Method and system for evaluating the relaxation rate of rock pore surface in low-field nuclear magnetic resonance experiments

Abstract

The present invention belongs to the field of rock physics and geophysics, and discloses a method and system for evaluating the relaxation rate of rock pore surface in low-field nuclear magnetic resonance experiment. A low-temperature nitrogen adsorption experiment is carried out on one core to obtain the specific surface area S; a pressurized saturated fluid experiment is carried out on another core to determine the total pore volume V0 of the sample by using the mass difference method of the sample before and after saturation; a nuclear magnetic resonance T2 spectrum experiment is carried out on the core in the saturated fluid state to determine the total nuclear magnetic signal N0 and the T2 average value T of the saturated core. 2LM0 After drying the saturated fluid core at different temperatures, the T2 spectrum of the core at each saturation was tested, and the fluid saturation f was determined based on the change in core mass or the total NMR signal. i , and the average value of NMR T2 T 2LMi The method of the present invention is not affected by human factors, and the evaluation result is more objective. . The conventional method of the present invention uses the average value of saturated nuclear magnetic T2 and the method of low-temperature nitrogen adsorption and mercury intrusion pore size distribution to calibrate the surface relaxation rate.

Description

Technical Field

[0001] The invention belongs to the field of rock physics and geophysics, and in particular relates to a method and a system for evaluating the relaxation rate of rock pore surfaces in a low-field nuclear magnetic resonance experiment. Background Art

[0002] Core samples are important first-hand data for the exploration, evaluation and development of underground oil and gas and other mineral resources. As a means of studying rock pore structure and fluid composition content, low-field nuclear magnetic resonance technology plays an important role, especially in revealing the mobility of reservoir fluids and combining nuclear magnetic logging to achieve continuous formation prediction. It has unique advantages and is considered to be an integrated evaluation method for rock reservoir properties, fluid occurrence state and mobility. However, the direct result of nuclear magnetic resonance detection is the time domain (including transverse relaxation time T2 and longitudinal relaxation time T1). How to convert time domain parameters into spatial domain (pore size) is one of the problems that must be solved in nuclear magnetic resonance characterization of rock physical properties, and the key parameter of the conversion process is the surface relaxation rate of rock pores. Therefore, in rock physical nuclear magnetic resonance evaluation, it is of great significance to establish a method to accurately obtain the surface relaxation rate of rock pores.

[0003] At present, there are three methods for evaluating the surface relaxation rate of rock pores: one is to match the rock pore size distribution curve with the rock nuclear magnetic resonance T2 spectrum curve; the pore size distribution curve of the rock is obtained by low-temperature nitrogen adsorption or mercury injection experiment, and the surface relaxation rate is calculated according to the translation coefficient by translating or scaling the nuclear magnetic resonance T2 spectrum of the saturated fluid state rock (saturated oil or saturated water). When the nuclear magnetic resonance T2 spectrum is closest to the pore size distribution curve of the rock, the main peak corresponds or the error is the smallest. However, there are large differences in the pore size ranges tested by low-temperature nitrogen, mercury injection and nuclear magnetic resonance experiments, and different experiments are based on different principles, resulting in large errors in this method. Especially for shale samples, the matching effect of low-temperature nitrogen, mercury injection and nuclear magnetic resonance T2 spectrum is poor, and it is more affected by human factors and uncertainties.

[0004] The second is to use nuclear magnetic resonance and centrifugal experiments, calculate according to the nuclear magnetic resonance T2 cutoff value and the pore size cutoff value (converted by centrifugal force), and specifically determine the conversion coefficient based on the nuclear magnetic resonance T2 cutoff value determined by the centrifugal experiment and its corresponding pore size cutoff value (Wshburn equation), and then estimate the surface relaxation rate. However, centrifugal experiments generally require plunger-shaped samples, which has high requirements on sample specifications. In addition, the nuclear magnetic resonance T2 cutoff values ​​determined by different centrifugal forces are different. The surface relaxation rate estimated by this method is not a fixed value, but is related to the size of the nuclear magnetic resonance T2 cutoff value (centrifugal force).

[0005] The third method is to use the average nuclear magnetic T2 value of the rock and the specific surface area and pore volume of the rock for calculation. The average T2 value is calculated using the nuclear magnetic T2 spectrum of the saturated fluid state rock, and the surface relaxation rate is reversely calculated in combination with the specific surface area and pore volume of the low-temperature nitrogen adsorption test. This method uses the average nuclear magnetic T2 value (one data point) to calculate the surface relaxation rate, and the result is greatly affected by the nuclear magnetic T2 spectrum of the saturated fluid state rock. In particular, when the rock contains microcracks and macropores, the pores reflected by the average nuclear magnetic T2 value are not in the pore size range of the low-temperature nitrogen adsorption test, resulting in deviations in the results.

[0006] In general, the above existing methods mostly reversely fit / evaluate the surface relaxation rate based on the known rock pore size distribution or pore size, and do not use nuclear magnetic resonance experiments to quantitatively evaluate the surface relaxation rate from a positive perspective. Summary of the invention

[0007] In view of the problems existing in the prior art, the present invention provides a method and system for evaluating the relaxation rate of rock pore surfaces in a low-field nuclear magnetic resonance experiment.

[0008] The present invention is achieved by a method for evaluating the relaxation rate of rock pore surfaces in a low-field nuclear magnetic resonance experiment, the method comprising:

[0009] S1: After oil washing, the core is divided into two parts: one part of the core is subjected to low-temperature nitrogen adsorption experiment to obtain the specific surface area S of the core pores; the other part of the core is first subjected to pressurized saturated fluid experiment. According to the change of core mass after different saturation times, the core is ensured to be completely saturated with fluid, and the total pore volume V0 of the sample is determined by the mass difference method of the sample before and after saturation;

[0010] S2: Conduct NMR T2 spectrum experiment on the core in saturated fluid state to determine the total NMR signal N0 and T2 average value T of the saturated core 2LM0 ;

[0011] S3: After drying the saturated fluid core at different temperatures, the T2 spectra of the core at each saturation are tested respectively. According to the change of core mass or the total NMR signal (N1, N2, N3…N i ) to determine the fluid saturation (f1,f2,f3…f i ), and the average value of nuclear magnetic T2 (T 2LM1 ,T 2LM2 ,T 2LM3 …T 2LMi ). According to the average value of NMR T2 (T 2LM1 ,T 2LM2 ,T 2LM3 …T 2LMi ) and fluid saturation (f1,f2,f3…f i ), calculate the surface relaxation rate ρ2.

[0012] Further, the surface relaxation rate ρ2 is calculated in S3, and the specific principle is as follows:

[0013] When NMR tests porous media such as rocks, there are two main relaxation mechanisms: longitudinal relaxation (T1) and transverse relaxation (T2);

[0014] Taking transverse relaxation as an example, for low-viscosity fluids in pores, when the diffusion effect is ignored, its relaxation time can be expressed as:

[0015]

[0016] Where, T 2b T 2s are the transverse bulk relaxation time and the transverse surface relaxation time, respectively; f s is the proportion of surface relaxation fluid to the total fluid volume.

[0017] When the pore fluid in the rock is in a saturated state, f s for:

[0018]

[0019] Combining formula (1) and (2), we can get:

[0020]

[0021] Let the transverse surface relaxation rate ρ2 be:

[0022]

[0023] The transverse relaxation time is:

[0024]

[0025] However, when the free fluid in the rock pores is in an unsaturated state, f s for:

[0026]

[0027] Among them, the fluid saturation f in the pore can be expressed as:

[0028]

[0029] Therefore, combining formulas (1)(4)(6)(7), the transverse relaxation time is:

[0030]

[0031] Obviously, the difference between formula (8) and (5) is mainly reflected in the influence of fluid saturation. For low-viscosity fluids in micro-nano pores, the bulk relaxation T 2b can be ignored, so

[0032]

[0033] In the above formula, λ is the length of surface relaxation, S is the pore surface area, V0 is the pore volume, and V a is the volume of fluid where surface relaxation occurs, V b is the volume of fluid in which body relaxation occurs.

[0034] Formula (9) can be regarded as a linear function between 1 / T2 and 1 / f, and the slope of the function is the product of the surface relaxation rate ρ2 and S / V0. For a certain rock core, the transverse surface relaxation rate ρ2, specific surface area S and pore volume V0 are its inherent properties and will not change with the change of fluid content. Therefore, the transverse surface relaxation rate ρ2 can be obtained according to the changes in the nuclear magnetic T2 spectrum of rocks with different fluid saturations and their fluid saturation f.

[0035] Another object of the present invention is to provide a system for evaluating the relaxation rate of rock pore surfaces in a low-field nuclear magnetic resonance experiment for implementing the method for evaluating the relaxation rate of rock pore surfaces in a low-field nuclear magnetic resonance experiment, the system comprising:

[0036] The core pretreatment module is used to divide the core into two parts after oil washing: one part of the core is subjected to low-temperature nitrogen adsorption experiment to obtain the specific surface area S of the core pores; the other part of the core is first subjected to pressurized saturated fluid experiment to ensure that the core is completely saturated with fluid according to the change of core mass after different saturation times, and the total pore volume V0 of the sample is determined by the mass difference method of the sample before and after saturation;

[0037] The NMR module is connected to the core preprocessing module to conduct NMR T2 spectrum experiments on the core in the saturated fluid state and determine the total NMR signal N0 and T2 average value T of the saturated core. 2LM0 ;

[0038] The surface relaxation rate calculation module is connected to the nuclear magnetic resonance module. After drying the saturated fluid core at different temperatures, the T2 spectrum of the core at each saturation is tested. According to the change of core mass or the total nuclear magnetic signal (N1, N2, N3…N i ) to determine the fluid saturation (f1,f2,f3…f i ), and the average value of nuclear magnetic T2 (T 2LM1 ,T 2LM2 ,T 2LM3 …T 2LMi ). According to the average value of NMR T2 (T 2LM1 ,T2LM2 ,T 2LM3 …T 2LMi ) and fluid saturation (f1,f2,f3…f i ), calculate the surface relaxation rate ρ2.

[0039] Another object of the present invention is to provide a computer device, which includes a memory and a processor, wherein the memory stores a computer program, and when the computer program is executed by the processor, the processor executes the steps of the method for evaluating the surface relaxation rate of rock pores in a low-field nuclear magnetic resonance experiment.

[0040] Another object of the present invention is to provide a computer-readable storage medium storing a computer program, which, when executed by a processor, enables the processor to execute the steps of the method for evaluating the surface relaxation rate of rock pores in a low-field nuclear magnetic resonance experiment.

[0041] Another object of the present invention is to provide an information data processing terminal, which is used to implement the system for evaluating the surface relaxation rate of rock pores in low-field nuclear magnetic resonance experiments.

[0042] In combination with the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solutions to be protected by the present invention are as follows:

[0043] First, the traditional method uses the average value of the nuclear magnetic T2 spectrum of saturated fluid rock (only one data point) to calculate the surface relaxation rate. Obviously, the traditional method is greatly affected by the nuclear magnetic T2 spectrum characteristics of saturated fluid rock. In particular, when there are microcracks or artificial cracks in the rock, T 2LM The abnormal increase leads to distortion of the evaluation result of the surface relaxation rate. In contrast, the method of determining the surface relaxation rate of rock pores by using nuclear magnetic T2 spectra of different saturations proposed in the present invention uses more data points, is less affected by the nuclear magnetic T2 spectrum of saturated fluid-state rocks, and the obtained results are more credible.

[0044] Second, the present invention establishes a method for evaluating the surface relaxation rate of rock pores from a positive perspective using nuclear magnetic resonance experiments. Based on the changes in the nuclear magnetic resonance T2 spectrum of rocks with different fluid saturations, a method and process for quantitatively evaluating the surface relaxation rate is established from a theoretical perspective. Compared with the previous method of calibrating the nuclear magnetic resonance T2 spectrum using the rock pore size distribution curve, the method of the present invention is less affected by human factors and the evaluation results are more objective.

[0045] Third, the expected benefits and commercial value of the technical solution of the present invention after transformation are as follows: the present invention proposes a method for determining the surface relaxation rate by utilizing the changes in the nuclear magnetic T2 spectrum under different saturations, without the need for mercury injection, scanning electron microscopy and other experiments, which greatly reduces experimental and technical costs.

[0046] The technical solution of the present invention overcomes technical prejudice: the present invention proposes a method for determining the surface relaxation rate from the perspective of nuclear magnetic T2 spectra with different saturation levels, and the method has a solid theoretical model foundation and is not affected by human interference.

[0047] Fourth, the method for evaluating the relaxation rate of rock pore surfaces in low-field nuclear magnetic resonance experiments provided by the embodiments of the present invention has brought significant technical progress in view of the problems existing in the prior art. The following is a specific analysis of the technical problems solved and the technical progress achieved:

[0048] Existing technical problems solved:

[0049] 1. Measurement accuracy problem: The traditional pore surface relaxation rate measurement method is interfered by many factors, such as the wettability of the rock, the shape and size of the pores, etc., resulting in inaccurate measurement results. The present invention can more accurately obtain the specific surface area and total pore volume of rock pores by comprehensively using low-temperature nitrogen adsorption experiments, pressurized saturated fluid experiments and nuclear magnetic resonance T2 spectrum experiments, thereby improving the accuracy of surface relaxation rate measurement.

[0050] 2. High experimental complexity: The traditional surface relaxation rate measurement method requires complex experimental operations and tedious data processing, resulting in poor experimental efficiency and repeatability. The present invention improves experimental efficiency and repeatability by simplifying the experimental steps and optimizing the data processing method.

[0051] Significant technological advances achieved:

[0052] 1. Improved measurement accuracy: By comprehensively using a variety of experimental methods and technical means, the present invention can more accurately measure the surface relaxation rate of rock pores, providing more reliable experimental data support for research in the fields of petroleum exploration, rock mechanics, etc.

[0053] 2. Simplified experimental steps: The present invention simplifies the experimental steps and operation flow by optimizing the experimental design and data processing method, thereby improving the experimental efficiency and repeatability. This makes the experimental process easier and reduces the experimental cost and time cost.

[0054] 3. Expanded application scope: The evaluation method of the present invention is not only applicable to the measurement of rock pore surface relaxation rate in low-field nuclear magnetic resonance experiments, but can also be extended to other similar experiments and research fields. For example, the method and principle for evaluating the transverse surface relaxation rate proposed in the present invention are also applicable to the evaluation of the longitudinal surface relaxation rate; in addition, in the fields of soil science and materials science, similar methods can also be used to measure the pore structure and surface properties of materials.

[0055] The method for evaluating the surface relaxation rate of rock pores in a low-field nuclear magnetic resonance experiment provided by the embodiment of the present invention not only solves the problems of the prior art, but also brings significant technical progress and application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 is a flow chart of a method for evaluating the relaxation rate of rock pore surfaces in a low-field nuclear magnetic resonance experiment provided by an embodiment of the present invention;

[0057] Figure 2 is a nuclear magnetic T2 spectrum of shale under different oil-bearing conditions provided by an embodiment of the present invention;

[0058] Figure 3 is 1 / T provided by the embodiment of the present invention 2LM Intersection diagram with 1 / f;

[0059] Figure 4 It is a system structure diagram for evaluating the relaxation rate of rock pore surfaces in a low-field nuclear magnetic resonance experiment provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0060] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0061] The present invention relates to a method for determining the surface relaxation rate of rock pores, and particularly focuses on observing and analyzing changes in nuclear magnetic T2 spectra by changing the fluid saturation of the rock. The method for determining the surface relaxation rate proposed by the present invention is not limited to different saturation cores obtained using a specific method (such as different temperature treatments), and other methods such as centrifugation, static volatilization, and self-imbibition are also applicable to core samples obtained. The versatility of this method makes it more flexible and practical in practical applications.

[0062] The core of the present invention is to determine the relaxation rate of the rock pore surface by controlling the fluid saturation of the rock and using the changes in the nuclear magnetic T2 spectrum. This method not only provides an effective means of evaluating the rock pore structure and fluid distribution, but also uses a variety of flexible methods that can meet different research needs.

[0063] Application Example 1:

[0064] When a geological exploration institution was conducting a study on rock physical properties, it needed to accurately evaluate the relaxation rate of rock pore surfaces in low-field nuclear magnetic resonance experiments in order to better understand the pore structure and fluid properties inside the rock.

[0065] 1) Core preparation:

[0066] Representative core samples were selected and subjected to oil washing treatment to remove the fluid in the original pores.

[0067] The core samples after oil washing were divided into two parts.

[0068] 2) Experiment 1: Low temperature nitrogen adsorption experiment

[0069] A portion of the cores were selected for low-temperature nitrogen adsorption experiments.

[0070] The specific surface area S of the core pores is obtained through experiments.

[0071] 3) Experiment 2: Pressurized saturated fluid experiment

[0072] Another part of the core was subjected to pressurized saturated fluid experiment.

[0073] Increase the pressure gradually, observe the change in core mass, and ensure that the core is fully saturated with fluid.

[0074] The total volume V0 of the core pores was calculated using the mass difference method.

[0075] 4) Nuclear magnetic resonance T2 spectrum experiment

[0076] Nuclear magnetic resonance T2 spectroscopy experiments were carried out on the cores saturated with fluid.

[0077] The total NMR signal N0 and T2 average value T2LM0 of the saturated core were measured.

[0078] 5) Drying and testing

[0079] The cores saturated with fluid were dried at different temperatures.

[0080] At each drying temperature point, the T2 spectrum of the core was tested, and the total NMR signal (N1, N2, N3…Ni) and the T2 average value (T2LM1, T2LM2, T2LM3…T2LMi) were recorded.

[0081] 6) Calculation of surface relaxation rate

[0082] The fluid saturation (f1, f2, f3…fi) is determined based on the total NMR signal after drying.

[0083] The surface relaxation rate ρ2 was calculated using the average NMR T2 value and fluid saturation.

[0084] Through this method, geological exploration institutions can accurately evaluate the relaxation rate of rock pore surfaces and further understand the internal pore structure and fluid properties of rocks.

[0085] The application of this method is helpful to optimize exploration strategy and improve exploration efficiency.

[0086] Application Example 2:

[0087] During the exploration of oil and gas fields, an oil company needs to evaluate the pore surface relaxation rate of core samples to assess the storage capacity and permeability of the oil and gas reservoirs.

[0088] 1) Core sample preparation:

[0089] Representative samples are selected from the cores drilled from the oil and gas fields for oil washing treatment.

[0090] The oil washed core was divided into two parts.

[0091] 2) Experimental steps:

[0092] According to the above steps S1 to S3, the experiment was carried out to obtain the specific surface area S of the core pores, the total pore volume V0, the total NMR signal N0 and the T2 average value T2LM0, as well as the fluid saturation and the NMR T2 average value after drying.

[0093] 3) Data analysis:

[0094] Using the acquired data, the surface relaxation rate ρ2 is calculated.

[0095] Combined with other rock physical properties (such as permeability, porosity, etc.), the storage capacity and permeability of oil and gas reservoirs are evaluated.

[0096] Oil companies optimize oil and gas field exploration and development strategies based on the evaluation results.

[0097] Reservoirs with high storage capacity and permeability are given priority for development to increase oil and gas production.

[0098] For reservoirs with unsatisfactory evaluation results, further geological analysis and exploration technology research are carried out to find ways to improve reservoir performance.

[0099] like Figure 1 As shown, an embodiment of the present invention provides a method for evaluating the surface relaxation rate of rock pores in a low-field nuclear magnetic resonance experiment, the method comprising:

[0100] S1: After oil washing, the core is divided into two parts: one part of the core is subjected to low-temperature nitrogen adsorption experiment to obtain the specific surface area S of the core pores; the other part of the core is first subjected to pressurized saturated fluid experiment. According to the change of core mass after different saturation times, the core is ensured to be completely saturated with fluid, and the total pore volume V0 of the sample is determined by the mass difference method of the sample before and after saturation;

[0101] S2: Conduct NMR T2 spectrum experiment on the core in saturated fluid state to determine the total NMR signal N0 and T2 average value T of the saturated core 2LM0 ;

[0102] S3: After drying the saturated fluid core at different temperatures, the T2 spectra of the core at each saturation are tested respectively. According to the change of core mass or the total NMR signal (N1, N2, N3…N i ) to determine the fluid saturation (f1,f2,f3…f i ), and the average value of nuclear magnetic T2 (T 2LM1 ,T 2LM2 ,T 2LM3 …T 2LMi ). According to the average value of NMR T2 (T 2LM1 ,T 2LM2 ,T 2LM3 …T 2LMi ) and fluid saturation (f1,f2,f3…f i ), calculate the surface relaxation rate ρ2.

[0103] The embodiment of the present invention provides a method for evaluating the surface relaxation rate (ρ2) of rock pores in low-field nuclear magnetic resonance experiments, aiming to accurately evaluate the microscopic characteristics of rock pore structure by different treatments and measurements of rock cores. The following is the detailed working principle of the method:

[0104] Step 1: Core processing and data acquisition

[0105] 1. Core preparation and segmentation:

[0106] After washing the core to remove oil, it is divided into two parts. The first part is used for low-temperature nitrogen adsorption experiments to obtain the specific surface area of ​​the core pores \(S\). This is estimated by measuring the amount of nitrogen adsorbed on the rock surface, reflecting the accessible area of ​​the rock pore surface.

[0107] The second part of the core is used for pressurized saturated fluid experiments. First, the core is saturated under controlled conditions to ensure that all pores are filled with fluid. Then, the total pore volume of the sample \(V_0\) is determined by measuring the mass difference before and after saturation.

[0108] Step 2: NMR T2 spectroscopy experiment

[0109] 2. Nuclear magnetic resonance T2 spectrum experiment:

[0110] For cores saturated with fluid, NMR T2 spectrum experiments are performed. This step can determine the total NMR signal of the saturated core, \(N_0\), and the average value of the T2 spectrum, \(T2LM_0\). The T2 spectrum reflects the distribution of fluid relaxation time in pores of different sizes, among which the T2 average value is an important parameter for evaluating pore size and distribution.

[0111] Step 3: Drying and relaxation rate calculation

[0112] 3. Drying treatment and subsequent testing:

[0113] The cores that have been subjected to the NMR test are then dried at different temperatures, and each time the cores are dried, a NMR T2 spectrum test is performed on the cores.

[0114] By measuring the mass change of the processed core or the total NMR signal \(N_i\), different fluid saturations \(f_i\) can be determined. At the same time, the T2 average value \(T2LM_i\) under each saturation state is recorded.

[0115] 4. Calculate the surface relaxation rate ρ2:

[0116] The surface relaxation rate \(ρ_2\) is calculated based on the obtained NMR T2 average value \(T2LM_i\) and fluid saturation \(fi\). This step reflects the physical and chemical properties of the rock pore surface by analyzing the relaxation behavior of the fluid in the rock pores and its interaction with the pore surface.

[0117] This method can provide a detailed understanding of the rock pore structure and its influence on fluid behavior, thus providing more accurate data support in the evaluation and research of rocks. This evaluation technology is particularly suitable for petroleum engineering and geological research, and is of great significance for optimizing production strategies and evaluating oil and gas reservoir characteristics.

[0118] The surface relaxation rate ρ2 is calculated in S3, and the specific principle is as follows:

[0119] When NMR tests porous media such as rocks, there are two main relaxation mechanisms: longitudinal relaxation (T1) and transverse relaxation (T2);

[0120] Taking transverse relaxation as an example, for low-viscosity fluids in pores, when the diffusion effect is ignored, its relaxation time can be expressed as:

[0121]

[0122] Where, T 2b T 2s are the transverse bulk relaxation time and the transverse surface relaxation time, respectively; f s is the proportion of surface relaxation fluid to the total fluid volume.

[0123] When the pore fluid in the rock is in a saturated state, f s for:

[0124]

[0125] Combining formula (1) and (2), we can get:

[0126]

[0127] Let the transverse surface relaxation rate ρ2 be:

[0128]

[0129] The transverse relaxation time is:

[0130]

[0131] However, when the free fluid in the rock pores is in an unsaturated state, f s for:

[0132]

[0133] Among them, the fluid saturation f in the pore can be expressed as:

[0134]

[0135] Therefore, combining formulas (1)(4)(6)(7), the transverse relaxation time is:

[0136]

[0137] Obviously, the difference between formula (8) and (5) is mainly reflected in the influence of fluid saturation. For low-viscosity fluids in micro-nano pores, the bulk relaxation T 2b can be ignored, so

[0138]

[0139] In the above formula, λ is the length of surface relaxation, S is the pore surface area, V0 is the pore volume, and V a is the volume of fluid where surface relaxation occurs, V b is the volume of fluid in which body relaxation occurs.

[0140] Formula (9) can be regarded as a linear function between 1 / T2 and 1 / f, and the slope of the function is the product of the surface relaxation rate ρ2 and S / V0. For a certain rock core, the transverse surface relaxation rate ρ2, specific surface area S and pore volume V0 are its inherent properties and will not change with the change of fluid content. Therefore, the transverse surface relaxation rate can be obtained according to the changes in the nuclear magnetic T2 spectrum of rocks with different fluid saturations and their fluid saturation f.

[0141] Taking the Shahejie Formation mud shale in the Bohai Bay Basin as an example, the NMR T2 spectrum test of the shale was carried out after oil washing, oil saturation and drying at different temperatures (40 / 60 / 80 / 100 / 120 / 140 / 160℃, drying at each temperature for 12h). The results are as follows: Figure 2As shown. Obviously, with the increase of temperature, the signal intensity of the nuclear magnetic T2 spectrum gradually decreases, reflecting the loss of fluid in the pores. The signal intensity of the saturated oil nuclear magnetic T2 spectrum is recorded as N0, at which time the fluid saturation is approximately considered to be 1; the total signal intensity N of the nuclear magnetic T2 spectrum at each temperature point is used to calculate the total signal intensity N i Divided by the total signal intensity N0 of the saturated oil NMR T2 spectrum, the fluid saturation f corresponding to each temperature point is obtained. i According to the nuclear magnetic T2 time and its corresponding signal amplitude, several average values ​​of each T2 spectrum are calculated using formula (10): 2LMi .

[0142]

[0143] In the formula, A j T 2j Corresponding to the NMR signal amplitude.

[0144] Using 1 / T 2LM When the intersection diagram is drawn with 1 / f, the two show a good linear relationship, such as Figure 3 As shown in the figure, the linear slope is the product of the surface relaxation rate ρ2 and S / V0. The specific surface area S of the sample tested in the low-temperature nitrogen adsorption experiment is 4.14m 2 / g, the pore volume V0 of the sample tested by the pressurized saturated oil experiment is 0.015cm 3 / g, using formula (9), we can determine the surface relaxation rate of the sample ρ2 = 3.83nm / ms.

[0145] like Figure 4 As shown, an embodiment of the present invention provides a system for evaluating the relaxation rate of rock pore surfaces in a low-field nuclear magnetic resonance experiment for implementing the method for evaluating the relaxation rate of rock pore surfaces in a low-field nuclear magnetic resonance experiment, the system comprising:

[0146] The core pretreatment module is used to divide the core into two parts after oil washing: one part of the core is subjected to low-temperature nitrogen adsorption experiment to obtain the specific surface area S of the core pores; the other part of the core is first subjected to pressurized saturated fluid experiment to ensure that the core is completely saturated with fluid according to the change of core mass after different saturation times, and the total pore volume V0 of the sample is determined by the mass difference method of the sample before and after saturation;

[0147] The NMR module is connected to the core preprocessing module to conduct NMR T2 spectrum experiments on the core in the saturated fluid state and determine the total NMR signal N0 and T2 average value T of the saturated core. 2LM0 ;

[0148] The surface relaxation rate calculation module is connected to the nuclear magnetic resonance module. After drying the saturated fluid core at different temperatures, the T2 spectrum of the core at each saturation is tested. According to the change of core mass or the total nuclear magnetic signal (N1, N2, N3…N i ) to determine the fluid saturation (f1,f2,f3…f i ), and the average value of nuclear magnetic T2 (T 2LM1 ,T 2LM2 ,T 2LM3 …T 2LMi ). According to the average value of NMR T2 (T 2LM1 ,T 2LM2 ,T 2LM3 …T 2LMi ) and fluid saturation (f1,f2,f3…f i ), calculate the surface relaxation rate ρ2.

[0149] Another object of the present invention is to provide a computer device, characterized in that the computer device includes a memory and a processor, the memory stores a computer program, and when the computer program is executed by the processor, the processor executes the steps of the method for evaluating the surface relaxation rate of rock pores in a low-field nuclear magnetic resonance experiment.

[0150] Another object of the present invention is to provide a computer-readable storage medium storing a computer program, which, when executed by a processor, enables the processor to execute the steps of the method for evaluating the surface relaxation rate of rock pores in a low-field nuclear magnetic resonance experiment.

[0151] Another object of the present invention is to provide an information data processing terminal, which is used to implement the system for evaluating the surface relaxation rate of rock pores in low-field nuclear magnetic resonance experiments.

[0152] It should be noted that the embodiments of the present invention can be implemented by hardware, software, or a combination of software and hardware. The hardware part can be implemented using dedicated logic; the software part can be stored in a memory and executed by an appropriate instruction execution system, such as a microprocessor or dedicated design hardware. It can be understood by a person of ordinary skill in the art that the above-mentioned devices and methods can be implemented using computer executable instructions and / or contained in a processor control code, such as a carrier medium such as a disk, CD or DVD-ROM, a programmable memory such as a read-only memory (firmware), or a data carrier such as an optical or electronic signal carrier. Such code is provided on the carrier medium. The device and its modules of the present invention can be implemented by hardware circuits such as very large-scale integrated circuits or gate arrays, semiconductors such as logic chips, transistors, etc., or programmable hardware devices such as field programmable gate arrays, programmable logic devices, etc., can also be implemented by software executed by various types of processors, and can also be implemented by a combination of the above-mentioned hardware circuits and software, such as firmware.

[0153] Theoretically, the model has a solid foundation, and from the analysis of implementation cases, 1 / T 2LM Correlation between R and 1 / f 2 Can reach 0.98 or above ( Figure 3 ), with high accuracy and reliable evaluation results. Compared with the traditional method of using the average value of nuclear magnetic T2 of saturated fluid rock (only one data point) and the method of calibrating the surface relaxation rate by low-temperature nitrogen adsorption and mercury injection pore size distribution, the method of the present invention is not affected by human factors and the evaluation results are more objective.

[0154] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for evaluating the surface relaxation rate of rock pores in a low-field nuclear magnetic resonance experiment, characterized in that: The method includes: S1: After washing the oil, the core was divided into two parts: one of the cores was subjected to a low-temperature nitrogen adsorption experiment to obtain the specific surface area of ​​the core pores. S For the other core, a pressurized saturated fluid experiment was first carried out to ensure that the core was completely saturated with fluid according to the change in core mass after different saturation times. The total pore volume of the sample was determined by the mass difference method of the sample before and after saturation. V 0; S2: Conduct NMR T2 spectrum experiments on the core in the saturated fluid state to determine the total NMR signal of the saturated core N 0 and saturation T2 logarithmic mean T 2LM0 ; S3: After drying the saturated fluid core at different temperatures, the T2 spectra of the core at each saturation are tested. N 1, N 2, N 3… N i Determining Fluid Saturation f 1, f 2, f 3… f i , and the logarithmic mean T2 2LM1 ,T 2LM2 ,T 2LM3 …T 2LMi ; Based on the logarithmic mean value of T2 2LM1 ,T 2LM2 ,T 2LM3 …T 2LMi and fluid saturation f 1, f 2, f 3… f i , calculate the transverse surface relaxation rate ρ 2; 1 / T 2 and 1 / f The linear function between is: (9) In the above formula, S is the pore surface area, V 0 is the total pore volume, and the slope of the function is the transverse surface relaxation rate ρ 2 and S / V 0; therefore, according to the nuclear magnetic T2 spectrum of rocks with different fluid saturation and their fluid saturation f The lateral surface relaxation rate can be obtained by analyzing the change of .

2. A system for evaluating the relaxation rate of rock pore surfaces in low-field nuclear magnetic resonance experiments implementing the method of claim 1, characterized in that: The system includes: Low temperature nitrogen adsorption experimental device, used to conduct low temperature nitrogen adsorption experiment on the core to obtain the specific surface area S of the core pores; The pressurized saturated fluid experimental device is used to conduct pressurized saturated fluid experiments on the core to ensure that the core is completely saturated with fluid, and to determine the total pore volume V0 of the sample by the mass difference method of the sample before and after saturation; The nuclear magnetic resonance T2 spectrum experimental device is used to conduct nuclear magnetic resonance T2 spectrum experiments on saturated fluid cores to determine the total nuclear magnetic signal N0 of the saturated core and the logarithmic average value T of the saturated T2 2LM0 ; The drying treatment device is used to dry the core saturated with fluid at different temperatures, and cooperate with the nuclear magnetic resonance T2 spectrum experimental device to test the T2 spectrum of the core at various saturations to determine the fluid saturation f1, f2, f3…f i and T2 logarithmic mean T 2LM1 , T 2LM2 , T 2LM3 … T 2LMi ; The data processing unit is used to calculate the transverse surface relaxation rate ρ2 based on the logarithmic mean value of T2 and the fluid saturation.

3. The system for evaluating the relaxation rate of rock pore surfaces in low-field nuclear magnetic resonance experiments according to claim 2, characterized in that: The drying processing device can automatically adjust the drying temperature and is equipped with equipment to monitor the core quality or changes in nuclear magnetic signal in real time.

4. The system for evaluating the relaxation rate of rock pore surfaces in low-field nuclear magnetic resonance experiments according to claim 2, characterized in that: The nuclear magnetic resonance T2 spectrum experimental device includes a magnetic field generator, a radio frequency pulse transmitter and receiver, and a signal processor, which are used to generate and receive nuclear magnetic resonance signals and process them to obtain T2 spectra.

5. The system for evaluating the relaxation rate of rock pore surfaces in low-field nuclear magnetic resonance experiments according to claim 2, characterized in that: The data processing unit also includes a memory and a display device for storing the experimental results and displaying the calculation results of the lateral surface relaxation rate ρ2 for further analysis and application.

6. A computer device, characterized in that: The computer device includes a memory and a processor, the memory stores a computer program, and when the computer program is executed by the processor, the processor executes the steps of the method for evaluating the surface relaxation rate of rock pores in a low-field nuclear magnetic resonance experiment as described in claim 1.

7. A computer-readable storage medium storing a computer program, wherein when the computer program is executed by a processor, the processor executes the steps of the method for evaluating the surface relaxation rate of rock pores in a low-field nuclear magnetic resonance experiment as described in claim 1.

Images