WO1995033980A1

WO1995033980A1 - Multiphase fluid flow rate and density measurement

Info

Publication number: WO1995033980A1
Application number: PCT/GB1995/001221
Authority: WO
Inventors: Miroslav M. Kolpak; Terry J. Rock
Original assignee: Atlantic Richfield Company; Arco British Limited
Priority date: 1994-06-07
Filing date: 1995-05-26
Publication date: 1995-12-14

Abstract

The density of a multiphase fluid mixture (gas and liquid) and the volumetric flow rate of the liquid and gas in the mixture may be determined by measuring the hydrostatic pressure difference across a span of a horizontal conduit section or a vertical conduit section by pressure transmitters spaced apart opposite each other on the horizontal conduit leg or spaced apart vertically along a vertical conduit leg. The pressure difference due to hydrostatic pressure only may be determined in the vertical conduit leg by also measuring the pressure difference along a span of a horizontal conduit leg of length equal to the span of the pressure difference measurements in the vertical leg to eliminate calculation of the pressure drop due to friction. Volumetric flow rates of gas and liquid may be determined based on the density measurements and the volumetric flow rate of the fluid mixture.

Description

MULTIPHASE FLUID FLOW RATE AND DENSITY MEASUREMENT

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention pertains to a differential pressure measurement system and method for measuring the flow rate and density of a mixture of gas and liquid and particularly useful in oil and gas production operations.

Background

In many oil and gas production operations, it is desirable to be able to measure with moderate accuracy the volumetric flow rate of a mixture of gas and liquid, the volumetric flow rate of the gas and liquid components of the mixture and the density of the mixture. For example, in the multiphase fluid flow splitting system described in copending patent application entitled: "Multiphase Fluid Flow Splitting and Measurement" by Miroslav M. Kolpa , filed concurrently herewith and assigned to the assignee of the present invention, mixtures of water and natural gas are reinjected into selected earth formations to store the gas in the formations and to also serve, to some extent, as a drive fluid for stimulating the production of petroleum liquids from the formations. It is considered necessary and desirable to measure the volumetric flow rate of the fluid mixture and the volumetric flow rate of one or both fluids in the mixture at particular points in such a mixture distribution system to control the flow to several injection wells and to maintain a desired gas to liquid ratio, for example.

Still further, there are applications for flow measurement in oil and gas production operations wherein a multiphase (gas and liquid) mixture is being conducted from a production well and it is desired to know the density as well as the volumetric flow rate of the fluids in the mixture. One such system is described in U.S. Patent 4,766,210, issued Oct. 11, 1988 to L. A. Baillie, et al and also assigned to the assignee of the present invention.

However, the present invention provides an improved system and method for measuring the density of a multiphase fluid mixture and also the volumetric flow rate of the mixture and its phase components.

SUMMARY OF THE INVENTION

The present invention provides an improved system for measuring the density and the flow rate of a multiphase fluid mixture comprising gas and liquid.

In accordance with an important aspect of the present invention, a system is provided for measuring the density of a multiphase fluid flowstream by measuring a pressure differential in the flowstream along a predetermined length of a vertical run of a conduit conducting the flowstream and also by measuring the pressure differential of the flowstream along a predetermined length of a horizontal run of the conduit to determine the pressure differential component which is due to friction flow losses.

In accordance with another important aspect of the present invention, an improved density measurement device is provided for measuring the density of a multiphase fluid flowing through a substantially horizontal run of a conduit at a predetermined point in the conduit by measuring the differential pressure across the width or diameter of the conduit more accurately than with certain prior art devices. The system for measuring the pressure differential across the conduit width or diameter includes spaced apart pressure transmitter devices which are effective to eliminate (a) errors due to changes in the liquid/gas interface in the pressure sensing device such as are experienced with so-called diaphragm type differential pressure transducers and (b) complicated runs of tubing required for installation, protection and effective operation of diaphragm type differential pressure measuring devices. Still further, the improved systems of the invention eliminate pressure biasing caused by the dynamic response of the pressure sensing tubes and diaphragm systems in prior art diaphragm type differential pressure measurement devices.

Still further, the present invention provides a device for measuring the density and flow rate of a multiphase fluid flow mixture which does not require a complicated densitometer such as the so-called gamma ray type. Elimination of the gamma ray type densitometer eliminates or reduces the requirements of a nuclear device safety program, such as wipe tests and procedures mandated by regulatory agencies. Other disadvantages of nuclear or gamma ray type densitometers include lack of sensitivity and large noise to signal ratio when measuring fluid density in a conduit which is conducting primarily gas.

Still further, the present invention provides an improved method for measuring the volumetric flow rate of the component phases (gas and liquid) in a multiphase fluid flowstream, which is particularly useful in certain operations such as injection of a water and gas mixture into a subterranean formation and in the production of petroleum liquids and/or gas wherein mixtures of both are emanating from an earth formation.

Those skilled in the art will recognize the above- described advantages and features of the invention, together with other superior aspects thereof upon reading the detailed description which follows in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a somewhat schematic view of one embodiment of a system for measuring the density and flow rate of a multiphase fluid mixture;

Figure 2 is a diagram showing another embodiment of a system for measuring the density and flow rate of a multiphase fluid mixture;

Figure 3 is a cross-sectional view of the conduit and pressure transmitter arrangement for the embodiment of Figure

2;

Figure 4 is a detail view of part of a pressure transmitter useful in the systems of the present invention;

Figure 5 is a section view taken generally along the line 5-5 of Figure 6;

Figure 6 is a longitudinal view, partially sectioned, of an alternate embodiment of a pressure transmitter; and

Figure 7 is a somewhat schematic diagram of a second alternate embodiment of a system for measuring multiphase fluid mixtures.

DESCRIPTION OF PREFERRED EMBODIMENTS In the description which follows, like elements are marked throughout the specification and drawing with the same reference numerals, respectively. The drawing figures are not necessarily to scale in the interest of clarity and conciseness.

Referring to Figure 1 there is illustrated a system in accordance with the present invention, generally designated by the numeral 10, which is operable to measure the density and flow rate of a multiphase fluid mixture flowing through a conduit 12. The conduit 12 has a vertical run or leg 14 and a horizontal run or leg 16 which are adapted to comprise part of the system 10. A multiphase fluid mixture, such as a mixture of natural gas and water, flows through the conduit 12 in the direction of the arrows indicated in Figure 1. A so called static fluid mixing device 18 is preferably interposed in the conduit 12 upstream of certain parts of the system 10 to be described herein to assure uniform mixing of a gas/liquid mixture passing through the system. The mixing device 18 may be of a type commercially available, for example, a type which is described in U.S. Patent 4,123,178, issued October 31, 1978 to R. N. Salzman, et al. A suitable static mixing device may also be of the type described in U.S. Patent 4,824,614 to J. A. Jones and issued April 25, 1989. For a conduit having a nominal diameter of 2.0 inches, the static mixing device 18 should be disposed just upstream of the pressure and flow rate measuring components of the system 10, preferably no more than about 2.0 feet, when measuring a mixture of water and gas wherein the gas content is about twenty percent (20%) or less by volume of the total mixture.

Referring further to Figure 1, the system 10 includes two spaced-apart pressure transmitters 20 disposed a distance L from each other along the vertical leg 14 of the conduit 12 and a second set of pressure transmitters 20 disposed apart the same distance L along the horizontal leg 16 of the conduit 12. A conventional volumetric flowmeter 22 is interposed in the conduit 12 where indicated. The flowmeter 22 may be of the so- called turbine type suitable for measuring a multiphase fluid mixture. The pressure transmitters 20 are preferably of a type which generates or modifies an electrical signal in response to sensing a pressure in the conduit 12 at the location of the transmitters, respectively. For example, one type of pressure transmitter which may be suitable for the system 10 is of a type which utilizes a piezoelectric sensing element, such as available from SDR of Lenexa, Kansas as their type 534 MP.

Figure 4 illustrates certain details of one embodiment of a pressure transmitter generally of the type described above having a shank portion 23 and a pressure transmitting surface 25 formed thereon and which may be exposed to the fluid mixture flowing through conduit 12. Although other types of pressure transmitters may be used in conjunction with the system and method of the present invention, the pressure transmitter 20 having a pressure responsive surface 25 which may be mounted substantially coplanar with the inside wall of the conduit 12 so as to minimize flow disturbances and the resultant transmitter signal errors which may be incurred. The output signals from the pressure transmitters of the system 10, comprising transmitters 20a, 20b, 20c and 20d, and the output signal from the flowmeter 22 may be processed by a suitable signal processing and computing device generally designated by the numeral 24.

The density of the fluid mixture flowing through the conduit 12 may be determined by applying the Bernoulli equation for the pressure differential between the respective sets of pressure transmitters as follows:

(Px_d-Px_c)= (den) (iL/D) (V²) / (2g) ( 1 )

(P₂o_a-P₂o_b)= (den) (fL/D) (V²) I (2g) + (DEN) (L) (2)

where den is the density of the fluid mixture in pounds per cubic foot, V is the superficial or indicated velocity of the mixture flowing the conduit 12 in feet per second, f is the friction factor of the inside wall of the conduit 12, g is the acceleration of gravity (32.2 feet per second squared), L is the length between the respective sets of pressure ports for the transmitters in feet and the pressures sensed at the ports for the transmitters 20a, 20b, 20c and 20d are measured in pounds per foot squared. Equations (1) and (2) above define the pressure differential across the length L for frictional pressure losses in the conduit 12 and equation (2) includes an additional term which is the hydrostatic pressure of a column of length L of material having a density den in the vertical leg 14 of the conduit 12. Combining the equations and solving for the density, den, yields:

den= [ (P_20a-P_20b) - (P_20d-P₂0 l /L ( 3 )

This equation (3) is particularly valid if the pipe diameter and pipe roughness are the same in both the vertical and horizontal legs 14 and 16 and the flow velocity is sufficient to minimize liquid fall back or plugging of the vertical leg.

The pressure differentials across the respective lengths L of the horizontal leg 16 and the vertical leg 14 are measured by the respective sets of pressure transmitters and a computation of the density, den, may be carried out by the signal processor and computer 24. Appropriate averaging or filtering techniques may be used as well as proper selection of transducer "zero" and "span" values to minimize signal bias. Still further, the flow rate of the multiphase fluid mixture may be measured directly by the flowmeter 22 and the flow rates of the components of liquid and gas in the fluid mixture can be calculated,as will be described hereinbelow, to determine, for example, if the desired gas-to-liquid ratio of fluid flow is being conducted through the system, or at least the volumetric flow rate of the liquid and the gas may be ascertained for accounting purposes. The densities of the liquid and the gas are assumed or may be separately measured at the flow conditions in the system 10. For example, if the system 10 is used to inject a mixture of natural gas and water into an earth formation for storage of the gas and for use as a drive fluid for petroleum liquids in the formation, the densities of the liquid and gas may easily be ascertained.

The gas-to-liquid ratio (GLR) is the ratio of the volumetric flow rate of gas (Qg) divided by the total flow (Qg + Ql) where (Ql) is the liquid volumetric flow rate. The gas-to-liquid ratio of the fluid mixture in the combined flowstream may be computed from the following equation:

wherein dl and dg are the densities of the liquid and the gas, respectively, (in pounds per cubic foot). Accuracies in the range of about ten percent (10%) variance from actual values should be achievable over much of a practical gas-to-liquid ratio range including a range wherein the volumetric fraction of gas in the total mixture is up to about twenty percent (20%).

The respective gas and liquid volumetric flow rates are thus:

Q^QJl -GLR) (5)

Q =Q_m (GLR) (6)

where Qm is the volumetric flow rate measured by the flowmeter 22. The system 10 for determining volumetric flow rates of liquid and gas in a multiphase fluid mixture is particularly advantageous in that the density of the fluid mixture is obtainable without the use of a nuclear type densitometer and the attendant maintenance and safety procedures that must accompany the use of such devices. Moreover, with the accuracy of the flowmeter 22 and the pressure transmitters 20a, 20b, 20c and 20d, measurement errors associated with certain other types of differential pressure transmitter or transducer devices are minimized. However, as indicated by equation (3), the fluid friction factor is not required to be known in order to make a determination of the fluid mixture density when a set of pressure transmitters is disposed in a horizontal run of the system and spaced apart the same distance as the distance between the pressure transmitters of the vertical leg of the conduit.

Referring now to Figures 2 and 3, an alternate embodiment of a system for determining density and flow rate of a multiphase fluid mixture is illustrated and generally designated by the numeral 40. The system 40 includes a generally horizontally extending cylindrical conduit 42 which, for sake of discussion, is in communication with a source of a fluid mixture comprising natural gas and water and is operable to conduct this fluid mixture to an injection well by way of a wellhead 44. A flowmeter 22 is interposed in the conduit 42 and a set of pressure transmitters 20e and 2Of,similar to the transmitters in the system 10, are arranged opposed to each other along a vertical line 46 intersecting the central longitudinal axis 47 of the conduit 42, see Figure 3. In order to minimize trapping of gas bubbles, or the accumulation of liquid or debris in the interior of the conduit 42, the pressure surfaces 25 of each of the pressure transmitters 20e and 20f are substantially coplanar with opposed planar surfaces 50 and 52 formed by respective filler or insert parts 54 and 56, which may be interposed in the conduit 42 as illustrated in Figures 2 and 3. These insert parts 54 and 56 may be faired into the surface of the interior of the conduit 42 at their inlet and discharge ends, respectively, to minimize turbulence in the flowstream of the fluid mixture passing through the conduit from a mixing device 18 to the flowmeter 22, for example.

As shown in Figure 3, the pressure transmitting surfaces 25 of the opposed transmitters 20e and 20f are spaced apart a vertical distance "d". In the system 40 the density, den, may be determined from the equation:

where P20f and P20e are the pressures sensed by the pressure transmitters 2Of and 20e, respectively. With the mixture flow rate and density of the fluid mixture known, the gas-to-liquid ratio (GLR) and the volumetric flow rates of the liquid and gas flowing through the system 40 may be determined from equations (4) through (6) above.

The pressure transmitters 20e and 20f may also be of a type illustrated in Figures 5 and 6. Referring to Figures

5 and 6, there is illustrated a modified conduit 42 which has interposed therein opposed segments 66 and 68 which are pressure sensitive but which also conform to the shape of the inside wall 43 of the conduit 42 so as not to provide a flow restriction or a point of accumulation of debris. The segments

66 and 68 may be flexible diaphragm members made of a suitable elastomeric material or made of material which, in response to deflection, emits an electrical signal. In the embodiment shown, the segments 66 and 68 define opposed cavities 70 and

72, respectively, which are delimited by an annular ring member

74, as shown. The cavities 70 and 72 may be filled with a suitable pressure fluid which may be operable to transmit differential pressure forces to a pressure differential gauge or sensor 78 whose output signal may be transmitted to a signal processor and computer, not shown. Still further, the pressure transmitters utilized in conjunction with the present invention may be of a type manufactured by Camille Bauer, Inc., Tempe,

Arizona, as their RS series.

Referring now to Figure 7, there is illustrated another embodiment of a system in accordance with the present invention and generally designated by the numeral 90. The system 90 is adapted to handle multiphase fluid flow wherein, from time to time, so-called slug flow is encountered and also wherein a relatively high gas fraction (as high as .99) may be encountered and further wherein the accuracy of measurement of the liquid and gas is required to be greater than the ten percent (10%) variance discussed for the systems 10 and 40. The system 90 includes a fluid inlet conduit 92 connected to an enlarged diameter conduit part 94 which may have a plurality of spaced apart transverse baffles 96 interposed therein. The baffles 96 extend substantially half the diameter of the conduit part 94 from the lower side thereof and are preferably perforated to thoroughly mix the fluid flowing through the conduit part 94. Accordingly, fluid flow leaving the conduit part 94 is essentially uniform even though the in-flow to the conduit part 94 may be in the so-called slug regime. Uniform liquid flow through the system 90 will provide for approximately five percent (5%) net oil measurement accuracy whereas slug flow will not provide such accuracy due to, among other things, rapid and large transient values in the differential pressure measurements.

Fluid flow leaving the enlarged diameter conduit part 94 enters a somewhat loop conduit arrangement comprising a smaller diameter horizontally extending conduit section 98 connected to a first vertical leg 100, a reverse horizontal leg 102, a second vertical leg 104 and a horizontal exit leg 106. A suitable venturi 108 is interposed in the inlet leg 98 and a differential pressure transmitter arrangement including a differential pressure measurement device 110 is provided and connected to pressure transmitters 112 and 114. The transmitter 114 is located approximately at the. throat of the venturi 108. The differential pressure transmitter 110 is operable to send an appropriate signal to a signal processor and computer unit 116 similar to the units 24. The vertical conduit leg 100 includes spaced apart pressure transmitters 120a and 120b which are suitably connected to a differential pressure signal transmitter device 122, also operably connected to the signal processor 116. Still further, the horizontal conduit leg 102 is provided with pressure transmitters 120c and 12Od which are suitably connected to a differential pressure signal transmitter 126, also operably connected to the processor 116. The arrangement of the pressure transmitters 120a, 120b, 120c and 120d is similar to the arrangement of the pressure transmitters in the embodiment of Figure 1.

The system 90 further includes a suitable meter 130 for measuring the water fraction of the liquid mixture flowing through the system. An inlet port 132 of the meter 130 is connected to the conduit 104 at a suitable flow mixing device 136. And a return port 138 is connected to a venturi 140 interposed in the conduit leg 104 to provide a pressure drop suitable to cause flow through the water fraction meter 130. The water fraction meter 130 may be of a type similar to that described in U.S. Patent 5,157,339, issued October 20, 1992 to Bentley N. Scott, et al and assigned to the assignee of the present invention. Still further, the system 90 includes a gas densitometer 144 adapted to measure a slipstream of gas taken through a conduit 146 from the top side of conduit part 94 and rejected to the system at the venturi 140. The gas densitometer 144 may be of a type manufactured by Automation Products, Inc., Houston, TX, under the trademark Dynatrol. The output signal from the gas densitometer 144 is also treated by the processor 116.

Fluid flow entering the conduit part 94 is made essentially uniform, then passes through the horizontal leg 98 and the venturi 108, then through the vertical leg 100, the horizontal leg 102, the vertical leg 104 and exits the system through the horizontal leg 106 which, in the view of Figure 7, is disposed behind the leg 98. The differential pressure measurement arrangement including the transmitters 112 and 114 and the signal measurement and transmitter device 110 effectively comprises a flowmeter since the differential pressure measurement of the total flow is proportional to the density of the total flow times the flow velocity squared. The cross sectional area of the pipe leg 98 may be easily determined and included in a calculation of the volumetric flow rate once the velocity has been determined based on the differential pressure measurement and the density measurement of the fluid flow from Equation (3) and the pressure differential measurements taken in the vertical leg 100 and the horizontal leg 102.

The systems 10, 40 and 90 may find several applications in measuring multiphase fluid mixtures. For example, the system 10 may be operated in conjunction with the liquid or gas outflow line from a liquid-gas separator vessel 60, Figure 1, (liquid outflow line shown) to determine whether or not gas or liquid carry-over is being experienced in the respective outflow lines. The system 40 in Figure 2 is, as previously described, interposed in a conduit system for conducting a gas and liquid mixture to an injection well. By monitoring the density and gas-to-liquid ratio of the mixture flowing through the conduit 42, any change in the prescribed mixture may be detected and suitable adjustments to the inputs of the flowstream may be carried out. Those skilled in the art will recognize that certain other applications of the systems 10, 40 and 90 may be advantageous.

The systems 10, 40 and 90 be constructed using conventional materials and components used in fluid flow measuring devices and including the components specifically identified above.

Although preferred embodiments of an improved flow rate and density measurement system and method have been described in detail herein, those skilled in the art will appreciate that certain substitutions and modifications may be made without departing from the scope and spirit of the appended claims.

What is claimed is:

Claims

1. A system for measuring the flow rate of gas and liquid in a gas and liquid mixture flowing through a conduit comprising: pressure transmitter means interposed in said conduit in a way which is operable to measure a pressure differential due to hydrostatic pressure of said fluid mixture; and a flowmeter interposed in said conduit for measuring the flow rate of said fluid mixture.

2. The system set forth in Claim 1 wherein: said conduit includes a vertical leg and said pressure transmitter means comprises means for measuring the pressure of the fluid mixture in said vertical leg at two points vertically spaced apart a predetermined span on said vertical leg.

3. The system set forth in Claim 2 including: pressure transmitter means connected to a horizontal leg of said conduit and operable to measure a pressure differential along a span of said horizontal leg which is equal to the span of the points of measurement of pressure differential along said vertical leg whereby the density of the fluid mixture flowing through said conduit may be determined by the difference between the differential pressures measured along said vertical leg and along said horizontal leg divided by the span.

4. The system set forth in Claim 1 including: a flow mixing device interposed in said conduit upstream of said pressure transmitter means with respect to the direction of flow of said fluid mixture through said system.

5. The system set forth in Claim 1 wherein: said pressure transmitter means comprises means for measuring a differential pressure across a predetermined vertical distance in a generally horizontal leg of said conduit.

6. The system set forth in Claims 3 or 5 wherein: said pressure transmitter means comprises a pair of pressure transmitters for measuring said pressure differential at selected points of measurement in said conduit.

7. The system set forth in Claim 6 wherein: said pressure transmitters are provided with piezoelectric sensing elements for sensing pressure applied to a pressure surface of said transmitters, respectively.

8. The system set forth in Claim 1 including: a gas densitometer operable to measure the density of a quantity of gas separated from said fluid mixture.

9. The system set forth in Claim 1 including: a water fraction meter adapted to measure the water fraction in the fluid mixture.

10. A system for measuring the density and flow rate of a fluid mixture comprising a gas and a liquid whose densities are known, respectively, comprising: a generally cylindrical conduit for conducting said fluid mixture; a flowmeter interposed in said conduit for measuring the flow rate of said fluid mixture; a pair of opposed pressure transmitters connected to said conduit and operable to sense a differential pressure due to the density of said fluid mixture across a vertical span of said conduit whereby the density of the fluid mixture may be determined by the differential pressure sensed by said pressure transmitters divided by the span between pressure sensing surfaces of said pressure transmitters, respectively.

11. The system set forth in Claim 10 wherein: said pressure transmitters include piezoelectric elements for sensing pressure of the fluid mixture in said conduit.

12. The system set forth in Claim 11 wherein: said conduit is provided with opposed planar surfaces which are substantially coplanar with the pressure sensing surfaces of said pressure transmitters, respectively.

13. A method for measuring the volumetric flow rate of gas and liquid in a multiphase fluid mixture flowing through a conduit having a vertical leg therein, comprising the steps of: measuring the pressure difference between the pressures of the fluid mixture in said vertical leg of said conduit at points spaced apart in said vertical leg a span L; measuring the pressure difference between points in said conduit along a horizontal leg thereof at points spaced apart also by a span L; and determining the density of the fluid mixture by dividing the difference between the pressure differentials measured along said vertical leg and said horizontal leg by said span.

14. The method set forth in Claim 13 including the step of: determining the density of the gas and the liquid in said fluid mixture and determining the gas-to-liquid ratio of the fluid mixture from the density of the gas, the density of the liquid and the density of the fluid mixture measured.

15. The method set forth in Claim 14 including the step of: measuring the volumetric flow rate of said fluid mixture flowing through said conduit and determining the volumetric flow rate of the liquid and the gas based on the volumetric flow rate of the fluid mixture and the gas-to-liquid ratio.

16. A method for determining the volumetric flow rate of at least one phase of a fluid mixture flowing through a conduit having a generally horizontal leg, comprising the steps of: measuring the difference in pressure across a vertical span, d, and determining the density of the fluid mixture by dividing the difference in pressure across said span by said span; measuring the volumetric flow rate of said fluid mixture flowing through said conduit; determining the gas-to-liquid ratio of said fluid mixture based on the density of the fluid mixture and the densities of the gas and the liquid in the fluid mixture; and determining the volumetric flow rate of at least one of the liquid and gas in the fluid mixture from the volumetric flow rate of the fluid mixture and the gas-to-liquid ratio.