, observation time t m and target parameter T 2 ). The two-dimensional inverse Laplace transform (2DILT) kernel function response equation is:

[0046]

[0047] Among them, k 1 and k 2 They represent the kernel functions corresponding to the case where the number of echoes is p and the number of echoes is q respectively; T 1 is the longitudinal relaxation time of the observed proton; T 2 is the transverse relaxation time of the observed proton; t E Indicates the echo interval corresponding to when the number of echoes is p or the number of echoes is q; t m Indicates the observation time. Figure 3 It contains the transmit RF pulses and receive windows required to execute the pulse sequence required for the T2exchange sequence. The received signal response conforms to the above two-dimensional inverse Laplace transform (2D ILT) kernel function response equation; where T 1 = p·t E ; T 2 =q·t E .

[0048] The pulse phases of the pulse sequence involved in the T2-T2 exchange experiment are shown in Table 1: Pulse90° means the 90-degree RF pulse phase is used; Pulse 180° means the 180-degree RF pulse phase is used; Acq means the acquisition phase used by the signal acquisition module. Figure 3 The RF pulse or signal receiving event shown, for example, the first column represents the first 90-degree RF pulse on the left of the pulse sequence, and the second column represents the 180-degree RF pulse in the left echo train; Acq represents the last signal receiving window. Each row represents the phase value executed for each signal transmission and reception event, which is recorded as one scan. A total of 8 scans are required to complete this experiment, so there are a total of 8 rows of executed phases. The design of these phases ensures that the two-dimensional inverse Laplace transform (2D ILT) kernel function response equation can be obtained during this two-dimensional acquisition process.

[0049] Table 1 Data of each pulse phase of T2 exchange pulse sequence

[0050]

[0051] Step S3 includes: setting different observation times t m According to the T2 exchange experiment, Figures 4a-4d The corresponding observation times t are shown m Down( Figure 4a , Figure 4b , Figure 4c , Figure 4d Medium m 0.1, 1, 10, 100m respectively s )'s two-dimensional distribution map; Figures 4a-4d The exchange peaks on the non-diagonal lines of the two-dimensional distribution spectrum shown in the figure are signal integrated (i.e., signal accumulation), and the signal integration peak value and the observation time t are established. m The corresponding relationship, from the figure Figures 4a-4d It can be seen that with the observation time t m The increase of transverse relaxation time T 2 = 0.01s corresponds to a gradual increase in the signal amount, indicating that there is exchange of pore fluid and the pores are connected; based on the signal integral peak and the observation time t mThe corresponding relationship between the two aperture sizes is obtained by exponential fitting, and the exchange rate R between different aperture sizes is calculated based on the exchange time R. E Once the exchange rate R is determined, the pore connectivity of the shale can be evaluated.

[0052] Wherein, the exchange time R is regarded as the internal magnetic field gradient g in and T 2 -T 2 The transverse relaxation time T obtained from the exchange experiment 2 The signal integration peak value and the observation time t m The corresponding relationship is:

[0053] E=E 0 *[1-exp(-t m / R)];

[0054] Where E is each observation time t m The signal integration peak value corresponding to the lower non-diagonal exchange peak; E 0 is the integrated initial peak value of the off-diagonal exchange peak; R is the apparent exchange time;

[0055] The exchange rate R E for:

[0056] R E =1 / R+1 / T 2 –Dγ 2 g in 2 t E 2 / 12;

[0057] Where D is the molecular diffusion coefficient, which is equal to 2.5*10 -9 m 2 / s;g in is the magnetic field gradient inside the pore space; γ is the proton gyromagnetic ratio; T 2 is the transverse relaxation time of the observed proton; t E Indicates the echo interval corresponding to when the number of echoes is p or the number of echoes is q.

[0058] In addition, the present invention also proposes a system for evaluating shale pore connectivity based on a nuclear magnetic resonance pulse sequence, such as Figure 5 As shown, the system includes a first measurement module, a second measurement module, an analysis module and an evaluation module; the first measurement module is used to edit and measure the internal gradient magnetic field distribution of shale based on the stimulated echo pulse sequence; the second measurement module is used to edit and measure the internal gradient magnetic field distribution of shale based on the T 2The exchange experiment pulse sequence measures the two-dimensional transverse relaxation time-transverse relaxation time distribution of shale; the analysis module is used to analyze the exchange rate of fluid between different pore sizes based on the internal gradient magnetic field distribution of shale and the two-dimensional transverse relaxation time-transverse relaxation time distribution of shale; the evaluation module is used to evaluate the pore connectivity of shale based on the exchange rate.

[0059] Specifically, the first measurement module includes an acquisition unit and a transformation unit, wherein the acquisition unit is used to increase the pulse interval time δ based on the stimulated echo pulse sequence, and the overall editing time T of the pulse sequence is d The stimulated echo amplitude is unchanged, and the stimulated echo amplitude is collected in sequence to obtain the stimulated echo amplitude under different pulse intervals δ; the transformation unit is used to establish the data acquisition parameters pulse interval δ, molecular diffusion observation window △, pulse sequence overall editing time T during the execution of the pulse sequence d , longitudinal and transverse relaxation time and internal magnetic field gradient g in The one-dimensional Laplace transform relationship between them can be used to obtain the internal gradient magnetic field distribution.

[0060] Specifically, the analysis module includes an acquisition unit, an integration unit, a fitting unit and a calculation unit; the acquisition unit is used to set different observation times t m , according to T 2 The exchange experiment obtains a two-dimensional distribution spectrum; the integration unit is used to perform signal integration on the exchange peak on the non-diagonal line of the two-dimensional distribution spectrum, and establish a signal integration peak value and observation time t m The fitting unit is used to fit the signal integral peak value and the observation time t m The corresponding relationship between the two aperture sizes is obtained by exponential fitting, and the visual exchange time R between the two aperture sizes is obtained; the calculation unit is used to calculate the exchange rate R between different aperture sizes according to the visual exchange time R E .

[0061] In summary, the present invention proposes a method and system for evaluating shale pore connectivity based on nuclear magnetic resonance pulse sequence, which combines the FFG-STE pulse sequence and the T2 exchange pulse sequence, and applies the method to the study of shale pore size distribution and pore connectivity, and studies the influence of the internal gradient field on different pore sizes in shale, as well as the exchange rate of fluids between different pore sizes under the action of the internal gradient field, ultimately helping petroleum geologists to better understand the rock physical properties of shale.

[0062] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent substitutions for some of the technical features therein; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for evaluating shale pore connectivity based on a nuclear magnetic resonance pulse sequence, characterized in that: The method comprises the following steps: Measure the gradient magnetic field distribution inside shale based on stimulated echo pulse sequence editing; The two-dimensional transverse relaxation time and transverse relaxation time distribution of shale are measured based on the T2 exchange experimental pulse sequence; According to the gradient magnetic field distribution inside the shale and the two-dimensional transverse relaxation time-transverse relaxation time distribution of the shale, the exchange rate of fluids between different pore sizes is analyzed; Evaluation of shale pore connectivity based on exchange rate.

2. The method according to claim 1, characterized in that The method of measuring the internal gradient magnetic field distribution of shale based on stimulated echo pulse sequence editing includes: Based on the stimulated echo pulse sequence, by increasing the pulse interval time δ, the overall editing time of the pulse sequence is T d The stimulated echo amplitudes are collected in sequence to obtain the stimulated echo amplitudes under different pulse intervals δ; Establish the data acquisition parameters during the pulse sequence execution process: pulse interval time δ, molecular diffusion observation window △, pulse sequence overall editing time T d , longitudinal and transverse relaxation time and internal magnetic field gradient g in The one-dimensional Laplace transform relationship between them can be used to obtain the internal gradient magnetic field distribution.

3. The method according to claim 2, characterized in that The kernel function of the response equation of the one-dimensional inverse Laplace transform relationship is: Where, k is the collected NMR signal; g in is the magnetic field gradient inside the pore space; γ is the proton gyromagnetic ratio; δ is the pulse interval time; D is the molecular diffusion coefficient; △ is the molecular diffusion observation window; T d is the overall editing time of the pulse sequence, T d Equal to the sum of δ and △, unit: ms; T1 is the longitudinal relaxation time of the observed proton; T2 is the transverse relaxation time of the observed proton.

4. The method according to claim 2 or 3, characterized in that: Based on the T2 exchange experimental pulse sequence, the two-dimensional transverse relaxation time and transverse relaxation time distribution of shale are measured, including: In the T2 exchange experiment pulse sequence, the number of echoes p in one dimension increases according to a certain rule, and in the other dimension, echo string data containing q echoes are collected at each editing time; then, through the two-dimensional inverse Laplace transform, the two-dimensional transverse relaxation time-transverse relaxation time distribution of shale is obtained.

5. The method according to claim 4, characterized in that The kernel function of the response equation of the two-dimensional inverse Laplace transform is: Wherein, k1 and k2 represent the kernel functions corresponding to the case where the number of echoes is p and the number of echoes is q, respectively; T1 is the longitudinal relaxation time of the observed proton; T2 is the transverse relaxation time of the observed proton; t E Indicates the echo interval corresponding to when the number of echoes is p or the number of echoes is q; t m Indicates the observation time.

6. The method according to claim 5, characterized in that According to the gradient magnetic field distribution inside the shale and the two-dimensional transverse relaxation time-transverse relaxation time distribution of the shale, the exchange rate of fluids between different pore sizes is analyzed, including: Set different observation time t m , obtain a two-dimensional distribution map based on the T2 exchange experiment; The signal of the exchange peak on the non-diagonal line of the two-dimensional distribution spectrum is integrated to establish the signal integration peak value and the observation time t m The corresponding relationship; Based on the signal integration peak value and observation time t m The corresponding relationship between the two aperture sizes is obtained by exponential fitting. The exchange rate R between different aperture sizes is calculated based on the visual exchange time R E .

7. The method according to claim 6, characterized in that The signal integration peak value and the observation time t m The corresponding relationship is: E=E0*[1-exp(-t m / R)]; Where E is each observation time t m The signal integral peak value corresponding to the lower non-diagonal exchange peak; E0 is the integral initial peak value of the non-diagonal exchange peak; R is the apparent exchange time; The exchange rate R E for: R E =1 / R+1 / T2–Dγ 2 g in 2 t E 2 / 12; Where D is the molecular diffusion coefficient, g in is the magnetic field gradient inside the pore space; γ is the proton gyromagnetic ratio; T2 is the transverse relaxation time of the observed proton; t E Indicates the echo interval corresponding to when the number of echoes is p or the number of echoes is q.

8. A system for evaluating shale pore connectivity based on a nuclear magnetic resonance pulse sequence, characterized in that: The system includes a first measurement module, a second measurement module, an analysis module and an evaluation module; wherein, The first measurement module is used to edit and measure the gradient magnetic field distribution inside the shale based on the stimulated echo pulse sequence; The second measurement module is used to measure the two-dimensional transverse relaxation time-transverse relaxation time distribution of shale based on the T2 exchange experimental pulse sequence; The analysis module is used to analyze the exchange rate of fluids between different pore sizes based on the internal gradient magnetic field distribution of the shale and the two-dimensional transverse relaxation time-transverse relaxation time distribution of the shale; The evaluation module is used to evaluate shale pore connectivity according to the exchange rate.

9. The system according to claim 8, characterized in that The first measurement module includes a collection unit and a transformation unit; wherein, The acquisition unit is used to increase the pulse interval time δ based on the stimulated echo pulse sequence, and the overall editing time T of the pulse sequence d The stimulated echo amplitudes are collected in sequence to obtain the stimulated echo amplitudes under different pulse intervals δ; The transformation unit is used to establish the data acquisition parameters pulse interval time δ, molecular diffusion observation window △, pulse sequence overall editing time T during the pulse sequence execution process. d , longitudinal and transverse relaxation time and internal magnetic field gradient g in The one-dimensional Laplace transform relationship between them can be used to obtain the internal gradient magnetic field distribution.

10. The system according to claim 8, characterized in that The analysis module includes an acquisition unit, an integration unit, a fitting unit and a calculation unit; wherein, The acquisition unit is used to set different observation times t m , obtain a two-dimensional distribution map based on the T2 exchange experiment; The integration unit is used to perform signal integration on the exchange peaks on the non-diagonal lines of the two-dimensional distribution spectrum, and establish a relationship between the signal integration peak value and the observation time t m The corresponding relationship; The fitting unit is used to fit the peak value of the signal integration and the observation time t m The corresponding relationship between the two aperture sizes is obtained by exponential fitting. The calculation unit is used to calculate the exchange rate R between different aperture sizes according to the visual exchange time R E .

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