A method for calculating the effective distance of injected gas in oil and gas reservoirs based on diffusion and seepage theory

Abstract

Aiming at the problem that the effective distance of injected gas is difficult to accurately predict during the development of oil and gas reservoirs, the present invention proposes a method for calculating the effective effective distance of injected gas in oil and gas reservoirs based on diffusion and seepage theory, which relates to the field of oil and gas development. The present invention combines Darcy's law with Fick's diffusion law to establish a comprehensive model of seepage and diffusion, comprehensively considers physical parameters such as the porosity, permeability, viscosity of the reservoir, and injection conditions such as gas injection pressure, gas injection time, and gas injection rate, and accurately calculates the effective range of the injected gas; the method has wide applicability, clear expressions, strong calculation operability, easy use, more accurate calculation results, avoids the complicated workload of human experience judgment and numerical simulation, and can provide theoretical support for research on optimizing the layout of gas injection wells, reducing ineffective gas injection and gas channeling, and improving recovery rate, thereby further improving the efficiency and economic benefits of oil and gas field development.

Description

Technical Field

[0001] The invention belongs to the field of oil and natural gas development, and in particular relates to a method for calculating the effective action distance of injected gas in oil and gas reservoirs based on diffusion and seepage theory. Background Art

[0002] Gas injection production increases oil and gas recovery or maintains formation pressure by injecting gas (such as carbon dioxide, nitrogen, natural gas, etc.) into oil and gas reservoirs, enhances oil and gas mobility, pushes crude oil or natural gas to production wells, reduces residual oil and gas, and significantly improves production efficiency. In this process, the distance from the injection point to the displacement front is the effective distance of the injected gas, which is a key indicator for evaluating the effect of gas injection, predicting production, and optimizing gas injection strategies.

[0003] At present, the methods for determining the effective action distance of injected gas in oil and gas reservoirs mainly include theoretical calculation method, reservoir numerical simulation method and experimental test method. In terms of theoretical calculation method, Zhan Jie et al., in "A method for predicting the dynamic changes of the gas front edge of carbon dioxide drive" (CN110863806B), based on the gas injection volume in different oil well directions, used a fan blade model to calculate the dynamic changes of the gas drive front position under different carbon dioxide injection volumes; this method relies on experience to judge the effectiveness of oil wells and the time of effectiveness, and has low accuracy. In terms of reservoir numerical simulation, Li Zhaomin et al. proposed in “A method for determining the limiting action radius of gas drive throughput in low-permeability closed sand bodies” (CN106894808A) that the starting pressure gradient is first measured, and then the data is integrated through the reservoir numerical simulation software CMG. The nitrogen throughput range is determined according to the position where the formation pressure gradient is greater than the starting pressure gradient, and the position where the formation pressure gradient is equal to the starting pressure gradient is determined as the limiting action radius; this method requires the establishment of a conceptual model of the target block and the production history fitting based on reservoir reserves, reservoir thickness, permeability, inclination, and oil well characteristics. This method is cumbersome to operate and has a large workload. In addition, Wang Zhaofeng et al. studied the effective influence radius of gas injection to displace coalbed methane, and used COMSOL numerical simulation software to obtain the relationship between the effective influence radius R and the gas injection time t, and derived an empirical formula for the effective influence radius of gas injection through regression analysis (Wang Zhaofeng, Chen Jinchao, Yang Hongmin. Study on the effective influence radius of gas injection to displace coalbed methane [J]. Coal Science and Technology, 2012, 40(09): 28-31.); However, the number of samples in this method is small and it is only applicable to the case of underground gas injection to displace coalbed methane. In terms of experimental testing methods, Liu Huang et al. proposed a method for determining the dissolution and diffusion distance of injected gas in crude oil during gas injection and oil production using nuclear magnetic resonance technology in "A test device and method for determining the diffusion distance of injected gas in the process of gas injection and oil production" (CN111239176A); this method determines the diffusion position of injected gas in crude oil by the change of T2 spectrum signal over time, and the actual reservoir core can be selected for experiment, but due to the large difference between the core scale and the reservoir scale, the experimental results are difficult to fully reflect the actual situation.

[0004] In summary, the existing theoretical calculation methods of the effective action distance of injected gas generally have the limitations of limited application scope and reliance on experience; the numerical simulation method has a large workload and is not convenient for practical engineering applications; the experimental method is difficult to overcome the problems caused by the scale difference between the physical model and the actual reservoir model. Therefore, it is urgent to establish a calculation method for the effective action distance of injected gas that is simple in calculation, has strong scalability, and is based on the action distance mechanism, so as to provide theoretical support and practical guidance for field production. Summary of the invention

[0005] In view of the problem that it is difficult to accurately predict the effective distance of injected gas during the development of oil and gas reservoirs, the present invention proposes a method for calculating the effective effective distance of injected gas in oil and gas reservoirs based on the seepage and diffusion theory. The present invention combines Darcy's law and Fick's diffusion law to establish a comprehensive model of seepage and diffusion, and uses the physical parameters of the reservoir such as porosity, permeability, viscosity, and injection conditions such as injection pressure and rate to accurately calculate the effective range of injected gas. Based on the content of the present invention, it is possible to optimize gas injection parameters (such as fluid properties, injection pressure difference, injection time, etc.), gas injection well network, judge gas channeling and its impact on production wells, and provide theoretical support for the design of gas injection schemes and the economic benefit evaluation of gas injection production.

[0006] In order to achieve the above technical objectives, the present invention adopts the following technical means.

[0007] A method for calculating the effective action distance of injected gas in oil and gas reservoirs based on diffusion and seepage theory comprises the following steps in sequence:

[0008] Step S1. Assume that the reservoir is a homogeneous isotropic porous medium, there is a perfect well in the circular stratum of equal thickness, and the seepage of the injected gas in the reservoir obeys the linear seepage law and presents a planar radial stable seepage;

[0009] Step S2. Obtain reservoir parameters, production parameters and fluid property parameters of the oil and gas reservoir, such as porosity φ, formation permeability K, gas phase relative permeability K rg , injected gas viscosity μ, injected gas diffusion coefficient D, injected gas pressure difference Δp and injected gas time t;

[0010] Step S3. In actual oil and gas reservoirs, gas flow and diffusion usually occur simultaneously and need to be considered together. The total gas flux can be expressed as the superposition of flow and diffusion:

[0011] J 总 =J 流动 +J 扩散 (1)

[0012] Step S4. Gas flow refers to the overall movement of gas in the oil and gas reservoir due to pressure difference or injection pressure, and gas diffusion refers to the spontaneous mixing movement of gas molecules due to concentration gradient. The gas flow flux and gas diffusion flux are superimposed to obtain:

[0013]

[0014] Step S5. Considering the overall movement of gas in the oil and gas reservoir due to pressure difference or injection pressure, the seepage effect of injected gas is described by Darcy's law, and the action distance r of injected gas seepage in the formation is derived. 流动 (t):

[0015]

[0016] Where: r 流动 is the fluid flow radius, m; K is the formation permeability, mD; K rg is the relative permeability of the gas phase, Δp is the injection pressure difference, MPa; μ is the viscosity of the fluid, mPa·s; φ is the porosity of the porous medium; t is the injection time, s.

[0017] Step S6. Considering that gas molecules diffuse due to concentration gradient, gas diffusion follows Fick's law, and according to the statistical equipartition theorem, the Gaussian solution of gas diffusion is obtained, and the analytical solution r of the injected gas diffusion distance is solved. 扩散 (t):

[0018]

[0019] Where: r 扩散 is the fluid flow radius, m; D is the diffusion coefficient, m 2 / s; t is the gas injection time, s.

[0020] Step S7. Superimpose the contributions of flow and diffusion to obtain the total gas action radius:

[0021]

[0022] Step S8: Determine the effective action distance of the injected gas in the reservoir under different production conditions.

[0023] Furthermore, the process of step S4 is as follows:

[0024] Step S41. When Darcy's law describes the gas flow in a porous medium, the flow flux and the flow velocity are in the same form, which is:

[0025]

[0026] Where: J 流动 is the flow flux, m / s; K is the formation permeability, mD; K rg is the relative permeability of the gas phase; Δp is the pressure difference, MPa; μ is the viscosity of the fluid, mPa·s; ΔL is the length of the seepage path, m.

[0027] Step S42. The diffusion flux of the gas along the flow direction can be expressed by Fick's first law. The diffusion flux of the gas per unit time and per unit area caused only by the diffusion phenomenon can be expressed as:

[0028]

[0029] Where: J 扩散is the diffusion flux, m / s; D is the diffusion coefficient, m 2 / s; C is the mole fraction concentration; x is the distance or spatial coordinate, m.

[0030] Step S43. Substitute formula (6) and formula (7) into formula (1) to obtain the total gas flux formula (2).

[0031]

[0032] Furthermore, the process of step S5 is as follows:

[0033] Step S51. According to the seepage characteristics of the injected gas in the porous medium, the relationship between the seepage velocity and the radius in the case of radial flow is:

[0034]

[0035] Where: v r is the injected gas seepage velocity, m / s; Δp is the injection pressure difference, MPa; r is the fluid flow radius, m.

[0036] Step S52. In order to make the model more consistent with the actual oil and gas reservoir conditions and empirical data, the present invention introduces a correction constant A. Although the gas front velocity is similar to the Darcy seepage velocity in form, in practical applications, the velocity directly calculated based on Darcy's law cannot accurately describe the propagation behavior of the gas front in a complex oil and gas reservoir environment. After introducing the correction coefficient, the Darcy velocity can be expressed as:

[0037]

[0038] Step S53. By introducing the correction constant A, the deviation caused by these complexities can be effectively compensated, so that the model is closer to the flow characteristics of the actual oil and gas reservoir, and the accuracy and applicability of the model in practical applications are improved. When the fluid passes through the porous medium, the porosity has a significant impact on the fluid's seepage path and time scale. In order to quantify the impact of porosity on the fluid, introduce:

[0039]

[0040] Where: A is the introduced constant; φ is the porosity of the porous medium.

[0041] Step S54. Assuming that the position of the gas front changes with time, the propagation velocity of the gas front can be related to the Darcy seepage velocity:

[0042]

[0043] in: is the propagation velocity of the gas front, m / s; t is the gas injection time, s.

[0044] Step S55. According to the propagation speed of the gas front, r and t are separated into variables to obtain the formula:

[0045]

[0046] Step S56. When the gas injection time is T0=0, the seepage radius R0=0, and when the gas injection time is T=t, the seepage radius R=r. Integrate the right side of equation (12) over the gas injection time interval (0, t), and integrate the left side over the seepage radius interval (0, r), and obtain equation (13):

[0047]

[0048] Step S57. After integral calculation, the relationship between the seepage radius is obtained as follows:

[0049]

[0050] Step S58. The formula for obtaining the effective distance of the injected gas seepage is:

[0051]

[0052] Furthermore, the process of step S6 is as follows:

[0053] Step S61. The gas diffusion process usually follows Fick's first law, and its diffusion rate is determined by the concentration gradient and diffusion coefficient of the gas molecules. Fick's first law is formula (7):

[0054]

[0055] Step S62: Based on Fick's first law, and considering the one-dimensional passive diffusion process, Fick's second law can be expressed as:

[0056]

[0057] Where: t is time, s; r is the distance in space, m.

[0058] Step S63. The classical Gaussian distribution solution of formula (15) is:

[0059]

[0060] in: is the normalization factor; is the variation of concentration with position and time; π is a constant of approximately 3.1416.

[0061] Step S64. Gaussian distribution describes the distribution of gas concentration in space r, with a mean value a=0, indicating that the concentration is maximum at r=0, which is the concentration point of the gas at the initial moment. The standard deviation during the diffusion process is:

[0062]

[0063] Step S65. The shape of the Gaussian distribution becomes wider and wider over time, which means that the gas concentration is distributed more and more widely in space. The gas diffusion distance is usually defined as the position where the gas concentration drops significantly to a certain level. Due to the symmetry of the Gaussian distribution, the gas diffusion distance can be estimated by the standard deviation. In the Gaussian distribution, the concentration changes with r and drops most significantly at the position where r is σ(t). Therefore, the standard deviation σ(t) can be regarded as the gas diffusion distance. The diffusion distance of the injected gas is:

[0064]

[0065] Furthermore, the process of step S7 is as follows:

[0066] Step S71. Superimpose the contribution of flow and diffusion to obtain the total gas action radius:

[0067] r 作用 (t) = r 流动 (t)+r 扩散 (t) (18)

[0068] Step S72: Substituting formula (3) and formula (4) into formula (18) can obtain the total gas action radius:

[0069]

[0070] Compared with the prior art, the present invention has the following beneficial effects:

[0071] The present invention proposes an innovative method for accurately calculating the distance of gas injection into oil and gas reservoirs by combining the seepage and diffusion theories. The main technical means is to establish a seepage model based on Darcy's law, and to construct a diffusion model in combination with Fick's law, and to calculate the reservoir physical parameters (such as porosity, permeability, viscosity, etc.) and gas injection conditions (gas injection pressure, gas injection time, etc.), so as to simultaneously consider the gas flow and diffusion process, and accurately predict the range of gas action in the reservoir. Compared with the prior art, the present invention couples seepage and diffusion to improve the prediction accuracy of the gas action range. Compared with the existing single seepage or diffusion model, empirical formula, numerical simulation and other models, the present method has wide applicability, clear expressions, strong computational operability, easy use, and more accurate calculation results. It avoids the complicated workload of artificial empirical judgment and numerical simulation. The present invention can provide theoretical support for optimizing the layout of gas injection wells, reducing ineffective gas injection and gas channeling, and improving the recovery rate, thereby further improving the efficiency and economic benefits of oil and gas field development. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 This is a flow chart of a method for calculating the effective action distance of injected gas in oil and gas reservoirs based on diffusion and seepage theory;

[0073] Figure 2 This is the seepage effect distance diagram of CO2 injected into the reservoir under different gas injection time and gas injection pressure difference;

[0074] Figure 3 The diffusion distance diagram of injected CO2 in the reservoir under different gas injection time and gas injection pressure difference;

[0075] Figure 4 This is the effective distance diagram of CO2 injected into the reservoir under different gas injection time and gas injection pressure difference;

[0076] Figure 5 The effective distance of injected gas under different reservoir physical properties;

[0077] Figure 6 It is the effective action distance of injected gas under different injected gas properties. DETAILED DESCRIPTION

[0078] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0079] A method for calculating the effective distance of injected gas in oil and gas reservoirs based on diffusion and seepage theory. Figure 1 , including the following steps in sequence:

[0080] Step S1. Assume that the reservoir is a homogeneous isotropic porous medium, there is a perfect well in the circular stratum of equal thickness, and the seepage of the injected gas in the reservoir obeys the linear seepage law and presents a planar radial stable seepage;

[0081] Step S2. Taking a certain tight oil reservoir as an example, the reservoir parameters, production parameters and fluid property parameters of the tight oil reservoir are obtained (the values ​​are shown in Table 1):

[0082] Table 1 Static parameters of a tight oil reservoir and properties of injected CO2 fluid

[0083] parameter Value Formation pressure P(MPa) 8 Average reservoir porosity φ 0.1 Permeability K(mD) 0.3 Formation temperature T(℃) 90 <![CDATA[Gas-phase relative permeability K rg > 0.26 <![CDATA[CO2 viscosity μ (mPa·s)]]> 0.0219 <![CDATA[CO2 diffusion coefficient D (10 -7 m 2 / s)]]> 1.618

[0084] Step S3. In actual oil and gas reservoirs, gas flow and diffusion usually occur simultaneously and need to be considered together; the total gas flux can be expressed as the superposition of flow and diffusion:

[0085] J 总 =J 流动 +J 扩散 (1)

[0086] Step S4. Gas flow refers to the overall movement of gas in the oil and gas reservoir due to pressure difference or injection pressure, and gas diffusion refers to the spontaneous mixing movement of gas molecules due to concentration gradient. The gas flow flux and gas diffusion flux are superimposed:

[0087] Step S41. When Darcy's law describes the gas flow in a porous medium, the flow flux and the flow velocity are in the same form, which is:

[0088]

[0089] Step S42. The diffusion flux of the gas along the flow direction can be expressed by Fick's first law. The diffusion flux of the gas per unit time and per unit area caused only by the diffusion phenomenon can be expressed as:

[0090]

[0091] Step S43. Substitute formula (6) and formula (7) into formula (1) to obtain the total gas flux formula (2).

[0092]

[0093] Step S5. Considering the overall movement of gas in the oil and gas reservoir due to pressure difference or injection pressure, the seepage effect of injected gas is described by Darcy's law, and the action distance r of injected gas seepage in the formation is derived. 流动 (t):

[0094] Step S51. The seepage characteristics of the injected gas in the porous medium, in the case of radial flow, the relationship between the seepage velocity and the radius is:

[0095]

[0096] Step S52. In order to make the model more consistent with actual oil and gas reservoir conditions and empirical data and accurately reflect the influence of porosity in the mathematical model, the present invention introduces a correction constant A:

[0097]

[0098] Step S53. By introducing the correction constant A, the deviation caused by these complexities can be effectively compensated, so that the model is closer to the flow characteristics of the actual oil and gas reservoir, and the accuracy and applicability of the model in practical applications are improved. When the fluid passes through the porous medium, the porosity has a significant impact on the fluid's seepage path and time scale. In order to quantify the impact of porosity on the fluid, introduce:

[0099]

[0100] Step S54. Assuming that the position of the gas front changes with time, the propagation velocity of the gas front can be related to the Darcy seepage velocity:

[0101]

[0102] Step S55. According to the propagation speed of the gas front, r and t are separated into variables to obtain the formula:

[0103]

[0104] Step S56. When the gas injection time is T0=0, the seepage radius R0=0, and when the gas injection time is T=t, the seepage radius R=r. Integrate the right side of equation (12) over the gas injection time interval (0, t), and integrate the left side over the seepage radius interval (0, r), and obtain equation (13):

[0105]

[0106] Step S57. After integral calculation, the relationship between the seepage radius is obtained as follows:

[0107]

[0108] Step S58. Separate the variables to obtain the effective distance of the injected gas seepage:

[0109]

[0110] Step S59. Under the injection conditions of 0.1 MPa injection pressure difference and 30 d (2592000 s) injection time, the reservoir parameters are shown in Table 1, the reservoir permeability K is 0.3 mD, the gas phase relative permeability K at residual oil saturation is rg is 0.26, the reservoir porosity φ is 0.1, and the injected CO2 viscosity μ is 0.0219 mPa·s. At this time, the effective distance of gas seepage is:

[0111]

[0112] like Figure 2 As shown, the gas flow action distances when the injection times are 30d (2592000s), 60d (5184000s), 90d (7776000s), 120d (10368000s) and 180d (15552000s) under the injection pressure differences of 0.1MPa, 1MPa, 5MPa and 10MPa are calculated.

[0113] Step S6. Considering that gas molecules diffuse due to concentration gradient, gas diffusion follows Fick's law, and according to the statistical equipartition theorem, the Gaussian solution of gas diffusion is obtained, and the analytical solution r of the injected gas diffusion distance is solved. 扩散 (t):

[0114] Step S61. The gas diffusion process usually follows Fick's law, and its diffusion rate is determined by the concentration gradient and diffusion coefficient of the gas molecules. Fick's first law is formula (7):

[0115]

[0116] Step S62. Based on Fick's first law, and considering the one-dimensional passive diffusion process, Fick's second law can be expressed as:

[0117]

[0118] Step S63. The classical Gaussian distribution solution of formula (15) is:

[0119]

[0120] Step S64. Gaussian distribution describes the distribution of gas concentration in space r, with a mean value a=0, indicating that the concentration is maximum at r=0, which is the concentration point of the gas at the initial moment. The standard deviation during the diffusion process is:

[0121]

[0122] Step S65. The gas diffusion distance is usually defined as the position where the gas concentration drops significantly to a certain level. Due to the symmetry of the Gaussian distribution, the gas diffusion distance can be estimated by the standard deviation. In the Gaussian distribution, the concentration changes with r and drops most significantly at the position where r is σ(t). Therefore, the standard deviation σ(t) can be regarded as the gas diffusion distance. The diffusion distance of the injected gas is:

[0123]

[0124] Step S66. Under the injection condition of 30 days of injection time, the diffusion coefficient D of injected CO2 is 1.618×10 -7 m 2 / s, at this time the gas diffusion distance is:

[0125]

[0126] The calculation results of gas diffusion distance when the injection time is 30d (2592000s), 60d (5184000s), 90d (7776000s), 120d (10368000s) and 180d (15552000s) are shown in Table 2 and Figure 3 shown.

[0127] Table 2 Calculation results of diffusion distance of injected CO2 in reservoir at different gas injection times

[0128]

[0129] Step S7. Superimpose the contributions of flow and diffusion to obtain the total gas action radius:

[0130] Step S71. The effective action distance of the gas is the superposition of the contribution of flow and diffusion:

[0131] r 作用 (t) = r 流动 (t)+r 扩散 (t) (18)

[0132] Step S72: Substituting formula (3) and formula (4) into formula (18) can obtain the total gas action radius:

[0133]

[0134] Step S73. Under the injection conditions of 0.1 MPa gas injection pressure difference and 30 d (2592000 s) gas injection time, the effective gas action distance is:

[0135] r 作用 (t) = 4.269 + 0.916 = 5.185 (m)

[0136] like Figure 4 As shown, the effective action distance of gas when the injection time is 30d (2592000s), 60d (5184000s), 90d (7776000s), 120d (10368000s) and 180d (15552000s) under the injection pressure difference of 0.1MPa, 1MPa, 5MPa and 10MPa.

[0137] Step S8. Determine the effective action distance of the injected gas in the reservoir under different production conditions. When the reservoir properties, injected gas properties, injection pressure difference, and injection time are different, the effective action distance of the injected gas needs to be recalculated according to S1 to S7.

[0138] Step S81. The results of the effective distance of injected gas under different reservoir physical parameters and conditions are shown in Table 3. The gas phase relative permeability K under residual oil saturation is calculated. rg is 0.26, the viscosity of injected CO2 is 0.0219 mPa·s, and the diffusion coefficient D of CO2 is 1.618×10 -7 m 2 / s, injection pressure difference of 10MPa, injection time of 180d (15552000s) under different reservoir physical conditions, the calculation results are as follows Figure 5 shown.

[0139] Table 3 Results of effective distance of injected gas under different reservoir physical property parameters and conditions

[0140]

[0141] S82. The results of the effective distance of injected gas under different injected gas property parameters and conditions are shown in Table 4. The gas phase relative permeability K is calculated under the reservoir permeability K of 0.3mD and residual oil saturation. rg The effective distance of gas under different injected gas properties is calculated as follows: Figure 6 shown.

[0142] Table 4 Results of effective distance of injected gas under different injected gas property parameters and conditions

[0143]

[0144] In summary, the present invention starts from the seepage and diffusion mechanism of injected gas and solves the analytical solution of the effective action distance of injected gas in oil and gas reservoirs. This method only requires some reservoir static parameters and basic data such as the viscosity and diffusion coefficient of the injected gas. The data collection and processing are simple and have a wide range of applications. This method can accurately and efficiently evaluate the effective action distance of injected gas, providing technical support for optimizing the gas injection scheme and improving the swept volume and recovery rate of gas.

[0145] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention. In the absence of conflict, those skilled in the art can combine the relevant technical features in the above examples according to actual conditions to achieve corresponding technical effects. The specific various combinations are not described here one by one.

[0146] It should be noted that all directional indications in the embodiments of the present invention (such as up, down, left, right, front, back, etc.) are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

Claims

1. A method for calculating the effective action distance of injected gas in oil and gas reservoirs based on diffusion and seepage theory, characterized in that: The following steps are included in sequence: Step S1. Assume that the reservoir is a homogeneous isotropic porous medium, there is a perfect well in the circular stratum of equal thickness, and the seepage of the injected gas in the reservoir obeys the linear seepage law and presents a planar radial stable seepage; Step S2. Obtain reservoir parameters, production parameters and fluid property parameters of the oil and gas reservoir, porosity φ, formation permeability K, gas phase relative permeability K rg , injected gas viscosity μ, injected gas diffusion coefficient D, injected gas pressure difference Δp and injected gas time t; Step S3. In actual oil and gas reservoirs, gas flow and diffusion usually occur simultaneously, and they need to be considered together. The total gas flux is expressed as the superposition of flow and diffusion: I 总 =J 流动 +J 扩散 (1) Step S4. Gas flow refers to the overall movement of gas in the oil and gas reservoir due to pressure difference or injection pressure. Gas diffusion refers to the spontaneous mixing movement of gas molecules due to concentration gradient. The gas flow flux and gas diffusion flux are superimposed to obtain: Step S5. Considering the overall movement of gas in the oil and gas reservoir due to pressure difference or injection pressure, the seepage effect of injected gas is described by Darcy's law, and the action distance r of injected gas seepage in the formation is derived. 流动 (t): Where: r 流动 is the fluid flow radius, m; K is the formation permeability, mD; K rg is the relative permeability of the gas phase; Δp is the injection pressure difference, MPa; μ is the viscosity of the fluid, mPa·s; φ is the porosity of the porous medium; t is the injection time, s; Step S6. Considering that gas molecules diffuse due to concentration gradient, gas diffusion follows Fick's law, and according to the statistical equipartition theorem, the Gaussian solution of gas diffusion is obtained, and the analytical solution r of the injected gas diffusion distance is solved. 扩散 (t): Where: r 扩散 is the fluid flow radius, m; D is the diffusion coefficient, m 2 / s; t is the gas injection time, s; Step S7. Superimpose the contributions of flow and diffusion to obtain the total gas action radius: S8. Determine the effective distance of injected gas in the reservoir under different production conditions.

2. The method for calculating the effective action distance of injected gas in oil and gas reservoirs based on diffusion and seepage theory according to claim 1, characterized in that: The process of step S4 is as follows: Step S41. When Darcy's law describes the gas flow in a porous medium, the flow flux and the flow velocity are in the same form, which is: Where: J 流动 is the flow flux, m / s; K is the formation permeability, mD; K rg is the relative permeability of the gas phase; Δp is the pressure difference, MPa; μ is the viscosity of the fluid, mPa·s; ΔL is the seepage path length, m; Step S42: The diffusion flux of the gas along the flow direction is expressed by Fick's first law. The diffusion flux of the gas per unit time and per unit area caused only by the diffusion phenomenon is expressed as: Where: J 扩散 is the diffusion flux, m / s; D is the diffusion coefficient, m 2 / s; C is the mole fraction concentration; x is the distance or spatial coordinate, m; Step S43. Substituting formula (6) and formula (7) into formula (1) to obtain the total gas flux formula (2); 3. The method for calculating the effective action distance of injected gas in oil and gas reservoirs based on diffusion and seepage theory according to claim 1, characterized in that: The process of step S5 is as follows: Step S51. According to the seepage characteristics of the injected gas in the porous medium, the relationship between the seepage velocity and the radius in the case of radial flow is: Where: v r is the injected gas seepage velocity, m / s; Δp is the injection pressure difference, MPa; r is the fluid flow radius, m; Step S52. In order to make the model more consistent with the actual reservoir conditions and empirical data, a correction constant A is introduced, and the Darcy velocity is expressed as: Step S53. By introducing the correction constant A, the deviation caused by these complexities can be effectively compensated, so that the model is closer to the flow characteristics of the actual oil and gas reservoir, and the accuracy and applicability of the model in practical applications are improved. When the fluid passes through the porous medium, the porosity has a significant impact on the fluid's seepage path and time scale. In order to quantify the impact of porosity on the fluid, the following is introduced: Where: A is the introduced constant; φ is the porosity of the porous medium; Step S54. Assuming that the position of the gas front changes with time, the propagation speed of the gas front is related to the Darcy seepage velocity: in: is the propagation velocity of the gas front, m / s; t is the gas injection time, s; Step S55. According to the propagation speed of the gas front, r and t are separated into variables to obtain the formula: Step S56. When the gas injection time is T0=0, the seepage radius R0=0, and when the gas injection time is T=t, the seepage radius R=r. Integrate the right side of equation (12) over the gas injection time interval (0, t), and integrate the left side over the seepage radius interval (0, r), and obtain equation (13): Step S57. After integral calculation, the relationship between the seepage radius is obtained as follows: Step S58. The formula for obtaining the effective distance of the injected gas seepage is:

4. The method for calculating the effective action distance of injected gas in oil and gas reservoirs based on diffusion and seepage theory according to claim 1, characterized in that: The process of step S6 is as follows: Step S61. The gas diffusion process usually follows Fick's first law, and its diffusion rate is determined by the concentration gradient and diffusion coefficient of the gas molecules. Fick's first law is formula (7): Step S62. Based on Fick's first law, and considering the one-dimensional passive diffusion process, Fick's second law is expressed as: Among them: t is time, s; r is the distance in space, m; Step S63. The classical Gaussian distribution solution of formula (15) is: in: is the normalization factor; is the variation of concentration with position and time; π is a constant of about 3.1416; Step S64. Gaussian distribution describes the distribution of gas concentration in space r, with a mean value a=0, indicating that the concentration is maximum at r=0, which is the concentration point of the gas at the initial moment. The standard deviation during the diffusion process is: Step S65. The shape of the Gaussian distribution becomes wider and wider over time, which means that the gas concentration is distributed more and more widely in space. The gas diffusion distance is usually defined as the position where the gas concentration drops significantly to a certain level. Due to the symmetry of the Gaussian distribution, the gas diffusion distance is estimated by the standard deviation. In the Gaussian distribution, the concentration changes with r and drops most significantly at the position where r is σ(t). Therefore, the standard deviation σ(t) is regarded as the gas diffusion distance. The diffusion distance of the injected gas is:

5. The method for calculating the effective action distance of injected gas in oil and gas reservoirs based on diffusion and seepage theory according to claim 1, characterized in that: The process of step S7 is as follows: Step S71. Superimpose the contribution of flow and diffusion to obtain the total gas action radius: r 作用 (t)=r 流动 (t)+r 扩散 (t) (18) Step S72: Substitute formula (3) and formula (4) into formula (18) to obtain the total gas action radius:

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