Shale carbon dioxide adsorption expansion damage experiment method considering hydration state

Abstract

The invention belongs to the field of carbon dioxide geological sequestration and oil-gas field development, and relates to a shale carbon dioxide adsorption expansion damage experiment method considering a hydration state, which comprises the following steps: processing a shale sample according to requirements; the shale is subjected to constant-humidity regulation and control pretreatment, and in the process, different relative humidity environments can be used for pretreatment, so that the influence of different clay mineral hydration interlayer spacing states on adsorption expansion is obtained; soaking the pretreated shale in supercritical carbon dioxide, adsorbing the sample when the temperature and pressure conditions reach a set value, soaking until the rock sample reaches adsorption saturation, and treating by using carbon dioxide with different pressures and temperature gradients; according to collection of sample microstructure and mechanical data, damage can be treated through crack initiation position comparison, porosity and gray scale quantitative analysis and macro-mechanical parameter evolution. The method provides an experimental method for carbon dioxide geological sequestration safety assessment and shale gas fracturing parameter optimization.

Description

Technical Field

[0001] This invention pertains to an experimental method for carbon dioxide adsorption, swelling, and damage in shale considering its hydration state. Background Technology

[0002] Global warming is a serious challenge we face today, and carbon dioxide geological sequestration (CCUS) is a way to reduce atmospheric carbon dioxide levels. The critical pathway to concentration. Shale, as a common caprock or shale gas reservoir, plays a crucial role in supercritical... ( The stability of the environment is directly related to the safety of storage and the effectiveness of fracturing modification.

[0003] Shale, as a typical caprock or reservoir medium, is rich in clay minerals such as montmorillonite and illite. Studies have shown that the rock... Significant adsorption-induced swelling occurs during immersion, with clay components, represented by montmorillonite, exhibiting high adsorption capacity and being the main contributors to the swelling deformation. However, this adsorption-induced swelling behavior of clay minerals is not static but is strongly regulated by their initial hydration state (i.e., the amount of water content in the interlayer).

[0004] Although existing technologies already contain information about Studies on rock interactions have largely focused on chemically driven mineral dissolution / precipitation processes or simple immersion / drying treatments to simulate rock states, lacking precise control and differentiation of the key variable of "different hydration states." Due to the lack of quantitative criteria regarding "hydration state—adsorption swelling—mechanical damage," existing techniques struggle to accurately predict rock mechanical responses under varying geological humidity conditions, leading to significant uncertainties in assessing the long-term sealing of caprocks or optimizing fracturing parameters. Therefore, an experimental method is urgently needed to accurately simulate and test the adsorption swelling damage of shale under different hydration states.

[0005] Furthermore, existing technologies struggle to observe microstructural damage. Precise repositioning and observation of the same micro-region on the rock surface before and after high-pressure treatment are crucial. Due to sampling and repositioning errors, researchers often can only compare different regions or statistical averages, making it difficult to establish direct causal evidence for "adsorption-induced expansion → microcrack evolution." Therefore, an experimental method that can precisely control the hydration state and achieve microscopic field-specific tracking is urgently needed. Summary of the Invention

[0006] To address the aforementioned problems, this invention proposes an experimental method for carbon dioxide adsorption, swelling, and damage in shale considering its hydration state, aiming to quantitatively reveal the effects of different degrees of hydration. Damage mechanism affecting the mechanical properties and microstructure of mudstone and shale.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: An experimental method for carbon dioxide adsorption swelling damage in shale considering hydration state, including... Step 1: Process the mudstone and shale rock samples according to the experimental requirements to prepare standard samples that meet the mechanical testing standards or small-sized samples for microscopic observation. Step 2: Perform constant humidity hydration treatment on mudstone and shale. By controlling the relative humidity of the environment, the interlayer water content of clay minerals in mudstone and shale is stably locked within a preset range to obtain initial rock samples under different precise hydration states. Step 3: Construct a supercritical carbon dioxide immersion environment. Place the pretreated rock sample in a high-pressure reactor and immerse it in supercritical carbon dioxide at a constant temperature and a set pressure gradient to simulate the reservoir storage environment until the set adsorption saturation time is reached. Step 4: Data acquisition and damage assessment. Macroscopic mechanical tests and microscopic structural observations were conducted on rock samples before and after soaking. In the microscopic observation, image processing technology based on rigid body registration was used to achieve repeated localization and comparison of the same micro-domain before and after processing. Combined with the changes in macroscopic mechanical parameters, the damage mechanism of adsorption expansion on mudstone and shale was quantitatively assessed.

[0008] Preferably, in step 2, the constant humidity control treatment differs from the direct immersion method. Instead, the rock sample is placed in a constant temperature and humidity chamber or a sealed container controlled by a saturated salt solution. By setting different relative humidity levels, the interlayer spacing of clay mineral crystals reaches a specific number of hydrated layers, thereby obtaining graded hydration samples.

[0009] Preferably, in step 3, the immersion ambient temperature is set to be slightly higher than [the specified temperature]. The critical temperature and pressure are set as gradient values ​​of the pressure at the depth of the covered reservoir. After soaking, the pressure is reduced to atmospheric pressure by a low-speed continuous depressurization method to avoid damage to the non-adsorbent structure caused by instantaneous pressure difference.

[0010] Preferably, in step 4, the microscopic observation method includes: pre-setting no less than 3 non-collinear micro-markers on the surface of the rock sample before processing in step 2; acquiring scanning electron microscope images of the same field of view before and after supercritical carbon dioxide immersion treatment; calculating the rotation matrix and translation vector using the coordinates of the markers, and accurately mapping the coordinates of the processed image back to the coordinate system before processing, so as to realize the reproduction of the same microcrack or pore region.

[0011] Preferably, step 4 also includes performing grayscale quantitative analysis on the registered microscopic image: through grayscale normalization, noise reduction, adaptive threshold segmentation and skeleton extraction, the crack area, aspect ratio, tortuosity and porosity index are automatically calculated, thereby quantifying the microstructure evolution caused by supercritical carbon dioxide immersion adsorption expansion.

[0012] Preferably, in step 4, the macroscopic mechanical tests include uniaxial compression tests, Brazilian splitting tests, and porosity measurements; combined with digital image correlation technology, the full-field displacement and strain of the rock sample surface are monitored to obtain the evolution of elastic modulus, tensile strength, cohesion, and internal friction angle.

[0013] Preferably, the different relative humidity levels include: 11%RH, 45%RH, 65%RH, and 95%RH.

[0014] The beneficial effects of using this invention are: 1. This invention, through constant humidity control technology, enables the preparation of shale samples with precisely controllable hydration states, filling a gap in the understanding of the effects of different water saturation levels on the hydration of shale. The gap in research on the effects of adsorption damage.

[0015] 2. The microscopic field of view reproduction method based on marker points proposed in this invention solves the problem of quantification in scanning electron microscopy observation, and can intuitively and accurately track the initiation, expansion or closure process of the same microcrack under the action of adsorption expansion.

[0016] 3. This invention combines macroscopic mechanical parameters with microscopic structural parameters to construct a cross-scale damage assessment system, providing... The study provides reliable data support for the safety assessment of caprock in geological storage and the optimization of shale gas fracturing parameters. Attached Figure Description

[0017] Figure 1 This is a flowchart of an experimental method for carbon dioxide adsorption, swelling, and damage in shale considering the hydration state, according to the present invention.

[0018] Figure 2 To obtain using this method A schematic diagram comparing the evolution of cracks in the same microscopic field before and after processing (after image registration).

[0019] Figure 3 Porosity of mudstone and shale under different humidity (RH) pretreatment conditions varies with A graph showing the changing patterns of the processed data. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this technical solution clearer, the following detailed description, in conjunction with specific embodiments, further illustrates this technical solution. It should be understood that these descriptions are merely exemplary and not intended to limit the scope of this technical solution.

[0021] like Figure 1 As shown in this embodiment, an experimental method for carbon dioxide adsorption swelling damage of shale considering hydration state is proposed, which includes the following steps: Step 1: Process the mudstone and shale samples according to requirements; usually, a standard cylinder with a diameter of 25 mm and a height of 50 mm is prepared for uniaxial compression testing, or a disc with a diameter of 25 mm and a thickness of about 10 mm is prepared for Brazilian splitting and microscopic observation.

[0022] Step 2: Perform constant humidity hydration treatment on the mudstone and shale.

[0023] Step 3: High pressure treatment of the hydrated mudstone and shale. soak.

[0024] Step 4: Acquisition of macro- and micro-scale data of samples and analysis of damage mechanisms.

[0025] Specifically, in step 2, the controllable factor is relative humidity (RH). For example, the samples are divided into four groups and placed in sealed environments with relative humidity of 11%, 45%, 65%, and 95% respectively to equilibrate to constant weight. RH 11% simulates a low hydration state (small spacing between clay layers), and RH 95% simulates a high hydration state (the clay layers have fully absorbed water and swelled). This treatment method reflects the true unsaturated state of the underground reservoir better than the traditional "complete immersion". In some feasible embodiments, in step 2, a constant temperature and humidity chamber is used to adjust the relative humidity (RH) to different hydration states such as 0W (dehydrated), 1W, 2W, or 3W (multi-layered water), simulating different geological environments from dry gas layers to water-rich layers.

[0026] In step 3, the experimental temperature is set to 35℃ (higher than 35℃). The critical point is 31.8℃, and the pressure can be set to 8 MPa, 10 MPa, or 12 MPa. Supercritical carbon dioxide is constructed at a pressure of 8 MPa. ) Immersion environment, It is in a supercritical state and has high permeability. The soaking time can be set from 2 hours to 48 hours to obtain the effect of adsorption time on damage. In some feasible embodiments, in step 3, The injection process uses a plunger pump to pressurize in stages, and after the experiment, a micro-flow control is used to slowly release the pressure to prevent artificial structural damage caused by excessive pressure difference, and to ensure that the observed damage mainly comes from adsorption expansion and chemical reaction.

[0027] In step 4, the focus is on the "in-situ" comparison of the microstructure. Before step 3, the sample surface is polished and sprayed with three non-collinear micro-markers for a first SEM scan. After step 3, a second SEM scan is performed. The markers are identified using a MATLAB algorithm, and rotation and translation parameters are calculated to "align" the second scan image to the coordinate system of the first scan. The location of microcracks is visually displayed through image subtraction or overlay. In some feasible embodiments, in step 4, the macroscopic mechanical testing further includes triaxial or uniaxial compression tests on the treated rock sample; by obtaining stress-strain curves under different confining pressures, the cohesion (c) and internal friction angle (θ) of the rock are calculated using the Mohr-Coulomb criterion. By comparing different hydration states and c value under soaking time and Value evolution is used to quantitatively characterize the weakening or strengthening mechanism of adsorption expansion on the shear strength of mudstone and shale.

[0028] Example 1

[0029] like Figure 2 and Figure 3 As shown, shale outcrop samples from a certain area of ​​the Longma System in Sichuan were selected and processed into standard test specimens.

[0030] Experimental procedure: 1. Prepare multiple sets of shale samples and perform surface polishing and marking.

[0031] 2. Divide the samples into two groups, A and B. Pre-treat both groups and allow them to equilibrate for 7 days.

[0032] 3. Place the B group sample into the high-pressure reactor and inject... Maintain immersion at 35℃ and 8 MPa for 2 hours.

[0033] 4. After depressurization and removal, groups A and B underwent mechanical strength testing and SEM observation.

[0034] Experimental results: Tests conducted using this method revealed the following: (1) After pretreatment, the porosity of Group A samples showed different trends, indicating that the pretreatment resulted in rock samples with different degrees of hydration.

[0035] (2) Samples in Group B were in After treatment, the porosity varied to different degrees due to pretreatment, indicating that the expansion of rocks in different hydration states has different effects on porosity. Microscopic images show that clay minerals are affected by adsorption. Significant expansion occurred, inducing the generation and connection of numerous microcracks.

[0036] (3) Combined with porosity testing, it was found that the porosity of the 45% RH sample increased significantly after the reaction, which verified the mechanism of adsorption expansion leading to cracking.

[0037] This experiment shows that mudstone and shale have... The sensitivity of adsorption to water strongly depends on its initial hydration state. Under moderate hydration, adsorption swelling and destructive effects are strongest; while under high hydration, competitive adsorption of water and compaction by confining pressure dominate. These conclusions are quantitatively confirmed by the constant humidity control and in-situ observation method provided by this invention. The selection of storage sites (avoiding medium-sized aquifers or undergoing pretreatment) and fracturing design are of great guiding significance.

[0038] The above content is only a preferred embodiment of the present invention. For those skilled in the art, many changes can be made in the specific implementation and application scope based on the ideas of the present invention. As long as these changes do not depart from the concept of the present invention, they all fall within the protection scope of this patent.

Claims

1. An experimental method for carbon dioxide adsorption swelling damage in shale considering hydration state, characterized in that: include Step 1: Process the mudstone and shale rock samples according to the experimental requirements to prepare standard samples that meet the mechanical testing standards or small-sized samples for microscopic observation. Step 2: Perform constant humidity hydration treatment on mudstone and shale. By controlling the relative humidity of the environment, the interlayer water content of clay minerals in mudstone and shale is stably locked within a preset range to obtain initial rock samples under different precise hydration states. Step 3: Construct a supercritical carbon dioxide immersion environment. Place the pretreated rock sample in a high-pressure reactor and immerse it in supercritical carbon dioxide at a constant temperature and a set pressure gradient to simulate the reservoir storage environment until the set adsorption saturation time is reached. Step 4: Data acquisition and damage assessment. Macroscopic mechanical tests and microscopic structural observations were conducted on rock samples before and after soaking. In the microscopic observation, image processing technology based on rigid body registration was used to achieve repeated localization and comparison of the same micro-domain before and after processing. Combined with the changes in macroscopic mechanical parameters, the damage mechanism of adsorption expansion on mudstone and shale was quantitatively assessed.

2. The experimental method for carbon dioxide adsorption swelling damage of shale considering hydration state according to claim 1, characterized in that: In step 2, the constant humidity control treatment differs from the direct immersion method. Instead, the rock sample is placed in a constant temperature and humidity chamber or a sealed container controlled by a saturated salt solution. By setting different relative humidity levels, the interlayer spacing of clay mineral crystals reaches a specific number of hydrated layers, thereby obtaining graded hydration samples.

3. The experimental method for carbon dioxide adsorption swelling damage of shale considering hydration state according to claim 1, characterized in that: In step 3, the soaking environment temperature is set to be slightly higher than... The critical temperature and pressure are set as gradient values ​​of the pressure at the depth of the covered reservoir. After soaking, the pressure is reduced to atmospheric pressure by a low-speed continuous depressurization method to avoid damage to the non-adsorbent structure caused by instantaneous pressure difference.

4. The experimental method for carbon dioxide adsorption swelling damage of shale considering hydration state according to claim 1, characterized in that: In step 4, the microscopic observation method includes: pre-setting no less than 3 non-collinear micro-markers on the surface of the rock sample before the treatment in step 2; acquiring scanning electron microscope images of the same field of view before and after supercritical carbon dioxide immersion treatment; calculating the rotation matrix and translation vector using the coordinates of the markers, and accurately mapping the coordinates of the processed image back to the coordinate system before processing, so as to realize the reproduction of the same microcrack or pore region.

5. The experimental method for carbon dioxide adsorption swelling damage of shale considering hydration state according to claim 4, characterized in that: Step 4 also includes gray-scale quantitative analysis of the registered microscopic image: through gray-scale normalization, noise reduction, adaptive threshold segmentation and skeleton extraction, the crack area, aspect ratio, tortuosity and porosity index are automatically calculated, thereby quantifying the microstructure evolution caused by supercritical carbon dioxide immersion adsorption expansion.

6. The experimental method for carbon dioxide adsorption swelling damage of shale considering hydration state according to claim 1, characterized in that: In step 4, the macroscopic mechanical tests include uniaxial compression tests, Brazilian splitting tests, and porosity measurements; combined with digital image correlation technology, the full-field displacement and strain of the rock sample surface are monitored to obtain the evolution of elastic modulus, tensile strength, cohesion, and internal friction angle.

7. The experimental method for carbon dioxide adsorption swelling damage of shale considering hydration state according to claim 2, characterized in that: The different relative humidity levels include: 11%RH, 45%RH, 65%RH, and 95%RH.

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