Scanning electron microscope sample preparation method for unconsolidated water-containing sediment

Abstract

The invention discloses a scanning electron microscope sample preparation method for unconsolidated water-containing sediments. The method sequentially comprises the following steps: drilling and plugging a rock core; slicing the rock; unfreezing the rock slices; drilling a scanning electron microscope sample; conducting treatment; and observing and analyzing by a scanning electron microscope. According to the method, the particularity of the unconsolidated water-containing sediment is fully considered from a series of sample preparation processes such as sample plug drilling, shape-preserving preparation and film coating, and an unconventional innovative technical process is adopted. Compared with a traditional sample preparation method, the method has the advantages that the sample structure is prevented from being damaged, the real underground state is preserved, the mineral components and content can be observed, the pore distribution and mineral distribution rule in the rock can be effectively shown, and the microscopic and in-situ observation and analysis advantages of the scanning electron microscope are fully exerted.

Description

Technical Field

[0001] This invention relates to the field of geological analysis technology, and in particular to a method for preparing unconsolidated water-bearing sediments for scanning electron microscopy. Background Technology

[0002] In the field of oil and gas exploration and development, scanning electron microscopes are often used to observe the microscopic in-situ distribution of mineral composition, pore characteristics and other related geological information in rock samples. Therefore, it is required that the samples be kept in their original state as much as possible.

[0003] Currently, electron microscopy sample preparation in the geological field is generally divided into block samples and powder samples. Block samples mainly come from drill cores and outcrops, and are used for in-situ observation and analysis of mineral composition and distribution, pore type and distribution, and diagenetic processes. Because rock samples undergo diagenetic alteration, they are highly consolidated and have low water content (a small amount of water content will not change the mineral composition and pore structure after drying). The preparation mainly involves making appropriately sized block samples with parallel observation and mounting surfaces. The mounting surface is then attached to the sample holder with conductive tape and placed in a coating instrument for gold spraying in high vacuum mode to achieve conductivity. Powder samples generally come from drill cuttings. Borrowing from powder sample preparation methods in materials science, the cuttings are evenly sprinkled onto the surface of conductive tape, and excess powder is blown away with an air gun before being placed in a coating instrument for gold spraying in high vacuum mode to achieve conductivity.

[0004] Traditionally, unconsolidated samples are typically prepared using electron microscopy (SEM) methods similar to those used for powder samples. These methods fail to preserve the rock's mineral composition and pore structure, hindering subsequent geological research. In 2024, a gas field with reserves exceeding 100 billion cubic meters was discovered in the Lingshui 36-1 ultra-deep-water, ultra-shallow reservoir in the Qiongdongnan Basin. The reservoir consists of weakly diagenetic—unconsolidated siltstone with a high mud content. Conducting SEM analysis to preserve the pore structure and morphology of the rocks under formation conditions is crucial for analyzing the mineral and pore structure of stratigraphic rocks. However, the unique properties of unconsolidated aquifer sediments present numerous challenges for SEM analysis. These sediments exhibit weak intergranular cementation, a loose structure, and high water content. In traditional sample preparation, improper water removal can easily lead to significant changes in the sample structure, such as grain displacement and pore collapse. This results in microscopic images that fail to accurately reflect the original state of the sediments, severely impacting the accuracy and scientific validity of the analytical results. Therefore, developing an efficient, accurate, and widely applicable SEM method for preparing unconsolidated aquifer sediments is of significant practical importance. Summary of the Invention

[0005] To address the problems of easily damaging sample structures, inaccurate analytical results, and complex operation associated with existing techniques, this invention provides a scanning electron microscope (SEM) sample preparation method for unconsolidated aquifer sediments. This method enables high-precision analysis of the original microstructure and composition of unconsolidated aquifer sediments, improving the accuracy and reliability of the analysis and providing reliable data support for geological research.

[0006] The present invention is achieved by the following technical solution.

[0007] A method for preparing unconsolidated aqueous sediments for scanning electron microscopy includes the following steps: S1: Core plug; S2: Rock slice; S3: Rock slices thawing; S4: SEM Sample Drilling: ① Using a rigid sampling tube with a diameter of 1-1.5 cm and a length of 1 cm, perpendicular to the sectioning direction, drill into the thawed rock section until the bottom; ② Remove excess rock sample around the tube, leaving only the tube and the sample it surrounds; ③ Drip soft glue onto the sample surface at both ends of the sampling tube, rotate the tube to cover the sample surface with glue, and let it stand for 50-90 seconds to allow the glue to solidify; S5: Scanning Electron Microscopy Sample Preparation and Coating: ① Peel off the soft adhesive from one end of the sample to obtain the fresh side of the sample; ② Attach and fix the sample to a 90° corner sample post; ③ Use copper conductive tape to tightly connect the fresh side of the sample to the sample post; ④ Place the sample on the post in a dust-free environment with the indoor temperature controlled below 25℃ and allow it to air dry naturally until it is semi-damp; ⑤ Process the fixed sample sheet using an ion sputtering coating device and set the coating parameters.

[0008] S6: Scanning electron microscopy observation and analysis.

[0009] Furthermore, step S1 specifically includes the following steps: ① Select a core sample collected after cutting to the target depth, place it on the drilling machine, and align the drill bit with the target position; ② Allow liquid nitrogen to spray the core sample for 10-15 seconds; ③ Allow the drill bit to drill into the core sample until the bottom, and remove the drilled plunger sample for freezing.

[0010] Furthermore, step S2 specifically includes the following steps: ① Place the plunger sample from step S1 in liquid nitrogen for more than 10 seconds to freeze the plunger sample solid; ② Take out the plunger sample and place it on a slicer to cut a 1cm thick rock slice.

[0011] Furthermore, step S3 specifically includes the following steps: placing a 1cm thick rock slice on the sample stage, making the circular cross-section of the sample parallel to the stage surface, controlling the room temperature below 25℃, and allowing the sample to thaw naturally for 90s-120s.

[0012] Furthermore, in step S5, the coating parameters are set as follows: gas pressure is 0.04mb-0.05mb; the sample coating method involves multiple platinum sprayings, each lasting 300 seconds. After each spraying, the sample is observed in a field emission scanning electron microscope (SEM). If the image is unclear, the platinum spraying is repeated until a clear image is obtained. The coating thickness is controlled between 3nm and 5nm. If the coating is too thin, the poor conductivity will cause the sample to charge easily, affecting the imaging; conversely, if the coating is too thick, it will obscure the sample details, affecting the observation of the sample's fine structure.

[0013] Furthermore, step S6 specifically includes the following steps: placing the processed sample into a high-performance field emission scanning electron microscope (SEM) for comprehensive observation and high-definition imaging at different magnifications from 500X to 50000X; using the energy dispersive spectroscopy (EDS) instrument attached to the SEM to perform component analysis on different regions; and deeply integrating and processing the collected microstructure images and component data to analyze the microscopic characteristics of unconsolidated marine sediments.

[0014] This application has the following beneficial effects.

[0015] (1) This invention innovatively solves the problems of sample sampling and shape preservation, and coating of water-containing samples, and solves the industry problem of conducting scanning electron microscopy analysis under the condition that unconsolidated water-containing sediments cannot maintain their original structure.

[0016] (2) This invention addresses the sampling needs of non-standard sized samples by using a hard plastic sampling tube with a diameter of 1-1.5 cm and a length of 1 cm. The tube is inserted into a thawed rock slice, and the sample-bearing tube is removed. Both ends of the tube are then sealed with a small amount of soft glue. This technique fixes the sample morphology and structure, and also prevents damage to the sample structure during subsequent experiments such as drying, fixing, coating, and instrumentation.

[0017] (3) In this invention, a platinum target is used instead of a gold target when sputtering gold onto the sample. The vacuum degree of the coating instrument and the gold sputtering time were experimentally verified. The optimal gas pressure is 0.04 mb and the optimal cumulative platinum sputtering time is more than 800 s. Too low or too high vacuum will lead to unsatisfactory experimental results. The sample coating method should adopt multiple platinum sputtering, with each platinum sputtering time being 300 s, until the sample imaging effect is ideal.

[0018] (4) This invention has a wide range of applications, achieving good analytical results for various types of unconsolidated aquifer sediments, including marine, riverine, and lacustrine sediments. Whether samples are collected in cold polar regions or hot tropical regions; whether they are fine-grained clay sediments or larger-grained sandy sediments, the method of this invention can achieve accurate analysis through flexible parameter adjustment and operation optimization. This allows it to meet the diverse needs of different geological research fields, providing strong technical support for geological science research worldwide and possessing broad application prospects. Attached Figure Description

[0019] Figure 1 This is a picture of the actual product of the sampling tube of this invention used for drilling rock samples and sealing both ends with glue; Figure 2 Here are scanning electron microscope (SEM) images of unconsolidated aqueous sediment samples prepared using the methods described in this invention. Figure 3 This is a scanning electron microscope image of an unconsolidated aqueous sediment sample prepared using existing methods. Detailed Implementation

[0020] The present patent application will be further described below with reference to the embodiments. Unless otherwise specified, the materials used in the preparation process in the following embodiments have not undergone further processing and have been commercially available.

[0021] Example A scanning electron microscope (SEM) sample preparation method for unconsolidated cement-bearing siltstone core samples includes the following steps: ① core plugging; ② rock slicing; ③ rock slice thawing; ④ SEM sample extraction; ⑤ conductivity treatment; ⑥ SEM observation and analysis.

[0022] (1) The method of core drilling plug is as follows: ① Select the core sample collected from the LS36A well in the deep water area of ​​Qiongdongnan Basin with a depth of 1763.63m (including a water depth of 1500m and a burial depth of 263.63m), place it on a stable drilling machine, and align the drill bit with the target position; ② Connect the liquid nitrogen tank inlet and the drill bit, open the liquid nitrogen tank valve, and let the liquid nitrogen spray the core for 12s; ③ Press down the handle at a uniform speed to make the drill bit slowly drill into the core until the bottom, and remove the drilled plunger sample and freeze it quickly.

[0023] (2) The method for rock slicing is as follows: ① Place the plunger sample from step (1) in a container filled with liquid nitrogen for 10 seconds to freeze the plunger sample; ② Remove the plunger sample and place it on the holder of the slicer, and cut a 1 cm thick rock slice (see Figure 1 ).

[0024] (3) The rock slice thawing method is as follows: Place the 1cm thick rock slice horizontally on a clean and flat sample stage (with the circular cut surface parallel to the stage surface), let it stand in the shade for about half a minute, and keep the room temperature below 25℃. Thaw the sample naturally for 120 seconds. The place where the rock slice is placed for thawing should avoid sunlight and vibration to reduce the possibility of the sample cracking or collapsing; the thawing time should not be too long to prevent excessive moisture evaporation and damage to the sample structure.

[0025] (4) The method for drilling samples for scanning electron microscopy is as follows: ① Using a hard plastic sampling tube with a diameter of 1 cm and a length of 1 cm, perpendicular to the direction of the thin section, slowly and evenly drill into the thawed rock thin section until the bottom is reached; ② Remove the excess rock sample around the tube, leaving only the tube and the sample it surrounds; ③ Drip a small amount of soft glue onto the sample surface at both ends of the tube, slowly rotate the tube to cover the sample surface with glue, and let it stand for 1 minute to allow the glue to solidify (see Figure 1 ).

[0026] (5) The method for obtaining fresh sample surface is as follows: ① Use tweezers to peel off the soft glue at one end of the sample to obtain the fresh sample surface; ② Attach the sample to the 90° corner sample post; ③ Use copper conductive tape to attach the fresh sample surface and the sample post tightly to prevent it from loosening in the sample chamber during testing and thus failing to obtain high-resolution, high-quality images.

[0027] (6) The dust-free drying method is as follows: Place the sample in a dust-free environment, let it cool and stand still, and control the indoor temperature below 25℃. Let it air dry naturally until it is semi-wet (the surface has no obvious moisture and the original morphology of the sample can be maintained). The dust-free drying method generally chooses natural air drying instead of vacuum drying, because vacuum drying does not allow for constant observation of the sample's state and can easily over-dry the sample, causing it to become loose. The placement location should avoid sunlight and vibration to reduce the possibility of the sample cracking or collapsing. The drying time should not be too long to prevent excessive moisture evaporation and damage to the sample structure.

[0028] (7) The coating method is as follows: ① Place the sample with the observation surface facing upwards in the coating instrument, set the gas pressure of the coating instrument to 0.04mb, check that the sample height is relatively consistent, and the distance between the sample and the target is 40mm. Generally, the effective sputtering distance is 30mm, and the working distance can be adjusted by adjusting the height of the sample stage; ② Set the first sputtering time to 300s, and the imaging under the microscope is not ideal; perform the second sputtering, set the time to 300s, and the imaging under the microscope is still not ideal; continue to perform the third sputtering, set the time to 200s, and the resulting sample imaging effect is good, and the fine structure is clearly observed.

[0029] The coating material used in this application is platinum. Using a platinum target instead of a gold target has the advantages of uniform coating, stronger adhesion to particulate samples, and the ability to obtain films with smaller particle sizes, making it more suitable for fine, unconsolidated deposits. However, platinum easily forms oxides in the plasma state, and improper argon treatment can lead to stress cracks due to oxidation. The chamber must be carefully flushed with argon before sputtering to remove any residual oxygen. Furthermore, the specific sputtering time needs to be analyzed based on the sample's moisture content, porosity, and other actual conditions. For porous samples with high porosity, gas releases from the sample pores during vacuuming during SEM coating, prolonging the vacuuming time and preventing metal deposition, thus affecting the coating effect. In addition, if the required vacuum level for argon flushing cannot be achieved, it can easily lead to large film particles and stress cracks. In such cases, the vacuuming time needs to be extended, and the argon flushing step repeated to achieve coating within the pores of porous sandstone. Alternatively, the time can be set based on SEM observations; if the effect is unsatisfactory, the number of platinum sputtering cycles and the time can be appropriately increased.

[0030] At this point, a complete scanning electron microscope (SEM) sample of an unconsolidated aqueous sediment has been prepared. The prepared sample is placed in the SEM sample chamber for SEM observation and analysis.

[0031] Experimental results are as follows Figure 2 , 3 As shown, the method of this invention preserves the original structure of the unconsolidated and loose sample completely, and the mineral morphology and pore structure characteristics are clear under high magnification. However, after processing with existing methods, the particles in the sample are loose, the structure is damaged, the particles are attached with glue, and the true signal on the particle surface is blurred.

[0032] Utilizing the above method, and building upon existing scanning electron microscopy (SEM) sample preparation techniques, this invention fully considers the unique characteristics of unconsolidated aquifer sediments and employs unconventional and innovative technologies. Compared to traditional sample preparation methods, this invention avoids damaging the sample structure, preserving a more authentic underground state. It not only allows for the observation of mineral composition and content but also effectively reveals the pore distribution and mineral distribution patterns within the rock, fully leveraging the advantages of SEM's microscopic, in-situ observation and analysis.

[0033] The embodiments described herein are preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. An unconsolidated water-bearing sediment scanning electron microscope specimen preparation method, characterized by: It comprises the following steps: S1: core drill plug; S2: rock slice; S3: rock slice thawing; S4: scanning electron microscope sample drilling: ①perpendicular to the direction of thin section slice, a 1-1.5cm diameter, 1cm long hard sampling tube is drilled into the thawed rock slice until the bottom end; ②peel off the excess rock sample around the tube, only keep the sample surrounded by the tube; ③drop soft glue on the sample surface at both ends of the sampling tube, rotate the sampling tube to make the sample surface covered with glue, and let the glue solidify for 50-90s; S5: scanning electron microscope sample preparation and coating: ①peel off the soft glue at one end of the sample to get a fresh surface; ②stick the sample to a 90° corner sample stake; ③use copper conductive tape to tightly connect the fresh surface of the sample and the sample stake; ④place the staked sample in a dust-free environment, control the indoor temperature within 25℃, and naturally dry to a semi-wet state; ⑤use ion sputtering coating equipment to process the fixed sample, and set the coating parameters; S6: scanning electron microscope observation and analysis.

2. A method of preparing a sample for scanning electron microscopy of unconsolidated water-bearing sediments according to claim 1, characterized in that: Step S1 specifically comprises the following steps: ①select the profiled core sample containing the target depth, place it on the drill bed, and align the drill bit with the target position; ②spray the core with liquid nitrogen for 10-15s; ③make the drill bit drill into the core until the bottom, and remove the drilled plug sample for freezing.

3. A method for preparing a sample of unconsolidated water-bearing sediment for scanning electron microscopy according to claim 1, wherein: Step S2 specifically comprises the following steps: ①place the plug sample in step S1 in liquid nitrogen for more than 10s to freeze the plug sample solid; ②remove the plug sample and place it on the slicer to cut a 1cm thick rock slice.

4. A method for preparing a sample of unconsolidated water-bearing sediment for scanning electron microscopy according to claim 1, wherein: Step S3 specifically comprises the following steps: place the 1cm thick rock slice on the sample stage with the circular cross section of the sample parallel to the stage surface, control the indoor temperature within 25℃, and naturally thaw the sample for 90-120s.

5. A method for preparing a sample of unconsolidated water-bearing sediment for scanning electron microscopy according to claim 1, wherein: In step S5, the coating parameters are set as follows: gas pressure is 0.04-0.05mb; sample coating method uses multiple platinum spraying, each time for 300s, and after each spraying, the sample is placed in a field emission scanning electron microscope for observation, if the image is not clear, repeat the platinum spraying until the image is clear, and the coating thickness is controlled within 3-5nm.

6. A method for preparing a sample of unconsolidated water-bearing sediment for scanning electron microscopy according to claim 1, wherein: Step S6 specifically comprises the following steps: place the processed sample into a high-performance field emission scanning electron microscope, and observe and take high-definition photos under different magnifications of 500X-50000X; use the energy dispersive spectrometer attached to the scanning electron microscope to analyze the composition of different areas, and deeply integrate and process the collected microstructure images and composition data to analyze the micro features of marine unconsolidated water-bearing sediments.

Images