Molecular simulation method for cracking behavior of mudstone in drawdown zone of strongly weathered reservoir slope

Abstract

The present invention is applicable to the technical field of bank slope protection, and in particular to a molecular simulation research method for cracking behavior of mudstone in the drawdown zone of a strongly weathered reservoir bank slope, the method comprising: determining the mineral composition of purple-red strongly weathered mudstone in the drawdown zone to obtain the determination result; establishing a microscopic model of mudstone cracks; simulating the adsorption of water molecules on the surface of montmorillonite crystal layers in the microscopic model of mudstone cracks; performing molecular dynamics simulation on the mudstone crack model of adsorbed water molecules; drawing images of the microstructure of mudstone cracks under different water concentrations and outputting them. The present invention obtains the coordinates and velocities of all atoms in the system at any time through Newton's laws of motion, and then uses statistical mechanics methods to calculate the relevant physical quantities and thermodynamic properties of the system, etc., which can clearly reveal the mechanism of generation and expansion of cracks in strongly weathered mudstone, quantitatively describe the interaction between clay minerals and feldspar and quartz in mudstone, and provide a theoretical reference for the modification of strongly weathered mudstone in the drawdown zone and the protection of the bank slope drawdown zone.

Description

Technical Field

[0001] The invention belongs to the technical field of bank slope protection, and in particular relates to a molecular simulation research method for cracking behavior of mudstone in a drawdown zone of a strongly weathered reservoir bank slope. Background Art

[0002] Strongly weathered purple-red mudstone is widely distributed on the slopes of the Three Gorges Reservoir. During the operation of the reservoir, the water level rises and falls repeatedly in the drawdown zone (water level 145 meters-175 meters) throughout the year, causing the strongly weathered mudstone to be in an environment of periodic "dry and wet" alternation.

[0003] At present, the research on the disintegration phenomenon of strongly weathered mudstone under the action of "wet-dry" alternation is mainly focused on indoor disintegration experiments and characterization tests of mudstone disintegration process. The results show that after the mudstone has experienced saturated state and dry state under different slope stress states (0 MPa, 0.5 MPa, 1.0 MPa), the surface cracks gradually expand. With the continuous increase of "wet-dry" alternation, the crack width continues to increase. At the same time, the unevenly distributed clay minerals in the strongly weathered mudstone expand when they encounter water, which also accelerates the expansion of cracks, and finally causes the mudstone to disintegrate into small fragments along the cracks.

[0004] Although the experimental results can clearly observe the disintegration phenomenon of strongly weathered mudstone under the alternating action of "dry and wet", the generation and expansion mechanism of cracks in mudstone under the action of water, such as how the expansive clay in mudstone expands after encountering water, how it shrinks after losing water, and the interaction with surrounding minerals (quartz, feldspar, etc.) during the expansion process cannot be clearly expressed. Summary of the invention

[0005] The purpose of the embodiment of the present invention is to provide a molecular simulation research method for the cracking behavior of mudstone in the drawdown zone of a strongly weathered reservoir slope, aiming to solve the problems raised in the third part of the background technology.

[0006] The embodiment of the present invention is implemented as follows: a molecular simulation research method for cracking behavior of mudstone in the drawdown zone of a strongly weathered reservoir slope, the method comprising:

[0007] The mineral composition of the purple-red strongly weathered mudstone in the water-falling zone was determined by X-ray diffraction spectrometer, and the results were obtained.

[0008] According to the test results, a microscopic model of mudstone fractures is established, which includes montmorillonite crystal layers with expansion characteristics and silica crystals, which are the main components of feldspar and quartz.

[0009] The adsorption of water molecules on the surface of montmorillonite crystal layers in the microscopic model of mudstone fractures was simulated;

[0010] Molecular dynamics simulations were performed on the mudstone fracture model with adsorbed water molecules at different substrate spacings;

[0011] Draw images of the microstructure of mudstone fractures at different water concentrations and output them.

[0012] Preferably, LAMMPS software is used to establish a microscopic model of mudstone fractures.

[0013] Preferably, the adsorption of water molecules on the surface of montmorillonite crystal layers in the mudstone fracture micromodel is simulated using the grand canonical Monte Carlo simulation method in LAMMPS.

[0014] Preferably, LAMMPS software is used for molecular dynamics simulation, wherein the molecular force field of montmorillonite and silica adopts CLAYFF force field, and the water molecule adopts single point charge SPC / E force field.

[0015] Preferably, the step of performing molecular dynamics simulation specifically includes:

[0016] The model was geometrically optimized and dynamic simulation was performed under the NPT ensemble with a temperature of 300 K and a pressure of 0.1 MPa to obtain a low-energy conformation and store the information in a dump file.

[0017] Preferably, the dump file is read using OVITO software, and the mudstone fracture structure under different water concentrations is plotted, and the influence of the adsorption behavior of water molecules on the montmorillonite surface on the spacing between the montmorillonite crystal layer and the silica substrate is analyzed by density distribution, radial distribution function, and adsorption energy change.

[0018] Preferably, in the microscopic model of mudstone fractures, the montmorillonite crystal molecular formula is Na 0.75 (Si 7.75 Al 0.25 )(Al 3.5 Mg 0.5 ) 20 (OH)4·nH2O, the montmorillonite model follows the rule that any two atomic substitution sites are not adjacent, and there is 1 Al replacement for every 32 Si in the silicon-oxygen tetrahedron and 1 Mg replacement for every 8 Al in the aluminum-oxygen octahedron, establishing a 4a×2b×1c montmorillonite supercell structure.

[0019] The molecular simulation research method for the cracking behavior of mudstone in the drawdown zone of a strongly weathered reservoir slope provided in an embodiment of the present invention obtains the coordinates and velocities of all atoms in the system at any time through Newton's laws of motion, and then uses statistical mechanics methods to calculate relevant physical quantities and thermodynamic properties of the system. It can clearly reveal the mechanism of generation and expansion of cracks in strongly weathered mudstone, quantitatively describe the interaction between clay minerals and feldspar and quartz in mudstone, and provide a theoretical reference for the modification of strongly weathered mudstone in the drawdown zone and the protection of the drawdown zone of the slope. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1A molecular simulation flow chart of a molecular simulation research method for cracking behavior of mudstone in the drawdown zone of a strongly weathered reservoir slope provided by the present invention;

[0021] Figure 2 A molecular simulation research method for cracking behavior of mudstone in the drawdown zone of a strongly weathered reservoir slope provided by an embodiment of the present invention, a structural snapshot image of the water absorption and expansion of a montmorillonite crystal layer;

[0022] Figure 3 A molecular simulation research method for cracking behavior of mudstone in the drawdown zone of a strongly weathered reservoir slope provided in an embodiment of the present invention provides a structural snapshot image of water loss shrinkage of montmorillonite crystal layers. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0024] The specific implementation of the present invention is described in detail below in conjunction with specific embodiments.

[0025] like Figure 1 As shown, the present invention is a molecular simulation research method for cracking behavior of mudstone in the drawdown zone of a strongly weathered reservoir bank slope, which is described in detail below in conjunction with specific examples and drawings. The process is as follows:

[0026] The mineral composition of the purple-red strongly weathered mudstone in the drawdown zone of the Three Gorges Reservoir was determined by X-ray diffraction spectrometer, in which montmorillonite, illite, kaolin, quartz, feldspar, chlorite and calcite accounted for 31.8%, 28.1%, 12.4%, 10.4%, 10.8%, 4.8% and 1.7% respectively.

[0027] According to the XRD measurement results, the LAMMPS software was used to establish a microscopic model of mudstone fractures containing montmorillonite crystal layers with expansion characteristics and silicon dioxide crystals, the main components of non-clay minerals feldspar and quartz. Among them, the molecular formula of montmorillonite crystals is Na 0.75 (Si 7.75 Al 0.25 )(Al 3.5 Mg 0.5 ) 20 (OH)4·nH2O, the montmorillonite model follows the rule that any two atomic substitution sites are not adjacent, and there is 1 Al replacement for every 32 Si in the silicon-oxygen tetrahedron and 1 Mg replacement for every 8 Al in the aluminum-oxygen octahedron, establishing a 4a×2b×1c montmorillonite supercell structure.

[0028] The grand canonical Monte Carlo simulation method in LAMMPS was used to simulate the adsorption of water molecules on the surface of montmorillonite crystal layers in the mudstone fracture micromodel.

[0029] LAMMPS software was used to perform molecular dynamics simulation on the mudstone fracture model with adsorbed water molecules at different substrate spacings. The molecular force field of montmorillonite and silica was CLAYFF force field, and the single point charge SPC / E force field was used for water molecules. The specific simulation steps were as follows: first, the model was geometrically optimized, and then dynamic simulation was performed under the NPT (isothermal and isobaric) ensemble, with a temperature of 300 K and a pressure of 0.1 MPa, to obtain a reasonable low-energy conformation, and the information was stored in the dump file;

[0030] Read the dump file, draw images of the microstructure of mudstone fractures under different water concentrations and output them.

[0031] like Figure 2 and Figure 3 As shown in the figure: In a wet environment, the montmorillonite crystal layer continuously absorbs water and expands, and the first hydration layer, the second hydration layer and the third hydration layer appear on the surface successively. The more water molecules on the montmorillonite surface, the farther the distance between montmorillonite and montmorillonite and the distance between montmorillonite and silica. In a dry environment, the montmorillonite crystal layer continuously loses water and shrinks. Since the interaction energy between montmorillonite crystal layers is much stronger than the interaction energy between montmorillonite and silica crystals, during the dehydration process, the upper montmorillonite crystal layer will gradually approach the lower montmorillonite crystal layer, making the distance between montmorillonite and silica increase continuously, forming initial cracks. Subsequently, the statistical mechanics method is used to qualitatively and quantitatively analyze the effects of montmorillonite water absorption and expansion, dehydration shrinkage on the basal distance between montmorillonite crystal layers and silica crystals through density distribution, radial distribution function and adsorption energy, and explain the mechanism of crack generation and expansion of mudstone microcracks in the "dry-wet" alternating environment.

[0032] The simulation results show that under the alternating action of "dry and wet", the water absorption and expansion and water loss and shrinkage characteristics of montmorillonite make it easy for the initial microcracks of mudstone to occur at the junction of clay mineral montmorillonite and non-clay minerals (feldspar, quartz, etc.), and the cracks will continue to expand as the montmorillonite loses water and shrinks.

[0033] The amount of mudstone disintegration in water was determined, and the disintegration experiment under the action of dry-wet alternation confirmed the disintegration behavior of strongly weathered mudstone, which poses a threat to the safety of the slope of the reservoir in the drawdown zone and needs to be protected. The water absorption rate and disintegration amount of mudstone under multiple dry-wet alternations were studied through mudstone water absorption rate tests and particle screening experiments, and the curve was fitted to obtain the disintegration law of mudstone.

[0034] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A molecular simulation research method for cracking behavior of mudstone in the drawdown zone of a strongly weathered reservoir slope, characterized in that: The method comprises: The mineral composition of the purple-red strongly weathered mudstone in the water-falling zone was determined by X-ray diffraction spectrometer, and the results were obtained. According to the test results, a microscopic model of mudstone fractures is established, which includes montmorillonite crystal layers with expansion characteristics and silica crystals, which are the main components of feldspar and quartz. The adsorption of water molecules on the surface of montmorillonite crystal layers in the microscopic model of mudstone fractures was simulated; Molecular dynamics simulations were performed on the mudstone fracture model with adsorbed water molecules at different substrate spacings; Draw and output images of the microstructure of mudstone fractures at different water concentrations; The LAMMPS software was used to establish a microscopic model of mudstone fractures; Molecular dynamics simulations were performed using LAMMPS software, where the molecular force fields of montmorillonite and silica were CLAYFF force fields, and the single point charge SPC / E force field was used for water molecules; The steps of molecular dynamics simulation include: The model was geometrically optimized and dynamic simulation was performed under the NPT ensemble at a temperature of 300 K and a pressure of 0.1 MPa to obtain a low-energy conformation and store the information in a dump file; OVITO software was used to read the dump file and draw the mudstone fracture structure under different water concentrations. The influence of the adsorption behavior of water molecules on the montmorillonite surface on the distance between the montmorillonite crystal layer and the silica substrate was analyzed through density distribution, radial distribution function and adsorption energy change.

2. The molecular simulation research method for cracking behavior of mudstone in the drawdown zone of a strongly weathered reservoir slope according to claim 1 is characterized in that: The grand canonical Monte Carlo simulation method in LAMMPS was used to simulate the adsorption of water molecules on the surface of montmorillonite crystal layers in the mudstone fracture micromodel.

3. The molecular simulation research method for cracking behavior of mudstone in the drawdown zone of a strongly weathered reservoir slope according to claim 1 is characterized in that: In the microscopic model of mudstone fractures, the molecular formula of montmorillonite crystal is Na 0.75 (Si 7.75 Al 0.25 )(Al 3.5 Mg 0.5 ) 20 (OH)4·nH2O, the montmorillonite model follows the rule that any two atomic substitution sites are not adjacent, and there is 1 Al replacement for every 32 Si in the silicon-oxygen tetrahedron and 1 Mg replacement for every 8 Al in the aluminum-oxygen octahedron, establishing a 4a×2b×1c montmorillonite supercell structure.

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