Method and system for judging fluid preservation effect of sealed frozen core sample based on nuclear magnetic resonance re-saturation contrast

Abstract

The present application relates to a method and system for determining the preservation effect of a sealed frozen core sample based on nuclear magnetic resonance re-saturation contrast, and belongs to the technical field of unconventional oil and gas exploration and development. The method comprises the following steps: first, obtaining and processing the sealed sample, performing a first T1-T2 two-dimensional nuclear magnetic resonance test after thawing, immediately performing re-saturation treatment, and immediately performing a second test after saturation; second, dividing each hydrogen-containing component fluid into regions and extracting signal amounts; third, calibrating the fluids of formation water, crude oil and dodecane, establishing a conversion relationship between oil-water hydrogen signal amounts and volumes, and then obtaining the oil-water volumes under sealed and re-saturated states; fourth, comparing the difference between the total fluid amount under the re-saturated state and the total fluid amount under the sealed state with the total fluid amount under the re-saturated state, and obtaining a ratio, which is defined as a re-saturation response coefficient, for quantitatively evaluating the preservation degree of the original pore fluid. The method can ensure the accuracy and reliability of the test results.

Description

Technical Field

[0001] This invention belongs to the field of unconventional oil and gas exploration and development and core analysis technology, specifically involving a method and system for judging the fluid preservation effect of sealed frozen coring samples based on nuclear magnetic resonance resaturation contrast. Background Technology

[0002] With the deepening of shale oil and gas exploration and development, the accurate preservation and quantitative characterization of shale pore fluids (free oil, adsorbed oil, and formation water) have become key factors restricting the accuracy of oil-bearing evaluation. Compared with conventional coring methods, closed cryogenic coring technology, through sealed containers and low-temperature conditions, effectively reduces drilling fluid contamination and the escape of light fluids, and has gradually become an important development direction for shale coring.

[0003] Currently, in engineering and experimental practice, it is generally assumed that sealed cryogenic coring samples can "fully preserve" formation pore fluids, and their NMR, pyrolysis, or solvent analysis results can be directly used for oil content evaluation. However, during the acquisition, cutting, transportation, thawing, and test preparation of sealed cryogenic coring samples, unavoidable loss of light fluids may still occur. The existing technical system lacks a quantitative verification method for the fluid preservation effect of sealed cryogenic coring samples themselves.

[0004] Existing quantitative methods for shale fluid mainly include: (1) pyrolysis and solvent extraction: the sample needs to be crushed and exposed for a long time, which can easily cause secondary loss of fluid; (2) centrifugation and heating: due to the pore structure and displacement efficiency, it is difficult to fully utilize the fluid in the closed pores; (3) nuclear magnetic resonance: it has the advantages of being non-destructive, fast and able to identify multiphase fluids, but existing applications are mostly focused on the "measurement results themselves", and a verification mechanism for the preservation of fluid in sealed frozen core samples has not yet been established.

[0005] Therefore, there is an urgent need to propose a method that can quantitatively determine the fluid preservation effect of sealed cryogenic core samples without damaging the samples, so as to provide technical support for the reliability of experimental results of sealed core samples. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for determining the fluid preservation effect of sealed frozen cored samples based on nuclear magnetic resonance (NMR) resaturation comparison. Specifically, it relates to a method for quantitatively determining the pore fluid preservation effect of shale cores through a resaturation comparison experiment using nuclear magnetic resonance (NMR) technology.

[0007] The present invention also provides a system for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast.

[0008] This invention quantitatively characterizes the loss of original pore fluid in a sample by comparing the "closed-state NMR test - resaturation treatment - saturated-state NMR test" approach, thereby calculating the degree of pore fluid loss during the acquisition and preservation of closed-state core samples. This verifies the accuracy and reliability of experimental results obtained using closed-state core samples and can also serve as a supplement and improvement to the application of NMR in the oil and gas field. The technical solution of this invention is as follows: A method for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast includes: Step S1: The sealed sample is cut and prepared, and after thawing, a T1-T2 two-dimensional nuclear magnetic resonance test is performed; Step S2: Immediately after the closed-state test is completed, a resaturation process is performed. After saturation, another T1-T2 two-dimensional nuclear magnetic resonance test is performed. Step S3: Divide the three hydrogen-containing fluid components—adsorbed oil, free oil, and free water—into regions and extract their signals in the closed and resaturated samples. Step S4: Establish the conversion relationship between hydrogen signal quantity and volume of each hydrogen-containing component fluid; Step S5: Calculate the oil and water volumes in the closed and resaturated states respectively, based on the conversion relationship; Step S6: Subtract the total fluid volume in the resaturated state from the total fluid volume in the closed state, and calculate the ratio of the difference to the total fluid volume in the resaturated state. The result is defined as the resaturation response coefficient, which is used to quantitatively evaluate the degree of fluid retention in the original pores.

[0009] According to a preferred embodiment of the present invention, in step S1, the sealed sample is kept in a cryogenic liquid nitrogen tank throughout the transportation process. After the transportation is completed, the sealed sample is taken out of the cryogenic liquid nitrogen tank and then immediately cut into block-shaped samples. The length of the block-shaped samples is 2.5~3cm, the width is 1~1.5cm, the height is 1~1.5cm, and the weight is 10~20g.

[0010] Meanwhile, wait for the block-sized sample to thaw. Once thawed, immediately perform a T1-T2 two-dimensional nuclear magnetic resonance test to obtain the nuclear magnetic spectrum data under sealed conditions.

[0011] Preferably according to the present invention, in step S1, the method for determining the thawing time is as follows: Conduct experimental analysis using a sealed sample. After taking out the sealed sample from a low-temperature liquid nitrogen tank, quickly place it into a nuclear magnetic resonance instrument for a T2 one-dimensional nuclear magnetic resonance test. Subsequently, repeat the test every 0.5 - 1.5 minutes, and observe the change in the signal amount after each test. After several T2 one-dimensional nuclear magnetic resonance tests, when the signal amount is stable (i.e., all signal amounts can be detected), record the total test time, which is the time to wait for the thawing process.

[0012] Preferably according to the present invention, in step S2, before the re-saturation treatment, the sample should be wrapped with filter paper and the sample number should be recorded. Subsequently, immediately place it into a saturation oil instrument, then pour the saturated fluid (dodecane) into the storage tank. After checking the closing status of each switch, turn on the vacuum pump for evacuation. After the evacuation is completed, perform a pressurization operation with the pressure set at 15 MPa. During this period, regularly supplement the pressure. After the pressure reading stabilizes at 15 MPa, end the re-saturation treatment.

[0013] Preferably according to the present invention, in step S2, after the sample completes the re-saturation treatment, immediately immerse the sample in a beaker containing dodecane. Before performing the T1-T2 two-dimensional nuclear magnetic resonance test, take out the sample, dry the surface fluid, and then perform a T1-T2 two-dimensional nuclear magnetic resonance test again to obtain the nuclear magnetic spectrum data in the re-saturated state.

[0014] Preferably according to the present invention, in step S3, T1 refers to longitudinal relaxation, T2 refers to transverse relaxation, and the hydrogen-containing component fluids in the region division include adsorbed oil, free oil, and free water; among them, when 0.2 ms < T2 < 1 ms and T1 / T2 > 10, it is the first region, corresponding to adsorbed oil; when T2 > 1 ms and T1 / T2 > 10, it is the second region, corresponding to free oil; when T2 > 0.2 ms and T1 / T2 < 10, it is the third region, corresponding to free water; The first, second, and third regions are the regions divided on the nuclear magnetic spectrum diagram, used to delineate and extract the signal amounts of adsorbed oil, free oil, and free water; Preferably according to the present invention, in step S4: Calibrate and analyze the crude oil, formation water, and the saturated fluid dodecane in the formation where the sample is located to establish the conversion relationship between the oil-water signal amount and the oil-water volume; The specific process of the calibration analysis is: Weigh the crude oil, water, and dodecane separately to make standard samples, and then perform a T2 one-dimensional nuclear magnetic resonance test. Divide the hydrogen signal amount of the obtained oil or water by the corresponding volume to obtain the signal amount - volume conversion coefficient k of the oil, water, and saturated fluid dodecane inside the sealed core shale; that is: ; ; ; In the formula, The volume of crude oil is expressed in ml. The volume of water is expressed in ml. Volume of saturated fluid dodecane, in ml; This is an oil signal, measured in au. This is a water signal, measured in au. The signal quantity for saturated fluid dodecane, in au; This is the crude oil calibration conversion factor; The conversion factor for formation water calibration; The calibration conversion factor for saturated fluid dodecane; the conversion factor k includes , and .

[0015] According to a preferred embodiment of the present invention, in step S5, the oil-water volume of the sample in the closed state and the resaturated state is calculated using the oil-water signal quantity delineated in step S3 and the oil-water conversion coefficient obtained in step S4; wherein, the oil component of the sample in the resaturated state includes crude oil and dodecane, and the conversion coefficients of the two are different, so they must be calculated separately when calculating the oil volume. The specific calculation process is as follows: ; ; ; ; In the formula, The volume of oil in the sample under closed conditions is expressed in ml. The volume of oil in the sample under resaturation is expressed in ml. The volume of water in the sample under closed conditions, in ml; The volume of water in the sample under resaturation, in ml; This refers to the oil signal quantity delineated by the sample in a closed state. The oil signal quantity delineated for the sample in the saturated state, in units of au; The water signal quantity delineated from the sample in a closed state, in au; This represents the water signal quantity delineated from the sample in the saturated state, in units of au.

[0016] According to a preferred embodiment of the present invention, in step S6, the total fluid volume of the same sample in both the closed and resaturated states is first calculated, whereby the total fluid volume is defined as the sum of the oil and water volumes in the sample. Subsequently, the difference between the total fluid volume in the resaturated state and the total fluid volume in the closed state is calculated to characterize the loss of the original pore fluid in the sample. This loss is then normalized to the total fluid volume in the resaturated state to obtain the resaturation response coefficient, which is used to quantitatively evaluate the retention degree of the original pore fluid. The calculation formula is as follows:

[0017] ; ; ; In the formula, The fluid volume of a sealed cryogenic cored sample before resaturation; The fluid volume of the same sample after resaturation treatment under controlled conditions; The resaturation response coefficient is used to quantitatively characterize the degree of preservation of the original pore fluid.

[0018] Further preferred methods for quantitatively characterizing the preservation level of the original pore fluid include: when When the value is ≤ 0.10, the original pore fluid preservation effect is considered excellent; When 0.10 < ≤ At 0.30, it is determined that part of the original pore fluid has been lost; when > At a value of 0.30, the original pore fluid retention effect is considered to be poor.

[0019] A system for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast includes: The T1-T2 two-dimensional nuclear magnetic resonance testing module is configured to: cut and prepare a sealed sample, perform a T1-T2 two-dimensional nuclear magnetic resonance test after thawing; immediately after the sealed state test is completed, perform a resaturation treatment, and after the saturation is completed, perform another T1-T2 two-dimensional nuclear magnetic resonance test. The region division and signal extraction module is configured to perform region division and signal extraction on three hydrogen-containing fluid components—adsorbed oil, free oil, and free water—in closed and resaturated samples based on two-dimensional nuclear magnetic relaxation characteristics. The oil and water volume calculation module is configured to: establish the conversion relationship between the hydrogen signal quantity and volume of each hydrogen-containing component fluid; and calculate the oil and water volumes under closed and resaturated states based on the conversion relationship. The preservation effect discrimination module is configured to: subtract the total fluid volume in the resaturated state from the total fluid volume in the closed state, calculate the ratio of the difference to the total fluid volume in the resaturated state, and define the result as the resaturation response coefficient, which is used to quantitatively evaluate the degree of fluid preservation in the original pores.

[0020] The beneficial effects of this invention are as follows: 1. This ensures the accuracy and reliability of experimental results obtained by using sealed samples for testing; 2. As an effective supplement to the existing NMR technology system (quantitative verification of the fluid preservation effect of sealed samples). Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the discrimination process for the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast according to the present invention; Figure 2 This is a test chart of the thawing time of the sealed frozen core sample according to the present invention; Figure 3 It is a one-dimensional NMR spectrum showing the change of the sample's Q NMR signal intensity with thawing time; Figure 4 This is a one-dimensional NMR spectrum of sample W as a function of thawing time; Figure 5 This is a one-dimensional NMR spectrum showing the change in NMR signal intensity of sample E over thawing time; Figure 6 It is a one-dimensional NMR spectrum showing the change of the NMR signal intensity of sample R with thawing time; Figure 7 These are one-dimensional NMR spectra of the samples provided in this invention under closed and resaturated states; Figure 8 This is a two-dimensional NMR spectrum of sample A in a closed state and a resaturated state, provided in an example of the present invention. Figure 9 This is a two-dimensional NMR spectrum of sample B in a closed state and a resaturated state, provided in this invention example; Figure 10 These are two-dimensional NMR spectra of sample C in closed and resaturated states provided in this invention example; Figure 11 This is a two-dimensional NMR spectrum of sample D in a closed state and a resaturated state, provided in an example of the present invention. Figure 12 This is a formation water calibration conversion diagram provided by an example of the present invention; Figure 13 This is a crude oil calibration conversion diagram provided in this invention example; Figure 14 This is a dodecane calibration conversion diagram provided in this invention example; Figure 15This is a schematic diagram illustrating the graded evaluation of the fluid preservation effect of samples A to D provided in this invention example. Detailed Implementation

[0022] The present invention will be further illustrated below with reference to embodiments and figures, but is not limited thereto.

[0023] Terminology Explanation: 1. T2 one-dimensional nuclear magnetic resonance testing can quickly evaluate the physical properties of a sample through a single T2 (lateral relaxation) dimension and obtain the magnitude of the total hydrogen signal in the sample. However, for complex shale samples, the T2 values ​​of different components may be difficult to distinguish due to overlap.

[0024] 2. T1-T2 two-dimensional nuclear magnetic resonance testing adds a T1 (longitudinal relaxation) dimension to the one-dimensional nuclear magnetic resonance. Based on the acquired signal, it can effectively identify fluids with different hydrogen-containing components such as free oil, adsorbed oil, and free water. 3. NMR spectral data refers to the two-dimensional spectrum and NMR data obtained after inversion of the two-dimensional raw data (data obtained directly after NMR testing) using NeoMed NMR data analysis software; the two-dimensional spectrum is used to define the boundary between oil and water components, and the NMR data is used for signal delineation and extraction; different colors are used to delineate the signal quantities of free water (light blue), adsorbed oil (orange), and free oil (light yellow) in the NMR data, and the sum of the delineated data quantities is the signal quantity of oil and water to be extracted; 4. Signal quantity refers to: hydrogen-containing signal quantity. Hydrogen atoms are widely distributed in formation fluids. In addition, the nucleus of this atom also has a strong nuclear magnetic resonance signal. Therefore, nuclear magnetic resonance testing uses hydrogen nuclei as fluid indicator objects. The magnitude of the signal quantity can be obtained directly by inversion on the nuclear magnetic resonance software after T2 one-dimensional nuclear magnetic resonance testing. 5. Signal quantity change refers to the fact that during the thawing test, the low temperature will cause some NMR signals to be undetectable. Therefore, during the test of gradually restoring from the low temperature to the normal temperature, the detected NMR signal quantity will continue to increase until all signals can be detected. At this time, the signal quantity should basically stop increasing or decrease slightly (light hydrocarbon loss).

[0025] Example 1 A method for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast, such as Figure 1 As shown, it includes: Step S1: The sealed sample is cut and prepared, and after thawing, a T1-T2 two-dimensional nuclear magnetic resonance test is performed; Step S2: Immediately after the closed-state test is completed, a resaturation process is performed. After saturation, another T1-T2 two-dimensional nuclear magnetic resonance test is performed. Step S3: Divide the three hydrogen-containing fluid components—adsorbed oil, free oil, and free water—into regions and extract their signals in the closed and resaturated samples. Step S4: Establish the conversion relationship between hydrogen signal quantity and volume of each hydrogen-containing component fluid; Step S5: Calculate the oil and water volumes in the closed and resaturated states respectively, based on the conversion relationship; Step S6: Subtract the total fluid volume in the resaturated state from the total fluid volume in the closed state, and calculate the ratio of the difference to the total fluid volume in the resaturated state. The result is defined as the resaturation response coefficient, which is used to quantitatively evaluate the degree of fluid retention in the original pores.

[0026] Example 2 The difference between the method for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast as described in Example 1 and the method described in Example 1 is as follows: In step S1, the sealed sample is kept in a cryogenic liquid nitrogen tank throughout the transportation process. After the transportation is completed, the sealed sample is taken out of the cryogenic liquid nitrogen tank and then immediately cut into block-shaped samples of 1.5*1.5*2.5cm. Each block-shaped sample weighs 10~20g to meet the requirements of nuclear magnetic resonance testing. Meanwhile, wait for the block-sized sample to thaw. Once thawed, immediately perform a T1-T2 two-dimensional nuclear magnetic resonance test to obtain the nuclear magnetic spectrum data under sealed conditions.

[0027] In step S1, the method for determining the thawing time is as follows: A sealed sample is used for experimental analysis. After the sealed sample is removed from the cryogenic liquid nitrogen container, it is quickly placed in an NMR instrument for a T2 one-dimensional NMR test. The test is then repeated every 1.5 minutes (test time points are 0, 1.5, 3, 4.5, 6, 7.5, 9, and 10.5 minutes). The signal changes are observed after each test. After several T2 one-dimensional NMR tests, when the signal stabilizes (i.e., all signals can be detected), the total test time is recorded, which is the time required for thawing (thawing time).

[0028] In step S2, the sample should be wrapped with filter paper and the sample number recorded before resaturation treatment. Then, it should be immediately placed in the saturated oil instrument, and then saturated fluid (dodecane) should be poured into the storage tank. After checking the closure of each switch, the vacuum pump should be turned on to evacuate the air. After the vacuum is completed, the pressure should be increased to 15 MPa. The pressure should be replenished periodically during the process. After the pressure reading stabilizes at 15 MPa, the resaturation treatment is completed.

[0029] In step S2, the instrument used for the resaturation treatment is the ZYB-II type vacuum pressurized saturation device. The concentration of the saturated fluid dodecane is 98%. The instrument used for vacuum pumping is the 2XZ-I type rotary vane vacuum pump. The vacuum pumping time is set to 4 hours, the saturation pressure is 15 MPa, and the saturation time is 24 hours. During the saturation process, the pressure needs to be manually maintained until the pressure indication is stable, and then the resaturation treatment is completed.

[0030] In step S2, after the sample completes the resaturation treatment, immediately immerse the sample in a beaker containing dodecane. The purpose is to prevent the loss of the saturated dodecane fluid in the sample. Before performing the T1-T2 two-dimensional nuclear magnetic resonance test, take out the sample, dry the surface fluid, and then perform the T1-T2 two-dimensional nuclear magnetic resonance test again to obtain the nuclear magnetic spectrum data in the resaturated state.

[0031] In steps S1 and S2, the instrument used for nuclear magnetic resonance testing is the MicroMR20-025V type nuclear magnetic resonance analyzer, with a main frequency of 20 MHz, a sampling frequency of 250 kHz, a magnet strength of 0.5 T, a coil diameter of 25 mm. The CPMG sequence is used for the T2 one-dimensional nuclear magnetic resonance test, and the IR-CPMG sequence is used for the T1-T2 two-dimensional nuclear magnetic resonance test.

[0032] The hydrogen-containing component fluids in the region division in step S3 include adsorbed oil, free oil, and free water. According to the two-dimensional nuclear magnetic relaxation characteristics: the heavier the fluid viscosity / component, the faster the corresponding T2 relaxation rate, the shorter the relaxation time, the slower the T1 relaxation rate, the longer the relaxation time, and the larger the T1 / T2 value; based on this, the three hydrogen-containing component fluids are regionally divided; Among them, when 0.2 ms < T2 < 1 ms and T1 / T2 > 10, it is the first region, corresponding to adsorbed oil; When T2 > 1 ms and T1 / T2 > 10, it is the second region, corresponding to free oil; When T2 > 0.2 ms and T1 / T2 < 10, it is the third region, corresponding to free water; The first, second, and third regions are the regions divided on the nuclear magnetic spectrum, used to delineate and extract the signal amounts of adsorbed oil, free oil, and free water; When performing nuclear magnetic resonance testing on porous media such as rocks, there are two relaxation mechanisms: Longitudinal relaxation (T1): Reflects the energy exchange efficiency between the hydrogen nuclei of the fluid and the surrounding lattice; Transverse relaxation (T2): Not only includes the same energy exchange mechanism as T1, but is also additionally affected by spin-spin interactions and magnetic field inhomogeneity, causing the transverse magnetization vector to decay faster, so T1 ≥ T2; For low-viscosity fluids in pores, the relaxation time is mainly related to the pore size; however, for fluids or solid-like molecules with higher viscosity, their relaxation characteristics are affected by intramolecular dipole-dipole interactions. For spherical molecules, the relaxation time can be expressed as: ; ; In the formula, ω is the Larmor frequency; C is a constant; τ is the rotational correlation time, which can be calculated by Debye theory: ; In the formula, μ is the viscosity of the fluid; a is the radius of the molecule; k is the Boltzmann constant; and T is the absolute temperature. In summary, T1 and T2 are functions of fluid molecule size and viscosity (T1≥T2). As molecule size and viscosity increase, rotation time increases, T1 increases, and T2 decreases. Therefore, T1 / T2 can be used to reflect the properties of the fluid. That is, when the fluid viscosity is low, T1 / T2 is close to 1. As the fluid viscosity and molecule size gradually increase, the ratio of T1 / T2 gradually increases. In step S4: Calibration analysis is performed on the crude oil, formation water, and dodecane fluid used for resaturation in the formation where the sample is located to establish the conversion relationship between oil-water signal quantity and oil-water volume; The specific process of calibration analysis is as follows: Sixteen small glass bottles, each with a capacity of 20 ml, were used. The first bottle was used for empty sample calibration. The remaining fifteen bottles were divided into three groups of five, corresponding to crude oil, formation water, and dodecane, respectively. For each group, fluids of 0.2 ml, 0.4 ml, 0.6 ml, 0.8 ml, and 1.0 ml were weighed to prepare standard samples. T2-dimensional nuclear magnetic resonance (NMR) testing was then performed. The hydrogen signal intensity of the obtained oil or water was compared with the corresponding volume to obtain the signal intensity-volume conversion coefficient k of the oil-water and saturated dodecane fluid inside the sealed cored shale. ; ; ; In the formula, The volume of crude oil is expressed in ml. The volume of water is expressed in ml. Volume of saturated fluid dodecane, in ml; This is an oil signal, measured in au. This is a water signal, measured in au. The signal quantity for saturated fluid dodecane, in au; This is the crude oil calibration conversion factor; The conversion factor for formation water calibration; The calibration conversion factor for saturated fluid dodecane; the conversion factor k includes , and .

[0033] In step S5, using the oil-water signal quantity delineated in step S3 and the oil-water conversion coefficient obtained in step S4, the oil-water volume of the sample in the closed state and the resaturated state is calculated. The oil component in the resaturated state includes crude oil and dodecane; therefore, different conversion coefficients are used to calculate the volumes of both states. The specific calculation process is as follows: ; ; ; ; In the formula, The volume of oil in the sample under closed conditions is expressed in ml. The volume of oil in the sample under resaturation is expressed in ml. The volume of water in the sample under closed conditions, in ml; The volume of water in the sample under resaturation, in ml; This refers to the oil signal quantity delineated by the sample in a closed state. The oil signal quantity delineated for the sample in the saturated state, in units of au; The water signal quantity delineated from the sample in a closed state, in au; This represents the water signal quantity delineated from the sample in the saturated state, in units of au.

[0034] In step S6, the total fluid volume of the same sample is first calculated in both the closed and resaturated states, with the total fluid volume defined as the sum of the oil and water volumes in the sample. Then, the difference between the total fluid volume in the resaturated state and the total fluid volume in the closed state is calculated to characterize the loss of the original pore fluid in the sample. This loss is further normalized to the total fluid volume in the resaturated state to obtain the resaturation response coefficient, which is used to quantitatively evaluate the retention degree of the original pore fluid. The calculation formula is as follows:

[0035] ; ; ; In the formula, The fluid volume of a sealed cryogenic cored sample before resaturation; The fluid volume of the same sample after resaturation treatment under controlled conditions; The resaturation response coefficient is used to quantitatively characterize the degree of preservation of the original pore fluid.

[0036] The physical meaning of the resaturation response coefficient is: The smaller R is, the more intact the original sample fluid is; the larger R is, the more severe the loss of the original sample fluid is. Based on this, the degree of preservation of the original pore fluid is graded and evaluated, including: when When the value is ≤ 0.10, the original pore fluid preservation effect is considered excellent; When 0.10 < ≤ At 0.30, it is determined that part of the original pore fluid has been lost; when > At a value of 0.30, the original pore fluid retention effect is considered to be poor.

[0037] Example 3 The method for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast, as described in Example 2, differs in that: The following will further illustrate this with examples and charts. Four sealed samples were randomly selected for the fluid preservation effect test and numbered A, B, C, and D; four sealed samples were also randomly selected for the thawing test and numbered Q, W, E, and R. The first step is to test the thawing time; For Q, W, E, and R, perform the following operations in sequence: After removing the sample from the liquid nitrogen container, quickly place it into the NMR instrument for a one-dimensional T2 NMR test. Repeat the test every 1.5 minutes thereafter (test time points are 0, 1.5, 3, 4.5, 6, 7.5, 9, and 10.5 minutes). Observe the changes in signal intensity after each test. Figure 2 This is a test chart of the thawing time of the sealed frozen core sample according to the present invention; Figures 3 to 6 The images show one-dimensional NMR spectra of the sample's Q, W, E, and R NMR signals as a function of thawing time. When the test time reaches 9 min or 10.5 min, the signal intensity tends to stabilize. Therefore, the thawing time can be set at 10 min.

[0038] The second step is nuclear magnetic resonance (NMR) testing in a closed environment; Samples A, B, C, and D were sequentially removed from the liquid nitrogen tank and subjected to the following procedures: First, a sample block was prepared, approximately 3 cm in length, 1.5 cm in width, and weighing 10-20 g; then, a thawing process was performed for approximately 10 minutes. After thawing, the surface moisture of the sample was wiped off, and the sample was weighed using a balance with a strength of 0.01 g; immediately afterwards, a T1-T2 two-dimensional nuclear magnetic resonance (NMR) test was performed, and after the test, the sample was returned to the liquid nitrogen tank. The instrument used for NMR testing was a MicroMR20-025V NMR analyzer with a main frequency of 20MHz, a sampling frequency of 250kHz, a magnet strength of 0.5T, and a coil diameter of 25mm. The T2 one-dimensional NMR test used the CPMG sequence, and the T1-T2 two-dimensional NMR test used the IR-CPMG sequence. The NMR test parameters of the sealed cored shale samples are shown in Table 1.

[0039] Table 1. NMR test parameters for sealed cored shale samples; The third step is resaturation treatment; After the NMR tests of samples A, B, C, and D were completed in a closed environment, they were all removed from the liquid nitrogen tank, wrapped with filter paper and their sample numbers were recorded. They were then immediately placed in a saturated oil instrument for resaturation. The saturating fluid was 98% dodecane, the vacuum period was 4 hours, the saturation pressure was 15 MPa, and the saturation time was 24 hours. During this period, the saturation pressure needed to be manually maintained until the pressure reading stabilized at 15 MPa, at which point saturation was complete. The fourth step is NMR testing under resaturation conditions; After the sample was resaturated, it was placed in a 2.5L beaker containing dodecane (98% volume concentration) and the sample was ensured to be submerged in the fluid. Then, a T1-T2 NMR test was immediately performed (before the test, excess fluid on the surface was wiped off and the sample was weighed with a balance of 0.01%). The test parameters were kept consistent with those of the closed-state test. The fifth step is the regionalization and extraction of oil and water signals; NMR data of A, B, C, and D in closed and saturated states were extracted and inverted to obtain two-dimensional spectra. Based on the two-dimensional NMR relaxation characteristics, the relative positions of the three major regions of free water, adsorbed oil, and free oil on the NMR spectrum were determined. Then, the oil-water signal quantities in closed and resaturated states were delineated and extracted. Figures 8 to 11 These are the two-dimensional NMR spectra of samples A to D provided in this invention under closed and resaturated states; Step 6: Establishing the relationship between hydrogen signal quantity and volume conversion for each hydrogen-containing component fluid; Sixteen small glass bottles, each with a capacity of 20 ml, were used. The first bottle was used for empty sample calibration and numbered 0. The remaining fifteen bottles were divided into three groups of five each, used for the calibration of crude oil, formation water, and dodecane, respectively, and numbered O (crude oil), W (water), and C (dodecane). For each group, fluids were weighed at volume gradients of 0.2 ml, 0.4 ml, 0.6 ml, 0.8 ml, and 1.0 ml to prepare standard samples. Subsequently, T2 one-dimensional nuclear magnetic resonance (NMR) tests were performed on each standard sample. The obtained oil (water) hydrogen signal was compared with its volume to obtain the signal-volume conversion coefficients for oil-water and saturated dodecane fluid inside the sealed cored shale. The calculation process is as follows: ; ; ; In the formula, The volume of crude oil is expressed in ml. The volume of water is expressed in ml. Volume of saturated fluid dodecane, in ml; This is an oil signal, measured in au. This is a water signal, measured in au. The signal quantity for saturated fluid dodecane, in au; This is the crude oil calibration conversion factor; The conversion factor for formation water calibration; The calibration conversion factor for saturated fluid dodecane is shown in Table 2. The calibration process records for crude oil, formation water, and dodecane fluid are shown in Table 2.

[0040] Table 2. Record of calibration process for crude oil, formation water and dodecane fluids; Figure 12 This is a formation water calibration conversion diagram provided in the example; Figure 13 This is a diagram of crude oil calibration conversion provided in the example; Figure 14 This is a diagram showing the dodecane calibration conversion relationship provided in the example; Step 7: Calculate the oil-water volume in the closed and resaturated states; Based on the oil-water signal quantities of the sample in the closed and resaturated states determined in step 5, and the oil-water conversion coefficient obtained from the fluid calibration analysis in step 6, the oil-water volume of the sample in the closed and resaturated states is obtained by comparing the signal quantities with the conversion coefficients. The oil component in the resaturated state includes crude oil and dodecane, which have different conversion coefficients; therefore, their volumes must be calculated separately. The calculation process is as follows: ; ; ; ; In the formula, The volume of oil in the sample under closed conditions is expressed in ml. The volume of the sample in its saturated state is expressed in ml. The volume of water in the sample under closed conditions, in ml; The volume of water in the sample at saturation is expressed in ml. The oil signal quantity delineated by the sample in a closed state, in au; The oil signal quantity delineated for the sample in a saturated state, in units of au; The water signal quantity delineated from the sample in a closed state, in au; The water signal quantity delineated from the sample in the saturated state, in au; Step 8: Calculate the resaturation response coefficient R; First, the total fluid volume of the same sample was calculated in both the closed and resaturated states, with the total fluid volume defined as the sum of the oil and water volumes in the sample. Then, the difference between the total fluid volume in the resaturated state and the total fluid volume in the closed state was calculated to characterize the loss of the original pore fluid in the sample (note that the loss of the original pore fluid cannot be simply replaced by the volume of dodecane entering the pores of the sample after resaturation, e.g., ...). Figure 7 As shown, during the saturation process, dodecane displaces a portion of the volume originally occupied by water, resulting in a higher resaturation response coefficient. Further normalization of this loss with the total resaturated fluid volume yields the resaturation response coefficient, which is used to quantitatively evaluate the preservation level of the original pore fluid. The calculation formula is as follows:

[0041] ; ; ; In the formula, The fluid volume of a sealed cryogenic cored sample before resaturation; The fluid volume of the same sample after resaturation treatment under controlled conditions; The resaturation response coefficient is used to quantitatively characterize the degree of preservation of the original pore fluid; the NMR data processing and calculation process is shown in Table 3.

[0042] Table 3. NMR data processing and calculation process; The ninth step is to grade and evaluate the fluid preservation effect, according to... Figure 15 It can be seen that the fluid preservation effect of this batch of sealed cryogenic core samples is good; Example 3 A system for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast includes: The T1-T2 two-dimensional nuclear magnetic resonance testing module is configured to: cut and prepare a sealed sample, perform a T1-T2 two-dimensional nuclear magnetic resonance test after thawing; immediately after the sealed state test is completed, perform a resaturation treatment, and after the saturation is completed, perform another T1-T2 two-dimensional nuclear magnetic resonance test. The region division and signal extraction module is configured to perform region division and signal extraction on three hydrogen-containing fluid components—adsorbed oil, free oil, and free water—in closed and resaturated samples based on two-dimensional nuclear magnetic relaxation characteristics. The oil and water volume calculation module is configured to: establish the conversion relationship between the hydrogen signal quantity and volume of each hydrogen-containing component fluid; and calculate the oil and water volumes under closed and resaturated states based on the conversion relationship. The preservation effect discrimination module is configured to: subtract the total fluid volume in the resaturated state from the total fluid volume in the closed state, calculate the ratio of the difference to the total fluid volume in the resaturated state, and define the result as the resaturation response coefficient, which is used to quantitatively evaluate the degree of fluid preservation in the original pores.

Claims

1. A method for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast, characterized in that, include: Step S1: The sealed sample is cut and prepared, and after thawing, a T1-T2 two-dimensional nuclear magnetic resonance test is performed; Step S2: Resaturate the sealed sample, and perform another T1-T2 two-dimensional nuclear magnetic resonance test after saturation. Step S3: Divide the three hydrogen-containing fluid components—adsorbed oil, free oil, and free water—into regions and extract their signals in the closed and resaturated samples. Step S4: Establish the conversion relationship between hydrogen signal quantity and volume of each hydrogen-containing component fluid; Step S5: Calculate the oil and water volumes in the closed and resaturated states respectively, based on the conversion relationship; Step S6: Subtract the total fluid volume in the resaturated state from the total fluid volume in the closed state, and calculate the ratio of the difference to the total fluid volume in the resaturated state. The result is defined as the resaturation response coefficient, which is used to quantitatively evaluate the degree of fluid retention in the original pores. In step S5, using the oil-water signal quantity delineated in step S3 and the oil-water conversion coefficient obtained in step S4, the oil-water volume of the sample in the closed state and the resaturated state is calculated; wherein, the oil component of the sample in the resaturated state includes crude oil and dodecane, and the specific calculation process is as follows: ； ； ； ； In the formula, The volume of oil in the sample under closed conditions is expressed in ml. The volume of oil in the sample under resaturation is expressed in ml. The volume of water in the sample under closed conditions, in ml; The volume of water in the sample under resaturation conditions is expressed in ml. This refers to the oil signal quantity delineated by the sample in a closed state. The oil signal quantity delineated for the sample in the saturated state, in units of au; The water signal quantity delineated from the sample in a closed state, in au; The water signal quantity delineated from the sample in the saturated state, in au; In step S4, the crude oil, formation water, and dodecane used for resaturation in the formation where the sample is located are calibrated and analyzed to establish the conversion relationship between oil-water signal quantity and oil-water volume. ； ； ； In the formula, The volume of crude oil is expressed in ml. The volume of water is expressed in ml. Volume of saturated fluid dodecane, in ml; This is an oil signal, measured in au. This is a water signal, measured in au. The signal quantity for saturated fluid dodecane, in au; This is the crude oil calibration conversion factor; The conversion factor for formation water calibration; The calibration conversion factor for saturated fluid dodecane; the conversion factor k includes , and ; In step S6, the total fluid volume of the same sample is first calculated in both the closed and resaturated states, with the total fluid volume defined as the sum of the oil and water volumes in the sample. Then, the difference between the total fluid volume in the resaturated state and the total fluid volume in the closed state is calculated to characterize the loss of the original pore fluid in the sample. This loss is then normalized to the total fluid volume in the resaturated state to obtain the resaturation response coefficient, which is used to quantitatively evaluate the retention degree of the original pore fluid. The calculation formula is as follows: ； ； ； In the formula, The fluid volume of a sealed cryogenic cored sample before resaturation; The fluid volume of the same sample after resaturation treatment under controlled conditions; The resaturation response coefficient is used to quantitatively characterize the degree of preservation of the original pore fluid. Quantitative characterization of the preservation level of the original pore fluid includes: when When the value is ≤ 0.10, the original pore fluid preservation effect is considered excellent; When 0.10 < ≤ At 0.30, it is determined that part of the original pore fluid was lost; when > At a value of 0.30, the original pore fluid retention effect is considered to be poor.

2. The method for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast according to claim 1, characterized in that, In step S1, the sealed sample is kept in a cryogenic liquid nitrogen tank throughout the transportation process. After transportation is completed, the sealed sample is taken out of the cryogenic liquid nitrogen tank and then immediately cut into block-shaped samples. The length of the block-shaped samples is 2.5~3cm, the width is 1~1.5cm, the height is 1~1.5cm, and the weight is 10~20g. Meanwhile, wait for the block-sized sample to thaw. Once thawed, immediately perform a T1-T2 two-dimensional nuclear magnetic resonance test to obtain the nuclear magnetic spectrum data under sealed conditions.

3. The method for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast according to claim 1, characterized in that, In step S1, the thawing time is determined as follows: A sealed sample is used for experimental analysis. After the sealed sample is removed from the cryogenic liquid nitrogen container, it is quickly placed in an NMR instrument for a T2 one-dimensional NMR test. The test is repeated every 0.5 to 1.5 minutes thereafter, and the signal changes are observed after each test. After several T2 one-dimensional NMR tests, when the signal stabilizes (i.e., all signals can be detected), the total test time is recorded, which is the thawing time required.

4. The method for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast according to claim 1, characterized in that, In step S2, the sample is wrapped with filter paper and the sample number is recorded before the resaturation treatment. Subsequently, it is immediately placed into the saturation oil instrument, and then the saturation fluid is poured into the storage tank. After checking the closing conditions of each switch, the vacuum pump is turned on for air extraction. After the vacuum extraction is completed, the pressurization operation is carried out.

5. The method for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast according to claim 1, characterized in that, In step S2, after the sample completes the resaturation treatment, the sample is immediately immersed in a beaker containing dodecane. Before performing the T1-T2 two-dimensional nuclear magnetic resonance test, the sample is taken out, the surface fluid is wiped dry, and then the T1-T2 two-dimensional nuclear magnetic resonance test is performed again to obtain the nuclear magnetic spectrum data in the resaturated state.

6. The method for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast according to claim 1, characterized in that, In step S3, T1 refers to longitudinal relaxation, and T2 refers to transverse relaxation. The hydrogen-containing component fluids in the region division include adsorbed oil, free oil, and free water. Among them, when 0.2 ms < T2 < 1 ms and T1 / T2 > 10, it is the first region, corresponding to adsorbed oil; when T2 > 1 ms and T1 / T2 > 10, it is the second region, corresponding to free oil; when T2 > 0.2 ms and T1 / T2 < 10, it is the third region, corresponding to free water. The first, second, and third regions are the regions divided on the nuclear magnetic spectrum diagram, which are used to delineate and extract the signal amounts of adsorbed oil, free oil, and free water.

7. The method for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast according to claim 1, characterized in that, The specific process of calibration analysis is as follows: Crude oil, water, and dodecane are weighed separately to prepare a standard sample, and then the T2 one-dimensional nuclear magnetic resonance test is performed. The ratio of the hydrogen signal amount of the obtained oil or water to the corresponding volume is calculated to obtain the conversion coefficient k of the signal amount - volume of the oil, water, and saturated fluid dodecane inside the sealed core shale.

8. A system for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast, used to implement the method for judging the fluid preservation effect of sealed cryogenic core samples based on nuclear magnetic resonance resaturation contrast as described in any one of claims 1-7, characterized in that, Including: The T1-T2 two-dimensional nuclear magnetic resonance test module is configured to: cut and prepare the sealed sample, and perform a T1-T2 two-dimensional nuclear magnetic resonance test after thawing; perform a resaturation treatment on the sealed sample, and perform a T1-T2 two-dimensional nuclear magnetic resonance test again after the saturation ends. The region division and signal amount extraction module is configured to: according to the two-dimensional nuclear magnetic relaxation characteristics, perform region division and signal amount extraction on the three hydrogen-containing component fluids of adsorbed oil, free oil, and free water in the sealed and resaturated samples. The oil and water volume calculation module is configured to: establish the conversion relationship between the hydrogen signal amount and volume of each hydrogen-containing component fluid; calculate the oil and water volumes in the sealed and resaturated states respectively according to the conversion relationship. The preservation effect discrimination module is configured to: subtract the total fluid amount in the resaturated state from the total fluid amount in the sealed state, and calculate the ratio of the obtained difference to the total fluid amount in the resaturated state. The result is defined as the resaturation response coefficient, which is used to quantitatively evaluate the preservation degree of the fluid in the original pores.

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