A method for characterizing wettability using NMR based on sandstone spontaneous imbibition experiments

Abstract

The application discloses a method for characterizing wettability based on a sandstone self-imbibition experiment using NMR, comprising the following steps: carrying out a self-imbibition experiment by immersing a sandstone sample in a solution; obtaining a fluid saturation distribution graph at different self-imbibition times and the mass of the imbibed fluid at different self-imbibition times; obtaining a T2 spectrum at different self-imbibition times by performing nuclear magnetic resonance; obtaining the volume of the imbibed liquid, dividing the volume by the effective pore volume of the sandstone sample to calculate a normalized self-imbibition volume; defining a relationship curve between the normalized self-imbibition volume and the self-imbibition time as a pore wettability index; characterizing the wettability of the sandstone fluid occurrence state in the self-imbibition process; and obtaining a contact angle, wherein the size of the contact angle represents the strength of the wettability. The application can quantitatively characterize the wettability change of a fracturing fluid after physical-chemical action of the fracturing fluid imbibed into a sandstone reservoir without damaging the sandstone sample, so as to predict which kind of fracturing fluid is beneficial to promote the recovery of unconventional oil and gas resources in actual engineering.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas development technology, specifically relating to a method for characterizing wettability using NMR based on sandstone self-absorption experiments. Background Technology

[0002] Sandstone reservoirs are a common type of geological reservoir, widely found in oil and gas exploration and development. During hydraulic fracturing in tight sandstone oil and gas reservoirs, fracturing fluids are absorbed into the porous medium, undergoing physicochemical reactions that alter the interfacial properties between the fracturing fluid and the reservoir rock. This changes the rock's wettability, ultimately affecting oil and gas recovery. Wettability refers to the interaction between rock and fluid, significantly influencing oil and gas migration and storage. Therefore, accurately assessing the wettability of sandstone is crucial for oil and gas exploration and development.

[0003] Currently, commonly used methods for characterizing wettability include contact angle measurement, static adsorption experiments, and surface energy calculation. Contact angle measurement assesses wettability by measuring the contact angle formed by a droplet on a solid surface; a smaller contact angle indicates better wettability. A relevant reference for using contact angle to assess wettability is "Nicolás Cabezudo, Jining Sun, Behnam Andi, et al. Enhancement of surface wettability via micro-and nanostructures by single point diamond turning[J]. Nanotechnology and Precision Engineering, 2019, 2(1): 8-14". Static adsorption experiments assess wettability by contacting a solid sample with a fluid and measuring the adsorption amount under equilibrium conditions; a larger adsorption amount indicates better wettability. A relevant reference for using static adsorption experiments to assess wettability is "Chunyuan Gu, Qinfeng Di, Haiping Fang. Slip velocity model of porous walls absorbed by hydrophobic nanoparticles SiO2[J]. Journal of Hydrodynamics, 2007, 19(3): 365-371”; Surface energy calculation is used to evaluate wettability by calculating the surface energy of a solid surface and the surface tension of a fluid. The smaller the surface energy difference, the better the wettability. The literature reference for evaluating wettability using surface energy calculation is "Rachel D, Davidson; Thomas E. O'Loughlin; Theodore EGAlivio; Soon-Mi Lim; Sarbajit Banerjee. Thermodynamics of wettability: a physical chemistry laboratory experiment[J]. Journal of Chemical Education, 2022, 99(7): 2689-2696".

[0004] However, the methods described above for characterizing wettability all have some drawbacks. For example, due to differences in experimental conditions and sample properties, there may be significant deviations between different experimental results, leading to inaccurate evaluation results; some require sample processing or destructive testing, which may affect the sample structure and properties; and some require complex experimental setups and operating procedures, which are cumbersome and time-consuming, making them unsuitable for large-scale applications.

[0005] Therefore, there is a need for an accurate, comprehensive, non-destructive, and easily computable method to characterize wettability. The method of characterizing wettability using nuclear magnetic resonance (NMR) based on sandstone self-absorption experiments can provide a new solution, overcoming some of the shortcomings of existing techniques. Summary of the Invention

[0006] To address the shortcomings of the existing technology, the present invention aims to provide a method for characterizing wettability using NMR based on sandstone self-absorption experiments. This method can dynamically represent the changes in wettability during the self-absorption process, thereby solving the problem that existing static contact angle measurements are severely affected by experimental conditions and cannot accurately reflect the dynamic changes in sandstone wettability during actual self-absorption.

[0007] This invention enables quantitative characterization of the changes in wettability after fracturing fluid has penetrated into sandstone reservoirs and undergone physical-chemical reactions, without damaging the sandstone sample. This allows for the prediction of which fracturing fluids will be beneficial for the recovery of unconventional oil and gas resources in practical engineering.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] The present invention provides a method for characterizing the wettability of sandstone using NMR based on a sandstone self-absorption experiment, comprising the following steps:

[0010] Step 1): Obtain sandstone samples, dry the sandstone samples, measure the dry weight, effective porosity φ, absolute permeability k and surface tension σ of the surfactant solution of the sandstone samples, and immerse the sandstone samples in the surfactant solution to carry out self-absorption experiments.

[0011] Step 2): Use magnetic resonance imaging to obtain fluid saturation distribution maps of sandstone samples at different self-absorption times along the vertical and parallel axes to qualitatively observe the fluid travel front, and use a balance to weigh the mass of the fluid absorbed at different self-absorption times.

[0012] Step 3): Perform nuclear magnetic resonance to obtain T2 spectra at different self-absorption times. The horizontal axis of the T2 spectrum represents the distribution of pore size in sandstone pores, in order to consider the changes in fluid flow in different types of microscopic pores during the self-absorption process.

[0013] Step 4): Divide the increase in mass of the sandstone sample after absorbing the surfactant solution by the density of the surfactant solution to obtain the volume V of the self-absorbed liquid; divide volume V by the effective pore volume Va of the sandstone sample. p Calculate the normalized self-absorption volume V n To eliminate the difference in pore volume caused by the heterogeneity of pore structure in sandstone samples;

[0014] Step 5): Establish the normalized self-absorption volume V nThe relationship curve between the self-absorption time and the porosity index was defined as the slope of the relationship curve.

[0015] Step 6): Centrifuge the sandstone sample under saturation and use nuclear magnetic resonance to measure the T2 spectrum of the centrifuged sandstone sample to obtain the saturation and porosity of the mobile and bound fluids in the sandstone sample; convert the effective porosity φ into the mobile fluid porosity φ. m Instead of optimizing, it characterizes the wettability of sandstone fluid occurrence state during the self-absorption process;

[0016] Step 7): Select the characteristic pore radius R from the T2 spectrum of the sandstone sample at different times of self-absorption obtained in Step 3) to represent the main water absorption pores. Use the characteristic pore radius R to correct the pore wettability index to obtain the contact angle θ. The size of the contact angle indicates the strength of wettability.

[0017] Further, in step 1), the surfactant solution has a mass concentration of 0.25 wt%, and the surfactant is sodium dodecyl sulfate.

[0018] Furthermore, in step 2), magnetic resonance imaging is used to obtain fluid saturation distribution maps of sandstone at different self-absorption times along the vertical and parallel axes. The warm-colored areas in the map indicate that the water content is high at the highlighted positions in the profile fluid saturation distribution map. The darker the color, the more surfactant solution the sandstone sample absorbs, and the lighter the color, the less surfactant solution the sandstone sample absorbs.

[0019] Furthermore, the different self-absorption times in steps 1), 2), and 3) are 1h, 2h, 4h, 8h, 16h, 24h, 36h, 48h, 72h, 96h, 120h, and 168h, respectively.

[0020] Further, step 3) specifically includes: placing a cylindrical sandstone sample with a height of 5-8 cm and a diameter of 2-5 cm into the magnet box of the nuclear magnetic resonance scanning device;

[0021] Adjust the parameters, perform a pre-scan, and stop after the scan is complete;

[0022] Perform a pre-scan for positioning, stop after the scan is complete, and display the image in the positioning image display area;

[0023] Select the parallel axial direction, set the scanning time to 15 min to 2 h, set the number of layers to 1, and select the layer thickness to 150 mm to 220 mm, and perform scanning to obtain grayscale and color images of the parallel axial direction.

[0024] Adjust the parameters, perform a pre-scan, and stop after the scan is complete;

[0025] Perform a pre-scan for positioning, stop after the scan is complete, and display the image in the positioning image display area;

[0026] Select the vertical axis direction, set the scanning time to 15 min to 2 h, and the number of layers to 1 to 8. Observe the location information of different layers along the axis. Select a layer thickness of 5 mm to 15 mm and a layer spacing of 1 to 3 mm to perform scanning and obtain grayscale and color images along the vertical axis.

[0027] Furthermore, in step 4), the effective pore volume V p =ALφ; where A is the bottom area of ​​the sandstone sample and L is the height of the sandstone sample.

[0028] Furthermore, the expression for the pore wettability index in step 5) is as follows:

[0029]

[0030] In the formula, σ is the surface tension; θ is the contact angle; r is the pore radius; k is the absolute permeability; φ is the effective porosity; v is the solution viscosity; and t is the self-absorption time.

[0031] Furthermore, the optimized pore wettability index formula in step 6) is as follows:

[0032]

[0033] Further, the centrifugation process in step 6) specifically includes: testing different rotational speeds of 2000 r / min, 4000 r / min, 6000 r / min, 8000 r / min, and 10000 r / min, with a centrifugation time of 1 hour, and calculating the movable fluid porosity based on the fluid signal quantity corresponding to different rotational speeds under stable fluid outflow conditions; the formula for calculating the movable fluid porosity is:

[0034] In the formula, V is the porosity of the movable fluid; v is the volume of the movable fluid; V sample This represents the volume of the sandstone sample.

[0035] Further, in step 7), the characteristic pore size is selected as 5 μm when the sandstone sample undergoes self-absorption for 1 hour, 6 μm for 2 hours, 7 μm for 4 hours, and 8 μm for 8 hours. The pore wettability index is corrected using the characteristic pore radius R, which represents the self-absorption process, to obtain the following formula:

[0036]

[0037] The beneficial effects of this invention are:

[0038] 1. Multiple methods: The method of this invention combines multiple experimental methods, including self-absorption experiment, magnetic resonance scanning experiment, balance weighing, nuclear magnetic resonance and centrifugation experiment, which can comprehensively obtain indicators for evaluating the wettability of fluid sandstone from different perspectives and provide more accurate and comprehensive results.

[0039] 2. Non-destructive: Nuclear magnetic resonance and magnetic resonance imaging are non-destructive characterization methods that do not damage the structure of sandstone samples, thus maintaining the integrity and properties of the samples.

[0040] 3. Quantitative Analysis: By combining self-absorption experiments and balance weighing, the interaction process between sandstone and fluids of different properties can be observed and quantified, revealing the dynamic changes in wettability and providing quantitative adsorption amounts and wettability indicators. Through nuclear magnetic resonance and centrifugation experiments, the fluids in sandstone samples can be quantitatively analyzed to obtain the distribution and properties of the fluids in the sandstone pores, thus obtaining more accurate wettability data and providing important references for oil and gas exploration and development.

[0041] 4. Operability: The method of this invention makes comprehensive use of a variety of experimental methods, but is relatively simple to operate and is suitable for large-scale testing and evaluation in practical applications.

[0042] 5. Practicality: The method of this invention is applicable to various types of sandstone samples. It not only has broad application prospects, but also provides accurate wettability assessment for oil and gas exploration and development, and provides important reference for the research and application of oil and gas migration and storage. Attached Figure Description

[0043] Figure 1 This is a flowchart of a specific embodiment of the method of the present invention based on characterizing the wettability of sandstone;

[0044] Figure 2 This is a schematic diagram of a combined experimental apparatus for acquiring experimental data using NMR characterization of wettability based on sandstone self-absorption experiments, as provided in an embodiment of the present invention.

[0045] Figure 3 This is a schematic diagram illustrating the self-absorption experiment for obtaining the self-absorption amount provided in an embodiment of the present invention;

[0046] Figure 4 This is a schematic diagram of the change in fluid saturation during self-absorption obtained by the magnetic resonance imaging device provided in this embodiment of the invention;

[0047] Figure 5 This is a schematic diagram of the nuclear magnetic resonance device provided in this embodiment of the invention acquiring the T2 spectrum during the self-absorption process using a CPMG sequence;

[0048] Figure 6 This is an index diagram of the wettability characterization method provided in the embodiments of the present invention;

[0049] Figure 7 This is a schematic diagram of the centrifugal device provided in an embodiment of the present invention for obtaining the porosity of flowing fluid and bound fluid;

[0050] Figure 8 This is an index diagram of the optimized wettability characterization method provided in this embodiment of the invention;

[0051] Figure 9 This is a schematic diagram illustrating the static measurement of contact angle to obtain the wettability magnitude in an embodiment of the present invention. Detailed Implementation

[0052] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present invention.

[0053] Reference Figure 1 As shown, the present invention provides a method for characterizing wettability of sandstone using NMR based on self-absorption experiments. In an example, the steps are as follows:

[0054] Step 1): Select a cylindrical sandstone sample with a diameter of 2.5 cm and a height of 6 cm, and dry the sample. Obtain the initial T2 spectrum using nuclear magnetic resonance (NMR). Refer to the national standard GB / T29172-2012 "Methods for Analysis of Rock Samples" to measure the dry weight, effective porosity φ, absolute permeability k, and surface tension σ of the surfactant solution. Prepare a 0.25 wt% sodium dodecyl sulfate solution as the self-absorption liquid for the self-absorption experiment, and measure the surface tension σ of the solution. Immerse the sandstone sample in the surfactant solution to change its surface wettability and conduct self-absorption experiments for different immersion times.

[0055] Figure 2 This is a schematic diagram of a combined experimental apparatus for acquiring experimental data using NMR characterization of wettability based on sandstone self-absorption experiments, as provided in an embodiment of the present invention. From Figure 2 As can be seen from the present invention, the experimental data of the method for characterizing wettability of sandstone based on self-absorption experiment using NMR consists of self-absorption experiment (1), balance weighing (2), centrifuge (3), nuclear magnetic resonance and magnetic resonance imaging (4), and contact angle measurement (5).

[0056] Step 2): Use magnetic resonance imaging (MRI) to obtain fluid saturation distribution maps of the sandstone sample along the vertical and parallel axes at different self-absorption times to qualitatively observe the fluid travel front, and use a balance to weigh the mass of the fluid absorbed at different self-absorption times to represent the change in self-absorption rate.

[0057] A cylindrical sandstone sample with a height of 5-8 cm and a diameter of 2-5 cm was placed inside the magnet box of the nuclear magnetic resonance scanning device.

[0058] Open the measurement software, set the parameters, put in the standard sample (oil sample) to find the center frequency and pulse width, replace the standard sample (oil sample) with a sandstone sample, click to acquire, and obtain the T2 spectrum.

[0059] Figure 3 This is a schematic diagram illustrating the self-absorption experiment used to obtain the self-absorption amount in an embodiment of the present invention. Figure 3 As can be seen, the amount of water absorbed by sandstone during the entire self-absorption process is constantly increasing. At the beginning of self-absorption, such as at 1h, 2h, and 4h, water is absorbed from the dry state. The capillary pressure in the sandstone pores leads to a fast water absorption rate. Until the later stage of self-absorption, the water absorption rate decreases significantly until it stabilizes.

[0060] Figure 4 This is a schematic diagram of the changes in fluid saturation during self-absorption obtained by the magnetic resonance imaging device provided in this embodiment of the invention. It can be seen that the hydrogen signal intensity at different positions along the axial and perpendicular axes of sandstone at different times of self-absorption in surfactant solution can be obtained. The transport and storage state of the spatial fluid during self-absorption can be obtained, and the spatial position change of the fluid infiltrating into the sandstone sample during self-absorption can be seen intuitively (MRI can obtain the change of water content in the sandstone sample during self-absorption by identifying water signal, i.e. hydrogen signal scanning).

[0061] Step 3): Perform nuclear magnetic resonance to obtain T2 spectra at different self-absorption times. The horizontal axis of the T2 spectrum represents the distribution of pore size in sandstone pores, which is used to consider the changes in fluid flow in different types of microscopic pores during the self-absorption process.

[0062] Figure 5 This is a schematic diagram of the T2 spectrum obtained by the nuclear magnetic resonance device using CPMG sequence during the self-absorption process in the embodiment of the present invention; it can be seen that during the self-absorption process of the sandstone sample, the content and flow of the self-absorbed liquid in its microscopic multi-scale pores are as follows.

[0063] Step 4): Divide the increase in mass of the sandstone sample after absorbing the surfactant solution by the density of the surfactant solution to obtain the volume V of the self-absorbed liquid; divide volume V by the effective pore volume Va of the sandstone sample. p The normalized self-absorption volume V was calculated. n To eliminate the pore volume differences in sandstone samples caused by the heterogeneity of pore structure; among which, the effective pore volume V p =ALφ; A is the bottom area of ​​the sandstone sample, and L is the height of the sandstone sample.

[0064] Step 5): Establish the normalized self-absorption volume V n The relationship curve between the self-absorption time and the porosity wettability index is defined as the slope of the relationship curve:

[0065]

[0066] In the formula, σ is the surface tension; θ is the contact angle; r is the pore radius; k is the absolute permeability; φ is the effective porosity; μ is the solution viscosity; and t is the self-absorption time.

[0067] Figure 6 This is an index diagram of the wettability characterization method provided in the embodiments of the present invention; it can be seen that the vertical axis is V n 2 ×((L 2 The slope of the curve with φμ) / 4σk) and the horizontal axis t is the porosity wettability index, which is 530028.

[0068] Step 6): Centrifuge the sandstone sample with long self-absorption time and saturation state, and use nuclear magnetic resonance to measure the T2 spectrum of the centrifuged sandstone sample to obtain the saturation and porosity of the mobile and bound fluids in the sandstone sample; based on equation (1), the effective porosity φ is expressed as the mobile fluid porosity φ m Instead of optimizing the wettability of sandstone fluids during the self-absorption process, the optimized pore wettability index formula is as follows:

[0069]

[0070] Figure 7 This is a schematic diagram of the centrifugation device provided in this embodiment of the invention for obtaining the porosity of flowing fluid and bound fluid. The centrifugation settings are set to different rotation speeds of 2k, 4k, 6k, 8k, and 1w, and the centrifugation time is 1h. Nuclear magnetic resonance measurement is performed when the outflow of fluid ejected from sandstone is stable and constant. That is, the porosity of movable fluid is calculated from the fluid signal quantity corresponding to the rotation speed of 1w.

[0071] according to Figure 7 It can be seen that the original porosity of the sandstone was 9.6%, and the movable fluid porosity obtained after centrifugation was 2.24%. This indicates that the movable fluid porosity φ... m A replacement for effective porosity φ is essential for characterizing the wettability of fluids within the pore structure of sandstone. The bound fluids are confined by the narrow pore throats, failing to adequately demonstrate the sandstone's wettability and affinity for fluids.

[0072] The centrifugation process specifically includes: testing at different rotational speeds of 2000 r / min, 4000 r / min, 6000 r / min, 8000 r / min, and 10000 r / min for a centrifugation time of 1 hour; and calculating the movable fluid porosity based on the fluid signal quantity corresponding to different rotational speeds under stable fluid outflow conditions. The formula for calculating the movable fluid porosity is as follows:

[0073] In the formula, V is the porosity of the movable fluid; v is the volume of the movable fluid; V sample This represents the volume of the sandstone sample.

[0074] Figure 8 This is an index graph of the optimized wettability characterization method provided in this embodiment of the invention; the vertical axis represents V. n 2 ×((L 2 φ m μ)4σk), the slope of the curve with the horizontal axis as t, i.e. the porosity wettability index, is 836.

[0075] Step 7): Select the characteristic pore radius R from the T2 spectra of the sandstone sample at different times of self-absorption obtained in Step 3) that can represent the main water absorption pores (i.e., the pore diameter of the mesopores). Use the characteristic pore radius R that can represent the self-absorption process to correct the pore wettability index to obtain cos(θ). Solve the inverse cosine function to obtain the contact angle θ. The size of the contact angle indicates the strength of wettability. The smaller the contact angle, the better the wettability.

[0076] For sandstone samples undergoing self-absorption for 1 hour, the characteristic pore size was selected as 5 μm; for 2 hours, 6 μm; for 4 hours, 7 μm; and for 8 hours, 8 μm. The porosity index was corrected using the characteristic pore radius R, which represents the self-absorption process, to obtain the following formula:

[0077]

[0078] Figure 5 This is a schematic diagram of the T2 spectrum obtained by the nuclear magnetic resonance device using CPMG sequence during the self-absorption process, as provided in an embodiment of the present invention. The diagram shows that the pores with a diameter range of 0.1–100 μm absorb the most water in the T2 spectrum, i.e., the peak value is high. Furthermore, as the self-absorption process progresses, the peak value tends to shift upwards and to the right, indicating that pore water is moving from smaller pores to larger pores. Therefore, the pore sizes corresponding to four moments in the self-absorption process, such as 1h, 2h, 4h, and 8h, are selected as the main pores representing the self-absorption moment, i.e., the characteristic pores R.

[0079] Figure 9 This is a schematic diagram of obtaining the wettability by static measurement of the contact angle in an embodiment of the present invention. It can be seen that the contact angle obtained by the contact angle measuring instrument before self-absorption is 64.45°, and the contact angle after self-absorption is 49.9°. The smaller the contact angle, the better the hydrophilicity.

[0080] Table 1 is a schematic table showing the experimental data provided in the embodiments of the present invention to verify the feasibility of the new wetting method; it can be seen from Table 1 that equation (2) and Figure 8The obtained pore wettability index was 836. Then, the pore wettability index was compared with... Figure 5 The cos(θ) values ​​obtained by multiplying the characteristic pores were 0.42, 0.5, 0.58 and 0.67, respectively; then the inverse cosine function was calculated to obtain the corresponding θ values ​​of 65.31°, 59.91°, 54.2° and 48.05°; this shows that the method of the present invention calculates that soaking the sandstone with the surfactant solution makes the sandstone more hydrophilic.

[0081] Furthermore, the contact angle measured by the contact angle measuring instrument before self-absorption (64.45°) and after self-absorption (49.9°) are very close to those measured using characteristic pore sizes of 5µm and 8µm as the main self-absorption parameters for 1 hour and 8 hours, respectively. This indicates that the method for calculating and characterizing wettability provided by this invention is accurate; Table 1 is as follows:

[0082] Table 1

[0083]

[0084] This invention has many specific applications. The above description is only a preferred embodiment of this invention. It should be noted that for those skilled in the art, several improvements can be made without departing from the principle of this invention, and these improvements should also be considered within the scope of protection of this invention.

Claims

1. A method for characterizing wettability of sandstone using NMR based on self-absorption experiments, characterized in that, The steps are as follows: Step 1): Dry the sandstone sample, measure the dry weight, effective porosity φ, absolute permeability k, and surface tension σ of the surfactant solution, and immerse the sandstone sample in the surfactant solution to carry out a self-absorption experiment. Step 2): Use magnetic resonance imaging to obtain the fluid saturation distribution maps of the sandstone sample along the vertical and parallel axes at different self-absorption times, and use a balance to weigh the mass of the aspirated fluid at different self-absorption times. Step 3): Perform nuclear magnetic resonance to obtain T2 spectra at different self-absorption times. The horizontal axis of the T2 spectrum represents the distribution of pore size in the sandstone. Step 4): Divide the increase in mass of the sandstone sample after absorbing the surfactant solution by the density of the surfactant solution to obtain the volume V of the liquid that was self-absorbed. Volume V divided by the effective pore volume V of the sandstone sample p Calculate the normalized self-absorption volume V n ; Step 5): Establish the normalized self-absorption volume V n The relationship curve between the self-absorption time and the porosity index was defined as the slope of the relationship curve. Step 6): Centrifuge the sandstone sample under saturation and use nuclear magnetic resonance to measure the T2 spectrum of the centrifuged sandstone sample to obtain the saturation and porosity of the mobile and bound fluids in the sandstone sample. The effective porosity φ is expressed as the movable fluid porosity φ m Instead of optimizing, it characterizes the wettability of sandstone fluid occurrence state during the self-absorption process; Step 7): Select the characteristic pore radius R from the T2 spectrum of the sandstone sample at different times of self-absorption obtained in Step 3) to represent the main water absorption pores. Use the characteristic pore radius R to correct the pore wettability index to obtain the contact angle θ. The size of the contact angle indicates the strength of wettability.

2. The method for characterizing wettability of sandstone using NMR based on self-absorption experiments according to claim 1, characterized in that, The surfactant solution in step 1) has a mass concentration of 0.25 wt%, and the surfactant is sodium dodecyl sulfate.

3. The method for characterizing wettability of sandstone using NMR based on self-absorption experiments according to claim 1, characterized in that, In step 2), magnetic resonance imaging is used to obtain fluid saturation distribution maps of sandstone at different self-absorption times along the vertical and parallel axes. The warm-colored areas in the map indicate that the water content is high at the highlighted positions in the profile fluid saturation distribution map. The darker the color, the more surfactant solution the sandstone sample absorbs, and the lighter the color, the less surfactant solution the sandstone sample absorbs.

4. The method for characterizing wettability of sandstone using NMR based on self-absorption experiments according to claim 1, characterized in that, The self-absorption times in steps 1), 2), and 3) are 1h, 2h, 4h, 8h, 16h, 24h, 36h, 48h, 72h, 96h, 120h, and 168h, respectively.

5. The method for characterizing wettability of sandstone using NMR based on self-absorption experiments according to claim 1, characterized in that, Step 3) specifically includes: placing a cylindrical sandstone sample with a height of 5-8 cm and a diameter of 2-5 cm into the magnet box of the nuclear magnetic resonance scanning device; Adjust the parameters, perform a pre-scan, and stop after the scan is complete; Perform a pre-scan for positioning, stop after the scan is complete, and display the image in the positioning image display area; Select the parallel axial direction, set the scanning time to 15 min to 2 h, set the number of layers to 1, and select the layer thickness to 150 mm to 220 mm, and perform scanning to obtain grayscale and color images of the parallel axial direction. Adjust the parameters, perform a pre-scan, and stop after the scan is complete; Perform a pre-scan for positioning, stop after the scan is complete, and display the image in the positioning image display area; Select the vertical axis direction, set the scanning time to 15 min to 2 h, the number of layers to 1 to 8, the layer thickness to 5 mm to 15 mm, and the layer spacing to 1 to 3 mm, and perform scanning to obtain grayscale and color images along the vertical axis.

6. The method for characterizing wettability of sandstone using NMR based on self-absorption experiments according to claim 1, characterized in that, In step 4), the effective pore volume V p =ALφ; where A is the bottom area of ​​the sandstone sample, L is the height of the sandstone sample, and φ is the effective porosity.

7. The method for characterizing wettability of sandstone using NMR based on self-absorption experiments according to claim 1, characterized in that, The expression for the pore wettability index in step 5) is as follows: In the formula, σ is the surface tension; θ is the contact angle; r is the pore radius; k is the absolute permeability; φ is the effective porosity; μ is the solution viscosity; and t is the self-absorption time.

8. The method for characterizing wettability using NMR based on sandstone self-absorption experiments according to claim 7, characterized in that, The optimized formula for the pore wettability index in step 6) is:

9. The method for characterizing wettability of sandstone using NMR based on self-absorption experiments according to claim 8, characterized in that, The centrifugation process in step 6) specifically includes: testing different rotational speeds (2000 r / min, 4000 r / min, 6000 r / min, 8000 r / min, 10000 r / min) for a centrifugation time of 1 hour; and calculating the movable fluid porosity based on the fluid signal quantity corresponding to different rotational speeds under stable fluid outflow conditions. The formula for calculating the movable fluid porosity is as follows: In the formula, V is the porosity of the movable fluid; v is the volume of the movable fluid; V sample This represents the volume of the sandstone sample.

10. The method for characterizing wettability of sandstone using NMR based on self-absorption experiments according to claim 9, characterized in that, In step 7), the characteristic pore size is selected as 5 μm after 1 hour of self-absorption in the sandstone sample, 6 μm after 2 hours, 7 μm after 4 hours, and 8 μm after 8 hours. The pore wettability index is corrected using the characteristic pore radius R, which represents the self-absorption process, to obtain the following formula:

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