A method for analyzing the interconnected pore structure of dense rocks

Abstract

This invention pertains to the field of oil and gas exploration, including shale gas and tight gas, and discloses a method for analyzing the interconnected pore structure of tight rocks. The method includes: preparing a saturated sodium chloroaurate solution as an injectant; preparing a rock sample, performing a first cleaning and drying treatment to obtain a sample to be tested; subjecting the rock sample to vacuuming, fluid immersion, and pressurization to allow the injectant to enter the pores of the sample; removing the sample after injection and drying it to convert the injectant into a gold-containing solid substance; performing a second cleaning treatment on the sample; then, obtaining the fracture distribution of the sample and analyzing the distribution characteristics of the injectant in the micro- and nano-scale pores to analyze the interconnected pore structure of the rock sample. This invention enhances the identification of pores and fractures in tight rocks under a scanning electron microscope (SEM) and combines SEM observation with the analysis of the interconnected pore structure of rock samples.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas exploration, such as shale gas and tight gas, and more specifically, relates to a method for analyzing the interconnected pore structure of tight rocks. Background Technology

[0002] With the exploration and development of shale gas, the study of nanoporous structures in shale and mudstone has become a hot topic. Micron-nanopores in shale and mudstone serve as storage spaces for shale gas, directly impacting its production capacity. Current research methods can be broadly categorized into two types. One type uses nitrogen adsorption, isothermal adsorption, and nuclear magnetic resonance to obtain quantitative information such as porosity and pore size distribution in shale and mudstone. The other type uses microscopic observation techniques such as argon ion polishing scanning electron microscopy, atomic force microscopy, and CT to directly obtain images of the pores, thereby analyzing their morphological and structural characteristics. While image analysis methods can reveal the structural features of the pores, they cannot reflect the connectivity of the pores; for example, they cannot distinguish between connected pores and relatively isolated pores, nor can they determine which pores gas will preferentially pass through. Furthermore, some methods used for conventional reservoirs, such as thin-section casting, cannot be applied to the study of nanoporous structures in shale and mudstone due to the dense nature of the rock.

[0003] Currently, several methods exist both domestically and internationally to analyze the connectivity characteristics of shale by injecting various substances into it and then detecting the distribution of these substances. These methods mainly fall into three categories: First, injecting rare gases such as xenon (Bolivia et al., 2014; S Mayo et al., 2015) or high-density liquids (such as diiodomethane) into sealed shale formations, followed by CT scanning to obtain the characteristics of the fluid-filled interconnected pores. Second, using different tracer fluids (such as non-adsorbent ReO4)... - and adsorbent Ce 3+ Co 2+ Cs + Eu 3+One method involves the self-absorption of tracers into shale, followed by ICP-MS analysis of ion characteristics to study the pore structure and diffusion characteristics of the tracer (Hu et al., 2014). A third method involves injecting a low-melting-point molten alloy into the pores of shale under high pressure, followed by analysis of the alloy's distribution within the pores using scanning electron microscopy and CT after cooling, thus revealing the connectivity characteristics of different pores (Hildenbrand et al., 2003; Hu et al., 2012, 2015; Klaver et al., 2015; Desbois et al., 2016). Of these methods, the first two, due to the fluid nature of the filling material, cannot be analyzed using nanoscale techniques such as scanning electron microscopy, thus limiting the analytical scale to the micrometer-millimeter level. The latter method requires very high pressure to inject the alloy, and the distribution of the fluid within the pores is limited by the pressure applied. Furthermore, the problem with analyzing pore connectivity by injecting chloroauric acid into dense rocks is that chloroauric acid is highly acidic and will react with carbonate minerals in shale during the injection process, thereby destroying the original internal structure.

[0004] Therefore, given the shortcomings of current research techniques, there is an urgent need to provide a better method for studying the interconnected pore structure of dense rocks. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method for analyzing the interconnected pore structure of dense rocks. This method enhances the identification of pores and fissures in dense rocks under a scanning electron microscope (SEM), and combined with SEM observation, allows for the analysis of pore connectivity and the statistical calculation of the proportion of interconnected pores.

[0006] To achieve the above objectives, the present invention provides a method for analyzing the interconnected pore structure of dense rocks, the method comprising the following steps:

[0007] S1: Prepare a saturated sodium chloroaurate solution as the injection material;

[0008] S2: The rock sample is prepared, cleaned and dried for the first time to obtain the sample to be tested;

[0009] S3: By vacuuming, immersing in fluid, and pressurizing, the injected material is brought into the pores of the sample to be tested. The sample is then removed, dried, and the injected material in the sample is transformed into a gold-containing solid substance.

[0010] S4: Perform a second cleaning process on the injection-completed sample after step S3; then, obtain the crack distribution of the injection-completed sample, analyze the distribution characteristics of the injected material in the micro-nano-level pores of the injection-completed sample, and analyze the interconnected pore structure of the rock sample.

[0011] The technical solution of the present invention has the following beneficial effects:

[0012] (1) The method of the present invention injects sodium chloroaurate fluid into the pores of dense rock through self-absorption and external pressure, and then converts the sodium chloroaurate in the pores into solid gold particles under heating conditions. Finally, the distribution of gold elements in the pores is analyzed by scanning electron microscopy to obtain the connectivity characteristics of nanoscale pores, which can enhance the identification of pores in dense rocks under scanning electron microscopy. Combined with scanning electron microscopy observation, the connectivity of pores can be analyzed and the proportion of connected pores can be statistically analyzed.

[0013] (2) Compared with the prior art, since the injected material is completely retained in the pores, the microscopic observation results can better reflect the true situation of the interconnected pores. Furthermore, high pressure injection is not required, which can avoid the damage of the original pore structure of the sample to high pressure. There is no need to prepare gold nanoparticles. The relatively low drying temperature can prevent the minerals and organic matter in the sample from undergoing physicochemical reactions. The process is relatively simple.

[0014] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0015] The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of exemplary embodiments of the invention in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments of the invention.

[0016] Figure 1 A schematic diagram of the fracture distribution of the injected sample obtained by a method for analyzing the interconnected pore structure of dense rock according to an embodiment of the present invention is shown.

[0017] Figure 2 A schematic diagram is shown illustrating the distribution characteristics of the injected material in the micro-nano scale pores of the completed injection sample obtained by a method for analyzing the interconnected pore structure of dense rock according to an embodiment of the present invention. Detailed Implementation

[0018] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0019] This invention provides a method for analyzing the interconnected pore structure of dense rocks, the method comprising the following steps:

[0020] S1: Prepare a saturated sodium chloroaurate solution as the injection material;

[0021] S2: The rock sample is prepared, cleaned and dried for the first time to obtain the sample to be tested;

[0022] S3: By vacuuming, immersing in fluid, and pressurizing, the injected material is brought into the pores of the sample to be tested. The sample is then removed, dried, and the injected material in the sample is transformed into a gold-containing solid substance.

[0023] S4: Perform a second cleaning process on the injection-completed sample after step S3; then, obtain the crack distribution of the injection-completed sample, analyze the distribution characteristics of the injected material in the micro-nano-level pores of the injection-completed sample, and analyze the interconnected pore structure of the rock sample.

[0024] In this invention, a saturated sodium chloroaurate solution (NaAuCl4·2H2O) is used as the injectant. Its characteristic is that under heating conditions, it readily decomposes to form solid AuCl3, and AuCl3 can further decompose under certain conditions to form the more stable elemental Au. The reaction process is as follows:

[0025] NaAuCl4→AuCl3+NaCl

[0026] AuCl3→Au+Cl2↑

[0027] According to the present invention, preferably, in step S2, the sample preparation includes using a core sampling device to prepare the rock sample into a cylindrical sample, the cylindrical sample having a diameter of 2-5 mm.

[0028] According to the present invention, preferably, in step S2, the first cleaning process includes polishing the two end faces of the cylindrical sample and removing debris and contaminants from the surface of the cylindrical sample using an ultrasonic device.

[0029] According to the present invention, preferably, in step S2, the drying process includes placing the cylindrical sample in an oven, and the drying temperature is room temperature, in order to remove moisture from the cylindrical sample.

[0030] In this invention, the parameters for vacuuming, fluid immersion, and pressurized placement can be set according to the different properties of the rock sample.

[0031] According to the present invention, preferably, in step S3, the vacuuming, fluid immersion, and pressurization treatments are performed in a metal or glass container; the drying temperature is below 160°C. The vacuuming time is 1-2 hours; the pressurization pressure is 5-10 MPa.

[0032] According to the present invention, preferably, in step S4, the second cleaning process includes polishing the two end faces of the injected sample by manual polishing and / or argon ion polishing to remove solutes and other substances adhering to the surface of the injected sample.

[0033] According to the present invention, preferably, in step S4, a CT scanning device is used to obtain the crack distribution of the injected sample, and a scanning electron microscope is used to analyze the distribution characteristics of the injected material in the micro-nano-level pores of the injected sample.

[0034] According to the present invention, preferably, the scanning electron microscope is a high-resolution scanning electron microscope.

[0035] According to the present invention, preferably, in step S4, the analysis of the interconnected pore structure of the rock sample includes calculating the porosity of the dominant interconnected pores in the micro-region, wherein the porosity of the dominant interconnected pores in the micro-region is the proportion of the injected material in the micro-region.

[0036] The present invention will be specifically illustrated below through examples.

[0037] Example

[0038] This embodiment provides a method for analyzing the interconnected pore structure of dense rocks. The method was applied to shale samples from the Sichuan Basin and includes the following steps:

[0039] S1: Prepare a saturated sodium chloroaurate solution as the injection material;

[0040] S2: The rock sample is prepared, cleaned, and dried to obtain the sample to be tested; the preparation includes using a core sampling device to make the rock sample into a cylindrical sample with a diameter of 3 mm; the first cleaning process is to polish the two end faces of the cylindrical sample and use an ultrasonic device to remove debris and contaminants from the surface of the cylindrical sample; the drying process includes placing the cylindrical sample in an oven and drying at room temperature.

[0041] S3: By vacuuming, immersing in fluid, and pressurizing in a glass container, the injected material is introduced into the pores of the sample to be tested. The sample is then removed, dried, and the injected material in the sample is converted into a gold-containing solid substance. The vacuuming time is 2 hours; the pressurization pressure is 10 MPa; and the drying temperature is 145°C.

[0042] S4: The injected sample processed in step S3 undergoes a second cleaning treatment; then, a CT scan is used to obtain the crack distribution of the injected sample, and a high-resolution scanning electron microscope is used to analyze the distribution characteristics of the injected material in the micro-nano-level pores of the injected sample. The interconnected pore structure of the rock sample is analyzed, and the porosity of the dominant interconnected pores in the micro-region (the proportion of the injected material in the micro-region) is calculated. The second cleaning treatment involves polishing the two end faces of the injected sample using manual polishing and / or argon ion polishing.

[0043] Figure 1 A schematic diagram of the crack distribution in the injected sample, as shown below. Figure 1 As can be seen, the injected material penetrates into the nanoscale pores, making these well-connected pores highly visible. Many grain edge seams were not clear before the injection, but became clearly visible afterward. Furthermore, this high-brightness display facilitates quantitative statistical analysis. Figure 2 A schematic diagram illustrating the distribution characteristics of the injected material in the micro-nano-scale pores of the completed sample, as shown below. Figure 2 It can be seen that the porosity of the dominant connected pores in the micro-region is 3.72%, which represents the distribution of the dominant connected pore structure in this region.

[0044] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A method for analyzing the interconnected pore structure of dense rocks, characterized in that, The method includes the following steps: S1: Prepare a saturated sodium chloroaurate solution as the injection material; S2: The rock sample is prepared, cleaned and dried for the first time to obtain the sample to be tested; S3: By vacuuming, immersing in fluid, and pressurizing, the injected material is brought into the pores of the sample to be tested. The sample is then removed, dried, and the injected material in the sample is transformed into a gold-containing solid substance. The pressure applied during pressurization is 5-10 MPa; the vacuuming time is 1-2 hours. S4: Perform a second cleaning process on the injection-completed sample after step S3; then, obtain the crack distribution of the injection-completed sample, analyze the distribution characteristics of the injected material in the micro-nano-level pores of the injection-completed sample, and analyze the interconnected pore structure of the rock sample. The analysis of the interconnected pore structure of the rock sample includes calculating the porosity of the dominant interconnected pores in the micro-region, wherein the porosity of the dominant interconnected pores in the micro-region is the proportion of the injected material in the micro-region.

2. The method for analyzing the interconnected pore structure of dense rocks according to claim 1, wherein, In step S2, the sample preparation includes using a core sampling device to make the rock sample into a cylindrical sample with a diameter of 2-5 mm.

3. The method for analyzing the interconnected pore structure of dense rocks according to claim 2, wherein, In step S2, the first cleaning process includes: polishing the two end faces of the cylindrical sample and removing debris and contaminants from the surface of the cylindrical sample using an ultrasonic device.

4. The method for analyzing the interconnected pore structure of dense rocks according to claim 3, wherein, In step S2, the drying process includes placing the cylindrical sample in an oven, and the drying temperature is room temperature.

5. The method for analyzing the interconnected pore structure of dense rocks according to claim 1, wherein, In step S3, the vacuuming, fluid immersion, and pressurization treatments are performed in a metal or glass container; the drying is carried out at a temperature below 160°C.

6. The method for analyzing the interconnected pore structure of dense rocks according to claim 1, wherein, In step S4, the second cleaning process includes polishing the two end faces of the injected sample using manual polishing and / or argon ion polishing.

7. The method for analyzing the interconnected pore structure of dense rocks according to claim 6, wherein, In step S4, a CT scanning device is used to obtain the crack distribution of the injected sample, and a scanning electron microscope is used to analyze the distribution characteristics of the injected material in the micro-nano-level pores of the injected sample.

8. The method for analyzing the interconnected pore structure of dense rocks according to claim 7, wherein, The scanning electron microscope is a high-resolution scanning electron microscope.

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