Tomographic multiphase fluid flow measurements

Abstract

The present invention relates to a solution for determining the content of a fluid flow, especially for determining the gas fraction or density of the fluid flow at a predetermined rate. It includes a data storage device storing an assumed water liquid ratio value related to the flow, a gamma densitometer configured to measure the density in at least a defined section of the fluid flow in a predetermined time period, thus providing a density measurement of the fluid flow in said defined section, and storing a corresponding first density value and tomographic impedance measuring unit configured for providing a sequence of discrete tomographic measurements of the gas / liquid distribution of the flow at discrete time steps within said time period. The tomographic measuring unit also calculating the gas fraction and density for the fluid flow in the defined section for each discrete time step.

Description

[0001] TOMOGRAPHIC MULTIPHASE FLUID FLOW MEASUREMENTS

[0002] Technical Field

[0003] The present invention relates to measuring the contents of a multiphase fluid flow, especially for measuring the gas fraction or density of the flow.

[0004] Multiphase flow measurements are well known within hydrocarbon production where mixtures of oil, gas and water is produced from wells or transported through the production systems. The measurements are performed with several different measuring technologies such as measuring the electrical characteristics of the flow using electrodes measuring permittivity, conductivity, capacitance etc in a flow, for example relying on the fact that water in the fluid has higher conductivity that the gas or oil, providing a water / liquid ratio WLR in the fluid. Such systems are described in NO 307393, W02006 / 132546, W02008 / 150180 and W02020 / 084132. These systems also provide multi electrode configurations that provide a tomographic cross section of the flow showing the distribution of the different phases in the flow.

[0005] Another well-known method for measuring the content of the flow is based on using gamma radiation to find the density of the fluid in the flow. Such solutions are for example described in W02020 / 168064, US7978815, WO2011 / 117426 and W090 / 02941 , where W02020 / 168064 use two detectors to take the distribution of the flow into account and W090 / 02941 also suggests a combination of gamma density measurements and a capacitance sensor. US2014 / 331783 describes a system where gamma measurements may be performed in the same cross section as the tomographic measurements but is primarily based on performing tomographic measurements the gamma densitometer may not be necessary.

[0006] The different measurements mentioned above have different advantages, where the impedance measurements may provide a fast measurement of the distribution of the flow but low accuracy measuring the gas content, while the density measurements using gamma radiation is relatively slow compared to electrical measurements. In addition, the gamma density measurements often do not cover the complete flow and therefore does not necessarily provide measurements representing the complete flow. Thus, it is an object of the present invention to provide a solution improving the accuracy and speed of the gas ratio and / or density measurements for a fluid flow. This is achieved as presented in the accompanying claims.

[0007] This way a solution is provided which can compensate for the situation where the density is not measured in the complete flow cross section, but only in a defined section of the flow. In order to compensate for this the tomographic measurement is used to find the gas fraction in that section so as to provide a relevant combination of the data sets for the same section of the flow. Once the relationship between the measurements in the section has been found it is possible to use this to find the gas fraction or density of the complete fluid flow.

[0008] The present invention thus provides a system and method combining the advantages of the fast tomographic impedance measurements providing a gas distribution with the density measurements based on a gamma measuring system. Based on an assumed initial value for the water liquid ratio (WLR) a parameter related to the flow content may be calculated, for example the density and the gas fraction. The parameter may be calculated using two different measurements which may give two different representations of the flow. Based on the difference between the two calculations of the parameter, a possibility is provided to adjust and improve the measurements based on the reliability and sampling rate of the measurements. Once the assumed density or gas fraction has been corrected or the difference between the two calculations of the same parameter is confirmed to be within a certain limit, it may be used as a basis for providing a gas fraction or density with sufficient accuracy at the measuring rate of the tomographic measurements even though the gamma densitometer has a lower measuring rate. In other words, the accuracy of high sampling rate density and / or gas fraction measurements is improved using the combination of the slow gamma measurements and the high sampling rate impedance measurements.

[0009] The present invention provides two measurements of the section of the flow measured by the gamma density measurements and the corresponding section of the flow measured by the tomographic measurement, providing a comparison between the two and an adjustment giving priority to the most accurate measurement, which typically is the gamma measurement. If a deviation between them is found one can assume that the deviation is also related to the parts of the flow not covered by the gamma density measurements, making it possible to adjust the measurement for the complete flow.

[0010] The correction may be as an alternative to performed by an iteration process based on the assumed WLR of the fluid, where WLR is varied until the deviation is under a predetermined value. This will provide an updated WLR for the fluid.

[0011] The invention will be described below with reference to the accompanying drawings, illustrating the invention by way of examples.

[0012] Figure 1 illustrates the measuring system according to the invention

[0013] Figure 2 illustrates the difference in time resolution of the measurements from the gamma measurements and tomography-based measurements.

[0014] Figure 3 is a flow chart illustrating a method according to the invention.

[0015] Figure 4 is a flow chart illustrating another method according to the invention.

[0016] As illustrated in figure 1 the system according to the invention relates to a system using two different measuring technologies measuring the content of a fluid flow 6 in a pipe 8. More specifically the measuring system includes a tomographic impedance measuring system comprising a number of electrodes 7 positioned around the circumference of the pipe and connected to a first tomographic measuring device 2. The illustrated solution shows six electrodes, but other numbers may of course be chosen based on available instruments, the required resolutions and / or the application. As is well known from the prior art the first tomographic impedance measuring device may provide a distribution of the characteristics of the fluid 6 in the pipe 8.

[0017] In addition a gamma densitometer as described in the abovementioned publications is used, including a source 1 and a detector 3 positioned on opposite sides of the fluid flow 6 and the pipe 8. The gamma densitometer only provides a measurement in a section 5 of the fluid flow 6. The gamma receiver 3 and the tomographic measuring device 2 are connected to a signal processor 4 using a combination of the measurements to provide a measure of the flow content, where both measurements are either related to the gas fraction or the density of the fluid, thus providing two measurements related to the same fluid property. The processor 4 is also configured to control the measurements and sampling the measured data, as well as communicating and / or storing the data.

[0018] As can be seen from figure 1 the gamma measurements cover only a section 5 of the flow 6 so that the impedance measurements and gamma measurements are not necessarily related. However, as the impedance measurements are tomographic it is possible for the processor 4 to select only the impedance measurements representing the section covered by the gamma measurements. This way there are two measurements relating to the same fluid volume 5.

[0019] As is known in the art it is possible to calculate a gas volume fraction based on the density of a fluid flow if you know the water liquid ratio (WLR). It is also possible, based on the impedance measurements and WLR, to calculate the gas fraction or density of a fluid. Thus, it is possible to provide two comparable measurements of the same fluid volume, being the density or gas volume fraction. Figure 2 illustrates an example of measurements of the gas fraction, where the dotted line 2b is the impedance measurement and the solid line 1a represents the gamma measurements, both measurements also representing the same fluid volume.

[0020] As is shown in figure 2 the time resolution of the two measurements are different. By assuming that the slow gamma measurement 1a is more accurate the mean value of the impedance measurements may be adjusted according to this. When the mean value has been adjusted this may be employed to the faster impedance measurements so that the accuracy of fast changes in the fluid flow will also be improved. This may also be used to provide a corrected measurement of the complete fluid flow 6 covered by the impedance measurements. In other words the fast fluctuations in the density may be adjusted or corrected every time a density measurement is provided. This will reduce the measurement error for the fast density measurements provided by the tomographic measurements.

[0021] More specifically a measure of the density and / or gas fraction may be obtained in the step-by-step process illustrated in figures 3 and 4:

[0022] 11 . Make a first assumption of WLR for the time-period of measurement usually assuming this to be constant for the period (predetermined at e.g. 5 seconds). As an alternative the WLR may be measured using a separate measuring instrument.

[0023] 12. Measure the attenuation of a gamma beam propagating though a section of the fluid flow using a gamma densitometer over the predetermined time-period.

[0024] 13. Measure high speed impedance in a multi-electrode system over the same time-period.

[0025] 14. Calculate the tomographic distribution of gas / liquid in short time-steps, e.g. at a rate of 10Hz over the same time period, using WLR from step 11 .

[0026] 15. For each tomogram; calculate the density and / or gas fraction in the same, known area or section covered by the gamma beam, using WLR from step 11 combined with the tomographic gas / liquid distribution from step 14. 16. Calculate the average density and / or gas fraction over the time-period based on the time-step densities and / or gas fractions calculated in step 15.

[0027] Based on steps 14-16 a time average density and / or gas fraction in the beam area is calculated from the impedance tomography and the WLR input.

[0028] 17. Calculate the density from the gamma ray attenuation measured in step 12, and / or calculate the Gas Fraction in the gamma beam area based on the WLR from step 11 combined with the calculated density.

[0029] 18. Compare the calculated density and / or gas fraction from step 16 with the calculated density D and / or gas fraction GF from step 17 and calculate the difference between the values, thus providing difference values AD and / or GF where AD= Dg- Dt, Dg being the density measured by the gamma densitometer and Dt being the time averaged density calculated from the tomographic measurements, and AGF=GFg-GFt, GFg being the gas fraction calculated from the gamma densitometer measurements and GFt being the time averaged gas fraction calculated from the tomographic measurements.

[0030] The difference values from step 18 may be used in one or more of the steps:

[0031] 19a, Providing a new fast density value from step 15 adjusted for the density difference value AD from step 18. This will provide a virtual fast density value at the same time resolution as the impedance measurements from the gamma section 5.

[0032] 19b. Providing a new, fast Gas Fraction value from step 14 adjusted for the density difference value AGF from step 18. The fast gas fraction is produced from the impedance tomography over the complete flow area.

[0033] Following step 19b an additional step may be performed:

[0034] 19c. Calculate the density over the complete pipe cross section using the WLR from step 11.

[0035] As illustrated in figure 4, if the difference values exceed a predetermined limit the method according to the invention may include an additional steps: 20. Adjust WLR and repeat steps 14 to 18 in an iterative process until the difference value(s) AD and / or GF is / are within a predetermined acceptance limit. This is performed by providing a new WLR in step 14 instead of the assumed WLR 11 and finding new difference value(s) AD and / or AGF corresponding to the new WLR, and repeating the iteration process until the difference values are below a limit. This way a measure of the WLR is provided.

[0036] After this the following may be extracted:

[0037] 21a, the fast density may be retrieved from step 15.

[0038] 21 b, the fast gas fraction may be retrieved from step 14, which may be followed by

[0039] 21c, the fast cross section density may be calculated using the WLR after the iteration process in step 20 and the gas fraction in step 21b.

[0040] The adjustments and process in step 20 may use any suitable iteration method otherwise known to the skilled in the art.

[0041] In some cases, e.g. if there are known reasons why the impedance and density measurements should deviate from each other, the adjustment may take such deviations into account. Thus, the average values from the impedance measurements may be chosen to deviate from the values from the density measurements.

[0042] When the deviation is sufficiently small the density or gas fraction calculated from the impedance measurements may be sufficiently accurate to provide measures of the density or gas fraction of the fluid flow at a rate significantly higher than typically supported by a gamma densitometer.

[0043] Steps 21a and 21 b of the process illustrated in Figure 4 may therefore be completely analogue to the same steps 19a and 19b of the process in Figure 3. In step 21c the cross-sectional gas fraction may be calculated for each timestep based on the time step Gas Fractions calculated in step 21a, combined with the adjusted WLR from step 20.

[0044] • The process will also result in a WLR improved from first assumption.

[0045] • In a running process it would be natural to use the WLR of the preceding timeperiod or an average of a number of previous WLRs as the first assumption for the next time-period.

[0046] • Alternatively, the first assumed WLR could be arrived at by anything from a wild guess to more advanced methods like described in NO 307393, WO 2006 / 132546, WO 2008 / 150180 or WO 2020 / 084132.

[0047] To summarize the present invention thus relates to a method and system for determining the content of a fluid flow, especially for determining the gas fraction or density of the fluid flow at a predetermined rate. The system includes the following: a data storage device storing an assumed or previously measured water liquid ratio (WLR) value related to the flow, a gamma densitometer configured to measure the density in at least a defined section of the fluid flow, essentially the section covered by the gamma ray though the flow, in a predetermined time period. This provides a density measurement of the fluid flow in the defined section which is stored as a first density value corresponding to a first density value,

[0048] A tomographic impedance measuring unit configured for providing a sequence of discrete tomographic measurements of the gas / liqu id distribution of the flow at discrete time steps within said time period.

[0049] The tomographic measuring unit is also configured for calculating the gas fraction and / or density for the fluid flow in the defined section for each discrete time step. This may also stored in a data storage for the further analysis.

[0050] A processor configured to calculate for each predetermined time period: An average density difference value between the first density value and an average density value for the defined section measured by the densitometer, calculated from the stored time step tomographic distributions using the stored WLR. The average of the tomographic measurements preferably being calculated over the same time as the gamma densitometer measurements.

[0051] A series of density values for the fluid flow in the defined section within the time period, the calculation including adjusting each calculated density value based on the calculated average density difference value, and / or

[0052] An average gas fraction difference value between a first gas fraction value calculated from the first density using the stored WLR and an average gas fraction value for the fluid flow in the defined section, calculated from said calculated time step gas fraction. The difference value is based on the gas fraction measured using the densitometer covering a section of the flow, and an average of the gas fraction calculated using the tomographic measurements for the same volume.

[0053] A series of gas fractions for the full flow area from each of said tomographic distributions within the predetermined time period, adjusting each calculated gas fraction value based on the calculated average gas fraction difference value. Thus the differences in the measurements in the same volume is used to adjust the values for the complete flow cross section.

[0054] If the average gas fraction difference value or said average density difference value exceeds a predefined limit, the system may be configured to adjust the assumed WLR value and to repeat the above steps in an iterative process until said average gas fraction difference value or density difference value is within said predefined limit.

Claims

Claims1 . System for determining the content of a fluid flow, especially for determining the gas fraction or density of the fluid flow at a predetermined rate, the system including a data storage device storing an assumed water liquid ratio (WLR) value related to the flow, a gamma densitometer configured to measure the density in a defined section of the fluid flow in a predetermined time period, thus providing a density measurement of the fluid flow in said defined section, and storing a corresponding first density value, a tomographic impedance measuring unit configured for providing a sequence of discrete tomographic measurements of the gas / liquid distribution of the flow at discrete time steps within said time period, the tomographic measuring unit also calculating the gas fraction and density for the fluid flow in the defined section for each discrete time step, a processor configured to calculate for each predetermined time period either■ an average density difference value between the first density value and an average density value for the defined section calculated from said time step tomographic distributions, and a series of density values for the fluid flow in the defined section within the time period by adjusting each calculated density value based on the calculated average density difference value, or■ an average gas fraction difference value between a first gas fraction value calculated from the first density, and an average gas fraction value for the fluid flow in the defined section calculated from said calculated time step gas fraction, and a series of gas fractions for the full flow area from each of said tomographic distributions within the predetermined time period, by adjusting eachcalculated gas fraction value based on the calculated average gas fraction difference value.

2. System according to claim 1 , wherein the system, if said average gas fraction difference value or said average density difference value exceeds a predefined limit, is configured to adjust the assumed WLR value and to repeat the steps of claim 1 in an iterative process until said average gas fraction difference value or density difference value is within said predefined limit.

3. Method for determining the content of a fluid flow, especially the gas fraction or density of the fluid flow at a predetermined rate, the method including the steps of: a) providing an assumed water liquid ratio (WLR) value for the fluid flow, b) measuring a first density in a defined section of the fluid flow during a predetermined time period using a gamma densitometer, c) during said time period measuring a sequence of tomographic impedance distributions in the fluid flow at discrete time-steps, and calculate the tomographic distribution of the gas fraction for each of said time-steps over the flow cross sections, also calculating the gas fraction and density for the fluid flow in the defined section for each discrete time step, d) for each time period either calculate an average density difference value between the first density value and an average density value for the defined section calculated from said time step tomographic distributions, and calculate a series of density values for the fluid flow in the defined section within the time period by adjusting each calculated density value based on the calculated average density difference value, or calculate an average gas fraction difference value between a first gas fraction value calculated from the first density, and an average gas fraction value for the fluid flow in the defined section calculated from said calculated time step tomographic distributions, andcalculate a series of gas fractions for the full flow area from each of said tomographic distributions within the predetermined time period, by adjusting each calculated gas fraction value based on the calculated average gas fraction difference value.

4. Method according to claim 3, wherein including the step of adjusting the assumed WLR if said average gas fraction difference value or said average density difference value exceeds a predefined limit and repeating the process of b) to d) until the difference is below the predetermined limit.

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