MXPA96002268A - A venturi flowmeter for measurement in a unassistance for circulation of a flu - Google Patents

Abstract

The present invention relates to a device for measuring the flow rate q of a fluid in a fluid passage, in a well, the device comprising a first venturi section and first means responding to a pressure difference P1 through the first venturi section, between two points that are separated from one another in the direction of flow, and a second venturi section and second means responding to a pressure difference P2 through the second venturi section, between two points that are separated in the direction of flow, the two venturi sections being disposed one with respect to the other in such a way that, for a given direction of fluid flow, the distance of one of them increases while the diameter of the other of they are decreased, the venturi sections being arranged in such a way that they can cancel the static pressure components present in the pressure differences P1 and

Description

A VENTURI FLOW METER FOR MEASUREMENT IN A PASADIZO FOR THE CIRCULATION OF A FLUID FIELD OF THE INVENTION AND PREVIOUS TECHNIQUE The invention refers to a device for measuring the flow rate of a fluid in a passageway, in particular in a well for extraction of hydrocarbons. A device known in the prior art is described in the European patent application document EP-A-234,747. Such a device is shown in Figure 1 and essentially comprises a first section 1 of a passage having a uniform diameter, followed by a second section 2 having a decreasing diameter such that a venturi is formed. Three pressure pick-up points 3, 4, 5 are arranged, point 4 being located at the venturi inlet, point 3 upstream of point 4 and point 5 downstream of the venturi. The flow direction of the fluid is marked by the arrow, which has been given the reference 6. A first measurement of pressure difference ΔPm can be obtained between points 3 and 4, at the ends of section 1 of diameter constant. This pressure difference? Pm serves to determine the average density of the circulating fluid. A second measurement of the pressure difference ΔPV can be carried out between points 4 and 5, that is through the venturi. This measurement is used to determine the flow rate of the fluid, as long as its density has been previously determined by measuring the? Pm * With greater precision, the flow rate v can be calculated using the relationship: ? PV = apv2 + b (? Q - p) where p is the density of the fluid and pQ corresponds to the density of a fluid present in the measurement circuit of the pressure difference sensors. The coefficient a is equal to (1 - d4 / D4), where d and D are respectively the diameter of the smallest section and the diameter of the largest section of the venturi. This relationship shows that the pressure difference that has been measured is the sum of two terms, one of which is proportional to the square of the flow rate while the other (the static component) is independent of the flow rate. Accordingly, when the flow rate is small, the static component predominates, so that the slightest error in the determination of the density immediately gives rise to an apparent flow. The use of two pressure difference sensors, which typically have an accuracy of 10 ~ 3 bar, can generate apparent flow rates in the order of 6.6 cubic meters per hour (mr / h), and this can happen even if there is no No fluid circulating in the passageway. This is shown in Figure 2 which, for two different densities (I: 1,250 kg / mr, n. 500 kg / pr *), shows how the two components of the pressure difference vary according to the flow: one of these components depends on the flow rate while the other component (the static component) is independent of it. From this graph, it can be seen that the static component is widely predominant for flow rates of less than about 3.96 m3 / h. Up to 13.2 mr /, the pressure difference measurement, and consequently the flow measurement, is very sensitive to the static component. In addition, the known device uses, as do all other systems using venturis, a reduction in the diameter of the passage or channel through which the fluid is circulating. However, the flow rate to be measured is the flow rate that occurs in the part of the passageway that has the diameter d, that is, in the normal section of the venturi. When test wells are submitted for hydrocarbon extraction (drilling rod test or "DST"), standard values need to be satisfied with respect to the production pipeline, ie the diameter d: this value is adjusted to 57 , 15 mm. Accordingly, the only way to achieve a restriction in order to form a venturi is to start by widening the internal diameter of the pipe to a diameter D and subsequently returning it to its standard diameter d. This has the consequence that in the expression given above for? PV, the term v is subject to a coefficient of (ld / D4) less than 1. Therefore, when d - 57.15 mm and D = 76.2 mm, the term v2 is subject to an attenuation coefficient of approximately 0.3. Accordingly, the sensitivity of? PV to the measured flow rate is small, particularly when the flow rate is small. Typically, for a flow rate of the order of 6.6 m3 / h, an error of 30% is common, and the error in flow rates of the order of 3.96 m3 / h can reach a value as high as 50% to 60 %. Errors smaller than 5% are obtained only with flow rates greater than approximately 33 m3 / h. Accordingly, the known device is subject to two main sources of error: one of them is associated with the fact that attempts are made to measure the flow rate in the small section of a venturi whose diameter can not be reduced to less than the nominal value of the d = 57.15mm: and the other is associated with the static component that is independent of the flow.

SUMMARY OF THE INVENTION The present invention creates a device for measuring the flow rate Q of a fluid in a passageway for the fluid, inside a well, the device comprising a first venturi section and first means sensitive to the pressure difference ΔP thr the first venturi section between two points that are separated from one another in the direction of circulation, a second venturi section, and a second means sensitive to the pressure difference ΔP2 thr the second venturi section between two points that they are separated in the direction of circulation, the two venturi sections being disposed one with respect to the other in such a way that for a given direction of fluid circulation, the diameter of one of them increases while the diameter of the other decreases Such a measurement device makes it possible to: completely eliminate the presence of the interfering static component, and substantially reduce the error r in the measured flow rate: for a given flow rate, error values can be obtained that are five to ten times lower than with the prior art device. The distances between the two pressure collection points of the two venturis are preferably equal. The same happens with the normal sections (and of course the larger sections of the venturis). In a particular embodiment, each of the first and second venturi sections may be constituted by a locally thicker portion of the wall that delimits the passage exit. In an apparatus of the present invention, the most accurate determination of the flow rate is obtained, however, at the expense of a small amount of loss in precision relative to density with high flow rates. However, this can be compensated for by adding a pressure difference measurement in a straight portion of the passageway. In such circumstances, an excellent density measurement is achieved, while very good flow measurements are obtained simultaneously. The two venturi sections can be fixed in a string of production test bars, and recording means can also be arranged in the bar string such that signals representative of? P ^ and? P2 are recorded. The invention also relates to a system for measuring the flow rate of a fluid, the system comprising a device as described above and means for calculating the fluid flow rate forming a linear combination of the pressure differences ΔP and ΔP2. Said system may also include means for determining the density of the fluid. Means may be provided for determining the flow rate Q i (i = 1,2) of the fluid in at least one of the two venturis on the basis of the pressure difference ΔP¿, and also, optionally, means for comparing Q¿ with Q. The invention also creates a method for measuring the flow rate of a fluid in a passageway for fluid within a well, which uses a device for a system as defined above, the method including the operations of: measuring a first difference of pressures? P ^ thr a first venturi section; measure a second pressure difference? P2 thr a second venturi section; and calculate the flow rate from said values? P and? P2 measured during the two previous operations, while eliminating the static component. The distances between the two pressure collection points of the two venturis are preferably equal. The same happens with the normal sections (and also the larger sections) of the venturis. The static component can be eliminated by a linear combination of? P ^ and? P2. The method may additionally include an operation to calculate the static component. In addition, it may also include an operation to determine the direction of flow of the fluid in the passageway, which method comprises the following sub-operations: assume a direction of fluid flow; determining, for said assumed direction, the flow QJL (i = 1,2) of the fluid thr at least one of the two venturis, on the basis of the pressure difference? P¿; compare Q with QA to verify the assumption concerning the direction of circulation.

The pressure differences corresponding to different instants j can be measured, and the corresponding data can be stored, optionally after being compressed, the Q-i values of the flow rate being subsequently calculated at different times t_? . This provides a data group Qj (.

BRIEF DESCRIPTION OF THE FIGURES In any case, the features and advantages of the invention are more clearly manifested in the light of the following description. The description refers to embodiments given in a non-limiting and explanatory manner, with reference to the accompanying drawings, in which: Figure 1 shows a prior art device for measuring the flow rate; Figure 2 shows the weights of the two components in a prior art measurement of the pressure difference; and Figures 3 and 4 show two devices of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION A first embodiment of a device of the invention is shown in Figure 3. In this Figure, references 12, 14 and 16 represent different sections of the internal wall of a string of pipes adapted for the DST (DST = from the English Drill Stem Test). These sections have a configuration such that a fluid circulating in the direction indicated by the arrow 13, for example, first passes through a divergent venturi: the inner wall of the column 6 is flared in such a way that the fluid passes through it. from a section of diameter d (normal diameter) to a section of diameter D (major section). In a drilling rod test, the diameter d is adjusted to a nominal value of 57.15 mm, while the diameter D of the central widening is 76.2 mm. After that, the fluid passes through a convergent venturi 20: the wall tapers conically so that the fluid passes from a section of diameter D to return to a section of diameter d. A pressure difference sensor 22 serves to measure the pressure difference between two pressure collection points 26 and 28, located respectively upstream and downstream of the divergent venturi 18. A pressure difference sensor 24 serves to measure the difference of pressures between two pressure collection points 30 and 32, located respectively upstream and downstream of the convergent venturi 20. Points 28 and 30 could coincide equally well. Another embodiment is shown in Figure 4. The fluid, flowing eg in the direction represented by the arrow 33, first passes through a convergent venturi 34 whose wall defines a larger section with a diameter D (76). , 2 mm) and a normal section with a diameter d (57.15 mm). A first pressure difference sensor 38 serves to measure the pressure difference between two pressure pickup points 40 and 42, located respectively at the inlet and outlet of the convergent venturi 34, and a pressure difference sensor 44 serves to measure the pressure between two points 46 and 48 of capturing pressures, located respectively at the entrance and exit of the divergent venturi 36. Also in this case, points 46 and 42 could coincide. In both cases, the distance (in the direction of circulation) between the two points of collection of pressures in a venturi is preferably equal to the distance between the two points of capture of pressures in the other venturi. However, the invention also extends to any embodiment in which these two distances are different. Similarly, the normal sections (or the larger sections, as the case may be) of the two venturis are preferably the same, but the invention also extends to the case where they are not equal.

In any case, the pressure difference sensors 22, 24, 38 and 44 can be connected, in a manner known to the person skilled in this technology, to means (not shown in the Figures) that make it possible for the supplied data by said sensors can be stored and / or manipulated. In particular, when working a well for hydrocarbon extraction, said means may include computer means located on the surface. On the basis of the signals produced by the pressure sensors at the various instants t.i, eg during a given test sequence, it is possible to obtain signals representative of the variation of the flow rate over time, Q (t ). In the two cases shown in Figures 3 and 4, these same equations govern the variations in pressures between the inlets and outlets of the venturis, and from the point of view of fluid circulation, the device presents the same advantages in relation to the prior art. From the point of view of the practical execution, the device of Figure 4 is easier to execute than the device of Figure 3. Considering the device shown in Figure 3, the pressure difference ΔP measured between the points 26 and 28 is given by the following relation: ? P? = a- ^ v2 + b (pQ - p?) (1) Similarly, the pressure difference between points 30 and 32 is given by the following relationship: ? P2 - a2p? V2 - b (pQ - P?) (2) In the previous equations, p? designates the density of the circulating fluid, p0 designates the density of a reference oil present in the conduits of the sensors 22, 24 of difference (or 38 and 44 for the embodiment shown in Figure 4), and v designates the flow velocity of the fluid. As well: aj »k / Cd] ^ 2 and a2 * k / Cd2" in which 1 2g d4 where Cd ^ and Cd2 are calibration coefficients for venturis. From the mean values of? P and? P2, it is possible to deduce the density and / or velocity of fluid circulation by the following relationships: P = (? P1? P2) (? P1? P2) (3) 2b + a " (? P?? P2) v2 = (4) (a, + a.

The relation (4) gives the flow velocity in the section of diameter D. The volumetric flow of the fluid is calculated using: Q = (t? D2v) (5) From this system of equations, it is possible to draw various consequences in relation to the characteristics of the double venturi device of the invention. In the first place, the static component, although of course it is presented in each of equations (1) and (2), has opposite signs in them, so that it disappears completely when they are added? P and? P2: consequently, Whatever the flow rate, this component has no influence on the result. It can be observed at this point in the memory that the above equations are given for the case in which the distance between the two points of capture of pressures of one venturi is equal to the distance between the two points of capture of pressures of the other venturi. Otherwise (ie in the case of different distances), the static component is not eliminated when P? ^ And? P2 are added, but this is eliminated when a linear combination of? P ^ and? P2 is established using the combination of coefficients that take into account the relationship between distances. The measurements? P and? P2, obtained with each of the sensors, can also suffer from an error or uncertainty that is associated with the sensors themselves. However, in comparison with the gradioventuri measuring system, the error associated with the sensor has a much smaller influence on the final result. In the gradioventuri system, the error necessarily varies with 1 / v, that is in a way that is inversely proportional to the flow rate. Using the double venturi of the invention, the overall error depends on the sign of the error in each sensor: it may happen that an error is obtained that varies in a way that is inversely proportional to the flow (but in this case the error is, nevertheless, around five to ten times less than that of a gradioventuri), but it is also possible to obtain an error that is constant throughout the range of measured flow rates, particularly when the error in one of the sensors compensates for the error in the other sensor. This possibility is mathematically impossible when only one venturi is used. Another advantage of a double venturi device according to the invention is that it makes it possible to achieve a very good estimation of the discharge coefficient by a wide range of flow rates. In a single venturi, the discharge coefficient is a function for which an analytical expression has not been rigorously established. Certain expressions make use of the Stolz equation, others are more empirical, but all of them share in common the fact of using the Reynolds number. With venturi flowmeters, the ISO-5167 standard provides a table that gives the approximate variation in the discharge coefficient according to the Reynolds number. This table is reproduced below as Table I.

TABLE I RE Cd 4 x 104 0,957 6 x 104 0,966 105 0,976 1.5 x 105 0,982 In the hydrocarbon production sector, the measured flows are located in the margin of approximately 3, 3 m3 / h to 99 m3 / h. For a flow rate of 6.6 m3 / h, corresponding to a circulation speed of approximately 0.7 meters per second (m / s), a Reynolds number is calculated which has a value of RE - 4 x 10, while for a flow rate of 63 m3 / h, this is approximately 7 m / s, the Reynolds number that is calculated is RE = 4 x 10 ^. In the comparison with the previous Table I, it can be deduced that the discharge coefficient Cd is not constant throughout the range of flows involved. The double venturi device of the present invention makes it possible to overcome this difficulty, since the coefficient of eguivalent discharge of the system as a whole can be given as the quadratic mean of the discharge coefficients Cd ^ and Cd2 of each of the venturis. With greater accuracy, the equivalent discharge coefficient is given by the ratio: (1 / Cd = J { L / Cd¡) + (1 / Cd) (6) This results in an attenuation of the variation in the discharge coefficient over the entire range of flows of interest. Table II below gives the value of the discharge coefficient at two different flow rates (6.6 m3 / h and 66 m3 / h) respectively for a convergent venturi (Cd ^), a divergent venturi (Cd2) and a double venturi system. invention (Cde). The error given in the lower part of each column corresponds to the error obtained in the flow when the discharge coefficient calculated for 66 m3 / h is applied to a small flow (6.6 m / h): this error decreases to 2.5 % for the double venturi of the invention, while it is approximately 5% for the convergent venturi and is greater than 15% for the divergent venturi. Accordingly, the double venturi of the invention makes it possible to use a single discharge coefficient throughout the range of flow rates of interest.

TAB LA II CD? Cd2 cde 6.6 m3 / h 0.95 1.43 0.785 66 m3 / h 0.998 1.21 0.765 ERROR 5.1% 15.4% 2.5% Since the discharge coefficients of a convergent venturi and a divergent venturi are not equal, a density measurement performed with a device of the invention suffers from an interfering component that is proportional to the sum of the signals from the two sensors (? P- ^ +? P), which in turn is proportional to the square of the velocity of the fluid (see equations 3 and 4 above). Accordingly, the error in determining the discharge coefficient for each venturi shows an increase in density, and this effect increases with an increasing flow velocity of the fluid. This means that the improvement in the determination of the flow rate is obtained at the cost of a reduced precision concerning the density. In order to remedy this disadvantage, it is possible to determine the density at low flow rates (eg with a zero speed of circulation), and subsequently use the density value, which has been obtained in this way, to determine the speed circulation with higher flow rates. Another method to compensate for such loss of accuracy relative to the flow rate is to add a pressure difference sensor in a section that has no diameter change (eg between points 28 and 30 of Figure 3 or between points 42). and 46 of Figure 4), thereby directing the measurement of the static component in a manner independent of the flow velocity of the fluid: this makes it possible to simultaneously obtain a very good density measurement and a good measurement of the flow rates. Because of the symmetrical configuration of the double venturi in a device of the present invention, fluid can flow through it in any direction, and the flow velocity can be determined under all circumstances. In particular, the invention is also applicable to injection wells. This is not possible with the gradioventuri structure of the prior art, in which the convergent venturi must extend in the direction of fluid circulation. Conversely, the device of the invention can be used to determine the flow direction of the fluid. This can be particularly advantageous under transient conditions, eg after a valve has been closed. It is possible to proceed in the following manner: it is assumed that the fluid is circulating in a particular direction, eg the direction indicated by the arrow 13 (or 33) in Figure 3 (or in Figure 4); after that, the values of? P- ^ and? P2 are measured and the circulation speed and density are derived from them using equations (3) and (4); Equation (1) is used to deduce the velocity of circulation v ^ through venturi 18 (a divergent venturi if the fluid is circulating in the direction 13) and from the value of the density p and the pressure difference? P- ^ it can be assumed that Cd- ^ = 1; Equation (2) is used to deduce the fluid circulation velocity v2 through the venturi 20 (a convergent venturi if the fluid is circulating in the direction 13), based on the density p and the pressure difference? P2; it can be assumed that Cd2 = 1; if the fluid is of course moving in the direction indicated by arrow 13 (Figure 3), then the following relations should be applied: Vj ^ > v and v2 > v; otherwise, v ^ < v and v2 < v, which means that the venturi 18 is convergent in the present direction of fluid flow, while the venturi 20 is divergent, and consequently the fluid is circulating in the opposite direction to that given by the arrow 13 (or arrow 33). ). Then the density must be recalculated, assuming the circulation of the fluid in the opposite direction. Then, the value of the circulation speed is corrected to take into account the new value of the density. All the methods described above, and in particular the methods for calculating the flow rate and / or the density of a fluid, or the method for determining the flow direction of a fluid, can be executed using appropriately programmed computer means of a type appropriate; for example, when producing hydrocarbons, these means may be the means that are located on the surface, and which have already been mentioned in the previous description. Finally, the invention has been described in its application to a well for hydrocarbon extraction. The measuring devices and methods that have been described are not limited to applications of that type, and the invention can be applied to the measurement of the circulation of a fluid in any non-horizontal passage (when the circulation is horizontal, there is no static component ).

Claims (19)

1. - A device for measuring the flow rate Q of a fluid in a passage for the fluid inside a well, the device comprising a first venturi section and first means sensitive to the pressure difference? through the first venturi section between two points which are separated from each other in the direction of circulation, and a second venturi section and a second means sensitive to the pressure difference ΔP through the second venturi section between two points that are separated in the direction of circulation, the two venturi sections being arranged one with respect to the other in such a way that for a given direction of fluid circulation the diameter of one of them increases while the diameter of the fluid decreases. another of them.

2. - A measuring device according to claim 1, wherein the distances between the two points of capture of pressures of the two venturis are equal.

3. A device according to claim 1 or 2, wherein the normal sections of the two venturis are equal.

4. A device according to any one of claims 1 to 3, wherein the larger sections of the two venturis are equal.

5. - A measuring device according to any one of claims 1 to 4, wherein each of the first and second venturi sections is constituted by a locally thicker portion of the wall defining the exterior of the passageway.

6. - A device according to any one of claims 1 to 5, which further includes means for measuring the pressure difference between two points of a portion of the passageway in which there is no variation in diameter.

7. - A device according to any one of claims 1 to 6, wherein the well is equipped with a string of production test bars, the two venturi sections being fixed in said bar string.

8. A device according to claim 7, in which are provided on the bar string recording means for recording signals representative of? P and? P2.

9 »- A system for measuring the flow rate of a fluid in a passage for fluid, the system including a device according to any one of claims 1 to 8, and means for calculating the flow rate of a fluid by means of a combination linear of the pressure differences? P- ^ and? P2.

10. A system for measuring the flow rate of the fluid in a passageway for fluid, comprising a device according to claim 2, and means for calculating the flow rate of a fluid by summing the differences in pressures? P ^ and? P2.

11. A measuring system according to claim 9 or 10, which further includes means for determining the density of the fluid.

12. A measuring system according to claim 11, which also includes means for determining the flow rate Q ± (i = 1,2) of the fluid through at least one of the two venturis on the base of the difference of pressures? P¿ and means to compare Q with 0 ^.

13. - A method for measuring the flow rate Q of a fluid in a passage for fluid inside a well, using a device according to any one of claims 1 to 8, or a system according to any one of claims 9 to 12, the method comprising the operations of: measuring a first pressure difference? P ^ through a first venturi section; measure a second pressure difference? P2 through a second venturi section; and calculate the flow rate from the values? P and? P2 measured during the two previous operations, while eliminating the static component.

14. A method to determine the flow rate Q of a fluid in a passageway for the fluid inside a well, which comprises the operations of: measuring a pressure differential P ^ obtained through a first venturi section between two points which are separated from one another in the direction of fluid circulation; measure a pressure difference? P2 obtained through a second venturi section between two points that are separated from one another in the direction of circulation; the two venturi sections being disposed one with respect to the other in such a way that for a given direction of circulation of the fluid the diameter of one of them increases while the diameter of the other decreases; obtaining the flow Q combining and deriving Pi (i «1,2) in such a way that each of the static components present in the expressions for? P ^ is eliminated.

15. A method according to claim 14, wherein the distance in the direction of circulation between the two points of the first venturi section is equal to the distance between the two points of the second venturi section.

16. A method according to claim 14 or 15, wherein the normal sections of the two venturis are equal.

17. A method according to any one of claims 14 to 16, wherein the larger sections of the two venturis are equal.

18. A method according to any one of claims 13 to 17, wherein the static component is eliminated by a linear combination of? P1 and? P2.

19. - A method according to claim 18, wherein the linear combination of? P and? P2 is the sum of? P- ^ y? 2- 20.- A method according to any one of claims 13 to 19, which further includes the operation of calculating the static component. 21. A method according to any one of claims 13 to 20, further including an operation to determine the direction of fluid flow and comprising the following sub-ops: assume a direction of fluid flow; determine, for said assumed direction, the flow Q (i = 1,2) of the fluid through at least one of the two venturis, on the basis of the pressure difference? P¿; compare Q with Q¿ to verify the assumption concerning the direction of circulation. 22. A method according to any one of claims 13 to 21, wherein the pressure differences corresponding to different instants t-¡are measured, the corresponding data being stored, optionally after being compressed, and being subsequently calculated the Q values. > of the flow for different instants. 23. A group of data Q-j (t.?) As obtained by a method according to claim 22.