Online correction method and system for underground supercritical carbon dioxide venturi tube flowmeter

Abstract

The invention provides an online correction method and system for an underground supercritical carbon dioxide venturi tube flowmeter. The online correction method comprises the steps that an initial curve of underground correction is obtained; a corrected zero drift value is obtained, and zero calibration is completed; performing flow coefficient correction on each to-be-corrected well layer of the target well, specifically, keeping a non-detection well layer closed, injecting supercritical carbon dioxide into the to-be-corrected well layer from the ground, and collecting fluid pressure and temperature data; the fluid density and expansion coefficient are calculated through a state equation or an NIST database, the first underground flow is calculated in combination with the outflow coefficient curve, and error comparison is conducted on the first underground flow and the ground injection flow; and if the error exceeds the preset range, calling a corresponding outflow coefficient based on the interval where the Reynolds number is located through an iteration method, and recalculating the second flow until the difference value of the two times of flow calculation meets the precision requirement. On-line and high-precision correction of the downhole flowmeter is achieved, and the measurement accuracy of the supercritical carbon dioxide injection flow is remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of oil and gas field development and measurement technology, specifically providing an online calibration method and system for downhole supercritical carbon dioxide venturi flowmeters. Background Technology

[0002] In the development of deep oil and gas reservoirs, supercritical carbon dioxide (SC2) is widely used to enhance oil recovery due to its unique physicochemical properties. To achieve efficient displacement, intelligent and precise stratified injection and production technology is crucial. This technology achieves independent measurement and control of SC2 injection flow rates at different layers by installing gas distributors and flow meters in each vertical oil layer. The accuracy of flow measurement directly determines the effectiveness of stratified control and the efficiency of oil sequestration.

[0003] Venturi tube differential pressure flow meters are a common choice for downhole flow measurement due to their simple structure, high reliability, and wide applicability. The flow calculation formula is as follows: Where d and D are structural parameters, C is the outflow coefficient, ε is the expansion coefficient, ρ is the fluid density, ΔP is the differential pressure, and K0 is the surface-calibrated flow coefficient. However, in practical downhole applications, severe challenges arise: First, the physical properties of supercritical carbon dioxide (such as density ρ and viscosity μ) are extremely sensitive to temperature and pressure. The operating conditions of different well formations (P1, T1; P2, T2) and even the same well formation at different times vary greatly, leading to significant changes in ρ, ε, and C. Second, impurities (water, oxygen, etc.) in SCO2 corrode the pipe wall and produce scale, which will irreversibly change the throat diameter d and pipe diameter D of the venturi tube over long-term operation. Third, the high-temperature and high-pressure environment downhole easily causes zero-point drift (ΔP0) in the differential pressure sensor. These factors work together to cause the fixed coefficient K0 calibrated on the surface to deviate significantly from the actual downhole coefficient K, introducing continuous measurement errors.

[0004] Existing technical solutions mostly focus on solving single problems. For example, patent CN104655213B relates to supercritical carbon dioxide flow measurement, focusing on density calculation based on the equation of state, but it does not address the structural parameter changes caused by long-term corrosion and scaling, nor does it provide systematic correction for sensor zero-point drift and nonlinear changes in the discharge coefficient. Other solutions either use corrosion-resistant materials or rely on periodic shutdowns for maintenance, neither of which can achieve low-cost, high-precision online real-time calibration.

[0005] Therefore, there is an urgent need for a method and system that can comprehensively cope with the complex and ever-changing downhole environment and perform online, adaptive calibration of flow meters. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, a calibrating method and system for Venturi flowmeters that can be calibrated online, in real time, and adaptively is provided to improve the flow measurement accuracy of Venturi flowmeters when measuring supercritical carbon dioxide downhole.

[0007] In a first aspect, the present invention provides an online calibration method for a downhole supercritical carbon dioxide venturi flowmeter, comprising the following steps: Obtain the initial curve for downhole correction.

[0008] Obtain the corrected zero drift value Complete the zero-point calibration; The flow coefficients of each formation to be corrected in the target well are calibrated. Specifically: a) Keep the non-detected wells closed, inject supercritical carbon dioxide at a flow rate of Q into the well to be calibrated from the ground, and simultaneously collect the fluid pressure P and temperature T and transmit the data to the ground host computer; b) Calculate the fluid density ρ using the equation of state or the NIST official database, and calculate the expansion coefficient. The first downhole flow rate was calculated by combining the outflow coefficient curve. Compared with the first traffic The error between the injected flow rate Q; c) If the error is not within the preset range, the error is corrected using an iterative method; through the first flow rate The fluid velocity V is calculated, and then the Reynolds number Re is calculated using the Reynolds number formula. The curve interval to which Re belongs is then determined. Based on the corresponding Reynolds number curve interval, the discharge coefficient C is retrieved, and the second flow rate is recalculated using the flow rate calculation formula. ; d) Compare the first flow rate and If the difference is within a preset range, it indicates that the accuracy meets the standard; otherwise, the above process is repeated to continue the iteration.

[0009] Preferably, the process of obtaining the initial curve for downhole correction includes: calculating the outflow coefficient C under different operating conditions based on the actual flow rate Q of the surface standard flow meter, the venturi differential pressure, and the fluid temperature T and pressure P; The outflow coefficient C-Reynolds number Re relationship curve is plotted based on the outflow coefficient C value, and the curve is piecewise linearized to obtain the initial curve for downhole correction.

[0010] Preferably, the flow rate calculation formula is: when when in, Let be the lower limit of the Reynolds number interval for the i-th segment and the corresponding outflow coefficient; K is the structural correction coefficient obtained from the surface calibration before well running. This is the downhole online correction coefficient for the flow meter. After the flow meter has been working downhole for a period of time, it is used to correct the deviation of the flow correction coefficient caused by changes in structural parameters and fluid. The calculation formula is: The calculation formula is: Where Q, ΔP, and ρ are the flow rate, differential pressure, and density obtained from surface calibration, respectively; and Q', ΔP', and ρ' are the flow rate, differential pressure, and density obtained from downhole online calibration, respectively.

[0011] Preferably, the corrected zero-point drift value is obtained. The zero-point calibration process includes: controlling the wellbore valves to close all well layers in the target well except for the layer to be calibrated, shutting off the surface gas injection system, and waiting for the fluid in the downhole pipeline to stagnate, at which point the flow rate Q=0; recording the downhole pressure value P and obtaining the differential pressure. According to the flow rate calculation formula, when the flow rate Q=0, Therefore, the corrected zero-point drift value is obtained. Complete the zero-point calibration.

[0012] Preferably, the In the formula, Re is the calculated Reynolds number, ρ is the fluid density, v is the fluid velocity, L is the fluid characteristic length, and μ is the fluid viscosity.

[0013] Preferably, the method further includes: real-time online correction. Specifically, under normal system operation, the downhole control board transmits data to the host computer every second, and the host computer automatically retrieves the data stored in the corresponding well layer for calculation to obtain the real-time downhole flow rate.

[0014] Preferably, the real-time online correction further includes: if the flow error exceeds a preset value, the flow is directly corrected via a host computer.

[0015] Preferably, calculating the fluid density ρ using the equation of state includes: The PR equation is: In the formula, P is pressure and T is temperature. Let be the molar volume, R be the universal gas constant, a be an energy parameter related to molecular attraction, which depends on the substance itself, and b be a volume parameter related to molecular size, which also depends on the substance itself. This is a temperature-dependent correction factor; The PR equation can be rearranged into a cubic equation for the molar volume Vm: By substituting the relevant parameters—that is, calculating the parameters of the cubic equation and solving the cubic equation—we can obtain the answer. Finally, the density ρ is calculated using the following formula: In the formula, M is the molar mass of the substance.

[0016] Preferably, the coefficient of thermal expansion is calculated. The process includes: for changes in the expansion coefficient caused by changes in operating conditions, the expansion coefficient is the ratio of the compressible theoretical flow rate to the incompressible theoretical flow rate, that is: In the formula, For compressible theoretical flow rate, Assuming the flow is incompressible, the theoretical flow rate is: In the formula, This represents the cross-sectional area of ​​the throat. Density at the entrance; For the compressible theoretical flow rate, the isentropic model is used for derivation: In the formula, For specific heat ratio, For the pressure at the entrance, Density at the entrance This refers to the pressure in the throat.

[0017] Secondly, the present invention provides an online calibration system for a downhole supercritical carbon dioxide venturi flowmeter, comprising: a first acquisition unit for acquiring an initial curve for downhole calibration; and a second acquisition unit for acquiring the zero-point drift value after calibration. The zero-point calibration is completed; the calibration unit is used to calibrate the flow coefficient of each well layer in the target well. Specifically: a) Keeping the non-detected well layers closed, supercritical carbon dioxide with a flow rate of Q is injected from the surface into the well layer to be calibrated, while simultaneously collecting the fluid pressure P and temperature T and transmitting the data to the ground host computer; b) Calculating the fluid density ρ using the equation of state or the NIST official database, and calculating the expansion coefficient. The first downhole flow rate was calculated by combining the outflow coefficient curve. Compared with the first traffic c) If the error is not within a preset range, the error is corrected using an iterative method; through the first flow... The fluid velocity V is calculated, and then the Reynolds number Re is calculated using the Reynolds number formula. The curve interval to which Re belongs is then determined. Based on the corresponding Reynolds number curve interval, the discharge coefficient C is retrieved, and the second flow rate is recalculated using the flow rate calculation formula. d) Compare the first flow rate and If the difference is within a preset range, it indicates that the accuracy meets the standard; otherwise, the above process is repeated to continue the iteration.

[0018] The beneficial effects of this invention are as follows: It proposes a flow calculation formula that can resist external interference in real time, and improves the calculation accuracy and precision of the formula by introducing structural correction parameters and zero points to correct for changing operating conditions and structural corrosion.

[0019] Using piecewise linearization in improved flow calculation methods can significantly reduce the increase in errors caused by the complexity of calibration curves. It can also expand the measurable range of calibration curves, making previously unusable curves usable through linearization.

[0020] The iterative method solves the problem of direct calculation due to the interrelationship between the flow coefficient and the flow rate. By using reasonable initial values ​​and a certain number of iterations, the actual flow rate value can be quickly approximated, improving the accuracy of flow rate calculation.

[0021] A method for correcting flow rate and zero point for different well layers was proposed, making flow rate correction more systematic and complete. Attached Figure Description

[0022] The disclosure of this invention will become more readily understood with reference to the accompanying drawings. It will be readily understood by those skilled in the art that these drawings are for illustrative purposes only and are not intended to limit the scope of protection of this invention. Furthermore, similar numbers in the drawings are used to denote similar components, wherein: Figure 1 This is a detailed algorithm flowchart of the iterative correction calculation steps according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the outflow coefficient C as a function of Reynolds number Re, showing the curve shift under different operating conditions (P,T); Figure 3 This is a schematic diagram of an implementation interface of the ground control center host computer software according to an embodiment of the present invention; Figure 4 This is a schematic diagram of an implementation interface of the ground control center host computer software according to an embodiment of the present invention. Detailed Implementation

[0023] Some embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0024] Example 1 like Figure 1-4 As shown, this invention provides an online calibration method for a downhole supercritical carbon dioxide venturi flowmeter, comprising the following steps: Step S1: Obtain the initial curve for downhole correction.

[0025] In this embodiment, a fluid delivery device and a temperature and pressure control unit are built on the ground to simulate downhole temperature and pressure. Supercritical carbon dioxide is controlled to flow through a throttling device at different flow rates (covering the actual downhole measurement range). The actual flow rate Q of the ground standard flow meter, the Venturi differential pressure, and the fluid T / P are recorded, and the discharge coefficient C under different operating conditions is calculated. Based on the discharge coefficient C value, a discharge coefficient C-Reynolds number Re relationship curve is plotted, and the curve is piecewise linearized to serve as the initial curve for downhole correction.

[0026] The downhole equipment installation and commissioning involves installing the calibrated venturi tube, pressure sensor, temperature sensor, control board, etc., into the pressure-resistant pipe, and then connecting it to the target well formation pipeline. The host computer is then started to test whether the downhole control board can function properly, correctly transmit P / T signals, and whether the surface host computer can calculate real-time flow and send relevant commands to the downhole control board.

[0027] Step S2: Obtain the corrected zero-point drift value Complete the zero-point calibration.

[0028] In this embodiment, the target well is closed except for the "well layer to be corrected" by controlling the well layer valve, and the surface gas injection system is shut down. The fluid in the downhole pipeline is then allowed to stand still, at which point the flow rate Q=0.

[0029] The downhole system uses dual pressure sensors to obtain differential pressure, records the downhole pressure value P, and obtains the differential pressure. From equation (6), it can be seen that when the flow rate Q = 0, Therefore, the corrected zero-point drift value is obtained. Complete the zero-point calibration and... Stored in the ground control system.

[0030] Step S3: Perform flow coefficient correction on each well layer to be corrected in the target well. Specifically: a) Keep the non-detected wells closed, inject supercritical carbon dioxide at a flow rate of Q into the well to be calibrated from the ground, and simultaneously collect the fluid pressure P and temperature T and transmit the data to the ground host computer; b) Calculate the fluid density ρ using the equation of state or the NIST official database, and calculate the expansion coefficient. The first downhole flow rate was calculated by combining the outflow coefficient curve. Compared with the first traffic The error between the injected flow rate Q; c) If the error is not within the preset range, the error is corrected using an iterative method; through the first flow rate The fluid velocity V is calculated, and then the Reynolds number Re is calculated using the Reynolds number formula. The curve interval to which Re belongs is then determined. Based on the corresponding Reynolds number curve interval, the discharge coefficient C is retrieved, and the second flow rate is recalculated using the flow rate calculation formula. ; d) Compare the first flow rate and If the difference is within a preset range, it indicates that the accuracy meets the standard; otherwise, the above process is repeated to continue the iteration.

[0031] In one embodiment, step S3 is repeated to correct other well layers of the target well.

[0032] In one embodiment, a structural correction factor K is introduced to correct the calculation deviation caused by changes in pipeline structural parameters, resulting in: By rearranging all the pipe structure parameters in the flow formula to the left side, we can see that when the structure parameters change, the left side of the formula can be approximated as a constant. Therefore, correcting the structure parameters is essentially correcting this constant to make it equal to the constant before corrosion or scaling. This yields: In the formula, The diameter of the venturi throat after scaling or corrosion. The inlet and outlet diameters of the venturi tube after scaling or corrosion.

[0033] Furthermore, the formula for calculating the structural correction coefficient K is derived: .

[0034] In one embodiment, for fluid density changes caused by altered operating conditions, the Peng-Robinson (PR) equation of state is used to calculate the density under changing conditions in real time. This equation is relatively simple in form, fast in calculation, widely applicable in engineering, and has good accuracy for gases such as carbon dioxide. The formula is as follows: In the formula, P is pressure (Pa) and T is temperature (K). molar volume ( / mol), R is the universal gas constant, and a is the energy parameter related to molecular attraction (Pa·m). 6 / mol 2 ), depending on the substance itself, b is a volume parameter (m) related to the molecular size. 3 ( / mol), depends on the substance itself. This is a temperature-dependent correction factor.

[0035] The PR equation can be rearranged into a cubic equation for the molar volume Vm: By substituting the relevant parameters, the parameters of the cubic equation can be calculated and the cubic equation can be solved to obtain the answer. Finally, the density ρ is calculated using the following formula, thereby eliminating the influence of changing operating conditions on the density.

[0036] In the formula, M is the molar mass of the substance (kg / mol).

[0037] In one embodiment, the formula is corrected for changes in the expansion coefficient caused by changes in operating conditions. The expansion coefficient is the ratio of the theoretical compressible flow rate to the theoretical incompressible flow rate, i.e. In the formula, For compressible theoretical flow rate, This is the incompressible theoretical flow rate.

[0038] Assuming the flow is incompressible, the theoretical flow rate is: In the formula, This represents the cross-sectional area of ​​the throat. This refers to the density at the entrance.

[0039] For the compressible theoretical flow rate, the isentropic model is used for derivation: In the formula, For specific heat ratio, For the pressure at the entrance, Density at the entrance This refers to the pressure in the throat.

[0040] In one embodiment, the nonlinear variation of the outflow coefficient is optimized. Specifically, a flow correction coefficient is introduced. This is a simple way to correct for measurement errors caused by changes in temperature and pressure. When temperature and pressure remain constant, the discharge coefficient C can be approximated as a constant when the flow rate is varied within a certain range. However, when temperature and pressure change slightly, the discharge coefficient will deviate to some extent. As follows: Figure 2 As shown, it can be observed that within a certain Reynolds number range, the discharge coefficients for different operating conditions can be approximately consistent, but a shift occurs within another Reynolds number range. To address this, a piecewise function is used for flow correction, dividing the Reynolds number into two parts: when when in, , Let be the lower limit of the Reynolds number interval for the i-th segment and the corresponding outflow coefficient, and K be the structural correction coefficient obtained from the surface calibration before well running. This is the downhole online correction coefficient for the flow meter. After the flow meter has been working downhole for a period of time, it is used to correct the deviation of the flow correction coefficient caused by changes in structural parameters and fluid. The calculation formula is: Where Q, ΔP, and ρ are the flow rate, differential pressure, and density obtained from surface calibration, respectively; and Q', ΔP', and ρ' are the flow rate, differential pressure, and density obtained from downhole online calibration, respectively.

[0041] When inconsistent operating conditions are observed after the flow meter is placed downhole, no special correction is needed for flow calculations before the Reynolds number critical point because the discharge coefficients are essentially the same under different operating conditions in this region. However, flow calculations after the Reynolds number critical point require correction. This is done by comparing the flow rate measured on the surface with the flow rate calculated from the downhole data to determine the flow correction factor. .

[0042] The above formula already includes the structural correction factor K and the downhole online flow correction factor. It integrates with the zero point, giving it excellent real-time anti-interference capabilities. It can easily handle changes in structural parameters caused by pipe scaling and corrosion due to prolonged placement in the well, as well as the zero-point drift problem of the differential pressure sensor.

[0043] In one embodiment, the concept of well layers is added to parameters such as structural parameters and differential pressure, extending the applicability of the formula from one well layer to multiple well layers: when when Where j represents the j-th well layer.

[0044] In one embodiment, the method further includes: real-time online correction. Specifically, under normal system operation, data is transmitted to the host computer in real time or within a preset time period. The host computer automatically retrieves the data stored in the corresponding well layer for calculation to obtain the real-time downhole flow rate. If the flow rate error exceeds a preset value, the flow rate is corrected based on the flow rate calculation formula.

[0045] In a possible online calibration method for a downhole supercritical carbon dioxide venturi flowmeter, the differential pressure flowmeter includes a throttling element (Venturi tube), a differential pressure sensor, a temperature sensor, a pressure sensor, a data transmission module, and a ground control center. The throttling element is installed inside the downhole gas pipeline. The differential pressure sensor is connected to the pressure taps upstream and downstream of the throttling element, respectively. The temperature sensor and pressure sensor are both installed in the pipeline upstream of the throttling element. Each sensor is connected to the ground control center via the data transmission module.

[0046] Before a differential pressure flowmeter is used downhole, a pre-calibration process must be completed in a surface laboratory. First, a simulated calibration system is built, which includes a fluid delivery device, a temperature control unit, a pressure control unit, a standard flow metering device, and a data acquisition and analysis unit. The fluid delivery device delivers supercritical carbon dioxide (the same medium being measured downhole). The temperature control unit adjusts the system temperature to 31.1℃~100℃ (simulating the downhole temperature range), and the pressure control unit adjusts the system pressure to 7.38MPa~30MPa (simulating the downhole pressure range). Then, the throttling element of the differential pressure flowmeter to be calibrated is installed in the test pipeline of the simulated calibration system. The fluid delivery device is started, allowing supercritical carbon dioxide to flow through the throttling element at different flow rates. The differential pressure flowmeter's sensor collects differential pressure, temperature, and pressure data, while simultaneously recording the corresponding flow rate values. The data acquisition and analysis unit calculates the flow coefficient deviation of the differential pressure flowmeter and generates a correction curve.

[0047] The calibrated differential pressure flowmeter is installed at the designated location in the downhole gas pipeline. The downhole supercritical carbon dioxide delivery system is then started. As the fluid flows through the throttling device, the differential pressure sensor collects the differential pressure signals upstream and downstream of the throttling device in real time, the temperature sensor collects the fluid temperature, and the pressure sensor collects the fluid pressure. The data transmission module transmits the collected differential pressure, temperature, and pressure data to the ground control center in real time via wired or wireless transmission.

[0048] After receiving the data, the ground control center initiates a built-in iterative calculation program. Based on real-time temperature and pressure data, this program calculates fluid parameters such as density and viscosity by calling a pre-set database of supercritical carbon dioxide physical properties. It then uses the thermodynamic equation of state to calculate the expansion coefficient of supercritical carbon dioxide. Finally, the differential pressure, expansion coefficient, fluid density, and throttling device geometry parameters are substituted into the optimized flow rate calculation formula.

[0049] Example 2 This invention discloses an online calibration system for a downhole supercritical carbon dioxide venturi flowmeter, comprising: The first acquisition unit is used to acquire the initial curve for downhole correction.

[0050] The second acquisition unit is used to acquire the corrected zero-point drift value. Complete the zero-point calibration; The correction unit is used to correct the flow coefficient of each well layer in the target well. Specifically: a) Keep the non-detected wells closed, inject supercritical carbon dioxide at a flow rate of Q into the well to be calibrated from the ground, and simultaneously collect the fluid pressure P and temperature T and transmit the data to the ground host computer; b) Calculate the fluid density ρ using the equation of state or the NIST official database, and calculate the expansion coefficient. The first downhole flow rate was calculated by combining the outflow coefficient curve. Compared with the first traffic The error between the injected flow rate Q; c) If the error is not within the preset range, the error is corrected using an iterative method; through the first flow rate The fluid velocity V is calculated, and then the Reynolds number Re is calculated using the Reynolds number formula. The curve interval to which Re belongs is then determined. Based on the corresponding Reynolds number curve interval, the discharge coefficient C is retrieved, and the second flow rate is recalculated using the flow rate calculation formula. ; d) Compare the first flow rate and If the difference is within a preset range, it indicates that the accuracy meets the standard; otherwise, the above process is repeated to continue the iteration.

[0051] Human-computer interface (UI): such as Figure 3-4 As shown, the interface design is intuitive.

[0052] Furthermore, it should be understood that since the various modules are only provided to illustrate the functional units of the device of the present invention, the physical devices corresponding to these modules may be the processor itself, or a part of the processor's software, hardware, or a combination of software and hardware. Therefore, the number of modules shown in the figures is merely illustrative.

[0053] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the original technical features, and the technical solutions resulting from these changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A method for online calibration of a downhole supercritical carbon dioxide venturi flowmeter, characterized in that, Includes the following steps: Obtain the initial curve for downhole correction; Obtain the corrected zero drift value Complete the zero-point calibration; The flow coefficients of each formation to be corrected in the target well are calibrated. Specifically: a) Keep the non-detected wells closed, inject supercritical carbon dioxide at a flow rate of Q into the well to be calibrated from the ground, and simultaneously collect the fluid pressure P and temperature T and transmit the data to the ground host computer; b) Calculate the fluid density ρ using the equation of state or the NIST official database, and calculate the expansion coefficient. The first downhole flow rate was calculated by combining the outflow coefficient curve. Compared with the first traffic The error between the injected flow rate Q; c) If the error is not within the preset range, the error is corrected using an iterative method; through the first flow rate The fluid velocity V is calculated, and then the Reynolds number Re is calculated using the Reynolds number formula. The curve interval to which Re belongs is then determined. Based on the corresponding Reynolds number curve interval, the discharge coefficient C is retrieved, and the second flow rate is recalculated using the flow rate calculation formula. ; d) Compare the first flow rate and If the difference is within a preset range, it indicates that the accuracy meets the standard; otherwise, the above process is repeated to continue the iteration.

2. The method according to claim 1, characterized in that, The process of obtaining the initial curve for downhole correction includes: calculating the discharge coefficient C under different operating conditions based on the actual flow rate Q of the surface standard flow meter, the venturi differential pressure, and the fluid temperature T and pressure P; The outflow coefficient C-Reynolds number Re relationship curve is plotted based on the outflow coefficient C value, and the curve is piecewise linearized to obtain the initial curve for downhole correction.

3. The method according to claim 1, characterized in that, The flow rate calculation formula is as follows: when when ; in, , Let be the lower limit of the Reynolds number interval for the i-th segment and the corresponding outflow coefficient, and K be the structural correction coefficient obtained from the surface calibration before well running. This is the downhole online correction coefficient for the flow meter. After the flow meter has been working downhole for a period of time, it is used to correct the deviation of the flow correction coefficient caused by changes in structural parameters and fluid. The calculation formula is: The calculation formula is: Where Q, ΔP, and ρ are the flow rate, differential pressure, and density obtained from surface calibration, respectively; and Q', ΔP', and ρ' are the flow rate, differential pressure, and density obtained from downhole online calibration, respectively.

4. The method according to claim 3, characterized in that, Obtain the corrected zero drift value The process of zero-point calibration includes: By controlling the well layer valves, close all well layers of the target well except the well layer to be corrected, shut down the surface gas injection system, and wait for the fluid in the downhole pipeline to stop, at which point the flow rate Q=0; Record the downhole pressure value P and obtain the differential pressure. According to the flow rate calculation formula, when the flow rate Q=0, Therefore, the corrected zero-point drift value is obtained. Complete the zero-point calibration.

5. The method according to claim 1, characterized in that, The In the formula, Re is the calculated Reynolds number, ρ is the fluid density, v is the fluid velocity, L is the fluid characteristic length, and μ is the fluid viscosity.

6. The method according to claim 1, characterized in that, The method also includes: real-time online correction. Specifically, under normal system conditions, data is transmitted to the host computer in real time or within a preset time period. The host computer automatically retrieves the data stored in the corresponding well layer for calculation to obtain the real-time downhole flow rate.

7. The method according to claim 6, characterized in that, The real-time online correction also includes: if the flow error exceeds a preset value, then correcting the flow based on the flow calculation formula.

8. The method according to claim 1, characterized in that, Calculating fluid density ρ using the equation of state includes: The PR equation is: In the formula, P is pressure and T is temperature. Let be the molar volume, R be the universal gas constant, a be an energy parameter related to molecular attraction, which depends on the substance itself, and b be a volume parameter related to molecular size, which also depends on the substance itself. This is a temperature-dependent correction factor; The PR equation can be rearranged into a cubic equation for the molar volume Vm: By substituting the relevant parameters—that is, calculating the parameters of the cubic equation and solving the cubic equation—we can obtain the answer. Finally, the density ρ is calculated using the following formula: In the formula, M is the molar mass of the substance.

9. The method according to claim 7, characterized in that, Calculate the expansion coefficient The process includes: for changes in the expansion coefficient caused by changes in operating conditions, the expansion coefficient is the ratio of the compressible theoretical flow rate to the incompressible theoretical flow rate, that is: In the formula, For compressible theoretical flow rate, Assuming the flow is incompressible, the theoretical flow rate is: In the formula, This represents the cross-sectional area of ​​the throat. Density at the entrance; For the compressible theoretical flow rate, the isentropic model is used for derivation: In the formula, For specific heat ratio, For the pressure at the entrance, Density at the entrance This refers to the pressure in the throat.

10. An online calibration system for a downhole supercritical carbon dioxide venturi flowmeter, characterized in that, include: The first acquisition unit is used to acquire the initial curve for downhole correction. The second acquisition unit is used to acquire the corrected zero-point drift value. Complete the zero-point calibration; The correction unit is used to correct the flow coefficient of each well layer in the target well. Specifically: a) Keep the non-detected wells closed, inject supercritical carbon dioxide at a flow rate of Q into the well to be calibrated from the ground, and simultaneously collect the fluid pressure P and temperature T and transmit the data to the ground host computer; b) Calculate the fluid density ρ using the equation of state or the NIST official database, and calculate the expansion coefficient. The first downhole flow rate was calculated by combining the outflow coefficient curve. Compared with the first traffic The error between the injected flow rate Q; c) If the error is not within the preset range, the error is corrected using an iterative method; through the first flow rate The fluid velocity V is calculated, and then the Reynolds number Re is calculated using the Reynolds number formula. The curve interval to which Re belongs is then determined. Based on the corresponding Reynolds number curve interval, the discharge coefficient C is retrieved, and the second flow rate is recalculated using the flow rate calculation formula. ; d) Compare the first flow rate and If the difference is within a preset range, it indicates that the accuracy meets the standard; otherwise, the above process is repeated to continue the iteration.

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