Phase behavior fitting method for gas-cap high CO2 oil and gas reservoirs and its application

Abstract

The present invention relates to a phase behavior fitting method for a gas-cap high CO₂ oil and gas reservoir, comprising the following steps: reading the data required for phase behavior fitting of the gas-cap high CO₂ oil and gas reservoir to be fitted; performing component splitting on the gas-cap high CO₂ oil and gas reservoir to be fitted; conducting similarity analysis based on its thermodynamic parameters, formulating a recombination scheme and carrying out recombination to achieve the best fitting accuracy; first fitting the saturation pressures of the gas zone and the oil zone of the gas-cap high CO₂ oil and gas reservoir to be fitted, and then fitting the phase behavior experiment to obtain the PVT parameters shared by the oil and gas phases of the gas-cap high CO₂ oil and gas reservoir. The phase behavior fitting method for the gas-cap high CO₂ oil and gas reservoir of the present invention is based on the oil and gas reservoir numerical simulation method, physical chemistry theory, and thermodynamic theory analysis method to obtain a set of unified critical parameters for the oil and gas two-phase fitting, so as to finely depict the characteristics of the oil-gas and oil-water interfaces and the component distribution law in the oil and gas reservoir, thereby determining a reasonable development mode.

Description

Technical Field

[0001] The present invention belongs to the fields of oil development technology and numerical simulation methods for oil and gas reservoirs, and particularly relates to a phase behavior fitting method for gas-cap high-CO2 oil and gas reservoirs and its application. Background Art

[0002] In recent years, with the continuous increase in the exploration intensity of offshore oil and gas resources, a large number of gas-cap high-CO2 carbonate oil and gas reservoirs have been discovered in the world's sea areas. Due to the extraction effect of CO2 on the light hydrocarbon components of the oil layer, such oil and gas reservoirs show the characteristic of "light on top and heavy on the bottom" in the component distribution, and a "fault" distribution is formed at the gas-oil two-phase interface, resulting in a high content of light components in the gas cap and a high content of heavy components in the oil layer of such oil and gas reservoirs, and the component distribution law is very complex.

[0003] Phase behavior fitting essentially provides fluid PVT (Pilot-run Verification Test) parameters for the component model. The higher the fitting accuracy, the more accurately the properties of the actual fluid in the oil and gas reservoir can be characterized. At present, there are phase behavior fitting techniques for condensate gas reservoirs and phase behavior fitting techniques for gas drive oil. These two methods are both used to study the phase behavior characteristics of single-phase fluids, but they have different characteristics and adaptabilities: The phase behavior fitting method for condensate gas reservoirs takes the gas phase as the fitting main body. Since the content of heavy fractions is small, the influence on the phase behavior fitting accuracy is small, and generally does not involve the recombination splitting technology of heavy components. The recombination splitting of pseudo-components generally only reaches C7+. The gas drive oil fitting method is mainly for heavy oils such as tight oil. Although the splitting and refinement of heavy fractions are involved in the fitting process, the influence of probability distribution is not considered. In the recombination process, the division scheme of pseudo-components is generally formulated only based on the content of components:

[0004] (1) Condensate gas reservoirs usually exist in high-temperature and low-pressure environmental conditions, and contain a large amount of light hydrocarbon components and more intermediate hydrocarbons in the gas. In the process of phase behavior fitting, it is usually not necessary to further split and refine the pseudo heavy fractions (Liu Changlin, Zhang Maolin, Mei Haiyan, etc. Phase Behavior Testing and Fitting of Oil and Gas Systems in Condensate Gas Reservoirs [J]. Petroleum Geology and Engineering, 2008(01): 65-68).

[0005] (2) The gas drive crude oil phase behavior fitting method is usually based on the phase behavior characteristic parameters of tight oil after gas injection. Tight oil has a relatively high content of heavy components, and sometimes it is necessary to split and refine the heavy fractions. In order to improve the flexibility and accuracy of phase behavior fitting, reduce the time spent on repeatedly adjusting parameters during the phase behavior fitting process, study the optimization of heavy component splitting, and assist in the completion of phase behavior fitting. After foreign scholars studied the method of combining single-carbon components extended from heavy components into multi-carbon pseudo-components, they believed that splitting heavy components C7+ or C11+ into 2-3 pseudo-components could well represent the fluid properties. They explored the distribution of the content of each pseudo-component and the components that each pseudo-component should include (Whitson C.H. Characterizing Hydrocarbon Plus Fractions[J]. SPE Journal, 1983, 23(4):683-694).

[0006] In gas-cap high-CO2 oil and gas reservoirs, gas and oil coexist in the same hydrodynamic system. Gas, oil, and water are in close contact and mass transfer occurs continuously between phases to reach dynamic equilibrium. Although the number of components in the gas cap and the oil layer is the same, the component content differences are very large. In the gas cap area, the content of light components is relatively high, and the C7+ component is significantly higher in the bottom oil. Using only 2-3 pseudo-components cannot meet the expected accuracy requirements. Therefore, during the phase behavior fitting process, the gas and oil phases must share a set of critical parameters, taking into account the phase behavior characteristics of both the gas cap and the oil layer. It is difficult for the phase behavior fitting method for single-phase fluids to simultaneously exhibit these two characteristics.

[0007] In summary, considering the characteristics of the coexistence and interaction of multi-phase fluids in gas-cap high-CO2 oil and gas reservoirs and the very complex component distribution law, the conventional condensate gas phase behavior fitting and gas drive crude oil phase behavior fitting methods are not applicable. Therefore, it is necessary to develop a more suitable phase behavior fitting method for gas-cap high-CO2 oil and gas reservoirs. Summary of the Invention

[0008] The object of the present invention is to solve the problems of unclear understanding of the phase behavior law and great fitting difficulty in complex oil and gas reservoirs with high CO2 content in the gas cap. Starting from the existing phase behavior fitting methods for condensate gas reservoirs and gas drive crude oil (heavy oil), based on oil and gas reservoir numerical simulation methods, physical chemistry theories, and thermodynamic theory analysis methods, a set of unified PVT parameters for fitting oil and gas phases is obtained to finely depict the oil-gas, oil-water interfaces, and the characteristics of component distribution laws in the oil and gas reservoir, thereby constructing an accurate component model.

[0009] In the first aspect, the present invention proposes a phase behavior fitting method for gas-cap high-CO2 oil and gas reservoirs, including the following steps:

[0010] S101: Read the data required for phase behavior fitting of the oil and gas reservoir with a high CO₂ content in the gas cap to be fitted;

[0011] S102: Perform component splitting on the oil and gas reservoir with a high CO₂ content in the gas cap to be fitted according to the data read in step S101;

[0012] S103: Conduct similarity analysis on the split components in step S102 according to their thermodynamic parameters, formulate a recombination plan and perform recombination to achieve the best fitting accuracy;

[0013] S104: First, fit the saturation pressure of the gas zone and the saturation pressure of the oil zone of the oil and gas reservoir with a high CO₂ content in the gas cap to be fitted, set regression variables to improve the fitting accuracy, and then fit the phase behavior experiment to obtain the PVT parameters shared by the oil and gas phases of the oil and gas reservoir with a high CO₂ content in the gas cap.

[0014] In chemical engineering thermodynamics, the PVT relationship of a fluid refers to the functional relationship among the pressure (P), volume (V), and temperature (T) of the fluid, and its functional formula is:

[0015] f(P, V, T) =0 (1)

[0016] The core of the present invention is to determine the principle of unifying the fitting of the two-phase fluids in the gas cap and the oil layer, establish an optimal splitting and recombination method, and select the regression variable setting method with the highest fitting accuracy.

[0017] According to the theory of seepage mechanics and the method of molecular dynamics analysis, it is determined that the gas, oil, and water phases of this type of oil and gas reservoir exist independently and are in close contact at the same time, in the same hydrodynamic system, continuously undergoing mass transfer between phases to reach dynamic equilibrium. And due to the strong extraction effect of CO₂, a large amount of light hydrocarbon crude oil is concentrated in the gas cap, and the heavy hydrocarbon components in the oil layer are relatively high. The gas cap fluid is in the supercritical range, and the fluid molecules continuously perform random thermal molecular motion; the oil layer fluid is fully miscible with CO₂, showing the characteristics of heavy oil with low viscosity. Only a component model can accurately characterize these complex phase changes. Therefore, it is necessary to unify the fitting of the oil and gas phases and share a set of PVT parameters during the fitting process. In addition, important parameters such as the viscosity and density of each component in the oil and gas reservoir are closely related to the PVT parameters. Therefore, to obtain accurate critical parameters, more precise phase behavior fitting must be carried out.

[0018] As a specific implementation manner of the present invention, in step S101, the data required by the present invention can be selected from the following types:

[0019] (1) Saturation pressure data

[0020] Determining the saturation pressures of the gas and oil zones in a gas-cap high CO₂ oil and gas reservoir is closely related to fluid saturation, density, viscosity, and relative volume, and is the basis for constructing an accurate phase PVT model. For the fluid in the gas-cap zone, the saturation pressure is the dew point pressure, which is expressed as the pressure at which the first liquid droplet appears as the pressure increases at the reservoir temperature during the phase fitting process; for the reservoir fluid, the saturation pressure is the bubble point pressure, which is expressed as the pressure corresponding to the first gas bubble as the pressure decreases at the reservoir temperature during the phase fitting process.

[0021] (2)Component composition of well fluids in the gas-cap zone and reservoir

[0022] Since there are significant differences in the component contents between the gas-cap zone and the oil-ring zone in a gas-cap high CO₂ oil and gas reservoir, it is necessary to read the contents of each component in the gas-cap and oil-ring. The key parameters of heavy fractions, including molecular weight and density, are crucial for subsequent splitting and recombination, especially the molecular weight and relative density of heavy fractions in the key oil-ring zone, which facilitate the adoption of suitable splitting methods for refinement in the later stage.

[0023] (3)Phase experiment data

[0024] Directly measure the PVT physical property parameters of the extracted well fluids through experiments. The main phase experiments include constant-composition expansion experiment (CCE experiment), constant-volume depletion experiment (CVD experiment), and differential liberation experiment (DL experiment). It mainly focuses on the correlation between pressure and fluid saturation, the relationship between pressure and relative volume, and the relationship between pressure and viscosity. The principle of phase fitting is to finally connect and fit the phase experiment observation values with a normalized curve to judge the fitting accuracy.

[0025] The fluid distribution law in a gas-cap high CO₂ oil and gas reservoir is complex, and due to the extraction effect of CO₂, the content of heavy components in the bottom oil is significantly higher, forming a heavy oil reservoir with a relatively large relative density. Therefore, the mole fractions of each component obtained through well fluid analysis and experiments cannot meet the fitting accuracy requirements for heavy hydrocarbon components, and a more suitable splitting method needs to be established.

[0026] The purpose of splitting is to weaken the experimental error of the last added component.

[0027] As a specific implementation manner of the present invention, in the step S102, the splitting method is at least selected from one of Multi-feed, Whitson, and PNA Distribution. Among them, the Multi-feed and PNA Distribution methods are applicable to conventional oil and gas reservoirs such as light oil and condensate oil, and there are limitations for heavy oil and gas reservoirs similar to those with a high CO2 content in the gas cap. The Whitson method has a wide range of applications, especially good adaptability to heavy oil and gas reservoirs; and this method introduces the gamma function and considers the influence of the gamma distribution function on the component distribution. The principle of the Whitson splitting method is as follows:

[0028] (1) Gamma distribution function:

[0029] , ,

[0030] (2) Let x = M , that is, x is the relative molecular mass of the single-carbon array. Therefore, the cumulative occurrence frequency of a certain component is:

[0031]

[0032] (3) Then the mole fraction Z i is proportional to the cumulative occurrence frequency:

[0033]

[0034] The splitting results of the C20+ and above heavy hydrocarbon components of the oil and gas reservoir with a high CO2 content in the gas cap can be obtained by the above-mentioned solution method using the Whitson splitting method.

[0035] As a specific implementation manner of the present invention, in the step S101, the oil and gas reservoir with a high CO2 content in the gas cap to be fitted at least includes one of an oil and gas reservoir with a high CO2 content in carbonate rock, an oil and gas reservoir with a high CO2 content in sandstone, and an oil and gas reservoir developed by injecting CO2.

[0036] Taking a typical oil and gas reservoir as an example, for a certain offshore gas-cap bottom-water oil and gas reservoir, the CO2 content in the gas-cap gas can reach 75%, and the heavy components above C7+ in the bottom oil are significantly high. The test analysis of the well stream adds components up to C20+. In order to reduce the fitting error and improve the fitting accuracy, splitting must be carried out after C20+. It is preferred to use the Whitson splitting method to split C20+ to C35+.

[0037] According to step S102, the heavy hydrocarbon pseudo-components are re-split into numerous components. For the sake of unification and convenient fitting, it is necessary to conduct similarity analysis based on the thermodynamic parameters of the original components, formulate a recombination plan, and ensure as many groups as possible to achieve the best fitting accuracy.

[0038] As a specific implementation manner of the present invention, in the step S103, the thermodynamic parameters include at least one of the critical temperature, critical pressure, molar mass, boiling point, or acentric factor of each component.

[0039] As a specific implementation manner of the present invention, in the step S103, the criterion for judging the reliability of recombination is that the component division plan after recombination satisfies the principle of a small change range of the phase diagram characteristic points, preferably the "four-point unification principle". Among them, the four phase diagram characteristic points include: (1) critical point; (2) critical condensation temperature point; (3) critical condensation pressure point; (4) saturation pressure point, one or more of them.

[0040] Considering comprehensively that in the development process of a gas-cap high-CO₂ oil and gas reservoir, for economic benefits and environmental protection, CO₂ may be selected for reinjection production, and a recombination plan is formulated according to the thermodynamic properties of each component as shown in Table 1. The different properties of the components have a great difference in the influence on grouping, which is mainly reflected in the division of light components and heavy components, while the division rule of intermediate components is basically the same. Then, the most suitable recombination plan for this oil and gas reservoir is selected according to the sensitivity of the thermodynamic parameters.

[0041] Table 1 Recombination plan according to the thermodynamic scheme

[0042]

[0043] Currently, there is no unified conclusion for the parameter adjustment and fitting method, and the subjective factor is relatively large. In view of the complex characteristics of the component distribution law of the gas-cap high-CO₂ oil and gas reservoir, the present invention adopts two major parameter adjustment ideas through comparison and optimization: one is to screen regression variables to greatly improve the fitting accuracy; the other is to optimize regression variables to further improve the fitting accuracy.

[0044] In the unified fitting process of the oil and gas two-phase of the gas-cap high-CO₂ oil and gas reservoir, due to the large difference in the component content of the oil and gas two-phase, it is difficult to fit the data. Solving this problem requires setting regression variables. Here, the Hessian matrix is introduced to optimize the regression parameters, and the Correlation matrix is used to optimize and fine-tune the parameters.

[0045] (1) Optimization of regression variables - Hessian matrix:

[0046] During the phase state fitting process, in order to obtain phase state parameters, a Hessian matrix is generated. Each element of the Hessian matrix represents the connection between some critical parameters and PVT parameters among various components. In each column of the Hessian matrix, those with an absolute value larger than the diagonal value of that column are selected as regression parameters, while those smaller than the absolute value are not used as regression variables, thus completing the selection of regression variables.

[0047] (2)Optimization of parameters - Correlation matrix:

[0048] After the preferred regression variables are completed, if there is a correlation between the regression parameters, it will increase the computational amount and cause difficulties in the convergence of the solution. In each column of the Correlation matrix, for those close to 1 (close to 1 indicates a good correlation between these parameters), only one needs to be retained. Which one to retain specifically needs to consider the sensitivity.

[0049] After simultaneously fitting the gas and oil phases of the gas-cap high CO2-containing oil and gas reservoir and splitting and refining the heavy fractions, and screening the best recombination plan based on thermodynamic parameters, the saturation pressure of the entire oil and gas reservoir fluid should be fitted next.

[0050] As a specific implementation manner of the present invention, in the step S104, when fitting the saturation pressure of the gas zone and the oil zone of the gas-cap high CO2-containing oil and gas reservoir to be fitted, the following steps are specifically included: Select the fitting phase with a large weight of the two-phase saturation pressure in the gas zone and the oil zone through calculation software, and set regression variables to improve the fitting accuracy.

[0051] After the saturation pressure fitting is completed, fit and adjust the parameters of the phase state experiments (such as constant-composition expansion experiment, constant-volume depletion experiment, flash separation experiment, etc.). After fitting using the phase state fitting method of the present invention, a high-precision phase state model is obtained (the fitting accuracy is above 95%, significantly better than about 80% of the traditional fitting idea).

[0052] After a high-precision phase state model of the gas-cap high CO2-containing oil and gas reservoir is completed using the method of the present invention, a set of PVT parameters common to the oil and gas phases is derived.

[0053] The phase behavior fitting method for gas-cap high CO₂ oil and gas reservoirs of the present invention is based on reservoir numerical simulation methods, physical chemistry theories, and thermodynamic theory analysis methods to obtain a set of common critical parameters for unified fitting of oil and gas phases, so as to finely depict the oil-gas, oil-water interfaces, and the characteristics of component distribution laws in the reservoir, thereby determining a reasonable development mode. At present, more research has been conducted on the fluid characteristics of condensate gas reservoirs at home and abroad, while there is little research on the fluid characteristics of high CO₂ oil and gas reservoirs. Moreover, the PVT analysis methods for reservoir fluids are all established based on low-content CO₂ hydrocarbon fluids, and the phase behavior fitting is based on experience or conventional reservoir numerical simulation methods, resulting in large errors and little practical significance for actual development. Using the method of the present invention, not only is the fitting accuracy relatively high, but also it respects the fact that the oil, gas, and water phases in the reservoir are in the same hydrodynamic system, providing theoretical and technical support for the actual development of oil and gas reservoirs, which is of extremely important significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The present invention will be further described in detail below with reference to the drawings.

[0055] Figure 1 It is a comparison diagram of the results after splitting the C7+ heavy fraction by the Whitson method and the results before splitting;

[0056] Figure 2 It is a standard schematic diagram for judging the reliability of recombination;

[0057] Figure 3 It is the fitting result of the saturation pressures of gas and oil phases;

[0058] Figure 4 It is a diagram of the phase behavior experiment fitting results, where: (a) is the phase behavior experiment fitting result of the gas sample CCE experiment pressure - relative volume; (b) is the fitting result of the oil sample DL experiment pressure - viscosity; (c) is the fitting result of the oil sample DL experiment pressure - relative volume;

[0059] Figure 5(a) is the phase diagram of the gas sample formed by the phase behavior fitting calculation of the gas-cap high CO₂ oil and gas reservoir;

[0060] Figure 5(b) is the phase diagram of the oil sample formed by the phase behavior fitting calculation of the gas-cap high CO₂ oil and gas reservoir. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0061] To make the present invention easier to understand, the present invention will be described in detail below in conjunction with embodiments and the drawings. These embodiments are only illustrative and not limited to the application scope of the present invention.

[0062]

Embodiment 1

[0063] Example 1 Taking a typical oilfield as an example, this oilfield is located in the Santos Basin offshore Brazil, with a water depth of 1000 m to 1500 m and a burial depth of about 5100 m to 5450 m. The oil and gas reservoirs can be divided into: carbonate rocks in the upper and lower depression periods of the Cretaceous under salt, as well as carbonate rocks in the upper rift period and carbonate rocks in the lower rift period. Among them, the carbonate rocks in the upper and lower depression periods and the upper rift period are the main gas storage layers, and a part of the carbonate rocks in the lower rift period are oil-bearing reservoirs. The depth of the oil and gas interface is -5390 m, the thickness of the gas layer is 268 m, and the CO2 content is 76.61%; the depth of the oil-water interface is -5435 m, the thickness of the oil layer is 45 m, and there is a relatively large bottom water. The API of the crude oil in the oil and gas reservoir is 17.6°-19.7° (0.934 g / cm 3 ~0.949 g / cm 3 ), belonging to heavy crude oil; the viscosity of the crude oil is relatively low, only 5 mPa·s - 7 mPa·s; the CO2 content in the oil sample reaches 57.2%, and the gas-oil ratio is 181 m 3 / m 3 -158 m 3 / m 3 .

[0064] Step 1: Read the data required for phase behavior fitting of the high CO2-containing gas cap of the oil and gas reservoir to be fitted.

[0065] (1) Saturation pressure data

[0066] Based on the gas samples and oil samples obtained on site, the saturation pressure of the gas sample is 55.0 MPa, and the saturation pressure of the oil sample is 42.0 MPa.

[0067] (2) Component composition of the well stream in the gas cap area and the oil layer

[0068] The component composition of the well stream of the gas sample and the oil sample is shown in Table 1.

[0069] Table 1 Component content of the gas sample sampling (mole percentage)

[0070]

[0071] (3) Phase behavior experimental data

[0072] Carry out an isocompositional expansion experiment (CCE experiment) and a differential liberation experiment (DL experiment), and the data results are shown in Tables 2-4:[[]]

[0073] Table 2 Change in the gas volume factor measured in the CCE experiment of the gas sample

[0074]

[0075] Table 3 Change in the viscosity of the crude oil measured in the DL experiment

[0076]

[0077] Table 4 Variation of volume coefficient in the DL experiment of oil samples

[0078]

[0079] Step 2: Using the Whitson method, split the recombined components of the gas-cap high CO₂ oil and gas reservoir to be fitted according to the data read in Step S101.

[0080] Step 3: Conduct a similarity analysis on the split components in Step 2 according to their thermodynamic parameters, formulate a recombination plan and carry out recombination to achieve the best fitting accuracy.

[0081] First, based on the similarity of the thermodynamic parameters of each component (the thermodynamic parameters can be calculated through the equation of state or obtained from the literature), they are divided into volatile components, intermediate components, and heavy components. The different properties of the components have a significant impact on the grouping, mainly reflected in the division of volatile components (light components) and heavy components, while the division rule of intermediate components is basically the same, and they are divided into two components.

[0082] For volatile components (light components) and heavy components, different types of combinations are carried out according to the molar mass, boiling point, critical temperature, critical pressure, and acentric factor characteristics of each component. By comparing the fitting accuracies of the critical point, critical condensation temperature point, critical condensation pressure point, and saturation pressure point, the recombination plan with the highest fitting degree is finally selected, as shown in Table 5.

[0083] Table 5 Recombination plan based on the thermodynamic scheme

[0084]

[0085] Step 4: First, fit the saturation pressure of the gas zone and the saturation pressure of the oil zone of the gas-cap oil and gas reservoir with high CO₂ to be fitted, set regression variables to improve the fitting accuracy, and then fit the phase behavior experiment to obtain the PVT parameters shared by the oil and gas phases of the gas-cap high CO₂ oil and gas reservoir.

[0086] The optimization of parameters by comparison is divided into two steps:

[0087] Step 401: Screen regression variables to greatly improve the fitting accuracy;

[0088] Step 402: Optimize the regression variables to further improve the fitting accuracy.

[0089] After completing a high-precision phase behavior model for a gas-cap high CO₂ oil and gas reservoir using the method of Embodiment 1, the phase diagrams of the gas sample and the oil sample can be established. Figure 5 shows the phase diagrams of the gas sample and the oil sample formed by the phase behavior fitting calculation for the gas-cap high CO₂ bottom-water oil and gas reservoir.

[0090] Since in the implementation process of Embodiment 1, the gas cap and the two different fluids in the oil and gas reservoir were considered separately and unifiedly fitted to construct a set of phase behavior models. By creating a splitting and recombining method for the gas-cap high CO₂ bottom-water oil and gas reservoir, introducing the influence of the gamma function on the component distribution law and referring to the similarity of thermodynamic parameters, and finally creating a method for setting regression variables to further improve the fitting accuracy, introducing the Hessian matrix and the Correlation matrix. Using the method of the present invention, not only the fitting accuracy is high and the results are reliable, but also it respects the fact that the oil, gas and water phases in the oil and gas reservoir are in the same hydrodynamic system, finely depicts the oil-gas and oil-water interfaces in the oil and gas reservoir, as well as the characteristics of the component distribution law, provides theoretical and technical support for the actual development of the oil and gas reservoir, and has extremely important significance. Using the method of the present invention, not only the gas and oil phases can be unifiedly fitted, but also the fitting accuracy can be improved to more than 95% compared with the traditional fitting method.

[0091] It should be noted that the above-described embodiments are only used to explain the present invention and do not constitute any limitation to the present invention. The present invention has been described by referring to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory words rather than limiting words. Modifications can be made to the present invention within the scope of the claims of the present invention as stipulated, and the present invention can be revised without departing from the scope and spirit of the present invention. Although the present invention described therein relates to specific methods, materials and embodiments, it does not mean that the present invention is limited to the specific examples disclosed therein. On the contrary, the present invention can be extended to all other methods and applications with the same functions.

Claims

1. A phase fitting method for gas-lifted high CO2 oil and gas reservoirs, characterized in that It includes the following steps: S101: Read the data required for phase behavior fitting of the gas-cap high CO₂-containing oil and gas reservoir to be fitted; S102: Split the components of the gas-cap high CO₂-containing oil and gas reservoir to be fitted according to the data read in step S101; S103: Conduct a similarity analysis on the split components in step S102 according to their thermodynamic parameters, formulate a recombination plan and conduct recombination to achieve the best fitting accuracy; S104: First, fit the saturation pressure of the gas zone and the saturation pressure of the oil zone of the gas-cap high CO₂-containing oil and gas reservoir to be fitted, set regression variables to improve the fitting accuracy, and then fit the phase behavior experiment to obtain the PVT parameters shared by the oil and gas phases of the gas-cap high CO₂-containing oil and gas reservoir; In step S101, the data includes: the saturation pressure of the gas zone and the saturation pressure of the oil zone in the gas-cap high CO₂-containing oil and gas reservoir to be measured, the content of each component in the gas zone and the oil zone of the gas-cap high CO₂-containing oil and gas reservoir to be measured, and the PVT physical property parameters measured according to the phase behavior experiment; In step S103, the thermodynamic parameters include the critical temperature, critical pressure, molar mass, boiling point and acentric factor of each component; In step S103, the criterion for judging the reliability of recombination is: the component division plan after recombination satisfies the principle that the change range of the phase diagram characteristic points is small.

2. The phase behavior fitting method for gas-lifted high CO2 oil and gas reservoirs according to claim 1, wherein In step S102, the splitting method is at least selected from one of Multi-feed, Whitson and PNA Distribution.

3. The phase state fitting method for gas-lifted high CO2 oil and gas reservoirs according to claim 1 or 2, characterized in that The phase diagram characteristic points at least include the critical point, critical condensation temperature point, critical condensation pressure point and saturation pressure point.

4. The phase state fitting method for gas-cap high CO2 oil and gas reservoirs according to claim 1 or 2, characterized in that In step S104, the regression variables at least include one of the Hessian matrix and the Correlation matrix.

5. The phase behavior fitting method for a gas-lifted high CO2 oil and gas reservoir according to claim 1 or 2, characterized in that In step S104, the phase behavior experiment is at least selected from one of the constant-composition expansion experiment, constant-volume depletion experiment and differential separation experiment.

6. The phase behavior fitting method for gas-lifted high CO2 oil and gas reservoirs according to claim 1 or 2, characterized in that In step S104, when fitting the saturation pressure of the gas zone and the saturation pressure of the oil zone of the gas-cap high CO₂-containing oil and gas reservoir to be fitted, it specifically includes the following steps: Select the fitting phase with a large weight of the two-phase saturation pressure of the gas zone and the oil zone through calculation software, and set regression variables to improve the fitting accuracy.

7. The phase state fitting method for gas-cap high CO2 oil and gas reservoirs according to claim 1 or 2, characterized in that In step S101, the gas-cap high CO₂-containing oil and gas reservoir to be fitted at least includes one of a gas-cap high CO₂-containing carbonate rock oil and gas reservoir, a gas-cap high CO₂-containing sandstone oil and gas reservoir and an oil and gas reservoir developed by injecting CO₂.

8. Application of the method according to any one of claims 1-7 in the phase behavior fitting of a gas-cap high CO₂-containing oil and gas reservoir.

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