A molecular simulation method and device for carbon dioxide huff and puff shale oil production

Abstract

The present application discloses a molecular simulation method and device for carbon dioxide huff and puff shale oil production, which belongs to the technical field of shale oil production. The method includes: constructing a CO2 bulk box; constructing a pore model, filling the target component oil molecules and pre-balancing to achieve a stable occurrence state; first performing MD relaxation simulation, and then combining to obtain an initial model; applying a constant injection pressure to the initial model, and performing NEMD relaxation simulation to obtain a CO2 huff and puff oil production model; reducing the applied constant pressure to the pressure of the depressurization production stage, and performing NEMD relaxation simulation of depressurization production. The device includes: a control module, which has a built-in program for executing the above molecular simulation method; a display module; and a pressure regulating module. The present application can simulate a downhole CO2 huff and puff shale oil state and pressure environment close to the actual state, and observe the action law of CO2 huff and puff shale oil from a microscopic level, thereby providing a theoretical research basis for improving the production and recovery rate of shale oil reservoirs.

Description

Technical Field

[0001] The present application belongs to the technical field of shale oil extraction, and in particular, relates to a molecular simulation method and device for extracting shale oil through carbon dioxide huff and puff. Background Art

[0002] With the continuous growth of global energy market demand and the depletion of conventional oil and natural gas reserves, people's demand for the exploration and development of unconventional oil and gas resources has increased greatly. Among them, shale oil and shale gas are both unconventional oil and gas resources. Shale oil refers to the oil resources contained in shale formations dominated by shale, including oil in the pores and cracks of mud shale, and also including oil resources in the adjacent layers and interlayers of dense carbonate or clastic rocks in the mud shale formation. Shale oil, as an important component of unconventional oil and gas resources, has shown great potential; while shale gas is mainly stored and preserved by organic matter adsorption or cracks and matrix pores in rocks. However, my country's continental shale oil reservoirs have the characteristics of strong heterogeneity, diverse storage space types, extensive development of nano-scale pores, extremely low porosity and permeability, and poor fluid mobility. The current one-time development of shale oil reservoirs after fracturing has the problems of low utilization, low single well production, rapid production decline, and low recovery rate.

[0003] In order to further increase the production of shale oil reservoirs, CO2 huff and puff is considered to be a powerful way to achieve efficient shale oil extraction. However, field practice has found that the effect of CO2 huff and puff on improving oil recovery varies greatly, which seriously restricts the promotion and application of this technology. Since traditional theory cannot explain the microscopic mechanism of action, it cannot provide a good theoretical basis for the efficient extraction of shale oil. The mechanism of CO2 huff and puff shale oil and shale gas is essentially different, and the mechanism research on shale gas cannot be directly applied to shale oil. Therefore, in order to study the microscopic mechanism of CO2 huff and puff shale oil, it is urgent to study CO2 huff and puff shale oil extraction from the molecular scale. However, there is currently a lack of CO2 huff and puff simulation methods for shale oil, which cannot provide a rigorous theoretical research basis. Summary of the invention

[0004] The present application aims to at least to some extent solve the technical problem of the lack of a simulation method for CO2 throughput of shale oil. To this end, the present application provides a molecular simulation method and device for carbon dioxide throughput to exploit shale oil, which can simulate the actual downhole CO2 throughput shale oil state and pressure environment, observe the action law of CO2 throughput shale oil from a microscopic level, and thus provide a theoretical research basis for improving the production and recovery rate of shale oil reservoirs.

[0005] In a first aspect, an embodiment of the present application provides a molecular simulation method for shale oil production by carbon dioxide huff and puff, which comprises:

[0006] According to the simulation requirements, a pure component CO2 bulk box is constructed under the injection temperature and pressure conditions. The bulk box is used to simulate the CO2 gas source during the CO2 huff-and-puff oil production process.

[0007] A pore model is constructed based on the evaluation results of shale oil reservoirs. The pore model is used to simulate a confined space. The pores of the pore model are filled with target component oil molecules and the pore model is pre-balanced so that the oil molecules in the pores reach a stable occurrence state under target temperature and pressure conditions.

[0008] The bulk box and the pore model of stable oil molecules are combined, and molecular dynamics relaxation simulation is first performed to make the combined model reach pre-equilibrium, and the part of the pore model that blocks the bulk box and the entrance of the confined space is removed. Then the bulk box is connected and combined with the remaining pore model to obtain the initial model of CO2 injection into shale oil.

[0009] A constant injection pressure is applied to the side of the bulk box in the initial model that is far away from the entrance of the confined space, and a non-equilibrium molecular dynamics relaxation simulation of the CO2 injection and well soaking process is performed to obtain a CO2 huff-and-puff oil production model after CO2 injection and well soaking.

[0010] The applied constant pressure is reduced to the pressure in the depressurization production stage, and the non-equilibrium molecular dynamics relaxation simulation of depressurization production is carried out on the CO2 huff-and-puff production model after CO2 injection and well soaking, completing a molecular simulation process of a CO2 huff-and-puff shale oil production cycle.

[0011] In some embodiments, it further comprises:

[0012] After a CO2 huff-and-puff cycle of shale oil production is completed, the bulk box is removed, and a new bulk box of pure CO2 under injection temperature and pressure conditions is reconstructed, and the new bulk box is recombined with the pore model without the bulk box to form the initial model.

[0013] The steps are repeated to obtain a CO2 huff and puff oil production model, and a molecular simulation process of a CO2 huff and puff shale oil production cycle is performed until multiple CO2 huff and puff shale oil production cycles are completed.

[0014] In some embodiments, it also includes: in the molecular simulation process of the CO2 huff and puff shale oil production cycle, by comparing the simulated configuration changes of the CO2 huff and puff oil production model, analyzing the microscopic production increase mechanism and molecular mass transfer behavior characteristics of the CO2 huff and puff shale oil production process.

[0015] In some embodiments, when constructing a bulk phase box, a fixed helium plate and a graphene piston plate of comparable size are placed, CO2 molecules are filled between the helium plate and the graphene piston plate, and the graphene piston plate is pressurized to perform non-equilibrium molecular dynamics relaxation simulation to obtain a bulk phase box under desired temperature and pressure conditions.

[0016] In some embodiments, when constructing a pore model, a target mineral component is constructed at the edge to form a confined space with one end open, and then a barrier plate is set at the interval from the open end of the confined space. The part enclosed by the target mineral component forms pores, and the pores are nanopores of pore scale.

[0017] In some embodiments, when constructing the target mineral component at the edge, the two rock walls of the target mineral component are first placed in parallel, and the rock wall of the target mineral component is used to seal one end to form a confined space that is open at the other end; when constructing the pores, a helium plate is used to seal the position opposite to the barrier plate on the periphery of the open end of the confined space.

[0018] In some embodiments, when the pore model is pre-balanced, target component oil molecules are filled in the pores between the baffle plate and the open end of the confined space, and pressure is applied to the oil molecules through the baffle plate to perform non-equilibrium molecular dynamics relaxation simulation.

[0019] In some embodiments, when the bulk box and the pore model for stably storing oil molecules are combined, the oil molecules and barrier plates outside the pores are removed to obtain the remaining pore model, at which time the pore model stably stores shale oil under target temperature and pressure conditions.

[0020] In some embodiments, when the bulk box is connected and combined with the remaining pore model: the free end face of the bulk box is first aligned with the open end of the confined space, and then structural optimization is performed after docking to obtain an initial model.

[0021] In a second aspect, an embodiment of the present application provides a molecular simulation device for shale oil production by carbon dioxide huff and puff, comprising:

[0022] A control module having a built-in program for executing the molecular simulation method for shale oil extraction by carbon dioxide huff and puff;

[0023] A display module, electrically connected to the control module, for displaying the execution result of the program;

[0024] The pressure regulating module is electrically connected to the control module and is used to regulate the pressure applied to the bulk box.

[0025] It can be seen from the above technical solution that the beneficial effects of this application are:

[0026] 1. This application constructs a pure component CO2 bulk box to simulate the CO2 gas source in the CO2 huff and puff oil production process, and constructs a pore model to form a stable occurrence state close to the target downhole temperature and pressure conditions. After the bulk box and the pore model are combined, the initial model of CO2 huff and puff shale oil in the formation is simulated. Then, by applying a constant injection pressure on one side of the bulk box and reducing the applied constant pressure, the kerogen expulsion pressure in the reservoir, the injection of CO2, and the depressurized production of CO2 are simulated. This can simulate a downhole CO2 huff and puff shale oil state and pressure environment close to the actual one, and at the same time Molecular dynamics relaxation simulation was used to obtain the initial model, and non-equilibrium molecular dynamics relaxation simulation was used to obtain the CO2 huff and puff oil production model. Through the molecular simulation method combining molecular dynamics (MD) + non-equilibrium molecular dynamics (NEMD), the physical process of CO2 huff and puff shale oil to improve the recovery rate in the microscopic pore structure of shale can be maximized and restored at the microscopic molecular scale. This is conducive to clarifying the microscopic mechanism of CO2 huff and puff shale oil production, observing the action law of CO2 huff and puff shale oil from a microscopic level, and thus providing a theoretical research basis for improving shale oil reservoir production and recovery rate.

[0027] 2. The present application executes the above-mentioned molecular simulation method for shale oil exploitation by carbon dioxide throughput through a control module, displays the output results of the simulation through a display module, and adjusts the pressure of the bulk box through a pressure regulating module. The pressure regulating module is controlled by a program, and the pressure regulating module can be used to quantitatively control the pressures of the injection and exploitation stages during the CO2 throughput process to achieve precise regulation, thereby simulating the downhole shale oil reservoir environment, which is more in line with actual conditions; and because the program adopts the above-mentioned molecular simulation method, the device can automatically simulate a downhole CO2 throughput shale oil state and pressure environment close to the actual state, and more intuitively reveal the microscopic effective mechanism of CO2 enhanced recovery rate due to the molecular mass transfer effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are some embodiments of the present application. For ordinary technicians in this field, other embodiments and drawings can be obtained based on these drawings without creative work.

[0029] Figure 1 A schematic diagram showing the steps of an embodiment of the molecular simulation method for shale oil production by carbon dioxide huff and puff according to the present invention;

[0030] Figure 2 A schematic diagram of an embodiment of the molecular simulation method for shale oil production by carbon dioxide huff and puff according to the present invention is shown;

[0031] Figure 2In the figure, (a) is a schematic diagram of the modeling of the bulk box simulating the external CO2 gas source; (b) is a schematic diagram of the pore model modeling of the stable storage of shale oil; (c) is a schematic diagram of the first cycle initial model of the combination of the bulk box and the pore model; (d) is a schematic diagram of the model simulation of CO2 injection and well blocking; (e) is a schematic diagram of the model simulation of pressure reduction production; (f) is a schematic diagram of the second cycle initial model of the combination of the bulk box and the pore model.

[0032] Figure 3 A schematic diagram showing the steps of an embodiment of the molecular simulation method for shale oil production by carbon dioxide huff and puff according to the present invention;

[0033] Figure 4 The density distribution diagram of the initial occurrence state of nC7 in the calcite nanopores according to the embodiment of the present invention is shown;

[0034] Figure 5 Two competitive adsorption modes and the evolution diagram of the adsorption state nC7 of the present invention are shown;

[0035] Figure 6 The cloud diagram of CO2 distribution density in the calcite nanopores and the curve diagram of CO2 density in the middle of the pores according to the embodiment of the present invention are shown;

[0036] Figure 7 A graph showing the change in the amount of CO2 and nC7 in the calcite nanopores over time according to an embodiment of the present invention. DETAILED DESCRIPTION

[0037] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] It should be noted that all directional indications in the embodiments of the present invention are only used to explain the relative position relationship, movement status, etc. between various components in a certain specific posture. If the specific posture changes, the directional indication will also change accordingly.

[0039] In the present invention, unless otherwise clearly specified and limited, the terms "connection", "fixation", etc. should be understood in a broad sense. For example, "fixation" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0040] In addition, in the present invention, descriptions such as "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In addition, the technical solutions between the various embodiments can be combined with each other, but they must be based on the ability of ordinary technicians in the field to implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0041] The present application is described below with reference to the accompanying drawings and specific embodiments:

[0042] Please refer to Figure 1 The first embodiment of the present application provides a molecular simulation method for shale oil production by carbon dioxide huff and puff, which includes steps S1 to S5:

[0043] S1. Construct a pure component CO2 bulk box under injection temperature and pressure conditions according to the simulation requirements. Please refer to Figure 2 Specifically, the model is built in the three-axis coordinate system of the modeling software, in which the Y axis can be ignored, and the simulation is performed in the XZ plane. A three-dimensional model can also be established. In this embodiment, a three-dimensional model is established, in which the model in the Y-axis direction is the same as the model in the Z-axis direction. The following description is taken as an example in the Z-axis direction. According to the simulation requirements, the specific size can be determined according to the design requirements. The bulk box is used to simulate the CO2 gas source in the CO2 injection and extraction process. Among them, the bulk phase refers to the structure or composition state inside the material or substance, which involves the molecular arrangement, interaction and various physical and chemical properties of the substance. Compared with the surface or interface, the bulk phase is more stable and not easily affected by the external environment. The bulk box is a virtual container used to simulate the bulk properties of a substance in molecular dynamics simulation. This container is usually a box with a fixed volume and shape, filled with particles representing the molecules of the substance. These particles move inside the box according to a specific force field and initial conditions, thereby simulating the behavior and state of the substance.

[0044] In some embodiments, modeling is performed in a three-axis coordinate system of a modeling software. When constructing a bulk phase box, a fixed helium plate is first placed perpendicular to the X-axis, and a graphene piston plate of comparable size is placed parallel to the plate. The helium plate is immovable, and the graphene piston plate has a certain range of motion under the injection pressure. CO2 molecules are filled between the helium plate and the graphene piston plate. A certain number of CO2 molecules are filled according to the density of CO2 under the target temperature and pressure conditions in the NIST website database. The injection pressure during the CO2 throughput process is simulated by pressurizing the graphene piston plate, and a non-equilibrium molecular dynamics relaxation simulation is performed to allow the system to reach thermodynamic equilibrium, thereby obtaining a bulk phase box under the desired temperature and pressure conditions.

[0045] In the specific implementation, the Materials Studio 2023 software was used to construct a graphene piston plate and a helium plate with a length and width of 89.45 Å and 24.59 Å respectively (where Å is the unit of length, 1Å=0.1nm), and the two plates were placed parallel to the X-axis at a distance of 100 Å in an amorphous unit cell of appropriate size. Then, the Amorphous Cell module was used, using the technical parameters provided by the NIST website: 0.8237 g / cm 3 The CO2 density (temperature and pressure conditions: 40MPa, 353K) was set, and 2413 CO2 molecules were filled between the two plates to optimize the structure and eliminate the unreasonable existence of molecules. The helium plate was fixed and a constant pressure of 40MPa was applied to the left end of the graphene piston plate using a Perl script. Then, a 1ns non-equilibrium molecular dynamics (NEMD) simulation was performed under the NVT ensemble to make the CO2 box as a whole reach a stable state under the temperature and pressure conditions of 353K and 40MPa. The obtained CO2 box was used to simulate the external CO2 gas source, such as Figure 2 As shown in part (a).

[0046] S2. Construct a pore model based on the evaluation results of shale oil reservoirs. The pore model is used to simulate confined space. The target component oil molecules are filled in the pores of the pore model and the pore model is pre-balanced so that the oil molecules in the pores reach a stable state under the target temperature and pressure conditions. The target component oil molecules refer to the component oil molecules of shale oil. Among them, confined space refers to a restricted or constrained spatial range. This space can be defined by physical boundaries (such as walls, containers, etc.). The purpose of constructing confined space is usually to control, optimize or study the physical, chemical or biological processes occurring therein. By simulating the confined space at the nanoscale, accurate simulation of material properties can be achieved, which is more in line with the shale oil reservoir environment downhole. Pre-balance is a state of dynamic equilibrium. In the formation, gas molecules are in a complex geochemical environment. They have extensive interactions with rocks, groundwater, other gas molecules, and the biosphere. These interactions may cause changes in the concentration, distribution and energy state of gas molecules. In some cases, the gas molecules in the formation may be in a relatively stable state, but this stability is not an absolute balance, but a dynamic balance restricted and influenced by multiple factors. At this time, this state is called "pre-equilibrium".

[0047] In some embodiments, when constructing a pore model, a target mineral component is constructed at the edge of the model to form a confined space with one end open. The shape of the model can be determined as needed, such as square or circular. It can be constructed in a shape close to the real pore, and then a baffle is set at the interval with the open end of the confined space. The baffle is a graphene piston plate, and the part surrounded by the target mineral component forms a pore, which is a nanopore of pore scale. In some embodiments, when constructing the target mineral component at the edge, first place the two rock walls of the target mineral component in parallel, parallel to the X-axis and spaced apart, and use the rock wall of the target mineral component to seal at one end along the X-axis to form a confined space with the other end open, so as to obtain a pore model of a single slit; when constructing the pore, a helium plate is used to seal the position opposite to the baffle plate at the periphery of the open end of the confined space, that is, the non-enclosed space above and below the pore in the figure is sealed in the X-axis direction, and the helium plate and the rock wall are all fixed to obtain a pore model.

[0048] In the specific implementation, the calcite wall model is obtained by cutting and expanding the calcite unit cell along the (1 0 4) direction, with a length, width and thickness of 125 Å, 24.59 Å and 12.5 Å respectively, and then two identical calcite walls are placed in parallel in another amorphous unit cell with a distance of 3.5 nm along the X-axis direction, and then the right end is blocked with a calcite wall with a smaller cross-section and the same thickness as the end of the pore. A helium plate is placed at the pore outlet to block the non-confined space above and below the pore, and finally all the walls and helium plates are fixed to obtain a single calcite slit nanopore model.

[0049] In some embodiments, when the pore model is pre-balanced, the target component oil molecules are filled in the pores between the baffle plate and the open end of the confined space. The target component oil molecules can be typical characterization component oil molecules, which can be determined according to chromatographic analysis, such as C7, C9, C12, etc. The baffle plate is a certain distance away from the open end of the confined space, and the open end of the confined space is its entrance. When filling, a certain number of oil molecules are filled according to the standard density of a certain oil component at the target temperature and pressure in the NIST website database, and then a force is applied on the side to apply pressure to the oil molecules through the baffle plate to simulate the reservoir pressure and perform non-equilibrium molecular dynamics relaxation simulation; in some embodiments, a part of the pore model can be removed in the next step S3, and in this step, after the model configuration is stable, the oil molecules and graphene piston plates outside the pores can be removed to obtain a pore model with stable shale oil.

[0050] In the specific implementation, the space inside the pore and 50 Å outside the pore is set according to the technical parameters provided by the NIST website: 0.6613 g / cm 3 The density of the pore (temperature and pressure conditions: 25MPa, 353K) was filled with 744 n-heptane molecules (nC7), and then a pressure of 25MPa was applied to nC7 through the graphene piston plate for NEMD relaxation. After the model configuration was stable, the nC7 and graphene piston plate outside the pore were deleted to obtain the calcite pore model with stable storage of shale oil, as shown in Figure 2 As shown in part (b).

[0051] S3. Combine the bulk box and the pore model for stably storing oil molecules, perform molecular dynamics relaxation simulation first to make the combined model reach pre-equilibrium, and remove the part of the pore model that blocks the bulk box and the entrance to the confined space. Molecular dynamics relaxation simulation is to explore the dynamic properties of matter by simulating the interaction between atoms and molecules. In MD simulation, matter is regarded as a basic unit composed of atoms, and the evolution of these basic units over time can reveal the dynamic behavior of matter.

[0052] In some embodiments, when the bulk box is combined with the pore model for stably storing oil molecules, the oil molecules and the barrier plates outside the pores are removed, such as deleting the model part outside the confined space in the model to obtain the pore model left after the deletion, at which time the pore model stably stores shale oil under the target temperature and pressure conditions. Then the bulk box is connected and combined with the remaining pore model, and the free end of the bulk box is docked with the pore model at the open end of the confined space, such as docking the other end of the bulk box opposite to the graphene piston plate with the entrance of the pore model to obtain the initial model of CO2 shale oil. In some embodiments, when the bulk box is connected and combined with the remaining pore model: first, the free end face of the bulk box is opposite to the open end of the confined space, and then the structure is optimized after docking to obtain the initial model.

[0053] In the specific implementation, after combining the CO2 box and the calcite nanopore model containing nC7, a 1ns MD relaxation simulation is performed to allow the combined model to reach pre-equilibrium. Then the helium plate that blocks CO2 and n-heptane is deleted, and then a 50,000-step structural optimization is performed to eliminate the unreasonable structure of the model after deleting the helium plate, and the initial CO2 throughput model is obtained, as shown in Figure 2 As shown in part (c).

[0054] S4. Apply a constant injection pressure to the side of the bulk box in the initial model that is away from the entrance of the confined space, and perform non-equilibrium molecular dynamics relaxation simulations of the CO2 injection and well-insertion process to obtain a CO2 injection and well-insertion model for CO2 oil production. Among them, the non-equilibrium molecular dynamics relaxation simulation is to explore the dynamic behavior and relaxation process of the system in a non-equilibrium state by simulating the motion trajectory and interaction of molecules or atoms under specific conditions. Relaxation refers to the process in which a system gradually evolves from a non-equilibrium state to an equilibrium state, which involves energy exchange and redistribution between molecules or atoms. Since the injection pressure P1 of CO2 is higher than the initial pressure P2 in the pores, CO2 can enter the nanopores under the action of pressure difference and concentration difference, and then simulate the thermodynamics and microscopic mass transfer behavior of CO2 huff-and-puff production of shale oil in the subsequent simulation process.

[0055] In the specific implementation, a constant injection pressure is applied to the graphene piston plate on the left side of the CO2 bulk box in the initial model of CO2 huff and puff oil production, and a NEMD relaxation simulation of the CO2 injection and well-insufflation process is performed for 1.5 ns to obtain the CO2 huff and puff oil production model after CO2 injection and well-insufflation; an injection pressure of 40 MPa is applied to the graphene piston plate on the left side of the CO2 bulk box in the initial model of CO2 huff and puff oil production, and then a NEMD relaxation simulation is performed using the COMPASSⅢ force field, in which the temperature control adopts the Andersen constant temperature mode, the electrostatic effect is calculated using the PPPM summation method, the van der Waals interaction is calculated using AtomBased, the cutoff radius is 12.5 Å, the NVT ensemble is used, the step size is 1fs, a frame of model state is recorded every 10 ps, ​​the total simulation time is 1.5 ns, and finally 150 frames of state results will be provided. During this period, since the overall pressure of the CO2 bulk box is 40MPa, which is higher than the 25MPa of the fluid in the pores, CO2 can be injected into the pores under the influence of pressure difference and concentration difference, and the CO2 injection and well soaking process are realized in the subsequent simulation process. Finally, the CO2 huff-and-puff oil production model after CO2 injection and well soaking is obtained, as shown in Figure 2 As shown in part (d).

[0056] S5. Reduce the applied constant pressure to the pressure of the depressurization production stage, perform non-equilibrium molecular dynamics relaxation simulation of depressurization production on the CO2 huff and puff oil production model after CO2 injection and well shut-in, and complete a molecular simulation process of CO2 huff and puff production shale oil cycle. For the CO2 huff and puff oil production model after CO2 injection and well shut-in, reduce the injection pressure originally applied to the graphene piston plate to the pressure P3 during the depressurization production process. Since the depressurization production pressure P3 is significantly lower than P1 and P2, during the non-equilibrium dynamic relaxation process, the fluid molecules in the nanopores will transfer mass to the outside of the pores under the action of expansion displacement, realize the simulation of the CO2 huff and puff oil production process during the depressurization production process, and complete a CO2 huff and puff oil production cycle.

[0057] In the specific implementation, the constant pressure of the graphene piston plate is reduced to the pressure of depressurization, and the non-equilibrium molecular dynamics relaxation of the CO2 huff and puff oil production model after the well is completed is performed to complete a molecular simulation process of the CO2 huff and puff oil production cycle; then the constant pressure of the graphene piston plate is reduced to 20MPa, and the NEMD relaxation simulation of the CO2 huff and puff oil production model after the well is completed is performed for 1ns. The relevant details of the process are set the same as those in step S4. The simulation of the depressurization production process is completed, thereby completing the first huff and puff cycle process, such as Figure 2 As shown in part (e).

[0058] At present, there is a lack of CO2 throughput simulation methods for shale oil. Analysis shows that the main reason for this phenomenon is the unclear understanding of the mechanism of microscopic phenomena such as the microscopic interface properties, miscibility laws, adsorption and desorption laws, and diffusion mass transfer processes of CO2-crude oil in the nano-confined space of shale reservoirs. The core-scale physical modeling experiment cannot accurately explain the microscopic process of the above complex phenomena, and it is difficult to explain the interaction mechanism of nano-confined pores CO2-pores-crude oil and the characteristics of crude oil utilization at the microscopic molecular scale. The existing CO2 throughput simulation methods for shale gas cannot be directly used in shale oil in the mechanism research of shale gas. This is because most of the reserves of shale oil are adsorbed gas. When simulating shale gas, it is set up by means of Grand Canonical Monte Carlo (GCMC) + Molecular Dynamics (MD). The basic logic of GCMC is to connect a hypothetical infinite box outside the simulation box, give this box a certain temperature and pressure state, and then let the target simulation box and the infinite box reach molecular dynamics equilibrium by themselves. This method is more It is suitable for the study of adsorption laws and is suitable for shale gas; but compared with shale gas, the adsorbed crude oil in shale oil reserves is much less than the free crude oil. The occurrence of shale oil does not rely on the adsorption principle, but is filled by the expulsion pressure generated when kerogen is cracked. Therefore, using the logic of GCMC for free adsorption simulation cannot achieve the stable occurrence of liquid shale oil under reservoir conditions (temperature and pressure). Therefore, according to this characteristic of shale oil, the occurrence and production simulation can only be carried out by adding external expulsion pressure.

[0059] In order to realize the simulation process of CO2 huff and puff production of shale oil, the present application simulates kerogen expulsion pressure, CO2 injection and CO2 depressurization production in the process of occurrence simulation and production simulation by means of external expulsion pressure. Specifically, the present application simulates the CO2 gas source in the CO2 huff and puff production process by constructing a pure component CO2 bulk phase box, and forms a stable occurrence state close to the target downhole temperature and pressure conditions by constructing a pore model. In this way, after the bulk phase box is combined with the pore model, an initial model of CO2 huff and puff shale oil in the formation is simulated, and then the kerogen expulsion pressure, CO2 injection and CO2 depressurization production in the reservoir are simulated by applying a constant injection pressure on one side of the bulk phase box and reducing the applied constant pressure. The depressurized production can simulate the actual downhole CO2 huff and puff shale oil state and pressure environment. At the same time, molecular dynamics relaxation simulation is used in obtaining the initial model, and non-equilibrium molecular dynamics relaxation simulation is used to obtain the CO2 huff and puff oil production model. Through the molecular simulation method combining molecular dynamics (MD) + non-equilibrium molecular dynamics (NEMD), the physical process of CO2 huff and puff shale oil to improve the recovery rate in the microscopic pore structure of shale can be maximized at the microscopic molecular scale. It is helpful to clarify the microscopic mechanism of CO2 huff and puff shale oil production, observe the action law of CO2 huff and puff shale oil from the microscopic level, and thus provide a theoretical research basis for improving the production and recovery rate of shale oil reservoirs.

[0060] Please refer to Figure 3 In some embodiments, the molecular simulation method further comprises the steps of:

[0061] S6, after completing a CO2 huff-and-puff cycle to produce shale oil, remove the bulk box, reconstruct a new bulk box of pure CO2 injected under temperature and pressure conditions, and reassemble it with the pore model without the bulk box to form an initial model. In specific implementation, the fluid molecules outside the pores of the oil production model after completing a huff-and-puff cycle and the graphene piston plate are deleted, and the CO2 bulk box in step S1 is re-placed and combined to form the initial model of the second cycle.

[0062] S7, repeat the steps to obtain a CO2 huff and puff oil production model, and perform a molecular simulation process of the CO2 huff and puff shale oil production cycle, such as Figure 2 As shown in part (f), multiple CO2 huff and puff cycles for shale oil production are completed. Specifically, steps S3 to S5 are repeated to complete multiple subsequent CO2 huff and puff cycles in this way.

[0063] In some embodiments, the molecular simulation method further comprises the steps of:

[0064] S8. In the molecular simulation process of the CO2 huff and puff shale oil production cycle, the microscopic increase in production mechanism and molecular mass transfer behavior characteristics of the CO2 huff and puff shale oil production process are analyzed by comparing the simulated configuration changes of the CO2 huff and puff oil production model. The configuration changes in the simulation process of the above steps S4 to S7 are analyzed. When specifically implemented: data analysis is performed in combination with the simulation results obtained in steps S4 to S7 to analyze the microscopic increase in production mechanism and molecular mass transfer behavior characteristics of the CO2 huff and puff shale oil production process. Figure 4 This is the density profile curve of the initial state of nC7 in the nanopores of calcite. It can be observed that there are three adsorption layers and a bulk region with very small density fluctuation in the nanopore nC7.

[0065] Please refer to Figure 5 , Figure 5 Two competitive adsorption modes of CO2 and the evolution of nC7 in the adsorption state over time are shown. One competitive adsorption process occurs in the short period of time before the intake and discharge, and carbon dioxide strips off the originally adsorbed nC7 in a triangular wedge pattern at the calcite wall to achieve a competitive adsorption process. The second competitive adsorption mode is that CO2 first enters the pore from the middle of the pore along the pore direction by relying on the pressure difference and concentration difference, and then migrates to the pore space where the wall has not yet been occupied by CO2. The overall migration direction of CO2 changes to migration toward the wall, and then occupies the adsorption sites on the wall, stripping off the nC7 adsorption layer; nC7 in the adsorption state is first stripped and migrated to the middle of the pore by mass transfer, and then migrates toward the outlet along the pore direction. Figure 6 Using the CO2 density cloud map and the density change of CO2 in the middle of the pore along the pore diameter direction, the distribution pattern of CO2 in the pore can be clearly seen. Combined with the CO2 migration pattern, the pores can be divided into three major areas in turn: the miscible zone, the miscible front zone, and the pure oil zone. Figure 7 The figure shows the variation of the amount of nC7 and CO2 in the calcite nanopores over time during five CO2 throughput cycles. The total time of a throughput cycle is 2.5ns. As the number of throughput cycles increases, the amount of nC7 in the calcite pores gradually decreases, and the amount of CO2 gradually increases. When the fifth cycle comes, the changes in the amount of the two molecules tend to be gentle.

[0066] In a second aspect of the present application, a molecular simulation device for carbon dioxide huff and puff production of shale oil is provided, which includes: a control module, a display module and a pressure regulating module. The control module has a built-in program. The control module adopts a single-chip microcomputer, a CPU, etc. The program is used to execute the above-mentioned molecular simulation method for carbon dioxide huff and puff production of shale oil. The program is written with the execution method in the above-mentioned steps S1 to S5, or steps S1 to S8; the program is executed by the control module, that is, the microscopic process of the CO2 huff and puff oil production model can be directly calculated and simulated; the display module is electrically connected to the control module, such as the display screen is connected to the control module Module, the display module is used to display the execution result of the program, and it is intuitively displayed in the form of pictures or model diagrams; the pressure regulation module is electrically connected to the control module, such as adding a module for applying pressure horizontally on the left side of the graphene piston plate of the bulk box, the module can perform actions to simulate the process of pressure application, the pressure regulation module is used to adjust the pressure applied to the bulk box, so as to accurately control the pressure of the injection and production stages during the CO2 huff and puff process of shale oil production; the storage module is electrically connected to the control module, such as the storage is connected to the control module, the storage is used to store the execution results, and the results of the program are stored.

[0067] At present, there is a lack of CO2 throughput simulation devices for shale oil. This is because there is a lack of CO2 throughput simulation methods for shale oil. The problem with existing devices is that they cannot accurately output results that reflect the complex microscopic phenomena in shale oil production, and are not suitable for the characteristics of shale oil. The present application executes the above-mentioned molecular simulation method for carbon dioxide throughput to produce shale oil through a control module, displays the output results of the simulation through a display module, and adjusts the pressure of the bulk box through a pressure regulating module. The pressure regulating module is controlled by a program. The pressure regulating module can quantitatively control the pressure of the injection and production stages during the CO2 throughput process to achieve precise regulation, thereby simulating the underground shale oil reservoir environment, which is more in line with the actual situation; and because the program adopts the above-mentioned molecular simulation method, the device can automatically simulate the underground CO2 throughput shale oil state and pressure environment close to the actual situation, and more intuitively reveal the microscopic effect mechanism of CO2 enhanced recovery rate of molecular mass transfer effect.

[0068] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", "optional example" or "optional implementation" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine different embodiments or examples described in this specification.

[0069] In addition, the technical solutions between the various embodiments can be combined with each other, but they must be based on the fact that they can be implemented by ordinary technicians in this field. When the combination of technical solutions is contradictory or cannot be implemented, it should be deemed that such combination of technical solutions does not exist and is not within the scope of protection required by this application.

[0070] Although the embodiments of the present application have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present application, and that the scope of the present application is defined by the claims and their equivalents.

Claims

1. A molecular simulation method for shale oil production by carbon dioxide huff and puff, characterized in that: include: Construct a pure component CO2 bulk phase box under injection temperature and pressure conditions according to simulation requirements; The bulk box is a box with a fixed volume and shape, filled with particles representing molecules of a substance, and these particles move inside according to a specific force field and initial conditions. The bulk box is used to simulate the CO2 gas source in the CO2 huff-and-puff oil production process; A pore model is constructed according to the evaluation results of the shale oil reservoir. When constructing the pore model, a target mineral component is constructed at the edge to form a confined space with one end open, and then a barrier plate is set at the interval with the open end of the confined space. The part surrounded by the target mineral component forms pores, and the pores are nanopores of pore scale; the pore model is used to simulate the confined space, and the target component oil molecules are filled in the pores of the pore model and the pore model is pre-balanced so that the oil molecules in the pores reach a stable occurrence state under target temperature and pressure conditions; The bulk box and the pore model for stably storing oil molecules are combined, molecular dynamics relaxation simulation is first performed to make the combined model reach pre-equilibrium, the portion of the pore model that blocks the bulk box and the entrance of the confined space is removed, and then the bulk box is connected and combined with the remaining pore model to obtain an initial model of CO2 injection and injection of shale oil; Applying a constant injection pressure to the side of the bulk box in the initial model that is away from the entrance of the confined space, and performing non-equilibrium molecular dynamics relaxation simulations of CO2 injection and well soaking processes to obtain a CO2 huff-and-puff oil production model after CO2 injection and well soaking; The applied constant pressure is reduced to the pressure of the depressurization production stage, and the non-equilibrium molecular dynamics relaxation simulation of depressurization production is performed on the CO2 huff and puff oil production model after CO2 injection and well soaking, completing a molecular simulation process of a CO2 huff and puff shale oil production cycle.

2. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to claim 1, characterized in that: Also includes: The bulk box after a CO2 huff-and-puff cycle of shale oil production is removed, a new bulk box of pure CO2 injected under temperature and pressure conditions is reconstructed, and the new bulk box is recombined with the pore model without the bulk box to form the initial model; Repeat the steps to obtain the CO2 huff and puff oil production model, and perform a molecular simulation process of the CO2 huff and puff shale oil production cycle until multiple CO2 huff and puff shale oil production cycles are completed.

3. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to claim 2, characterized in that: Also includes: In the molecular simulation process of the CO2 huff and puff shale oil production cycle, the microscopic production increase mechanism and molecular mass transfer behavior characteristics of the CO2 huff and puff shale oil production process are analyzed by comparing the simulated configuration changes of the CO2 huff and puff oil production model.

4. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to any one of claims 1 to 3, characterized in that: When constructing the bulk box, a fixed helium plate and a graphene piston plate of equivalent size are placed, CO2 molecules are filled between the helium plate and the graphene piston plate, and the graphene piston plate is pressurized to perform non-equilibrium molecular dynamics relaxation simulation to obtain the bulk box under the required temperature and pressure conditions.

5. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to claim 1, characterized in that: When constructing the target mineral component at the edge, the two rock walls of the target mineral component are first placed in parallel, and the rock wall of the target mineral component is used to seal one end to form a confined space that is open at the other end; when constructing the pores, a helium plate is used to seal the position opposite to the barrier plate at the outer periphery of the open end of the confined space.

6. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to claim 5, characterized in that: When the pore model is pre-balanced, target component oil molecules are filled in the pores between the baffle plate and the open end of the confined space, and pressure is applied to the oil molecules through the baffle plate to perform non-equilibrium molecular dynamics relaxation simulation.

7. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to claim 1, characterized in that: When the bulk box and the pore model for stably storing oil molecules are combined, the oil molecules outside the pores and the barrier plates are removed to obtain the remaining pore model, at which point the pore model can stably store shale oil under target temperature and pressure conditions.

8. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to claim 7, characterized in that: When the bulk box is connected and combined with the remaining pore model: firstly, the free end face of the bulk box is aligned with the open end of the confined space, and then the structure is optimized after docking to obtain the initial model.

9. A molecular simulation device for shale oil exploitation by carbon dioxide huff and puff, characterized in that: include: A control module having a built-in program, wherein the program is used to execute the molecular simulation method for shale oil extraction by carbon dioxide huff and puff as claimed in any one of claims 1 to 8; A display module, electrically connected to the control module, for displaying the execution result of the program; The pressure regulating module is electrically connected to the control module and is used to regulate the pressure applied to the bulk box.

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