CN108226002A - The method that secondary porosity is obtained based on scale Electrical imaging secondary porosity spectrum cutoff value - Google Patents

Abstract

The invention discloses it is a kind of based on scale Electrical imaging secondary porosity spectrum cutoff value obtain secondary porosity method, including：S1, secondary porosity spectrum cutoff value calculation formula is established：Secondary porosity composes cutoff value=a × δ+porosity intermediate value, and a is fixed coefficient, and δ is the porosity variance in selected window in minimal amount of porosity and porosity median value range on electric imaging logging image；S2, rock core secondary porosity is calculated using rock core nuclear magnetic resonance T 2 spectrum；S3, the numerical value using fixed coefficient a in the rock core secondary porosity scale secondary porosity spectrum cutoff value calculation formula of rock core nuclear magnetic resonance T 2 spectrum calculating gained, to obtain secondary porosity after scale.The present invention uses rock core nuclear magnetic resonance T 2 spectrum scale, the fixed coefficient a in secondary porosity spectrum cutoff value calculation formula is obtained using the method for Approach by inchmeal, the reliability of secondary porosity spectrum cutoff value result of calculation is increased, and then obtains the high secondary porosity acquisition methods of accuracy.

Description

A Method for Acquiring Secondary Porosity Based on the Cutoff Value of Scaled Electric Imaging Secondary Porosity Spectrum

technical field

The invention relates to the technical field of acquiring secondary porosity of carbonate reservoirs, in particular to a method for acquiring secondary porosity based on the cut-off value of the secondary porosity spectrum of calibration electrical imaging.

Background technique

Carbonate reservoirs have the characteristics of simultaneous development of matrix pores (primary pores) and secondary pores, complex pore-throat structures, diverse storage spaces, and strong reservoir heterogeneity. Due to diagenesis and other effects, it is often difficult to effectively preserve the primary pores, while the secondary pores are usually larger in diameter and better in permeability than the primary pores, and are the main storage space and space for oil and gas in carbonate reservoirs. Therefore, it is of great significance for the evaluation of the effectiveness of carbonate reservoirs to carry out the research on the logging evaluation method of secondary porosity in carbonate reservoirs, especially the accurate calculation of secondary porosity, and it can also contribute to the Provide a theoretical basis for formulating and adjusting plans and effectively enhancing oil recovery.

Since the formation of secondary pores is controlled by a series of factors such as dissolution, cementation, diagenesis, and biological processes, the response relationship between a single logging parameter and secondary porosity is generally poor, and conventional logging data cannot effectively evaluate the secondary porosity. Therefore, electrical imaging logging data are used to conduct secondary porosity logging evaluation research at home and abroad, and the calculation method of secondary porosity is mainly divided into the following three steps:

(1) Calculation of reservoir porosity spectrum using electrical imaging logging data: first select an electrical imaging logging image window, convert the conductivity value of the borehole wall flushing zone in the window into the resistivity value of the borehole wall flushing zone, and use Archie The formula converts the resistivity matrix of the wellbore flushing zone into the porosity of each image logging pixel point, and then performs frequency statistics on the porosity values in a certain depth range of the reservoir in different porosity value intervals, and The statistical results are drawn into a frequency histogram in the coordinate system, and the frequency histogram is the electrical imaging porosity spectrum of the formation.

(2) Calculating the cut-off value for distinguishing primary pores and secondary pores in the porosity spectrum: the Newberry porosity spectrum cut-off value calculation method based on the median value of porosity proposed by B.M.Newberry of Schlumberger in 1996 and T.S.Ramakrishnan The discriminant porosity spectrum cut-off value calculation method based on standard linear discriminant analysis proposed by et al. in 1997 is the current mainstream calculation method. It can reflect the porosity characteristics of the reservoir and is not affected by the morphological characteristics of the porosity spectrum. However, the calculation accuracy of the cut-off value of the porosity spectrum depends on the value of the regional empirical coefficient. The applicability of the calculation method and the reliability of the calculation results are not high. It affects the calculation accuracy of secondary porosity in the next step. In order to solve these problems, Shi Feizhou and others in China proposed a calculation method based on the Gaussian function fitting porosity spectrum cut-off value method based on the shape of the porosity spectrum, but the calculation accuracy of this method is closely related to the shape of the porosity spectrum, and it is not suitable for In the case of a large distance between the double peaks in the spectrum, the application in carbonate reservoirs is limited.

(3) Calculation of reservoir secondary porosity based on the cut-off value of the secondary porosity spectrum: according to the calculation result of the cut-off value of the porosity spectrum, the boundary line between primary pores and secondary pores is determined in the porosity spectrum. Integrate the area enclosed by the envelope of the secondary pore spectrum, and the integral calculation result is the secondary porosity.

Based on the above methods and steps for calculating the secondary porosity of carbonate rock reservoirs, with the help of electrical imaging logging data, the problem that conventional logging cannot be used to calculate the secondary porosity of carbonate rocks has been effectively solved. However, in the calculation of secondary porosity, or the calculation accuracy is too dependent on regional empirical parameters, the selection of parameters lacks theoretical basis. In addition, the calculation accuracy will also be affected by the shape of the porosity spectrum, which limits the applicability of the method.

Therefore, it is necessary to establish a new secondary porosity acquisition method to meet the demand for improving the accuracy of secondary porosity.

Contents of the invention

The technical problem to be solved by the present invention is to provide a method for obtaining secondary porosity based on the cut-off value of the secondary porosity spectrum of the scale electrical imaging with improved reliability and accuracy.

The technical solution adopted by the present invention to solve the technical problem is to provide a method for obtaining secondary porosity based on the cut-off value of the secondary porosity spectrum of scale electric imaging, comprising the following steps:

S1. Establish the formula for calculating the cut-off value of the secondary porosity spectrum:

Secondary porosity spectrum cut-off value = a × δ + median value of porosity

Among them, a is a fixed coefficient, δ is the porosity variance within the range between the minimum porosity and the median porosity in the selected window on the electrical imaging logging image, and the median porosity is the median porosity in the selected window;

S2, using the core nuclear magnetic resonance T2 spectrum to calculate the secondary porosity of the core;

S3. The value of the fixed coefficient a in the calculation formula of the secondary porosity scale scale secondary porosity spectrum cut-off value calculated by using the core nuclear magnetic resonance T2 spectrum:

Using the formula for calculating the cut-off value of the secondary porosity spectrum in step S1, adjust the value of the fixed coefficient a between 1-3, control the calculation result of the cut-off value of the secondary porosity spectrum, and calculate the envelope of the porosity spectrum after the cut-off value The enclosed area is integrated to calculate the secondary porosity to obtain the secondary porosity of the porosity spectrum; the error analysis is carried out between the secondary porosity of the porosity spectrum calculated successively and the secondary porosity obtained in step S2, and the corresponding The value of a is determined after calibration, and the calculation result of the corresponding porosity spectrum secondary porosity is the secondary porosity after calibration.

Preferably, in step S1, a window is selected on the electrical imaging logging image, and the sum of the porosity count rate within the minimum porosity and porosity median range within the window is calculated; the count rate of each porosity within the range is calculated The percentage of the total count rate, based on which the porosity variance δ within the range between the minimum porosity and the median porosity is calculated; a fixed coefficient a is multiplied by the variance to obtain the cut-off value of the secondary porosity spectrum, and the secondary porosity Spectral cutoff value calculation formula.

Preferably, in step S2, according to the size of the secondary porosity of the core is equal to the volume of the large-diameter pore fluid discharged after centrifugation, and according to the peak characteristics in the T2 spectrum distribution diagram of the core NMR, it is obtained that the relaxation time of the porosity component before centrifugation is the longest The peak start time, the start time corresponds to the secondary porosity spectrum T2 cut-off value, obtain the core secondary porosity = cumulative total porosity before centrifugation - cumulative porosity before centrifugation corresponding to the secondary porosity spectrum T2 cut-off value.

Preferably, the following steps are also included:

S4, calculating the secondary porosity of the cast body thin section;

S5 , verifying the accuracy of the secondary porosity of the porosity spectrum obtained in step S3 by using the secondary porosity of the cast thin section.

Preferably, in step S4, the reservoir layer corresponding to the selected window position on the electrical imaging logging image in step S1 is taken to prepare a cast thin section;

According to the identification results of the casting flakes, the type of secondary pores and the corresponding relative content of the casting flakes are obtained, and the relationship between the total surface porosity and the total porosity of the casting flakes is fitted to obtain the conversion between the total surface porosity and the total porosity of the casting flakes The relational expression converts the surface porosity of secondary pores into the secondary porosity of cast thin slices.

Preferably, the types of secondary pores in the casting sheet include intergranular dissolution pores, intragranular dissolution pores, matrix dissolution pores, and structural dissolution fractures.

Preferably, in step S5, the average absolute error between the secondary porosity calculated from the cast thin section and the porosity spectrum is used to verify the accuracy of the acquired porosity spectrum secondary porosity.

Preferably, in step S5, the mean absolute error is less than 5%

The present invention provides another method for obtaining secondary porosity based on the cut-off value of the secondary porosity spectrum of the scale electric imaging, comprising the following steps:

S1. Establish the formula for calculating the cut-off value of the secondary porosity spectrum:

Secondary porosity spectrum cut-off value = a + average porosity

Among them, a is a fixed coefficient, and the average porosity is the arithmetic mean value of all porosities in the selected window on the electrical imaging logging image;

S2, using the core nuclear magnetic resonance T2 spectrum to calculate the secondary porosity of the core;

S3. The value of the fixed coefficient a in the calculation formula of the secondary porosity scale scale secondary porosity spectrum cut-off value calculated by using the core nuclear magnetic resonance T2 spectrum:

Using the formula for calculating the cut-off value of the secondary porosity spectrum in step S1, adjust the value of the fixed coefficient a between 1-3, control the calculation result of the cut-off value of the secondary porosity spectrum, and calculate the envelope of the porosity spectrum after the cut-off value The enclosed area is integrated to calculate the secondary porosity to obtain the secondary porosity of the porosity spectrum; the error analysis is carried out between the secondary porosity of the porosity spectrum calculated successively and the secondary porosity obtained in step S2, and the corresponding The value of a is determined after calibration, and the calculation result of the corresponding porosity spectrum secondary porosity is the secondary porosity after calibration.

Preferably, in step S2, according to the size of the secondary porosity of the core is equal to the volume of the large-diameter pore fluid discharged after centrifugation, and according to the peak characteristics in the T2 spectrum distribution diagram of the core NMR, it is obtained that the relaxation time of the porosity component before centrifugation is the longest The peak start time, the start time corresponds to the secondary porosity spectrum T2 cut-off value, obtain the core secondary porosity = cumulative total porosity before centrifugation - cumulative porosity before centrifugation corresponding to the secondary porosity spectrum T2 cut-off value.

Beneficial effects of the present invention: adopt rock core nuclear magnetic resonance T2 spectrum scale, utilize the method of successive approximation to obtain the fixed coefficient a in the calculation formula of secondary porosity spectrum cut-off value, greatly increase the reliability of the calculation result of secondary porosity spectrum cut-off value Therefore, a highly accurate secondary porosity acquisition method can be obtained.

Using the secondary porosity of the casting thin section to verify the results of the secondary porosity acquisition of the porosity spectrum effectively guarantees the accuracy of the secondary porosity acquisition results.

Description of drawings

The present invention will be further described below in conjunction with accompanying drawing and embodiment, in the accompanying drawing:

Fig. 1 is a schematic flowchart of a method for obtaining secondary porosity based on the cut-off value of the secondary porosity spectrum of the scale electric imaging according to the first embodiment of the present invention;

Fig. 2 is carbonate reservoir rock core nuclear magnetic resonance T2 spectrum distribution figure among the present invention;

Fig. 3 is a fitting relationship diagram between the total surface porosity (slit) ratio and the total porosity of the cast thin slices in the present invention.

Detailed ways

Referring to FIG. 1 , the method for obtaining secondary porosity based on the cut-off value of the scale electric imaging secondary porosity spectrum according to the first embodiment of the present invention includes the following steps:

S1. Establish the formula for calculating the cut-off value of the secondary porosity spectrum:

Secondary porosity spectrum cut-off value = a × δ + median value of porosity

Among them, a is a fixed coefficient; δ is the porosity variance between the minimum porosity and the median porosity in the selected window on the electrical imaging logging image; the median porosity is the median porosity in the selected window, and the accumulated porosity The porosity corresponding to the degree distribution percentage reaches 50%, its physical meaning is that the porosity larger than it accounts for 50%, and the porosity smaller than it also accounts for 50%.

Specifically, in step S1, a window is selected on the electrical imaging logging image, and the sum of the porosity count rate within the range of the minimum porosity and the median porosity in the window is calculated; the minimum porosity and the median porosity in the window are calculated The count rate of each porosity in the range accounts for the percentage of the total count rate, and the porosity variance δ in the range between the minimum porosity and the porosity median is calculated accordingly; a fixed coefficient a is multiplied by the variance to obtain the secondary porosity Spectrum cut-off value, the calculation formula of secondary porosity spectrum cut-off value is obtained.

S2. Calculating the secondary porosity of the core by using the T2 spectrum of the nuclear magnetic resonance of the core.

Specifically, according to the size of the secondary porosity of the core is equal to (or approximately equal to) the volume of large-diameter pore fluid discharged after centrifugation, and according to the peak characteristics of the core NMR T2 spectrum distribution, the minimum relaxation time in the porosity component before centrifugation is obtained. Long peak start time, the start time corresponds to the cut-off value of secondary porosity spectrum T2, the obtained core secondary porosity = cumulative total porosity before centrifugation - cumulative porosity before centrifugation corresponding to the cut-off value of secondary porosity spectrum T2.

S3. Use the numerical value of the fixed coefficient a in the calculation formula of the secondary porosity spectrum cut-off value of the core NMR secondary porosity scale:

Using the formula for calculating the cut-off value of the secondary porosity spectrum in step S1, adjust the value of the fixed coefficient a between 1-3, control the calculation result of the cut-off value of the secondary porosity spectrum, and calculate the envelope of the porosity spectrum after the cut-off value The enclosed area is integrated to calculate the secondary porosity to obtain the secondary porosity of the porosity spectrum; the error analysis is carried out between the secondary porosity of the porosity spectrum calculated successively and the secondary porosity obtained in step S2, and the corresponding The value of a is determined after calibration, and the calculation result of the corresponding porosity spectrum secondary porosity is the secondary porosity after calibration.

Further, the method for obtaining secondary porosity based on the cut-off value of the scaled electrical imaging secondary porosity spectrum in the first embodiment of the present invention further includes the following steps:

S4. Calculating the secondary porosity of the thin section of the cast body.

Wherein, corresponding to the window selected on the electrical imaging logging image in step S1, the cast thin slice is prepared.

According to the identification results of the casting flakes, the type and relative content of the secondary pores of the casting flakes are obtained, and the relationship between the total surface porosity and the total porosity of the casting flakes is fitted, and the conversion relation between the total surface porosity and the total porosity of the casting flakes is obtained , the surface porosity of the secondary pores is converted to the secondary porosity of the casting sheet. The types of secondary pores in casting thin sections include intergranular dissolution pores, intragranular dissolution pores, matrix dissolution pores, and structural dissolution fractures. The above-mentioned total surface porosity of the cast sheet includes the total surface porosity of holes and slot-like holes in the cast sheet.

S5. Verify the accuracy of the secondary porosity of the porosity spectrum obtained in step S3 by using the secondary porosity of the cast thin section:

The average absolute error between the secondary porosity and the porosity spectrum calculation of the cast thin section is used to verify the accuracy (ie precision) of the acquired porosity spectrum secondary porosity.

If the verification result meets the accuracy required, output the secondary porosity acquisition result; if not, return to step S3 to determine the cut-off value of the secondary porosity spectrum again until it meets the acquisition accuracy.

The method for obtaining secondary porosity based on the cut-off value of the secondary porosity spectrum of the scale electric imaging in the second embodiment of the present invention includes the following steps:

S1. Establish the formula for calculating the cut-off value of the secondary porosity spectrum:

Secondary porosity spectrum cut-off value = a + average porosity

Among them, a is a fixed coefficient, and the average porosity is the arithmetic mean value of all porosities in the selected window on the electrical imaging logging image.

S2. Calculating the secondary porosity of the core by using the T2 spectrum of the nuclear magnetic resonance of the core.

Specifically, according to the size of the secondary porosity of the core is equal to (or approximately equal to) the volume of large-diameter pore fluid discharged after centrifugation, and according to the peak characteristics of the core NMR T2 spectrum distribution, the minimum relaxation time in the porosity component before centrifugation is obtained. Long peak start time, the start time corresponds to the cut-off value of secondary porosity spectrum T2, the obtained core secondary porosity = cumulative total porosity before centrifugation - cumulative porosity before centrifugation corresponding to the cut-off value of secondary porosity spectrum T2.

S3. The value of the fixed coefficient a in the calculation formula of the secondary porosity scale scale secondary porosity spectrum cut-off value calculated by using the core nuclear magnetic resonance T2 spectrum:

Using the formula for calculating the cut-off value of the secondary porosity spectrum in step S1, adjust the value of the fixed coefficient a between 1-3, control the calculation result of the cut-off value of the secondary porosity spectrum, and calculate the envelope of the porosity spectrum after the cut-off value The enclosed area is integrated to calculate the secondary porosity to obtain the secondary porosity of the porosity spectrum; the error analysis is carried out between the secondary porosity of the porosity spectrum calculated successively and the secondary porosity obtained in step S2, and the corresponding The value of a is determined after calibration, and the calculation result of the corresponding porosity spectrum secondary porosity is the secondary porosity after calibration.

Further, the method for obtaining secondary porosity based on the cut-off value of the secondary porosity spectrum of the second embodiment of the present invention further includes the following steps:

S4. Calculating the secondary porosity of the thin section of the cast body.

S5 , verifying the accuracy of the secondary porosity of the porosity spectrum obtained in step S3 by using the secondary porosity of the cast thin section.

For the specific operations of steps S4 and S5, reference may be made to the relevant descriptions of the above-mentioned first embodiment.

Taking the Liuhua 11-1 reef limestone oilfield in the Pearl River Mouth Basin as an example, and taking LH11-1-D5P1 well as an example to obtain the secondary porosity of the target interval, the present invention will be described in detail below:

Step 1. Establish the formula for calculating the cut-off value of the secondary porosity spectrum: select a window on the electrical imaging logging image, and the size of the window is determined according to the actual situation. Calculate the sum of the porosity count rates within the range of the minimum porosity and the median porosity in the window, calculate the percentage of the count rate of each porosity within the range to the total count rate, and calculate the minimum porosity and porosity median The porosity variance within the value range, set a fixed coefficient a and multiply this variance to obtain the cut-off value of the secondary porosity spectrum. The calculation formula is:

Secondary porosity spectrum cut-off value = a × δ + median value of porosity

In the formula: δ—porosity variance within the range of minimum porosity and porosity median;

a—Fixed coefficient, which determines the calculation accuracy of cut-off value of secondary porosity spectrum.

Step 2. Calculate the core secondary porosity using the core NMR T2 spectrum: as shown in Figure 2, it is the distribution map of the carbonate reservoir core NMR T2 spectrum, and curve 1 is the porosity component before centrifugation of the rock sample, showing It is generally believed that the third peak with a longer relaxation time represents large-diameter pores, that is, secondary pores; Curve 2 is the porosity component after centrifugation of the rock sample, showing obvious double-peak characteristics, and the porosity before centrifugation The third peak with the longest relaxation time in the component disappears, indicating that the fluid in the large-diameter pores is discharged due to centrifugal action. Curve 3 and curve 4 are the accumulation before centrifugation and the accumulation after centrifugation, respectively.

The size of secondary porosity is approximately equal to the volume of large-diameter pore fluid discharged after centrifugation. Find the start time of the third peak with the longest relaxation time in the porosity component before centrifugation, which corresponds to the T2 cut-off value of the secondary porosity spectrum, and the cumulative total porosity before centrifugation minus the T2 cut-off value of the secondary porosity spectrum corresponds to The cumulative porosity before centrifugation is the secondary porosity.

Step 3, use the fixed coefficient a value in the secondary porosity calculation formula of the core nuclear magnetic resonance secondary porosity scale: adopt the secondary porosity spectrum cut-off value calculation formula in step 1, by adjusting the fixed coefficient a in the calculation formula The value (between 1 and 3) controls the calculation result of the cut-off value of the secondary porosity spectrum, and the area enclosed by the envelope of the porosity spectrum after the cut-off value is integrated to calculate the secondary porosity.

The error analysis of the secondary porosity of the porosity spectrum and the secondary porosity calculated by the nuclear magnetic resonance T2 spectrum of the target interval of the LH11-1-D5P1 well is carried out. When a is set to 1.01, the calculated secondary porosity and nuclear magnetic resonance T2 spectrum calculation of secondary porosity has the smallest error, so the fixed coefficient a in the cut-off value formula of secondary porosity spectrum is determined to be 1.01, and the calibration of secondary porosity is completed.

Step 4. Calculating the secondary porosity of the casting sheet: According to the identification results of the casting sheet at 1399.5m in Well LH11-1-D5P1, the intergranular dissolution pores, intragranular dissolution pores, matrix dissolution pores, and structural dissolution fractures of the sheet are obtained as 2%, 5%, 2%, 3%, that is, the secondary face (seam) rate is 12%.

Fig. 3 is a fitting relationship diagram of the total surface porosity (seam) rate and the total porosity of the cast body sheet, thus drawing the transformation relationship y=0.61x of an embodiment of the cast body sheet surface (seam) rate and the total porosity +6.75, the secondary porosity surface (slit) ratio is converted to the secondary porosity of the casting sheet.

Step 5. Verify the accuracy of the secondary porosity of the porosity spectrum by using the secondary porosity of the casting thin section: calculate the secondary porosity and porosity of the casting thin section in the 1389-1470m target interval of Well LH11-1-D5P1 as shown in Table 1 below The average absolute error and average relative error between the calculated secondary porosity of the spectrum are 0.86% and 7.7%, respectively, which proves that the acquired secondary porosity spectrum has high accuracy.

Table 1 Accuracy analysis of secondary porosity

By comparing the secondary porosity obtained by casting thin section and nuclear magnetic resonance two calibration methods, the secondary porosity obtained by porosity spectrum is in good agreement with the secondary porosity of casting thin section and the secondary porosity calculated by nuclear magnetic resonance T2 spectrum.

The above is only an embodiment of the present invention, and does not limit the patent scope of the present invention. Any equivalent structure or equivalent process transformation made by using the description of the present invention and the contents of the accompanying drawings, or directly or indirectly used in other related technologies fields, are all included in the scope of patent protection of the present invention in the same way.

Claims (10)

1. A method for obtaining secondary porosity based on scale electric imaging secondary porosity spectrum cut-off value, is characterized in that, comprises the following steps: S1. Establish the formula for calculating the cut-off value of the secondary porosity spectrum: Cut-off value of secondary porosity spectrum = a × δ + median value of porosity Among them, a is a fixed coefficient, δ is the porosity variance within the range between the minimum porosity and the median porosity in the selected window on the electrical imaging logging image, and the median porosity is the median porosity in the selected window; S2, using the core nuclear magnetic resonance T2 spectrum to calculate the secondary porosity of the core; S3. The value of the fixed coefficient a in the calculation formula of the secondary porosity scale scale secondary porosity spectrum cut-off value calculated by using the core nuclear magnetic resonance T2 spectrum: Using the formula for calculating the cut-off value of the secondary porosity spectrum in step S1, adjust the value of the fixed coefficient a between 1-3, control the calculation result of the cut-off value of the secondary porosity spectrum, and calculate the envelope of the porosity spectrum after the cut-off value The enclosed area is integrated to calculate the secondary porosity to obtain the secondary porosity of the porosity spectrum; the error analysis is carried out between the secondary porosity of the porosity spectrum calculated successively and the secondary porosity obtained in step S2, and the corresponding The value of a is determined after calibration, and the calculation result of the corresponding porosity spectrum secondary porosity is the secondary porosity after calibration. 2. The method for obtaining secondary porosity based on the cut-off value of the scale electrical imaging secondary porosity spectrum according to claim 1, characterized in that, in step S1, a window is selected on the electrical imaging logging image, and the minimum value in the calculation window is The sum of porosity count rates within the range of porosity and porosity median; calculate the percentage of the count rate of each porosity within the range to the total count rate, and calculate the minimum porosity and porosity median within the range Porosity variance δ; set a fixed coefficient a and multiply this variance to obtain the cut-off value of the secondary porosity spectrum, and obtain the calculation formula of the cut-off value of the secondary porosity spectrum. 3. the method for obtaining secondary porosity based on the cut-off value of scale electric imaging secondary porosity spectrum according to claim 1, it is characterized in that, in step S2, according to the size of core secondary porosity is equal to the large amount discharged after centrifugation According to the peak characteristics in the core NMR T2 spectrum distribution diagram, the pore fluid volume of the pore size is used to obtain the peak start time of the longest relaxation time in the porosity component before centrifugation. This start time corresponds to the T2 cutoff value of the secondary porosity spectrum, and the core second Primary porosity = cumulative total porosity before centrifugation - cumulative porosity before centrifugation corresponding to T2 cut-off value of secondary porosity spectrum. 4. according to any one of claim 1-3, obtain the method for secondary porosity based on scale electrical imaging secondary porosity spectrum cut-off value, it is characterized in that, also comprise the following steps: S4, calculating the secondary porosity of the cast body thin section; S5 , verifying the accuracy of the secondary porosity of the porosity spectrum obtained in step S3 by using the secondary porosity of the cast thin section. 5. The method for obtaining secondary porosity based on the cut-off value of the scale electrical imaging secondary porosity spectrum according to claim 4, characterized in that, in step S4, the selected window position on the electrical imaging logging image in step S1 is taken For the corresponding reservoir, prepare cast thin slices; According to the identification results of the casting flakes, the type of secondary pores and the corresponding relative content of the casting flakes are obtained, and the relationship between the total surface porosity and the total porosity of the casting flakes is fitted to obtain the conversion between the total surface porosity and the total porosity of the casting flakes The relational expression converts the surface porosity of secondary pores into the secondary porosity of cast thin slices. 6. The method for obtaining secondary porosity based on the cut-off value of the scale electric imaging secondary porosity spectrum according to claim 5, wherein the type of secondary pores in the cast sheet comprises intergranular dissolved pores, intragranular dissolved pores pores, matrix dissolution pores, and structural dissolution fractures. 7. The method for obtaining secondary porosity based on the cut-off value of the scale electric imaging secondary porosity spectrum according to claim 4, characterized in that, in step S5, the secondary porosity and porosity spectrum are calculated by casting thin slices The mean absolute error between the primary porosities is used to verify the accuracy of the acquired porosity spectrum secondary porosity. 8. The method for obtaining secondary porosity based on the cut-off value of the secondary porosity spectrum of the scale electric imaging according to claim 7, characterized in that, in step S5, the average absolute error is less than 5%. 9. A method for obtaining secondary porosity based on the cut-off value of the scale electric imaging secondary porosity spectrum, characterized in that, comprising the following steps: S1. Establish the formula for calculating the cut-off value of the secondary porosity spectrum: Secondary porosity spectrum cutoff = a + average porosity Among them, a is a fixed coefficient, and the average porosity is the arithmetic mean value of all porosities in the selected window on the electrical imaging logging image; S2, using the core nuclear magnetic resonance T2 spectrum to calculate the secondary porosity of the core; S3. The value of the fixed coefficient a in the calculation formula of the secondary porosity scale scale secondary porosity spectrum cut-off value calculated by using the core nuclear magnetic resonance T2 spectrum: Using the formula for calculating the cut-off value of the secondary porosity spectrum in step S1, adjust the value of the fixed coefficient a between 1-3, control the calculation result of the cut-off value of the secondary porosity spectrum, and calculate the envelope of the porosity spectrum after the cut-off value The enclosed area is integrated to calculate the secondary porosity to obtain the secondary porosity of the porosity spectrum; the error analysis is carried out between the secondary porosity of the porosity spectrum calculated successively and the secondary porosity obtained in step S2, and the corresponding The value of a is determined after calibration, and the calculation result of the corresponding porosity spectrum secondary porosity is the secondary porosity after calibration. 10. The method for obtaining secondary porosity based on the cut-off value of scale electrical imaging secondary porosity spectrum according to claim 9, characterized in that, in step S2, according to the size of the core secondary porosity is equal to the large According to the peak characteristics in the core NMR T2 spectrum distribution diagram, the pore fluid volume of the pore size is used to obtain the peak start time of the longest relaxation time in the porosity component before centrifugation. This start time corresponds to the T2 cutoff value of the secondary porosity spectrum, and the core second Primary porosity = cumulative total porosity before centrifugation - cumulative porosity before centrifugation corresponding to T2 cut-off value of secondary porosity spectrum.