CO2 adsorption induced kerogen composite nanopore deformation prediction method

Abstract

The invention discloses a CO2 adsorption induced kerogen composite nanopore deformation prediction method. The method comprises the following steps: S1, establishing an adsorption-deformation theoretical model suitable for representing kerogen adsorption deformation; s2, constructing a kerogen composite matrix model and a kerogen composite pore model; s3, determining the gas adsorption quantity of deformed kerogen according to the kerogen composite matrix model and the kerogen composite pore model; s4, determining a kerogen bulk modulus according to the kerogen composite pore model; s5, determining an adsorption-deformation coupling coefficient according to the gas adsorption quantity of the deformed kerogen; and S6, according to the kerogen bulk modulus and the adsorption-deformation coupling coefficient, combining the adsorption-deformation theoretical model to predict the kerogen composite nanopore deformation. According to the method, the deformation of the kerogen mesoporous-microporous composite nanopores after CO2 adsorption can be more accurately and quickly predicted.

Description

Technical Field

[0001] This invention relates to the field of oil and gas reservoir development technology, and in particular to a method for predicting the deformation of kerogen composite nanopores induced by CO2 adsorption. Background Technology

[0002] In shale gas reservoirs, gases such as CO2 and methane are primarily found within kerogen. Kerogen, a major component of shale organic matter, possesses well-developed nanoscale pores, a flexible structure, and mixed wettability. This leads to a strong fluid-structure interaction between CO2 adsorption and pore deformation: CO2 adsorption induces expansion or compression deformation in the kerogen, which, by altering the pore structure, further influences the gas adsorption capacity, forming a dynamic coupling mechanism between gas adsorption and pore deformation. Clarifying this coupling behavior is crucial for accurately understanding the microscopic characteristics of CO2 and other gases in shale.

[0003] Currently, in experiments considering the influence of geostress, studies have conducted shale adsorption-swelling experiments under confining pressure conditions, revealing the anisotropic deformation behavior and adsorption of shale. While the coupling characteristics of deformation have been studied, current work primarily focuses on the macroscopic scale, with very limited research on adsorption deformation in the nanoscale pores of shale kerogen. Experimental testing is currently the most direct characterization method for the adsorption-deformation coupling behavior in rock media. This method typically involves attaching strain gauges to the rock surface to record the strain during the adsorption process in real time. This technique is currently applied to both shale and coal, but it is mainly suitable for structurally stable cores or small rock columns. For shale kerogen, due to its amorphous and unstable structure, this technique is not applicable. Currently, researchers in this field mainly employ two techniques to study the gas adsorption-pore deformation coupling behavior in the nanopores of shale kerogen: theoretical model prediction and molecular simulation.

[0004] Theoretical modeling methods characterize the adsorption deformation behavior of porous media by introducing constitutive equations. Previous researchers, based on the fundamental theories of porous media mechanics and thermodynamics, derived a theoretical model for the adsorption deformation of microporous media that considers the compressive effect of confining pressure. Subsequent researchers applied this theoretical model to predict the adsorption deformation behavior of shale kerogen matrix. By using molecular simulations to obtain the adsorption amount of shale kerogen matrix under zero-strain conditions (fixed pore structure), and substituting this into the adsorption deformation theoretical model, the corresponding volumetric strain of the shale kerogen matrix under this adsorption amount can be predicted.

[0005] However, the theoretical model method has the following main drawbacks: (1) The key parameters (bulk modulus and adsorption-deformation coupling coefficient) of the adsorption-deformation theoretical model on which this technology is based are determined based on the molecular structure of coal samples and pure carbon. However, shale kerogen differs significantly from coal and pure carbon in terms of physical pore structure and surface chemical properties, and the key parameters of the existing theoretical model cannot accurately reflect the adsorption-deformation coupling behavior of shale kerogen. (2) This scheme is mainly for a single type of microporous medium, in which the gas in the porous medium is in the adsorption state, and there is no need to distinguish the gas occurrence state. However, for the microporous-mesoporous composite pores of shale kerogen, the gas has multiple occurrence states, and it is necessary to accurately distinguish the adsorption state in order to characterize its coupling behavior with kerogen deformation. Therefore, the theoretical model method cannot be applied to the characterization of the gas adsorption and pore deformation coupling behavior in the microporous-mesoporous composite pores of shale kerogen.

[0006] Molecular simulation methods based on GCMC-MD coupling are used, where GCMC realizes gas adsorption in kerogen, and MD realizes the deformation of kerogen after gas adsorption. Adsorption and deformation are dynamically coupled until an equilibrium state is reached. Currently, most studies use the GCMC-MD simulation method to explore the coupling characteristics of fluid adsorption and pore deformation in shale kerogen. The canonical ensemble (NVT) can realize pore structure deformation during kerogen adsorption, where the kerogen cell volume is fixed and the kerogen does not expand or contract. The isothermal-isobaric ensemble (NPT) can simultaneously realize volume expansion and pore structure deformation during kerogen adsorption, and is therefore the main molecular simulation method for studying gas adsorption-induced kerogen deformation.

[0007] However, molecular simulation has the following main drawbacks: (1) It is very time-consuming and computationally expensive. Under normal workstation configuration, the simulation of one pressure point in the conventional matrix system of kerogen often takes more than 10 days. (2) Due to the adsorption-deformation dynamic simulation, it is impossible to quickly determine the adsorption result under the target deformation amount. In addition, both the adsorption amount and the deformation amount are variables in the simulation process, and the deformation amount fluctuates greatly. Its small changes will have a significant impact on the volumetric strain rate. (3) For the microporous-mesoporous composite pore model of kerogen, it is difficult to realize the deformation of kerogen through the GCMC-MD method, and the deformation result fluctuates greatly, making it difficult to obtain general rules. At present, most studies set the fluid pressure of kerogen during the adsorption process to be consistent with the magnitude of the confining pressure (effective stress equals zero). However, in the actual production process, the fluid pressure of shale kerogen will gradually decrease, and it is difficult to achieve the condition that the confining pressure is always equal to the fluid pressure. Therefore, the results of this technical solution have a large degree of uncertainty. Summary of the Invention

[0008] To address the above problems, this invention aims to provide a method for predicting the deformation of kerogen composite nanopores induced by CO2 adsorption.

[0009] The technical solution of the present invention is as follows: A method for predicting CO2 adsorption-induced deformation of kerogen composite nanopores includes the following steps: S1: Establish an adsorption-deformation theoretical model suitable for characterizing the adsorption deformation of kerogen; S2: Construct a kerogen composite matrix model and a kerogen composite pore model; S3: Determine the amount of gas adsorption by deformed kerogen based on the kerogen composite matrix model and the kerogen composite pore model. S4: Determine the bulk modulus of kerogen based on the kerogen complex pore model; S5: Determine the adsorption-deformation coupling coefficient based on the amount of gas adsorbed by the deformed kerogen; S6: Based on the bulk modulus of kerogen and the adsorption-deformation coupling coefficient, and combined with the adsorption-deformation theoretical model, predict the deformation of the kerogen composite nanopores.

[0010] Preferably, in step S1, the adsorption-deformation theoretical model is: (1) In the formula: The strain is volumetric and dimensionless. is the confining stress on the porous medium, MPa; K is the kerogen bulk modulus, GPa; is the maximum gas adsorption capacity of a single layer, mmol / g; R is the universal gas constant, J / (mol·K); T is the temperature, K; p is the gas volume pressure, MPa; Z is the compressibility factor, dimensionless. The adsorption-deformation coupling coefficient is dimensionless. For Langmuir pressure, MPa.

[0011] Preferably, when the adsorption-deformation coupling coefficient does not depend on fluid pressure, the adsorption-deformation theoretical model is as follows: (2) In the formula: C is a constant, dimensionless.

[0012] Preferably, when the gas phase pressure is converted into fugacity, the adsorption-deformation theoretical model is as follows: (3) In the formula: f Fugacity, MPa; f L ν is the fugacity corresponding to Langmuir pressure, in MPa.

[0013] Preferably, in step S2, the kerogen composite matrix model is constructed through the following sub-steps: S21: Based on the evolution law of chemical structure of shale kerogen during thermal maturation, and combined with the degree of thermal evolution of the shale kerogen to be studied, the fragment structure in the shale kerogen composite model is determined; based on the constructed fragment structure, combined with geometric optimization and structural relaxation methods, the configuration with the lowest energy is obtained, and this is used as a typical kerogen fragment. S22: Select a pure component elemental unit cell as the substrate in the unit cell library, and generate a cluster model of the target pore shape by adjusting the lattice parameters, which serves as the mesoporous pore template for subsequent pore casting. S23: Construct a cubic empty box and place the mesoporous template of the mold into the geometric center of the box; then, load the typical kerogen fragment into the box containing the mold template; finally, based on molecular dynamics methods, control the temperature and pressure under the NPT ensemble to obtain a density-converged and energy-stable kerogen composite matrix model.

[0014] Preferably, in step S2, the kerogen composite pore model is constructed through the following sub-steps: S21': Construct an empty box with the same side length as the kerogen composite matrix model, place the casting template in the center of the box and fix it; S22': Establish a vacuum layer along the Z-axis of the box, and place partitions at both ends of the Z-axis. One partition is fixed, and the other partition is compressed using a piston script. S23': Load the same number of typical kerogen fragment structures as the kerogen composite matrix model into the box, compress them using a piston script and fully relax them under the NVT ensemble until the distance between the baffles is equal to the side length in the XY direction; S24': Remove the baffles at both ends, place the kerogen structure into an empty box with the same side length as the kerogen composite matrix model, allow it to fully relax, and then remove the fixed mold template to obtain the kerogen composite pore model.

[0015] Preferably, step S3 specifically includes the following sub-steps: S31: Based on an undeformed and mesoporous kerogen matrix model, the number of kerogen fragments is quantified; S32: Set the target mesopore size of the kerogen composite pore model, use the same number of kerogen segments as the kerogen matrix model, construct a kerogen composite pore model containing mesopores of this size, and calculate the volumetric strain rate of the model relative to the kerogen matrix model. S33: Gradually increase the size of the target mesopore and repeat step S32 to obtain kerogen composite pore models with different mesopore sizes under different volumetric strain rates; S34: Gas occurrence simulation based on GCMC method in kerogen composite pore model with different volumetric strain rates; S35: Based on the configuration after equilibrium, quantify the absolute gas adsorption amount in the kerogen composite pore model. This absolute gas adsorption amount is the gas adsorption amount of the modified kerogen.

[0016] Preferably, step S4 specifically includes the following sub-steps: S41: Eliminate the symmetry of the kerogen complex pore model and perform geometric optimization on the structure of the kerogen complex pore model; S42: Apply the set strain to the kerogen composite pore model and perform relaxation equilibrium under the NVT ensemble to obtain the kerogen model under different strains after equilibrium. S43: Based on the kerogen model under different strains, the elastic stiffness matrix and elastic compliance matrix of the kerogen are calculated by finite difference. S44: The bulk modulus was calculated using the Vogit and Reuss methods respectively, and the results of the two methods were averaged to obtain the bulk modulus of kerogen with different degrees of deformation.

[0017] Preferably, step S5 specifically includes the following sub-steps: S51: Plot the relationship between gas adsorption and kerogen volumetric strain rate as the abscissa and the amount of gas adsorbed by deformed kerogen as the ordinate. S52: Calculate the slope of the curve in the relationship diagram and calculate the adsorption-deformation coupling coefficient under the pressure using the small strain assumption; S53: Repeat step S52 to calculate the adsorption-deformation coupling coefficient under different pressures, and fit the relationship between the adsorption-deformation coupling coefficient and the pressure. S54: Determine the adsorption-deformation coupling coefficient at the target pressure based on the relationship between the adsorption-deformation coupling coefficient and the pressure.

[0018] Preferably, step S6 specifically includes the following sub-steps: S61: Substitute the kerogen bulk modulus and the adsorption-deformation coupling coefficient into the adsorption-deformation theoretical model to obtain an adsorption-deformation theoretical model with clear key parameters; S62: The amount of gas adsorbed in the pores of a zero-strain kerogen composite structure was obtained using molecular simulation methods. S63: Substitute the amount of gas adsorbed in the kerogen composite pores of the zero-strain structure into the adsorption-deformation theoretical model with well-defined key parameters to predict the amount of kerogen deformation corresponding to the amount of gas adsorbed in the kerogen composite pores of the zero-strain structure.

[0019] The beneficial effects of this invention are: This invention can more accurately and quickly predict the deformation of kerogen mesoporous-microporous composite nanopores after CO2 adsorption. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a flowchart of the method for predicting the deformation of kerogen composite nanopores induced by CO2 adsorption according to the present invention. Figure 2 This is a schematic diagram of a fatty chain structural unit in a specific embodiment; Figure 3 This is a schematic diagram of a spherical casting mold model in a specific embodiment; Figure 4 This is a flowchart of the kerogen matrix model construction process in a specific embodiment; Figure 5 A flowchart illustrating the construction of a kerogen composite pore model in a specific embodiment; Figure 6 This is a schematic diagram of the kerogen composite pore model and pore structure in a specific embodiment; Figure 7 A schematic diagram illustrating the process of determining the adsorption-deformation coupling coefficient of a kerogen composite pore model in a specific embodiment; Figure 8 This represents the bulk modulus of the kerogen fatty chain structure at different mesopore sizes in a specific embodiment. Figure 9 This is a curve showing the relationship between CO2 adsorption and volumetric strain in deformed kerogen in a specific embodiment. Figure 10 This is a graph showing the relationship between the adsorption-pore deformation coupling coefficient and pressure of the kerogen fatty chain structure in a specific embodiment. Figure 11 This is a volumetric strain rate curve of the kerogen fatty chain structure in a specific embodiment. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specified, the embodiments and technical features described in this application can be combined with each other. It should also be pointed out that, unless otherwise indicated, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. The terms "comprising" or "including" and similar words used in this invention refer to elements or objects preceding the word that encompass the elements or objects listed following the word and their equivalents, without excluding other elements or objects.

[0023] like Figure 1 As shown, this invention provides a method for predicting the deformation of kerogen composite nanopores induced by CO2 adsorption, comprising the following steps: S1: Establish an adsorption-deformation theoretical model suitable for characterizing the adsorption deformation of kerogen.

[0024] In one specific embodiment, the adsorption-deformation theoretical model is as follows: (1) In the formula: The strain is volumetric and dimensionless. is the confining stress on the porous medium, MPa; K is the kerogen bulk modulus, GPa; is the maximum gas adsorption capacity of a single layer, mmol / g; R is the universal gas constant, J / (mol·K); T is the temperature, K; p is the gas volume pressure, MPa; Z is the compressibility factor, dimensionless. The adsorption-deformation coupling coefficient is dimensionless. For Langmuir pressure, MPa.

[0025] In the above embodiments, the adsorption-deformation theoretical model is derived based on the fundamental thermodynamic theory and the near-linear porous media mechanics model, taking into account the influence of gas non-idealities. The main ideas and steps are as follows: (1) The Helmholtz free energy change of the porous media system is transformed into a function of fluid adsorption and pore deformation; (2) Using Maxwell's relation, the equations of stress-chemical potential partial derivative and adsorption amount-strain partial derivative are derived; (3) Based on the small strain assumption and isotropic assumption of porous media, the constitutive equations of porous media are derived; (4) Combining the Gibbs-Duhem relation under isothermal conditions, the chemical potential in the constitutive equations is replaced with the gas phase pressure; (5) Based on the small strain assumption, the gas adsorption amount in the deformed structure is transformed into a function of adsorption amount and adsorption-deformation coupling coefficient under zero strain conditions, and substituted into the constitutive equations to derive the extended porous media mechanical equations; (6) Combining the non-ideal gas equation of state, the non-ideality of the gas is considered in the extended porous media mechanical equations; (7) Combining the Langmuir adsorption model, the adsorption amount under zero strain conditions in the extended porous media mechanical equations is eliminated. Specifically: For typical porous media, the change in Helmholtz free energy caused by fluid adsorption and pore deformation is: (4) In the formula: h is the Helmholtz free energy, J / m 3 ; The confining stress on the porous medium, in MPa; The strain is volumetric and dimensionless. The stress is deviatoric stress, MPa; It is a partial strain, dimensionless; Let be the chemical potential of the fluid, in J / mol; , where is the Lagrange molar number of gas molecules per unit volume of the porous medium, in mol / m³. 3 N represents the actual number of moles of molecules in the porous medium, in mol. For the volume of the undeformed porous medium, m 3 ; Equation (4) can be rewritten as equation (5): (5) Equation (5) can be obtained based on Maxwell's relation: (6) (7) Based on the small strain assumption, if the porous material is isotropic, the adsorption of gas molecules does not depend on the partial strain. If the bulk modulus K and shear modulus G of the porous medium material are constants, then the above regarding... , and Integrating equation (4) yields the constitutive equations for porous media: (8) (9) (10) According to the Gibbs-Duhem relation under isothermal conditions ( ,in The molar volume of the bulk gas, in cm³. 3 / mol), by introducing the gas phase pressure p as a state variable, the equation set (8)~(10) can be rewritten as: (11) (12) (13) The isothermal adsorption capacity of heterogeneous porous media is related to gas pressure and strain. Assuming that the porous media undergoes small strain deformation (ɛ<<1), a first-order expansion of equation (13) yields: (14) in, It is the isothermal adsorption capacity of porous media under zero strain, mmol / cm². 3 If the above equation holds, substituting equation (14) into the system of equations (11) to (13) yields the extended mechanical equations for porous media: (15) Rewrite equation (15) as a function of strain: (16) According to the state equation under non-ideal conditions, we know that... Equation (13) can be further rewritten as: (17) For kerogen, the isothermal adsorption curve of CO2 under zero strain conditions conforms to the Langmuir model: (18) Substituting equation (18) into equation (17), we can obtain the adsorption-deformation theoretical model shown in equation (1) that is suitable for characterizing the adsorption deformation of kerogen.

[0026] In a specific embodiment, when the adsorption-deformation coupling coefficient does not depend on fluid pressure, the adsorption-deformation theoretical model is as follows: (2) In the formula: C is a constant, dimensionless.

[0027] In the molecular simulation of kerogen adsorption, fluid fugacity is typically used instead of fluid pressure. Therefore, in a specific embodiment, the adsorption-deformation theoretical model is as follows: (3) In the formula: f Fugacity, MPa; f L ν is the fugacity corresponding to Langmuir pressure, in MPa.

[0028] In the above embodiments, the fluid phase pressure can be converted into fugacity using the Peng-Robinson equation.

[0029] The confining stress on the porous medium in the adsorption-deformation theoretical model Under indoor isothermal adsorption experimental conditions, it is equal to the fluid pressure; during shale reservoir development, it is greater than the fluid pressure; and under the ideal conditions of maximum free deformation of kerogen, it can be approximately equal to atmospheric pressure.

[0030] S2: Construct a kerogen composite matrix model and a kerogen composite pore model.

[0031] In one specific embodiment, the kerogen composite matrix model is constructed through the following sub-steps: S21: Based on the evolution law of chemical structure of shale kerogen during thermal maturation, and combined with the degree of thermal evolution of the shale kerogen to be studied, the fragment structure in the shale kerogen composite model is determined; based on the constructed fragment structure, combined with geometric optimization and structural relaxation methods, the configuration with the lowest energy is obtained, and this is used as a typical kerogen fragment. S22: Select a pure component elemental unit cell as the substrate in the unit cell library, and generate a cluster model of the target pore shape by adjusting the lattice parameters, which serves as the mesoporous pore template for subsequent pore casting. S23: Construct a cubic empty box and place the mesoporous template of the mold into the geometric center of the box; then, load the typical kerogen fragment into the box containing the mold template; finally, based on molecular dynamics methods, control the temperature and pressure under the NPT ensemble to obtain a density-converged and energy-stable kerogen composite matrix model.

[0032] In one specific embodiment, the kerogen composite pore model is constructed through the following sub-steps: S21': Construct an empty box with the same side length as the kerogen composite matrix model, place the casting template in the center of the box and fix it; S22': Establish a vacuum layer along the Z-axis of the box, and place partitions at both ends of the Z-axis. One partition is fixed, and the other partition is compressed using a piston script. S23': Load the same number of typical kerogen fragment structures as the kerogen composite matrix model into the box, compress them using a piston script and fully relax them under the NVT ensemble until the distance between the baffles is equal to the side length in the XY direction; S24': Remove the baffles at both ends, place the kerogen structure into an empty box with the same side length as the kerogen composite matrix model, allow it to fully relax, and then remove the fixed mold template to obtain the kerogen composite pore model.

[0033] S3: Determine the amount of gas adsorption by deformed kerogen based on the kerogen composite matrix model and the kerogen composite pore model.

[0034] In a specific embodiment, step S3 specifically includes the following sub-steps: S31: Based on an undeformed and mesoporous kerogen matrix model, the number of kerogen fragments is quantified; S32: Set the target mesopore size of the kerogen composite pore model, use the same number of kerogen segments as the kerogen matrix model, construct a kerogen composite pore model containing mesopores of this size, and calculate the volumetric strain rate of the model relative to the kerogen matrix model. S33: Gradually increase the size of the target mesopore and repeat step S32 to obtain kerogen composite pore models with different mesopore sizes under different volumetric strain rates; S34: Gas occurrence simulation based on GCMC method in kerogen composite pore model with different volumetric strain rates; S35: Based on the configuration after equilibrium, quantify the absolute gas adsorption amount in the kerogen composite pore model. This absolute gas adsorption amount is the gas adsorption amount of the modified kerogen.

[0035] It should be noted that in the above embodiments, the structure of the kerogen matrix model is a well-known structure in the art. The kerogen composite matrix model of the present invention contains a mesoporous template for casting, while the kerogen matrix model does not, and it does not contain mesopores.

[0036] S4: Determine the bulk modulus of kerogen based on the kerogen complex pore model.

[0037] In a specific embodiment, step S4 specifically includes the following sub-steps: S41: Eliminate the symmetry of the kerogen complex pore model and perform geometric optimization on the structure of the kerogen complex pore model; S42: Apply the set strain to the kerogen composite pore model and perform relaxation equilibrium under the NVT ensemble to obtain the kerogen model under different strains after equilibrium. S43: Based on the kerogen model under different strains, the elastic stiffness matrix and elastic compliance matrix of the kerogen are calculated by finite difference. S44: The bulk modulus was calculated using the Vogit and Reuss methods respectively, and the results of the two methods were averaged to obtain the bulk modulus of kerogen with different degrees of deformation.

[0038] In the above embodiments, the formula for calculating the bulk modulus using the Vogit method is: (19) Where: K v The bulk modulus, calculated using the Vogit method, is given in GPa. is a constant of the elastic stiffness matrix.

[0039] The formula for calculating the bulk modulus using the Reuss method is: (20) Where: K R The bulk modulus, calculated using the Reuss method, is given in GPa. is the constant of the elasticity compliance matrix.

[0040] The final bulk modulus of kerogen is calculated using the following formula: (twenty one) S5: Determine the adsorption-deformation coupling coefficient based on the amount of gas adsorbed by the deformed kerogen.

[0041] In one specific embodiment, step S5 specifically includes the following sub-steps: S51: Plot the relationship between gas adsorption and kerogen volumetric strain rate as the abscissa and the amount of gas adsorbed by deformed kerogen as the ordinate. S52: Calculate the slope of the curve in the relationship diagram and calculate the adsorption-deformation coupling coefficient under the pressure using the small strain assumption; S53: Repeat step S52 to calculate the adsorption-deformation coupling coefficient under different pressures, and fit the relationship between the adsorption-deformation coupling coefficient and the pressure. S54: Determine the adsorption-deformation coupling coefficient at the target pressure based on the relationship between the adsorption-deformation coupling coefficient and the pressure.

[0042] It should be noted that in the above embodiments, in step S52, the adsorption-deformation coupling coefficient under the pressure is calculated using the small strain assumption only when the slope of the curve in the relational graph is constant.

[0043] S6: Based on the bulk modulus of kerogen and the adsorption-deformation coupling coefficient, and combined with the adsorption-deformation theoretical model, predict the deformation of the kerogen composite nanopores.

[0044] In a specific embodiment, step S6 specifically includes the following sub-steps: S61: Substitute the kerogen bulk modulus and the adsorption-deformation coupling coefficient into the adsorption-deformation theoretical model to obtain an adsorption-deformation theoretical model with clear key parameters; S62: The amount of gas adsorbed in the pores of a zero-strain kerogen composite structure was obtained using molecular simulation methods. S63: Substitute the amount of gas adsorbed in the kerogen composite pores of the zero-strain structure into the adsorption-deformation theoretical model with well-defined key parameters to predict the amount of kerogen deformation corresponding to the amount of gas adsorbed in the kerogen composite pores of the zero-strain structure.

[0045] In a specific embodiment, taking a shale reservoir as an example, the CO2 adsorption-induced deformation of kerogen composite nanopores described in this invention is used to predict the CO2 adsorption-induced deformation of kerogen composite nanopores. Specifically, the following steps are included: (1) Establish the adsorption-deformation theoretical model shown in equation (1) suitable for characterizing the adsorption deformation of kerogen; (2) Constructing a kerogen composite matrix model Based on the chemical structural evolution pattern of decreasing fatty chain proportion during thermal maturation, and considering that fatty chains are in an immature thermal evolution stage, a pure component fatty chain structure (C) was constructed. 25 H 50 O2). Based on the constructed fragment structure, combined with geometric optimization and structural relaxation methods, the energy-minimum configuration is finally obtained, such as... Figure 2 As shown. In the cell library, a Si elemental cell is selected as the substrate. By adjusting the lattice parameters, a spherical cluster model with a radius gradient of 0-2 nm (step size 0.5 nm) is generated as the geometric template for subsequent porosity casting, as shown. Figure 3 As shown. Construct a cubic unit cell with side lengths of 60 Å and 100 Å ( ). Figure 4 (a) A silicon spherical model was placed at the geometric center of the unit cell, with centroid fraction coordinates of (0.5, 0.5, 0.5). Subsequently, 220 adipose chain fragments were loaded using a fixed loading method. Figure 4(b)). Finally, based on the NPT ensemble, the temperature and pressure were controlled at 363.15 K and 20 MPa, respectively, with a total simulation time of 1 ns ( Figure 4 (c)).

[0046] (3) Constructing a kerogen composite pore model In the kerogen composite matrix model with spherical porous template constructed in step (2), the spherical molded template will deviate from the geometric center of the unit cell due to uneven stress, so a piston script is used for correction. First, an empty box with the same side lengths as the X and Y directions of the kerogen aliphatic chain structure (the side length of the aliphatic chain structure is 54.57 Å) is constructed, and the silica spherical model is placed at the geometric center of the empty box. Figure 5 (a)), and then a vacuum layer with a thickness of 40 Å is established along the Z direction ( Figure 5 (b) then, the same number of adipose chain fragments as in step (2) were adsorbed using a fixed loading method. Figure 5 (c) A carbon atomic layer is used as a baffle for the piston script. Baffles are placed on both sides of the Z-axis, one end fixed, and the baffle at the other end is continuously compressed by the piston script. Each advance of the baffle is 0.5 Å. After each advance, NVT ensemble relaxation (200 ps) is performed until the system reaches equilibrium. Compression stops when the distance between the baffles matches the cell edge length in the XY direction. Figure 5 (d) Finally, the end plates were removed, and the kerogen structure was placed in an empty box with the same side length as the initial kerogen model. After NVT ensemble relaxation for 2 ns, the spherical casting hole template was removed. The final result was a kerogen composite pore model with different spherical pore sizes. Figure 5 (e)). Schematic diagram of kerogen composite pore model and nanopore structure as shown below. Figure 6 As shown.

[0047] (4) Determine the amount of gas adsorbed by modified kerogen. First, the volumetric strain rate of the kerogen composite pore model was determined. Based on the above step (3), kerogen composite pore models with different spherical pore sizes were constructed, with the pore size range being 0-2 nm and the pore size growth gradient being 0.5 nm. Taking a spherical pore model with a radius of 1 nm as an example: when the pore radius is less than 1 nm, the kerogen exhibits compressive deformation, and when it is greater than 1 nm, it transforms into expansion deformation ( Figure 7(a)). Referring to the undeformed kerogen matrix model, the number of segments in the kerogen composite pore model was determined to be 220. The target mesopore sizes of the kerogen fatty chain composite pore model were set to 0, 0.5, 1, 1.5, and 2 nm, and spherical Si elemental cluster models of corresponding sizes were established. Based on the 220 kerogen fatty chain segments and the spherical Si elemental cluster models of the target sizes, kerogen fatty chain composite pore models with different strains were established in combination with steps (2) and (3). Figure 7 (b) Taking kerogen composite porosity models containing 1 nm and 2 nm mesopores as examples, the corresponding volumetric strain rates were calculated. For kerogen composite porosity models with different volumetric strain rates, CO2 occurrence simulations were conducted under different pressures (2, 8, 14, 20 MPa) using the GCMC method. Figure 7 (c)). After CO2 storage equilibrium is reached, the absolute CO2 adsorption amount in the kerogen complex pore model is identified and quantified by combining its density profile.

[0048] (5) Determine the bulk modulus of kerogen The elastomechanical properties of a kerogen fatty chain complex porosity model were calculated using a constant strain simulation method. This method first eliminates the symmetry of the kerogen complex porosity model and performs geometric optimization of its structure. Then, a set strain is applied to the model, and relaxation equilibrium is achieved under an NVT ensemble, yielding the kerogen model under different strains after equilibrium. Based on the kerogen configuration under different strains, the elastic stiffness and elastic compliance matrices of the kerogen are calculated using finite differences. Based on the elastic stiffness constant matrix, the bulk modulus is calculated using the Vogit method. Based on the elastic compliance constant matrix, the bulk modulus can be calculated using the Reuss method. The average of the two methods yielded the bulk modulus K of different types of kerogen matrices, as shown in the following figures. Figure 8 As shown.

[0049] (6) Determine the adsorption-deformation coupling coefficient Based on step (4), the absolute adsorption amount of kerogen under different volumetric strain rates and pressures has been obtained. Using the volumetric strain rate of kerogen as the abscissa and the absolute CO2 adsorption amount as the ordinate, a graph showing the relationship between the absolute CO2 adsorption amount and the volumetric strain rate of kerogen is plotted. Figure 9 At this point, the CO2 adsorption amount and the volumetric strain rate of kerogen are approximately linearly related. The adsorption-deformation coupling coefficient of the kerogen fatty chain structure is calculated according to equation (22): (twenty two) Where: n 0(p) represents the CO2 adsorption capacity in the undeformed kerogen composite pores, which is a function of pressure, mmol / g; C( ,p) is the adsorption-deformation coupling coefficient of the kerogen composite pore model, which is a function of volumetric strain and pressure.

[0050] In equation (22), the single vertical line represents the slope of the absolute adsorption amount and the volumetric strain rate of kerogen calculated under different pressures. The adsorption-deformation coupling coefficient under different pressure conditions is as follows: Figure 10 As shown, with the increase of pore size, the adsorption capacity is enhanced, but the stress sensitivity decreases, therefore the coupling coefficient C of CO2... The coefficient of coupling of the aliphatic chain structure gradually decreases. The average uncertainty of the coupling coefficient under different pressures is less than 15%. For ease of calculation, the coupling coefficient under different pressures can be approximated as a constant when the pore size is constant.

[0051] (7) Predicting gas adsorption-pore deformation behavior in kerogen The calculated adsorption-deformation coupling coefficient and the bulk modulus of the model were substituted into the adsorption-deformation theoretical model. Combining the gas adsorption amount in the composite pores of shale kerogen fatty chains under zero strain conditions obtained by molecular simulation, the volumetric strain rate of the kerogen fatty chains under different pressures and different spherical pore sizes was calculated using formula (23): (twenty three) For kerogen with different pore sizes, the coupling coefficient can be approximated as a constant under different pressures; the calculated volumetric strain rate of the kerogen is as follows: Figure 11 As shown.

[0052] In summary, this invention enables more accurate and rapid prediction of the deformation of shale kerogen mesoporous-microporous composite nanopores after CO2 adsorption. The proposed kerogen composite pore model construction approach and the method for determining the adsorption-deformation coupling coefficient effectively simulate the dynamic deformation process of kerogen composite pores while maintaining the target shape mesopores at the geometric center of the unit cell and ensuring geometric stability, which is beneficial for accurate and rapid calibration of CO2 adsorption. By changing the mesopore size to achieve the expansion and compression deformation of kerogen, it can more accurately simulate the local expansion or compression deformation caused by gas adsorption, which better matches the deformation characteristics of shale kerogen induced by gas adsorption in actual reservoirs. Compared with existing technologies, this invention represents a significant advancement.

[0053] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for predicting the deformation of kerogen composite nanopores induced by CO2 adsorption, characterized in that, The method comprises the following steps: S1: establishing an adsorption-deformation theory model suitable for characterizing kerogen adsorption deformation; S2: constructing a kerogen composite matrix model and a kerogen composite pore model; S3: determining the gas adsorption amount of deformed kerogen according to the kerogen composite matrix model and the kerogen composite pore model; S4: determining the bulk modulus of kerogen according to the kerogen composite pore model; S5: determining the adsorption-deformation coupling coefficient according to the gas adsorption amount of the deformed kerogen; S6: predicting the deformation amount of the kerogen composite nanopore according to the bulk modulus of the kerogen and the adsorption-deformation coupling coefficient in combination with the adsorption-deformation theory model.

2. The method of claim 1, wherein the CO2-sorption-induced dry kerogen composite nanoporosity deformation amount is predicted by the following equation: ###0001### wherein, A is the CO2-sorption-induced dry kerogen composite nanoporosity deformation amount, A0 is the initial kerogen composite nanoporosity deformation amount, and A0 is the initial kerogen composite nanoporosity deformation amount. In step S1, the adsorption-deformation theory model is: （1） wherein: is the volumetric strain, dimensionless; is the confining stress on the porous medium, MPa; K is the kerogen bulk modulus, GPa; is the maximum gas adsorption capacity of a single layer, mmol / g; R is the universal gas constant, J / (mol K); T is the temperature, K; p is the gas bulk pressure, MPa; Z is the compressibility factor, dimensionless; is the adsorption-deformation coupling coefficient, dimensionless; is the Langmuir pressure, MPa.

3. The method of claim 2, wherein the CO2-sorption-induced dry kerogen composite nanoporosity deformation amount is predicted by the following equation: ###0001### wherein, A is the CO2-sorption-induced dry kerogen composite nanoporosity deformation amount, A0 is the initial dry kerogen composite nanoporosity deformation amount, and A0 is the initial dry kerogen composite nanoporosity deformation amount. When the adsorption-deformation coupling coefficient is independent of fluid pressure, the adsorption-deformation theory model is: （2） In the formula: C is a constant, dimensionless.

4. The method of claim 3, wherein the CO2-sorption-induced dry kerogen composite nanoporosity deformation amount is predicted by the following equation: ###0002### wherein, A is the CO2-sorption-induced dry kerogen composite nanoporosity deformation amount, A0 is the initial dry kerogen composite nanoporosity deformation amount, and A0 is the initial dry kerogen composite nanoporosity deformation amount. When the gas bulk pressure is converted into fugacity, the adsorption-deformation theory model is: （3） wherein: f is the fugacity, MPa; f L is the fugacity corresponding to the Langmuir pressure, MPa.

5. The method for predicting CO2 adsorption-induced deformation of kerogen composite nanopores according to claim 1, characterized in that, In step S2, the kerogen composite matrix model is constructed through the following sub-steps: S21: based on the evolution law of the chemical structure of shale kerogen in the thermal maturation evolution process, and combined with the thermal evolution degree of the shale kerogen to be studied, the fragment structure in the shale kerogen composite model is determined; based on the constructed fragment structure, geometric optimization and structure relaxation method are combined to ensure that the lowest energy configuration is obtained, and the lowest energy configuration is taken as a typical fragment of kerogen; S22: selecting a pure component unit cell as a base in a cell library, generating a cluster model with a target pore shape by adjusting the lattice parameters, and taking the cluster model as a mesopore template for subsequent pore casting; S23: establishing a cubic empty box, placing the mesopore template of the casting mold in the geometric center of the box, then loading the typical fragment of kerogen in the box containing the casting mold template, and finally controlling the temperature and pressure under the NPT ensemble based on the molecular dynamics method to obtain a density-converged and energy-stable kerogen composite matrix model.

6. The method of predicting the amount of CO2-sorption-induced complex nanoporosity deformation of kerogen according to claim 1, wherein, In step S2, the kerogen composite pore model is constructed through the following sub-steps: S21': an empty box with the same edge length as the kerogen composite matrix model is established, the casting mold template is placed in the center of the box and fixed; S22': a vacuum layer is established along the Z-axis direction of the box, and a partition plate is placed at both ends of the Z-axis, one end of the partition plate is fixed, and the other end of the partition plate is compressed using a piston script; S23': the same number of typical kerogen fragment structures as the kerogen composite matrix model are loaded in the box, and the piston script is used for compression and relaxation under the NVT ensemble until the distance between the baffles is equal to the edge length in the XY direction; S24': the two end baffles are deleted, the kerogen structure is placed in an empty box with the same edge length as the kerogen composite matrix model, and after sufficient relaxation, the fixed casting mold template is deleted to obtain a kerogen composite pore model.

7. The method of claim 1, wherein the CO2-sorption-induced dry kerogen composite nanoporosity deformation amount is predicted by the following equation: ###0002### where, A is the CO2-sorption-induced dry kerogen composite nanoporosity deformation amount, A0 is the initial kerogen nanoporosity, and A0 is the initial kerogen nanoporosity. Step S3 specifically includes the following sub-steps: S31: taking the undeformed and mesopore-free kerogen matrix model as the basis, the number of kerogen fragments is quantified; S32: setting a target mesopore size of the kerogen composite pore model, using the same number of kerogen fragments as the kerogen matrix model, constructing a kerogen composite pore model containing the size of the mesopore, and calculating the volume strain rate of the model relative to the kerogen matrix model; S33: gradually increasing the size of the target mesopore, repeating step S32 to obtain kerogen composite pore models with different mesopore sizes under different volume strain rates; S34: based on the GCMC method, simulating gas storage in kerogen composite pore models under different volume strain rates; S35: combining the configuration after the storage balance, quantifying the absolute gas adsorption amount in the kerogen composite pore model, and the absolute gas adsorption amount is the gas adsorption amount of the deformed kerogen.

8. The method of predicting the amount of CO2-sorption-induced complex nanoporosity deformation of kerogen according to claim 1, wherein, Step S4 specifically includes the following sub-steps: S41: eliminating the symmetry of the kerogen composite pore model, and geometrically optimizing the structure of the kerogen composite pore model; S42: applying a set strain to the kerogen composite pore model and relaxing under the NVT ensemble to obtain the kerogen model under different strains after equilibrium; S43: based on the kerogen model under different strains, calculating the elastic stiffness matrix and the elastic flexibility matrix of the kerogen by finite difference; S44: calculating the bulk modulus by Vogit and Reuss methods respectively, and averaging the calculation results of the two methods to obtain the bulk modulus of the deformed kerogen under different deformations.

9. The method of claim 1, wherein the CO2-sorption-induced dry kerogen composite nanoporosity deformation amount is predicted by the following equation: ###0002### wherein, A is the CO2-sorption-induced dry kerogen composite nanoporosity deformation amount, A0 is the initial kerogen composite nanoporosity deformation amount, and A0 is the initial kerogen composite nanoporosity deformation amount. 0 Step S5 specifically includes the following sub-steps: S51: taking the volume strain rate of the kerogen as the abscissa and the gas adsorption amount of the deformed kerogen as the ordinate to draw a relationship chart between the gas adsorption amount and the volume strain rate of the kerogen; S52: calculating the slope of the curve in the relationship chart, and calculating the adsorption-deformation coupling coefficient under the pressure by using the small strain assumption; S53: repeating step S52 to calculate the adsorption-deformation coupling coefficient under different pressures, and fitting to obtain the relationship between the adsorption-deformation coupling coefficient and the pressure; S54: determining the adsorption-deformation coupling coefficient under the target pressure according to the relationship between the adsorption-deformation coupling coefficient and the pressure.

10. The method of predicting the amount of CO2-sorption-induced kerogen composite nanoporosity deformation according to any one of claims 1 to 9, wherein, Step S6 specifically includes the following sub-steps: S61: substituting the bulk modulus of the kerogen and the adsorption-deformation coupling coefficient into the adsorption-deformation theoretical model to obtain the adsorption-deformation theoretical model with key parameters; S62: using a molecular simulation method to obtain the gas adsorption amount in the kerogen composite pore of the zero strain structure; S63: substituting the gas adsorption amount in the kerogen composite pore of the zero strain structure into the adsorption-deformation theoretical model with key parameters to predict the deformation amount of the kerogen corresponding to the gas adsorption amount in the kerogen composite pore of the zero strain structure.

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