GB2335494A

GB2335494A - Venturi Flow Meter

Info

Publication number: GB2335494A
Application number: GB9805626A
Authority: GB
Inventors: Neil Irwin Douglas; Lloyd Stan Bailey; James Brian Wilson
Original assignee: ABB Seatec Ltd
Current assignee: Baker Hughes International Treasury Services Ltd
Priority date: 1998-03-16
Filing date: 1998-03-16
Publication date: 1999-09-22
Also published as: AU2847699A; GB9805626D0; WO1999047895A1

Abstract

A Venturi flow meter comprises generally an inlet section 1, a divergent cone 2, a mixer section 3, a convergent cone 4 and an outlet section 5. The divergent cone 2 provides a gradual progression from the circular cross-section of the inlet section 1 to an elliptical cross-section of a larger cross-sectional area than the inlet section 1. The mixer section 3 maintains this elliptical cross-section for the whole of its length. The convergent cone 4 provides a shallow progression from the elliptical cross-section of the mixer section 3 to the circular cross-section of the outlet section 5. This feature makes the flow meter particularly suitable for use in oil production tubing in locations where there is limited space, such as within a borehole. By locating a channel 8 and a channel 13 at positions where fully developed flow would be experienced under usual flow conditions, measurement transducers 10 and 12 will experience substantially constant static pressures. Electronic flow measurement means 14 comprises means to detect the pressure differential between the first position and the second position and means to derive from this pressure differential a measurement of the flow rate of a fluid through the insert. This is achieved by applying Bernoulli's equation to the detected pressure differential having regard to the density of the fluid and the known internal dimensions of the insert. The device is arranged such that the inlet and outlet sections are inserted in sections of well casing, whilst the middle section is located against the walls of the borehole.

Description

1 A FLOW METER 2335494 The present invention relates to a flow meter which

can be applied to any environment where measurement of the flow of any liquid or gas is required; and to a borehole having tubing including such a flow meter.

The invention arose, however, in the design of a flow meter for measuring the flow of oil and water mixtures in the production tubing of a well.

It is usual, when it is desired to extract oil from an underground reservoir, to drill and subsequently line a borehole with a casing. This casing comprises a steel pipe which is inserted and cemented into position within the borehole. Production tubing is passed down inside the casing such as to extend from the mouth of the borehole to the reservoir, near to the bottom of the borehole; and the space or annulus between the casing and the tubing may be sealed at points along the length of the tubing using devices known as packers. In use, oil enters the production tubing either through its open lower end or through a choke device located at some position along the tubing's length between two packers. In a sophisticated systen74 it is expected that flow rate, temperature and pressure measurements will be made of the oil at points within the borehole, and these measurements will be used in controlling the choke devices. Accordingly, the need arises to measure oil flow rate at different positions within the production tubina. A problem found with certain conventional flow meters is that they reduce the effective diameter of the tubina and must be removed to allow the passage of tools to upstream tubin.g. The removal process is costly, time consuming, and risky 2 In accordance with this invention, there is provided a flow meter for measuring fluid flow in a duct comprising: means to provide a region of increased cross-sectional area in the duct; means for detecting a pressure differential between a first position outside of the region and a second position within the region; and means for deriving from said detection a measurement of flow rate within the duct.

The invention can thus be viewed as being an inverse Venturi flow meter. Ordinary Venturi flow meters are known, in which by narrowing the cross-sectional area of the flow path and measuring the pressure differential between a position within the reduced cross-sectional area and a position outside of that area and by applying Bernoulli's equation to the result, a measure of flow rate can be obtained but at the expense of restricting the flow and, worse still, obstructing the movement of tools and instrumentation that may be desired to be carried through the tubing.

The present invention contemplates, for the first time, that the Venturi effect can be applied in inverse by enlarging the cross-sectional area of the flow path, rather than reducing it, to provide a flow meter.

C1 In this flow meter, the second position may be a position at which, under usual flow conditions, a substantially constant pressure would be observed. This arrangement may allow more accurate measurements to be made by the deriving means and/or may allow the use of less complex deriving means than would be used with respect to a non constant pressure position.

3 Either of the above flow meters may be constructed such that the insert has first and second channels allowing communication of pressure information from respective ones of the first and second positions on its inner surface to its outer surface. This allows pressure measurement transducers and any associated electronics to be located outside of the duct, away from the fluid flow, which could increase its lifelreliability, especially if the fluid flow presents a harsh environment. These channels are preferably drilled in the body of the insert. Preferably, the second channel is arranged so as to pass first radially and then longitudinally. This feature may allow the insert to occupy minimal space radially from the axis of the duct. This is of particular significance where the insert is to be used within an oil producing borehole, or other narrow bore applications.

In accordance with a second aspect of the present invention, a borehole for producing C oil or gas comprises an outer casing surrounding an oil or gas production tubing comprising a flow meter in accordance with the first aspect of the invention installed in the tubina, the outer diameter of an insert providing the region of increased cross- C1 sectional area being such as to form a sliding fit within the outer casing.

The invention is particularly applicable to use in oil wells because the area of increased diameter can be dimensioned so as to fit as a sliding fit into the outer casing and thus provide support helpful to prevent vibrations interfering with the measurements obtained by the flow meter. Also, the insert can be fitted into the production tubing so that it slides throuah the casing as the production tubing, which may be up to 6 km long, is installed or lowered into the borehole.

4 The space between the casing and the tubing in the borehole may be used to accommodate service lines designed to provide electrical and/or hydraulic services to control or instrumentation equipment down the borehole. These service lines may include hydraulic power tubes, electrical power conductors andlor electrical communications conductors needed to supply control signals and power to chokes and other control and instrumentation devices which may be down the borehole. The insert at the region of increased cross-sectional area preferably has at least one recess on its outer surface to accommodate such service lines. This may in turn make it desirable to make the region of increased cross-sectional area have a non-circular cross-section, having a smaller diameter adjacent the or each recess. The internal cross- section of the insert may, for example, be elliptical or oval.

The preferred form of pressure sensor is a high accuracy quartz sensor, or could be a strain gauge, a displacement gauge or any other type.

In accordance with a third aspect of the present invention, A method of measuring fluid flow in a duct having a region of increased cross-sectional area, comprises: detecting a pressure differential between a first position outside of the region and a second position within the region; and deriving from said detection an indication of flow rate within the duct, the second position being such as to experience fully developed flow under usual flow conditions.

An embodimept of the present invention will now be described, by way of example only, with reference to the accompanying drawings of which:

Z-- 1 t Fl(zure 1 shows a longitudinal cross-section of a flow meter in accordance with the present invention; and Figure 2 shows a side view of the Figure 1 flow meter and a cross-section of the flow meter installed in a borehole.

In Figure 1, an insert comprises generally an inlet section 1, a divergent cone 2, a mixer section 3, a convergent cone 4 and an outlet section 5. The insert is formed by the welding together of separately machined stainless steel sections. The inlet section 1 includes a threaded portion 6 which, in use, is threaded into the oppositely-threaded section of upstream production tubing (not shown). The outlet section 5 similarly includes a threaded portion 7 for threading into an oppositely-threaded portion of downstream production tubing (not shown). The connections between the inlet section 1 and the outlet section 5 and their respective production tubing is preferably such as to provide substantially no resistance to the flow of fluid both on entry to and on exit from the insert and no restriction to the movement of tools andlor instrumentation into and out of the insert.

The inner bore of the inlet section 1 has a circular cross-section of constant diameter, as has the inner bore of the outlet section 5. The divergent cone 2 provides a gradual prooTession from the circular cross-section of the inlet section 1 to an elliptical cross section of a larger cross-sectional area than the inlet section 1. The mixer section 3 maintains this elliptical cross-section for the whole of its length. The convergent cone 4 provides a shallow progression from the elliptical cross-section of the mixer section 6 3 to the circular cross-section of the outlet section 5. Thus, the insert does not provide at any point along its length a cross-section having narrower dimensions than the circular cross-section of the inlet section 1, which is the same as the crosssection of the outlet section 5. A channel 8 is formed by drilling to extend radially from the inner surface of the insert at an apex of the elliptical cross-section of the mixer section 3 to the outer surface of the insert. This channel is intercepted by a second channel 9 which extends longitudinally through the wall of the insert to a pressure measurement transducer 10.

A bung 11 is located in the first channel 8 such as to cause the pressure sensor 10 to be in fluid contact only with the inner bore of the mixer section 3 of the insert. A second pressure transducer 12 is in fluid contact with the inner bore of the outlet section 5 by way of a further channel 13 which extends axially through the wall of the insert. The measurement transducer 10 and the measurement transducer 12 are connected by way of respective ones of twisted pairs 15 and 16 to an electronic flow measurement means 14 attached to the outer surface of the insert adjacent to the outer section 5. The pressure measurement transducers 10 and 12 and the electronic flow measurement means 14 are mechanically protected from the environment by way of a protective cover 17.

A similar flow arrangement 18 is located on the opposite side of the insert to the arrangement 8-17. This arrangement 18 will be used in the event of failure of the first arrangement 8-17. This redundancy is of particular use in borehole applications, where replacement of the flow meter may necessitate the extraction of the whole of the production tubing from the well, which can be very expensive, time consuming and risky.

7 The channel 8 is located at a second position a distance from the divergent cone 2 calculated such as to be sufficient to allow fully developed flow of the fluid thereat under all expected flow conditions. It will be understood that fully developed flow occurs when the velocity profile of the fluid does not change with distance along the direction of flow. Flow can be fully developed even though there may still be turbulence. The distance between the second position, at which the first channel 8 intersects with the inner bore of the mixer section 3, and the intersection of the mixer section 3 with the divergent cone 2 is dependent on the nature of the fluid, the maximum expected fluid flow, the cross-sectional area of the inlet section 1, the cross-sectional area and shape of the cross-section of the mixer section 3, the length of the divergent cone 2 and the accuracy of flow measurement required.

The second position must also be at sufficient distance from the intersection of the rrdxer section 3 and the convergent cone 4 such that the changes to the flow brought about by the convergent section 4 do not have a significant bearing on the static pressure experienced at the second position.

The radial channel 13 is located in the outlet section 5 at a sufficient distance from the intersection of that section 5 with the convergent cone 4 such that the pressure measurement transducer 12 experiences fully developed flow of the fluid in the outlet section 5. By locating the channel 8 and the channel 13 at positions where fully developed flow would be experienced under usual flow conditions, the measurement transducers 10 and 12 will experience substantially constant static pressures. The pressure and measurement transducers 10 and 12 may each be a high accuracy; quartz 8 sensor, a strain gauge measurement transducer. or any other type.

The electronic flow measurement means 14 comprises means to detect the pressure differential between the first position and the second position and means to derive from the pressure differential so detected a measurement of the flow rate of a fluid through the insert. This is achieved by applying Bernoulli's equation to the detected pressure differential having regard to the density of the fluid and the known internal dimensions of the insert.

Referring now to Figure 2, the insert is shown from a sideways view and in which the elements of Figure 1 have retained their reference numerals where they are shown. The outer diameter of a portion 20 of the insert is approximately equal to that of the outer diameter of the production tubing in which the insert is to be included. The outer diameter of a portion 21, however, is greater than the diameter of the portion 20 to accommodate the region of increased cross-sectional area provided by the divergent cone 2, the mixer section 3 and the convergent cone 4. The outer diameter of this portion 21 is selected so as to form a sliding fit within the outer casin C of the borehole. In this t> 9 way, the portion 21 of the insert will fit closely within the casing C and will prevent movement of the insert within the casing C, other than movement longitudinally along the casing C. Such longitudinal movement may also be reduced by the contact between the outer surface of the portion 21 and the casing C. The outer diameter of a portion 22 of the insert is larger than the outer diameter of the portion 20 because it must mate with a threaded portion of downstream production tubing on the inner surface thereof.

9 It will be observed that recesses 23 and 24 extend longitudinally from the section 20 to Z:1 1 the section 22 to allow service lines to extend past the insert within the borehole. These service lines may include hydraulic power tubes H, electrical power cables E and/or electrical communications conductors CC.

The orientation of the elliptical cross-section of the inner bore of the mixer section 3 dictates the location of the recesses 23 and 24 in the portion 21.

It will be appreciated that, instead of the electronic flow measurement means 14 being located within the protective casing 17 of the flow meter,signals from the transducers and 12 could be communicated up the borehole, possibly within the or another plastic casing referred to above, for processing in a flow measurement means located near the mouth of the borehole, or on a platform or the like some distance from the borehole. This would remove at least some electronics components from the environment of the borehole, which may therefore provide increased reliability.

Furthermore, pressure transducers could be used which, instead of detecting pressure at each of the first and second positions, detect pressure differentials. Such transducers may even be preferable to multiple discrete sensors.

Claims

2. A flow meter according to Claim 1 in which the second position is a position at which, under usual flow conditions, a substantially constant pressure would be observed.

A flow meter for measuring fluid flow in a duct comprising: means to provide a region of increased cross-sectional area in the duct; means for detecting a pressure differential between a first position outside of the region and a second position within the region; and means for deriving from said detection an indication of flow rate within the duct.
3. A flow meter according to Claim 1 or Claim 2 in which the region providing means has first and second channels allowing communication of pressure information from respective ones of the first and second positions on its inner surface to its outer surface.
4. A flow meter according to Claim 3 in which the second channel is arranged so as to pass first radially and then longitudinally.
5. A borehole for producing oil or gas comprising an outer casino, surrounding an CI o'I or gas production tubing comprising a flow meter in accordance with any of Claims l c 1 to 5 installed in the tubing, the outer diameter of an insert providing the region of It, C increased cross-sectional area beina such as to form a slidine, fit within the outer casino, c C C" 11
6. A borehole according to Claim 5 further comprising service lines designed to provide electrical and/or hydraulic services to control or instrumentation equipment down the borehole.
7. A borehole according to Claim 6 in which the insert at the region of increased cross-sectional area has at least one recess on its outer surface to accommodate the service lines.
8. A borehole according to Claim 7 in which the region of increased crosssectional area have a non-circular cross-section, having a smaller diameter adjacent the or each recess.
9. A flow meter substantially as shown herein or as described with reference to Figure 1 or Figure 2 of the accompanying drawings.

0
10. A method of measuring fluid flow in a duct having a region of increased crosssectional area, comprising: detecting a pressure differential between a first position outside of the region and a second position within the region; and deriving from said detection an indication of flow rate within the duct, the second position being such as to experience fully developed flow under usual flow CI conditions.