Abstract:
A flowmeter is described which has a retrievable housing conduit which is locatable within a conduit or pipe and which seals against the pipe so that fluid only flows through the housing conduit, and a flow restriction is created within the housing conduit to create a Venturi when in use. At least two spaced pressure measuring stations are provided along the housing conduit, one of the stations located at an unrestricted flow location, the other at a restricted flow location, whereby pressures measured at these stations can be used to calculate fluid flow rates. Embodiments of the invention are described.

Description:
RELATED APPLICATIONS  
       [0001]    This application is a continuation of U.S. patent application Ser. No. 09/889,261 filed Sep. 21, 2001, which is the U.S. national phase of PCT/GB00/00069 filed Jan. 13, 2000, which claims priority of United Kingdom Patent Application No. 9900569.6 filed Jan. 13, 1999. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to flow monitoring apparatus for, and to a method of, monitoring the flow of a fluid in a pipe.  
         BACKGROUND OF THE INVENTION  
         [0003]    In particular, the present invention relates to a flowmeter for monitoring the flow of a liquid, gas, or multi-phase fluid through a conduit or pipe, particularly, but not exclusively, for use in downhole oilfield applications.  
           [0004]    It is known to measure the flow rate of fluid flowing through a conduit or pipe in a downhole oilfield environment using flowmeters based on the Venturi principle which have throat restriction portions. A Venturi flowmeter is disclosed in U.K. Patent 2186981B in which a constant diameter section of pipe called a gradiomanometer merges smoothly with a converging section of pipe to act as a Venturi. Applicant&#39;s own U.K. Patent No. 2261519B discloses a hydrocarbon flow rate monitor which measures the flow of fluid through a production pipe by using a drill string during drill string testing (DST) of the production pipe. Measurements of pressure are taken from various points along the pipe from which fluid density and ultimately flow rate of hydrocarbons in the production pipe can be calculated. A modification of this flowmeter is disclosed in applicant&#39;s International Patent Publication No. WO 94/16295 where the Venturi flowmeter is provided with pressure measurement stations on either side of the flow restrictor to provide increased flexibility of use and accuracy of flow rate measurement.  
           [0005]    Venturi flowmeters with throat restrictions including those mentioned above generally work well in practice but suffer from the main disadvantage of bore restriction which, in many cases, prevents tools being run through the flowmeter. These tools also require pressure sensors to be provided in the wall of the tubing where failure of a sensor can lead to abandonment of the flowmeter.  
           [0006]    An alternative Venturi flowmeter is disclosed in the applicant&#39;s PCT Publication No. WO 97/25594, in which a flow restrictor insert is located centrally in the bore of a conduit or pipe, the flow restrictor insert being arranged so as to create an annular flow path around the insert. A remote pressure sensor downstream is used in conjunction with one of the pressure sensors at the Venturi, so as to be able to calculate fluid density p from the pressure difference as disclosed.  
           [0007]    The flowmeter of WO 97/25594 suffers from the same disadvantage of bore restriction as the throat restrictor type. Wireline retrievable versions of both types can be used but these still require sensors in the tubing wall with the associated problems of sensor failure. One embodiment of the flowmeter in WO 97/25594 provides sensors in the insert. When the insert is retrievable this provides an advantage over other embodiments in the event of sensor failure.  
           [0008]    However, the prior art flowmeters, including those disclosed in of WO 97/25594, suffer from the significant disadvantage that the accuracy of the calculated flow rates is highly dependent upon the spacing between the outer surface of the flow restrictor insert and the inner surface of the conduit or pipe in which the insert is located. This spacing is difficult to determine accurately in the downhole oilfield environment and, indeed, in other such similar applications where the insert is located in a conduit because the conduit is typically a well casing or production tubing, the surface of which tends to become coated over time by drilling materials, oil and the like adhering to the inner surface. This causes variation in tubing diameter which, in turn, causes variation in the accuracy of the pressure measurements and therefore upon the accuracy of the flow rate calculations.  
           [0009]    Furthermore, when it is desired to measure the flow rates in conduits or pipes of different internal diameters, it is necessary to adjust the diameter of the flow restrictor insert to achieve the required spacing between the outer surface of the insert and the inner surface of the conduit or pipe. This is a time consuming process, typically requiring component parts of the insert to be changed, for example, by removing the leading portion of the insert which is shaped to create an annular flow path around the insert, and replacing this with a portion of different shape. The leading portion comprises an entry cone, a cylindrical body attached to the entry cone, and an exit cone at the trailing end of the body, attached to the remaining portion of the insert. Thus replacement of the leading portion is a complex and time-consuming procedure and is therefore expensive, requiring removal of the entry cone, the cylindrical body and the exit cone, followed by the installation of replacement parts of different outside diameter, depending on the diameter of the conduit or pipe receiving the insert.  
           [0010]    It is amongst the objects of the present invention to obviate or mitigate at least one of the disadvantages of known inventions.  
         SUMMARY OF THE INVENTION  
         [0011]    This is achieved, in the broadest sense, by locating a retrievable housing conduit within a conduit or pipe and which seals against the pipe so that fluid only flows through the housing conduit, and a flow restriction is created within the housing conduit to create a Venturi when in use. At least two spaced pressure measuring stations are provided along the housing conduit, one of the stations located at an unrestricted flow location, the other at a restricted flow location, whereby pressures measured at these stations can be used to calculate fluid flow rates as described above.  
           [0012]    The housing conduit diameter is machined accurately as is the insert so that the spacing in the flow conduit is carefully controlled to provide flow rate calculations of much greater accuracy. The retrievability of the housing conduit is also an advantage.  
           [0013]    According to a first aspect of the present invention, there is provided a flowmeter for measuring the flow rate of a fluid flowing through a conduit having an inside diameter, the flowmeter comprising:  
           [0014]    a housing conduit for location within a portion of the conduit, the housing conduit having an inner surface and an outer surface, said housing conduit having an outside diameter less than the inside diameter of the portion of the conduit;  
           [0015]    locating means for locating the housing conduit within the conduit to provide sealing engagement between the housing conduit and the conduit;  
           [0016]    flow restriction means disposed within the housing conduit to create a flow restriction zone at a portion of the housing conduit to provide a restriction to fluid flow through said housing conduit, and pressure measurement means disposed in the housing conduit for measuring pressure of the fluid at least at two spaced pressure measurement locations along the length of the housing conduit to measure a pressure at an unrestricted flow location and at the flow restriction location, said housing conduit having predetermined internal diameters at the first and second pressure measuring locations.  
           [0017]    Preferably, the flow restriction means is a generally cylindrical insert having a leading end on which fluid flowing through the housing conduit impinges, said leading end being coupled to a body of substantially constant diameter, the diameter of the leading end being less than the diameter of the body of the insert, an annular flow path being created around said insert, one of said pressure measurement locations being in said body at said substantially constant diameter to measure pressure at said flow restriction location.  
           [0018]    Alternatively, the flow restriction means is an annular body having a leading end coupled to a generally cylindrical restriction portion having a circular cross-section orifice, wherein the diameter of the circular cross-section is less than the diameter of the leading end, and wherein the annular body lies adjacent to the inner surface of the housing conduit such that fluid flowing through the housing conduit flows through the circular cross-section.  
           [0019]    Preferably, the annular body is formed integrally with the housing conduit.  
           [0020]    Alternatively, the annular body is an insert adapted to be sealingly engaged within the housing conduit.  
           [0021]    Advantageously, the locating means includes locking means and sealing means for locking and sealing the housing conduit to the flow conduit, such that when the flowmeter is located in the flow conduit, fluid can only flow through the housing conduit.  
           [0022]    Preferably, the locking means and the sealing means are integral, such that the housing conduit is simultaneously engaged by the locking means and sealed to the conduit by the sealing means.  
           [0023]    Alternatively, the locking means and the sealing means are separate, such that the housing conduit is separately engaged by the locking means and sealed to the conduit by the sealing means.  
           [0024]    Preferably, flow restriction centralising means are provided for centralising the flow restriction means within the housing conduit.  
           [0025]    Preferably, the pressure measurement means are differential pressure sensors. Alternatively, the pressure measurement means are absolute pressure sensors.  
           [0026]    Preferably, one of the pressure measurement means is located at the leading edge of the flow restriction means for measuring the pressure at the unrestricted flow location, and the other pressure measurement means is located at a point along the length of the body of substantially constant diameter of the flow restriction means, for measuring the pressure at the restricted flow location.  
           [0027]    According to a second aspect of the present invention, there is provided a method of measuring the flow rate of a fluid flowing through a flow conduit, comprising the steps of:  
           [0028]    locating a housing conduit having an internal bore within the flow conduit;  
           [0029]    sealing the housing conduit in the flow conduit to force fluid flowing through the flow conduit to flow through the internal bore of the housing conduit;  
           [0030]    providing a restriction in the internal bore of the housing conduit for restricting the flow of fluid through the internal bore;  
           [0031]    measuring the pressure of the fluid at least at a first unrestricted flow location and a second restricted flow location, and  
           [0032]    determining the fluid flow rate using measurements of fluid pressure taken at the first and second locations.  
           [0033]    A flowmeter for measuring a range of flow rates in flow conduits having different diameters, said flowmeter comprising:  
           [0034]    a) a housing conduit selectable from a range of housing conduits of different diameters;  
           [0035]    b) sealing means coupled to said selected housing conduit for sealingly engaging the internal surface of said flow conduit;  
           [0036]    c) a flow restrictor element releasably engageable with said housing conduit, said flow restrictor element being selectable from a range of flow restrictor elements of different diameters, the flow restrictor element having at least two spaced pressure measurement means for measuring pressure at an unrestricted flow location and a restricted flow location, said housing conduit and said flow restrictor being interchangeable with other housing conduits and flow restrictors from the respective ranges to allow the flowmeter to be assembled for use with a range of flow rates and flow conduit diameters.  
           [0037]    Preferably the flow restrictor insert is a generally cylindrical annular element centrally disposed in the housing conduit for creating an annular flowpath. Alternatively, the flow restrictor insert is a tubular insert with a restriction throat of a reduced diameter.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]    Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0039]    [0039]FIG. 1 is a diagrammatic sectional view of a wellbore incorporating a flowmeter in accordance with a preferred embodiment of the present invention;  
         [0040]    [0040]FIG. 2 is a longitudinal sectional view of the flowmeter of FIG. 1 in more detail and shown to a larger scale;  
         [0041]    [0041]FIG. 3 is an enlarged cross-sectional view of the flowmeter of FIG. 2 taken along lines A-A, and  
         [0042]    [0042]FIG. 4 is a longitudinal sectional view of a flowmeter in accordance with an alternative embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0043]    Referring firstly to FIG. 1, there is shown a flowmeter indicated generally by reference numeral  20  for measuring the flow rate of a fluid  22  flowing through a wellbore  24  provided by well tubing  26 . The flowmeter comprises a housing conduit  28 , a locking seal  30  and flow restriction insert  32  centrally located in housing conduit so that fluid flows in an annulus  34  between the insert and housing conduit  28 .  
         [0044]    Referring now to FIG. 2 of the drawings, the housing conduit  28  is a cylindrical sleeve having an open upper end  36  and a lower generally conical perforated end  38  to facilitate location of the flowmeter  20  in the well tubing  26 , and to permit fluid to flow through the conduit  28 . The conduit  28  has a first cylindrical sleeve portion  28   a  and a second cylindrical sleeve portion  28   b  which are threaded together at location  40  to form the conduit  28 . Furthermore, the second sleeve portion  28   b  can be unscrewed from its threaded engagement with the first sleeve portion  28   a , and be replaced with an alternative second sleeve portion as will be later described.  
         [0045]    The conduit  28  is located within the well tubing  26  by the locking seal  30  which, as will be described, locks the conduit  28  to a mating surface (not shown) in well tubing  26 , and simultaneously seals the conduit  28  to the well tubing  26 , so that fluid is prevented from flowing along the annulus  34  and flows through the conduit  28  past the insert  32 , in the direction shown by the arrows  33  in FIG. 2.  
         [0046]    The flow restriction insert  32  is in the general shape of a dart, i.e. having a pointed leading end in front of a wider body portion, the body portion tapering at its rear end towards an elongate cylindrical portion of smaller diameter which extends from the body. The insert comprises a sensor housing  42 , an electronics housing  44  and a battery housing  46  arranged in series along the length of the conduit  28 .  
         [0047]    The sensor housing  42  has a tip  48  which forms part of a conical leading end  50  of the housing  42 , a cylindrical body  52  of constant outside diameter, and a trailing end  54  which tapers in a direction from the body  52  towards the electronics housing  44 . Thus an annulus  34  of constant width is formed between the outer surface of body  52  and the inner surface  58  of conduit  28 .  
         [0048]    The flow restriction insert  32  is centralised within the conduit  28  by spaced flowrings  60  and  62 , as best seen in FIG. 2 and FIG. 3 whilst permitting fluid to flow through the conduit  28 . In particular, flowring  60 , shown at FIG. 3, includes a tubular collar  64  which is retained within conduit  28  at the location  40  between the first sleeve portion  28   a  and the second sleeve portion  28   b . The flowring  60  includes four spring-loaded arms  66   a - d , spaced at 90° around the circumference of the conduit  28 . Arms  66   a -d define quadrant gaps  68 , through which fluid  22  flows.  
         [0049]    Referring again to FIG. 2 locking seal  30  has a first threaded female opening  70  to receive a threaded end of the conduit  28  so that seal  30  and conduit  28  can be locked together. The locking seal  30  has a second female opening with a latching ring  80  for engaging wireline or coiled tubing (not shown) for lowering/raising the flowmeter within the well tubing  26 . The locking seal  30  has latching protrusions  81  for engaging a suitable packer (not shown) in the well tubing  26  to both lock and seal the conduit  28  to the well tubing  26  in a known way.  
         [0050]    Thus, as will be appreciated by persons skilled in the art, when the insert  32  is located in the conduit  28 , and the conduit  28  is located in the well tubing  26 , a Venturi is formed by the restriction in the cross-sectional flow area in flowmeter  20  from full bore width at the tip  48  of the sensor housing  42 , to the annulus  34 .  
         [0051]    In use, well fluid flowing through the well tubing  26  flows through the conduit  28 , impinging initially on the tip  48  of sensor housing  42 , where the fluid flow is unrestricted. As fluid flows over the insert  32 , the fluid flow becomes restricted, and the velocity of the fluid increases. In accordance with well known hydraulic flow principles, this increase in velocity causes a decrease in the pressure of the fluid from full bore pressure at the tip  48 , to a reduced pressure in annulus  34 .  
         [0052]    A first pressure sensor  82  is disposed within the tip  48  to measure the full bore pressure and second pressure sensor  84  is disposed within the body  52  to measure the fluid pressure in annulus  34 . Thus the pressure difference between the pressure of the fluid at unrestricted flow and the pressure at a restricted flow can be determined, either by measuring a differential pressure between the two points. This enables the fluid flow rate to be calculated using flow equations disclosed in WO 94/16295. It will be understood that there are simplified versions of general fluid mechanics equations which are well known in the art and which require to be modified to take account of actors such as pipe friction, temperature and hence thermal expansion, the particular dimensions of the pipes and Venturi, discharge coefficients and the like. The general fluid mechanics analysis can be found from the following references: (1) Kay J. M., An Introduction to Fluid Mechanics and Heat Transfer, 2nd Ed., Cambridge U Press, 1968; (2) ISO 5167 (BS 1042), Measurement of Fluid Flow in Closed Conduits, Part 1, BSI; (3) Miller R. W., Flow Measurement Engineering Handbook, 2nd Ed., McGraw Hill Pub., 1983; (4) Coulson J. M., Richardson J. F., Chemical Engineering, Vol 1, 2nd Ed., 1970; (5) Gregory, G. A., Fogarasi M., Alternate to Standard Friction Factor Equation, Oil &amp; Gas J. Apr. 1 1985, pp 120-127, and (6) Jorissen A. L., Discharge Measurements by Means of Venturi Tubes, Trans ASME, May 1951, pp 403-408.  
         [0053]    Pressure measurements are made at pre-programmed time intervals and the data obtained is recorded in memory electronics provided in the electronics housing  44  of the flow restriction insert  32 . Data stored in the memory electronics can be retrieved by removing the flowmeter  20  to the surface and interfacing the flowmeter with a computer (not shown), for the transferral of stored data to the computer for calculation of the fluid flow rates, or the flowmeter  20  may be connected to the surface via a wireline connection (not shown) for downloading stored data. In a further alternative arrangement, suitable electronics (not shown) may be provided within the flowmeter  20  to enable the flowmeter to perform the necessary calculations to determine the flow rates.  
         [0054]    Power for the operation of the electronic equipment in the electronics housing  44  and for other operations of the flowmeter  20  is provided by an internal battery located in the battery housing  46  of flowmeter  20 . Additional pressure sensors may be provided, such as the sensors  86  and  88 , disposed within the trailing end  54  of the sensor housing  42 . It will be understood by persons skilled in the art that the flow rate of fluid flowing through conduit  28  may be calculated by measuring the pressure of the fluid at least at two spaced locations within the conduit  28 , including a first location where fluid flow is unrestricted, and a second location where fluid flow is restricted. Thus sensors  82 , 84 , 86  and  88  may be provided to obtain the optimum flow rate calculations.  
         [0055]    Referring now to FIG. 4, there is shown an alternative embodiment of a flowmeter in accordance with the present invention, indicated generally by reference  100 , which comprises a housing conduit  102 , a locking seal  104  and a flow restrictor  106 .  
         [0056]    The housing conduit is a cylindrical sleeve  102  similar to the conduit  28  of FIGS.  1  to  3 , having an open upper end  108  and a conical perforated lower end  110 . The sleeve  102  is located in well tubing  28  by the locking seal  104  in the same way as described with reference to the first embodiment.  
         [0057]    The flow restrictor  106  is provided by a tubular insert  112  which is located within the sleeve  102 . The tubular insert  112  has an upstream opening  114 , which tapers from its leading end where the fluid flow is unrestricted, towards a throat  116  of constant reduced diameter disposed centrally within the sleeve  102 , where fluid flow is restricted, then opens to downstream opening  118 .  
         [0058]    A first pressure sensor  120  is disposed within a wall  122  of sleeve  102  to measure the pressure of the fluid where flow is unrestricted, the wall  122  being thick enough to be machined to create wall conduits  123 . A second pressure sensor  124  is disposed within the insert  112 , to measure the pressure of the fluid in throat  116 . Pressure sensors  120 , 124  are linked such that a differential pressure measurement of the pressure difference between the sensors is obtained, thus enabling the fluid flow rate to be calculated as described above.  
         [0059]    Various modifications may be made to the flowmeter of the first embodiment. For example, absolute pressure sensors can be used to sense pressure and the pressure difference then calculated. Also, power may be provided by a wireline connection to the surface. An alternative second sleeve portion of different diameter may be connected to the first sleeve portion to vary the annulus spacing within the sleeve. Similarly, the entire insert can be unscrewed from the sleeve and inserted into a sleeve of different diameter. A range of sleeves of different diameters and a range of inserts of different diameters are provided; the sleeves and inserts being advantageously interchangeable to create a flowmeter for use with different sizes of well tubing with a range of flowmeters. The insert may also contain temperature measurement sensors. The annulus spacing may be varied by locating a cylindrical sheath within the sleeve of the flowmeter to space the annulus to a predetermined value.  
         [0060]    Various modifications may be made to the second embodiment without departing from the scope of the invention. For example, the second pressure sensor  124  may be disposed at any point along the throat  116  where fluid flow is restricted. Pressure sensors  120  and  124  may measure the absolute fluid pressures, the pressure difference being subsequently calculated. If desired, additional pressure sensors may be provided within the sleeve  102  and spaced along its length, to measure the pressure of the fluid flowing through the sleeve  102  at various points.  
         [0061]    The sleeve  102  includes an electronics housing  126  for storing data obtained, and a battery housing  128  for providing power for operation of the flowmeter  100  as required. In the same way as in the embodiment of FIGS.  1 - 3 , the electronics and battery may be provided at the surface or externally from the flowmeter. The flow restrictor of flowmeter  100  is provided integrally with the sleeve  102 . In this modification, sensors  120 , 124 , the electronics housing  126  and battery housing  128  are provided within the wall  130  of the housing sleeve  102 .  
         [0062]    It will be appreciated by persons skilled in the art that sensors may be provided for measuring other parameters of the fluid flowing through the flowmeter such as temperature, fluid viscosity and the like.  
         [0063]    The main advantage of providing a flowmeter  20  as a two-part component including a conduit  28  and a flow restriction insert  32  as shown in FIGS.  1  to  3  is that the relative spacing between the inner surface  58  of the conduit  28 , and the outer surface of the sensor housing portion  42  is carefully controlled by precise machining of the component parts, and careful location of the flow restriction insert  32  within the housing conduit  28 , prior to insertion of the flowmeter  20  into the well tubing  26 . This permits more accurate flow rate measurements to be made than has hitherto been possible.  
         [0064]    Other advantages are provided by flowmeter retrievability and insert/sleeve interchangeability which allows the flowmeter to be used with differently sized well tubing bore and over a range of flow rates.