WO2005040732A1

WO2005040732A1 - Wet gas measurement apparatus and method

Info

Publication number: WO2005040732A1
Application number: PCT/NL2003/000729
Authority: WO
Inventors: Martin Van Werven
Original assignee: Elster-Instromet Ultrasonics B.V.
Priority date: 2003-10-27
Filing date: 2003-10-27
Publication date: 2005-05-06
Also published as: AU2003276777A1

Abstract

An apparatus and method for determining fluid flow velocities of a flow comprising at least two phases in a conduit are disclosed. By measuring a pressure difference between two measurement points and measuring the flow at these measurement points using ultrasonic flow meters, an accurate calculation of the flow rates of the gas phase and liquid phase is allowed.

Description

Wet gas measurement apparatus and method

The present application relates to an apparatus and method for determining fluid flow velocities of a flow comprising at least two phases in a conduit. When natural gas fields are developed, the gas that is produced from the reservoir is frequently accompanied by hydrocarbon liquids, generally referred to as condensate. These liquids are considered to be of high value to the operating companies. Maximum recovery of available hydrocarbon gas and condensate in the reservoir is of great importance. The produced natural gas can also be accompanied by water. In the context of this specification a reference to two-phase flow covers both gas-condensate and gas-water mixtures. As a consequence of this, wet gas flow measurement technology that enables the continuous and simultaneous measurement of both the gas and the liquids being produced is of key importance to the industry. The availability of wet gas flow measurement technology has a big impact on the economics of gas field developments. One way to measure the flow rates of two, or more, different fluids flowing together in a pipeline is to first separate them and then measure the single phase flow rates by means of traditional flow measurement devices. For instance, separator tanks or cyclones can be used to separate liquid from a gas stream and a range of flowmeters is available to determine the single phase flows. Nowadays, common practice in the world of wet gas flow measurement is the use of Venturi flowmeters. Typically, a Venturi meter will overestimate the gas volume flow rate when applied in wet gas flow. An other existing and more advanced technique to measure the flow rates of a two-phase flow is to correct the reading from a Venturi differential pressure meter to account for the amount of liquid present in the gas. With this method the overreading and thus the actual gas flow rate can be calculated. For successful application of this correction method, the amount of liquid needs to be known. In order to determine this liquid amount the so called tracer dilution technique can be used. A liquid tracer is injected in the gas and will mix with the flowing liquids in the pipeline. After a certain distance downstream of the injection point a sample of liquid is taken from the conduit and will be analysed. The dilution of the tracer is determined by comparing the measured concentration of the downstream liquid sample to the known concentration of the injected tracer. The disadvantages of this technique are that it is labour intensive and that the liquid flow rate is only determined at the time of the tracer sampling and assumed to be constant thereafter. The current state of the art in the flow measurement of wet gas is dominated by methods that combine different measurement devices and/or measurement principles. The need for two different measurements can be appreciated by considering that, in order to solve the problem of determining both the gas and liquid flow rates, at least two independent equations are needed. Here, it is also noted that in the state of the art so-called multi-phase meters are known. However, these meters are used as process control means. Their accuracy is in the range from 10-15%, which is sufficient for that particular application. However, an accuracy in this range is insufficient for accounting procedures. As far as ultrasonic flowmeters are concerned, the most recent developments in ultrasonic measurement of wet gas flow are those by Letton & Zanker, as disclosed in US patents US-A-6, 151, 958, US-B1- 6,209,388 and US-Bl-6, 550, 345. The first mentioned patents describe a method and apparatus in order to correlate the measured ultrasonic parameters such as the velocity of sound, gain levels and standard deviations in time differences, to the amount of liquid that is flowing in the gas stream. US~Bl-6, 550, 345 discloses a method for .detecting a stratified liquid layer at the bottom of a pipeline by ultrasonic measurement along an ultrasonic path that reflects off the interface between the two phases . A disadvantage of both methods are the practical limitations of their application. In the first method this limitation is due to the assumptions made. In the second method the liquid layer is not stable in time, but fluctuates, resulting in inadequate results. One object of the invention is to provide a measuring apparatus and method in which the flow velocities and/or (volumetric) flow rates of both fluids, gas and liquid, in a two-phase flow can be determined simultaneously and continuously. Another object of the invention is to provide such an apparatus and method allowing the accurate determination of flow velocities in a two-phase flow, in a wide range of applications. Yet another object of the invention is to provide information about the flow pattern of the two-phase flow in the conduit from the data obtained. According to the invention the apparatus for measuring gas and liquid flow rates of a two-phase gas-liquid flow in a pipeline, comprises a pipeline part having a non-constant cross-section over its length, and a measurement device for determining the differential pressure between two measurements points spaced apart in the longitudinal direction of said pipeline part, as well as at both measurement points an ultrasonic flowmeter for measuring flow velocity at a measurement point, and data processing means for calculating the gas and liquid flow rates from the data obtained by the measurement device and the ultrasonic flowmeters. The apparatus according to the invention comprises measurement devices for measurement of at least two independent variables from which the gas and liquid flow rates can be derived. One measurement device is capable of determining the pressure difference between two measurement points in the conduit. The ultrasonic flowmeters are arranged at the measurement points, such that they enable the determination of the flow rates at these measurement points. An ultrasonic flowmeter comprise at least one pair of transducers, each of which can act individually as a transmitter for transmitting sound waves along an acoustic interrogation path and as a receiver for receiving sound waves. The ultrasonic flowmeter measures the transit time of a transmitted sound wave over the acoustic path. From the transit time a flow rate can be calculated. Hereinafter it will be .explained in more detail how the flow velocities can be calculated from the pressure difference and transit times. The pipeline part having a non-constant cross-section over its length provides a discontinuity to the flow, thereby inducing an effect on flow characteristics, which effect is to be measured. For example, a reduction of the diameter of a circular conduit results in an acceleration of the flow velocity. With the application of a simultaneous and continuous wet gas flowmeter, capital expenditures can significantly be reduced because the need for an expensive separating unit is eliminated. Furthermore, commonly shared pipelines can be used for the transportation of the gas and condensate when flow measurement can be applied on, or shortly after, the wellhead. Such a simultaneous and continuous wet gas flowmeter provides valuable information about the production rates from the reservoir, which can be used for optimising production and reservoir management. Compared to the multi-phase eter discussed above the apparatus and method according to the invention provide more accurate results. Preferably the measurement device is a Venturi tube, because compared to an orifice plate or nozzle that are also differential pressure measuring devices, such a device allows for the fast and almost complete recovery of the pressure downstream from the Venturi tube. The Venturi tube also provides for the non-constant cross- section of the conduit part. A Venturi tube is a device which consists of an inlet having a constant diameter, a convergent section, connected to a cylindrical throat having a constant cross- section, which is in turn connected to a conical expanding section called the "divergent", and finally an outlet section having a constant diameter, generally the same as the inlet section. Preferably one of the measurement points is located in the throat section, while the other measurement point is situated in one of the other sections. More preferably this other measurement point is located upstream of the throat section, preferably in the inlet section. In a preferred embodiment of the apparatus according to the invention at a measurement point an ultrasonic flowmeter having at least two pairs of transducers is provided in such an arrangement that their acoustic interrogation paths cross each other. In order to obtain a more complete image along the cross-section of the conduit, the interrogation paths thereof are situated in horizontal planes at different heights. This also ensures that if one of transducer pairs fails, the other one is able to perform the measurement operation. With respect to the height above the bottom part of the conduit it is noted that the height level is set such that at that height an impact of liquid to gas is expected to be present. A second aspect of the invention relates to a method for measuring gas and liquid flow rates of a two-phase gas-liquid flow in a pipeline part having a non-constant cross-section over its length, comprising determining the differential pressure between two measurements points spaced apart in the longitudinal direction of said pipeline part, and at both measurement points measuring the transit time of a soundwave from an ultrasonic flowmeter, and calculating the gas and liquid flow of the two-phase gas-liquid flow from the measured differential pressure and transit times. The calculation comprises a comparison and combination of the measured parameters, as these parameters are the gas flow velocity at and the differential pressure between two measurement points respectively, most preferably the inlet section and throat section of the Venturi as explained hereinabove. In such a way, a variety of flow characteristics can be determined. First, the presence of liquid in the gas flow can be determined. Furthermore, the flow rates of the gas and liquid phase can be calculated based on the ultrasonic flow measurement and the differential pressure measurement. Furthermore, a prediction can be made of the two-phase flow pattern present in the conduit . Preferred embodiments of the method according to the invention are defined in the dependent claims, and have already been explained hereinabove regarding the apparatus according to the invention. The invention will now be illustrated by reference to the attached drawings, wherein: FIG. 1 is a schematic drawing of a side view of a Venturi tube with ultrasonic transducers in the inlet and throat sections; FIG. 2 is a schematic drawing of the top view of the Venturi tube with ultrasonic transducers from FIG. 1; FIG. 3 is a drawing of the cross view of the inlet pipe section of the Venturi tube with the ultrasonic transducers on the sides, the pressure tapping on top and the horizontal acoustic paths depicted; FIG. 4 is a flowchart that represents the calculation scheme to derive the gas and liquid flow rates from the measured gas velocities upstream and in the throat and the measured differential pressure; FIG. 5 is a graph of characteristic pressure profiles in single phase flow [dry gas] and in two-phase flow [wet gas] along the axial co-ordinate of a classical Venturi tube; FIG. 6 is a graph of a characteristic velocity profile of gas and dispersed liquid droplets in two-phase flow along the axial coordinate of a classical Venturi tube. FIG. 1 shows a schematic side view of a Venturi tube 10 having an inlet section 12 having a constant diameter D, a convergent section 14, a throat section 16 having a constant diameter d, a divergent section 18 and a constant diameter outlet section 20, assuming a fluid flow from left to right. The tube 10 is provided at both ends with flanges 22 for insertion into a conduit. The Venturi tube is preferably constructed in accordance with ISO 5167. In the preferred embodiment as depicted in FIG. 1 the pressure tappings of the differential pressure measurement meter, in the inlet section 12 and throat section 16 respectively, are positioned at measurement points 24, 26 at a distance D/2 and d/2 respectively from the convergent section 14 in agreement with the above standard. At the positions of these measurement points in the inlet section 12 and the throat section 16, ultrasonic flow measurements are also carried out. As is apparent from the side view in FIG. 1 and the top view in FIG. 2, in the inlet section a first pair of co-operating transducers 30, 32 defines a first horizontal acoustic interrogation path 34 between them at a certain height. A second pair of co-operating transducers 36, 38 defines a second horizontal acoustic interrogation path 40 at a different height. The pairs of transducers are arranged such that the acoustic paths thereof, in horizontal projection, cross each other. The intersection is in the measurement plane, perpendicular to the longitudinal direction of the tube at the measurement point 24. A similar construction and arrangement of transducers is present in the throat section. A third pair of co-operating transducers 42, 44 defines a third horizontal acoustic interrogation path 46 between them at a certain height. A second pair of co-operating transducers 48, 50 defines a fourth horizontal acoustic interrogation path 52 at a different height. The pairs of transducers are arranged such that the acoustic paths thereof, in horizontal projection, cross each other. Again the intersection is in the measurement plane, perpendicular to the longitudinal direction of the tube, located at the associated measurement point 26. The heights are such that it is expected that at least one of the transducer pairs, preferably both pairs, is expected not to be in the liquid flow. The diameter ratio of the throat section to the inlet section (d/D) is defined as β. Fig. 3 shows diagrammatically a cross-section at a measurement point in the inlet section 12. The ultrasonic transducers are inserted through the wall of the tube, like the pressure tapping. This drawing also represents the configuration in the throat section of the Venturi tube 10, replacing the dimensions of the inlet section by those of the throat section. For single phase flow through a Venturi flowmeter, when dissipation is neglected, the Bernoulli equation gives the relation between the flow velocity of the fluid and the pressure difference between inlet section and throat section. In an actual, dissipative, flow situation the dissipation of energy is accounted for by using the discharge coefficient. When the Venturi device is designed and manufactured following the standards described in ISO 5167 the discharge coefficient can be calculated and the flow rate can directly be derived from the measured differential pressure. In wet gas two-phase flow, the theoretical description of the differential pressure, and in particular the dissipation, in the Venturi is more complex than in single phase flow. The presence of liquid in the gas stream gives rise to a higher differential pressure compared to the single phase situation, and this higher 'reading' of the differential pressure meter is called the ' overreading' . Furthermore, the measurement of flow velocity by means of an ultrasonic flow meter in two-phase flow is different and more complicated than in the single phase situation. In gas-liquid flow, for example, the available cross sectional area for one phase to flow through is partly occupied by the other phase. To calculate the volumetric flow rate of one of the phases from the measured flow velocity of that phase the cross sectional area occupied by that particular phase needs to be known. In a perfect mist flow (i.e. gas flow with dispersed droplets that flow with the same velocity as the gas) the error resulting from an ultrasonic flow meter is directly proportional to the reduced flow area available for the gas phase, i.e. the error equals the Liquid Volume Fraction (LVF) . In other words, in a perfect mist flow the total flow rate of gas and liquid is measured. In a stratified flow, in which the liquid phase is flowing as a layer at the bottom of the conduit, the velocity difference or slip between the two phases will be significant, with the gas velocity being higher than the liquid velocity, and this will result in liquid hold-up and a higher error from the ultrasonic meter. The method of this invention is based on the comparison of measured gas flow velocities at two measuring positions: preferably, one velocity measurement in the inlet section of the Venturi tube and another velocity measurement in the throat section of the Venturi tube. By considering the ratio of these two measured velocities a conclusion can be drawn concerning the difference in slip between Venturi inlet and Venturi throat and thus information is provided about the possible flow patterns. A summary of possible flow patterns in the two relevant sections of the Venturi with the corresponding changes in slip is given in table 1 below. Table 1. Possible flow patterns upstream and in throat Venturi concluded from gas velocity ratio

Here it is assumed that stratified flow is not present in the throat section. Further assumptions are that the gas phase and liquid phase have the same velocity (no slip) , when there is a mist flow, upstream of the Venturi tube, that the densities of gas and liquid are known, and that the measured gas velocities reflect the mean influence of the liquid fraction. If necessary, suitable measures can be taken in order to condition the flow to a mist flow, e.g. by increasing the velocity of the gas flow using a so called gradual precontraction . Here it is noted that if the flow pattern is stable in time and is known, one of the ultrasonic flow meters may be omitted from the apparatus according to the invention. E.g. when one knows with reasonable certainty that homogeneous mist flow is present in both the inlet and throat sections, then the ultrasonic flow meter, respectively ultrasonic flow measurement in one of these sections is not required. An apparatus and method to this end are claimed in claims 17 and 18 respectively. The method of this invention combines velocity measurements with the differential pressure measurement using the law of conservation of energy and the law of conservation of mass in order to calculate the gas and liquid volume fractions (GVF and LVF) and from these volume fractions the flow rates of gas and liquid. A schematic calculation procedure, based on three measured parameters, is presented in the flowchart of FIG. 4. In a first step the flow velocity in the inlet section (V_G i) and in the throat section (V_{G 2}) are measured, and the velocity ratio V_{G 2}/V_{G 1} is compared to the inversed square cross-section ratio 1/β². Three different situations may occur, V_{G 2}/V_{G 1} < 1/β², V_{G 2}/V_{G x} « 1/β², and V_{G 2}/V_G > 1/β². In the second situation, two further subsituations can be recognized, one where there is one-phase flow, and one where there is no slip between the two phases. These subsituations can be differentiated using the differential pressure data obtained from the Venturi measurement. If

ΔP « p_G V² _{G !} (1/β⁴ - 1)/C² _d then there is a single phase, wherein p_G is the density of the gas phase and C_d is a correction factor for the dissipation. This correction factor is a fixed discharge factor in the case of a Venturi according to ISO standards. In this subsituation the equation Q_G = A_pipe V_{G 1} = A_throat V_{G 2} applies, wherein A is the cross-section. If not, then there is mist flow without slip having the meaning of a homogeneous flow with volume weighted average density. In this case, ΔP equals p^* V² _{G 1} (1/β⁴ - 1), wherein p^* = GVF p_G + (1-GVF) p_L, P_L being the density of the liquid present in the two-phase system. Solving this equation provides GVF, and subsequently the volumetric flow rates of both phases can be calculated according to Q_G = GVF A_pip_e V_{G 1} and Q_L = (1-GVF) A_pipe V_{G 1#} In the situation where V_{G 2}/V_{G x} > 1/β² , the area occupied by the gas can be calculated from the law of conservation of mass for the gas and liquid

_{L 1} V_{L !} = A_{L 2} V_{L 2} and the law of conservation of energy

ΔP = ϋ p_G GVF (V² _{G 2} - V² _G χ)+ p_L LVF (V² _{L 2} - V² _{L t}) .

Here dissipation (discharge coefficient) is not incorporated into this formula, because of the complexity in two-phase flow. Taking into account that GVF = Q_G/ (Q_G + Q_G) = A_{G x}/ A_pipe and similar equation for LVF the combination of the formulas gives ΔP as function of the unknown parameter Ag _{l r} which can be solved by iteration. Once As _x is known, then the other A_{L x}, AQ₂, A_{L 2}, V_{L 2}, and subsequently the volumetric flow rates of gas and liquid can be calculated. In the situation where V_{G 2}/ _{G 1} < 1/β², it can be concluded that the flow upstream of the Venturi is stratified. The assumption made in the "mist flow" case that the gas and liquid velocity is equal in the upstream section is not valid here. In order to solve this problem of unknown upstream liquid velocity an assumption has to be made about the velocity slip between the gas and liquid phase in the inlet section. Different models can be used to derive an equation for the slip upstream, for example a model for equal shear stress at the gas-liquid interface or a model assuming minimal energy of the flowing fluids. These models result in an equation for the slip depending only on the densities of gas and liquid. Once the slip is known, the liquid velocity can be expressed in terms of the gas velocity and the same calculation procedure as in the "mist flow" case can be followed. The operation and differential pressure measurement by means of the Venturi meter described in this invention are preferably carried out following the standard operating principles for this device, as described in ISO 5167. Preferably, the horizontal acoustic paths between the ultrasonic transducers in the inlet and throat sections, as shown in Figures 1 and 2, are symmetrical relative to the horizontal upstream and throat diameter. In addition to the paths described, an acoustic path with, for instance, an inscribed square can be used for the detection of a liquid layer at the bottom of the pipe. For this preferred embodiment, the gas density and the liquid density should be determined, in addition to the measurement of the parameters as mentioned above, as they are needed as input parameters for the algorithm. However, when gas and liquid densities are unknown, these densities can also be estimated based on the ultrasonic measurement of other parameters as, for example, the velocity of sound. The flowmeter and measurement method can be applied in both stratified and mist flow conditions as long as the liquid volume fraction remains below 10%. Fig 5 shows pressure profiles along a classical Venturi tube. W_L and W_G are the mass flow rates of liquid and gas respectively. Fig 6 shows the gas and liquid flow velocity. Symbols used in the flowchart of Figure 4a.

Claims

C L A I M S

1. Apparatus for measuring gas and liquid flow rates of a two- phase gas-liquid flow in a pipeline, comprising a pipeline part having a non-constant cross-section over its length, and a measurement device for determining the differential pressure between two measurements points spaced apart in the longitudinal direction of said pipeline parts, as well as at both measurement points an ultrasonic flowmeter for measuring flow velocity at a measurement point, and data processing means for calculating the gas and liquid flow rates from the data obtained by the measurement device and the ultrasonic flowmeters .

2. Apparatus according to claim 1, characterized in that said pipeline part and said measurement device comprise a Venturi differential pressure device (10) for measuring the differential pressure between two measurement points (24, 26) .

3. Apparatus according to claim 2, characterized in that one of the measurement points (26) is positioned in the throat section (16) of the Venturi tube.

4. Apparatus according to claim 2 or 3, characterized in that one of the measurement points (24) is positioned in the inlet section (12) of the Venturi tube.

5. Apparatus according to one of the preceding claims, characterized in that at a measurement point (24, 26) an ultrasonic flowmeter having at least two pairs of transducers (30, 32, 36, 38; 42, 44, 48, 50) is provided in such an arrangement that their acoustic interrogation paths (34, 40; 46, 52) cross each other.

6. Apparatus according to claim 5, characterized in that the pairs of transducers (30, 32, 36, 38; 42, 44, 48, 50) are arranged such that the acoustic interrogation paths thereof (34, 40; 46, 52) are in horizontal planes at different heights.

7. Method for measuring gas and liquid flow rates of a two-phase gas-liquid flow in a pipeline part having a non-constant cross- section over its length, comprising determining the differential pressure between two measurements points spaced apart in the longitudinal direction of said pipeline, and at both measurement points measuring the transit time of a soundwave from an ultrasonic flowmeter at a measurement point, and calculating the gas and liquid flow of the two-phase gas-liquid flow from the differential pressure and the transit times.

8. Method according to claim 7, characterized in that said pipeline part is a Venturi differential pressure measurement device (10) for measuring the differential pressure between two measurement points .

9. Method according to claim 8, characterized in that the differential pressure is measured between a measurement point (26) in the throat section (16) of the Venturi tube and another measurement point.

10. Method according to claim 8 or 9, characterized in that the differential pressure is measured between a measurement point (24) in the inlet section (12) of said Venturi tube and another measurement point .

11. Method according to one of the preceding claims 7-10, characterized in that at a measurement point (24, 26) transit times are measured along two acoustic interrogation paths (34, 40; 46, 52) , which paths cross each other.

12. Method according to one of the preceding claims 7-11, characterized in that at a measurement point (24, 26) transit times are measured along two acoustic interrogation paths (34, 40; 46, 52) in horizontal planes at different heigths .

13. Method according to one of the preceding claims 7-12, characterized in that the method also comprises the calculation of the volumetric flow rates of gas and liquid.

14. Method according to one of the preceding claims 7-13, characterized in that the method also comprises the determination of the distribution of liquid present in the gas.

15. Method according to one of the preceding claims 7-14 characterized in that the method also comprises the determination of the asymmetry in the velocity profile of the gas due to the presence of liquid.

16. Method according to one of the preceding claims 6-15, characterized in that the method also comprises the determination of the flow pattern of the two-phase flow.

17. Apparatus for measuring gas and liquid flow rates of a two- phase gas-liquid flow in a pipeline, comprising a pipeline part having a non-constant cross-section over its length, and a measurement device for determining the differential pressure between two measurements points spaced apart in the longitudinal direction of said pipeline parts, as well as, at one of the measurement points, an ultrasonic flowmeter for measuring flow velocity at that measurement point, and data processing means for calculating the gas and liquid flow rates from the data obtained by the measurement device and the ultrasonic flowmeters.

18. Method for measuring gas and liquid flow rates of a two-phase gas-liquid flow in a pipeline part having a non-constant cross- section over its length, comprising determining the differential pressure between two measurements points spaced apart in the longitudinal direction of said pipeline, and at one measurement point measuring the transit time of a soundwave from an ultrasonic flowmeter at a measurement point, and calculating the gas and liquid flow of the two-phase gas-liquid flow from the differential pressure and the transit time.