Molecular simulation method and device for exploiting shale oil through carbon dioxide huff and puff

Abstract

The invention discloses a molecular simulation method and device for exploiting shale oil through carbon dioxide huff and puff, and belongs to the technical field of shale oil exploiting. Constructing a pore model, filling target component oil molecules, and pre-balancing to achieve a stable occurrence state; mD relaxation simulation is firstly carried out, and then an initial model is obtained through combination; applying constant injection pressure to the initial model, and performing NEMD relaxation simulation to obtain a CO2 huff and puff oil production model; and reducing the applied constant pressure to the pressure in the depressurization mining stage, and performing NEMD relaxation simulation of depressurization mining. The device comprises: a control module having a built-in program for executing the molecular simulation method; a display module; and a pressure adjusting module. According to the method, the similar actual underground CO2 huff and puff shale oil state and pressure environment can be simulated, and the action rule of CO2 huff and puff shale oil is observed from the microscopic level, so that a theoretical research basis is provided for improving the yield and recovery ratio of a shale oil reservoir.

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, the demand for the exploration and development of unconventional oil and gas resources has increased significantly. 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. This includes oil in the pores and fractures of mudstone, as well as oil resources in the adjacent layers and interlayers of tight carbonate or clastic rocks within mudstone formations. As a key component of unconventional oil and gas resources, shale oil has demonstrated tremendous potential. Shale gas, on the other hand, is primarily stored and preserved by organic matter adsorption or in fractures and matrix pores within the rock. However, continental shale oil reservoirs in my country are characterized by strong heterogeneity, diverse reservoir space types, widespread nanoscale pores, extremely low porosity and permeability, and poor fluid mobility. Current primary development of shale oil reservoirs after fracturing is generally plagued by low utilization, low single-well production, rapid production decline, and low recovery factors.

[0003] To further increase shale oil reservoir production, CO2 huff-and-puff is considered a powerful approach for efficient shale oil extraction. However, field practice has revealed significant variability in the effectiveness of CO2 huff-and-puff in enhancing oil recovery, severely hindering its widespread application. Traditional theories cannot explain the microscopic mechanisms of action, providing a sound theoretical basis for efficient shale oil extraction. Furthermore, the mechanisms of CO2 huff-and-puff in shale oil and shale gas are fundamentally different, and mechanistic research on shale gas cannot be directly applied to shale oil. Therefore, to understand the microscopic mechanisms of CO2 huff-and-puff in shale oil, molecular-scale studies are urgently needed. However, currently, there is a lack of CO2 huff-and-puff simulation methods specifically for shale oil, which provide no rigorous theoretical basis for this research. 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 extract 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, embodiments of the present application provide a molecular simulation method for shale oil production using carbon dioxide huff and puff, comprising: Construct a pure component CO2 bulk box under injection temperature and pressure conditions according to the simulation requirements. The bulk box is used to simulate the CO2 gas source during the CO2 huff-and-puff oil production process. A pore model is constructed based on the shale reservoir evaluation results. 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 to ensure that the oil molecules in the pores reach a stable state under the target temperature and pressure conditions. The bulk box and the pore model that stably stores oil molecules are combined. Molecular dynamics relaxation simulation is first performed to pre-equilibrate the combined model. The portion of the pore model that blocks the bulk box and the entrance to the confined space is removed. The bulk box is then connected to the remaining pore model to obtain an initial model for CO2 huff-and-puff shale oil. A constant injection pressure was applied to the side of the bulk box in the initial model that was away from the confined space entrance, and non-equilibrium molecular dynamics relaxation simulations of the CO2 injection and well soaking processes were performed 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 the depressurization production is carried out on the CO2 huff and puff production model after CO2 injection and soaking, completing the molecular simulation process of a CO2 huff and puff production shale oil cycle.

[0006] In some embodiments, further comprising: After completing a CO2 huff-and-puff cycle for shale oil production, the bulk box is removed, and a new bulk box of pure CO2 under the 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. 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 until multiple CO2 huff and puff shale oil production cycles are completed.

[0007] 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.

[0008] In some embodiments, when constructing a bulk 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 box under the desired temperature and pressure conditions.

[0009] 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.

[0010] 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 with an opening 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.

[0011] In some embodiments, when pre-balancing the pore model, target component oil molecules are filled in the pores between the barrier plate and the open end of the confined space, and pressure is applied to the oil molecules through the barrier plate to perform non-equilibrium molecular dynamics relaxation simulation.

[0012] 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 point the pore model stably stores shale oil under target temperature and pressure conditions.

[0013] In some embodiments, when connecting and combining the bulk box and the remaining pore model: first, the free end face of the bulk box is aligned with the open end of the confined space, and then structural optimization is performed after docking to obtain an initial model.

[0014] In a second aspect, an embodiment of the present application provides a molecular simulation device for shale oil extraction using carbon dioxide huff and puff, comprising: A control module having a built-in program for executing the molecular simulation method for carbon dioxide huff and puff shale oil extraction; 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.

[0015] It can be seen from the above technical solution that the beneficial effects of this application are: 1. This application simulates the CO2 gas source in the CO2 huff-and-puff oil production process by constructing a pure component CO2 bulk 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 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. In this way, a state and pressure environment of CO2 huff-and-puff shale oil close to the actual downhole can be simulated. 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 at the microscopic molecular scale. This is conducive to clarifying the microscopic mechanism of CO2 huff-and-puff shale oil production and observing the action laws of CO2 huff-and-puff shale oil from a microscopic level, thereby providing a theoretical research basis for improving shale oil reservoir production and recovery rate.

[0016] 2. The present application executes the above-mentioned molecular simulation method of carbon dioxide throughput for shale oil production 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 pressure of the injection and production 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

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present application, a brief introduction will be given below to 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 any creative work.

[0018] Figure 1 A schematic diagram showing the steps of an embodiment of the molecular simulation method for carbon dioxide huff-and-puff shale oil production according to the present invention is shown; Figure 2 A schematic diagram of an embodiment of the molecular simulation method for carbon dioxide huff-and-puff shale oil production according to the present invention is shown; 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 modeling of the pore model for 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.

[0019] Figure 3 A schematic diagram showing the steps of an embodiment of the molecular simulation method for carbon dioxide huff-and-puff shale oil production according to the present invention is shown; Figure 4 shows the density distribution diagram of the initial occurrence state of nC7 in the calcite nanopores according to an embodiment of the present invention; Figure 5 Shown are two competitive adsorption modes and the evolution diagram of the adsorption state nC7 of the present invention; Figure 6 The cloud diagram of CO2 distribution density in calcite nanopores and the CO2 density curve in the middle of the pores according to the embodiment of the present invention are shown; 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 is shown. DETAILED DESCRIPTION

[0020] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0021] It should be noted that all directional indications in the embodiments of the present invention are only used to explain the relative position relationship and movement status of various components in a certain specific posture. If the specific posture changes, the directional indication will also change accordingly.

[0022] In the present invention, unless otherwise specified or limited, the terms "connection" and "fixation" should be understood in a broad sense. For example, "fixation" can mean fixed connection, detachable connection, or integration; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; internal communication between two elements or interaction between two elements, unless otherwise specified. Those skilled in the art will be able to understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0023] In addition, in the present invention, descriptions such as "first" and "second" are for descriptive purposes only and should not 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 such features. In addition, the technical solutions between the various embodiments can be combined with each other, but this 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 the present invention.

[0024] The present application is described below with reference to specific embodiments and with reference to the accompanying drawings: Please refer to Figure 1 The first embodiment of the present application provides a molecular simulation method for shale oil production using carbon dioxide huff and puff, which includes steps S1 to S5: 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, modeling is done in the three-axis coordinate system of the modeling software, in which the Y-axis can be ignored and 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 explanation takes the Z-axis direction as an example. According to the simulation requirements, the specific dimensions 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 less susceptible to 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.

[0025] In some embodiments, modeling is performed in the three-axis coordinate system of the modeling software. When constructing a bulk 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. By pressurizing the graphene piston plate, the injection pressure during the CO2 throughput process is simulated, and non-equilibrium molecular dynamics relaxation simulation is performed to bring the system into thermodynamic equilibrium and obtain a bulk box under the desired temperature and pressure conditions.

[0026] In the specific implementation, a graphene piston plate and a helium plate with a length and width of 89.45 Å and 24.59 Å, respectively, were constructed using Materials Studio 2023 software (where Å is a unit of length, 1 Å = 0.1 nm). 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, using the Amorphous Cell module, the technical parameters provided by the NIST website were used: 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 resulting CO2 box was used to simulate an external CO2 gas source, such as Figure 2 As shown in part (a).

[0027] S2. Based on the shale reservoir evaluation results, a pore model is constructed. The pore model simulates a confined space. The pores of the pore model are filled with target component oil molecules and pre-equilibrated to achieve a stable distribution state within the pores under target temperature and pressure conditions. The target component oil molecules refer to the components of shale oil. A confined space is a restricted or constrained spatial range. This space can be defined by physical boundaries (such as walls or containers). The purpose of constructing a confined space is typically to control, optimize, or study the physical, chemical, or biological processes occurring within it. By simulating a confined space at the nanoscale, accurate simulations of material properties can be achieved, more accurately matching the downhole shale reservoir environment. Pre-equilibrium is a state of dynamic equilibrium. In a formation, gas molecules are exposed to a complex geochemical environment. They interact extensively with rock, groundwater, other gas molecules, and the biosphere. These interactions can cause changes in the concentration, distribution, and energy state of the 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".

[0028] 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 using a shape close to a real pore, and then a baffle is set at the interval with the open end of the confined space. The baffle is, for example, a graphene piston plate. The part enclosed by the target mineral component forms a pore, and the pore is a nanopore of pore scale. In some embodiments, when constructing the target mineral component at the edge, the two rock walls of the target mineral component are first placed parallel to each other, parallel to the X-axis and spaced apart, and the rock wall of the target mineral component is used 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 outer periphery of the open end of the confined space, that is, the non-enclosed space above and below the pore in the figure is blocked in the X-axis direction, and the helium plate and the rock wall are all fixed to obtain a pore model.

[0029] In practice, a calcite unit cell was cut and expanded along the (1 0 4) direction to create a calcite wall model with length, width, and thickness of 125 Å, 24.59 Å, and 12.5 Å, respectively. Two identical calcite walls were then placed parallel to each other in another amorphous unit cell, 3.5 nm apart along the X-axis. A calcite wall with a smaller cross-section and consistent thickness was then used to block the right end of the pore, forming the end of the pore. A helium plate was placed at the pore exit to block the non-enclosed space above and below the pore. Finally, all the walls and helium plates were fixed to create a single calcite slit nanopore model.

[0030] In some embodiments, when pre-equilibrating the pore model, 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 characterized using typical component oil molecules, which can be determined based on chromatographic analysis, such as C7, C9, or C12. The baffle plate is located a certain distance from the open end of the confined space, which serves as its entrance. During 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. A force is then applied to the side, exerting pressure on the oil molecules through the baffle plate to simulate reservoir pressure and perform non-equilibrium molecular dynamics relaxation simulations. In some embodiments, a portion of the pore model can be removed in the next step S3. Alternatively, in this step, after the model configuration is stabilized, the oil molecules and graphene piston plate outside the pores can be removed to obtain a pore model with stable shale oil.

[0031] In the specific implementation, the space within the pore and 50 Å outside the pore is set according to the technical parameters provided by the NIST website: 0.6613 g / cm 3The density of the pore (temperature and pressure conditions: 25 MPa, 353 K) was filled with 744 n-heptane molecules (nC7), and then a pressure of 25 MPa was applied to the 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 a calcite pore model with stable shale oil, as shown in the figure. Figure 2 As shown in part (b).

[0032] S3. Combine the bulk box and the pore model that stably stores oil molecules, perform molecular dynamics relaxation simulation first to bring the combined model to 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 explores the dynamic properties of matter by simulating the interactions between atoms and molecules. In MD simulation, matter is regarded as a basic unit composed of atoms, and the temporal evolution of these basic units can reveal the dynamic behavior of matter.

[0033] In some embodiments, when combining a bulk box with a pore model that stably stores oil molecules, the oil molecules and barrier plates outside the pores are removed, such as by deleting the portion of the model outside the confined space. This results in a pore model that stably stores shale oil under target temperature and pressure conditions. The bulk box is then 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 by docking the other end of the bulk box opposite the graphene piston plate with the inlet of the pore model, to obtain an initial model for CO2 injection into shale oil. In some embodiments, when combining the bulk box with the remaining pore model, the free end 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 the initial model.

[0034] 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 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 the following figure. Figure 2 As shown in part (c).

[0035] S4. A constant injection pressure is applied to the side of the bulk box in the initial model that is away from the entrance of the confined space, and non-equilibrium molecular dynamics relaxation simulations of the CO2 injection and well-simmering process are performed to obtain a CO2 huff-and-puff oil production model after CO2 injection and well-simmering. 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 the exchange and redistribution of energy 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.

[0036] During 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-suffusing process is performed for 1.5 ns to obtain the CO2 huff and puff oil production model after CO2 injection and well-suffusing; 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 is controlled using the Andersen constant temperature mode, the electrostatic interaction 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 1 fs, and 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 and concentration differences, and the CO2 injection and well soaking process is 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 the figure. Figure 2 As shown in part (d).

[0037] S5. Reduce the applied constant pressure to the pressure of the depressurization production stage, and perform non-equilibrium molecular dynamics relaxation simulation of depressurization production on the CO2 huff-and-puff production model after CO2 injection and well shut-in, completing the molecular simulation process of a CO2 huff-and-puff production cycle for shale oil. For the CO2 huff-and-puff 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 due to expansion displacement, realizing the simulation of the CO2 huff-and-puff production process during the depressurization production process, thus completing a CO2 huff-and-puff production cycle.

[0038] In specific implementation, the constant pressure of the graphene piston plate is reduced to the pressure of depressurization production, 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).

[0039] Currently, 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, adsorption and desorption laws, and diffusion mass transfer processes of CO2-crude oil in the nano-confined space of shale reservoirs. Core-scale physical model experiments cannot accurately explain the microscopic processes of the above complex phenomena, and it is difficult to elucidate 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 research on the mechanism of shale gas. This is because most of the reserves of shale oil are adsorbed gas. When shale gas is simulated, it is set up using the Grand Canonical Monte Carlo (GCMC) + Molecular Dynamics (MD) method. The basic logic of GCMC is to connect a hypothetical infinite box to 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. This method is more It is suitable for the study of adsorption laws and is suitable for shale gas; however, 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 GCMC logic for free adsorption simulation cannot achieve the stable occurrence of liquid shale oil under reservoir conditions (temperature and pressure). Therefore, based on this characteristic of shale oil, the occurrence and production simulation can only be carried out by applying external expulsion pressure.

[0040] In order to realize the simulation process of CO2 huff and puff production of shale oil, this application simulates the kerogen expulsion pressure, CO2 injection and CO2 pressure reduction production in the process of occurrence simulation and production simulation by means of external expulsion pressure. Specifically, this application simulates the CO2 gas source in the CO2 huff and puff production process by constructing a pure component CO2 bulk 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 box and the pore model are combined, the initial model of CO2 huff and puff shale oil in the formation is simulated, and then the kerogen expulsion pressure, CO2 injection and CO2 in the reservoir are simulated by applying a constant injection pressure on one side of the bulk box and reducing the applied constant pressure. Pressure-reducing production is carried out, which 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 when obtaining 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. 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.

[0041] Please refer to Figure 3 In some embodiments, the molecular simulation method further comprises the steps of: S6. After completing a CO2 huff-and-puff cycle of shale oil production, the bulk box is removed. A new bulk box of pure CO2 components injected under the temperature and pressure conditions is reconstructed and reassembled with the pore model from which the bulk box was removed to form the initial model. Specifically, after completing a huff-and-puff cycle, the fluid molecules outside the pores of the oil production model and the graphene piston plate are removed. The CO2 bulk box from step S1 is then re-placed and reassembled to form the initial model for the second cycle.

[0042] 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.

[0043] In some embodiments, the molecular simulation method further comprises the steps of: S8. During the molecular simulation of the CO2 huff-and-puff shale oil production cycle, analyze the microscopic production-increasing mechanism and molecular mass transfer behavior characteristics of the CO2 huff-and-puff shale oil production process by comparing simulated configuration changes in the CO2 huff-and-puff oil production model. This analysis is performed based on the configuration changes during the simulation of steps S4 through S7. Specifically, data analysis is performed on the simulation results obtained in steps S4 through S7 to analyze the microscopic production-increasing 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 occurrence state of nC7 in calcite nanopores. It can be observed that there are three adsorption layers and a bulk region with very small density fluctuation in the nanopore nC7.

[0044] Please refer to Figure 5 , Figure 5 The two competitive adsorption modes of CO2 and the evolution of nC7 in the adsorbed state over time are demonstrated. One competitive adsorption process occurs in the short period of time before the huff and puff, when carbon dioxide strips off the originally adsorbed nC7 in a triangular wedge-shaped pattern on the calcite wall, realizing the competitive adsorption process. The second competitive adsorption mode is that CO2 first enters the pore from the middle of the pore along the pore diameter direction based 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; the nC7 in the adsorbed state is first stripped and transported to the middle of the pore by mass transfer, and then migrates along the pore diameter direction toward the outlet. 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 pore can be divided into three major areas in turn: the miscible area, the miscible front area, and the pure oil area. Figure 7 The figure shows the change 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 flat.

[0045] 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 into the above-mentioned steps S1 to S5, or the execution method in steps S1 to S8; by executing the program through the control module, the microscopic process of the CO2 huff and puff 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 results of the program, and 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 horizontal pressure-applying module 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 to accurately control the pressure of the injection and production stages during the CO2 huff-and-puff production of shale oil; the storage module is electrically connected to the control module, such as the memory is connected to the control module, and the memory is used to store the execution results and store the results of the program.

[0046] Currently, there is a lack of CO2 throughput simulation devices for shale oil. This is due to the 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. However, this application uses a control module to execute the above-mentioned molecular simulation method for carbon dioxide throughput shale oil production, a display module to display the simulation output results, and a pressure regulation module to regulate the pressure of the bulk box. The pressure regulation module is controlled by a program and can quantitatively control the pressure during the injection and production stages of the CO2 throughput process, achieving precise regulation, thereby simulating the downhole shale oil reservoir environment, which is more in line with actual conditions. Moreover, because the program uses the above-mentioned molecular simulation method, the device can automatically simulate the downhole CO2 throughput shale oil state and pressure environment close to the actual state, more intuitively revealing the microscopic effect mechanism of CO2 enhanced recovery through molecular mass transfer effect.

[0047] 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" 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 can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification.

[0048] 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 this field to implement them. When the combination of technical solutions is mutually 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 this application.

[0049] 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 intent 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 extraction by CO2 huff and puff, It is characterized in that include: According to the simulation requirements, pure component CO injected under temperature and pressure conditions was constructed. 2 Phase box, which is used to simulate CO 2 CO in Huff and Puff Oil Production 2 Gas source; A pore model is constructed according to the evaluation results of the shale oil reservoir, wherein the pore model is used to simulate a confined space, 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, and molecular dynamics relaxation simulation is first performed to make the combined model reach pre-equilibrium, and 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 CO 2 Initial model for shale oil production; A constant injection pressure is applied to the side of the volume box in the initial model away from the entrance of the confined space, and CO 2 Nonequilibrium molecular dynamics relaxation simulation of injection and well soaking process, and the CO 2 CO after injection and well shut-down 2 Huff and puff oil production model; Reducing the applied constant pressure to the pressure of the depressurization stage has a significant impact on CO 2 The CO after injection and well shut-down 2 The huff-and-puff production model performs non-equilibrium molecular dynamics relaxation simulation of depressurization to complete a CO 2 Molecular simulation of the huff-and-puff shale oil cycle.

2. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to claim 1, It is characterized in that Also includes: Will complete a CO 2 After the huff-and-puff production of shale oil, the bulk box is removed and the pure component CO injected under temperature and pressure conditions is reconstructed. 2 a new volume box, and recombining it with the pore model without the volume box into the initial model; Repeat the steps to obtain the CO 2 Huff and puff oil production model, and conduct CO 2 Molecular simulation of the huff-and-puff shale oil production cycle until multiple CO 2 Huff and puff shale oil production cycle.

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

4. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to any one of claims 1 to 3, It is characterized in that When constructing the bulk box, a fixed helium plate and a graphene piston plate of equivalent size are placed, and CO is filled between the helium plate and the graphene piston plate. 2 The molecules are subjected to non-equilibrium molecular dynamics relaxation simulation by pressurizing the graphene piston plate 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 any one of claims 1 to 3, It is characterized in that When constructing the pore model, the 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 enclosed by the target mineral component forms pores, and the pores are nanopores of pore scale.

6. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to claim 5, It is 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.

7. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to claim 6, It is 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.

8. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to claim 5, It is 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.

9. The molecular simulation method for shale oil production by carbon dioxide huff and puff according to claim 8, It is 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.

10. A molecular simulation device for shale oil extraction using carbon dioxide. It is 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 9; 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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