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PRIORITY CLAIM  
     The present application claims priority to European Patent Application 05107316.1 filed Aug. 9, 2005. 
     FIELD OF THE INVENTION 
     The present invention relates to a system for injecting an injection fluid into an earth formation via a wellbore formed in the earth formation and for producing hydrocarbon fluid from the earth formation via the wellbore. The injection fluid can be, for example, steam that is injected into the formation at high temperature and pressure to lower the viscosity of heavy oil present in the formation so as to enhance the flow of the oil through the pores of the formation during the production phase. In one such application, steam is injected through one or more injector wells drilled in the vicinity of one or more production wells, and oil is produced from the production wells. 
     BACKGROUND OF THE INVENTION 
     Instead of using separate wells for steam injection and oil production, a single well can be used for the injection of steam and the production of oil. In such operation the injection of steam and the production of oil occur in a cyclic mode generally referred to as Cyclic Steam Simulation (CSS) process. In the CSS process, the well is shut in and steam is injected through the well into the oil-bearing formation to lower the viscosity of the oil. During a next stage, oil is produced from the formation through the same well. In order that the steam is injected substantially uniformly along the portion of the well penetrating the reservoir zone, i.e. without a concentration of injected steam at one location at the cost of another location, the steam is generally pumped through spaced outlet ports having a relatively small diameter, generally referred to as Limited Entry Perforations (LEP). This is done to ensure that the steam exits the outlet ports at a velocity approaching sonic velocity and is therefore choked or throttled. The size of the outlet ports typically is of the order of 0.5-1.0 inch. 
     U.S. Pat. No. 6,158,510 suggests a wellbore liner for CSS including a base pipe provided with a plurality of LEP ports spaced in longitudinal direction and circumferential direction of the liner. The liner is provided with several sandscreens spaced along the liner, each sandscreen extending around the base pipe at short radial distance therefrom. During each steam injection cycle, the well is shut in and steam is injected into the rock formation via the LEP ports. The steam flows through the LEP ports at sub-critical velocity so that the flow rate of steam in the LEP ports is independent from pressure variations downstream the ports, thus ensuring a uniform outflow of steam along the liner. After a period of steam injection, a production cycle is started whereby oil from the surrounding rock formation flows via the LEP ports into the liner and from there to a production facility at surface. 
     It is a drawback of the known system that, during the production cycle, the volumetric flow rate of oil through the LEP ports is relatively low. The amount of oil produced from the well in a given period of time is therefore also low. 
     U.S. Pat. No. 5,865,249. discloses a system configured to flush debris from the bottom of a wellbore by injecting water via a water injection conduit into the plugged zone and inducing the debris to flow up through the wellbore through the production conduit. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a system for injecting an injection fluid into an earth formation via a wellbore formed in the earth formation and for producing hydrocarbon fluid from the earth formation via the wellbore, the system comprising an injection conduit extending into the wellbore and being in fluid communication with a plurality of outlet ports for injection fluid, the system further comprising a production conduit extending into the wellbore and being in fluid communication with at least one inlet section for hydrocarbon fluid, wherein the injection conduit is arranged to prevent fluid communication between the injection conduit and each said inlet section, characterised in that the injection fluid is a heated fluid which is injected into the formation in order to reduce the viscosity of hydrocarbon fluids within the formation. 
     By virtue of the feature that the injection conduit is arranged to prevent fluid communication between the injection conduit and each inlet section, it is achieved that the injection fluid can be injected through the LEP ports of small size, whereas oil can be produced through each inlet section of a much larger size. Suitably the injection conduit and the production conduit are separate conduits. 
     Furthermore, it is preferred that the outlet ports are comprised in a plurality of series of outlet ports, wherein the system comprises a plurality of said inlet sections, and wherein said inlet sections and said series of outlet ports are arranged in alternating order in longitudinal direction of the wellbore. In this manner it is achieved that injection fluid is injected at locations along the liner inbetween the inlet sections thereby ensuring substantially uniform heating of the rock formation along the length of the liner. 
     The invention will be described hereinafter in more detail by way of example, with reference to the accompanying drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE INVENTION 
         FIG. 1  schematically shows a wellbore for the production of hydrocarbon fluid from an earth formation, provided with an embodiment of the system of the invention; 
         FIG. 2  schematically shows a portion of a liner used in the system of  FIG. 1 ; 
         FIG. 3  schematically shows side view  3 - 3  of  FIG. 2 ; and 
         FIG. 4  schematically shows an upper portion of the liner used in the system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the Figures like reference numerals relate to like components. 
     Referring to  FIG. 1  there is shown a wellbore  1  for the production of hydrocarbon oil and gas from an earth formation  2 . The wellbore  1  has an upper section  3  extending substantially vertical and a lower section  4  extending substantially horizontal. A wellhead  5  is arranged at the earth surface  5   a  above the well  1 . The lower wellbore section  4  penetrates a reservoir zone  2 A of the earth formation  2 . A conventional casing  6  extends from surface into the vertical wellbore section  3 , and a production liner  8  extends from the lower end of the casing  6  into the horizontal wellbore section  4 . A packer  10  seals the outer surface of the liner  8  relative to the inner surface of the casing  6 . The liner  8  comprises a plurality of inlet sections in the form of tubular sandscreens  12  for reducing inflow of solid particles, and a plurality of tubular bodies  14 . As is shown in  FIG. 1 , the screens  12  and the tubular bodies are arranged in alternating order in the horizontal wellbore section  4 . Each tubular body  14  is provided with a series of outlet ports  16  of relatively small diameter for injection of fluid into the reservoir zone  2 A of the earth formation  2 . As discussed hereinbefore, outlet ports of this type are referred to as Limited Entry Perforations (LEP) which limit the flow rate of injection fluid into a zone at a given injection pressure by virtue of the fact that the velocity of injection fluid exiting the outlet ports approaches the sonic velocity. The outlet ports  16  of a series are regularly spaced in circumferential direction of the tubular body  14 . 
     The sandscreens  12  are of conventional type, including a perforated base pipe (not shown) and a tubular filter layer  13  extending around the perforated base pipe. The base pipe of each sandscreen  12  is connected to the respective tubular bodies  14  adjacent the base pipe by conventional screw connectors (not shown) or by any other suitable means, for example by welding. 
     The wellbore  1  is further provided with a production conduit  18  for the transportation of produced hydrocarbon fluid through the wellbore  1  to surface, the conduit  18  having an inlet opening  19  near the upper end of the liner  8 , and an injection conduit in the form of a coiled tubing  20  for the injection of injection fluid into the reservoir zone  2 A of the earth formation  2 . 
     Reference is further made to  FIG. 2  in which one of the tubular bodies  14  is shown in longitudinal section. The tubular body  14  is provided with a central through-passage  22  extending in longitudinal direction, the through-passage  22  having a mid-portion of enlarged diameter forming a chamber  24  that is in fluid communication with the exterior of the tubular body  14  by means of the outlet ports  16 . The coiled tubing  20  extends through the through-passage  22  and has a slightly smaller outer diameter than the diameter of the through-passage  22  so as to allow the coiled tubing to slide through the through-passage  22 . The coiled tubing  20  has one or more outlet openings  26  debouching in the chamber  24  of the tubular body  14 . Annular seals  28 ,  30  are provided at either side of the chamber  24  to seal the coiled tubing  20  relative to the passage  22 . 
     Thus, the coiled tubing  20  passes through the liner  8 , with the openings  26  being located in the respective chambers  24  of the tubular bodies  14 . A plug (not shown) closes the lower end of the coiled tubing  20  at a location below the chamber  24  of the lowermost tubular body  14 . 
     Referring further to  FIG. 3  there is shown a side view of the tubular body  14  that is provided with a series of through-bores in the form of production ports  32  fluidly connecting the respective ends  34 ,  36  ( FIG. 2 ) of the tubular body  14 . As shown, the production ports  32  are regularly spaced in circumferential direction of the tubular body  14 . The outlet ports  16  for injection fluid (indicated in phantom in  FIG. 3 ) do not intersect the production ports  32 . 
     In  FIG. 4  is shown the upper end of the liner  8  extending into the casing  6 , with the packer  10  sealing the upper end of the liner  8  relative to the casing  6 . As shown, the inlet opening  19  of the production conduit  18  is located in the lower end part of the casing  6 . 
     During a first stage of normal operation, the well  1  is shut in and an injection fluid, such as high temperature steam, is pumped at surface into the coiled tubing  20  by means of a suitable injection facility (not shown). The steam flows downwardly through the coiled tubing  20 , and via the outlet openings  26  into respective chambers  24  of the tubular bodies  14 . Leakage of steam along the through-passages  22  of the tubular bodies  14  is prevented by the annular seals  28 . From the chambers  24 , the steam flows through the outlet ports  16  and into the wellbore  1 . From there, the steam flows into the reservoir zone  2 A of the surrounding earth formation  2 . As discussed before, the outlet ports  16  are Limited Entry Perforations (LEP) which have a relatively small diameter so as to limit the flow rate of steam through the outlet ports  16 . The pressure at which the steam is injected into the coiled tubing  20  is sufficiently high to ensure that the flow rate of steam in the outlet ports  16  approaches sonic velocity, so that the flow rates are independent of pressure differences downstream the outlet ports  16 . It is thus achieved that the steam is substantially uniformly distributed over the various outlet ports  16 , and that increased flow through one port  16  at the cost of another port  16  is prevented. The steam heats the reservoir zone  2 A whereby the viscosity of the oil in the reservoir zone  2 A is lowered. 
     During a second stage of normal operation, after a period of continued steam injection into the reservoir zone  2   a , the injection of steam is stopped. The coiled tubing  20  is then retrieved from the wellbore  1  or, alternatively, can remain in the wellbore  1  for the next cycle of steam injection. The well  1  is then opened to start oil production from the reservoir zone  2 A, whereby the oil flows into the sandscreens  12  and, from there, via the production ports  32  of the respective tubular bodies  14  towards the production conduit  18 . The oil enters the production conduit  18  at its inlet opening  19 , and flows to surface to a suitable production facility (not shown). It will be understood that injected steam initially flows back into the well  1  before oil starts flowing into the well  1 . 
     Thus, by the separate arrangement of production conduit  18  and the injection conduit  20  it is achieved that the production of oil is not limited to inflow of oil through the small outlet ports  16  for injection fluid. Instead, oil is produced at flow rates comparable to oil production from wells that do not require injection of steam into the formation. 
     After a period of continued oil production from the well  1 , a next cycle of steam injection is started. The coiled tubing  20  is to be re-installed in the well  1  in case it was retrieved from the well  1  after the previous steam injection cycle. The aforementioned first and second stages of operation are then repeated in cyclic order.

Summary:
A system is provided for injecting an injection fluid into an earth formation via a wellbore formed in the earth formation and for producing hydrocarbon fluid from the earth formation via the wellbore. The system comprises an injection conduit extending into the wellbore and being in fluid communication with a plurality of outlet ports for injection fluid, and a production conduit extending into the wellbore and being in fluid communication with at least one inlet section for hydrocarbon fluid. The injection conduit is arranged to prevent fluid communication between the injection conduit and each said inlet section.