A method for quantitatively characterizing the miscibility degree of CO2 flooding based on three front edges

Abstract

The application discloses a method for quantitatively characterizing the miscibility degree of CO2 flooding based on three frontiers. The method comprises the following steps: determining the CO2 component frontier and the CO2 phase frontier of a target oilfield; establishing a three-dimensional space mechanism model of the target oilfield by using a CMG numerical simulation software, and performing numerical simulation operation; in the three-dimensional space mechanism model of the target oilfield, the miscibility range is the overlapping area between the CO2 component frontier and the CO2 phase frontier and between the miscibility pressure frontier and the injection well; setting the numerical simulation time step, starting from the beginning to the end, a total of i steps; determining the total miscibility area of each time step according to the numerical simulation result, and obtaining the miscibility degree of the target oilfield. The application determines the CO2 component frontier and the phase frontier through a one-dimensional core numerical model, determines the miscibility range judgment standard, and obtains the method for quantitatively characterizing the miscibility degree of CO2 flooding based on three frontiers according to the numerical simulation result.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of oil and gas field development, and relates to a CO2 flooding miscibility degree quantitative characterization method based on three front edges. BACKGROUND

[0002] At present, offshore resources suitable for CO2 flooding have great potential, and the implementable CO2 flooding oil reserves are up to 1.7 billion tons, which is 8% higher than that of water flooding, and the CO2 flooding recoverable reserves are increased by more than 130 million tons. The Bohai Bay Basin is the most important target for CO2 flooding, and through CO2 flooding, the low-permeability reservoir reserves in the Bohai Bay can be produced. The effect of CO2 flooding miscibility determines the ultimate recovery of CO2 flooding, and the research on the CO2 flooding miscibility range is also particularly important. At present, the methods for characterizing the CO2 flooding miscibility range are mostly to judge the miscibility degree by using a single parameter of pressure, which has certain limitations. In view of the problems encountered in the above CO2 flooding miscibility range judgment method, the application innovatively proposes a CO2 flooding miscibility degree quantitative characterization method based on three front edges, which can more accurately quantitatively characterize the CO2 flooding miscibility degree of the target reservoir and greatly improve the recovery rate. SUMMARY

[0003] The application aims to provide a CO2 flooding miscibility degree quantitative characterization method based on three front edges.

[0004] The application provides a CO2 flooding miscibility degree quantitative characterization method based on three front edges, which comprises the following steps: 1) determining a target oilfield, collecting its field data, performing fluid fitting through Fluid software to obtain fitted fluid data, and establishing a core numerical model through Petrel software, and substituting the fitted fluid data into the core numerical model to perform core displacement numerical simulation, so that the position where the CO2 concentration is greater than 0 is the CO2 component front edge;

[0005] The field data comprises oil layer temperature, saturation pressure, gas-oil ratio, average dissolved gas system number of formation oil, volume coefficient, compression coefficient, shrinkage rate, crude oil density, viscosity and crude oil components;

[0006] 2) along the direction from the production well to the injection well, the position where the CO2 and crude oil interfacial tension in the numerical simulation result is greater than 0 is the CO2 phase front edge;

[0007] 3) collecting actual underground three-dimensional space data of the target oilfield, establishing a three-dimensional space mechanism model of the target oilfield through CMG numerical simulation software, and performing numerical simulation operation, and the simulation result shows that in the early stage of injection, the CO2 pressure front and the CO2 component front do not intersect, and the swept area is completely miscible; in the middle stage of injection, the CO2 pressure front and the CO2 component front contact, and reach the critical miscible displacement; in the late stage of injection, the CO2 pressure front moves to the production well, and is in a non-complete miscible state; in the three-dimensional space mechanism model of the target oilfield, the miscible range is the overlapping area between the CO2 component front and the CO2 phase front and between the miscible pressure front and the injection well;

[0008] 4) setting a numerical simulation time step, starting to ending, a total of i steps; according to the numerical simulation result, determining the total miscible area of each time step, denoted as S i , calculated according to the following formula I-II, that is, the miscibility degree of the target oilfield is obtained;

[0009] Formula I ΔS n = S n -S n-1

[0010] In formula I, n is a time step, 1-i;

[0011] S n is the total miscible area at the nth step, m 2 ;

[0012] ΔS n is the newly added miscible area at the nth step, m 2 ;

[0013] Formula II

[0014] In formula II, M is the miscibility degree of the target oilfield, %;

[0015] S is the total area, m 2 .

[0016] In the above method, the CO2 molar percentage concentration is 0-0.01%, but 0 is not included.

[0017] In the above method, in step 2), the CO2 and crude oil interfacial tension is 0-0.01 mN·m -1 .

[0018] The present application has the following beneficial effects:

[0019] (1) The present application proposes a set of technical methods, and the application of the method can quantitatively characterize the miscibility degree of CO2 flooding based on three fronts.

[0020] (2) The present application provides a quantitative and operable technical method and implementation steps. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is a fluid fitting accuracy graph in Example 1.

[0022] Figure 2 is a core displacement numerical simulation result graph.

[0023] Figure 3 is a global miscibility graph in the early injection stage.

[0024] Figure 4 is a critical miscible displacement graph in the middle injection stage.

[0025] Figure 5 is a non-complete miscible state graph in the late injection stage. DETAILED DESCRIPTION

[0026] The experimental methods used in the following examples are conventional methods unless otherwise specified.

[0027] The materials, reagents, etc. used in the following examples can be obtained from commercial channels unless otherwise specified.

[0028] EXAMPLE

[0029] The present application mainly uses reservoir engineering and numerical simulation methods to establish a quantitative characterization method for determining the miscibility degree based on three fronts, including the following steps:

[0030] 1. Determine the CO2 component front and phase front through a one-dimensional core numerical model

[0031] (1) Determine the CO2 component front

[0032] Taking the Sha 3 member of Bozhong 25-1 oilfield as an example, first, fluid fitting is performed based on field data (reservoir temperature, saturation pressure, gas-oil ratio, average dissolved gas system number of formation oil, volume coefficient, compression coefficient, shrinkage, crude oil density, viscosity and crude oil components, as shown in Tables 1 and 2) through Fluid software, to obtain a fitting accuracy graph (as shown in FIG. 1), second, a core numerical model is established through Petrel software, and finally the data file generated by fluid fitting is substituted into the core numerical model for core displacement numerical simulation, and the result is shown in FIG. 2, where the position where the CO2 concentration is about to be greater than 0 is the CO2 component front. Figure 1

[0033] Table 1 Field data and crude oil parameters

[0034]

[0035] Table 2 Natural gas components ​

[0036]

[0037] (2) Clearly define the CO2 phase front

[0038] Along the direction from the production well to the injection well, the position where the interfacial tension between CO2 and crude oil is about to exceed 0 is the CO2 phase front. Considering the difficulty of dividing the three-dimensional field interface, the first value of the interfacial tension greater than 0 (0.01mN·m -1 ) as a parameter to determine the position of the phase front, such as Figure 2 shown.

[0039] 2. Determine the miscibility of CO2 flooding

[0040] (1) Establish a three-dimensional mechanism model

[0041] Establish the mechanism model of the third section of the Bozhong 25-1 Shahejie Formation and conduct numerical simulation. Figure 3 As shown in the figure, in the early stage of injection, the pressure front and the component front do not intersect, and the whole phase is mixed. In the middle stage of injection, the pressure front and the component front contact, and the critical miscible displacement is achieved (as shown in the figure). Figure 4 At the late injection stage, the pressure front migrates toward the production well and is in a non-completely miscible state (as shown in Figure 5 shown).

[0042] (2) Clarify the criteria for determining the miscibility range

[0043] In the three-dimensional model, the miscible range is the overlapping area between the component front and the phase front and between the miscible pressure front and the injection well, e.g. Figures 3-5 This demonstrates the limitations of using pressure alone to determine miscibility, reflecting the mechanism by which the CO2 front evaporatively contributes to miscibility. In low-permeability reservoirs, as production wells depressurize, the miscible pressure front retreats. When the miscible pressure front intersects the CO2 component front, the miscible zone begins to shrink. When the miscible pressure front separates from the CO2 phase front, miscibility in the reservoir becomes unattainable.

[0044] (3) Determine the degree of miscibility

[0045] Set the numerical simulation time step (a total of i steps from the start to the end), and determine the total miscible area S for each time step based on the numerical simulation results. i ,make:

[0046] ΔS n =S n -S n-1

[0047] Where: n—time step, 1~i;

[0048] S n —Total miscible area at step n, m 2 ;

[0049] ΔS n Newly added miscible area at the nth step, m 2 .

[0050] The final miscibility degree is calculated according to the following formula:

[0051]

[0052] In the formula, M is the miscibility degree of the target oilfield, %.

[0053] S is the total area, m 2 .

[0054] Combining Figures 3-5 That is the result of simulation. The miscible zone marked in the figure is the result. According to the above formula, the miscibility degree of the target oilfield can be obtained, which is 58%.

Claims

1. A method for quantitatively characterizing the miscibility of CO2 flooding based on three fronts, comprising the following steps: 1) identifying a target oilfield, collecting field data therefrom, performing fluid fitting using Fluid software to obtain fitted fluid data, establishing a core numerical model using Petrel software, substituting the fitted fluid data into the core numerical model for core flooding numerical simulation, and determining the location where the CO2 concentration is about to exceed 0 as the CO2 component front; The field data include oil layer temperature, saturation pressure, gas-oil ratio, average dissolved gas coefficient of formation oil, volume coefficient, compressibility, shrinkage, crude oil density, viscosity and crude oil composition; 2) Along the direction from the production well to the injection well, the position where the interfacial tension between CO2 and crude oil is about to exceed 0 in the numerical simulation results is the CO2 phase front; 3) Collecting actual underground three-dimensional spatial data of the target oilfield, establishing a three-dimensional spatial mechanism model of the target oilfield using CMG numerical simulation software, and performing numerical simulation operations. The simulation results show that in the early stage of injection, the CO2 pressure front and the CO2 component front do not intersect, and the affected area is completely miscible; in the middle stage of injection, the CO2 pressure front contacts the CO2 component front, achieving critical miscible displacement; in the late stage of injection, the CO2 pressure front migrates toward the production well and is in a non-completely miscible state; in the three-dimensional spatial mechanism model of the target oilfield, the miscible range is the overlapping area between the CO2 component front and the CO2 phase front and between the miscible pressure front and the injection well; 4) Set the numerical simulation time step, from the beginning to the end, which is recorded as i steps; according to the numerical simulation results, determine the total miscible area of ​​each time step, which is recorded as S i , calculated according to the following formula I-II, the miscibility of the target oil field is obtained; Formula IΔS n =S n -S n-1 In formula I, n is the time step, 1 to i; S n —Total miscible area at step n, m 2 ; ΔS n —The newly added miscible area at step n, m 2 ; Formula II In formula II, M is the miscibility of the target oil field, %; S—total area, m 2 .

2. The method according to claim 1, characterized in that The CO2 molar percentage concentration is 0 to 0.01%, but does not include 0.

3. The method according to claim 1 or 2, characterized in that In step 2), the interfacial tension between CO2 and crude oil is 0-0.01 mN·m -1 .

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