A molecular simulation method for evaluating the production enhancement performance of CO2 co-solvents in shale oil reservoirs

Abstract

The application provides a kind of molecular simulation method for evaluating the performance of CO2 solubilizer in shale oil reservoir, which uses Materials Studio software to construct nanopore model, shale oil system box and CO2-solubilizer composite system box; temperature equilibrium and pressure equilibrium are carried out on shale oil system box and CO2-solubilizer composite system box; three-phase contact model for simulating the process of CO2-solubilizer composite system replacing shale oil and model for calculating interfacial tension are constructed; combined with radial distribution function, diffusion coefficient, interfacial tension, interaction energy and density distribution, the effects of solubilizer on the dissolution, diffusion, miscibility, competitive adsorption of CO2 in shale oil and the improvement of shale oil displacement efficiency in nanopores are analyzed, and the various stimulation performances of solubilizer are comprehensively evaluated. Through the analysis of multiple parameters, the various stimulation performances of solubilizer are determined, which provides a theoretical basis for the selection of solubilizer in the process of CO2 injection development in shale oil reservoir.

Description

Technical Field

[0001] This invention belongs to the field of shale oil reservoir development technology, specifically relating to a molecular simulation method for evaluating the production enhancement performance of CO2 co-solvents in shale oil reservoirs. Technical Background

[0002] To meet the ever-increasing energy demand, shale oil development has attracted widespread attention. However, shale reservoirs have extremely low porosity and permeability, strong heterogeneity, and low formation energy, resulting in extremely low recovery rates during depletion development. Shale oil reservoirs are commonly developed using CO2 injection, and the effectiveness of shale oil extraction largely depends on the various interaction mechanisms between CO2 and crude oil within nanopores. However, the production-enhancing performance of CO2 in shale oil reservoirs is limited by reservoir conditions and crude oil properties. To achieve effective shale oil reservoir development, co-solvents can be added to CO2. Due to limitations in experimental equipment and technology, indoor physical simulations cannot clearly define the various production-enhancing properties of CO2 co-solvents in shale oil reservoirs from a microscopic perspective. Molecular simulation, with its unique advantages, has become an effective method for simulating the process of CO2 displacement of shale oil within nanopores.

[0003] Currently, there is no systematic evaluation method for the production enhancement performance of CO2 co-solvents in shale oil reservoirs. Problems and limitations of existing technologies include:

[0004] (1) Most shale oil is buried deep with high formation temperature and pressure. Indoor physical simulation is difficult to achieve the actual temperature and pressure conditions of the reservoir, which will increase the risk of experiments and increase costs.

[0005] (2) Existing experimental and simulation methods are not systematic and comprehensive enough to simultaneously evaluate the multiple production-enhancing properties of CO2 co-solvents in shale oil reservoirs.

[0006] (3) Most of the existing experimental methods rely on some parameters to evaluate the effect of co-solvents on CO2 displacement of shale oil. Even microscopic visualization experiments are difficult to reveal multiple oil displacement mechanisms at the molecular level, especially the molecular diffusion mechanism and competitive adsorption mechanism which are very important for shale oil reservoir development. It is difficult to combine various factors to comprehensively evaluate the production enhancement performance of co-solvents.

[0007] The significance of solving the above problems and defects is as follows: (1) It can quickly simulate the interaction process between the CO2-co-solvent composite system in nanopores and the shale oil system under different types of shale reservoirs and different temperature and pressure conditions; (2) This simulation method can analyze various micro-mechanisms of CO2-co-solvent composite system injection into shale oil in nanopores; (3) It can comprehensively evaluate the production enhancement performance of different types of co-solvents based on the promoting effect of different co-solvents on various micro-mechanisms of CO2 injection into shale oil and make the best selection. Summary of the Invention

[0008] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a molecular simulation method for evaluating the CO2 co-solvent enhancement performance in shale oil reservoirs. This method can reveal the promoting effect of co-solvents on various mechanisms of CO2 extraction from shale oil in nanopores from a microscopic perspective, providing a theoretical basis for the optimal selection of co-solvents during CO2 injection development of shale oil reservoirs.

[0009] To achieve the aforementioned objective, this specification provides a molecular simulation method for evaluating the production enhancement performance of CO2 co-solvents in shale oil reservoirs, comprising the following steps:

[0010] Step 1: Select different types of shale reservoir rock minerals, shale oil components and CO2 co-solvents, and use Materials Studio software to construct different types of nanopore, shale oil system boxes and CO2-co-solvent composite system boxes;

[0011] Step 2: Temperature and pressure equilibrium are performed on the established shale oil system box and CO2-cosolvent composite system box to make the model meet the actual formation temperature and formation pressure conditions of the shale oil reservoir. During the equilibrium process, the size of the box remains unchanged in the x and y directions, and only the size in the z direction can change.

[0012] Step 3: By placing two shale oil system boxes at both ends of the nanopores and attaching them tightly to the mineral wall, and inserting a CO2-cosolvent composite system box in the middle space of the two shale oil system boxes, a three-phase contact model is constructed to simulate the process of CO2 replacing shale oil under the action of a cosolvent. In addition, by placing the shale oil system box in the middle of the two CO2-cosolvent composite system boxes, a model is constructed to calculate the interfacial tension.

[0013] Step four involves performing kinetic simulations on each model. By analyzing the radial distribution function, diffusion coefficient, interfacial tension, interaction energy, and density distribution, the effects of the co-solvent on the dissolution, diffusion, miscibility, competitive adsorption of CO2 in shale oil within nanopores, and the improvement of shale oil replacement efficiency are clarified. Based on this, the various production-enhancing properties of the co-solvent are comprehensively evaluated.

[0014] In the molecular simulation method described above for evaluating the production enhancement performance of CO2 co-solvents in shale oil reservoirs, step one involves constructing different nanopores, shale oil system boxes, and CO2-co-solvent composite system boxes. The dimensions of the nanopore walls, shale oil system boxes, and CO2-co-solvent composite boxes should be consistent in both the x and y directions to avoid software errors when constructing the various boxes together.

[0015] In the molecular simulation method for evaluating the production enhancement performance of CO2 co-solvents in shale oil reservoirs, step two involves temperature and pressure equilibration of the established shale oil system box and CO2-co-solvent composite system box. The system temperature is equilibrated to the actual formation temperature of the shale oil reservoir using a Nosé-Hoover temperature controller under the NVT ensemble using ForciteDynamics, and the system pressure is equilibrated to the actual formation pressure of the shale oil reservoir using a Berendsen pressure controller under the NPT ensemble.

[0016] In the molecular simulation method for evaluating the production enhancement performance of CO2 co-solvents in shale oil reservoirs, step three involves constructing a three-phase contact model to simulate the process of CO2 replacing shale oil under the action of co-solvents and a model for calculating interfacial tension. When constructing the three-phase contact model and the model for calculating interfacial tension using Build Layers, the distance between different boxes should be kept at least 0.4 nm to avoid atomic overlap.

[0017] In the molecular simulation method described above for evaluating the production-enhancing performance of CO2 co-solvents in shale oil reservoirs, step four includes:

[0018] (1) The radial distribution function of CO2-cosolvent composite system and shale oil was statistically analyzed using the Radial distribution function in ForciteAnalysis. By comparing the magnitude of the radial distribution function of CO2-cosolvent composite system and shale oil under different cosolvent conditions, the miscibility between the two phases was indirectly evaluated. The larger the radial distribution function, the stronger the miscibility between the two phases, the lower the difficulty of CO2 injection in actual development, and the better the improvement effect of shale oil fluidity.

[0019] (2) The mean square displacement of CO2 is statistically analyzed using the Mean square displacement in ForciteAnalysis, and the diffusion coefficient of CO2 is calculated using formula (1) to evaluate the diffusion performance of CO2 under the influence of the co-solvent. The larger the CO2 diffusion coefficient, the more obvious the promoting effect of the co-solvent on the diffusion of CO2 in shale oil, and the wider the range of CO2 injected into the shale oil reservoir in actual development.

[0020]

[0021] In the formula, D is the diffusion coefficient, and m is the diffusion coefficient. 2 / s;r i (t) — the position of the molecule at time t; < > — the average of the squares of the molecular displacements in the component;

[0022] (3) Use Forcite Energy to calculate the single-point energy of different models, and combine it with formula (2) to calculate the interaction energy between CO2-cosolvent composite system and shale oil. When the interaction energy is negative, it indicates that the two phases attract each other. The greater the interaction energy between CO2-cosolvent composite system and shale oil, the stronger the promoting effect of cosolvent on CO2 dissolution into shale oil. By calculating the interaction energy between CO2-cosolvent composite system and nanopore wall and the interaction energy between shale oil and nanopore wall, the difference between the two is determined to quantitatively evaluate the competitive adsorption effect between CO2-cosolvent composite system and shale oil at nanopore wall. The greater the difference, the stronger the competitive adsorption effect, which is more conducive to CO2 stripping the oil film adsorbed on nanopore wall.

[0023] E int =E total -(E1+E2) Formula (2)

[0024] In the formula, E int —Interaction energy, kcal / mol; E total —The total energy of substance 1 and substance 2, kcal / mol; E1—Energy of substance 1, kcal / mol; E2—Energy of substance 2, kcal / mol;

[0025] (4) Calculate the interfacial tension between the CO2-cosolvent composite system and shale oil using formula (3) to clarify the effect of the cosolvent on improving the interfacial tension between the CO2 and shale oil phases, so as to indirectly evaluate the improvement of the cosolvent on the miscibility of the gas and oil phases. The smaller the interfacial tension, the better the effect of the cosolvent on improving the miscibility of the gas and oil phases, and the more conducive it is to hindering the gas channeling of CO2 in the actual reservoir.

[0026]

[0027] In the formula, γ—interfacial tension, mN / m; p ii (i = x, y, z) — Pressure components in different directions, MPa; L z —The box dimension perpendicular to the interface along the Z direction, in nm; l z —The length of any segment containing the interface along the Z direction, in nm;

[0028] (5) The relative concentration distribution of the shale oil system in the z direction is statistically analyzed using the Concentration profile in ForciteAnalysis. Then, the density distribution of the shale oil system is obtained by conversion using formula (4). The replacement efficiency of shale oil is calculated by the density change of the adsorbed phase at the beginning and end. The greater the replacement efficiency, the more obvious the improvement of the oil washing efficiency of the cosolvent in actual development.

[0029]

[0030] In the formula, C r —Relative concentration, dimensionless quantity; m—Mass of a component, g; V—Unit volume of a component, cm³ 3 ρ(slab) — density of shale oil in a specific block, g / cm³ 3 ρ(bulk) — Macroscopic density of shale oil, g / cm³ 3 .

[0031] The advantages and positive effects of this invention are as follows:

[0032] The molecular simulation method for evaluating the production-enhancing performance of CO2 co-solvents in shale oil reservoirs provided by this invention is based on advanced computer simulation technology. It can easily simulate the actual high-temperature and high-pressure conditions of shale reservoirs, and is safe, efficient, and low-cost. This method can comprehensively and systematically analyze the promoting effect of co-solvents on various mechanisms of CO2 replacement of shale oil in nanopores at the molecular level, and provides a theoretical basis for the selection of co-solvents in the CO2 injection development process of shale oil reservoirs. Attached Figure Description

[0033] Figure 1 This is a flowchart of the molecular simulation method for evaluating the production enhancement performance of CO2 co-solvents in shale oil reservoirs according to the present invention.

[0034] Figure 2(a) shows a three-phase contact model according to an embodiment of the present invention.

[0035] Figure 2(b) shows a model for calculating interfacial tension according to an embodiment of the present invention.

[0036] Figure 3 This represents the radial distribution function between the shale oil system and the CO2-cosolvent composite system under different cosolvent conditions in the embodiments of the present invention.

[0037] Figure 4 The mean square displacement of CO2 under different co-solvent conditions in the embodiments of the present invention is shown.

[0038] Figure 5A This represents the interaction energy between the shale oil system and the CO2-cosolvent composite system under different cosolvent conditions in the embodiments of the present invention.

[0039] Figure 5B This invention presents the interaction energy between the rock wall and the CO2-cosolvent composite system and the rock wall and the shale oil system under different cosolvent conditions in embodiments of the invention, as well as the difference between the two.

[0040] Figure 6 This is the density distribution of shale oil systems under different co-solvent conditions in embodiments of the present invention. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0042] To address the problems existing in the prior art, this invention provides a molecular simulation method for evaluating the production enhancement performance of CO2 co-solvents in shale oil reservoirs. The invention will be described in detail below with reference to the accompanying drawings.

[0043] like Figure 1 As shown, the molecular simulation method for evaluating the CO2 co-solvent enhancement performance in shale oil reservoirs provided in this embodiment of the invention includes the following steps:

[0044] Step 1: Select different types of shale reservoir rock minerals, shale oil components and CO2 co-solvent, and use Materials Studio software to construct nanopores of different types of rock walls, shale oil system boxes and CO2-co-solvent composite system boxes;

[0045] First, the quartz unit cell is cut along the (100) plane, and then expanded. The expanded size is... Subsequently, the selected molecular formula C was... 210 H 184 O 20 The N4S4 kerogen molecule underwent geometric optimization with a PCFF force field. An initial kerogen box was constructed using the Amorphous Cell module, with both length and width being [missing information]. The constructed box needs to have its structure optimized using the Material Depth (MD) method. First, temperature equilibration is performed under the NVT ensemble, with the temperature set at 323.15K and controlled using the Nosé-Hoover method. Ewald electrostatic forces are used, and Atom-based van der Waals forces are employed. The cutoff radius is set to... Pressure equilibrium was then performed under the NPT ensemble, with the pressure set at 20 MPa and controlled using the Berendsen method, while other conditions remained constant. The step size was 1 fs, and the total step size was 0.5 ns. A nanopore structure was constructed by combining quartz and kerogen walls, leaving a vacuum layer between them sufficient to accommodate the CO2-cosolvent composite system box and the shale oil system box. The contents of each component in the shale oil and the CO2-cosolvent are shown in Table 1. CO2 was mixed with ethanol, propane, and ethyl acetate to obtain the CO2-ethanol composite system box, CO2-propane composite system box, and CO2-ethyl acetate composite system box, respectively. The shale oil system box was obtained by mixing the various components of the shale oil. The length and width of the CO2-ethanol composite system box, CO2-propane composite system box, CO2-ethyl acetate composite system box, and shale oil system box are...

[0046] Table 1

[0047]

[0048] Step 2: Temperature and pressure equilibrium are performed on the established shale oil system box and CO2-cosolvent composite system box to make the model meet the actual formation temperature and formation pressure conditions of the shale oil reservoir. During the equilibrium process, the size of the box remains unchanged in the x and y directions, and only the size in the z direction can change.

[0049] The temperature of the system is balanced using a Nosé-Hoover temperature controller under the NVT ensemble, and the pressure of the system is balanced using a Berendsen pressure controller under the NPT ensemble. In this embodiment, the temperature is 323.15 K and the pressure is 20 MPa.

[0050] Step 3: By placing two shale oil system boxes at both ends of the nanopores and attaching them tightly to the mineral wall, and inserting a CO2-cosolvent composite system box in the middle space of the two shale oil system boxes, a three-phase contact model is constructed to simulate the process of CO2 replacing shale oil under the action of a cosolvent. In addition, by placing the shale oil system box in the middle of the two CO2-cosolvent composite system boxes, a model is constructed to calculate the interfacial tension.

[0051] A three-phase contact model and a model for calculating interfacial tension were constructed using Build Layers, and the distance between different boxes was kept to be at least 0.4 nm to avoid atomic overlap. The resulting models are shown in Figure 2(a) and Figure 2(b).

[0052] Step four involves fixing the wall surface and using Forcite Geometry Optimization to geometrically optimize the three-phase contact model, ensuring that each molecule is positioned on the minimum potential energy surface. Subsequently, dynamic simulations are performed on each model. The total simulation step size for the three-phase contact model is 4 ns, and for the interfacial tension model, it is 15 ns. By analyzing the radial distribution function, diffusion coefficient, interaction energy, interfacial tension, and density distribution, the promoting effect of the co-solvent on various mechanisms of CO2 replacement of shale oil in shale nanopores is clarified.

[0053] (1) The radial distribution function (RDF) of the CO2-cosolvent composite system and the shale oil system was statistically analyzed using the Radial Distribution Function (RDF) in ForciteAnalysis. By comparing the magnitudes of the RDF of the CO2-cosolvent composite system and the shale oil system under different cosolvent conditions, the miscibility between the two phases was indirectly evaluated. The radial distribution functions of the CO2-cosolvent composite system and the shale oil system under different types of cosolvent conditions are shown below. Figure 3 As shown, the RDF of the CO2-ethyl acetate composite system with shale oil is greater than that of the CO2-ethanol composite system with shale oil and the CO2-propane composite system with shale oil, indicating that the CO2-ethyl acetate composite system has the best solubility in the shale oil system. Compared to ethanol and propane, CO2 injection performance is the best and the improvement effect on shale oil fluidity is the greatest under the action of ethyl acetate.

[0054] (2) The mean square displacement (MSD) of CO2 was statistically analyzed using the Mean squared placement in ForciteAnalysis, and the diffusion coefficient of CO2 was calculated using formula (1) to evaluate the diffusion performance of CO2 under the influence of cosolvent. Figure 4 The mean square displacement of CO2 under different co-solvent conditions was used to calculate the diffusion coefficients of CO2 under the influence of ethanol, ethyl acetate, and propane, which were 3.42 × 10⁻⁶. -8 m 2 / s, 1.07×10 -8 m 2 / s and 1.78×10 -8 m 2 The value per second indicates that CO2 has a relatively large diffusion coefficient and good diffusion performance in this type of shale oil system under the influence of ethanol. For this type of shale oil system, the saturation range under the influence of ethanol is even wider during actual CO2 injection.

[0055] (3) Single-point energy calculations were performed on different models using Forcite Energy, and the interaction energy between the CO2-cosolvent composite system and the shale oil system was calculated using formula (2). The greater the interaction energy between the CO2-cosolvent composite system and the shale oil system, the stronger the promoting effect of the cosolvent on the dissolution of CO2 into the shale oil. By calculating the interaction energy between the CO2-cosolvent composite system and the nanopore wall and the interaction energy between the shale oil system and the nanopore wall, the difference between the two was determined to quantitatively evaluate the competitive adsorption effect between the CO2-cosolvent composite system and the shale oil system at the nanopore wall. Figure 5A The interaction energy represents the interaction energy between the shale oil system and the CO2-cosolvent composite system under different cosolvent conditions. A negative interaction energy indicates adsorption between the two phases. From... Figure 5A As can be seen from the data, the interaction between the CO2-ethyl acetate composite system and the shale oil system is the strongest, providing a greater impetus for mass transfer between the two phases. Figure 5B The interaction energies between the rock wall and the CO2-cosolvent composite system and between the rock wall and the shale oil system under different cosolvent conditions, and the differences between the two, are shown. The larger the difference, the stronger the competitive adsorption between the CO2-cosolvent composite system and the shale oil system at the rock wall. Figure 5B As can be seen, compared with ethanol and propane, the adsorption of CO2-cosolvent composite system by the rock wall is stronger under the influence of ethyl acetate, resulting in better competitive adsorption and thus promoting the stripping of more adsorbed shale oil molecules from the rock wall.

[0056] (4) The interfacial tension between the CO2-cosolvent composite system and the shale oil system was calculated using formula (3) to clarify the effect of the cosolvent on improving the interfacial tension between the CO2 and shale oil phases, so as to indirectly evaluate the improvement of the cosolvent on the miscibility of the gas and oil phases. The final calculation results showed that, under the conditions of temperature 323.15K and pressure 20MPa, the interfacial tensions between the CO2-ethanol composite system, the CO2-ethyl acetate composite system, and the CO2-propane composite system and the shale oil system were 0.05376mN / m, 0.04355mN / m, and 0.04924mN / m, respectively. Based on this, it was determined that ethyl acetate had a better effect on improving the miscibility of CO2 and shale oil than ethanol and propane, and ethyl acetate was more conducive to reducing the degree of CO2 channeling in this type of shale oil. Under the same temperature and pressure, the interfacial tension results follow the same pattern as the interaction energy between the CO2-cosolvent composite system and the shale oil system. The stronger the interaction between the CO2-cosolvent composite system and the shale oil system, the better the cosolvent's effect on improving interfacial tension.

[0057] (5) The relative concentration distribution of the shale oil system in the z-direction was statistically analyzed using the Concentration Profile in Forcite Analysis. Then, the density distribution of the shale oil system was calculated using formula (4). Finally, the replacement efficiency of the shale oil was calculated based on the density changes of the adsorbed phase at both the beginning and end states. The density distribution of the shale oil system in the z-direction within the nanopores under different co-solvent conditions is shown below. Figure 6 As shown, compared to ethanol and propane, using ethyl acetate as a CO2 co-solvent resulted in a greater decrease in the density of the shale oil adsorbed phase at both ends of the pores, and the oil molecules were more evenly distributed within the pores. By comparing the density changes of the shale oil adsorbed phase before and after simulation, the replacement efficiency of the CO2-co-solvent composite system at both ends of the pores for the shale oil system was calculated. The replacement efficiencies of the CO2-ethanol composite system, the CO2-ethyl acetate composite system, and the CO2-propane composite system for the shale oil system were 13.70%, 28.26%, and 11.92%, respectively. This indicates that ethyl acetate has a better promoting effect on CO2 replacement of this type of shale oil system and a more significant improvement in the oil washing efficiency during actual CO2 injection.

[0058] In summary, using ethyl acetate as a co-solvent results in better CO2 solubility in this type of shale oil system, leading to a more significant improvement in CO2 injection performance and shale oil flowability. Ethanol, on the other hand, results in a wider CO2 diffusion range. Ethyl acetate provides greater impetus for CO2 dissolution in this shale oil system and better promotes the competitive adsorption of CO2 at the rock wall. Compared to the CO2-ethanol and CO2-propane composite systems, the CO2-ethyl acetate composite system exhibits lower interfacial tension and better miscibility with this type of shale oil system. Furthermore, ethyl acetate has a better promoting effect on CO2 displacement in this type of shale oil system, resulting in a more significant improvement in oil washing efficiency. Therefore, ethyl acetate should be selected as the CO2 co-solvent for this type of shale oil system under the conditions of 323.15 K and 20 MPa.

[0059] As can be seen from the above embodiments, the molecular simulation method provided in this specification for evaluating the production-enhancing performance of CO2 co-solvents in shale oil reservoirs can construct shale oil systems with different mineral compositions, nanopores, and diverse components, as well as CO2-co-solvent composite systems with different types and contents of co-solvents. Different models constructed using different combination methods can be used to calculate different parameters. By comparing the RDF between different types of CO2-co-solvent composite systems and shale oil systems, the miscibility between the two can be indirectly evaluated, thereby evaluating the improvement effect of co-solvents on CO2 injection and shale oil flowability. The diffusion coefficient of CO2 in shale oil under the influence of co-solvents is calculated by MSD, thereby evaluating the influence of different types of co-solvents on CO2 diffusion performance, and thus evaluating the influence of co-solvents on the CO2 sweep range. By calculating the interaction energy between the CO2-co-solvent composite system and the shale oil system, the promoting effect of different types of co-solvents on the dissolution performance of CO2 in the shale oil system can be evaluated. Under normal circumstances, the result of this interaction energy is related to the interfacial tension. Correspondingly, by calculating the interaction energy between the CO2-co-solvent composite system and the rock wall, and the interaction energy between the shale oil system and the rock wall, as well as the difference between the two, the changes in competitive adsorption at the rock wall can be quantitatively described, thereby evaluating the promoting effect of co-solvents on CO2 stripping of adsorbed shale oil from the rock wall. By calculating the interfacial tension between the CO2-co-solvent composite system and the shale oil system, the improvement effect of different types of co-solvents on the miscibility of the two systems can be evaluated, and this can be used to evaluate the improvement effect of co-solvents on gas channeling during CO2 injection development of shale oil reservoirs. The replacement efficiency can be calculated by the density change of the shale oil system, evaluating the effect of co-solvents on improving CO2 washing efficiency. This method is systematic, comprehensive, efficient, and low-cost. Through the analysis of a series of parameters, it systematically evaluates the various production-enhancing properties of different types of co-solvents, providing a theoretical basis for the optimal selection of co-solvents during CO2 injection development of shale oil reservoirs.

[0060] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention, and within the spirit and principles of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A molecular simulation method for evaluating the production-enhancing performance of CO2 co-solvents in shale oil reservoirs, characterized in that, It includes the following steps: Step 1: Select different types of shale reservoir rock minerals, shale oil components and CO2 co-solvent, and use MaterialsStudio software to construct different types of nanopore, shale oil system boxes and CO2-co-solvent composite system boxes; Step 2: Temperature and pressure equilibrium are performed on the established shale oil system box and CO2-cosolvent composite system box to make the model meet the actual formation temperature and formation pressure conditions of the shale oil reservoir. During the equilibrium process, the size of the box remains unchanged in the x and y directions, and only the size in the z direction can change. Step 3: By placing two shale oil system boxes at both ends of the nanopores and attaching them tightly to the mineral wall, and inserting a CO2-cosolvent composite system box in the middle space of the two shale oil system boxes, a three-phase contact model is constructed to simulate the process of CO2 replacing shale oil under the action of a cosolvent. In addition, by placing the shale oil system box in the middle of the two CO2-cosolvent composite system boxes, a model is constructed to calculate the interfacial tension. Step four: Perform kinetic simulations on each model. By analyzing the radial distribution function, diffusion coefficient, interfacial tension, interaction energy, and density distribution, clarify the effects of the co-solvent on the dissolution, diffusion, miscibility, competitive adsorption of CO2 in nanopores in shale oil, and the improvement of shale oil replacement efficiency. Based on this, comprehensively evaluate the various production-enhancing properties of the co-solvent. Diffusion coefficient calculation formula: Formula (1); In the formula, D is the diffusion coefficient, and m is the diffusion coefficient. 2 / s;r i (t) — The position of the molecule at time t; —Average the squares of the molecular displacements in the component; Formula for calculating interaction energy: Formula (2); In the formula, E int —Interaction energy, kcal / mol; E total —The total energy of substance 1 and substance 2, kcal / mol; E1—Energy of substance 1, kcal / mol; E2—Energy of substance 2, kcal / mol; Formula for calculating interfacial tension: Formula (3); In the formula, γ—interfacial tension, mN / m; p ii (i=x,y,z) — Pressure components in different directions, MPa; L z —The box dimension perpendicular to the interface along the Z direction, in nm; l z —The length of any segment containing the interface along the Z direction, in nm; Density distribution calculation formula: Formula (4); In the formula, C r —Relative concentration, dimensionless quantity; m —Mass of a component, g; V —Unit volume of a component, cm³ 3 ρ(slab) — density of shale oil in a specific block, g / cm³ 3 ρ(bulk) — Macroscopic density of shale oil, g / cm³ 3 .

2. The molecular simulation method for evaluating the CO2 co-solvent enhancement performance in shale oil reservoirs according to claim 1, characterized in that, Step 1 constructs different nanopore, shale oil system boxes, and CO2-cosolvent composite system boxes. The dimensions of the nanopore walls, shale oil system boxes, and CO2-cosolvent composite boxes are consistent in the x and y directions.

3. The molecular simulation method for evaluating the CO2 co-solvent enhancement performance in shale oil reservoirs according to claim 1, characterized in that, Step 2 involves balancing the temperature and pressure of the established shale oil system box and CO2-cosolvent composite system box. Using Forcite Dynamics, the system temperature is balanced to the actual formation temperature of the shale oil reservoir under the NVT ensemble using a Nosé-Hoover temperature controller, and the system pressure is balanced to the actual formation pressure of the shale oil reservoir under the NPT ensemble using a Berendsen pressure controller.

4. The molecular simulation method for evaluating the production-enhancing performance of CO2 co-solvents in shale oil reservoirs according to claim 1, characterized in that, Step 3 involves constructing a three-phase contact model to simulate the process of CO2 displacing shale oil under the action of a cosolvent, and a model for calculating interfacial tension. When constructing the three-phase contact model and the model for calculating interfacial tension using Build Layers, the distance between different boxes should be kept at least 0.4 nm to avoid atomic overlap.

5. The molecular simulation method for evaluating the CO2 co-solvent enhancement performance in shale oil reservoirs according to claim 1, characterized in that, Step four includes: (1) The radial distribution function of CO2-cosolvent composite system and shale oil is statistically analyzed using the radial distribution function in Forcite Analysis. By comparing the magnitude of the radial distribution function of CO2-cosolvent composite system and shale oil under different cosolvent conditions, the miscibility between the two phases can be indirectly evaluated. The larger the radial distribution function, the stronger the miscibility between the two phases. (2) The mean square displacement of CO2 is statistically analyzed using the Mean square displacement in Forcite Analysis, and the diffusion coefficient of CO2 is calculated using formula (1) to evaluate the diffusion performance of CO2 under the influence of the co-solvent. The larger the CO2 diffusion coefficient, the more obvious the promoting effect of the co-solvent on the diffusion of CO2 in shale oil, and the wider the ripple range of injected CO2 in shale oil reservoirs in actual development. (3) Use Forcite Energy to calculate the single-point energy of different models, and combine it with formula (2) to calculate the interaction energy between CO2-cosolvent composite system and shale oil. When the interaction energy is negative, it indicates that the two phases attract each other. The greater the interaction energy between CO2-cosolvent composite system and shale oil, the stronger the promoting effect of cosolvent on CO2 dissolution into shale oil. By calculating the interaction energy between CO2-cosolvent composite system and nanopore wall and the interaction energy between shale oil and nanopore wall, the difference between the two is determined to quantitatively evaluate the competitive adsorption effect between CO2-cosolvent composite system and shale oil at nanopore wall. The greater the difference, the stronger the competitive adsorption effect, which is more conducive to CO2 stripping the oil film adsorbed on nanopore wall. (4) Calculate the interfacial tension between the CO2-cosolvent composite system and shale oil using formula (3) to clarify the effect of the cosolvent on improving the interfacial tension between the CO2 and shale oil phases, so as to indirectly evaluate the improvement of the cosolvent on the miscibility of the gas and oil phases; the smaller the interfacial tension, the better the effect of the cosolvent on improving the miscibility of the gas and oil phases, and the more conducive it is to hindering the gas channeling of CO2 in the actual reservoir. (5) The relative concentration distribution of the shale oil system in the z direction is statistically analyzed using the Concentration profile in Forcite Analysis. Then, the density distribution of the shale oil system is obtained by conversion using formula (4). The replacement efficiency of shale oil is calculated by the density change of the adsorbed phase at the beginning and end. The greater the replacement efficiency, the more obvious the improvement of the oil washing efficiency of the cosolvent in actual development. By combining a series of parameters, the effects of co-solvents on the dissolution, diffusion, miscibility, competitive adsorption of CO2 in nanopores in shale oil, and the improvement of shale oil replacement efficiency were clarified, so as to comprehensively evaluate the various production-enhancing properties of co-solvents.

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