A nondestructive quantitative characterization method for the full pore throat size distribution of rocks based on nuclear magnetic resonance

Abstract

The present invention relates to a non-destructive quantitative characterization method for the distribution of the full pore throat size of a rock based on nuclear magnetic resonance. The method comprises the following steps: using a nuclear magnetic resonance instrument to test the nuclear magnetic resonance T2 spectrum of a saturated rock sample; performing a first centrifugation on the saturated rock sample, subtracting the nuclear magnetic resonance T2 spectrum of the saturated rock sample from the rock sample after the first centrifugation to obtain a difference T2 spectrum, and obtaining a critical value of the T2 spectrum change; increasing the centrifugal speed in sequence, obtaining the centrifugal pressure and the corresponding critical value of the T2 spectrum change, and fitting and establishing a correlation formula between the inverse of the centrifugal pressure and the critical value of the T2 spectrum change; and establishing a throat diameter d in combination with the Washburn equation. t The conversion relationship between the value of T2 and the throat size distribution of the rock sample is obtained; the pore-throat ratio is obtained according to the observation results of the casting thin section, and the pore diameter d is established. p The conversion relationship between the value of T2 and the pore size distribution of the rock sample is obtained. The invention can accurately test the full pore throat size distribution of the rock, the test process is harmless to the rock sample, the measurement range is wide, and the uniformity and comparability of the test results are guaranteed.

Description

Technical Field

[0001] The invention relates to the technical field of oil and natural gas exploration and development, and in particular to a non-destructive quantitative characterization method for full pore throat size distribution of rocks based on nuclear magnetic resonance. Background Art

[0002] Rock pore structure refers to the shape, size, type and interconnectedness of pores and throats inside rocks, which can be specifically divided into pore size distribution and throat size distribution. Based on the principles and characteristics of the test methods, rock pore structure characterization methods can be divided into three categories: image analysis method, fluid intrusion statistical method and non-destructive statistical method.

[0003] Image analysis method uses high-resolution imaging techniques such as optical microscope (OM), transmission electron microscope (TEM), scanning electron microscope (SEM), field emission scanning electron microscope (FESEM), atomic force microscope (AFM) to directly observe the contact of rock skeleton particles, the size, morphology, connectivity of pores and throats, etc. Image analysis method has the advantages of high resolution and intuitive and reliable observation results, but it can only observe the pore throat development characteristics of local areas of the sample. It has the disadvantages of high sample preparation requirements, small imaging range, and insufficient representativeness. It is limited to qualitative or qualitative-semi-quantitative analysis, and it is difficult to achieve a comprehensive and quantitative analysis of rock pore structure. At the same time, in the process of scanning imaging using a microscope, it is usually necessary to adjust parameters such as brightness and contrast according to the characteristics of the sample itself to achieve a better imaging effect, resulting in changes in the grayscale distribution characteristics of images of different samples and different fields of view, so that there are differences in the threshold for distinguishing rock skeletons and pores.

[0004] The fluid intrusion statistical method uses non-wetting liquids such as mercury or gases such as nitrogen and carbon dioxide to inject rock samples at different pressures and record the injection volume. It obtains information such as pore size distribution and specific surface area through certain theoretical calculation methods. It is a quantitative research method, mainly including mercury injection and gas adsorption. Rock samples tested by mercury injection cannot be reused, and mercury is harmful to both the human body and the environment. With the improvement of environmental protection requirements, the use of mercury injection is gradually restricted. The gas adsorption method requires that the rock samples be crushed into samples with a particle size of 250 to 425 μm. It mainly tests the pore size distribution of nano-scale pores. The original cracks will be destroyed during the crushing of the rock samples, resulting in large errors in the test results when reflecting the characteristics of the original pore structure.

[0005] The nondestructive statistical method is a relatively new pore testing technology and belongs to a quantitative research method. Its biggest advantage is that it does not damage the rock sample and can obtain quantitative pore structure data. The rock sample after the test can also carry out other experiments. The nondestructive statistical method is currently the mainstream rock pore structure characterization method, mainly including CT scanning technology and nuclear magnetic resonance technology. Similar to the image analysis method, CT scanning technology also distinguishes pores and matrix by analyzing the grayscale of the image. The difference in the setting of the resolution threshold has a great influence on the test results. For rock samples with a size of 20 to 50 mm commonly used in conventional rock physics experiments, Macro70 detection lenses are required for CT scanning, and the distribution information of pores with a size less than 70 μm cannot be effectively obtained. Compared with CT scanning technology, nuclear magnetic resonance technology has the advantages of high precision, simple operation, wide application range, high safety, and simple data processing. It is being used more and more widely in rock pore space testing.

[0006] When using nuclear magnetic resonance technology to measure rock pore structure, it is necessary to use an accurate and single geometric shape factor and the transverse surface relaxation rate of rock particles for calculation. However, these two parameters are difficult to obtain accurately: (1) Rock pore shapes are diverse, and there are some irregular pores. The geometric shape factor cannot be simply set to a single fixed value to represent the shape characteristics of all pores inside the rock sample; (2) The transverse surface relaxation rate is affected by many factors, such as lithology, paramagnetic mineral content, clay mineral content, etc., and the transverse surface relaxation rate cannot be simply set to a single fixed value.

[0007] In order to avoid the difficulties in obtaining the geometric shape factor and the transverse surface relaxation rate separately, most scholars currently define the product of the two as a conversion coefficient, and use nuclear magnetic resonance technology in combination with high-pressure mercury injection to determine the conversion coefficient. However, high-pressure mercury injection actually reflects the throat size distribution, not the pore size distribution. Therefore, the combination of nuclear magnetic resonance technology and high-pressure mercury injection can only obtain the throat size distribution inside the rock, but not the pore size distribution inside the rock. At the same time, rock samples after high-pressure mercury injection testing cannot be reused, which greatly limits the reliability and comparability of different experimental test results. Summary of the invention

[0008] In order to overcome the problems in the prior art, the present invention provides a non-destructive quantitative characterization method for the full pore throat size distribution of rocks based on nuclear magnetic resonance. The method combines nuclear magnetic resonance technology with centrifugation and casting thin section observation to establish T 2 The correlation between the value and the centrifugal pressure, T 2 The correlation between the value and the throat diameter, T 2 The correlation between the value and the pore diameter is obtained, and finally the rock throat diameter distribution and the pore diameter distribution are obtained. To achieve the above purpose, the present invention provides the following scheme:

[0009] A non-destructive quantitative characterization method for the full pore throat size distribution of rocks based on nuclear magnetic resonance comprises the following steps:

[0010] S1. Collect cylindrical rock samples, dry them to constant weight, and measure the length l of the rock samples;

[0011] S2. Vacuum and pressurize the rock sample to saturate it, and use a nuclear magnetic resonance instrument to perform nuclear magnetic resonance of the saturated rock sample. 2 Spectrum test, obtain the nuclear magnetic resonance T of saturated rock samples 2 spectrum;

[0012] S3, put the saturated rock sample into the centrifuge, set the centrifugal speed to the first speed n, centrifuge the rock sample for the first time, and use the centrifugal pressure calculation formula to obtain the centrifugal pressure P corresponding to the centrifugal speed c ;

[0013] S4. After centrifugation, take out the rock sample and weigh it. Use nuclear magnetic resonance to carry out nuclear magnetic resonance of the rock sample after the first centrifugation. 2 Spectrum test, obtain the nuclear magnetic resonance T of the rock sample after the first centrifugation 2 spectrum;

[0014] S5. NMR of saturated rock samples 2 Spectra and NMR T of rock samples after the first centrifugation 2 Spectral subtraction gives the difference T 2 spectrum, obtain T 2 Spectral change threshold;

[0015] S6. Put the rock sample into the centrifuge again, increase the centrifugal speed in steps of Δn, repeat S3, S4, and S5, and obtain the centrifugal pressure and the corresponding T 2 The critical value of spectrum change is until the rock sample quality, NMR T 2 The spectrum remains essentially unchanged;

[0016] S7. Plot the inverse of centrifugal pressure and T 2 The relationship between the critical value of the spectrum change is shown in the figure, and the inverse of the centrifugal pressure and T are fitted. 2 Correlation formula of critical value of spectrum change;

[0017] S8. Combined with the Washburn equation, establish the throat diameter d of the rock sample t With T 2 The conversion relationship of values;

[0018] S9, Substitute the rock sample pore-throat ratio into the rock sample throat diameter and T 2 The conversion relationship of the values ​​is used to establish the pore diameter d of the rock sample. p With T 2 The conversion relationship of values;

[0019] S10, according to the throat diameter d of the rock sample t With T 2 The conversion relationship of the value is to convert the saturated rock sample NMR T 2 The spectrum is converted into the throat size distribution of the rock sample;

[0020] S11, according to the pore diameter d of the rock sample p With T 2 The conversion relationship of the value is to convert the saturated rock sample NMR T 2 The spectrum is converted into the pore size distribution of the rock sample.

[0021] Furthermore, in step S3, the first rotation speed n is 1000 r / min.

[0022] Furthermore, in step S3, the centrifugal pressure calculation formula is:

[0023]

[0024] Where: P c is the centrifugal pressure, Pa; n is the centrifugal speed, r / min; ρ w is the density of water, kg / m 3 ; l is the length of the rock sample, m; r 2 is the distance between the outer end face of the rock sample and the rotation axis, m.

[0025] Furthermore, in step S5, the T 2 The critical value of spectrum change is the difference T 2 The value of the first intersection of the spectrum with the 0 scale line from right to left.

[0026] Furthermore, in step S6, Δn is 500 r / min.

[0027] Furthermore, in step S7, the inverse of the centrifugal pressure and T 2 The correlation equation of the critical value of spectrum change is a power function relationship.

[0028] Furthermore, in step S7, the inverse of the centrifugal pressure and T 2 The correlation formula of the critical value of spectrum change is:

[0029]

[0030] Where: T 2 T 2 Spectral change critical value, ms; a, b are fitting parameters, dimensionless.

[0031] Furthermore, in step S8, the throat diameter d of the rock sample is t With T 2 The conversion relationship of the value is:

[0032]

[0033] Where: α, β are fitting parameters and are dimensionless.

[0034] Furthermore, in step S9, the pore diameter d of the rock sample p With T 2 The conversion relationship of the value is:

[0035]

[0036] Where: A, B are fitting parameters and are dimensionless.

[0037] Furthermore, in step S9, the pore-throat ratio of the rock sample is obtained based on the observation results of the rock casting thin section.

[0038] On the other hand, the present invention provides a computer device, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that when the processor executes the computer program, the steps of the method for non-destructive quantitative characterization of the full pore throat size distribution of rocks as described in any one of the above claims are implemented.

[0039] On the other hand, the present invention provides a computer-readable storage medium having a computer program stored thereon, characterized in that when the computer program is executed by a processor, the steps of the method for non-destructive quantitative characterization of the full pore throat size distribution of rocks as described in any one of the above claims are implemented.

[0040] The present invention has the following advantages: (1) The entire testing process of the method provided by the present invention is harmless to the test personnel and the environment, and does not damage the rock samples. The rock samples can be reused, which makes it convenient to use the same rock sample to carry out different experiments to ensure the uniformity of the test results. (2) The method provided by the present invention has a wide range of measurement size (1nm~10mm), and can simultaneously obtain the throat diameter distribution and pore diameter distribution, overcoming the defect that CT scanning technology cannot achieve simultaneous measurement of nano-scale to millimeter-scale pore structures. (3) The test of the present invention facilitates the use of the same rock sample to carry out the same type of experiment continuously, and reflects the continuous changes of the rock during the experiment in real time (such as the evolution of acid wormholes caused by acid-rock reaction, the expansion of cracks inside freeze-thaw rocks, etc.), to ensure the comparability of the test results. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0042] Figure 1 The water-saturated nuclear magnetic resonance (T) of the rock sample in the embodiment of the present invention 2 spectrum;

[0043] Figure 2 is the difference T between the saturated state of the rock sample and the first centrifugation in the embodiment of the present invention 2 spectrum;

[0044] Figure 3 T is the rock sample at different centrifugal speeds in the embodiment of the present invention. 2 Relationship curve between the critical value of spectrum change and the inverse of centrifugal pressure;

[0045] Figure 4 This is a distribution diagram of the throat size of the rock sample in the embodiment of the present invention;

[0046] Figure 5 1 is a pore size distribution diagram of the rock sample in the embodiment of the present invention. DETAILED DESCRIPTION

[0047] The present invention is further described below in conjunction with the accompanying drawings and embodiments. It should be noted that, in the absence of conflict, the embodiments in this application and the technical features in the embodiments can be combined with each other. It should be noted that, unless otherwise specified, all technical and scientific terms used in this application have the same meanings as those generally understood by those of ordinary skill in the art to which this application belongs. The words "including" or "comprising" and the like used in the disclosure of the present invention mean that the elements or objects appearing before the word cover the elements or objects listed after the word and their equivalents, without excluding other elements or objects.

[0048] In order to solve the technical problem that it is difficult to accurately obtain the geometric shape factor and the lateral surface relaxation rate when using the nuclear magnetic resonance technology to measure in the prior art, the present invention provides a non-destructive quantitative characterization method for the full pore throat size distribution of rocks based on nuclear magnetic resonance, and the main testing steps are as follows: collecting cylindrical rock samples, exhausting and drying them to constant weight, and measuring the length of the rock samples; vacuuming and pressurizing the rock samples, and using a nuclear magnetic resonance instrument to obtain the nuclear magnetic resonance T of the saturated rock samples 2 Spectrum; put the saturated rock sample into the centrifuge, centrifuge it for the first time, and calculate the corresponding centrifugal pressure; use the nuclear magnetic resonance instrument to obtain the nuclear magnetic resonance T of the rock sample after the first centrifugation 2 Spectrum; The water-saturated rock sample NMR T 2 Spectra and NMR T of rock samples after the first centrifugation 2 Spectral subtraction gives the difference T 2 Spectrum, difference T 2 The first intersection of the spectrum from right to left with the 0 scale line is T 2 Spectrum change critical value; increase the centrifugal speed in sequence to obtain the centrifugal pressure and the corresponding T 2The critical value of the spectrum change is fitted to establish the inverse of the centrifugal pressure and T 2 The correlation formula of the critical value of spectrum change; combined with the Washburn equation, the throat diameter d of the rock sample is established t With T 2 The conversion relationship of the value is to convert the saturated rock sample NMR T 2 The spectrum is converted into the throat size distribution of the rock sample; the pore-throat ratio of the rock sample is obtained based on the observation results of the rock casting thin section, and the pore diameter d of the rock sample is further established. p With T 2 The conversion relationship of the value is to convert the saturated rock sample NMR T 2 The spectrum is converted into the pore size distribution of the rock sample.

[0049] Specifically, in a measurement example, the following measurement steps are included:

[0050] S1. Collect cylindrical rock samples, dry them to constant weight with ventilation, and measure the length of the rock samples to be 4.47 cm;

[0051] S2. Vacuum and pressurize the rock sample to saturate it, and use a nuclear magnetic resonance instrument to perform nuclear magnetic resonance of the saturated rock sample. 2 Spectrum test, obtain the nuclear magnetic resonance T of saturated rock samples 2 The measurement results are as follows Figure 1 As shown;

[0052] S3. Place the saturated rock sample in a centrifuge, set the centrifugal speed to 1000 r / min, centrifuge the rock sample for the first time, and use the centrifugal pressure calculation formula to obtain the centrifugal pressure P corresponding to the centrifugal speed. c :

[0053]

[0054] Where: P c is the centrifugal pressure, Pa; n is the centrifugal speed, r / min; ρ w is the density of water, kg / m 3 ; l is the length of the rock sample, m; r 2 is the distance between the outer end face of the rock sample and the rotation axis, m.

[0055] The centrifugal pressure corresponding to the centrifugal speed of 1000r / min is 0.046MPa;

[0056] S4. After centrifugation, take out the rock sample and weigh it. Use nuclear magnetic resonance to carry out nuclear magnetic resonance of the rock sample after the first centrifugation. 2 Spectrum test, obtain the nuclear magnetic resonance T of the rock sample after the first centrifugation 2 spectrum;

[0057] S5. NMR of saturated rock samples 2 Spectra and NMR T of rock samples after the first centrifugation2 Spectral subtraction gives the difference T 2 spectrum, the difference T 2 The first intersection of the spectrum from right to left with the 0 scale line is defined as T 2 Spectral change threshold, such as Figure 2 As shown, the T corresponding to the centrifugal speed of 1000r / min 2 The critical value of spectrum change is 11.1ms;

[0058] S6. Continue to put the rock sample into the centrifuge, increase the centrifugal speed in steps of 500 r / min, and repeat S3, S4, and S5 at centrifugal speeds of 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000, 5500, 6000, 6500, and 7000 r / min, respectively, to obtain the centrifugal pressure and the corresponding T 2 The critical value of spectrum change is until the rock sample quality, NMR T 2 The spectrum remains essentially unchanged;

[0059] S7. Plot the inverse of centrifugal pressure and T 2 The relationship curve of the critical value of spectrum change, such as Figure 3 As shown in the figure, the inverse of the centrifugal pressure and T 2 The critical value of spectrum change is a power function relationship, and the relevant relationship is:

[0060]

[0061] Where: T 2 T 2 Spectral change threshold, ms.

[0062] S8. Under the high-speed centrifugal action of air-water, the water inside the rock sample is thrown out, which conforms to the Washburn equation:

[0063]

[0064] Where: σ is the interfacial tension of water in air, N / m; θ is the wetting contact angle of water in air, °; d t is the throat diameter of the rock sample, μm.

[0065] Under laboratory conditions, σ takes the value of 0.072N / m and θ takes the value of 0°, then:

[0066]

[0067] Therefore, the throat diameter d t With T 2 The conversion relationship of the value is:

[0068]

[0069] S9. According to the observation results of rock casting thin section, the pore-throat ratio of the rock sample is obtained to be 6.0. Substitute the pore-throat ratio of the rock sample into the throat diameter and T 2 The conversion relationship of the values ​​is used to establish the pore diameter d of the rock sample. p With T 2 The conversion relationship of the value is:

[0070]

[0071] Where: d p is the pore diameter of the rock sample, μm.

[0072] S10, according to the throat diameter d of the rock sample t With T 2 The conversion relationship of the value is to convert the saturated rock sample NMR T 2 The spectrum is converted into the throat size distribution of the rock sample, such as Figure 4 As shown, the throat diameter of the rock sample is distributed in the range of 0.002 to 358.1 μm;

[0073] S11, according to the pore diameter d of the rock sample p With T 2 The conversion relationship of the value is to convert the saturated rock sample NMR T 2 The spectrum is converted into the pore size distribution of the rock sample, such as Figure 5 As shown, the pore diameters of the rock samples are distributed in the range of 0.012 to 2148.4 μm.

[0074] The present invention provides a non-destructive quantitative characterization method for the full pore throat size distribution of rocks based on nuclear magnetic resonance. The method can obtain the full pore throat size distribution of a rock sample in a non-destructive state, and has a wide measurement range. It can realize the simultaneous measurement of pore structures from nanometer to millimeter scale, and provides a scientific and reliable test method for the characterization of rock pore structure, ensuring the uniformity and comparability of the test results.

[0075] The above description is only a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with the profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention still falls within the scope of the technical solution of the present invention.

Claims

1. A non-destructive quantitative characterization method for the full pore throat size distribution of rocks based on nuclear magnetic resonance, characterized in that: The following steps are involved: S1. Collect cylindrical rock samples, dry them to constant weight, and measure the length of the rock samples. l ; S2. Vacuum and pressurize the rock sample to saturate it, and use a nuclear magnetic resonance instrument to perform nuclear magnetic resonance on the saturated rock sample. T 2 Spectrum test, obtain NMR of saturated rock samples T 2 spectrum; S3. Place the saturated rock sample into the centrifuge and set the centrifugal speed to the first speed. n , centrifuge the rock sample for the first time, and use the centrifugal pressure calculation formula to obtain the centrifugal pressure corresponding to the centrifugal speed P c ; The centrifugal pressure calculation formula is: P c =1.097*0.01*n*n* ρ w * l *( r 2-0.5* l ) Where: P c is the centrifugal pressure, Pa; n is the centrifugal speed, r / min; ρ w is the density of water, kg / m 3 ; l is the length of the rock sample, m; r 2 is the distance between the outer end of the rock sample and the rotation axis, m S4. After centrifugation, take out the rock sample and weigh it. Use nuclear magnetic resonance to carry out nuclear magnetic resonance of the rock sample after the first centrifugation. T 2 Spectrum test, obtain the nuclear magnetic resonance of the rock sample after the first centrifugation T 2 spectrum; S5. Nuclear magnetic resonance of saturated rock samples T 2 spectrum and NMR of rock samples after the first centrifugation T Subtract the two spectra to get the difference T 2 spectrum, obtain T 2 Spectral change critical value; in T 2 The critical value of spectrum change is the difference T 2 The first intersection value of the spectrum from right to left with the 0 scale line; S6. Put the rock sample into the centrifuge again. n Increase the centrifugal speed in steps, repeat S3, S4, S5, and obtain the centrifugal pressure and the corresponding T 2 Spectral change critical value, until the rock sample quality, NMR T 2The spectrum remains basically unchanged; The inverse of centrifugal pressure and T 2 The correlation equation of the critical value of spectrum change is: （1 / P c ）=a*( T 2) b Where: T 2 for T 2 Spectral change threshold, ms; a , b is the fitting parameter, dimensionless; S7. Plot the inverse of centrifugal pressure and T 2 spectrum change critical value relationship curve, and fit to establish the inverse of centrifugal pressure and T 2 Correlation formula of critical value of spectrum change; S8. Combined with the Washburn equation, establish the throat diameter of the rock sample d t and T 2-value conversion relationship; S9, Substitute the rock sample pore-throat ratio into the rock sample throat diameter and T 2 values ​​to establish the pore diameter of the rock sample d p and T 2-value conversion relationship; S10, according to the throat diameter of the rock sample d t and T 2 value conversion formula, the saturated rock sample NMR T 2 spectrum is converted into rock sample throat size distribution; S11, according to the pore diameter of the rock sample d p and T 2 value conversion formula, the saturated rock sample NMR T 2 spectrum is converted into the pore size distribution of the rock sample.

2. The nondestructive quantitative characterization method for the full pore throat size distribution of rock according to claim 1, wherein in step S3, the first rotation speed n It is 1000r / min.

3. The nondestructive quantitative characterization method for the full pore throat size distribution of rock according to claim 1, wherein in step S6, Δ n It is 500r / min.

4. The nondestructive quantitative characterization method for the full pore throat size distribution of rock according to claim 1, wherein in step S7, the inverse of the centrifugal pressure is T 2 The correlation equation of the critical value of spectrum change is a power function relationship.

5. According to the non-destructive quantitative characterization method for the full pore-throat size distribution of rocks according to claim 1, in said step S9, the pore-throat ratio of the rock sample is obtained according to the observation results of the rock casting thin section.

6. A computer device comprising a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that: When the processor executes the computer program, the steps of the method for non-destructive quantitative characterization of the full pore throat size distribution of rocks as described in any one of claims 1 to 5 are implemented.

7. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the method for non-destructive quantitative characterization of the full pore throat size distribution of rocks as described in any one of claims 1 to 5 are implemented.

Images