Abstract:
A method and system are disclosed for dispersing gas bubbles in a multiphase mixture in a production tubing in an crude oil production well or in a riser connected to such a well, by means of one or more bubble breaker assemblies in which a plurality of orifices are arranged that are located in a substantially eccentric position relative to a central axis of the tubing. The use of eccentric orifices promotes the breaking up of large gas bubbles into a large amount of smaller gas bubbles, which are finely dispersed in the fluid stream and only re-coalesce slowly into larger bubbles.

Description:
PRIORITY CLAIM 
   The present application claims priority to European Patent Application 03104118.9 filed Nov 7,2003. 
   FIELD OF THE INVENTION 
   The invention relates to a bubble breaker assembly for dispersing gas bubbles in a multiphase fluid transportation conduit, such as a production tubing in a crude oil production well into which lift gas is injected to decrease the density of the produced fluid. 
   More particularly, the invention relates to a method and system for dispersing gas bubbles in a multiphase fluid transportation conduit, wherein the gaseous and liquid fluid fractions are intensively mixed to produce a low density froth or foam comprising small and uniformly distributed gas bubbles in a liquid matrix. 
   BACKGROUND OF THE INVENTION 
   Such a method and system are known from International patent application WO00/05485. 
   In the known method and system one or more bubble breaker assemblies are arranged in the conduit to create alternating flow zones of small and large cross-sectional areas with abrupt transition from the small cross-sectional areas to the large cross-sectional areas to produce a turbulent flow in which swirls and eddies are generated. The known bubble breaker assemblies consist either of venturi-like orifices that are concentric to the central axis of the conduit or of annular flow passages which are formed between the inner wall of the conduit and a central mandrel which is arranged in a concentric position. 
   U.S. Pat. No. 4,544,207 discloses a method for the uniform distribution of a two phase mixture by one or more orifice containing turbulence promoters which may comprise plates containing orifices of various shapes. 
   It is an object of the present invention to provide a method and bubble breaker assembly, which further enhance the mixing of gaseous and liquid fractions in the conduit such that the size of the gas bubbles is further decreased and the gas bubbles are distributed as a finely dispersed froth in the multiphase fluid stream. 
   SUMMARY OF THE INVENTION 
   The method according to preferred embodiments of the invention for dispersing gas bubbles in a multiphase fluid transportation conduit comprises inserting at least one bubble breaker assembly in the conduit, which assembly comprises a plurality of orifices that are located in a substantially eccentric position relative to a central axis of the tubing, characterised in that lift gas is injected at one or more downhole gas injection points spaced along the length of the production tubing to enhance oil production from the well, and that one or more bubble breaker assemblies with eccentric orifices are arranged at selected distances downstream of the lift gas injection points. 
   The present invention includes a method of producing crude oil, wherein large gas slugs, that are known as are Taylor bubbles, are broken up into finely dispersed smaller gas bubbles by means of one or more bubble breaker assemblies with eccentric orifices in accordance with the method for dispersing gas bubbles in a production tubing in an oil production well, the method comprising inserting at least one bubble breaker assembly in the tubing, which assembly comprises a plurality of orifices that are located in a substantially eccentric position relative to a central axis of the tubing, wherein lift gas is injected at one or more downhole gas injection points spaced along the length of the production tubing to enhance oil production from the well, and that one or more bubble breaker assemblies with eccentric orifices are arranged at selected distances downstream of the lift gas injection points. 
   It has been found that the use of a bubble breaker assembly in which a plurality of eccentric orifices are arranged significantly enhances the dispersion of relatively large gas bubbles into a large amount of small gas bubbles, which are uniformly distributed in the multiphase fluid stream. 
   In an embodiment a flow restriction may comprise a disk-shaped plate in which at least two eccentric orifices are arranged, and which disk may be removably secured to the inner wall of the conduit, for example by a clamping assembly which can be contracted if the plate needs to be removed. 
   Preferably a plurality of flow restrictions are arranged at selected distances along the length of the conduit, wherein at least two of said flow restrictions comprise disk-shaped plates in which different patterns of eccentric orifices are arranged 
   In an embodiment at least one flow restriction may comprise a pair of eccentric orifices that are located substantially symmetrically relative to a plane of symmetry in which the central axis of the conduit lies. 
   Alternatively at least one flow restriction may comprise three or more equidistant eccentric orifices that are arranged at regular angular intervals relative to a longitudinal axis of the conduit. 
   In the fluid stream downstream of the gas-injection point(s) the gas bubbles will tend to coalesce into steadily growing larger gas bubbles, known as gas slugs or Taylor bubbles, and by arranging a series of bubble breakers according to the invention, each with eccentric orifices, an intensively mixed low density multiphase stream of crude oil and uniformly distributed small gas bubbles is created throughout the length of the production tubing. 
   The invention also relates to a system for dispersing gas bubbles in a multiphase fluid transportation conduit, which system comprises at least one bubble breaker assembly which is arranged within the tubing, which assembly comprises a plurality of orifices that are located in a substantially eccentric position relative to a central axis of the tubing characterised in that one or more downhole lift gas injection points are arranged along the length of the production tubing to enhance oil production from the well, and that one or more bubble breaker assemblies with eccentric orifices are arranged at selected distances downstream of the lift gas injection points. 
   Further features, advantages and embodiments of the method and system according to the present invention are detailed in the following detailed description of preferred embodiments and in the appended claims, abstract and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some preferred embodiment of the method and system according to the present invention will be described by way of example with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic three-dimensional view of a production tubing in a well into which lift gas is injected and which comprises downstream of the gas injection point a bubble breaker assembly with eccentric orifices according to the present invention which serve to break up coalesced large gas bubbles into a large amount of finely dispersed small gas bubbles; 
       FIG. 2  is a schematic three-dimensional view of a production tubing in a well in which an alternative embodiment of a bubble breaker with four eccentric orifices is arranged; 
       FIG. 3  is a longitudinal sectional view of a bubble breaker which is clamped between a pair of retrievable well tubulars; 
       FIG. 4A  is a side view of the bubble breaker plate shown in  FIG. 3 ; 
       FIG. 4B  is a cross sectional view of the bubble breaker plate shown in  FIG. 4A , taken along line B-B and seen in the direction of the arrows; 
       FIG. 5  is a diagram which provides a comparison of the oil production rate in a 3000 m deep well with and without a bubble breaker according to the invention; 
       FIG. 6  is a diagram which illustrates the improvement of oil production in the well of  FIG. 5 ; and 
       FIG. 7  is a plotted diagram in which the improvement in mean gas hold up of a conventional bubble breaker with a central orifice is compared with that of a bubble breaker with eccentric orifices according to the invention. 
       FIG. 8  is a schematic drawing of a well in which a plurality of bubble breaker assemblies are arranged at selected distances along the tubing. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  shows an underground oil production well  1  passing through an underground formation  2 . 
   The well  1  comprises a well casing  3  and a production tubing  4  into which lift gas bubbles  5  are injected through an assembly of lift gas injection nozzles  6  that are arranged in a lift gas injection mandrel  7  which is retrievably inserted into a side pocket  8  in the production tubing  4 . The lift gas may be natural gas which is separated from the produced hydrocarbon stream and which is reinjected via the wellhead (not shown) into the annular space  9  between the production tubing  4  and surrounding well casing  3 . The lift gas flows from the annular space  9  via an orifice  11  in the production tubing  4  into the interior of the side pocket  8  and via openings  12  through the interior of the gas lift injection mandrel towards the orifices  6  as illustrated by arrows  13 . The orifices  6  may be surrounded by a porous membrane (not shown) as disclosed in European patent application EP 1278938. 
   The injected gas bubbles  5  may gradually coalesce into large gas slugs or Taylor bubbles  15  and in the region where such coalescence may take place a bubble breaker assembly  16  according to the invention is arranged, which comprises at least one disk shaped plate  17  in which twelve eccentric orifices  18  is arranged. 
   The twelve orifices  18  are arranged at regular angular intervals relative to the central longitudinal axis of the production tubing  4 . 
   The bubble breaker assembly  16  further comprises a tubular carrier body  19  which is retrievably clamped and sealed within the production tubing  4  by an expandable clamping mechanism  20  and inflatable seals  21 . The bubble breaker assembly  16  further comprises a pulling nose  22  which can be coupled to a wireline tool or well robot (not shown) which is configured to expand the clamping mechanism  20  and inflate the seals  21  during installation of the bubble breaker assembly  16  and to contract the clamping mechanism  20  and deflate the seals  21  if the bubble breaker assembly  16  is retrieved for maintenance of the assembly itself or of well components, such as the gas lift injection mandrel  7 , that are located below the bubble breaker assembly  16 . 
     FIG. 2  depicts an alternative embodiment of a bubble breaker assembly  26  according to the invention, wherein the assembly  26  comprises a disk shaped plate  27  in which four eccentric orifices  28  are arranged at regular angular intervals relative to a longitudinal axis of the production tubing  34 . The tubing  34  is suspended within a well casing  33  of a crude oil production well  31 , which passes through a subsurface earth formation  32 . Natural gas may be injected into the tubing  34  via the annular space  29  between the tubing  34  and well casing  33  and one or more orifices (not shown) in the wall of production tubing  34  below the bubble breaker assembly  26 . Alternatively or additionally natural gas which is dissolved in the crude oil at reservoir pressure may be released and form gas bubbles  35  in the stream of crude oil within the production tubing  34 . The injected and/or released gas bubbles  35  may coalesce in to large gas slugs that are known as Taylor bubbles  36 , which are broken up into a large number of finely dispersed small gas bubbles by the bubble breaker assembly  26  according to the invention. 
   In the configuration shown in  FIG. 2  the disk shaped plate  27  is inserted in an annular recess between two tubular sections  37  and  38 . The upper tubular section  38  is screwed below a tubular carrier body  39  which is suspended and sealed within the production tubing  34  by sealing rings  40  and an expandable locking mechanism  41  that fits within a recess  42  in the inner wall of the production tubing  34 . The bubble breaker assembly  26  shown in  FIG. 2  is inserted into the production tubing  34  by a wireline tool or well robot which is configured to release the locking mechanism  41  when it is located adjacent to the annular recess  42  and expand the sealing rings  40  during installation of the assembly  26  and which contracts the locking mechanism  41  and sealing rings  40  when the assembly  26  is to be retrieved from the well  31 . 
   The eccentric orifices  18 ,  28  break up the gas slugs of Taylor bubbles  15 ,  36  into a large amount of finely dispersed smaller gas bubbles  25 ,  37  that only re-coalesce slowly into larger bubbles. Preferably the gas bubbles formed have a diameter less than about 1 millimeter, so that microbubbles are formed which are highly resistant to re-coalescence into large Taylor bubbles  15 ,  36 . 
   A benefit of creating small bubbles is that residence time of the gas in a bubbly flow is higher than in a slug flow, resulting in less slip between the gas and crude oil stream and a corresponding higher gas hold-up in the tubing downstream of the bubble breaker assembly  16 ,  26 . The higher gas hold-up results in a lower average fluid density and therefore a lower pressure drop in the tubing  4 ,  34 . The lower pressure drop in the tubing  4 ,  34  leads to a lower flowing bottom hole pressure and an increase of the crude oil production rate. 
   Experiments revealed that the pressure loss associated with the bubble breaker assembly  16 ,  26  with eccentric orifices  18 ,  28  according to some embodiments of the invention is small compared to the beneficial pressure effect of the low density bubbly flow it creates, often only one-tenth the magnitude. Therefore there is a net reduction in the bottom hole pressure in the crude oil inflow region of the well  1 ,  31  and an increase in the crude oil production rate of the well  1 ,  31 . 
     FIG. 3  illustrates how a bubble breaker plate  50  can be installed using a specially designed carrier, consisting of two tubular sections  51  and  52  screwed together with the plate  50  in between. The inner surface  51 A of the top part of the upper tubular section  51  can be threaded to match a standard lock mandrel or other installation device. The bubble breaker plate  50  can easily be interchanged when loosening the lower tubular section  52 , the installation tool will not be damaged. 
     FIG. 4A  and  FIG. 4B  show that the bubble breaker plate  50  has eight circumferentially spaced eccentric orifices  53  and is weakened around the periphery by milling a ring-shaped groove  53  into the upper surface of the plate  50  such that the groove  53  intersects the orifices  53 . 
   This enables an operator to punch out the inner part of the plate  50  in case of emergency. The groove  54  is not milled all the way through the plate  50  so that the fluids can still only pass through the eccentric orifices  53 . 
   Computer simulations of the method according to some embodiments of the invention indicate that crude oil production increase of as much as 20% can result. 
     FIG. 5  shows the gas-lift performance curve for a typical 3000 m deep gas lifted oil well with and without bubble breakers according to the invention. The lower curve  55  shows the gas lift performance of a gas lifted oil well without bubble breakers and the upper curve  56  shows the gas lift performance of a gas lifted well with a bubble breaker assembly  16 ,  26 , or  50  according to some embodiments of the invention as illustrated in  FIGS. 1-4 . 
   In the simulated crude oil production well lift gas is injected at the bottom of a 3000 m deep production tubing, with a tubing head pressure of 10 bar. The tubing diameter is 76 mm. The crude oil API is 30° and crude oil density is 850 kg/m 3 . The specific density of the lift gas is 0.65 and the reservoir pressure is 220 bar. 
   In  FIG. 5  the horizontal axis represents the gas injection rate Qg (sm 3 /day) and it can be seen that for gas injection rates less than 80.000 sm 3 /day the amount of crude oil Ql (m 3 /day) produced by a gas-lifted oil production well equipped with a bubble breaker assembly  16 ,  26  according to some embodiments of the invention is significantly higher than of the same gas lifted well without bubble breakers according to the invention. It is observed that the unit sm 3  refers to standard cubic meters, which is the volume of the injected gas at atmospheric pressure. 
     FIG. 6  is a diagram, which depicts the improvement in production resulting from application of the bubble breaker assembly  16 , 26  in the oil well production diagram of  FIG. 5 . In  FIG. 6  the horizontal axis represents the gas lift injection rate Qg (sm 3 /day), and the vertical axis represents the percentage of improvement Δ (%) in oil production for the curve  56  with bubble breaker, when compared with the curve  55  without bubble breaker.  FIG. 6  indicates that at a lift gas injection rate of about 15.000 sm 3 /day a production improvement Δ of about 18% is generated by application of the bubble breaker with eccentric orifices according to the invention. 
   Experiments were done with bubble breaker assemblies with various patterns of orifices in an 18 m high transparent perspex test conduit having an internal diameter of 72 mm and through which a water-ethanol mixture was pumped in an upward direction at a flow rate of 15-70 l/minute. Air was injected at the bottom of the conduit and a disk shaped plate in which one or more orifices were made was inserted in the conduit at about 5 m above the bottom. 
   Several experiments were carried out with a bubble breaker assembly with a single central orifice and with a number of eccentric orifices. 
   The experiments revealed that a bubble breaker plate with eccentric orifices breaks up gas bubbles more efficiently into finely dispersed small bubbles than a conventional bubble breaker plate with a central orifice. 
     FIG. 7  shows the results of an experiment where the improvement in mean gas hold up of a bubble breaker with a single central orifice is plotted and represented by dotted curve  70  and that of a bubble breaker with a series of eight eccentric orifices as shown in  FIG. 4  is plotted and represented by dotted curve  71 .  FIG. 7  illustrates the improvement in gas hold up downstream of the bubble breaker as a function of gas flow rate for a constant liquid flow rate of 54 l/minute. The dotted curve  71  for the device with eccentric orifices is higher than the curve  70  for the device with a single central orifice. The cross-sectional area and local pressure loss is the same for the device with eccentric orifices and for the device with a single central orifice. 
     FIG. 7  indicates that the increase in gas hold up was higher for the experiments with the number of eccentric orifice keeping the pressure drop over the device constant. On the horizontal axis of  FIG. 7  the difference in gas hold up downstream of the bubble breaker is plotted against the gas injection rate.  FIG. 7  shows that the improvement in mean gas hold up is larger for a bubble breaker with several eccentric orifices around the periphery, while keeping the pressure drop over the device constant. 
   Observations with a high speed camera revealed that the eccentric orifices according to some embodiments of the invention generated a large amount of turbulent eddies in the fluid stream and that the air bubbles were broken over and over again by these eddies in the region of the bubble breaker until they had a diameter of one or a few millimeters.