Abstract:
A system for producing hydrocarbon fluids from a subsea formation comprises at least one producing well penetrating the formation for producing hydrocarbon fluids. At least one dummy well is hydraulically connected to the at least one producing well for routing the hydrocarbon fluids from the producing well to the dummy well. At least one pump is disposed in the at least one dummy well. The pump takes suction flow from the dummy well and boosts the flow energy of the discharge flow of hydrocarbon fluids.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to subsea production of hydrocarbons. More specifically, the invention relates to a system and method to provide flow/pressure boosting in a subsea environment.  
           [0003]    2. Description of the Related Art  
           [0004]    Petroleum development and production must be sufficiently profitable over the long term to withstand a variety of economic uncertainties. Booster pumping is increasingly being used to aid in the production of wellhead fluids. Subsea installations of these pumps are particularly helpful in producing remote fields and many companies are considering their use for producing remote pockets of oil and for producing deep water reservoirs from remote facilities located in shallower water. Such booster pumps allow producers to transport multiphase fluids (oil, water, and gas) from the wellheads to remote processing facilities (instead of building new processing facilities near the wellheads and often in deep water). These booster pumps also allow fluid recovery at lower final reservoir pressures before abandoning production. Consequently, there is a greater total recovery from the reservoir.  
           [0005]    For deep water reservoirs, booster pumps are used to transport wellhead fluids from deep water wellheads to remote processing facilities located in shallower water. While there are a number of technical difficulties in this type of production, the cost savings are very large. Consequently, producers would like to transport wellhead fluids from the sea-floor in deep waters through pipelines to remote processing facilities in moderate water depths. Transport distances of tens of kilometers are not uncommon with longer distances currently in the planning stages.  
           [0006]    Commonly available booster pumping systems commonly include a submersible pump installed in the producing well or a pumping system connected to a subsea Christmas tree manifold attached to the wellheads from which fluids flow as a result of indigenous reservoir energy. The other end of the pumps are connected to a pipeline which transports the fluids from the wellhead to the remote processing site. Submersible pumps and their operation, including their installation, are well known and understood in the art. A problem, however, is that should a failure occur, valuable production flow can be interrupted while the pump is repaired. Subsea pumping systems connected externally to the producing well are typically unique to each application and require modifications and adaptations of surface pumps to the subsea environment. Such systems are typically more expensive and more difficult to install than submersible pumps.  
           [0007]    Wellhead fluids can exhibit a wide range of chemical and physical properties. These wellhead fluid properties can differ from zone to zone within a given field and can change with time over the course of the life of a well. Furthermore, well bore flow exhibits a well-known array of flow regimes, including slug flow, bubble flow, stratified flow, and annular mist, depending on flow velocity, geometry, and the aforementioned fluid properties. Consequently, the ideal pumping system should allow for a broad range of input and output parameters without unduly compromising pumping efficiency and service life. Submersible pumps typically operate at conditions of lower gas fractions than seafloor mounted systems and thus exhibit fewer problems from such multi-phase flows.  
           [0008]    The methods and apparatus of the present invention overcome the foregoing disadvantages of the prior art by providing a submersible pump system that provides flow/pressure boosting and does not jeapordize production flow during downtime.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention contemplates a subsea pumping system for boosting the flow energy of a production flow.  
           [0010]    In one aspect, the present invention is a system for producing hydrocarbon fluids from a subsea formation, comprising at least one producing well penetrating the formation for producing hydrocarbon fluids. At least one dummy well is hydraulically connected to the at least one producing well for routing the hydrocarbon fluids from the producing well to the dummy well. At least one pump is disposed in the at least one dummy well. The pump takes suction flow from the dummy well and boosts the flow energy of the discharge flow of hydrocarbon fluids.  
           [0011]    In another aspect, the present invention describes a method for producing hydrocarbon fluids from a subsea formation, comprising installing at least one pump in at least one dummy well where the dummy well is hydraulically connected to at least one producing well. The at least one dummy well acts as a suction reservoir for the pump. Production flow is routed from the producing well to the dummy well where the pump is used for imparting flow energy to the production flow.  
           [0012]    Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    For detailed understanding of the present invention, references should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein:  
         [0014]    [0014]FIG. 1 is a schematic drawing of a subsea flow system according to one preferred embodiment of the present invention;  
         [0015]    [0015]FIG. 2 is a schematic drawing of a booster pumping system according to one preferred embodiment of the present invention;  
         [0016]    [0016]FIG. 3 is a flow diagram according to one preferred embodiment of the present invention; and  
         [0017]    [0017]FIG. 4 is a flow diagram according to another preferred embodiment of the present invention.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]    [0018]FIG. 1 shows a production system according to one embodiment of the present invention. Producing well  1  is shown penetrating a hydrocarbon bearing formation  2  at some depth below the seafloor  18 . Well  1  is completed using any of the myriad of common techniques known in the art. Well  1  may be a vertical well as shown or, alternatively, may be highly inclined including horizontal. Formation fluid  3  flows up the wellbore  19  to a wellhead  4 . The fluid  3  may be single phase or multiphase. Multiphase as used herein means (i) oil, water, and gas; (ii) oil and water; (iii) oil and gas; and (iv) water and gas. Well  1  is located at some distance from subsea processing station  12  where the distance may be on the order of tens of kilometers. As previously indicated, in many such cases, the pressure of the formation driving the flow of fluid, such as fluid  3 , is insufficient to force adequate flow to reach processing station  12 . Booster pumping system  40  is installed to provide sufficient flow energy to force adequate flow to reach the processing station  12 .  
         [0019]    Booster pumping system  40  (see FIG. 1 and FIG. 2), in one preferred embodiment, comprises a dummy well  7  extending to a predetermined depth below the seafloor  18 . Dummy well  7  is drilled and cased with casing  44  using techniques known in the art. Dummy well  7  may be drilled and cased at the time that producing well  1  is drilled using the same rig (not shown) used to drill well  1 . Alternatively, dummy well  7  may be drilled at any time using coiled tubing supported by a surface vessel equipped with coiled tubing equipment and using techniques known in the art. Wellhead  6  is attached to the top of dummy well  7  and flow conduit  5  connects producing wellhead  4  to dummy wellhead  6 . Conduit  5  enables flow of fluid  3  from producing well  1  to dummy well  7 . Liner  10  is hung off from wellhead  6  and extends to near the bottom of dummy well  7 . A pump string  60  comprising submersible pump  8 , motor  9 , and tubing  17  is run into liner  10  on tubing  17  and hung off from wellhead cap  41 . Tubing  17  may be a length of coiled tubing. Alternatively, tubing  17  may be lengths of threaded tubing joined together. In a preferred embodiment, electrical conductors  43  are run inside tubing  17  and connect motor  9  to power source  21  through a wet-mateable connector  49 . Such connectors are known in the art and are not described here. Alternatively, electrical conductors  43  may be attached to the outside of tubing  17  using techniques known in the art. The inlet to pump  8  is sealed to the liner  10  by seal  42  directing fluid  3  in the annulus  45  between casing  44  and liner  10  to enter pump  8 . Well  7  depth is selected to provide sufficient suction pressure to allow pump  8  to operate at a desired efficiency. For example, for cases where a substantial portion of fluid  3  is liquid, the dummy well  7  may be just deep enough to fit a submersible pump string, on the order of 100 to 200 feet. In cases where there is a substantial gas fraction in fluid  3 , the depth of dummy well  7  may be significantly deeper, on the order of 1000 feet. The increased depth reduces the gas-oil ration (GOR) due to increased pressure, and may also act to drive the gas back into solution in the liquid, both such conditions resulting in significantly increased pump efficiency. Such determinations are specific to each application. For cases where there is substantial gas entrained in the flow, vanes  47  may be attached to the outside of liner  10  to break up any large bubbles and mix the gas in the liquid phase as the flow passes the vanes  47 . Vanes  47  may be spirally attached to the liner  10 .  
         [0020]    Flow  50  exits pump  8  with increased flow energy as compared to the inlet flow  3 . Flow  50  travels up in annulus  46  and exits through pipeline  11  and travels to subsea processing station  12  (see FIG. 1) for further processing and distribution. Sensors  50 , 51 , 30 , 31 , 32 , and  33  may be placed in the flow lines at multiple locations to characterize the flow conditions. Such sensors may be adapted to measure parameters of interest including, but not limited to (i) wellhead pressure at the producing well; (ii) hydrocarbon flow rate at the producing well; (iii) gas fraction at the wellhead; (iv) pressure in the dummy well; (v) pump discharge pressure; and (vi) pump discharge flow rate. Additional sensors may be connected to motor  9  for performance monitoring.  
         [0021]    As shown in FIG. 1 and FIG. 2, the submersible pump  8  and motor  9  are insertable and extractable using a coiled tubing reel  14  and coiled tubing  15  operated from a surface vessel  16  which may be a light intervention type vessel of a type known in the art.  
         [0022]    In one preferred embodiment, shown in FIG. 3, three booster pumping systems  40   a - c  are connected with producing well  1  through manifold system  50 . Manifold system  50  comprises valves  22  and  23  for directing flow from producing well  1  to any combination of booster pumping systems  40   a - c . Typically each booster pumping system will be sized such that two of pumping systems  40   a - c  are always used, thereby providing a spare pump for high reliability. It should also be noted that multiple pumps may be installed in a pump string to increase reliability and/or flow output.  
         [0023]    In operation, for example, if one of pumps  40   a - c  fails, the failed pump system may be isolated using the appropriate valves  22  and  23 . Coiled tubing  15  is lowered from vessel  16 . A suitable connector (not shown) on coiled tubing  15  is attaches to connector  48  on cap  41  and extracts pump string  60  from dummy well  7 . The pump string  60  is repaired or replaced and reinserted back in dummy well  7  and put back in service as needed.  
         [0024]    A subsea controller  65  (see FIG. 2) controls the pumping systems  40   a - c . The controller may contain circuits for interfacing with various sensors and controlling the motor  9  and the valves  23  and  22  according to sensor data and programmed instructions. The subsea controller also contains communication circuits and communicates with other subsea systems such as processing station  12  and/or surface controllers (not shown).  
         [0025]    In another preferred embodiment, see FIG. 4, three producing wells  1   a - c  are connected to flow control manifold  70 . Manifold  70  directs the flow to pumping systems  140   a - c  as required. While three producing wells  1   a - 1   c  are shown, any number of producing wells may be connected to such a booster pumping system. In any such system, the pumps will be sized for the appropriate flows.  
         [0026]    In all of the previously disclosed booster pump systems, a flow bypass system such as bypass  67  (see FIG. 3) may be incorporated to bypass the booster pump and allow natural (unboosted) production flow should such a need arise, for example with a failure of pump power. Such a bypass may have a remotely operated valve  66  for enabling such a bypass flow.  
         [0027]    A system has been disclosed wherein a number of industry proven devices and techniques are combined in a novel arrangement to provide pressure/flow boosting to a production flow in a subsea environment. A dummy well is used to act as a suction reservoir for a submersible pump disposed in the dummy well. The dummy well has a case dependent depth to provide increased suction pressure resulting in improved pump efficiency, especially for flows with high gas content. The submersible pump is insertable and retrievable from a surface vessel using coiled tubing techniques. Multiple pumps may be inserted in one dummy well. In addition, multiple dummy wells with pumps may be manifolded to one or more producing wells.  
         [0028]    The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope of the invention. It is intended that the following claims be interpreted to embrace all such modifications and changes.