Molecular dynamics simulation method for CO2-ion water flooding

Abstract

The invention discloses a molecular dynamics simulation method for CO2-ion oil displacement, and belongs to the technical field of CO2 oil displacement. The method comprises the steps that an oil molecule model, a water molecule model, a CO2 molecule model, a CaCl2 molecule model and a graphene plate model corresponding to target oil reservoir characteristics are established; carrying out geometric structure optimization on each model; the optimized molecular model is assembled into a 'CO2-CaCl2-H2O' ternary displacement system, and molecular dynamics simulation is executed; during simulation, an NVT ensemble is selected, periodic boundary conditions are applied, a COMPASS II force field is adopted, atomic potential parameters are configured, and after operation energy is minimized and temperature is initialized, displacement process simulation is executed; and quantitatively evaluating the oil displacement effect of the displacement medium of the 'CO2-CaCl2-H2O' ternary system on the basis of the trajectory file output by simulation. According to the method, the behavior of molecules under the synergistic effect of multiple displacement media can be better explained, the oil displacement micromechanism under the synergistic effect of the multiple displacement media is embodied, and the method has important significance for optimizing the compact oil reservoir development effect and improving the recovery efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of CO2 flooding, in particular to a CO2-ion water flooding molecular dynamics simulation method. BACKGROUND

[0002] The tight oil reservoir reserves in China are huge, but most of the tight oil reservoirs have strong heterogeneity, low porosity and permeability, and complex pore throat structure, so it is difficult to observe the surface of oil displacement from a macroscopic perspective and reveal the action mechanism at the molecular level from a microscopic perspective.

[0003] For the development direction of tight oil reservoirs, the commonly used technologies include ion water flooding and CO2 flooding. At present, the research and application of ion water flooding and CO2 flooding technologies are relatively lagging behind, and there are many problems. The traditional experimental method can only explore the effect of ion concentration and CO2 displacement mechanism in a certain simulated oil by adjusting the experimental parameters, such as adjusting the ion concentration and pH value in water, selecting different CO2 injection pressures and depressurization rates, so as to propose a more reasonable injection scheme and process flow at the macroscopic level, improve the oil and gas recovery and production, and have a good effect on the development of tight oil reservoirs. And the comprehensive application of the two technologies is less, and more is the mechanism analysis through single displacement medium experiment and reservoir numerical simulation.

[0004] In recent years, with the application of molecular dynamics simulation in the development process, many macroscopic production laws and reservoir percolation characteristics can be calculated and simulated at the reservoir microstructure and pore scale by using molecular dynamics related software. At present, the molecular dynamics simulation research on the development effect of tight oil reservoirs is mostly through single medium simulation displacement, and the behavior of displacement medium in nanometer pores is directly observed through molecular scale, and the density distribution, diffusion coefficient, mean square displacement, interaction energy and other data are calculated to reasonably explain the behavior of molecules, but it cannot explain the behavior of molecules under the synergistic action of multiple displacement media, and cannot well reflect the micro mechanism of oil displacement under the synergistic action of multiple displacement media, which is not conducive to further optimizing the development effect of tight oil reservoirs and improving the recovery rate. SUMMARY

[0005] The purpose of the present application is to overcome the problems in the prior art, and to provide a molecular dynamics simulation method capable of simulating CO2-ion water flooding. The present application uses CO2, CaCl2 and H2O to construct displacement medium and construct a new model system, which can better explain the behavior of molecules under the synergistic action of multiple displacement media, reflect the micro mechanism of oil displacement under the synergistic action of multiple displacement media, optimize the development effect of tight oil reservoirs, and has important significance for improving the recovery rate.

[0006] The application provides a CO2-ion water flooding molecular dynamics simulation method. The dense oil reservoir microcosmic model is constructed, including establishment of a water molecule model, a CO2 molecule model, a CaCl2 molecule model, and an oil molecule model corresponding to the characteristics of the target oil reservoir, a graphene plate model and a rock pore wall model; The COMPASS II force field is used to optimize the geometric structure of each molecule model; The optimized water molecule model, CO2 molecule model and CaCl2 molecule model are assembled into a "CO2-CaCl2-H2O" ternary system, then the "CO2-CaCl2-H2O" ternary system is placed on one side of the oil molecule model, and the rock pore wall model is fixed on the upper side and the lower side of the "CO2-CaCl2-H2O" ternary system respectively, the graphene plate is placed on one side of the "CO2-CaCl2-H2O" ternary system, and then the molecular dynamics simulation is performed; l2 When the molecular dynamics simulation is performed, the NVT ensemble is selected, the periodic boundary condition is applied, the COMPASS II force field is used and the atomic potential parameters are configured, after energy minimization and temperature initialization, the displacement process simulation is performed; Based on the trajectory file of the simulation output, the displacement front position, the mean square displacement, the radial distribution function and the system energy change are calculated, and the oil displacement effect of the displacement medium of the "CO2-CaCl2-H2O" ternary system is quantitatively evaluated.

[0007] As a preferred mode, the steps of constructing the displacement medium model of the "CO2-CaCl2-H2O" ternary system are as follows: In the molecular dynamics simulation software, a box containing a plurality of water molecule models, a box containing a plurality of CO2 molecule models and a CaCl2 molecule model containing a plurality of calcium ions and chlorine ions are constructed respectively; the size of each box is consistent with the pore size of the dense oil reservoir; When the above box is constructed, the water molecule, CO2 molecule model and CaCl2 molecule model need to be optimized under the force field COMPASS II, the structure is optimized by using the SMART algorithm, the cutoff radius is 12Å, the convergence standard is set to 0.001 kcal / mol, the step number is 5000, and then the NVT system simulation of 100 ns is performed, and the water molecule model, CO2 molecule model and calcium ion and chlorine ion model are obtained respectively; 2+ and Cl - , using the SMART algorithm for structure optimization, the cutoff radius is 12Å, the convergence standard is set to 0.001 kcal / mol, the step number is 5000, and then the NVT system simulation of 100 ns is performed, and the water molecule model, CO2 molecule model and calcium ion and chlorine ion model are obtained respectively; The water molecule model, the CO2 molecule model and the CaCl2 molecule model are adjusted into a box, a structure optimization is carried out by using a SMART algorithm, a truncation radius is 12Å, a convergence standard is set to 0.001 kcal / mol, a step number is 5000 steps, and then, a 100 ns long NVT system simulation is carried out, so that a displacement medium model of a "CO2-CaCl2-H2O" ternary system is obtained.

[0008] As a preferred mode, when the oil molecule is subjected to the structure optimization, the Task selects Geometry Optimization, the force field selects COMPASS II, the truncation radius is 12Å, the convergence standard is set to 0.001 kcal / mol, the step number is 5000 steps, the temperature is set to 300 K, the pressure is set to 0.1 GPa, the model is ensured to converge while ensuring the accuracy, and the geometry optimization is carried out for 2-3 times; and then, the 100 ns long NVT system simulation structure optimization is carried out.

[0009] As a preferred mode, when the molecular dynamics simulation is carried out, the temperature is determined according to the actual oil reservoir condition, and the temperature is controlled by using a Nosé-Hoover temperature control method.

[0010] As a preferred mode, when the molecular dynamics simulation is carried out, the C8H 18 As a main component of crude oil.

[0011] As a preferred mode, when the geometry structure of each model is optimized, the oil molecule is first subjected to the structure optimization.

[0012] As a preferred mode, when the molecular dynamics simulation is carried out, the right movement is added to the graphene plate, so that the displacement medium can be moved under the push of the graphene plate, and the oil phase is driven to be discharged from the pore of the rock wall model.

[0013] Compared with the prior art, the present application has the beneficial effects that: The application has obvious optimization effect on development of tight oil reservoirs from the molecular perspective based on ion water flooding and CO2 flooding, after oil displacement of the ternary system of "CO2-CaCl2-H2O" is constructed, the micro interaction mechanism of the ternary displacement medium of "CO2-CaCl2-H2O" and the oil reservoir can be revealed from the molecular level through displacement process and data processing after displacement, the dynamic displacement process of the oil phase in the rock pore of the tight oil reservoir is simulated, and the influence of different displacement medium types and concentrations on the oil displacement speed and displacement degree is analyzed, compared with the current single displacement medium molecular dynamics simulation research, the application realizes the combination of the molecular dynamics software and LAMMPAS by using the multiple displacement medium, the force field auxiliary and parameterization are more accurate, the behavior of the molecules under the synergistic action of the multiple displacement medium can be better explained, and the micro mechanism research of the oil displacement under the synergistic action of the multiple displacement medium is realized through the multiple level optimization, the development effect optimization of the tight oil reservoir is important for improving the recovery ratio.

[0014] The application is different from the experimental method, the traditional experimental method only explores the displacement mechanism in the crude oil by adjusting the experimental parameter method, such as adjusting the ion concentration and pH value of water and CO2, selecting different CO2 injection pressure and pressure reduction rate, and the like, so that a more reasonable injection scheme and process flow are proposed in the macro level, and the oil and gas recovery ratio and yield are improved.

[0015] The application provides a new ternary composite oil displacement simulation method by using the molecular dynamics simulation, the micro mechanism of the ternary composite displacement is clear, and the reference for the development of future oil displacement technology is provided. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is the schematic diagram of the embodiment of the application.

[0017] Figure 2 is the oil molecule structure diagram of the embodiment of the application.

[0018] Figure 3 is the ternary system molecular structure diagram of "CO2-CaCl2-H2O" of the embodiment of the application.

[0019] Figure 4 is the rock wall surface structure diagram of the embodiment of the application.

[0020] Figure 5 is the CO2-ion water oil displacement structure diagram of the embodiment of the application.

[0021] Figure 6is a three-component system oil displacement model structure diagram of the embodiment of the present application.

[0022] Figure 7 is a displacement structure diagram of the embodiment of the present application.

[0023] Figure 8 is a displacement front diagram of different displacement media of the embodiment of the present application.

[0024] Figure 9 is a three-component system oil displacement average displacement curve diagram of the embodiment of the present application.

[0025] Figure 10 is a different displacement medium oil displacement radial distribution function curve diagram of the embodiment of the present application. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical scheme and advantages of the embodiments of the present application clearer, the technical scheme of the embodiments of the present application will be described clearly and completely below with reference to the drawings of the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the described embodiments of the present application, all other embodiments obtained by those of ordinary skill in the art without any creative effort fall within the scope of protection of the present application.

[0027] Unless otherwise defined, technical terms or scientific terms used herein should be understood as having the common meaning in the field of the present application to which they pertain. The terms "first", "second", and similar terms used herein do not denote any order, quantity, or importance, but are used to distinguish different components. The terms "include" or "contain" and similar terms mean that the elements or objects before the "include" or "contain" cover the elements or objects listed after the "include" or "contain" and their equivalents, and do not exclude other elements or objects. The terms "connect" or "connected" and similar terms are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. The terms "up", "down", "left", "right", and the like are used only to represent relative positional relationships, and when the absolute positions of the described objects change, the relative positional relationships may also change accordingly.

[0028] As shown in Figures 1-10 In the present embodiment, a CO2-ion water displacement molecular dynamics simulation method is provided, comprising the following steps: Step one, model establishment An oil molecule model is constructed. In the molecular dynamics simulation software, a box of 24x28x80 Å is constructed. Considering that tight oil is mainly light oil, and the typical characteristics of tight oil density and composition, C8H 18As the main component of crude oil, the thickness of the oil film is set to 5 nm. Then use Amorphous Cell Calculation module, build 200 oil molecules, Task option Construction, Quality option Medium, force field COMPASS II. Use Forcite module 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 number of 5000 steps, followed by a 100 ns long NVT system simulation, and the oil molecule model is obtained after completion;

[0029] Constructing a H2O molecule model, a 48x28x68 Å box is constructed in the molecular dynamics simulation software, and a single water molecule model is constructed, with the chemical formula H2O. Then use Amorphous Cell Calculation module, build 1800 water molecules, Task option Construction, Quality option Medium, force field COMPASS II. Use Forcite module to optimize the structure of the water molecules using the SMART algorithm, with a cutoff radius of 12 Å, a convergence criterion of 0.001 kcal / mol, and a step number of 5000 steps, followed by a 100 ns long NVT system simulation, and the water molecule model is obtained after completion;

[0030] Constructing a CO2 molecule model, a 48x28x68 Å box is constructed in the molecular dynamics simulation software, and a single CO2 molecule model is constructed, with the chemical formula CO2. Then use Amorphous Cell Calculation module, build 450 CO2 molecules, Task option Construction, Quality option Medium, force field COMPASS II. Use Forcite module to optimize the structure of the water molecules using the SMART algorithm, with a cutoff radius of 12 Å, a convergence criterion of 0.001 kcal / mol, and a step number of 5000 steps, followed by a 100 ns long NVT system simulation, and the water molecule model is obtained after completion;

[0031] Constructing a CaCl2 model, a 48x28x68 Å box is constructed in the molecular dynamics simulation software, and calcium and chloride ion models are constructed, with the chemical formula CaCl2. Then use Amorphous Cell Calculation module, build 75 Ca 2+ , 150 Cl -In Properties, select Charge and set the charge amount. The charge of the calcium ion is set to +2, and the charge of the chloride ion is set to -1. Select Construction as the Task option, Medium as the Quality option, and COMPASSⅡ as the force field. Use the Forcite module to optimize the structure of the ion using the SMART algorithm, with a cutoff radius of 12Å, a convergence standard of 0.001kcal / mol, and 5000 steps. Then perform a 100ns NVT system simulation. After completion, the calcium ion Ca 2+ 、Chloride ion Cl - Model;

[0032] A rock wall model was constructed by importing the SiO2_quartz (silicon dioxide-quartz crystal) structure into molecular dynamics simulation software. A quartz surface with dimensions (xyz) of 2.8 × 10.00 × 1.2 nm was constructed, and hydroxylated quartz nanopores were symmetrically constructed to simulate the tight oil reservoir wall model. Based on the characteristics of low-porosity and low-permeability reservoirs, the pore width was set to 2.8 nm. Construct a graphene plate model. Use Graphite in molecular dynamics simulation software to build a graphene molecular model. Use the Build module Clear surface to cut and obtain a graphene single layer unit. Use the Build module Supercell to construct a graphene surface area of ​​2.8nm × 4.8nm, with a total of two layers, and the spacing between each graphene layer is 0.5nm. Through molecular dynamics software, the Amorphous Cell Calculation module was used to construct the CO2, CaCl2, and H2O molecules in the above steps into a box to obtain a "CO2-CaCl2-H2O" ternary system model to simulate the molecular types contained in the displacement medium. Based on the above model construction, the Forcite module was used to perform structural optimization of the system using the SMART algorithm, with the convergence standard set to 0.001 kcal / mol and the number of steps set to 5000. Subsequently, a 100 ns NVT system simulation was performed to obtain the oil displacement medium model of the "CO2-CaCl2-H2O" ternary system, thereby providing a basic model for the analysis of the displacement mechanism of crude oil by this system in tight oil reservoirs.

[0033] Step 2: Force field parameter optimization According to the actual reservoir conditions, the temperature was kept at 344.15 K and controlled by the Nosé-Hoover temperature control method. The simulation was performed in the NVT ensemble (the number of molecules, volume and temperature are constant). To obtain a reasonable initial oil phase, the C8H 18Structure optimization was performed using the SMART algorithm with a convergence criterion of 0.001 kcal / mol, a cutoff radius of 12 Å, and 5000 steps, followed by a 100 ns long NVT system simulation structure optimization. The "CO2-CaCl2-H2O" ternary system was placed in the C8H 18 On the left side of the phase, the rock wall was placed on the upper and lower sides of the "CO2-CaCl2-H2O" ternary system, respectively, and was close to the ternary system to simulate the pore structure characteristics of the actual dense oil reservoir. The Modify module Constraints was used for fixation to prevent the wall from moving during displacement. The graphene plate was placed on the left side of the "CO2-CaCl2-H2O" ternary system, and subsequent movement of the graphene plate to the right was added to enable the displacement medium to move under the push of the graphene plate, driving the oil phase to be expelled from the pores composed of rock walls. The Modify module Constraints was used for fixation to facilitate subsequent molecular dynamics simulation equilibrium calculation. Subsequently, the Forcite module was used to optimize the structure of the overall system, adjusting the initial structure to a physically reasonable state to ensure the stability, efficiency, and reliability of the results. The CVFF force field was selected, and the SMART algorithm was used with a convergence criterion of 0.001 kcal / mol and a step number of 5000.

[0034] Step three, software simulation In Lammps, the model was set up with Units Real unit system, Boundary p p p periodic boundary, Atom_style Full atom type, time step 1 fs, and adjacent cutoff distance 2bin. According to the system file, the potential parameters were set, Pair_style lj / cut, Bond_style, Angle_style, and Dihedral_style were selected as Harmonic, and the atom potential parameters were set in detail. The system was energy minimized and temperature initialized, and the NVT ensemble was selected with a control temperature of 300K for 10000 step relaxation simulation. Subsequently, in the NVT ensemble, the temperature was controlled at 300K, and the graphene plate was added with a positive displacement of 0.001 Å per step to drive the displacement of the ternary system. After 80000 steps, the atomic coordinates were output.

[0035] Step four, data processing OVITO was used to visualize and analyze the data, and the radial distribution function and mean square displacement were output.

[0036] Oil displacement effect evaluation: By studying the displacement of the simulated "CO2-CaCl2-H2O" ternary compound system to crude oil in the molecular dynamics force field, using mean square displacement, crude oil displacement front graph, radial distribution function and other performance evaluation parameters, the oil displacement effect of the "CO2-CaCl2-H2O" ternary compound system is evaluated, so as to reflect the influence of the development effect of the oil displacement method on the tight oil reservoir in the molecular dynamics software.

[0037] According to the trajectory file of the simulated "CO2-CaCl2-H2O" ternary compound system in LAMMPS, the peeling dynamic diagram of crude oil in the pore at different displacement times can be drawn by using the OVITO visualization tool, and the displacement front diagram of different displacement media is as shown in Figure 8 Compared with the pure water system and the "H2O+CO2" system, the "CO2-CaCl2-H2O" ternary compound system has better peeling degree of crude oil, less residual oil in the rock pore, and better oil displacement effect.

[0038] According to the trajectory file of the simulated "CO2-CaCl2-H2O" ternary compound system in LAMMPS, the mean square displacement of crude oil in the displacement process under the compound system can be obtained by using the Compute command, and the micro-motion characteristics of the crude oil molecules can be obtained by the change of the mean square displacement. The mean square displacement refers to the deviation of the position of the particle after moving with time relative to the reference position. The slope of the mean square displacement curve represents the flow ability of the molecules. The greater the curve slope, the stronger the flowability of the molecules, and the faster the diffusion. The mean square displacement curve under the ternary compound system is as shown in Figure 9 Compared with the pure water system and the "H2O+CO2" system, the "CO2-CaCl2-H2O" ternary compound system has better peeling degree of crude oil, less residual oil in the rock pore, and better oil displacement effect.

[0039] According to the trajectory file of the simulated "CO2-CaCl2-H2O" ternary compound system in LAMMPS, the motion of the displacement medium under the compound system can be analyzed, and the difference of the oil displacement effect of the "CO2-CaCl2-H2O" ternary compound system and other single systems or compound systems is analyzed by the radial distribution function. The radial distribution function can reflect the atomic distribution around the specified atom. The radial function distribution curve is as shown in Figure 10 Compared with the pure water system and the "H2O+CO2" system, the "CO2-CaCl2-H2O" ternary compound system has higher peak value of the radial distribution function, which means that the system can maintain higher binding strength with crude oil during the oil displacement process, and better oil displacement effect is achieved.

[0040] While embodiments of the application have been shown and described, it is to be understood that the embodiments described are merely exemplary of the principles and application of the present application. Numerous modifications and adaptions can be effected without departing from the spirit and scope of the present application, which is not limited to the exact construction and arrangement described. It is intended, therefore, to cover all modifications and adaptions that fall within the scope of the claims and their equivalents.

Claims

1. A molecular dynamics simulation method for CO2-ion water flooding, characterized in that: The following steps are involved: Construct a microscopic model of a tight oil reservoir, including establishing a water molecule model, a CO2 molecule model, a CaCl2 molecule model, and an oil molecule model, a graphene plate model, and a rock pore wall model corresponding to the target reservoir characteristics; The COMPASS II force field was used to optimize the geometric structure of each molecular model; The optimized water molecule model, CO2 molecule model, and CaCl2 molecule model were assembled into a "CO2-CaCl2-H2O" ternary system. The "CO2-CaCl2-H2O" ternary system was then placed on one side of the oil molecule model. The rock pore wall models were fixed on the "CO2-CaCl2-H2O" ternary system. l2 The graphene sheet was placed on the upper and lower sides of the "CO2-CaCl2-H2O" ternary system, and a molecular dynamics simulation was performed. The NVT ensemble was selected for the molecular dynamics simulation, periodic boundary conditions were applied, the 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 files output by the simulation, the displacement front position, mean square displacement, radial distribution function, and system energy change are calculated to quantitatively evaluate the oil displacement effect of the displacement medium of the "CO2-CaCl2-H2O" ternary system.

2. The molecular dynamics simulation method for CO2-ion water flooding according to claim 1, characterized in that: The steps for constructing the displacement medium model of the "CO2-CaCl2-H2O" ternary system are as follows: In molecular dynamics simulation software, a box containing multiple water molecule models, a box containing multiple CO2 molecule models, and a box containing multiple CaCl2 molecule models (calcium and chloride ions) were constructed. The size of each box was consistent with the pore size of the tight oil reservoir. When constructing the above box, it is necessary to use the force field COMPASSⅡ to model the water molecule, CO2 molecule, and Ca 2+ and Cl - , the SMART algorithm was used for structure optimization, with a cutoff radius of 12Å, a convergence criterion of 0.001kcal / mol, and 5000 steps. Then, a 100ns NVT system simulation was performed. After completion, the water molecule model, CO2 molecule model, and calcium ion and chloride ion models were obtained respectively; The water molecule model, CO2 molecule model, and CaCl2 molecule model were adjusted into a box, and the structure was optimized using the SMART algorithm with a cutoff radius of 12Å, a convergence standard of 0.001kcal / mol, and 5000 steps. Subsequently, a 100ns NVT system simulation was performed to obtain the displacement medium model of the "CO2-CaCl2-H2O" ternary system.

3. The molecular dynamics simulation method of CO2-ion water flooding according to claim 1, characterized in that: When optimizing the structure of oil molecules, we selected Geometry Optimization as the Task, COMPASSⅡ as the force field, a cutoff radius of 12 Å, a convergence criterion of 0.001 kcal / mol, 5000 steps, a temperature of 300 K, and a pressure of 0.1 GPa to ensure model convergence and accuracy. We performed 2-3 geometry optimizations, followed by a 100 ns simulation of the NVT system structure optimization.

4. The molecular dynamics simulation method for CO2-ion water flooding according to claim 1, characterized in that: During the molecular dynamics simulation, the holding temperature is determined according to the actual reservoir conditions and controlled by the Nosé-Hoover temperature control method.

5. The molecular dynamics simulation method for CO2-ion water flooding according to claim 1, characterized in that: In molecular dynamics simulations, C8H 18 As the main component of crude oil.

6. The molecular dynamics simulation method for CO2-ion water flooding according to claim 1, characterized in that: When optimizing the geometric structure of each model, the structure of the oil molecules is optimized first.

7. The molecular dynamics simulation method for CO2-ion water flooding according to claim 1, characterized in that: When performing molecular dynamics simulations, a rightward movement is added to the graphene plate, allowing the displacement medium to migrate under the push of the graphene plate, driving the oil phase out of the pores of the rock wall model.

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