Measuring and testing – Volume or rate of flow – Using differential pressure
Reexamination Certificate
1999-05-28
2001-11-13
Patel, Harshad (Department: 2855)
Measuring and testing
Volume or rate of flow
Using differential pressure
C073S861630
Reexamination Certificate
active
06314821
ABSTRACT:
The present invention relates to flow monitoring apparatus for monitoring the flow of fluid in pipes. In particular, the invention is concerned with flow monitoring apparatus in which there are no moving parts and which is based on the venturi principle. The invention also relates to monitoring the flowrate of hydrocarbon products through the downhole pipeline which carries the products from an underground hydrocarbon reservoir and also relates to injection-type wells. In addition, the invention relates to the measurement of the downhole scale thickness. It will be understood that the hydrocarbon products can be in single-phase flow or multi-phase flow and can be an oil/water/gas mixture or a wet or dry/gas mixture.
Applicant's own co-pending British Patent Application No. 9600699.4 discloses an annular flow measurement apparatus and method which is achieved by locating a flow restrictor insert into the bore or a conduit of a pipe with a flow restrictor insert being arranged so as to create an annular flowpath around the insert. In a preferred arrangement, the flow restrictor insert has a leading upstream end which is disposed between first and second pressure monitoring stations, whereby pressures which are measured at these stations can be used to calculate flowrate in accordance with known established methodology. The flow restrictor insert is centrally and axially located in the bore and annular flowrates are created between the flow restrictor insert and the conduit bore wall, thereby eliminating the requirement for seals. With this arrangement the flow restrictor insert may be permanent or wireline retrievable. In addition, with this arrangement it is disclosed in one embodiment that the central flow restrictor insert may be modified to carry pressure gauges so that the entire flow monitoring tool could be wireline retrievable with the central part containing appropriate electronics and memory gauges.
A common problem in oil field production operations is the downhole deposition of solid scales. Such scales include, for example, calcium carbonate and calcium sulphate. Under suitable conditions these scales adhere to the production tubing and casing strings exposed to production fluids. These deposits exhibit considerable strength and commonly entirely coat the production string, thereby reducing the internal diameter for flow. All components exposed to the fluid, including the downhole flowmeter are susceptible to scaling. In most cases, the existence of downhole scaling is unknown, although it may be suspected from scaling tendency calculations. An exact knowledge of scale thickness may therefore not be known. Measured flowrate is extremely sensitive to scale deposition, being proportional to the diameter of the pipe squared and, accordingly, such scale deposition will result in major flowrate errors. Existing flow measurement apparatus does not provide any indication of scale thickness nor of flowrate errors caused by scale deposition.
An object of the present invention is to provide an improved apparatus and method of measuring flowrate which obviates or mitigates at least one of the disadvantages associated with the aforementioned apparatus.
A further object of the present invention is to provide an improved apparatus and method of measuring flowrate which improves flowmeter accuracy by calculating scale thickness and taking account of scale thickness when calculating flowrate measurement.
This is achieved, in one aspect of the invention, by providing a flow restrictor insert into the bore of a conduit or pipe with the flow restrictor insert being arranged so as to create an annular flowpath around the insert, and the flow restrictor insert having at least two substantially co-axial portions of different diameters with sufficient pressure monitoring stations being provided so as to allow the calculation of at least two pressure differentials. This may be implemented by providing three absolute pressure gauges which may be located at three appropriate locations in the bore of a carrier or by using two differential sensors connected to the monitoring stations to calculate pressure differentials from the three bores. The three pressure monitoring bores and two differential pressure sensors may be located within the flow restrictor insert, and pressure sensor being located in respective co-axial portions.
When three pressure sensors are used a third pressure differential can be calculated, although this is not required in the calculation. Redundancy can also be achieved by providing more than two differential pressure sensors. Using standard fluid mechanics analysis, a mathematical model is derived which computes gross flowrate from the three pressure measurements (non-differential) in a scaling environment. A value of scale thickness is a byproduct of the calculation and this is used as the-basis for a downhole scale indicator.
In the absence of scale deposition, or where the scale thickness is known and is assumed to be constant, then the aforementioned design provides flowrate backup in the event of gauge failure. Although scale thickness is not generally known, the thickness is available from this tool prior to gauge failure and can then be used post-failure to generate more accurate flowrates. As an approximation it is possible to extrapolate future deposition behaviour based on data gathered prior to failure.
A solution to the aforementioned problem is also achieved in accordance with another aspect of the invention wherein a multiple venturi flowmeter is used to calculate scale deposition. In this case, the multiple venturi flowmeter has a first throat restriction portion of a first diameter which leads to a second co-axial throat restriction of a second smaller diameter than the first diameter. Two pressure measurement stations are located in the main bore and in the areas opposite the first and second diameters and a third gauge is also located in the flowmeter, either in a converging portion or in a third co-axial portion bore, thus providing three absolute pressure measurements (which can be used to calculate pressure differentials) or two differential pressure measurements by coupling two differential pressure sensors to the three stations. With this arrangement, and from the differential pressures, it is possible to not only calculate flowrate but to take account of the scale thickness using the same mathematical model as described above.
According to a first aspect of the present invention, there is provided a method of monitoring the rate of fluid flow through a conduit, said method comprising the steps of:
providing first, second and third pressure monitoring stations axially spaced apart along said conduit, said first, second and third pressure monitoring stations permitting the measurement of pressure thereat, disposing within the conduit bore a fluid flow restrictor insert, said fluid flow restrictor insert having a first substantially cylindrical section and a second substantially cylindrical section, the first and second cylindrical sections being adapted to be disposed in said bores such that the sections are parallel and the first section is located at a first bore diameter and the second section is located at a second bore diameter when the leading end of the flow restrictor insert is disposed between the first and the second pressure monitoring stations and, when the insert is in this location, the first cylindrical section is disposed substantially opposite the second pressure sensor and the second cylindrical section is disposed substantially opposite the third pressure monitoring station, creating annular flow by the flow restrictor insert in the vicinity of the second and the third pressure monitoring stations to provide a venturi effect thereat and measuring pressures at said first, second and third pressure monitoring stations for deriving at least two pressure differentials for use in determining the flow rate of said fluid and for determining the thickness of any scale disposed in the interior of the carrier bore.
According to a sec
Expro North Sea Limited
Patel Harshad
Piper Marbury Rudnick & Wolfe
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