A method for predicting the miscibility degree of carbon dioxide flooding

Abstract

This invention relates to a method for predicting the miscibility of carbon dioxide flooding, belonging to the field of oilfield development technology. The method of this invention, from a microscopic perspective of component changes, directly and quantitatively characterizes the reservoir miscibility after CO2 flooding. After CO2 injection into the reservoir, the miscibility coefficient is used to characterize the changes of various components in the crude oil within the affected area, thereby calculating the CO2 flooding miscibility within that area. This allows for the evaluation of the CO2 flooding effect in the target block. This method improves the prediction accuracy of carbon dioxide flooding miscibility, provides a basis and technical support for evaluating the effect of injected gas blocks, and can also guide the preparation of CO2 flooding reservoir engineering plans.

Description

Technical Field

[0001] This invention relates to a method for predicting the degree of miscibility in carbon dioxide flooding, belonging to the field of oilfield development technology. Background Technology

[0002] CO2 exists in a supercritical state at temperatures above its critical temperature of 31.26℃ and pressures above its critical pressure of 7.2 MPa. Its properties change; its density approaches that of a liquid, its viscosity approaches that of a gas, and its diffusion coefficient is 100 times that of a liquid, thus exhibiting significant dissolving power. When CO2 is dissolved in crude oil, the properties of the crude oil change, reservoir properties improve, and oil recovery is enhanced. Upon initial contact with crude oil, CO2 extracts light hydrocarbon components, forming a gas phase rich in light hydrocarbons and a liquid phase with decreasing light hydrocarbon content. With continued CO2 injection, CO2 condenses in the liquid phase, further decreasing the light hydrocarbon content; the gas phase, rich in light hydrocarbons, extracts crude oil, further increasing the light hydrocarbon content. After multiple rounds of repeated contact, the light hydrocarbon content in the gas and liquid phases eventually becomes equal, achieving miscibility. Carbon dioxide flooding is a technology that primarily enhances oil recovery by reducing the interfacial tension between oil and water, crude oil viscosity, and extracting and vaporizing light hydrocarbons from crude oil. Miscibility is a crucial indicator for evaluating the effectiveness of carbon dioxide flooding.

[0003] Chinese patent document CN108798614A discloses a method for determining the miscibility of CO2-assisted oil recovery, comprising the following steps: 1) establishing a numerical simulation model for CO2 injection based on the actual conditions of the reservoir and performing numerical simulation calculations; 2) obtaining parameter field data at different times after CO2 injection based on the numerical simulation results; 3) calculating the miscibility volume factor C based on the parameter field data at different times. p Near miscible volume coefficient C s and the sweep efficiency C of CO2 components c ;4) Calculate the degree of miscibility C under reservoir conditions based on the results of step 3);5) Calculate the relative degree of miscibility C based on the results of steps 3) and 4). x .

[0004] However, during carbon dioxide flooding, the composition of various light hydrocarbons in the crude oil changes as the carbon dioxide flooding proceeds, and the composition of light hydrocarbons affects the miscibility volume factor C, which is used to evaluate the degree of miscibility. p Near miscible volume coefficient C s and the sweep efficiency C of CO2 components c Therefore, the miscibility predicted by the above-mentioned carbon dioxide flooding miscibility characterization method has a large error. Summary of the Invention

[0005] The purpose of this invention is to provide a method for predicting the degree of miscibility in carbon dioxide flooding, which can solve the problem of large prediction errors in current methods for predicting the degree of miscibility in carbon dioxide flooding.

[0006] To achieve the above objectives, the technical solution adopted by the carbon dioxide flooding miscibility prediction method of the present invention is as follows:

[0007] A method for predicting the degree of miscibility in carbon dioxide flooding includes the following steps:

[0008] (1) Based on the PVT parameter field reflecting the actual changes in formation fluid properties in the study area and the mole fraction of each component in the typical well oil phase, combined with the reservoir physical parameters, well spacing and injection-production parameters in the study area, a three-dimensional gridded numerical model of the miscible flooding in the study area is established.

[0009] (2) A three-dimensional gridded numerical model for CO2 flooding was used to simulate CO2 flooding. The mole fractions of all components in the oil and gas phases at each three-dimensional grid in the study area were determined at the target time after CO2 flooding. The miscibility coefficients at each three-dimensional grid were calculated to characterize the degree of difference between the gas and oil phase components.

[0010] (3) Based on the mixing coefficient at each three-dimensional grid, calculate the ratio of the volume of all three-dimensional grids with a mixing coefficient of 1 to the total volume of CO2 driving wave in the study area, and obtain the degree of CO2 driving mixing in the study area.

[0011] The method for predicting the miscibility of carbon dioxide flooding in this invention, starting from the microscopic perspective of component changes, directly and quantitatively characterizes the reservoir miscibility after CO2 flooding. After CO2 injection into the reservoir, the miscibility coefficient is used to characterize the changes in various components in the crude oil within the affected area, thereby calculating the CO2 flooding miscibility within that area. This allows for the evaluation of the CO2 flooding effect in the target block. This method improves the prediction accuracy of carbon dioxide flooding miscibility, provides a basis and technical support for evaluating the effect of injected gas blocks, and can also guide the preparation of CO2 flooding reservoir engineering plans.

[0012] Preferably, the PVT parameters include saturation pressure, relative volume of formation crude oil, crude oil viscosity at saturation pressure, dissolved gas-oil ratio, and crude oil density at saturation pressure.

[0013] Preferably, the PVT parameter field is determined by the following method: collecting fluid characteristic data from typical wells in the study area, conducting experiments using the fluid characteristic data, and obtaining the PVT parameter field that reflects the actual changes in the properties of the formation fluid.

[0014] Preferably, the fluid characteristic data includes the mole fraction of each component in the oil phase, and the temperature, pressure, and viscosity of the oil phase under formation conditions.

[0015] Preferably, the experiment is an equal-component expansion CCE experiment, a flash evaporation experiment, or a CO2 injection expansion experiment.

[0016] Preferably, the reservoir physical properties include pressure, porosity, permeability, and oil saturation.

[0017] Preferably, the method for predicting the degree of miscibility of carbon dioxide flooding further includes the following steps: performing pseudo-component processing on all components other than CO2 in the oil phase of typical wells in the study area to determine the composition and mole fraction of each pseudo-component; and then using the mole fraction of each pseudo-component to establish a three-dimensional gridded numerical model of miscibility flooding in the study area.

[0018] Preferably, the pseudo-component treatment involves merging two or more components with similar molecular weights and properties to form a pseudo-component.

[0019] Preferably, the miscibility coefficient is calculated as follows:

[0020]

[0021] Where α is the miscibility coefficient; n is the number of pseudo-components at the 3D mesh; i represents the i-th pseudo-component at the 3D mesh; x i y represents the mole fraction of the i-th pseudo-component in the gas phase at the three-dimensional grid after CO2 displacement; i x represents the mole fraction of the i-th pseudo-component in the oil phase at the 3D grid after CO2 flooding; ic The mole fraction of the i-th pseudo-component in the gas phase at the CO2 precursor 3D grid is equal to 0; y ic This represents the mole fraction of the i-th pseudo-component in the oil phase at the CO2-driven three-dimensional grid.

[0022] Preferably, the formula for calculating the degree of CO2 flooding miscibility in the study area is as follows:

[0023]

[0024] Among them, V 混相程度 V represents the degree of CO2 flooding miscibility in the study area. 混相 V represents the total volume of all three-dimensional meshes with a miscibility coefficient of 1 within the study area after CO2 flooding. 总 This represents the CO2-driven wave and total volume within the study area. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the carbon dioxide flooding miscibility prediction method according to an embodiment of the present invention.

[0026] Figure 2 This refers to the three-dimensional meshed numerical model of the mixed-phase drive established in this embodiment of the invention;

[0027] Figure 3 This is a diagram showing the distribution of miscibility coefficients in an embodiment of the present invention;

[0028] Figure 4 This is a schematic diagram of CO2 wave propulsion and overall volume in an embodiment of the present invention;

[0029] Figure 5 This is a schematic diagram of the total volume of all three-dimensional meshes with a mixing coefficient of 1 within the CO2-driven wave range in an embodiment of the present invention.

[0030] Figure 6 This is a schematic diagram illustrating the relationship between improved recovery rate and improved miscibility in this invention;

[0031] Figure 7 This is a schematic diagram of the CO2-driven miscibility of each block in this invention. Detailed Implementation

[0032] The method for predicting the miscibility of carbon dioxide flooding in this invention is an improved invention. This method addresses the problem that current numerical simulations used to predict the miscibility of carbon dioxide flooding, which rely on factors such as the miscibility volume factor Cp, near-miscibility volume factor Cs, and CO2 component sweep efficiency Cc, are subject to component changes during the carbon dioxide flooding process, resulting in poor prediction accuracy. Instead, this method uses the difference in components between the gas and oil phases after CO2 flooding to characterize the miscibility of the carbon dioxide flooding.

[0033] Example

[0034] The method for predicting the miscibility of carbon dioxide flooding in this embodiment takes an oilfield as the study area, such as... Figure 1 As shown, the specific steps include:

[0035] (1) Collect fluid characteristic data from typical wells in the study area (fluid characteristic data includes the mole fraction of each component in the oil phase, and the temperature, pressure, and viscosity of the oil phase under formation conditions). Use the fluid characteristic data to conduct isocomponent expansion CCE experiments, flash evaporation experiments, and CO2 injection expansion experiments to obtain PVT parameter fields reflecting the actual changes in formation fluid properties. PVT parameters include saturation pressure, relative volume of formation crude oil, crude oil viscosity at saturation pressure, dissolved gas-oil ratio, and crude oil density at saturation pressure. Among them, the isocomponent expansion CCE experiment refers to the determination of the expansion capacity of the fluid and the changes in fluid physical parameters above saturation pressure when the formation system pressure begins to gradually decrease and the saturation pressure and relative volume of the fluid change, under the condition that the formation temperature and system composition remain unchanged. The specific method of the flash evaporation experiment is as follows: ① Heating; heat the liquid to be treated to a certain temperature to make it saturated. ② Decompression; reduce the pressure of the system to make the pressure of the liquid lower than its saturated vapor pressure, thereby initiating the boiling of the liquid. ③ Evaporation; after the liquid begins to boil, the molecules in the liquid are rapidly converted into vapor, thereby achieving separation and concentration. During evaporation, the lighter, more volatile components in the liquid evaporate first, while the heavier, less volatile components remain in the liquid. ④ Separation: The evaporated vapor and the unevaporated liquid are separated by a separator. The separator is typically a condenser, which condenses the vapor into liquid and then separates it from the unevaporated liquid. ⑤ Concentration: By controlling the conditions of the flash evaporation operation, liquid concentration can be achieved. The evaporated vapor contains volatile components from the liquid; after condensation into liquid by a condenser, a concentrated liquid can be obtained. The specific method for the CO2 expansion experiment is as follows: A known amount of CO2 gas is transferred to a PVT container containing a constant amount of crude oil. The pressure is then increased until all the CO2 gas dissolves in the crude oil. The container pressure is then gradually reduced until a small amount of bubbles are observed. The pressure reduction is stopped, and the saturation pressure and expansion volume at this state are recorded. Then, a larger amount of gas than in the previous experiment is injected, and the new saturation pressure and expansion volume are recorded.

[0036] In this embodiment, the mole fraction of each component in a typical well oil phase was determined by chemical analysis, and the results are shown in Table 1.

[0037] Table 1. Mole fractions of various components in typical well oil phases.

[0038]

[0039] (2) In order to reduce the number of equations to be solved and improve computational efficiency, pseudo-component processing can be performed on all components in the typical well oil phase except CO2. Pseudo-component processing is to merge two or more components in the oil phase to form a new pseudo-component. The principle of merging components during pseudo-component processing is to merge two or more components with similar molecular weights and properties to form a pseudo-component.

[0040] In this embodiment, following the above principles, during the pseudo-component processing, N2 and methane are grouped into one pseudo-component, ethane and propane into one pseudo-component, n-butane, isobutane, n-pentane, and isopentane into one pseudo-component, hexane, heptane, octane, and nonane into one pseudo-component, hydrocarbons with 10 to 15 carbon atoms into one pseudo-component, and hydrocarbons with 16 or more carbon atoms into one pseudo-component. The composition and mole fraction of each pseudo-component obtained from the pseudo-component processing are shown in Table 2. The mole fraction of a pseudo-component is equal to the sum of the mole fractions of all components within that pseudo-component.

[0041] Table 2. Composition and mole fraction of each pseudo-component obtained from the pseudo-component treatment.

[0042] Composition of pseudo-components mole fraction <![CDATA[CO2]]> 0.0329 <![CDATA[N2-C1]]> 0.1453 <![CDATA[C2-C3]]> 0.1064 <![CDATA[C4-C5]]> 0.0866 <![CDATA[C6-C9]]> 0.2038 <![CDATA[C 10 -C 15 ]]> 0.1813 <![CDATA[C 16+ ]]> 0.2439 total 1

[0043] (3) Using the PVT parameter field determined in step (1) and the mole fraction of each pseudo-component determined in step (2), combined with the reservoir physical properties (pressure, porosity, permeability, oil saturation), well spacing and injection-production parameters of the study area, a three-dimensional gridded numerical model of the study area for miscible flooding is established on the basis of the three-dimensional geological model (structural model, sedimentary facies model, attribute model).

[0044] In this embodiment, the reservoir physical parameters (pressure, porosity, permeability, oil saturation), well spacing, and injection-production parameters are as follows: original formation pressure is 34.5 MPa, average porosity is 12.5%, average permeability is 3.9 mD, average oil saturation is 0.53, average well spacing is 270 m, and cumulative injection-production ratio is 1.3. In this embodiment, the specific steps for establishing a three-dimensional meshed numerical model of the miscible flooding in the study area using the commercial tNavigator numerical simulation software are as follows:

[0045] ① Import the completion data of relevant wells into the three-dimensional geological grid model, such as perforation date, perforation depth and skin coefficient of the well, and complete the production dynamic settings of relevant wells;

[0046] ② Input the three-phase permeability test data of oil, gas and water obtained from the indoor core displacement experiment, and complete the permeability curve setting;

[0047] ③ The properties of the three-phase fluids (oil, gas, and water) obtained from the indoor high-temperature and high-pressure physical property test were fitted using PVT fitting software, including the fitting of parameters such as viscosity, density, bubble point pressure, dew point pressure, and gas-oil ratio of the three phases (oil, gas, and water).

[0048] ④ Input the PVT software fitting results into the above mesh model to complete the setting of the physical properties of the three-phase high-temperature and high-pressure fluids of oil, gas and water;

[0049] ⑤ Set the development strategy, select gas injection development as the development method, select CO2 as the injected gas component, and complete the establishment of the miscible flooding numerical model.

[0050] The three-dimensional meshed numerical model of the mixed-phase drive established in this embodiment is as follows: Figure 2 As shown. The principle of dividing the three-dimensional grid is: taking into account the well network density and the oil layer thickness of each well in the vertical direction, reasonably dividing the planar and vertical grids, and subdividing them into smaller layers.

[0051] (4) Using the three-dimensional gridded numerical model of miscible flooding established in step (3), CO2 flooding numerical simulation was performed to determine the mole fraction of all pseudo-components in the oil and gas phases at each three-dimensional grid in the study area at the target time during the CO2 flooding oil production stage. The miscibility coefficient used to characterize the degree of difference between gas and oil phase components at each three-dimensional grid was calculated. The calculation method of the miscibility coefficient is as follows:

[0052]

[0053] Where α is the miscibility coefficient; n is the number of pseudo-components at the 3D mesh; i represents the i-th pseudo-component at the 3D mesh; x i y represents the mole fraction of the i-th pseudo-component in the gas phase at the three-dimensional grid after CO2 displacement; i x represents the mole fraction of the i-th pseudo-component in the oil phase at the 3D grid after CO2 flooding; ic The mole fraction of the i-th pseudo-component in the gas phase at the CO2 precursor 3D grid is equal to 0; y ic This represents the mole fraction of the i-th pseudo-component in the oil phase at the three-dimensional grid before CO2 flooding. In this embodiment, the degree of miscibility at the three-dimensional grid is evaluated by the miscibility coefficient. When carbon dioxide has not been flooded, the miscibility coefficient is equal to 0. During CO2 flooding, light components begin to be extracted, and the higher the value, the higher the degree of extraction, gradually becoming miscible. At this time, the miscibility coefficient is greater than 0 and less than 1. After CO2 flooding, when the gas phase and oil phase have the same composition, they are miscible, and the miscibility coefficient is equal to 1.

[0054] In this embodiment, during the CO2 flooding numerical simulation, well Q58-5 was used as the injection well, and wells Q58-6 and Q58-7 were used as production wells. The three-dimensional grid A of the study area was obtained through numerical simulation and calculation in the third year after CO2 flooding (e.g., ...). Figure 2The miscibility coefficient at point A (as shown in the figure) is calculated as follows: The x-axis coordinate of point A in the 3D mesh is 3, the y-axis coordinate is 75, and the z-axis coordinate is 1. The mole fraction of the oil phase for pseudo-component N2-C1 is 0.0833, and the mole fraction of the gas phase is 0.42; the mole fraction of the oil phase for pseudo-component C2-C3 is 0.04259, and the mole fraction of the gas phase is 0.1722; the mole fraction of the oil phase for pseudo-component C4-C5 is 0.1431, and the mole fraction of the gas phase is 0.1976; the mole fraction of the oil phase for pseudo-component C6-C9 is 0.0435, and the mole fraction of the gas phase is 0.0253; the mole fraction of the oil phase for pseudo-component C... 10 -C 15 The oil phase mole fraction was 0.1106, and the gas phase mole fraction was 0.0073; pseudo-component C 16+ The oil phase mole fraction is 0.5322, and the gas phase mole fraction is 0. Substituting these components into the above formula, the miscibility coefficient is calculated. The miscibility coefficients at other 3D meshes are calculated using the same method, and the results are plotted as shown below. Figure 3 The diagram shows the distribution of the miscibility coefficient.

[0055] (5) Based on the miscibility coefficient at each three-dimensional grid, calculate the ratio of the volume of all three-dimensional grids with a miscibility coefficient equal to 1 to the total volume of CO2-driven wave in the study area, and obtain the degree of CO2-driven miscibility in the study area. The calculation formula is as follows:

[0056]

[0057] Among them, V 混相程度 V represents the degree of CO2 flooding miscibility in the study area. 混相 V represents the total volume of all three-dimensional meshes with a miscibility coefficient of 1 within the study area after CO2 flooding. 总 This represents the total volume swept by CO2 within the study area. The volume of the 3D meshes with a miscibility coefficient of 1 within the study area after CO2 displacement is determined statistically in the numerical model. The total volume of all 3D meshes with a miscibility coefficient of 1 within the study area after CO2 displacement is equal to the sum of the volumes of all 3D meshes with a miscibility coefficient of 1 within the study area after CO2 displacement. The total volume swept by CO2 within the study area is determined statistically within the CO2-sweeped area in the numerical model.

[0058] In this embodiment, the schematic diagram of CO2 wave drive and overall volume is as follows: Figure 4 As shown, the total volume of all three-dimensional meshes with a miscibility coefficient of 1 within the CO2-driven wave range is as follows: Figure 5 As shown in the figure. The calculation results show that the miscibility of CO2 flooding in the study area is 83%. To visually demonstrate the effect of CO2 flooding, a numerical simulation of CO2 flooding was performed using the method described above. By simulating the miscibility and recovery rate at different times after CO2 flooding, the results are plotted as shown in the figure. Figure 6The diagram illustrates the relationship between improved recovery rate and miscibility. Figure 6 It can be seen that when the miscibility of CO2 flooding is 80%, the recovery rate is increased by 13.2%, which is a relatively high recovery rate.

[0059] Based on the simulation results, using well Q58-5 as the gas injection well and wells Q58-6 and Q58-7 as production wells, CO2 flooding was carried out. The production data are as follows: the cumulative gas injection volume is 13,694 t, and the cumulative oil increase volume is 4,268 t. The above results show that the prediction method for the degree of miscibility of carbon dioxide flooding in this invention has high prediction accuracy.

[0060] Simultaneously, the CO2 flooding miscibility of other blocks was predicted using the same method. The CO2 flooding miscibility of each block is as follows: Figure 7 As shown in the figure. Based on the prediction results, CO2 flooding was carried out on oil wells with high miscibility. The results showed that the higher the miscibility, the better the CO2 flooding effect and the higher the recovery rate of the block.

Claims

1. A method for predicting the degree of miscibility in carbon dioxide flooding, characterized in that, Includes the following steps: (1) Based on the PVT parameter field reflecting the actual changes in formation fluid properties in the study area and the mole fraction of each component in the typical well oil phase, combined with the reservoir physical parameters, well spacing and injection-production parameters in the study area, a three-dimensional gridded numerical model of the miscible flooding in the study area is established. (2) A three-dimensional gridded numerical model for CO2 flooding was used to simulate CO2 flooding. The mole fractions of all components in the oil and gas phases at each three-dimensional grid in the study area were determined at the target time after CO2 flooding. The miscibility coefficients at each three-dimensional grid were calculated to characterize the degree of difference between the gas and oil phase components. (3) Based on the mixing coefficient at each three-dimensional grid, calculate the ratio of the volume of all three-dimensional grids with a mixing coefficient of 1 to the total volume of CO2 driving wave in the study area, and obtain the degree of CO2 driving mixing in the study area.

2. The method for predicting the degree of miscibility in carbon dioxide flooding as described in claim 1, characterized in that, The PVT parameters include saturation pressure, relative volume of formation crude oil, crude oil viscosity at saturation pressure, dissolved gas-oil ratio, and crude oil density at saturation pressure.

3. The method for predicting the degree of miscibility in carbon dioxide flooding as described in claim 1 or 2, characterized in that, The PVT parameter field is determined by the following method: collecting fluid characteristic data from typical wells in the study area, conducting experiments using the fluid characteristic data, and obtaining the PVT parameter field that reflects the actual changes in the properties of the formation fluid.

4. The method for predicting the degree of miscibility in carbon dioxide flooding as described in claim 3, characterized in that, The fluid characteristic data includes the mole fraction of each component in the oil phase, and the temperature, pressure, and viscosity of the oil phase under formation conditions.

5. The method for predicting the degree of miscibility in carbon dioxide flooding as described in claim 3, characterized in that, The experiments included equal-component expansion CCE experiment, flash evaporation experiment, and CO2 injection expansion experiment.

6. The method for predicting the degree of miscibility in carbon dioxide flooding as described in claim 1, characterized in that, The reservoir physical properties include pressure, porosity, permeability, and oil saturation.

7. The method for predicting the degree of miscibility in carbon dioxide flooding as described in claim 1, characterized in that, The method for predicting the degree of miscibility in carbon dioxide flooding also includes the following steps: performing pseudo-component processing on all components other than CO2 in the oil phase of typical wells in the study area to determine the composition and mole fraction of each pseudo-component; and then using the mole fraction of each pseudo-component to establish a three-dimensional gridded numerical model of miscibility flooding in the study area.

8. The method for predicting the degree of miscibility in carbon dioxide flooding as described in claim 7, characterized in that, The pseudo-component processing involves grouping two or more components with similar molecular weights and properties into a single pseudo-component.

9. The method for predicting the degree of miscibility in carbon dioxide flooding as described in claim 7 or 8, characterized in that, The method for calculating the miscibility coefficient is as follows: Where α is the miscibility coefficient; n is the number of pseudo-components at the 3D mesh; i represents the i-th pseudo-component at the 3D mesh; x i y represents the mole fraction of the i-th pseudo-component in the gas phase at the three-dimensional grid after CO2 displacement; i x represents the mole fraction of the i-th pseudo-component in the oil phase at the 3D grid after CO2 flooding; ic The mole fraction of the i-th pseudo-component in the gas phase at the CO2 precursor 3D grid is equal to 0; y ic This represents the mole fraction of the i-th pseudo-component in the oil phase at the CO2-driven three-dimensional grid.

10. The method for predicting the degree of miscibility in carbon dioxide flooding as described in claim 1, characterized in that, The formula for calculating the miscibility of CO2 flooding in the study area is as follows: Among them, V 混相程度 V represents the degree of CO2 flooding miscibility in the study area. 混相 V represents the total volume of all three-dimensional meshes with a miscibility coefficient of 1 within the study area after CO2 flooding. 总 This represents the CO2-driven wave and total volume within the study area.

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