A molecular simulation research method for carbon dioxide huff and puff exploitation of shale gas

Abstract

The present application relates to the technical field of shale gas exploitation simulation, in particular to a molecular simulation research method for carbon dioxide huff and puff exploitation of shale gas, which can maximize reduction of adsorption and desorption physical processes of gas in complex nanometer pore structure of shale at a molecular scale by constructing a carbon dioxide huff and puff simulation device and using the proposed GCMC-MD combined molecular simulation method, reproduce the production process of injected carbon dioxide for shale gas production, and reveal the micro mechanism of carbon dioxide storage for strengthening shale gas exploitation technology, which is beneficial to figuring out the mechanism of carbon dioxide huff and puff gas recovery of shale gas considering the molecular mass transfer effect, and further laying a theoretical foundation for shale gas production potential tapping and optimization of shale gas exploitation path, thereby solving the problem that the existing molecular simulation method cannot effectively reproduce the process of carbon dioxide huff and puff exploitation of shale gas.

Description

Technical Field

[0001] This invention relates to the field of shale gas extraction simulation technology, and in particular to a molecular simulation research method for shale gas extraction using carbon dioxide huff and puff. Background Technology

[0002] Shale gas resources are abundant globally. Compared to coal, shale gas is a cleaner energy source, producing only 45% of the carbon dioxide. Shale gas can fully meet the energy needs of developing economies that continue to grow over the next few decades and make a significant contribution to reducing carbon dioxide emissions. Currently, shale gas reservoirs are mostly developed using depletion-based methods, with recovery rates generally below 30%. Carbon dioxide enhanced shale gas extraction (CO2-EGR) technology is considered a very promising method for shale gas extraction.

[0003] Previous experiments have verified the effectiveness of carbon dioxide injection in enhancing shale gas production, but research on CO2 huff and puff for shale gas extraction is currently scarce. In recent years, some scholars have conducted research using numerical simulations, molecular simulations, and laboratory experiments, verifying that CO2 huff and puff is effective and can increase shale gas recovery by 6%–26%. Molecular simulation is a powerful tool for exploring the microscopic mechanisms of CO2-enhanced shale gas extraction; however, most current research is based on single rock medium pore models, studying the competitive adsorption behavior of CO2 / CH4 under static conditions. This reflects the relative adsorption affinity of CH4 and CO2 for the adsorbent, but it cannot reflect the process of CO2 injection for shale gas extraction. Some scholars have conducted research on the CO2 huff and puff process for shale gas extraction, but they all solely use the simulation path based on the GCMC method proposed by Zhou Juan. This method has limitations, failing to consider the molecular thermal motion behavior affected by the degree of pore throat connectivity in real reservoirs. A deep understanding of the microscopic mechanisms of CO2 injection for enhanced shale gas recovery under real reservoir conditions is a prerequisite for tapping shale gas production potential and designing targeted measures. Currently, there is no suitable molecular simulation method that can effectively reproduce the CO2 injection process for shale gas extraction. Summary of the Invention

[0004] The purpose of this invention is to provide a molecular simulation research method for CO2 huff and puff shale gas extraction, aiming to solve the problem that existing molecular simulation methods cannot effectively reproduce the CO2 huff and puff shale gas extraction process.

[0005] To achieve the above objectives, this invention provides a molecular simulation study method for carbon dioxide huff and puff extraction of shale gas, comprising the following steps:

[0006] S1 is a molecular simulation system for carbon dioxide injection and extraction of shale gas based on the coupling of slit pores and large fractures.

[0007] S2 sets the molecular simulation method for the molecular simulation system for carbon dioxide injection and extraction of shale gas;

[0008] S3 performs gas-water pre-equilibrium based on the algorithm and the slit pore region to obtain the initial condition recovery model for gas-water desorption in shale pores;

[0009] S4 simulates the attenuation mining of the shale pore gas-water desorption initial condition recovery model to obtain the gas reservoir pore model after exhaustion mining.

[0010] S5 uses the gas reservoir pore model after exhaustion to simulate the huff and puff process and obtains the molecular dynamics equilibrium configuration of the fluid at each extraction stage.

[0011] S6 analyzes the mechanism of CO2 huff and puff extraction of shale gas based on the molecular dynamics equilibrium configuration results of the fluids at each extraction stage.

[0012] The slit pores are composed of layers of quartz media and kerogen sheets of the same size stacked together.

[0013] Among them, based on the algorithm and the gas-water pre-equilibrium in the slit pore region, an initial condition recovery model for shale pore gas-water desorption is obtained, including:

[0014] S31 uses a Monte Carlo and molecular dynamics combined algorithm to achieve water storage balance in the slit pores and establishes a water-bearing composite shale model.

[0015] S32 recovers CH4 gas using the Monte Carlo algorithm and then achieves gas-water balance using the molecular dynamics algorithm, thus obtaining the initial condition recovery model for gas-water desorption in shale pores.

[0016] The step of performing attenuation mining simulation on the initial condition recovery model of shale pore gas-water desorption to obtain the gas reservoir pore model after depletion mining includes:

[0017] S41 reduces the pressure of the middle CH4 bulk phase in the large fracture using the Monte Carlo algorithm;

[0018] S42 uses molecular dynamics combined with algorithms to simulate the depressurization desorption process under given pressure conditions, and obtains the gas reservoir porosity model after exhaustion.

[0019] The simulation of the huff and puff process using the post-exhaustion gas reservoir pore model to obtain the molecular dynamics equilibrium configuration results of the fluid at each extraction stage includes:

[0020] S51 calculates the methane phase density and pore system pressure in the fractures of the gas reservoir pore model after exhaustion.

[0021] S52 uses the Peng-Robinson equation of state to determine the amount of CH4 / CO2 mixed gas to be injected based on the bulk density of CH4 and the molar ratio of CH4 / CO2 mixed gas. Then, it injects CH4 / CO2 mixed gas into the crack using the Monte Carlo algorithm and realizes the CO2 pressurization process using the molecular dynamics algorithm.

[0022] S53 uses the Monte Carlo algorithm to simulate the pressure in the fracture, removes the sealing layer between the pores and the fracture, and performs molecular dynamics algorithm simulation to achieve depressurized shale gas extraction.

[0023] S54 repeats steps S51 to S53 twice to obtain the molecular dynamics equilibrium configuration results of the fluid at each mining stage.

[0024] This invention discloses a molecular simulation method for carbon dioxide huff and puff extraction of shale gas, comprising the following steps: constructing a molecular simulation system for carbon dioxide huff and puff extraction of shale gas based on the coupling of slit pores and large fractures; setting the molecular simulation method for the molecular simulation system; performing gas-water pre-equilibrium based on the algorithm and the slit pore region to obtain a recovery model of the initial conditions for gas-water desorption in shale pores; performing attenuation extraction simulation on the recovery model of the initial conditions for gas-water desorption in shale pores to obtain a pore model of the gas reservoir after exhaustion extraction; using the pore model of the gas reservoir after exhaustion extraction to simulate the huff and puff extraction process, obtaining the molecular dynamics equilibrium configuration results of the fluids at each extraction stage; and performing a mechanism analysis of CO2 huff and puff extraction of shale gas based on the molecular dynamics equilibrium configuration results of the fluids at each extraction stage. This method, by constructing a carbon dioxide huff and puff simulation device and utilizing the proposed GCMC-MD combined molecular simulation method, can maximize the adsorption and desorption physical processes of reducing gas in the complex nanoporous structure of shale at the molecular scale. It can reproduce the production process of shale gas extraction by injecting carbon dioxide and reveal the microscopic mechanism of carbon dioxide sequestration-enhanced shale gas extraction technology, thereby solving the problem that existing molecular simulation methods cannot effectively reproduce the CO2 huff and puff process of shale gas extraction. Attached Figure Description

[0025] The present invention can be further illustrated by the non-limiting embodiments given in the accompanying drawings.

[0026] Figure 1 Molecular simulation research process for shale gas extraction using carbon dioxide injection.

[0027] Figure 2 A microscopic simulation device for carbon dioxide injection and release in shale gas extraction.

[0028] Figure 3 A model for restoring the initial conditions of shale pore gas-water desorption.

[0029] Figure 4CH4 recovery rate under different pressure differentials during depletion mining.

[0030] Figure 5 This represents the molecular dynamic equilibrium configuration of the fluids at each mining stage.

[0031] Figure 6 A shows the evolution of CH4 density distribution in the pores of high-water-bearing shale during the throughput mining process. Figure 6 B is a diagram showing the evolution of CO2 density distribution in the pores of high-water-bearing shale during the throughput mining process.

[0032] Figure 7 A represents the CH4 extraction efficiency at different mining stages in shale pores. Figure 7 B represents the CO2 sequestration efficiency in shale pores at different mining stages.

[0033] Figure 8 This is a flowchart of a molecular simulation study method for carbon dioxide injection and release in shale gas extraction according to the present invention. Detailed Implementation

[0034] Embodiments of the present invention are described in detail below, examples of which are illustrated in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention.

[0035] Please see Figures 1 to 8 This invention provides a molecular simulation study method for carbon dioxide injection and release in shale gas extraction, comprising the following steps:

[0036] S1 is a molecular simulation system for carbon dioxide injection and extraction of shale gas based on the coupling of slit pores and large fractures.

[0037] The slit pores are composed of layers of quartz media and kerogen sheets of the same size stacked together.

[0038] Specifically, the shale quartz-kerogen composite slotted nanopore model mainly consists of composite slotted pores coupled with large fractures. The composite slotted pores are formed by stacking quartz and kerogen sheets of the same size. Based on the composite slotted nanopore model, two sealing layers are superimposed to expand and construct a vacuum layer. The dimensions of the sealing layers in the x and y directions are the same as those in the slotted pore model. The vacuum layer represents large fractures in the reservoir, enabling control of the external pressure of the composite pores. He (He) layers are chosen as the sealing layers because they can act as a barrier without interacting with fluid molecules. The composite slotted pore region and the vacuum layer region constitute a carbon dioxide huff-and-puff gas production system.

[0039] Construct an initial cuboid simulation box with dimensions in the x, y, and z directions as follows: First, six kerogen molecules (C1) from the deep Longmaxi Formation of the Luzhou Block in southern Sichuan were randomly introduced into the simulation box. 320 H 263 O 19 Using the steps in Table 1, kerogen structural relaxation was performed to obtain an equilibrium kerogen matrix molecular model. A 4a×4b×4c supercell structure was established along the x, y, and z directions. A matrix block was then cut out from the center of the matrix model. It consists of organic lamellar structures. Using an α-quartz unit cell, the (100) crystal plane of the quartz crystal was first cut, and hydrogen was added to the unbonded oxygen atoms to achieve hydroxylation. Then, a supercell structure was constructed to obtain a structure with dimensions of [missing information]. Quartz media layers. A deep shale quartz-kerogen composite slit nanopore model with a pore size of 4 nm was constructed by superimposing organic and inorganic layers. Based on the composite slit nanopore model, a sealing layer was superimposed to expand and build a vacuum layer. The vacuum layer represents large fractures in the reservoir, enabling control of the external pressure of the composite pores, thus obtaining a CO2 injection-puffing production simulation device. Figure 2 ).

[0040] Table 1. Relaxation steps of kerogen structure

[0041]

[0042] S2 sets the molecular simulation method for the molecular simulation system for carbon dioxide injection and extraction of shale gas;

[0043] Specifically, based on the molecular simulation system for shale gas extraction using carbon dioxide huff and puff constructed in step S1, the proposed Grand Canonical Monte Carlo-Molecular Dynamics Combined Method (GCMC-MD) is adopted. In the GCMC simulation, fluid fugacity replaces pressure as the input parameter, and fugacity is calculated using the Peng-Robinson equation of state. Specifically, within the slit pores, the GCMC-MD method is used to realize the insertion, deletion, and movement of fluids, achieving accurate determination of the gas chemical potential; in the external volume control region, the GCMC method is used to control the external gas chemical potential and gas composition; for the entire simulation system, the MD method is used to realize the diffusion and transport of gas molecules, considering the diffusion and transfer of gas mass.

[0044] S3 performs gas-water pre-equilibrium based on the algorithm and the slit pore region to obtain the initial condition recovery model for gas-water desorption in shale pores;

[0045] S31 uses a Monte Carlo and molecular dynamics combined algorithm to achieve water storage balance in the slit pores and establishes a water-bearing composite shale model.

[0046] Specifically, a predetermined amount of water is first loaded into the slit pore using the Monte Carlo (GCMC) algorithm, and then the water content equilibrium is achieved using the molecular dynamics (MD) algorithm to establish a water-bearing composite shale model.

[0047] S32 recovers CH4 gas using the Monte Carlo algorithm and then achieves gas-liquid balance using the molecular dynamics algorithm.

[0048] Specifically, the moisture content was 23.21%. Next, under conditions of 80 MPa pressure and 393.15 K temperature, a gas recovery simulation was conducted within the water-bearing nanopores. First, the CH4 gas under these temperature and pressure conditions was recovered using the GCMC algorithm, and then the gas-water recovery equilibrium was achieved using the MD algorithm, resulting in the initial condition recovery model for gas-water desorption in shale pores. Figure 3 ).

[0049] S4 simulates the attenuation mining of the shale pore gas-water desorption initial condition recovery model to obtain the gas reservoir pore model after exhaustion mining.

[0050] S41 reduces the pressure of the middle CH4 bulk phase in the large fracture using the Monte Carlo algorithm;

[0051] Specifically, based on the shale pore gas-water desorption initial condition recovery model obtained in step 3, the CH4 pressure in the large fracture is reduced by the GCMC algorithm, and the He layer blocking the pores and vacuum layer is removed to connect the slit pore region and the fracture region.

[0052] S42 uses molecular dynamics algorithms to simulate the depressurization desorption process under given pressure conditions to obtain a gas reservoir porosity model after exhaustion.

[0053] Specifically, the MD method is used to simulate the depressurization desorption process of the system under a given pressure condition. The pressure outside the pores (fractures) is gradually reduced, and the above simulation process is repeated to calculate the CH4 recovery rate under different pressure differentials. Figure 4 The equilibrium model corresponding to the point where the CH4 recovery rate no longer increases significantly is selected as the gas reservoir porosity model after exhaustion. In this example, the equilibrium configuration is set to a pressure difference of 65 MPa.

[0054] S5 uses the gas reservoir pore model after exhaustion to simulate the huff and puff process and obtains the molecular dynamics equilibrium configuration of the fluid at each extraction stage.

[0055] S51 calculates the methane phase density and pore system pressure in the fractures of the gas reservoir pore model after exhaustion.

[0056] Specifically, the methane bulk density in the fracture (vacuum layer) after depletion mining was calculated, and the corresponding system pressure was looked up in the database of the National Institute of Standards and Technology (NIST). The pressure of the shale pore system was determined to be 26 MPa. CO2 was then injected with an initial pressure of 26 MPa.

[0057] S52 uses the Peng-Robinson equation of state to determine the amount of CH4 / CO2 mixed gas to be injected based on the bulk density of CH4 and the molar ratio of CH4 / CO2 mixed gas. Then, it uses the Monte Carlo algorithm to inject CH4 / CO2 mixed gas into the crack and then uses the molecular dynamics algorithm to realize the CO2 pressurization process.

[0058] Specifically, based on the known parameters of CH4 bulk density and CH4 / CO2 mixed gas molar ratio (0.75:0.25), the amount of CO2 injected and the system pressure of 33 MPa were determined using the Peng-Robinson equation of state. CH4 / CO2 mixed gas was injected into the fracture using the GCMC method, and then the first CO2 injection pressurization process (stage III) was achieved using the MD method, in which carbon dioxide was injected into the shale gas reservoir.

[0059] S53 uses the Monte Carlo algorithm to simulate the pressure in the fracture, removes the sealing layer between the pores and the fracture, and performs molecular dynamics algorithm simulation to achieve depressurized shale gas extraction.

[0060] Specifically, the adsorption amount of CH4 / CO2 mixed gas in the fracture was obtained through GCMC simulation when the pressure was P0. The system pressure in the fracture (26MPa) was lower than the system pressure in the pore (33MPa). The sealing layer between the pore and the fracture was deleted and MD simulation was performed to realize the depressurization extraction of shale gas (stage IV).

[0061] S54 repeats steps S51 to S53 twice to obtain the molecular dynamics equilibrium configuration results of the fluid at each mining stage.

[0062] Specifically, the above describes one round of CO2 pressurization-repressurization process, which is repeated twice. Here, it is assumed that after CO2 injection, the CH4 density in the CH4 / CO2 binary mixture in the bulk phase (vacuum layer) is equal to the density of pure CH4 after depressurization. This work was conducted in models with dry and 23.21% moisture content, and the specific details of the throughput simulation are listed in Table 2. The simulation yielded the molecular dynamics equilibrium configurations of the fluid at each extraction stage (…). Figure 5 ).

[0063] Table 2. Detailed information on fluids in the fractures during the mining process (T=393K)

[0064]

[0065] S6 analyzes the mechanism of CO2 huff and puff extraction of shale gas based on the molecular dynamics equilibrium configuration results of the fluids at each extraction stage.

[0066] Specifically, data analysis is performed using the molecular dynamics results obtained in step S5 to study the microscopic mechanism of CO2 injection for shale gas extraction, considering the impact of different injection cycles and water content on CH4 recovery and CO2 sequestration rates. The simulation results can effectively distinguish and quantify different fluid occurrence forms, allow for direct observation of the kinetic state of fluid molecules, and statistically quantify CH4 recovery and CO2 sequestration rates from a molecular perspective. Figure 5 The simulation results show the molecular dynamics equilibrium configuration of fluids at various stages of shale gas extraction using carbon dioxide injection. The simulation results show that as CO2 is injected, water molecules in the pores migrate to the fractures, indicating that CO2 gas extraction considering molecular thermal motion can utilize adsorbed water in the shale pores. Figure 6 This is a diagram showing the evolution of gas density distribution in the pores of water-bearing shale during the injection and extraction process. The evolution of CH4 gas density distribution in different occurrence states within the pores of the composite shale after CO2 injection can be observed from the diagram. Figure 6 A) and the change in CO2 density distribution within the pores ( Figure 6 B). By quantitatively calculating CH4 recovery rate and CO2 sequestration rate, the effect of CH4 enhanced extraction during the extraction process was studied. Figure 7 A) and CO2 sequestration effect ( Figure 7 B). Figure 7 To assess the CH4 recovery and CO2 storage efficiency at different mining stages in shale pores, quantitative results can be used to compare the mining mechanisms during pressurization and depressurization processes, explore the necessity of multiple rounds of huff and puff, and demonstrate that both CH4 recovery and CO2 storage rates increase with increasing huff and puff rounds. Furthermore, the study investigates the impact of different moisture contents within the pores on CH4 recovery and CO2 storage rates, finding that moisture contributes to CO2 storage.

[0067] Beneficial effects:

[0068] The invention aims to maximize the adsorption and desorption physical processes of reducing gas in the complex nanoporous structure of shale at the molecular scale by constructing a carbon dioxide huff-and-puff simulation device and utilizing the proposed GCMC-MD combined molecular simulation method. This allows for the reproduction of the shale gas production process through carbon dioxide injection and reveals the microscopic mechanism of carbon dioxide sequestration-enhanced shale gas extraction (CS-EGR) technology. The technical solution provided by this invention is beneficial for elucidating the shale gas CO2 huff-and-puff extraction mechanism considering molecular mass transfer effects, thereby laying a theoretical foundation for tapping shale gas production potential and optimizing shale gas extraction pathways.

[0069] The above-disclosed embodiments are merely one or more preferred embodiments of this application and should not be construed as limiting the scope of this application. Those skilled in the art can understand that all or part of the processes for implementing the above embodiments and equivalent changes made in accordance with the claims of this application still fall within the scope of this application.

Claims

1. A molecular simulation method for carbon dioxide huff and puff extraction of shale gas, comprising the following steps: S1 is a molecular simulation system for carbon dioxide injection and extraction of shale gas based on the coupling of slit pores and large fractures. S2 sets the molecular simulation method for the molecular simulation system for carbon dioxide injection and extraction of shale gas; S3 is based on the algorithm and the slit pore region to perform gas-water pre-equilibrium, and obtain the initial condition recovery model for gas-water desorption in shale pores, including: S31 achieving water storage equilibrium in the slit pores through a Monte Carlo and molecular dynamics combined algorithm, and establishing a water-bearing composite shale model; S32 recovering CH4 gas through the Monte Carlo algorithm, and then achieving gas-water equilibrium through the molecular dynamics algorithm, to obtain the initial condition recovery model for gas-water desorption in shale pores; S4 simulates the attenuation mining of the shale pore gas-water desorption initial condition recovery model to obtain the gas reservoir pore model after exhaustion mining. S5 uses the gas reservoir pore model after exhaustion to simulate the huff and puff process and obtains the molecular dynamics equilibrium configuration of the fluid at each extraction stage. S6 analyzes the mechanism of CO2 huff and puff extraction of shale gas based on the molecular dynamics equilibrium configuration results of the fluids at each extraction stage.

2. The molecular simulation research method for carbon dioxide huff and puff extraction of shale gas as described in claim 1, characterized in that, The slit pores are composed of layers of quartz media and kerogen sheets of the same size stacked together.

3. The molecular simulation research method for carbon dioxide huff and puff extraction of shale gas as described in claim 2, characterized in that, The process of performing attenuation mining simulation on the initial condition recovery model of shale pore gas-water desorption to obtain the gas reservoir pore model after depletion mining includes: S41 reduces the CH4 bulk pressure in the large crack using the Monte Carlo algorithm; S42 uses molecular dynamics algorithms to simulate the depressurization desorption process under given pressure conditions to obtain a gas reservoir porosity model after exhaustion.

4. The molecular simulation research method for carbon dioxide huff and puff extraction of shale gas as described in claim 3, characterized in that, The simulation of the huff and puff process using the post-exhaustion gas reservoir porosity model yields the molecular dynamics equilibrium configurations of the fluids at each extraction stage, including: S51 calculates the methane phase density and pore system pressure in the fractures of the gas reservoir pore model after exhaustion. S52 uses the Peng-Robinson equation of state to determine the amount of CH4 / CO2 mixed gas to be injected based on the bulk density of CH4 and the molar ratio of CH4 / CO2 mixed gas. Then, it injects CH4 / CO2 mixed gas into the crack using the Monte Carlo algorithm and realizes the CO2 pressurization process using the molecular dynamics algorithm. S53 uses the Monte Carlo algorithm to simulate the pressure in the fracture, removes the sealing layer between the pores and the fracture, and performs molecular dynamics algorithm simulation to achieve depressurized shale gas extraction. S54 repeats steps S51 to S53 twice to obtain the molecular dynamics equilibrium configuration results of the fluid at each mining stage.

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