Abstract:
A gravity water separation system that may be integrated within a well completion. A diverted flowpath is provided for produced hydrocarbons, external to the completion tubing. As produced hydrocarbons travel through the diverted flowpath, they pass through separation stages wherein gravity separation ensues by migration through predefined flow ports which extend from produced oil “separation chamber(s)” into separated “water chamber(s).”

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to provisional application 61/047,243, filed Apr. 23, 2008. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This disclosure relates to a water separator, and in particular, to a downhole gravitational water separator for subsea well operations. 
       BACKGROUND OF THE INVENTION 
       [0003]    Growing emphasis on increasing the reservoir recovery factor for subsea well operations provides a stimulus for separation of water from produced hydrocarbons. Additionally, onshore wells very often have to cope with significant water breakthrough (70-80%+ of water in oil (WiO)). Fundamentally, water separation provides significant operational efficiency gains. 
         [0004]    Water separation provides for reduction of back pressure on the reservoir by reduction of static hydraulic head (i.e., lower specific gravity of produced fluid in the pipeline, which can be significant in deeper waters and deeper reservoirs) and reduced frictional effects in the subsea pipeline. It may operate at a lower relative flowrate than for the combined oil+effluent volume. The reduction of back pressure on the reservoir and the reduced frictional effects in the subsea pipeline provide an opportunity for increasing total reservoir recovery over field life, by reducing field abandonment pressure, and/or deferring the time at which pressure boosting might be considered necessary, where feasible. 
         [0005]    Water separation allows for the reduction in size of export flowline(s) for a given scenario. Reduction in size of export flowline(s) can significantly reduce the total installed cost of the pipeline, particularly on subsea developments where pipeline costs are always a predominant cost factor. Water separation also reduces dependence on chemical injection, which is otherwise required for hydrate mitigation. By eliminating dependence on chemical injection, consumables cost over field life may be reduced. 
         [0006]    A need exists for a technique that addresses the emphasis on increasing the reservoir recovery factor for subsea well operations by separation of water from produced hydrocarbons. A new technique in necessary to simplify total system installation and to provide available separation capacity at the earliest point in field life without disruption to production. The following technique may solve one or more of these problems. 
       SUMMARY OF THE INVENTION 
       [0007]    A gravity water separation system that may be integrated within a well completion. A diverted flowpath is provided for produced hydrocarbons, external to the completion tubing. As produced hydrocarbons travel through the diverted flowpath, they pass through separation stages wherein gravity separation ensues by migration through predefined flow ports which extend from produced oil “separation chamber(s)” into separated “water chamber(s).” 
         [0008]    An operable full bore isolation valve is provided, maintaining access to the wellbore for through-tubing operations over field life, while also providing the means for flow diversion under a “separation enabled” mode. The full bore isolation valve also provides a “separator by-pass” mode for early field production (i.e., prior to water cut) and over field life in the case of flow disruption through the separator for whatever reason. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic view of a wellbore with a downhole water separation unit installed. 
           [0010]      FIG. 2  is a schematic view of a wellbore with a downhole water separation unit and water pump installed. 
           [0011]      FIG. 3  is a vertical cross sectional view of a downhole gravitational water separation unit with labyrinth chambers. 
           [0012]      FIG. 4  is an isometric view of a downhole gravitational water separation unit with labyrinth chambers. 
           [0013]      FIG. 5  is a vertical cross sectional view of the final chamber in a gravitational water separation unit with labyrinth chambers. 
           [0014]      FIG. 6  is a lateral cross sectional view of the separation chamber of  FIG. 5 . 
           [0015]      FIG. 7  is a vertical cross sectional view of a downhole helical water separation unit. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    Referring to  FIG. 1 , an exemplary embodiment of a wellbore completion assembly, represented by reference numeral  10 , is shown in side view and includes production tubing  12 , which extends into a formation  11 . Production tubing  12  runs from tubing hanger  27  in the wellhead  26  down into fluid communication with a producing formation. Production casing or liner  15  extends downward from a liner hanger  17 , or otherwise from a casing hanger of suitable size in the wellhead. Production packer  13  isolates an annulus between the production tubing  12  and the production casing  15 . 
         [0017]    Water separation unit  20  is installed within surface casing  19  downhole, and is connected to production tubing  12 . Surface casing  19  extends downward from casing hanger  25 . A surface controlled, subsurface safety valve (SCSSSV)  22  is located on the production tubing  12 , above the water separation unit  20 . SCSSSV  22  is a downhole safety valve that is operated from surface facilities through a control line strapped to the external surface of the production tubing  12 . The control system operates in a fail-safe mode, with hydraulic control pressure used to hold open a ball or flapper assembly that will close if the control pressure is lost. This means that when closed, SCSSSV  22  will isolate the reservoir fluids from the surface. 
         [0018]    In  FIGS. 1 and 2 , flow from the formation  11  travels up the production tubing  12  and enters the separation unit  20 . Once the flow reaches separation unit  20 , a separation device removes water (i.e., the more dense fluid) from the oil and water mixture (i.e., production fluid) as it flows through the unit  20 . Once the desired amount of separation has occurred, the flow (i.e., less dense fluid) reenters the production tubing  12  and is directed to the surface. The water (i.e., more dense fluid) that was removed from the flow (i.e., production fluid) in the separation unit  20  can be further processed or re-injected. 
         [0019]    In  FIG. 1 , the water removed from the flow in the separation unit  20  travels through water disposal line  23 , and then into an external separation device  31 . External separation device  31  may also receive water from other sources  29 , before further separating the water, and dispersing it to the sea through a sea exit line  33 , or re-injecting it through a re-injection line  35 . As  FIG. 2  illustrates, in an alternate embodiment, the water removed from the flow in the separation unit  20  travels through water disposal line  23 , is pumped through a downhole water pump  37 , and re-injected to an injection zone through re-injection line  39 . 
         [0020]      FIG. 3  illustrates a separation unit  21  comprised of a gravitational water separator with labyrinth chambers radially circumscribing a length of production tubing  12 . An operable full bore isolation valve (FBIV)  41  is located in the production tubing  12  within the separation unit  21 . FBIV  41  allows access to be maintained to the wellbore for through tubing operations over field life, while providing the means for flow diversion through the separator  21  under “Separation Enabled” mode. The FBIV  41  additionally provides a “Separator By-Pass” mode for early field production (i.e. prior to water cut) and over field life in case of flow disruption through the separator  21 . FBIV  41  may be replaced by an alternative closure mechanism such as a remotely installed plug. 
         [0021]    Referring to  FIGS. 3 and 4 , when FBIV  41  is closed and in “Separation Enabled” mode, flow (i.e., production fluid) from the formation travels up the production tubing  12 , where it is blocked by the closed FBIV  41 , thus forcing the flow to enter the separation unit  21 . The flow then enters initial flow chamber  49  and travels upwards through oil flow tube  51 , which carries the oil and water mixture through water chamber  50 . It is important to note that the flow is completely isolated from water chamber  50  by flow tube  51 . Flow tube  51  terminates in a separation chamber  52 . The separation chamber  52  comprises a plurality of small holes  55  on its lower surface. As the flow passes over holes  55 , the gravitational forces exerted on the fluid mixture causes water (i.e., more dense fluid) within the flow to drop out and to travel through holes  55  and into water chamber  50  below. After flowing over the holes  55 , the mixture (i.e., less dense fluid) continues upward through flow tube  54 . Flow tube  54  then passes through water chamber  56  before opening to separation chamber  57 . 
         [0022]    When the flow reaches separation chamber  57 , the oil and water mixture again passes over a grate-like floor that has a number of small holes  55  on its surface. As the flow passes over holes  55 , the gravitational forces exerted on the fluid mixture causes water within the flow to drop out and to travel through holes  55  and into water chamber  56  below. Once the flow has passed over the holes  55 , it continues upward through flow tube  59 . Flow tube  59  then passes through water chamber  60  before opening to separation chamber  61 . When the flow reaches separation chamber  61 , the oil and water mixture again passes over a grate-like floor that has a number of small holes  55  on its surface. As the flow passes over holes  55 , the gravitational forces exerted on the fluid mixture causes water within the flow to drop out and to travel through holes  55  and into water chamber  60  below. Once the flow has passed over the holes  55 , it continues upward through flow tube  63 . Flow tube  63  then passes through water chamber  64  before opening to the final separation chamber  65 . 
         [0023]    Referring to  FIGS. 4 and 5 , when the flow reaches the final separation chamber  65 , the oil and water mixture again passes over a grate-like floor that has a number of small holes  55  on its surface. As the flow passes over holes  55 , the gravitational forces exerted on the fluid mixture causes water within the flow to drop out and to travel through holes  55  and into water chamber  64  below. Once the oil flow (i.e., less dense fluid) has passed over the holes  55 , it reenters the production tubing  12  above the FBIV  41 , and is directed to the surface. 
         [0024]    Referring to  FIG. 4 , water chambers  50 ,  56 ,  60 ,  64  in the separation unit  21  are connected to one another by water flow tubes  53 ,  58 ,  62 . The water that enters water chamber  50  travels through water flow tube  53  which is connected to water chamber  56 . The water that enters water chamber  56  travels through water flow tube  58  which is connected to water chamber  60 . The water that enters water chamber  60  travels through water flow tube  62  which is connected to water chamber  64 . As previously illustrated in  FIGS. 1 and 2 , the water disposal line can flow upwards or downwards from the separation unit, and may be attached to a water pump or an additional separation unit before being disposed of or re-injected into the aquifer. For example, in  FIGS. 4 and 5  the water that enters water chamber  64  travels through outgoing water flow tube  66 , and then travels from separation unit  21  through water disposal line  67 . 
         [0025]      FIG. 6  illustrates a cross sectional view of  FIG. 5  along line  6 - 6 . Fluid flows into the final separation chamber  65  through flow tube  63 , and passes over holes  55 . Water from the water chambers flows upward and out of the separation unit  21  through outgoing water line  66 . The remaining oil and water mixture reenters production tubing  12 , and continues on. 
         [0026]    Although this embodiment of a separation unit contains four separation “stages,” the number of separation “stages,” including accompanying water chambers, depends on the desired oil to water ratio of the flow leaving the separation unit. The length of the separation unit is also dictated by the number of separation “stages” desired. 
         [0027]      FIG. 7  illustrates an alternate embodiment separation unit  24 . In this embodiment, flow from production line  12  enters a helical flow tube  43 , which wraps upwards and around production tubing  12 . An operable full bore isolation valve (FBIV)  41  is located in the production tubing  12  within the separation unit  24 . The FBIV  41  operates as previously discussed, to selectively direct the flow to pass through the separation unit  24 . As the water and oil mixture enters the helical tube  43 , the flow travels over holes  44  in the bottom of the tube  43 . As the flow passes over holes  44 , the gravitational forces exerted on the fluid mixture causes water within the flow to drop out and to travel through holes  44  and into water chamber  45  below. The water chamber  45  is comprised of the annulus between the production line  12  and the surface casing  19 . The flow continues upward through the helical tubing  43 , until it reconnects with production line  12 . As previously discussed, the water captured in water chamber  45  can be removed from the separation unit  24  by a number of different methods. The length of helical tubing  43  and separation unit  24 , depends on the desired oil to water ratio of the fluid leaving the separation unit  24 . 
         [0028]    The gravitational water separator system as comprised by the technique has significant advantages. The gravitational water separator system may be integrated within the well completion, simplifying total system installation (i.e., no separate structure needed as required for a seabed installed system, with attendant installation costs, and reduced topsides costs), and providing available separation capacity at the earliest point in field life without disruption to production. 
         [0029]    While the technique has been described in only one of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the technique.