A molecular dynamics simulation method for co2-ionized water flooding
By constructing a ternary displacement medium model of CO2, CaCl2 and H2O, and combining it with molecular dynamics simulation software, the microscopic mechanism problem of oil displacement under the synergistic effect of multiple displacement media in tight oil reservoirs was solved, and the oil recovery rate was improved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- XI'AN PETROLEUM UNIVERSITY
- Filing Date
- 2025-07-18
- Publication Date
- 2026-06-23
AI Technical Summary
Existing technologies are insufficient to effectively explain the microscopic mechanisms of oil displacement in tight reservoirs under the synergistic effect of multiple displacement media through molecular dynamics simulations, thus limiting the improvement of oil recovery.
A ternary displacement medium model consisting of CO2, CaCl2, and H2O was constructed. The displacement process was simulated and the displacement effect was calculated using the molecular dynamics simulation software LAMMPS and COMPASS II force fields.
This study aims to reveal the microscopic mechanism of oil displacement under the synergistic effect of multiple displacement media at the molecular level, optimize the development effect of tight oil reservoirs, and improve the recovery rate.
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Figure CN120808914B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of CO2 flooding technology, and more specifically to a molecular dynamics simulation method for CO2-ion water flooding. Background Technology
[0002] my country has huge reserves of tight oil reservoirs, but most of these reservoirs are highly heterogeneous, with low porosity and permeability and complex pore-throat structures. It is difficult to reveal the molecular-level mechanism of action from a microscopic perspective, as the crude oil displacement can only be observed from a macroscopic perspective.
[0003] For tight oil reservoir development, commonly used technologies include ion water flooding and CO2 flooding. Currently, research and application of ion water flooding and CO2 flooding technologies are relatively lagging, with many problems. Traditional experimental methods for understanding the ion concentration and CO2 displacement mechanism during the flooding process only involve adjusting experimental parameters to explore their effects in a specific simulated oil. For example, adjusting the ion concentration and pH value in water, and selecting different CO2 injection pressures and depressurization rates, etc., can lead to more rational injection schemes and process flows at a macroscopic level, improving oil and gas recovery and production, which has shown good results for tight oil reservoir development. Furthermore, the integrated application of these two technologies is relatively rare; more often, mechanistic analysis is conducted through experiments with a single displacement medium and reservoir numerical simulations.
[0004] In recent years, with the application of molecular dynamics simulations in the development process, many macroscopic production laws and reservoir seepage characteristics can be comprehensively calculated and simulated at the reservoir microstructure and pore scale using molecular dynamics software. Currently, most molecular dynamics simulation studies on the development effect of tight oil reservoirs simulate displacement through a single medium, directly observing the behavior of the displacement medium within nanopores at the molecular scale and calculating data such as density distribution, diffusion coefficient, mean square displacement, and interaction energy to provide a relatively reasonable explanation for molecular behavior. However, these studies cannot explain the behavior of molecules under the synergistic effect of multiple displacement media, and cannot well reflect the oil displacement microstructure mechanism under the synergistic effect of multiple displacement media, which is not conducive to further optimizing the development effect of tight oil reservoirs and improving recovery rate. Summary of the Invention
[0005] The purpose of this invention is to overcome the problems in the prior art and provide a molecular dynamics simulation method that can simulate CO2-ion water flooding. This invention uses CO2, CaCl2 and H2O to construct the displacement medium and build a new model system, which can better explain the behavior of molecules under the synergistic effect of multiple displacement media, reflect the oil displacement micro-mechanism under the synergistic effect of multiple displacement media, and has important significance for optimizing the development effect of tight oil reservoirs and improving the recovery rate.
[0006] This invention provides a molecular dynamics simulation method for CO2-ion water flooding, comprising the following steps:
[0007] Constructing microscopic models of tight oil reservoirs includes establishing water molecule models, CO2 molecule models, CaCl2 molecule models, as well as oil molecule models, graphene plate models, and rock pore wall models corresponding to the characteristics of the target oil reservoir.
[0008] The geometry of each molecular model was optimized using the COMPASS II force field.
[0009] The optimized water molecule model, CO2 molecule model, and CaCl2 molecule model were assembled into a "CO2-CaCl2-H2O" ternary system. This "CO2-CaCl2-H2O" ternary system was then placed on one side of the oil molecule model, and the rock pore wall model was fixed to the "CO2-CaCl2-H2O" ternary system. l2 The graphene plate was placed on one side of the "CO2-CaCl2-H2O" ternary system, with the upper and lower sides of the "-H2O" ternary system. Then, molecular dynamics simulation was performed. When performing molecular dynamics simulation, the NVT ensemble was selected, periodic boundary conditions were applied, COMPASS II force field was used and atomic potential parameters were configured. After running energy minimization and temperature initialization, the displacement process simulation was performed.
[0010] Based on the trajectory file output by the simulation, the displacement leading edge position, mean square displacement, radial distribution function and system energy change are calculated to quantitatively evaluate the oil displacement effect of the displacement medium in the "CO2-CaCl2-H2O" ternary system.
[0011] As a preferred method, the steps for constructing the displacement medium model of the “CO2-CaCl2-H2O” ternary system are as follows:
[0012] Within the molecular dynamics simulation software, a box containing multiple water molecule models, a box containing multiple CO2 molecule models, and a box containing multiple calcium and chloride ion CaCl2 molecule models were constructed respectively; the size of each box is consistent with the pore size of tight oil reservoirs.
[0013] When constructing the aforementioned box, it is necessary to model the water molecule, CO2 molecule, and Ca under the force field COMPASS II. 2+ and Cl - The SMART algorithm was used for structural optimization with a cutoff radius of 12 Å, a convergence criterion of 0.001 kcal / mol, and a step count of 5000. Then, an NVT system simulation was performed for 100 ns. After the simulation, water molecule model, CO2 molecule model, calcium ion model and chloride ion model were obtained.
[0014] The water molecule model, CO2 molecule model, and CaCl2 molecule model were combined into one box, and the structure was optimized using the SMART algorithm with a cutoff radius of 12 Å, a convergence criterion of 0.001 kcal / mol, and a step count of 5000. Then, an NVT system simulation with a duration of 100 ns was performed to obtain the displacement medium model of the "CO2-CaCl2-H2O" ternary system.
[0015] As a preferred approach, when optimizing the structure of oil molecules, the Task is Geometry Optimization, the Force Field is COMPASS II, the cutoff radius is 12 Å, the convergence criterion is set to 0.001 kcal / mol, the number of steps is 5000, the temperature is set to 300 K, and the pressure is set to 0.1 GPa to ensure model convergence while maintaining accuracy. Two to three geometric optimizations are performed. Subsequently, a 100 ns NVT system simulation is conducted for structural optimization.
[0016] As a preferred approach, the holding temperature is determined based on the actual reservoir conditions during molecular dynamics simulations and controlled using the Nosé-Hoover temperature control method.
[0017] As a preferred method, in molecular dynamics simulations, C8H 18 As a major component of crude oil.
[0018] As a preferred approach, when optimizing the geometry of each model, the structure of the oil molecules is optimized first.
[0019] As a preferred approach, during molecular dynamics simulations, a rightward movement is added to the graphene plate, allowing the displacing medium to migrate under the influence of the graphene plate, driving the oil phase to drain from the pores of the rock wall model.
[0020] Compared with the prior art, the beneficial effects of the present invention are:
[0021] This invention, from a molecular perspective, demonstrates significant optimization effects of ion water flooding and CO2 flooding on tight oil reservoir development. After constructing a ternary system of "CO2-CaCl2-H2O" for oil displacement, this invention, through displacement process and post-displacement data processing, reveals the microscopic interaction mechanism between the "CO2-CaCl2-H2O" ternary displacement medium and the reservoir at the molecular level. It simulates the dynamic displacement process of the oil phase in the pores of tight oil reservoir rocks, thereby analyzing the influence of different types and concentrations of displacement media on the displacement rate and degree. Compared with current molecular dynamics simulation studies using single displacement media, this invention achieves the use of multiple displacement media, combining molecular dynamics software with LAMMPAS, resulting in more precise force field assistance and parameterization. This allows for a better explanation of molecular behavior under the synergistic effect of multiple displacement media. Furthermore, multi-level optimization enables the study of the microscopic mechanism of oil displacement under the synergistic effect of multiple displacement media, which is of great significance for optimizing the development effect of tight oil reservoirs and improving recovery rates.
[0022] This invention differs from traditional experimental methods. Traditional experimental methods investigate the displacement mechanism of ion concentration and CO2 in oil displacement processes solely by adjusting experimental parameters, such as the ion concentrations of water and CO2, pH value, and different CO2 injection pressures and depressurization rates. This approach aims to propose more rational injection schemes and processes at the macroscopic level, thereby improving oil and gas recovery and production. This invention explains the mechanisms of action of each component in the displacement process at the microscopic level. Compared to experiments, it has lower overall economic costs, shorter time cycles, and provides visualized displacement results at the molecular level.
[0023] This invention provides a novel ternary composite displacement simulation method using molecular dynamics simulation, which clarifies the microscopic mechanism of ternary composite displacement and provides a reference for the future development of displacement technology. Attached Figure Description
[0024] Figure 1 This is a schematic diagram illustrating an embodiment of the present invention.
[0025] Figure 2 This is a diagram of the oil molecule structure according to an embodiment of the present invention.
[0026] Figure 3 This is a molecular structure diagram of the "CO2-CaCl2-H2O" ternary system according to an embodiment of the present invention.
[0027] Figure 4 This is a diagram of the rock wall structure according to an embodiment of the present invention.
[0028] Figure 5 This is a structural diagram of CO2-ion water-driven oil recovery according to an embodiment of the present invention.
[0029] Figure 6This is a structural diagram of a ternary system oil displacement model according to an embodiment of the present invention.
[0030] Figure 7 This is a displacement structure diagram according to an embodiment of the present invention.
[0031] Figure 8 These are displacement front diagrams of different displacement media in embodiments of the present invention.
[0032] Figure 9 This is a mean square displacement curve of the ternary system for oil displacement according to an embodiment of the present invention.
[0033] Figure 10 This is a radial distribution function curve of oil displacement by different displacement media according to an embodiment of the present invention. Detailed Implementation
[0034] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.
[0035] Unless otherwise defined, the technical or scientific terms used herein shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms “first,” “second,” and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as “comprising” or “including” indicate that the elements or objects preceding “comprising” or “including” encompass the elements or objects listed following “comprising” or “including” and their equivalents, and do not exclude other elements or objects. Terms such as “connected” or “linked” are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as “upper,” “lower,” “left,” and “right” are used only to indicate relative positional relationships; when the absolute position of the described objects changes, the relative positional relationship may also change accordingly.
[0036] like Figures 1-10 As shown in this embodiment, a molecular dynamics simulation method for CO2-ion water drive is provided, including the following steps:
[0037] Step 1: Model Building
[0038] An oil molecule model was constructed using molecular dynamics simulation software, creating a 24×28×80 Å box. Considering that tight oil is predominantly light oil and the typical characteristics of its density and composition, C8H was selected for this study. 18 As the main crude oil component, the oil film thickness was set to 5 nm. Then, using the Amorphous Cell Calculation module, 200 oil molecules were constructed, with the Task option set to Construction, the Quality option to Medium, and the force field to COMPASS II. The Forcite module was used to optimize the structure of the oil molecules using the SMART algorithm, with a cutoff radius of 12 Å, a convergence criterion of 0.001 kcal / mol, and a step count of 5000. A 100 ns NVT system simulation was then performed to obtain the oil molecule model.
[0039] A molecular model of H2O was constructed. Within the molecular dynamics simulation software, a 48×28×68Å box was built to construct a single water molecule model, with the chemical formula H2O. Then, using the Amorphous Cell Calculation module, 1800 water molecules were constructed, with the Task option set to Construction, the Quality option to Medium, and the force field to COMPASS II. The Forcite module was used to optimize the structure of the water molecules using the SMART algorithm, with a cutoff radius of 12Å, a convergence criterion of 0.001 kcal / mol, and a step count of 5000. A 100 ns NVT system simulation was then performed to obtain the water molecule model.
[0040] A CO2 molecule model was constructed using molecular dynamics simulation software. A 48×28×68Å box was built to create a single CO2 molecule model, with the chemical formula CO2. Then, using the Amorphous Cell Calculation module, 450 CO2 molecules were constructed, with the Task option set to Construction, the Quality option to Medium, and the force field to COMPASS II. The water molecule structure was optimized using the SMART algorithm in the Forcite module, with a cutoff radius of 12Å, a convergence criterion of 0.001 kcal / mol, and 5000 steps. A 100 ns NVT system simulation was then performed to obtain the water molecule model.
[0041] A CaCl2 model was constructed using molecular dynamics simulation software. A 48×28×68 Å box was built to construct calcium and chloride ion models, respectively, with the chemical formula CaCl2. Then, using the Amorphous Cell Calculation module, 75 CaCl2 ions were constructed.2+ 150 Cl - In Properties, under Charge, set the charge amount: +2 for calcium ions and -1 for chloride ions. Select Construction for Task, Medium for Quality, and COMPASS II for the force field. Use the SMART algorithm in the Forcite module for structure optimization of the ions, with a cutoff radius of 12 Å, a convergence criterion of 0.001 kcal / mol, and 5000 steps. Then, perform a 100 ns NVT system simulation. The calcium ion Ca2+ is obtained after this simulation. 2+ Chloride ions Cl - Model;
[0042] A rock wall model was constructed, and the SiO2_quartz (silica-quartz crystal) structure was imported into the molecular dynamics simulation software to construct a quartz surface with dimensions (xyz) of 2.8×10.00×1.2 nm. Hydroxystocidal quartz nanopores were symmetrically constructed to simulate the wall model of a tight oil reservoir. Based on the characteristics of low-porosity and low-permeability reservoirs, the pore width was set to 2.8 nm.
[0043] A graphene plate model was constructed. The graphene molecular model was constructed using the graphite software in the molecular dynamics simulation. The graphene monolayer unit was obtained by cutting the clear surface using the Build module. The graphene surface area was constructed using the Supercell module of the Build module, with a total of two layers of graphene with a surface area of 2.8nm×4.8nm and a spacing of 0.5nm between each graphene layer.
[0044] Using molecular dynamics software, the Amorphous Cell Calculation module was used to construct the CO2, CaCl2, and H2O molecules from the above steps into a single container, obtaining a "CO2-CaCl2-H2O" ternary system model to simulate the types of molecules contained in the displacement medium. Based on the above model construction, the SMART algorithm was used in the Forcite module to optimize the structure of the system, with a convergence criterion of 0.001 kcal / mol and a step count of 5000. Subsequently, a 100 ns NVT system simulation was performed to obtain the oil displacement medium model of the "CO2-CaCl2-H2O" ternary system, thus providing a basic model for the analysis of the displacement mechanism of this system on crude oil in tight oil reservoirs.
[0045] Step 2: Optimization of Force Field Parameters
[0046] Based on actual reservoir conditions, the temperature was maintained at 344.15 K and controlled using the Nosé-Hoover temperature control method. The simulation was conducted in the NVT ensemble (with constant molecular number, volume, and temperature). To obtain a reasonable initial oil phase, the C8H phase was first analyzed using the Forcite module. 18 Structure optimization was performed using the SMART algorithm, with a convergence criterion of 0.001 kcal / mol, a cutoff radius of 12 Å, and a step count of 5000. This was followed by a 100 ns NVT system simulation for further structure optimization. The "CO2-CaCl2-H2O" ternary system was then placed in a C8H... 18 To the left of the phase, rock walls were placed above and below the "CO2-CaCl2-H2O" ternary system, close to the system to simulate the pore structure of actual tight oil reservoirs. They were fixed using the Modify module's Constraints to prevent wall migration during displacement. A graphene plate was placed to the left of the "CO2-CaCl2-H2O" ternary system. Subsequent rightward movement of the graphene plate allowed the displacement medium to migrate under its influence, driving the oil phase out of the pores formed by the rock walls. This was also fixed using the Modify module's Constraints for subsequent molecular dynamics equilibrium calculations. The Forcite module was then used to optimize the overall system structure, adjusting the initial structure to a physically reasonable state to ensure simulation stability, efficiency, and result reliability. The force field was CVFF, and the SMART algorithm was used with a convergence criterion of 0.001 kcal / mol and a step count of 5000.
[0047] Step 3, Software Simulation
[0048] In Lammps, basic model settings were configured: Units were set to Real units, Boundary to ppp periodic boundary, Atom_style to Full all-atom type, time step to 1 fs, and adjacent cutoff distance to 2 bins. Potential parameters were set according to the system file: Pair_style was set to lj / cut potential, Bond_style, Angle_style, and Dihedral_style to Harmonic, and detailed settings were configured for each atom potential parameter. Energy minimization and temperature initialization were performed on the system using the NVT ensemble, with the temperature controlled at 300K, for 10,000 relaxation steps. Subsequently, under the NVT ensemble, with the temperature controlled at 300K, a positive displacement of 0.001 Å per step was added to the graphene plate to drive the displacement of the ternary system, and after 80,000 steps, the coordinates of each atom were output.
[0049] Step 4, Data Processing
[0050] The required data is visualized and processed using OVITO, and calculations and analyses are performed to output radial distribution function and mean square displacement, etc.
[0051] Evaluation of oil displacement effect:
[0052] By studying the displacement of crude oil by the simulated "CO2-CaCl2-H2O" ternary composite system in a molecular dynamics force field, and using performance evaluation parameters such as mean square displacement, crude oil displacement front diagram, and radial distribution function, the oil displacement effect of the "CO2-CaCl2-H2O" ternary composite system is evaluated, thereby reflecting the impact of using this oil displacement method on the development effect of tight oil reservoirs in molecular dynamics software.
[0053] This invention, based on the trajectory file of the simulated "CO2-CaCl2-H2O" ternary composite system in LAMMPS, uses the OVITO visualization tool to plot the dynamic map of crude oil stripping in pores at different displacement times. Displacement front diagrams for different displacement media are shown below. Figure 8 As shown. Compared with the pure water system and the "H2O+CO2" system, the "CO2-CaCl2-H2O" ternary composite system proposed in this invention has a better degree of crude oil stripping under the same time, less residual oil in rock pores, and a better oil displacement effect.
[0054] This invention utilizes a trajectory file of a simulated "CO2-CaCl2-H2O" ternary composite system in LAMMPS. The Compute command can be used to obtain the mean square displacement (MSD) of crude oil during the displacement process within this composite system. Changes in the MSD can reveal the microscopic motion characteristics of crude oil molecules. The MSD measures the deviation of a particle's position from a reference position over time. The slope of the MSD curve indicates the molecular mobility. A steeper slope indicates stronger molecular mobility and faster diffusion. The MSD curve for the ternary composite system is shown below. Figure 9 As shown. Compared with the pure water system and the "H2O + CO2" system, the "CO2-CaCl2-H2O" ternary composite system proposed in this invention has stronger diffusion ability and better oil displacement effect.
[0055] This invention, based on the trajectory file of the simulated "CO2-CaCl2-H2O" ternary composite system in LAMMPS, can analyze the movement of the displacement medium in the composite system. By analyzing the radial distribution function, it can differentiate the oil displacement effect of the "CO2-CaCl2-H2O" ternary composite system from other single or composite systems. The radial distribution function reflects the atomic distribution around a specified atom. The radial function distribution curve is shown below. Figure 10As shown. Compared with the pure water system and the "H2O+CO2" system, the radial distribution function of the "CO2-CaCl2-H2O" ternary composite system proposed in this invention has a higher peak value, which means that the system can maintain a higher binding strength with crude oil during the oil displacement process and achieve a better oil displacement effect.
[0056] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.
Claims
1. A molecular dynamics simulation method for CO2-ion water flooding, characterized in that, Includes the following steps: Constructing microscopic models of tight oil reservoirs includes establishing water molecule models, CO2 molecule models, CaCl2 molecule models, as well as oil molecule models, graphene plate models, and rock pore wall models corresponding to the characteristics of the target oil reservoir. The geometry of each molecular model was optimized using the COMPASS II force field. The optimized water molecule model, CO2 molecule model, and CaCl2 molecule model were assembled into a "CO2-CaCl2-H2O" ternary system. This system was then placed on one side of the oil molecule model, with rock pore wall models fixed above and below it. A graphene plate was placed on the side of the "CO2-CaCl2-H2O" ternary system furthest from the oil molecule model. Molecular dynamics simulations were then performed, with a force applied to the graphene plate to move towards the oil molecule model. This allowed the displacement medium to migrate under the influence of the graphene plate, driving the oil phase out of the pores formed by the rock walls. During the molecular dynamics simulation, the NVT ensemble was selected, periodic boundary conditions were applied, a COMPASS II force field was used, and atomic potential parameters were configured. After energy minimization and temperature initialization, the displacement process simulation was performed. Based on the trajectory file output by the simulation, the displacement leading edge position, mean square displacement, radial distribution function and system energy change are calculated to quantitatively evaluate the oil displacement effect of the displacement medium in the "CO2-CaCl2-H2O" ternary system.
2. The molecular dynamics simulation method for CO2-ion water flooding as described in claim 1, characterized in that, The steps for constructing the displacement medium model of the "CO2-CaCl2-H2O" ternary system are as follows: Within the molecular dynamics simulation software, a box containing multiple water molecule models, a box containing multiple CO2 molecule models, and a box containing multiple calcium and chloride ion CaCl2 molecule models were constructed respectively; the size of each box is consistent with the pore size of tight oil reservoirs. When constructing the aforementioned box, it is necessary to model the water molecule, CO2 molecule, and Ca under the force field COMPASS II. 2+ and Cl - The SMART algorithm was used for structural optimization with a cutoff radius of 12 Å, a convergence criterion of 0.001 kcal / mol, and a step count of 5000. Then, an NVT system simulation was performed for 100 ns. After the simulation, water molecule model, CO2 molecule model, calcium ion model and chloride ion model were obtained. The water molecule model, CO2 molecule model, and CaCl2 molecule model were combined into one box, and the structure was optimized using the SMART algorithm with a cutoff radius of 12 Å, a convergence criterion of 0.001 kcal / mol, and a step count of 5000. Then, an NVT system simulation with a duration of 100 ns was performed to obtain the displacement medium model of the "CO2-CaCl2-H2O" ternary system.
3. The molecular dynamics simulation method for CO2-ion water flooding as described in claim 1, characterized in that, When optimizing the structure of oil molecules, the task was Geometry Optimization, the force field was COMPASS II, the cutoff radius was 12 Å, the convergence criterion was set to 0.001 kcal / mol, the number of steps was 5000, the temperature was set to 300 K, and the pressure was set to 0.1 GPa to ensure model convergence while maintaining accuracy. Two to three geometric optimizations were performed. Subsequently, a 100 ns NVT system simulation was conducted for structural optimization.
4. The molecular dynamics simulation method for CO2-ion water flooding as described in claim 1, characterized in that, During molecular dynamics simulations, the holding temperature is determined based on actual reservoir conditions and controlled using the Nosé-Hoover temperature control method.
5. The molecular dynamics simulation method for CO2-ion water flooding as described in claim 1, characterized in that, In molecular dynamics simulations, C8H 18 As a major component of crude oil.
6. The molecular dynamics simulation method for CO2-ion water flooding as described in claim 1, characterized in that, When optimizing the geometry of each model, the structure of oil molecules is optimized first.
7. The molecular dynamics simulation method for CO2-ion water flooding as described in claim 1, characterized in that, During the molecular dynamics simulation, the graphene plate was placed on the left side of the "CO2-CaCl2-H2O" ternary system. A force was applied to the graphene plate to move it to the right, causing the displacing medium to move under the push of the graphene plate, driving the oil phase to be discharged from the pores of the rock wall model.