Calculation method of carbon dioxide storage capacity in gas reservoirs considering pore expansion and component effects

Abstract

The present invention discloses a calculation method for the carbon dioxide storage capacity of gas reservoirs considering pore expansion and component effects, belonging to the technical field of carbon dioxide sequestration. This method comprehensively considers the pore expansion effect caused by CO2 injection. By introducing the pore pressure sensitivity coefficient, a pore expansion model is established to effectively characterize the pore expansion effect caused by the increase in fluid pressure, thereby more accurately evaluating the CO2 storage potential of gas reservoirs. Secondly, the Peng-Robinson equation of state is adopted, considering the thermodynamic interaction between CO2 and residual CH4, and accurately characterizing the property changes of the mixed gas. The present invention makes up for the deficiency of the existing method in simplifying the CO2-CH4 system to a single-component gas when calculating gas properties, avoiding the calculation error of the structural carbon dioxide storage capacity caused by this simplification. At the same time, due to the more accurate calculation of the CO2 partial pressure, the calculation error of the dissolved CO2 storage capacity is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of carbon dioxide storage, and in particular relates to a method for calculating the storage amount of carbon dioxide in a gas reservoir by taking into account pore expansion and component effects. Background Art

[0002] In recent years, with economic development and the exploitation of oil and gas resources, environmental issues have received more and more attention. Due to the combustion of fossil fuels, the impact of carbon dioxide (CO2) emissions on the greenhouse effect of the atmosphere has attracted widespread attention. Carbon dioxide capture and storage (CCS) technology, as an effective means to reduce CO2 emissions and slow global warming, has gradually become one of the key technologies for addressing climate change. The geological storage of CO2, that is, injecting CO2 into deep underground gas reservoirs, oil reservoirs or salt caverns for long-term storage, is a potential solution for achieving large-scale CO2 emission reductions. The capacity assessment of CO2 storage in gas reservoirs is a key issue in gas reservoir CO2 storage projects, which is directly related to the storage capacity of CO2 and the safety of long-term storage.

[0003] At present, the evaluation method of CO2 storage capacity in gas reservoirs is mainly based on the geometric volume, porosity, gas reservoir pressure, temperature and other parameters of the gas reservoir. Commonly used evaluation models include the storage capacity calculation formula based on the law of conservation of matter. However, the existing CO2 storage capacity calculation methods generally have certain limitations, especially in the calculation process, the interaction between CO2 and other components in the gas reservoir and the pore expansion effect are not fully considered, which may lead to large deviations in the calculation results and affect the accurate evaluation of CO2 storage potential.

[0004] 1. Ignoring the pore expansion effect: CO2 injection will not only increase the gas pressure in the gas reservoir, but may also cause changes in porosity. After a gas reservoir is exploited to exhaustion, its reservoir pressure is often low. Injecting CO2 into it can increase the reservoir pressure several times. Studies have shown that such a pressure increase will significantly increase the reservoir porosity. This effect is often ignored in existing assessment models. Pore expansion will cause the gas reservoir's capacity to increase, so at the same pressure, the total amount of CO2 that can be stored in the gas reservoir is higher. Ignoring this change may underestimate the storage capacity of the gas reservoir, affecting the optimization of the CO2 injection amount and the stability of long-term storage.

[0005] 2. Interaction between CO2 and residual methane: There is still residual methane (CH4) gas in depleted gas reservoirs, and CO2 injection will form a mixed system of CO2-CH4. The thermodynamic interaction between CO2 and CH4 has an important influence on the physical and chemical properties of the gas, such as gas density, deviation factor and its solubility. When calculating the gas properties, the existing CO2 storage calculation method often assumes that the reservoir gas after storage is a single component CO2, ignoring the influence of residual CH4, because the concentrations of CO2 and CH4 are unknown at the final state of storage, thereby oversimplifying the calculation of CO2 gas properties. The interaction between CO2 and CH4 will not only affect the state equation of CO2 fluid, but also affect the calculation of CO2 concentration and solubility, thereby affecting the estimation of CO2 storage.

[0006] Therefore, the existing CO2 storage capacity assessment methods have certain problems in accuracy and reliability, especially when the porosity of the gas reservoir is easily deformed, these problems are more prominent. In order to solve these problems, the present invention proposes a new calculation method that takes into account the pore expansion effect and the thermodynamic interaction between CO2 and residual methane in the gas reservoir. By adopting a more accurate thermodynamic model and the principle of conservation of matter, the method of the present invention can fully consider the changes in the physical and chemical properties of the gas during the assessment of the CO2 storage capacity, thereby improving the accuracy and reliability of the calculation results, and providing a more scientific basis for the scale assessment, injection volume optimization and storage safety analysis of CO2 geological storage. This is of great significance for promoting the widespread application of CCS technology and achieving large-scale CO2 emission reductions. Summary of the invention

[0007] In view of the above problems existing in the prior art, the present invention proposes a method for calculating the storage capacity of carbon dioxide in gas reservoirs taking into account pore expansion and component effects. The method has a reasonable design, solves the shortcomings of the prior art, and has good effects.

[0008] The method for calculating the carbon dioxide storage capacity of gas reservoirs considering the effects of pore expansion and components includes the following steps:

[0009] Step 1: Calculate the porosity after CO2 injection into the gas reservoir;

[0010] Step 2: Calculate the CH4 concentration before and after CO2 injection into the gas reservoir;

[0011] Step 3: Calculate the CO2 concentration after CO2 is injected into the gas reservoir using Newton-Raphson iteration;

[0012] Step 4: Calculate the solubility of CO2 in the water phase;

[0013] Step 5: Calculate the total amount of CO2 stored.

[0014] Furthermore, the step 1 is specifically as follows: after CO2 is injected into the gas reservoir, the gas pressure increases, and the pores expand due to the pressure increase. Therefore, the porosity after CO2 injection will be greater than the initial state. The porosity of the gas reservoir under different pressures is calculated:

[0015] ; (1)

[0016] in, f b is the porosity after CO2 injection, f a is the porosity before CO2 injection, exp is the exponential function, c is the porosity pressure sensitivity coefficient, p b is the expected reservoir pressure after CO2 injection, p a is the initial reservoir pressure before CO2 injection.

[0017] Furthermore, the step 2 is specifically as follows: in the initial state, that is, before CO2 injection, the gas reservoir contains only CH4, and the single-component Peng-Robinson state equation is used to calculate p a Reservoir temperature T The pure CH4 concentration under the above conditions specifically includes the following sub-steps:

[0018] Step 2.1: Solve the following about Z Cubic equation to obtain the deviation factor of CH4 in the initial state Z 1a :

[0019] ; (2)

[0020] in, ; ;

[0021] T r To compare the temperature, T r = T / T c1 , T is the reservoir temperature, T c1 is the critical temperature, T c1 =190.56;

[0022] p ra To compare pressure, p ra = p0 / p c1 , p 0 is the reservoir pressure, p c1 is the critical pressure, p c1 =4.6MPa;

[0023] d A and d B is the empirical parameter of the state equation, d A =1+√2, d B =1-√2;

[0024] If a cubic equation has three real roots, then Z 1a is equal to the largest real root. If there is only one real root, then Z 1a is equal to the only real root;

[0025] Step 2.2: Calculate the CH4 concentration at the initial state using the following formula:

[0026] ; (3)

[0027] in, c 1a is the initial molar concentration of CH4, Z 1a is the compressibility factor of CH4 in the initial state, R is the universal gas constant, R = 8.314 J / (mol·K);

[0028] Step 2.3: Calculate the CH4 concentration after CO2 injection:

[0029] ; (4)

[0030] in, c 1b is the total molar concentration of CH4 after CO2 injection, S wa It is the water saturation before CO2 injection.

[0031] Furthermore, the step 3 is specifically as follows: after CO2 is injected, the gas in the gas reservoir is a mixed gas of CO2 and residual CH4, and the pressure of the mixed gas is calculated using a multi-component Peng-Robinson model that considers the interaction between components:

[0032] ; (5)

[0033] in,v is the specific volume of the mixed gas, v =1 / ( c 1b + c 2b );

[0034] α m is an empirical parameter to characterize the attraction between component molecules. ;

[0035] α i To characterize the i The empirical parameters of the gravitational force between the components of the molecule, , α j To characterize the j Empirical parameters of the gravitational forces between the components; i , j =1,2, i , j =1 represents CH4 component, i , j =2 represents the CO2 component;

[0036] T ci For the i The critical temperature of each component, p ci For the i The critical pressure of each component, T ri For the i Components vs. temperature;

[0037] m i is an empirical parameter, , oh i For the i The eccentricity factor of each component, oh 1=0.0104, oh 2=0.2667;

[0038] z i For the i The mole fraction of each component, z j For the j The mole fraction of each component, i , j =1,2,z1= c 1b / ( c 1b + c2b ), z2= c 2b / ( c 1b + c 2b );

[0039] b m is an empirical parameter to characterize the molecular exclusion volume, ; b i To characterize the i Empirical parameters of component molecular exclusion volumes, b i =0.0778 RT ci / p ci .

[0040] Furthermore, in the multi-component Peng-Robinson model, the known condition is the reservoir temperature T ,pressure p b 、CH4 concentration c 1b , the unknown quantity is CO2 concentration c 2b , the Newton-Raphson iteration method is used to numerically solve the CO2 molar concentration c 2b , the specific method is:

[0041] Step 3.1: In the Newton iteration, first assume a CO2 concentration value , into the following formula to calculate the dimensionless objective function :

[0042] ; (6)

[0043] Step 3.2: If Less than or equal to 10 -6 , then stop the calculation and ;like Greater than 10 -6 , then use formula (7) to update ;

[0044] ; (7)

[0045] Among them, the derivative of the objective function is calculated using the numerical difference method :

[0046] ; (8)

[0047] Step 3.3: Exploit the updated , use formula (6) again to calculate the objective function ;

[0048] Step 3.4: Repeat steps 3.2 and 3.3 until the objective function g Less than 10 -6 The iteration ends when the concentration value after iteration is the CO2 concentration after CO2 injection. c 2b .

[0049] Furthermore, the step 4 includes the following sub-steps:

[0050] Step 4.1: Calculate the partial pressure of CO2 after CO2 injection p 2b , the calculation formula is:

[0051] ; (9)

[0052] Step 4.2: Calculate the solubility of CO2 in water according to Henry's law c 2w :

[0053] ; (10)

[0054] in, c 2w is the solubility of CO2 in water; H is the Henry constant for CO2.

[0055] Furthermore, the step 5 includes the following sub-steps:

[0056] Step 5.1: Calculate water saturation after CO2 injection S wb :

[0057] ; (11)

[0058] in, S wa is the water saturation before CO2 injection;

[0059] Step 5.2: Calculate the total amount of CO2 stored after CO2 injection according to the following formula:

[0060] ; (12)

[0061] in, M g2 is the molecular mass of CO2, Mg2 =44 g / mol; V is the total volume of the gas reservoir, m co2 is the total mass of final CO2 storage.

[0062] Beneficial technical effects brought by the present invention:

[0063] The method for calculating the CO2 storage capacity of gas reservoirs that considers the interaction between pore expansion and components proposed in the present invention can significantly improve the calculation accuracy of the CO2 storage potential of depleted gas reservoirs. First, the method comprehensively considers the pore expansion effect caused by CO2 injection, and establishes a pore expansion model by introducing the porosity pressure sensitivity coefficient, which effectively characterizes the pore expansion effect caused by the increase in fluid pressure, thereby more accurately evaluating the CO2 storage potential of the gas reservoir. Secondly, the method adopts the Peng-Robinson equation of state, systematically considers the thermodynamic interaction between CO2 and residual CH4, and accurately characterizes the changes in the properties of the mixed gas. The present invention makes up for the deficiency of the existing method that simplifies the CO2-CH4 system into a single-component gas when calculating gas properties, avoids the calculation error of the CO2 structural storage capacity caused by the simplification, and at the same time, because the calculation of the CO2 partial pressure is more accurate, the calculation error of the dissolved CO2 storage capacity is reduced.

[0064] In addition, the present invention adopts rigorous mathematical iteration methods (such as Newton-Raphson iteration) to ensure the stability of the calculation process and the reliability of the results. In the calculation of the final CO2 storage volume, the gas phase concentration and solubility are fully combined to further improve the scientificity and accuracy of the results. Through the above innovations, the present invention provides more sophisticated theoretical support and calculation tools for CO2 geological storage technology, which can be widely used in key links such as storage scale assessment, injection volume optimization and safety analysis, significantly promoting the engineering development of carbon dioxide capture and storage technology, and providing important technical support for mitigating global climate change. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 This is a flow chart of the method for calculating the storage capacity of carbon dioxide in gas reservoirs taking into account the effects of pore expansion and components in the present invention.

[0066] Figure 2 This is a Newton-Raphson iteration flow chart for solving the CO2 concentration after storage in the present invention. DETAILED DESCRIPTION

[0067] The specific implementation of the present invention is further described below in conjunction with specific embodiments:

[0068] The calculation method of carbon dioxide storage capacity in gas reservoirs considering pore expansion and component effects is as follows: Figure 1 As shown, the following steps are included:

[0069] Step 1: Calculate the porosity after CO2 injection into the gas reservoir;

[0070] Step 2: Calculate the CH4 concentration before and after CO2 injection into the gas reservoir;

[0071] Step 3: Calculate the CO2 concentration after CO2 is injected into the gas reservoir using Newton-Raphson iteration;

[0072] Step 4: Calculate the solubility of CO2 in the water phase;

[0073] Step 5: Calculate the total amount of CO2 stored.

[0074] Specifically, step 1 is as follows: After CO2 is injected into the gas reservoir, the gas pressure increases, and the pores expand due to the pressure increase. Therefore, the porosity after CO2 injection will be larger than the initial state. The porosity of the gas reservoir under different pressures is calculated:

[0075] (1);

[0076] in, f b is the porosity after CO2 injection, dimensionless, f a is the porosity before CO2 injection, exp is the exponential function, c is the porosity pressure sensitivity coefficient, Pa -1 , p b is the reservoir pressure expected to be reached after CO2 injection, Pa; p a is the initial reservoir pressure before CO2 injection, Pa.

[0077] Specifically, step 2 is as follows: in the initial state, i.e. before CO2 injection, the gas reservoir contains only CH4, and the single-component Peng-Robinson state equation is used to calculate p a Reservoir temperature T The pure CH4 concentration under the above conditions specifically includes the following sub-steps:

[0078] Step 2.1: Solve the following about Z Cubic equation to obtain the deviation factor of CH4 in the initial state Z 1a :

[0079] ; (2)

[0080] in, ; ;

[0081] Tr To compare the temperature, T r = T / T c1 , T is the reservoir temperature, k; T c1 is the critical temperature, T c1 =190.56; p ra To compare pressure, p ra = p 0 / p c1 , p 0 is the reservoir pressure, p c1 is the critical pressure, p c1 =4.6MPa; d A and d B is the empirical parameter of the state equation, d A =1+√2, d B =1-√2; if the cubic equation has three real roots, then Z 1a is equal to the largest real root. If there is only one real root, then Z 1a is equal to the only real root;

[0082] Step 2.2: Calculate the CH4 concentration at the initial state using the following formula:

[0083] ; (3)

[0084] in, c 1a is the initial molar concentration of CH4, mol / m 3 ; Z 1a is the compression factor of CH4 in the initial state, dimensionless; R is the universal gas constant, R=8.314J / (mol·K).

[0085] Step 2.3: After CO2 injection, the pores expand, resulting in an increase in the total pore volume. The number of moles of CH4 remains unchanged and the volume of water remains unchanged (the compressibility of water is negligible), so the total molar concentration of CH4 will decrease. Since the concentration is inversely proportional to the volume, the CH4 concentration after CO2 injection is:

[0086] ; (4)

[0087] in, c 1b is the total molar concentration of CH4 after CO2 injection, mol / m 3 ; S wa It is the water saturation before CO2 injection, dimensionless.

[0088] Specifically, step 3 is as follows: after CO2 is injected, the gas in the gas reservoir is a mixture of CO2 and residual CH4. The thermodynamic interaction strength of the two components is different under different CO2-CH4 concentrations, and the pressure of the mixed gas is also different. The mixed gas cannot be used as a single-component gas to calculate gas parameters. The multi-component Peng-Robinson model considering component interactions is used to calculate the properties and pressure of the mixed gas:

[0089] ; (5)

[0090] in, p b is the reservoir pressure after CO2 injection, v is the specific volume of the mixed gas, v =1 / ( c 1b + c 2b );

[0091] α m is an empirical parameter to characterize the attraction between component molecules. ;

[0092] α i To characterize the i The empirical parameters of the gravitational force between the components of the molecule, , α j To characterize the j Empirical parameters of the gravitational forces between the components; i , j =1,2, i , j =1 represents CH4 component, i , j =2 represents the CO2 component;

[0093] T ci For the i The critical temperature of each component, p ci For the i The critical pressure of each component, Tri For the i Components vs. temperature;

[0094] m i is an empirical parameter, , oh i For the i The eccentricity factor of each component, oh 1=0.0104, oh 2=0.2667;

[0095] z i For the i The mole fraction of each component, z j For the j The mole fraction of each component, i , j =1,2,z1= c 1b / ( c 1b + c 2b ), z2= c 2b / ( c 1b + c 2b );

[0096] b m is an empirical parameter to characterize the molecular exclusion volume, ; b i To characterize the i Empirical parameters of component molecular exclusion volumes, b i =0.0778 RT ci / p ci .

[0097] In the multi-component Peng-Robinson model, the known condition is the reservoir temperature T ,pressure p b 、CH4 concentration c 1b , the unknown quantity is CO2 concentration c 2b , the Newton-Raphson iteration method is used to numerically solve the CO2 molar concentration c 2b ,like Figure 2 The specific method is as follows:

[0098] Step 3.1: In the Newton iteration, first assume a CO2 concentration value , into the following formula to calculate the dimensionless objective function :

[0099] (6);

[0100] Step 3.2: If Less than or equal to 10 -6 , then stop the calculation and ;like Greater than 10 -6 , then use formula (7) to update ;

[0101] (7);

[0102] Among them, the derivative of the objective function is calculated using the numerical difference method :

[0103] (8).

[0104] Step 3.3: Exploit the updated , use formula (6) again to calculate the objective function .

[0105] Step 3.4: Repeat steps 3.2 and 3.3 until the objective function g Less than 10 -6 The iteration ends when the concentration value after iteration is the CO2 concentration after CO2 injection. c 2b .

[0106] Specifically, step 4 includes the following sub-steps:

[0107] Step 4.1: Calculate the partial pressure of CO2 after CO2 injection p 2b , the calculation formula is:

[0108] (9).

[0109] Step 4.2: Calculate the solubility of CO2 in water according to Henry's law c 2w :

[0110] ; (10)

[0111] in, c 2w is the solubility of CO2 in water, mol / m 3; H is the Henry constant for CO2, mol / (Pa·m 3 ).

[0112] Specifically, step 5 includes the following sub-steps:

[0113] Step 5.1: Calculate water saturation after CO2 injection S wb :

[0114] (11);

[0115] in, S wa It is the water saturation before CO2 injection, dimensionless.

[0116] Step 5.2: Calculate the total amount of CO2 stored after CO2 injection according to the following formula:

[0117] (12);

[0118] in, M g2 is the molecular mass of CO2, M g2 =44 g / mol; V is the total volume of the gas reservoir, m 3 ; m co2 is the total mass of final CO2 storage, t.

[0119] Example 1, select a target depleted gas reservoir where CO2 storage is required, and obtain relevant reservoir parameters of the target gas reservoir, including the initial pressure, initial porosity, initial water saturation, total reservoir volume V, reservoir temperature, pressure sensitivity coefficient of porosity, Henry's constant of CO2 in water at reservoir temperature, and the reservoir pressure expected to be reached at the end of CO2 storage.

[0120] In this example, the initial pressure of the target gas reservoir is 3 MPa, the initial porosity is 0.15, the initial water saturation is 0.3, the temperature is 343 K (i.e., 60 °C), and the total volume of the target reservoir is 8 × 10 7 , porosity stress sensitivity coefficient 2×10 -8 Pa -1 , Henry constant is 0.00076mol / (Pa·m 3 ). The expected reservoir pressure reached by the end of CO2 is 12MPa.

[0121] Step 1: Calculate the porosity after CO2 injection into the gas reservoir;

[0122] After CO2 storage is completed, the reservoir pressure rises from 3MPa to 12MPa. f a =0.15, p a =3·10 6 Pa, p b =12·10 6 Pa, γ=2; 10 -8 Pa -1 According to formula (1), we can calculate f b =0.1796. It can be seen that the reservoir porosity increased significantly after CO2 injection.

[0123] Step 2: Calculate the CH4 concentration before and after CO2 injection into the gas reservoir;

[0124] In the initial state (i.e. before CO2 injection), there is only CH4 in the reservoir, which can be directly calculated using the Peng-Robinson state equation p a =3MPa and T = Pure CH4 concentration at 60℃, the following are the detailed steps:

[0125] Step 2.1: Solve for Z The cubic equation (2) only has one real root ( Z =0.9577), so the deviation factor of CH4 in the initial state is Z 1a =0.9575;

[0126] Step 2.2: Calculate the initial CH4 concentration using formula (3) p a =3·10 6 Pa, Z 1a =0.9575, T =333K can be obtained by substituting c 1a =1131.5mol / m 3 ;

[0127] Step 2.3: Calculate the CH4 concentration after CO2 injection according to formula (4). Substitute c 1a =1131.5mol / m 3 , f a =0.15, f b =0.1796, Swa =0.3, calculated c 1b =882.898mol / m 3 .

[0128] Step 3: Use the Newton-Raphson iteration method to numerically solve the CO2 concentration. After Newton iteration, the CO2 concentration after burial is 6702.4 mol / m 3 .

[0129] Step 4: Calculate the amount of dissolved CO2 stored;

[0130] Step 4.1: c 1b , c 2b and p b Substitute the value into the above formula (9) to obtain the partial pressure of CO2 p 2b 10.6032MPa;

[0131] Step 4.2: Calculate the solubility of CO2 in water according to Henry's law c 2w , the Henry constant H= 0.00076mol / (Pa·m 3 ) into formula (10), we can get c 2w =8058.4mol / m 3 .

[0132] Step 5: Calculate the total amount of CO2 stored;

[0133] Step 5.1: Change the water saturation before injection S wa =0.3, Porosity before injection f a =0.15, Porosity after injection f b =0.1796 Substituting into formula (11) we can get the water saturation after injection S wb =0.2506;

[0134] Step 5.2: According to formula (12), the total amount of CO2 stored after CO2 injection is calculated to be 4.4520×10 6 t.

[0135] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for calculating the storage capacity of carbon dioxide in gas reservoirs taking into account the effects of pore expansion and components, characterized in that: The following steps are involved: Step 1: Calculate the porosity after CO2 injection into the gas reservoir; Step 2: Calculate the CH4 concentration before and after CO2 injection into the gas reservoir; Step 3: Calculate the CO2 concentration after CO2 is injected into the gas reservoir using Newton-Raphson iteration; Step 4: Calculate the solubility of CO2 in the water phase; Step 5: Calculate the total amount of CO2 stored; Step 1 is as follows: After CO2 is injected into the gas reservoir, the gas pressure increases. The pores will expand due to the increased pressure. Therefore, the porosity after CO2 injection will be larger than the initial state. The porosity of the gas reservoir under different pressures is calculated: ； (1) in, φ b is the porosity after CO2 injection, φ a is the porosity before CO2 injection, exp is the exponential function, γ is the porosity pressure sensitivity coefficient, p b is the expected reservoir pressure after CO2 injection, p a is the initial reservoir pressure before CO2 injection; The step 3 is specifically as follows: after CO2 is injected, the gas in the gas reservoir is a mixed gas of CO2 and residual CH4, and the pressure of the mixed gas is calculated using a multi-component Peng-Robinson model that considers the interaction between components: ；(5) in, v is the specific volume of the mixed gas, v =1 / ( c 1b + c 2b ); α m is an empirical parameter to characterize the attraction between component molecules. ; α i To characterize the i The empirical parameters of the gravitational force between the components, , α j To characterize the j Empirical parameters of the gravitational forces between the components; i , j =1,2, i , j =1 represents CH4 component, i , j =2 represents the CO2 component; T ci For the i The critical temperature of each component, p ci For the i The critical pressure of each component, T ri For the i Components vs. temperature; m i is an empirical parameter, , ω i For the i The eccentricity factor of each component, ω 1=0.0104, ω 2=0.2667; z i For the i The mole fraction of each component, z j For the j The mole fraction of each component, i , j =1,2,z1= c 1b / ( c 1b + c 2b ), z2= c 2b / ( c 1b + c 2b ); b m is an empirical parameter to characterize the molecular exclusion volume, ; b i To characterize the i Empirical parameters of component molecular exclusion volumes, b i =0.0778 RT ci / p ci ; In the multi-component Peng-Robinson model, the known conditions are the reservoir temperature T ,pressure p b 、CH4 concentration c 1b , the unknown quantity is CO2 concentration c 2b , the Newton-Raphson iteration method is used to numerically solve the CO2 molar concentration c 2b , the specific method is: Step 3.1: In the Newton iteration, first assume a CO2 concentration value , into the following formula to calculate the dimensionless objective function : ；(6) Step 3.2: If Less than or equal to , then stop the calculation and ;like Greater than 10 -6 , then use formula (7) to update ; ；(7) Among them, the derivative of the objective function is calculated using the numerical difference method : ；(8) Step 3.3: Exploit the updated , use formula (6) again to calculate the objective function ; Step 3.4: Repeat steps 3.2 and 3.3 until the objective function g Less than 10 -6 The iteration ends when the concentration value after iteration is the CO2 concentration after CO2 injection. c 2b .

2. The method for calculating the storage capacity of carbon dioxide in gas reservoirs considering pore expansion and component effects according to claim 1, characterized in that: The specific step 2 is as follows: in the initial state, that is, before CO2 injection, the gas reservoir contains only CH4, and the single-component Peng-Robinson state equation is used to calculate p a Reservoir temperature T The pure CH4 concentration under the above conditions specifically includes the following sub-steps: Step 2.1: Solve the following about Z Cubic equation to obtain the deviation factor of CH4 in the initial state Z 1a : ；(2) in, ; ; T r To compare the temperature, T r = T / T c1 , T is the reservoir temperature, T c1 is the critical temperature, T c1 =190.56; p ra To compare pressure, p ra = p 0 / p c1 , p 0 is the reservoir pressure, p c1 is the critical pressure, p c1 =4.6MPa; d A and d B is the empirical parameter of the state equation, d A =1+√2, d B =1-√2; If a cubic equation has three real roots, then Z 1a is equal to the largest real root. If there is only one real root, then Z 1a is equal to the only real root; Step 2.2: Calculate the CH4 concentration at the initial state using the following formula: ；(3) in, c 1a is the initial molar concentration of CH4, Z 1a is the compressibility factor of CH4 in the initial state, R is the universal gas constant, R = 8.314 J / (mol·K); Step 2.3: Calculate the CH4 concentration after CO2 injection: ；(4) in, c 1b is the total molar concentration of CH4 after CO2 injection, S wa It is the water saturation before CO2 injection.

3. The method for calculating the storage capacity of carbon dioxide in gas reservoirs considering pore expansion and component effects according to claim 2, characterized in that: The step 4 includes the following sub-steps: Step 4.1: Calculate the partial pressure of CO2 after CO2 injection p 2b , the calculation formula is: ； (9) Step 4.2: Calculate the solubility of CO2 in water according to Henry's law c 2w : ；(10) in, c 2w is the solubility of CO2 in water; H is the Henry constant for CO2.

4. The method for calculating the storage capacity of carbon dioxide in gas reservoirs considering pore expansion and component effects according to claim 3, characterized in that: The step 5 includes the following sub-steps: Step 5.1: Calculate water saturation after CO2 injection S wb : ； (11) in, S wa is the water saturation before CO2 injection; Step 5.2: Calculate the total amount of CO2 stored after CO2 injection according to the following formula: ；(12) in, M g2 is the molecular mass of CO2, M g2 =44 g / mol; V is the total volume of the gas reservoir, m co2 is the total mass of final CO2 storage.

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