Method for analyzing miscible phase degree of multi-component crude oil developed by volume phase space gas injection

Abstract

According to the method, an equilibrium molecular dynamics (EMD) calculation method is adopted, the miscible phase behavior difference between different components in the mixed crude oil fluid in the bulk phase space is deeply discussed, and the influence of the multi-component synergistic effect on the miscible phase behavior of a single component is analyzed. The invention provides a novel analysis method, and the method quantitatively evaluates the miscible state of each component by comprehensively analyzing the behavior of the crude oil molecules in the miscible process. The quantitative evaluation standard of the mixed phase behavior of each component in the mixed system is established by acquiring the adsorption state difference of each component in the mixed system. According to the method, the influence of the synergistic effect of the multi-component fluid in the bulk phase space on the physical property change is deeply researched, and an important theoretical basis is provided for calculating the miscible phase degree of the mixed fluid in the bulk phase space. The research provides important guidance for deep exploration of fluid properties in bulk phase space and further development of related applications, and has remarkable theoretical and practical significance.

Description

Technical Field

[0001] The present invention relates to the analysis and research of fluids in bulk phase space, and specifically to a method for analyzing the mixing degree of multi-component crude oil and injected gas by equilibrium molecular dynamics (EMD) simulation. The method studies the miscible behavior of the mixed fluid during the gas injection process and calculates the miscible degree of each component by quantitative evaluation. Background Art

[0002] The intensity of miscible behavior between multi-component crude oil and injected gas under bulk conditions is a crucial research area in oil and gas production and secondary recovery technology. Miscible behavior refers to the process of interaction, molecular diffusion and dissolution between crude oil and injected gas (such as CO2, CH4, N2, etc.) under reservoir conditions, and its intensity directly affects oil displacement efficiency, recovery rate and crude oil production capacity. Especially in the production process of tight oil and shale oil, the complexity of crude oil components and the physicochemical properties of injected gas make the analysis and prediction of miscible behavior more complex and important. Multi-component crude oil usually contains multiple components such as hydrocarbons, aromatic hydrocarbons, and colloid components of different molecular weights. The chemical properties and physical behaviors of these components will lead to different miscible characteristics due to their different molecular structures and interaction forces. For example, lighter alkane components are usually easy to dissolve and interact with injected gas, while heavier components may be limited by their lower solubility and higher intermolecular interactions, resulting in complexity and heterogeneity of miscible behavior.

[0003] In this context, the interaction between injected gas and crude oil depends not only on the type of gas, but also on crude oil composition, reservoir pressure, temperature, pore structure and fluid dynamic conditions. However, traditional analytical methods usually focus on the observation of the gas-liquid interface and use interfacial tension, solubility or other macroscopic properties for qualitative evaluation. These methods are often difficult to accurately capture the microscopic details and complex behaviors of multi-component crude oil and gas miscibility. Under bulk conditions, the miscibility strength of multi-component crude oil and injected gas not only involves the direct dissolution of gas and oil molecules, but also needs to take into account the interaction between multiple components and the distribution state between solute molecules and solvent molecules during the miscibility process.

[0004] In recent years, the development of molecular dynamics simulation has enabled us to more accurately describe and predict the miscible behavior of multi-component systems under bulk conditions. By simulating the interaction between crude oil components and injected gas, the miscible strength and phase distribution of different gases at different temperatures and pressures can be revealed. Molecular dynamics simulation can provide a more microscopic and detailed description than traditional experimental methods, helping us understand the impact of gas injection on crude oil phase composition, physical properties and fluidity. By combining molecular dynamics simulation and experimental data, we can comprehensively analyze the interaction between multi-component crude oil and injected gas and the strength of their miscible behavior in the reservoir. This not only provides theoretical support for improving oil and gas recovery, but also provides a theoretical basis for optimizing gas flooding and chemical flooding processes, which helps to adjust gas injection parameters and improve gas injection strategies, thereby improving oil recovery efficiency and ultimate recovery. In addition, this study is also of great significance for the optimization of reservoir evaluation models. It can more accurately predict the fluidity and permeability of crude oil, providing a reliable theoretical basis for the formulation of oil production plans and development strategies. Summary of the invention

[0005] The present invention proposes a method for simulating theoretical calculations using equilibrium molecular dynamics (EMD) to study the miscibility of bulk fluids. This study uses the miscible molecular adsorption state of each component under ideal conditions as the standard for complete miscibility, and by analyzing the difference between the molecular adsorption state of each component in the mixed system and that under pure component conditions, quantitative calculation of the miscibility of each component of crude oil in bulk space is achieved. The computer simulation software used in the method of the present invention is Materials Studio (MS), a new generation of material calculation software developed by Accelrys Corporation of the United States, and LAMMPS (Large-scale Atomic / Molecular Massively Parallel Simulator), an open source program package for molecular dynamics simulation developed by Sandia National Laboratories.

[0006] The main contents of the present invention include two parts: first, the construction of the oil and gas system in the phase space of the single component system and the mixed component system; second, the difference in the adsorption quantity of crude oil molecules on gas molecules under the ideal conditions of the pure component and the mixed system is analyzed, and the miscibility strength of a certain component in the mixed system is further calculated based on this. The specific contents are as follows:

[0007] (1) Construction of phase space oil and gas system of single component system and mixed component system

[0008] Materials Studio software was used to create crude oil molecules and injected gas molecules CO2, and these molecules were used to construct a simulated gas-in-oil simulation box for structural and energy optimization. For the mixed system, a sandwich structure bulk simulation system was formed by placing the mixed system in one dimension in the order of gas phase / oil phase / gas phase.

[0009] Then, equilibrium molecular dynamics (EMD) simulation was performed in LAMMPS software. In this process, the OPLS-AA force field was used for crude oil components. The EPM2 force field was used for CO2 molecules. The simulation was set under the temperature and pressure conditions of the tight oil reservoir environment, specifically 318.15K and 30MPa pressure. Energy minimization was performed under the set simulation temperature and pressure conditions, and the cutoff radius was set to The simulation time of pure component system lasts for 6 ns of NPT ensemble simulation, and the simulation time of mixed system lasts for 25 ns of NPT ensemble simulation.

[0010] (2) Calculation of the amount of gas adsorbed by crude oil molecules in single-component systems and mixed systems

[0011] First, a pure component system is selected and the 3ns data of the later stage of simulation is used as the basis. With each oil molecule as the center, the radial distribution function (RDF) of the surrounding injected gas molecules is calculated. By analyzing the RDF curve, if there is an obvious trough after the first adsorption peak, the position of the trough is used as the criterion for determining the adsorption radius; if there is no obvious trough in the RDF curve, the 90% principle is used to select the adsorption radius, that is, the position where the RDF curve reaches the 90% maximum value in the descending area is selected as the adsorption radius; in the pure component system, the number of injected gas molecules within the adsorption radius is counted, and this number is used as the complete miscibility state of the component. That is, when the number of gas molecules around a component molecule reaches a certain number, the component is considered to be in a completely miscible state; in the mixed system, all molecules of the component are selected, and the number of injected gas molecules within the adsorption radius is counted. If the number of injected gas molecules around the component molecule is greater than the number of adsorbed gas molecules under the pure component system, the molecule is considered to have entered a miscible state; if it does not exceed, the molecule is considered to be in an immiscible state; finally, the proportion of all molecules of the component that have entered a miscible state in the mixed system is calculated to obtain the miscible strength of the component. Through this proportion, the miscible behavior strength of each component in the mixed system can be evaluated, and the interaction between the crude oil component and the injected gas and its influence on the miscible strength can be further analyzed.

[0012] The advantages of the present invention are: providing a simple and effective method for calculating the miscibility of mixed crude oil in bulk phase space. Since the miscible dissolution process in the bulk phase space of a mixed system is complex and difficult to observe experimentally, the molecular dynamics simulation method can achieve accurate simulation of the bulk phase space fluid system of a complex mixed system. The method has accurate calculation results and can deeply analyze the miscible behavior of each component at the molecular level, solving problems that are difficult to solve experimentally. Compared with the traditional method of judging the miscibility by interfacial tension and solubility, this method has higher accuracy and is closer to the actual proportion of the components of the produced crude oil. This method provides theoretical guidance for gas injection production and process parameter optimization of oil reservoirs, and has the characteristics of low cost and high efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0014] Figure 1 It is a schematic diagram of the technical process of this patent;

[0015] Figure 2 It is the single-component miscible phase configuration adopted in bulk phase space;

[0016] Figure 3 is the radial distribution function curve of single-component oil molecules to injected gas molecules in bulk space;

[0017] Figure 4 is the number of gas molecules within the single-component adsorption radius of the bulk phase space;

[0018] Figure 5 It is the bulk space mixed component gas injection diffusion configuration adopted;

[0019] Figure 6 It is the miscibility of the four components in the mixed system; DETAILED DESCRIPTION

[0020] The method of the present invention is described in detail below with reference to the drawings in the specification, and the technical route and purpose of the present invention are further explained by taking 318.15K, 30MPa conditions as an example, so as to further understand the scheme, technical route and characteristics of the present invention.

[0021] As shown in the accompanying drawings, a schematic diagram of the technical process designed by the present invention is provided.

[0022] The specific trial plan is divided into the following three steps:

[0023] (1) Model construction

[0024] Materials Studio software was used to create oil molecules and CO2 molecules. For the pure components, a gas-in-oil phase space simulation system was constructed. For the mixed system, a sandwich structure was formed by placing the mixture in one dimension in the order of gas phase / oil phase / gas phase. The structure and energy of the two were optimized. The density of CO2 is 0.88533 g / ml.

[0025] (2) Equilibrium molecular dynamics (EMD) simulation

[0026] In the LAMMPS software, equilibrium molecular dynamics (EMD) simulations were performed. The OPLS-AA force field was used for the crude oil component, and the EPM2 force field was used for the CO2 gas phase. The simulations were performed under specific temperature and pressure conditions, with a temperature of 318.15 K and a pressure of 30 MPa. Under these set temperature and pressure conditions, energy minimization was first performed, with the cutoff radius set to The simulation time for the single-component system is 6ns, and the simulation time for the mixed system is 25ns, ensuring that the system reaches a fully balanced state.

[0027] (3) Calculation of the number of injected gas adsorbed by crude oil molecules

[0028] First, data extraction is performed on the pure component system and the mixed phase space system. In the pure component system, the radial distribution function (RDF) between the oil component molecules and the injected gas molecules is calculated. If there is a trough in the RDF curve after the first adsorption peak, the trough position is used as the adsorption radius to determine the distance; if there is no obvious trough, the adsorption radius is selected according to the 90% principle to define the adsorption area. Next, the number of gas molecules adsorbed by oil molecules in the adsorption area in the last 3ns of the simulation is statistically simulated, and the mean is calculated as the miscible state judgment standard for this type of crude oil molecules. If the number of adsorbed molecules is greater than or equal to this standard, it is considered to be a miscible state. Then, in the last 5ns of the mixed phase space simulation system, the adsorption area of ​​this type of oil molecules is statistically analyzed to obtain the number of molecules that reach the standard miscible state and calculate the mean. The ratio of this mean to the total number of molecules in the mixed system is the miscible degree of this type of crude oil molecules. By calculation, the adsorption radii of nonane molecules and aromatic hydrocarbon molecules in the pure component system are respectively and Within the adsorption radius, the number of CO2 molecules is 22 and 11. Subsequently, we calculated the miscibility of the nonane component and the aromatic hydrocarbon component in the mixed system and found that their miscibility strengths are 97.4% and 79.9%, respectively.

[0029] The above is only an embodiment of the present invention. It does not limit the present invention in any form. Any ordinary technician familiar with the profession can smoothly implement the present invention according to the drawings and the steps described above. However, any technician familiar with the profession can make some changes, modifications and evolutions of the technical content described above without departing from the scope of the technical solution of the present invention, which are equivalent embodiments of the present invention. At the same time, any equivalent changes, modifications and evolutions made to the above embodiments based on the implementation technology of the present invention are within the protection scope of the technical solution of the present invention.

Claims

1. A method for calculating the miscibility of multi-component crude oil by using molecular dynamics simulation theory to analyze the miscibility of multi-component crude oil by gas injection in bulk space. The method comprises the following steps: (1) Molecular model construction: Use Materials Studio software to construct multiple types of crude oil molecules and injected gas molecules. This step includes establishing a single-component gas phase box and a mixed-component gas phase box, and combining the oil phase and gas phase boxes into a sandwich structure to construct a bulk space oil / gas molecular diffusion model; (2) Diffusion model construction: Using the molecules obtained in step (1), the oil phase and gas phase boxes are constructed using Packmol software, and the oil phase and gas phase boxes are sandwiched to construct a bulk space oil / gas molecular diffusion model; (3) Charge distribution: Assign force field and charge parameters to the model constructed in step (2) (4) Equilibrium molecular dynamics (EMD) simulation: LAMMPS software is used to perform molecular mechanics optimization and equilibrium molecular dynamics simulation to select a reasonable cutoff radius. Under specific temperature and pressure conditions, the NPT method is used to perform molecular dynamics simulation to obtain a uniformly dispersed structure of the oil and gas phases. The interaction between crude oil molecules and CO2 molecules is calculated using the Lennard-Jones potential function and electrostatic interaction. The interaction between different atoms satisfies the Lorentz-Bellot mixing rule. (5) Calculation of radial distribution function: Calculate the radial distribution function curve of oil molecules to injected gas molecules in the phase space of pure components (6) Division of adsorption radius and adsorption area: According to the calculation result obtained in step (4), the trough position of the curve is selected as the adsorption radius, and the area within the adsorption radius is the adsorption area. (7) Determination of miscibility: Count the number of gas molecules in the adsorption area of ​​crude oil molecules in the pure component system and use this as the criterion for determining the miscibility. (8) Calculation of miscibility: Under the mixed crude oil system, the crude oil components selected in step (2) to construct the pure component system are counted, the number of molecules in the adsorption area obtained in step (5) of the component is counted, and the molecules that meet the miscibility are recorded. The proportion of miscible molecules in the molecules of the component is calculated.

2. The method for calculating the miscibility of multi-component crude oil according to claim 1, characterized in that: In the step (2), the cross section of the gas phase is of square cross-section and length 3. The method for calculating the miscibility of multi-component crude oil according to claim 1, characterized in that: In the step (2), the mixed oil phase system is constructed by a four-component crude oil characterization method, comprising saturated hydrocarbons, aromatic hydrocarbons, colloids, and asphaltenes.

4. The method for calculating the miscibility of multi-component crude oil according to claim 1, characterized in that: In the step (3), the gas phase molecule may be CO2, but is not limited to CO2. Any gas that can be miscible with the oil phase can be calculated using the method described in this patent.

5. The method for calculating the miscibility of multi-component crude oil according to claim 1, characterized in that: In step (3), the OPLS-AA force field is used for the oil phase molecules, and the EPM2 force field is used for the CO2 molecules.

6. The method for calculating the miscibility of multi-component crude oil according to claim 1, characterized in that: In the step (5), during the molecular mechanics optimization, equilibrium molecular dynamics simulation and stretching molecular dynamics simulation, the van der Waals interaction is calculated using the Lennard-Jones 12-6 potential, the LJ potential between different types of atoms is calculated using the Arithmetic mixing rule, the electrostatic interaction is calculated using the Coulomb potential, and the long-range electrostatic interaction is calculated using the PPPM method.

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