A method for sandstone core spontaneous imbibition experiment based on NMR scanning

Abstract

The application discloses a kind of sandstone core spontaneous imbibition experimental methods based on NMR scanning, comprising: using nuclear magnetic resonance scanning device scans to obtain the gray scale of sandstone core sample in initial state when not absorbing water vertically axial stratification and parallel axial direction;Using nuclear magnetic resonance scanning device scans to obtain the gray scale of sandstone core sample vertically axial stratification imaging and parallel axial direction and T2 curve under different spontaneous imbibition time under different time corresponding moment when soaking in spontaneous imbibition device;DCM data of the imaging result obtained is adjusted to window width window position, the color scale of all gray scale is adjusted to obtain the best false color map, and the hydrogen signal intensity of sandstone core sample along axial direction different position and parallel axial direction is analyzed to obtain fluid migration and occurrence state in sample space at different time in complete process of spontaneous imbibition.The application can be not destroyed core sample as premise, and space positioning is carried out by analyzing a large number of profile image data, and then movable water migration speed and reached position in spontaneous imbibition process are predicted.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of geological exploration engineering, and particularly relates to a sandstone core spontaneous imbibition experiment method based on NMR scanning. BACKGROUND

[0002] At present, the exploitation of unconventional energy is conducive to solving the serious energy crisis problem in the world, and hydraulic fracturing is a necessary reservoir reconstruction technology applied in the exploitation of unconventional low-permeability oil and gas reservoirs, that is, water-based fracturing fluid is injected into the reservoir under high pressure to form a complex fracture network to improve the reservoir permeability. In the above hydraulic fracturing process, part of the water-based fracturing fluid will be retained in the reservoir after the well is closed. This part of the retained fracturing fluid spontaneously imbibes into the rock matrix under the action of capillary pressure, and finally affects the oil and gas reservoir recovery. Among them, spontaneous imbibition generally refers to the process of replacing the reservoir oil and gas (non-wetting phase) with the fracturing fluid (wetting phase) without external force, and the spontaneous imbibition can be divided into forward imbibition and reverse imbibition. At the same time, spontaneous imbibition is one of the basic mechanisms for understanding the complex liquid-reservoir interaction in the process of hydraulic fracturing operation in tight sandstone reservoirs.

[0003] Nuclear magnetic resonance (NMR) can reveal the influence of sandstone water absorption on the spontaneous imbibition process from the microscopic perspective, which is helpful to explore the reasons for the fast and slow spontaneous imbibition rate. Using spontaneous imbibition can explore the mobile water migration speed and arrival position in the imbibition process of sandstone core. Therefore, the combination of spontaneous imbibition experiment and nuclear magnetic resonance (NMR) test can analyze the fluid flow in the tight core sample at the pore scale level. Among them, NMR scanning is performed at different times in the spontaneous imbibition experiment process, such as 1h, 2h, 4h, 8h, 16h, 24h, etc., and nuclear magnetic resonance is used to measure the T2 curve at different times, X, Y and Z three-axis independent gradient field is used to distinguish the position, and imaging sequence is matched to image from different positions and different layers, and to observe the fluid migration and occurrence state in the porous medium at different times.

[0004] Currently, the traditional spontaneous imbibition experimental methods are divided into weighing method and volume method. The weighing method is to record the mass change of the core at different time periods during the spontaneous imbibition process to analyze the imbibition effect (Lai F, Li Z, Wei Q, Zhang T, et al. Experimental investigation of spontaneous imbibition in a tight reservoir with nuclear magnetic resonance testing [J]. Energy & Fuels, 2016, 30(11), 8932-8940.); the volume method is to place a saturated oil core in a water absorption instrument and record the change of oil displacement volume at different time (Makhanov K, Dehghanpour H, Kuru E. An experimental study of spontaneous imbibition in Horn River shales. In: Paper SPE 162650 Presented at the SPE Canadian Unconventional Resources Conference, Calgary, Alberta, 2012, October 30-November 1.). With the upgrading of observation means, some scholars use the ratio of the envelope area between the spontaneous imbibition NMR T2 curve and the horizontal coordinate at a certain moment and the envelope area of the T2 curve before spontaneous imbibition (the increment of the nuclear magnetic curve) to obtain the recovery rate of the spontaneous imbibition experiment (Lai F, Li Z, Zhang T, et al. Characteristics of microscopic pore structure and its influence on spontaneous imbibition of tight gas reservoir in the Ordos Basin, China [J]. Journal of Petroleum Science Engineering, 2019, 172:23-31.). Since there are few methods for using NMR scanning to monitor and quantitatively characterize the spontaneous imbibition of sandstone cores, it is necessary to combine NMR scanning technology and spontaneous imbibition experiment to analyze the fluid flow in porous media from the pore scale, and to explore the factors affecting the gas / water / sandstone spontaneous imbibition recovery rate by observing the experimental results, so as to provide a basis for the exploitation and recovery prediction of unconventional energy. SUMMARY

[0005] In view of the above deficiencies of the prior art, the present application aims to provide a sandstone core spontaneous imbibition experiment method based on NMR scanning, so as to solve the problem that the mechanism process of capillary spontaneous imbibition of fracturing fluid in the process of hydraulic fracturing technology cannot be microscopically described, and the recovery cannot be quantitatively and qualitatively analyzed when the conventional oil and gas resources are exploited by using the hydraulic fracturing technology; the present application can space-locate by analyzing a large amount of profile image data under the premise of not damaging the core sample, and further predict the movable water migration speed and the position reached in the spontaneous imbibition process.

[0006] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0007] The sandstone core spontaneous imbibition experiment method based on NMR scanning of the present application is based on a spontaneous imbibition device and a nuclear magnetic resonance scanning device, and the steps are as follows:

[0008] Step 1: obtain a sandstone core sample, and use a nuclear magnetic resonance scanning device to scan to obtain the gray scale images of the vertical axial stratification and the parallel axial direction of the sandstone core sample in the initial state without water absorption;

[0009] Step 2: use the nuclear magnetic resonance scanning device to scan to obtain the gray scale images of the vertical axial stratification and the parallel axial direction of the sandstone core sample at different time points after soaking in the spontaneous imbibition device for different times, and obtain the T2 curve at different imbibition times;

[0010] Step 3: adjust the window width and window level of the DCM data of all the imaging results obtained in steps 1 and 2, adjust the color scale of all the gray scale images to obtain the best pseudo-color image, and analyze the hydrogen signal intensity of the sandstone core sample at different positions along the axial direction and the parallel axial direction in combination with the T2 curve obtained in step 2 to obtain the fluid migration and occurrence state in the space of the sandstone core sample at different time points in the complete spontaneous imbibition process.

[0011] Further, the spontaneous imbibition device comprises a reverse U-shaped cover, a cylindrical glass cup, a sealing plug, spontaneous imbibition liquid and a ground glass mouth. The reverse U-shaped cover is connected with the cylindrical glass cup through the ground glass mouth to realize the sealing of the spontaneous imbibition device. The lower end of the cylindrical glass cup is provided with the sealing plug for discharging the spontaneous imbibition liquid at the end of the experiment. The cylindrical glass cup is filled with the spontaneous imbibition liquid, and the sandstone core sample is placed in the spontaneous imbibition liquid in the cylindrical glass cup. After the spontaneous imbibition liquid is injected from the top end of the reverse U-shaped cover, 1-2 drops of black oil droplets are injected to float on the uppermost end of the liquid surface of the spontaneous imbibition liquid. After soaking for different times, the scale value of the rising oil droplets on the top end of the reverse U-shaped cover is observed, the inner diameter of the thin tube at the top end of the reverse U-shaped cover is 0.2-10 mm, and the content of the fluid discharged by the sandstone core sample after spontaneous imbibition is calculated. After the required soaking time of the sandstone core sample is reached, the sealing plug is opened to discharge the spontaneous imbibition liquid, the reverse U-shaped cover and the cylindrical glass cup are disassembled from the ground glass mouth, and the sandstone core sample is taken out.

[0012] Further, the inverted U-shaped outer cover is specifically an inverted U-shaped quartz glass outer cover, and the top of the scale tube can amplify the small volume change of the liquid level.

[0013] Further, the cylindrical glass cup is specifically a cylindrical quartz glass cup with scales.

[0014] Further, the nuclear magnetic resonance scanning device comprises a holder, a magnet, a coil, a nuclear magnetic resonance analyzer, a data acquisition and control system, and a magnet box; the holder, the magnet, and the coil are arranged in the magnet box, the magnet comprises a first magnetic pole and a second magnetic pole, the two poles are symmetrically arranged and are opposite poles for providing a high-strength magnetic field; the coil comprises a first coil and a second coil, which are arranged in the first magnetic pole and the second magnetic pole respectively; the holder is arranged in the coil, a sandstone core sample is placed in the holder, one end of the holder is connected to the data acquisition and control system through a line, and the other end of the holder is connected to the nuclear magnetic resonance analyzer through a line; the nuclear magnetic resonance scanning device emits electromagnetic waves to the sandstone core sample from the coil, when the frequency of the radio frequency electromagnetic waves changes near the resonance frequency of the sandstone core sample, the nuclear magnetic resonance analyzer is used to set parameters of the sandstone core sample under different time conditions for scanning, so as to obtain a vertical axial layered and parallel axial imaging gray scale image and a color image of the sandstone core sample, and the spatial fluid migration and occurrence state in the self-absorption process are obtained by analyzing the hydrogen signal intensity of the sandstone core sample at different positions along the axis.

[0015] Further, the step 1 specifically comprises:

[0016] Step 1.1: a cylindrical sandstone core sample with a height of 5-8 mm, a diameter of 2-5 mm, and an initial state without water absorption is placed in the magnet box of the nuclear magnetic resonance scanning device;

[0017] Step 1.2: adjust the parameters, perform pre-scanning, and stop after scanning is completed;

[0018] Step 1.3: perform positioning pre-scanning, stop after scanning is completed, and display the image in the positioning image display area;

[0019] Step 1.4: select the parallel axial direction, set the scanning time to 15 min-2 h, perform scanning, and obtain the parallel axial gray scale image;

[0020] Step 1.5: adjust the parameters, perform pre-scanning, and stop after scanning is completed; perform positioning pre-scanning, stop after scanning is completed, and display the image in the positioning image display area; select the vertical axial direction, set the scanning time to 15 min-2 h, perform scanning, and obtain the vertical axial gray scale image.

[0021] Further, the step 2 specifically comprises:

[0022] Step 2.1: The sandstone core sample is immersed in the self-suction liquid. During the self-suction immersion of the sandstone core sample, the volume of gas discharged from the sandstone core sample is observed through the scale on the inverted U-shaped cover. The sandstone core sample is immersed for different times to meet the setting of the self-suction experiment time condition. After reaching the corresponding time period, the sealing plug is opened to discharge the self-suction liquid. The sandstone core sample is taken out and wiped with a water absorption paper. The sandstone core sample is wrapped in a safety film and placed in a nuclear magnetic scanning device;

[0023] Step 2.2: The parameters for vertical axial imaging are set. From the front end to the rear end of the scanned sandstone core sample, the number of layers is set to 1-8. The information at different layer positions along the axial direction is observed. The layer thickness is selected to be 5-15 mm, and the layer spacing is 1-3 mm. The sandstone core sample at different times during the self-suction process is set with the same parameters, and then scanning imaging is performed;

[0024] Step 2.3: The parameters for parallel axial imaging are set. The number of layers is set to 1, and the layer thickness is selected to be 150-220 mm. The sandstone core sample at different times during the self-suction process is set with the same parameters, and then scanning imaging is performed;

[0025] Step 2.4: Based on the scanning parameter settings of steps 2.2 and 2.3, the gray scale images of the sandstone core sample at different times during the spontaneous imbibition experiment are obtained. By comparing the scanning images of the vertical axial layered sections after sequentially immersing for different time periods with the scanning images of the parallel axial sections after sequentially immersing for different time periods, the spatial positions of the self-suction liquid penetrating into the sandstone core sample at different time periods are observed;

[0026] Step 2.5: The T2 curve of the sandstone core sample at different times corresponding to the self-suction state is measured to obtain the pore size distribution of different pore sizes in the sandstone core sample, the corresponding porosity component, and the sample porosity. Combined with the vertical axial layered and parallel axial imaging at the corresponding time, the migration change and flow path evolution of the fluid in the pore structure of the sandstone core sample during the entire self-suction process are obtained.

[0027] Further, the step 3 specifically comprises:

[0028] Step 3.1: Based on the initial state and the imaging gray scale images at different times during the self-suction process obtained by scanning the sandstone core sample during the entire self-suction process according to steps 1 and 2, the following image data processing is performed: using the window technique to utilize the display effective value range of 0-255 to reduce the signal part that should not be lost; by setting the window number value in the range of 2500-3600 and the window level value in the range of 3400-4000, clear scanning images are obtained;

[0029] Step 3.2: The NMR images obtained from all different time points in the imbibition process are rendered using the Jet color scale, and the NMR images are composed of a series of pixels. The blue area in the image is the imaging background without imaging signal. The warm color area in the pseudo-color image is used to represent the water content, i.e. the hydrogen signal intensity. The darker the color, the higher the content of imbibition fluid, and the better the connectivity between pores. The cold color area in the pseudo-color image is used to represent the water content, i.e. the hydrogen signal intensity. The darker the color, the lower the content of imbibition fluid, and the worse the connectivity between pores or the area of sandstone matrix. Then, the pseudo-color image and the T2 curve obtained in step 2 are combined to analyze the hydrogen signal intensity of the sandstone core sample at different positions along the axial direction and parallel to the axial direction to obtain the fluid migration and occurrence state in the space of the sandstone core sample at different time points in the imbibition process.

[0030] The beneficial effects of the present application are:

[0031] (1) The imbibition device of the present application can be used to collect dry sandstone cores (initial state) soaked in imbibition fluid due to imbibition to absorb imbibition fluid, while expelling the gas in the original sample, realizing quantitative recording of the volume of expelled gas. Compared with the traditional imbibition experiment soaked in an open glass container, which cannot record the gas content in the original sample, the present application can quantitatively monitor the amount of absorption and expulsion, which has high innovative significance.

[0032] (2) The present application sets two NMR scanning imaging modes, vertical axial stratification and parallel axial imaging, respectively. By observing the water saturation changes of the sandstone core sample at different profile positions along the vertical axial direction during the imbibition process, and by comparing the saturation changes of each region in the pseudo-color images of the vertical axial profile at the same profile and the parallel axial profile at different time points, the water migration of the sample during the water absorption process can be visualized, the water migration dynamic study can be realized, the water absorption performance can be analyzed, and the spatial analysis of the spatial fluid distribution and the pore connectivity of the sandstone core soaked in the imbibition tank at different times can be carried out, which is beneficial to improving the accuracy and reliability of the experimental results.

[0033] (3) The present application can quantitatively predict the imbibition capacity and recovery rate of the tight sandstone core. Field operation can scale the sandstone imbibition recovery data obtained from the experiment to realize the prediction and analysis of oil and gas production capacity without other production capacity testing equipment, effectively reducing the development cost. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 The present application is a schematic diagram of the imbibition device structure in the present application.

[0035] Figure 2 The present application is a schematic diagram of the imbibition device structure in the present application.

[0036] Figure 3 Structure diagram of the nuclear magnetic resonance scanning device in the application;

[0037] Figure 4 Position parameter setting diagram of the vertical axial layered imaging of the sandstone core in the application;

[0038] Figure 5 Position parameter setting diagram of the vertical axial layered imaging of the sandstone core in the application;

[0039] Figure 6 Position parameter setting diagram of the vertical axial layered imaging of the sandstone core in the application;

[0040] Figure 7 Pore size distribution diagram (T2 curve) of the sandstone core at different self-sorption times in the application;

[0041] Figure 8 Pseudo-color result diagram of the vertical axial fixed layered profile imaging of the sandstone core at different self-sorption times in the application;

[0042] Figure 9 Pseudo-color result diagram of the vertical axial fixed layered profile imaging of the sandstone core at different self-sorption times in the application;

[0043] Figure 10 Pseudo-color result diagram of the vertical axial fixed layered profile imaging of the sandstone core at different self-sorption times in the application;

[0044] Figure 11 Pseudo-color result diagram of the vertical axial fixed layered profile imaging of the sandstone core at different self-sorption times in the application;

[0045] Figure 12 Pseudo-color result diagram of the vertical axial fixed layered profile imaging of the sandstone core at different self-sorption times in the application. DETAILED DESCRIPTION

[0046] In order to facilitate the understanding of those skilled in the art, the application will be further described below in combination with the embodiments and the drawings. The content mentioned in the embodiments is not a limitation of the application.

[0047] Referring to Figures 1-3 The sandstone core self-sorption experiment method based on NMR scanning in the application is based on a self-sorption device and a nuclear magnetic resonance scanning device.

[0048] The self-suction device comprises a reverse U-shaped cover 1, a cylindrical glass cup 2, a sealing plug 4, self-suction liquid 5 and a frosted mouth 6; the reverse U-shaped cover 1 is connected with the cylindrical glass cup 2 through the frosted mouth 6 to realize the sealing of the self-suction device; the lower end of the cylindrical glass cup 2 is provided with the sealing plug 4 for discharging the self-suction liquid at the end of the experiment; the cylindrical glass cup 2 is filled with the self-suction liquid 5, and the sandstone core sample 3 is placed in the self-suction liquid 5 in the cylindrical glass cup 2; after the self-suction liquid 5 is injected from the top end of the reverse U-shaped cover 1, 1-2 drops of black oil drops 7 are injected to float on the uppermost end of the liquid surface of the self-suction liquid 5; after soaking for different time (such as 1 h, 2 h, 4 h, 8 h, 16 h, 24 h, etc.), the scale value of the rising oil drops 7 at the upper end of the reverse U-shaped cover 1 is observed, the inner diameter of the thin tube at the upper end of the reverse U-shaped cover 1 is 0.2-10 mm (when the inner diameter is 0.5 mm, the experimental data is more accurate), and the content of the self fluid (gas) discharged after the sandstone core sample absorbs the self-suction liquid due to the spontaneous imbibition is calculated; after reaching a required soaking time of the sandstone core sample 3, the sealing plug 4 is opened to discharge the self-suction liquid 5, the reverse U-shaped cover 1 and the cylindrical glass cup 2 are disassembled from the frosted mouth 6, and the sandstone core sample 3 is taken out;

[0049] The reverse U-shaped cover 1 is specifically a reverse U-shaped quartz glass cover, which can better contact the cylindrical glass cup 2 in a frosted manner; and the thin tube with a scale at the top can amplify the small volume change of the liquid surface.

[0050] The cylindrical glass cup 2 is specifically a cylindrical quartz glass cup, which is provided with a scale to facilitate the recording of the volume of the contained liquid.

[0051] The nuclear magnetic resonance scanning device comprises a magnet 8, a coil 9, a nuclear magnetic resonance analyzer 10, a data acquisition and control system 11, a magnet box 12 and a holder 13; the magnet 8, the coil 9 and the holder 13 are arranged in the magnet box 12; the magnet 8 is divided into a first magnetic pole and a second magnetic pole, which are symmetrically arranged and are opposite poles for providing a high-strength magnetic field; the coil 9 is divided into a first coil and a second coil, which are respectively arranged in the first magnetic pole and the second magnetic pole; the holder 13 is arranged in the coil 9, the sandstone core sample 3 is placed in the holder 13, one end of the holder 13 is connected with the data acquisition and control system 11 through a line, and the other end of the holder 13 is connected with the nuclear magnetic resonance analyzer 10 through a line; the nuclear magnetic resonance scanning device emits electromagnetic waves from the coil 9 to the sandstone core sample 3; when the frequency of the radio frequency electromagnetic waves changes near the resonance frequency of the sandstone core sample, the nuclear magnetic resonance analyzer 10 is used to set the parameters of the sandstone core sample 3 under different time conditions to scan, so as to obtain the vertical axial layering and parallel axial imaging gray value image and color image of the sandstone core sample, and the spatial fluid migration and occurrence state in the self-suction process are obtained by analyzing the hydrogen signal intensity of the sandstone core sample at different positions along the axis.

[0052] Referring toFigure 4 The method steps are as follows:

[0053] Step 1: Obtain a sandstone core sample, and use a nuclear magnetic resonance scanning device to scan to obtain a gray scale image of the sandstone core sample in a vertical axial layered and parallel axial state without water absorption in an initial state;

[0054] Specifically, the step 1 specifically includes:

[0055] Step 1.1: Place a cylindrical sandstone core sample with a height of 5-8 mm, a diameter of 2-5 mm, and preferably a size of 6 mm in height and 2.5 mm in diameter in the magnet box 12 of the nuclear magnetic resonance scanning device, and the measured data of the core sample in the initial state without water absorption is more accurate;

[0056] Step 1.2: Adjust the parameters, and perform a pre-scan, and stop after the scanning is completed;

[0057] Step 1.3: Perform a positioning pre-scan, and stop after the scanning is completed, and the positioning image is displayed in the display area;

[0058] Step 1.4: Select a parallel axial direction, set the scanning time to 15 min-2 h, and preferably select 1 h in the preferred example to obtain the best data, because the data obtained by scanning for less than 1 h is more accurate and has higher precision, and the data obtained by scanning for more than 1 h has similar quality and saves time cost, and perform scanning to obtain a gray scale image in the parallel axial direction;

[0059] Step 1.5: Adjust the parameters, and perform a pre-scan, and stop after the scanning is completed; perform a positioning pre-scan, and stop after the scanning is completed, and the positioning image is displayed in the display area; select a vertical axial direction, set the scanning time to 15 min-2 h, and preferably select 1 h in the preferred example to obtain the best data, and the precision is higher, because the data obtained by scanning for less than 1 h is more accurate, and the data obtained by scanning for more than 1 h has similar quality and saves time cost, and perform scanning to obtain a gray scale image in the vertical axial direction.

[0060] Step 2: Use a nuclear magnetic resonance scanning device to scan to obtain a gray scale image of a sandstone core sample in a vertical axial layered and parallel axial state at a corresponding time when the sandstone core sample is soaked in a self-suction device for different times, and obtain a T2 curve corresponding to different self-suction times;

[0061] Specifically, the step 2 specifically includes:

[0062] Step 2.1: Immerse the sandstone core sample in the self-absorbing liquid 5. During the process of the sandstone core sample continuously absorbing water through self-absorption, observe the volume of gas discharged from the sandstone core sample through the scale on the inverted U-shaped outer cover 1. Set different immersion times to meet the self-absorption experiment time conditions. After reaching the corresponding immersion time, open the sealing plug 4 to discharge the self-absorbing liquid 5, take out the sandstone core sample and wipe it with absorbent paper. Wrap the sandstone core sample in a protective film and place it in the NMR scanning device (to prevent the surface moisture of the sandstone core sample from evaporating during the subsequent NMR scanning process and affecting the accuracy of the experimental data).

[0063] Step 2.2: Refer to Figure 5 As shown, the parameters for vertical axis imaging were set from the front end to the back end of the scanned sandstone core sample, with the number of layers set to 1 to 8 (when the number of layers is 4, the imaging effect of each layer profile after NMR scanning is the best, and the number of layers is 4, which can make the profile evenly distributed throughout the core sample; the more layers there are, the weaker the water signal that can be identified by NMR in each layer, and the worse the imaging effect). The location information of different layers along the axis was observed, with layer thickness of 5 mm to 15 mm and layer spacing of 1 to 3 mm (the data obtained when the layer thickness is 10 mm and the layer spacing is 2 mm is more accurate). The same parameter settings were applied to the sandstone core samples at different times during the self-absorption process, and then scanning imaging was performed separately.

[0064] Step 2.3: Refer to Figure 6 As shown, the parameters for parallel axis imaging were set, the number of layers was set to 1, and the layer thickness was selected from 150mm to 220mm (the data obtained when the layer thickness is 200mm is more accurate). The same parameter settings were applied to sandstone core samples at different times of the self-absorption process, and then scanning imaging was performed separately.

[0065] Step 2.4: Based on the scanning parameter settings in Steps 2.2 and 2.3, obtain grayscale images of the spontaneous absorption state of the sandstone core sample at different times during the spontaneous absorption experiment (the duration of spontaneous absorption varies from the beginning to 30 days, taking 1h, 2h, 4h, 8h, 16h, 24h as examples). By comparing the scanning images of each section of the vertical axis after soaking for different time periods with the scanning images of the parallel axis section after soaking for different time periods, observe the spatial location of the spontaneously absorbed liquid infiltrating into the sandstone core sample at different soaking time periods (because NMR scans by identifying the intensity of water signals, i.e., hydrogen signals, to obtain the changes in water content of the sample before and after spontaneous absorption).

[0066] Step 2.5: Refer to Figure 7As shown, the T2 curves of the sandstone core sample in the self-suction state at different time points are measured, the pore size distribution of different pore sizes in the sandstone core sample and the corresponding porosity component and the sample porosity are obtained, and the migration change and flow path evolution of the fluid in the pore structure of the sandstone core sample during the entire self-suction process are obtained by combining the vertical axial layering and parallel axial imaging at the corresponding time points.

[0067] Step 3: Refer to Figures 8-12 As shown, the DCM data of all the imaging results obtained in steps 1 and 2 are adjusted in window width and window level, the color scale of all the gray scale images is adjusted to obtain the best pseudo-color image, and the hydrogen signal intensity of the sandstone core sample at different positions along the axial direction and the parallel axial direction is analyzed in combination with the T2 curve obtained in step 2 to obtain the fluid migration and occurrence state in the sandstone core sample space at different time points during the entire self-suction process.

[0068] Specifically, the step 3 specifically includes:

[0069] Step 3.1: According to the initial state and the imaging gray scale images at different time points during the entire self-suction process of the sandstone core sample obtained in steps 1 and 2, the following image data processing is performed: (since the display has only 8-bit (0-255) gray scale that can be displayed, and the DCM data of the NMR imaging has 12-16 bits, normalizing the nuclear magnetic resonance imaging to 0-255 will cause a lot of image detail information loss, and the highlighted part in the output image is noise), the window technology is used to utilize the display effective value range of 0-255 to reduce the signal part that should not be lost; by setting the window number value in the range of 2500-3600 and the window level value in the range of 3400-4000, clear scanning images are obtained.

[0070] Step 3.2: The NMR imaging at all different time points during the self-suction process is rendered using the Jet color scale, the NMR imaging is composed of a series of pixel points, the blue area in the processed image is the imaging background without imaging signal, the warm color area in the obtained pseudo-color image is used to represent the water content, i.e., the hydrogen signal intensity, the deeper the color, the higher the self-suction liquid content and the better the connectivity between pores; the cold color area in the obtained pseudo-color image is used to represent the water content, i.e., the hydrogen signal intensity, the deeper the color, the lower the self-suction liquid content and the worse the connectivity between pores or the sandstone matrix area; in combination with the pseudo-color image and the T2 curve obtained in step 2, the hydrogen signal intensity of the sandstone core sample at different positions along the axial direction and the parallel axial direction is analyzed to obtain the fluid migration and occurrence state in the sandstone core sample space at different time points during the entire self-suction process.

[0071] The application has many specific application approaches, and the above description is only the preferred embodiment of the application, and it should be pointed out that, for ordinary skilled in the art, several improvements can be made without departing from the principles of the application, and these improvements should also be considered as the protection scope of the application.

Claims

1. A self-absorption experimental method for sandstone cores based on NMR scanning, comprising a self-absorption device and a nuclear magnetic resonance scanning device, characterized in that, The steps are as follows: Step 1: Obtain sandstone core samples and use a nuclear magnetic resonance (NMR) scanner to obtain grayscale images of vertical and parallel axial layering of the sandstone core samples in their initial, water-free state. Step 2: Use a nuclear magnetic resonance scanning device to scan and obtain vertical axial layered imaging and parallel axial grayscale images of sandstone core samples at different times corresponding to different self-absorption times, and obtain T2 curves corresponding to different self-absorption times. Step 3: Adjust the window width and window level of all imaging results obtained in Step 1 and Step 2, adjust the color scale of all grayscale images to obtain the best pseudo-color image, and combine the T2 curve obtained in Step 2 to analyze the hydrogen signal intensity of sandstone core samples at different positions along the axial direction and parallel to the axial direction to obtain the fluid transport and occurrence state in the space of sandstone core samples at different times of the complete self-absorption process. Step 2 specifically includes: Step 2.1: Immerse the sandstone core sample in the self-absorbing liquid. During the process of the sandstone core sample continuously absorbing water through self-absorption, observe the volume of gas discharged from the sandstone core sample through the scale on the inverted U-shaped outer cover. Set different immersion times to meet the self-absorption experiment time conditions. After reaching the corresponding immersion time, open the sealing plug to discharge the self-absorbing liquid, take out the sandstone core sample and wipe it with absorbent paper. Wrap the sandstone core sample in a protective film and place it in the nuclear magnetic resonance scanning device. Step 2.2: Set the parameters for vertical axis imaging. From the front end to the back end of the scanned sandstone core sample, set 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. Set the same parameters for sandstone core samples at different times during the self-absorption process, and then perform scanning imaging respectively. Step 2.3: Set the parameters for parallel axis imaging, set the number of layers to 1, and select a layer thickness of 150mm~220mm. Set the same parameters for sandstone core samples at different times of the self-absorption process, and then perform scanning imaging respectively. Step 2.4: Based on the scanning parameter settings in Step 2.2 and Step 2.3, grayscale images of the spontaneous absorption state of the sandstone core sample at different times during the spontaneous absorption experiment are obtained. By comparing the scanning images of each section of the vertical axis after being soaked for different time periods with the scanning images of the parallel axis section after being soaked for different time periods, the spatial location of the spontaneously absorbed liquid infiltrating into the sandstone core sample at different soaking time periods is observed. Step 2.5: Measure the T2 curves of sandstone core samples under self-absorption conditions at different times to obtain the pore size distribution of different pore sizes in the sandstone core samples and their corresponding porosity components and sample porosity. Combine the vertical axial stratification and parallel axial imaging at the corresponding times to obtain the migration changes and flow path evolution of fluid in the pore structure of the sandstone core samples during the entire self-absorption process.

2. The method for self-absorption experiments on sandstone cores based on NMR scanning according to claim 1, characterized in that, The self-absorption device includes: an inverted U-shaped outer cover, a cylindrical glass cup, a sealing plug, self-absorption liquid, and a ground glass inlet; the inverted U-shaped outer cover and the cylindrical glass cup are connected through the ground glass inlet to achieve a seal for the self-absorption device; a sealing plug is provided at the lower end of the cylindrical glass cup for discharging the self-absorption liquid after the experiment; the cylindrical glass cup contains the self-absorption liquid, and the sandstone core sample is placed in the self-absorption liquid in the cylindrical glass cup. After injecting the self-absorption liquid from the top of the inverted U-shaped outer cover, black oil droplets are injected; by observing the scale value of the rising oil droplets at the top of the inverted U-shaped outer cover at different soaking times, the self-fluid content of the sandstone core sample after spontaneously aspirating the self-absorption liquid is calculated; after the required soaking time for the sandstone core sample is reached, the sealing plug is opened to discharge the self-absorption liquid, and the inverted U-shaped outer cover and the cylindrical glass cup are disassembled from the ground glass inlet to remove the sandstone core sample.

3. The method for self-absorption experiments on sandstone cores based on NMR scanning according to claim 2, characterized in that, The inverted U-shaped outer cover is specifically an inverted U-shaped quartz glass outer cover, and the fine tube with graduations on the top can magnify the minute volume changes of the liquid surface.

4. The method for self-absorption experiments on sandstone cores based on NMR scanning according to claim 2, characterized in that, The cylindrical glass cup is specifically a cylindrical quartz glass cup with graduations.

5. The method for self-absorption experiments on sandstone cores based on NMR scanning according to claim 1, characterized in that, The nuclear magnetic resonance (NMR) scanning device includes: a holder, a magnet, a coil, an NMR analyzer, a data acquisition and control system, and a magnet box. The holder, magnet, and coil are housed within the magnet box. The magnet has a first magnetic pole and a second magnetic pole, which are symmetrically arranged and are opposite magnetic poles, used to provide a high-intensity magnetic field. The coil has a first coil and a second coil, which are respectively placed within the first and second magnetic poles. The holder is placed within the coil, and the sandstone core sample is placed within the holder. One end of the holder is connected to the data acquisition and control system via a circuit, and the other end is connected to the NMR analyzer via a circuit. The NMR scanning device emits electromagnetic waves from the coil to the sandstone core sample. When the frequency of the radio frequency electromagnetic wave changes near the resonant frequency of the sandstone core sample, the NMR analyzer is used to scan the sandstone core sample under different time conditions with set parameters to obtain vertical axial layering and parallel axial imaging grayscale images and color images of the sandstone core sample. By analyzing the hydrogen signal intensity at different positions along the axial direction of the sandstone core sample, the spatial fluid transport and storage state during the self-absorption process can be obtained.

6. The method for self-absorption experiments on sandstone cores based on NMR scanning according to claim 1, characterized in that, Step 1 specifically includes: Step 1.1: Place a cylindrical sandstone core sample with a height of 5-8 mm and a diameter of 2-5 mm, which has not absorbed water in its initial state, into the magnet box of the nuclear magnetic resonance scanning device; Step 1.2: Adjust the parameters, perform a pre-scan, and stop after the scan is complete; Step 1.3: Perform a pre-scan for positioning. Stop the scan after it is completed, and display the image in the positioning image display area; Step 1.4: Select the parallel axial direction, set the scanning time to 15 min to 2 h, perform the scan, and obtain the grayscale image of the parallel axial direction; Step 1.5: Adjust the parameters, perform a pre-scan, and stop after the scan is complete; perform a positioning pre-scan, and stop after the scan is complete; the positioning image display area displays the image; select the vertical axis direction, set the scan time to 15min~2h, perform the scan, and obtain the grayscale image of the vertical axis.

7. The method for self-absorption experiments on sandstone cores based on NMR scanning according to claim 1, characterized in that, Step 3 specifically includes: Step 3.1: Based on the initial state and the imaging grayscale images obtained from the scanning of the sandstone core sample during the self-absorption process obtained in Steps 1 and 2, the following image data processing is performed: Windowing techniques are used to utilize the effective display value range of 0 to 255 to reduce the signal portion that should not be lost; clear scanned images are obtained by setting the window width value in the range of 2500 to 3600 and the window level value in the range of 3400 to 4000. Step 3.2: Use the Jet color scale to render NMR images at all different times during the self-absorption process. NMR images consist of a series of pixels. The blue area in the processed image is the imaging background with no imaging signal. The warm-colored area in the obtained pseudo-color image is used to represent water content, i.e., a strong hydrogen signal. The darker the color, the higher the content of the self-absorbed fluid and the better the connectivity between pores. The cool-colored area in the obtained pseudo-color image is used to represent water content, i.e., a weak hydrogen signal. The darker the color, the lower the content of the self-absorbed fluid and the worse the connectivity between pores, or the area where the sandstone matrix is ​​located. Then, combine the pseudo-color image and the T2 curve obtained in Step 2 to analyze the hydrogen signal intensity of the sandstone core sample at different positions along the axial direction and parallel to the axial direction to obtain the fluid transport and occurrence state in the space of the sandstone core sample at different times during the complete self-absorption process.

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