Abstract:
A drilling system is provided for drilling a borehole into an earth formation, the drilling system comprising pump means for pumping drilling fluid into the borehole and discharge means for discharging drilling fluid from the borehole. The discharge means comprises at least one pressure chamber for temporarily accommodating drilling fluid being discharged from the borehole, and control means for controlling the fluid inflow into each pressure chamber.

Description:
The present application claims priority on European Patent Application 01307865.4 filed on 14 Sep. 2001. 
     FIELD OF THE INVENTION 
     The present invention relates to a drilling system and a method for drilling a borehole into an earth formation, the drilling system comprising pump means for pumping drilling fluid into the borehole and discharge means for discharging drilling fluid from the borehole. 
     The drilling system may furthermore comprise a drill string extending into the borehole whereby an annular space is formed between the drill string and the borehole wall, the annular space containing a body of drilling fluid. The drill string generally has a longitudinal passage for pumping drilling fluid into the annular space through a opening near the lower end of the drill string. The drilling fluid can be discharged from the borehole through a discharge conduit connected with the borehole near the upper end of said annular space. The flow of drilling fluid through said annular space can be controlled by said discharge means, for example by a controllable resistance in said discharge conduit. 
     Therefore the discharge conduit can be provided with a choke valve providing a controllable throttle opening. However, because of rock debris and contaminated mud in the drilling fluid a throttle opening in the discharge conduit shall be worn out soon. 
     BACKGROUND OF THE INVENTION 
     WO-A-0079092 discloses such drilling system, whereby the discharge means control the discharge of drilling fluid,and therewith the flow of drilling fluid through the annular space. Therefore the discharge conduit is provided with a controllable outlet valve. As an alternative WO-A-0079092 describes an injection pump arranged to pump injection fluid via an injection nozzle into the discharge conduit in a direction opposite to the direction of flow of drilling fluid through the discharge conduit. By controlling the injection fluid, the resistance in the discharge conduit can be controlled. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a drilling system for drilling a borehole into an earth formation, the drilling system comprising pump means for pumping drilling fluid into the borehole and discharge means for discharging drilling fluid from the borehole, wherein the discharge means comprises at least one pressure chamber for temporarily accommodating drilling fluid being discharged from the borehole, and control means for controlling the fluid inflow into each pressure chamber. 
     Thereby it is achieved that the drilling fluid from the borehole is transported to the pressure chamber and the inflow of it into the pressure chamber can be controlled without a restriction through which the drilling fluid has to flow. 
     Preferably said control means is arranged to control the fluid pressure in the pressure chamber. 
     The inflow of drilling fluid into the pressure chamber can be controlled by controlling the outflow of gas or liquid which is expelled from the pressure chamber by the inflow of drilling fluid. Such gas or liquid, hereinafter referred to as expel fluid, can be led through a controllable throttle opening, thereby controlling the inflow of drilling fluid. In fact thereby the said control means control fluid pressure above the drilling fluid in the pressure chamber. 
     In fact the discharge of drilling fluid from the borehole is controlled by throttling the expel fluid in stead of throttling the drilling fluid. And because the expel fluid does not contain rock debris or contaminated mud, there is no wear problem in the choke valve throttling the fluid. 
     Preferably the pressure chamber is provided with two compartments separated by a flexible membrane, whereby one of the compartments is to be filled with drilling fluid and the other compartment contains an expel fluid, whereby said control means control the outflow of said expel fluid from the pressure chamber. 
     In a preferred embodiment two or more pressure chambers being alternately filled with drilling fluid from the borehole, whereby said control means control the inflow of drilling fluid in each of the pressure chambers. By making use of more than one pressure chamber, the drilling fluid can be removed from a pressure chamber while the drilling fluid from the borehole can be led to another pressure chamber. 
     Preferably two pressure chambers are interconnected by an expel fluid conduit for transporting an expel fluid between the two pressure chambers, whereby said control means comprise a control valve in said expel fluid conduit. 
     In a preferred embodiment the discharge means comprise two pressure chambers, each provided with a membrane to form a drilling fluid compartment and an expel fluid compartment, both having a variable content, the expel fluid compartments being interconnected by an expel fluid conduit provided with a control valve for controlling flow through said expel fluid conduit, the system furthermore being provided with inlet valve means to direct the drilling fluid to be discharged alternately to one of said drilling fluid compartments and with outlet valve means to remove drilling fluid from the other drilling fluid compartment. 
     The invention furthermore relate to a method for drilling a borehole into an earth formation, whereby drilling fluid is pumped into the borehole and whereby drilling fluid is discharged from the borehole and transported to a pressure chamber, whereby the inflow of drilling fluid into the pressure chamber is controlled. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in more detail and by way of example with reference to the accompanying drawing in which: 
         FIG. 1  schematically shows an embodiment of a drilling system; and 
         FIGS. 2 and 3  schematically show the drilling fluid discharge means. 
       In the figures like reference numerals relate to like components. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In  FIG. 1  is shown a drill string  1  extending into a borehole  3  formed in an earth formation  5  and provided with a drill bit  7  and a bottom hole assembly (BHA, not shown). The drill string  1  is made up of a plurality of drill string joints, whereby each pair of adjacent joints is interconnected by a releasable connector. For the purpose of clarity only one of the uppermost connectors  9   a ,  9   b , which connects the uppermost joint to the remainder of the drill string  1 , is shown (in disconnected mode). In the description hereinafter, the upper drill string joint is referred to as the upper drill string section  10  and the remainder of the drill string  1  is referred to as the lower drill string section  12 . The lower drill section  12  is supported at rig floor  14  of a drilling rig (not shown) by power slips  16 . The upper drill string section  10  is supported by a top drive  18  which is capable of supporting the entire drill string  1  and which is provided with a drive system (not shown) for rotating the drill string  1  during drilling. A primary pump  19  is in fluid communication with the upper drill string section to pump drilling fluid through the drill string  1  when the connector  9   a ,  9   b  is in connected mode. 
     A fluid chamber  20  is supported by a support column  22  provided at rig floor  14  in a manner allowing the fluid chamber  20  to move up or down along the column  22 , and means (not shown) are provided to control such movement. The upper drill string section  10  extends into the fluid chamber  20  through an upper opening  24  of the fluid chamber  20  so that the open lower end of the upper drill string section  10  is located in an upper portion  25  of the chamber  20 . The lower drill string section  12  extends into the fluid chamber  20  through a lower opening  26  of the fluid chamber  20  so that the open upper end of the lower drill string section  12  is located in a lower portion  27  of the chamber  20 . Both upper opening  24  and de lower opening  26  are of a sufficiently large diameter to allow passage of the drill string connectors (which generally are of slightly larger diameter than the drill string sections) therethrough. Furthermore, the upper and lower openings  24 ,  26  are provided with seals  29   a ,  29   b  which are controllable so as to be moved radially inward and thereby to seal against the respective upper and lower drill string sections  10 , 12 . The lower portion  27  of chamber  20  is provided with a fluid inlet  28  in fluid communication with a secondary pump  30  to pump drilling fluid through the lower drill string section  12  when the connector  9   a ,  9   b  is in disconnected mode. 
     The upper portion  25  and the lower portion  27  of the fluid chamber  20  are selectively sealed from each other by a partitioning means in the form of a valve  32 . A control device (not shown) is provided to open or close the valve  32 , whereby in its open position the valve  32  allows passage of drill string  1  through the valve  32 . Furthermore, in the open position of the valve  32 , the upper portion  25  and the lower portion  27  of the fluid chamber  20  are in fluid communication with each other. A pair of power tongues  34 , 36  connecting and disconnecting the connector  9   a , 9   b  is attached to the fluid chamber  20  at the lower side thereof. 
     An annular space  38  is defined between the lower drill string section  12  on one hand and the borehole wall and a wellbore casing  42  on the other hand, which annular space is filled with a body of drilling fluid  40 . The annular space  38  is at its upper end sealed by a rotating blowout preventor (BOP)  46  which allows rotation and vertical movement of the drill string  1 . A drilling fluid discharge conduit  48  is provided at the upper end of the annular space  38 , which discharge conduit  48  debouches into a drilling fluid reservoir (not shown) via discharge means  50 , which discharge means shall be elucidated hereinafter referring to  FIGS. 2 and 3 . A tertiary pump  52  is arranged in parallel with the discharge means  50 , which pump  52  is in fluid communication with the discharge conduit  48  at a branch connection  54  located between the discharge means  50  and the rotating BOP  46 . The pump  52  is operable so as to pump drilling fluid from a drilling fluid reservoir (not shown) into the annular space  38 . The lower part of the drill string  1  is provided with means for controlling the flow of drilling fluid from the body of drilling fluid  40  into the drill string  1  in the form of a non-return valve (not shown) which prevents such return flow. 
     During normal operation the drill string  1  is rotated by the top drive  18  to further drill the borehole  3  whereby the connector  9   a , 9   b  is in connected mode. A stream of drilling fluid is pumped by primary pump  19  via the drill string  1  and the drill bit  7  into the annular space  38  where drill cuttings are entrained into the stream. The stream then flows in upward direction through the annular space  38  and via the discharge conduit  48  and the discharge means  50  into the drilling fluid reservoir (not shown). The fluid pressure in the annular space  38  is controlled by controlling the pump rate of pump  19  and/or by controlling the discharge means  50  and/or the tertiary pump  52 . 
     When it is desired to remove the drill string from the borehole  3 , the individual drill strings joints are to be disconnected and removed from the drill string  1  in sequential order. This is done by disconnecting and removing the uppermost joint, moving the drill string  1  upwardly to a position wherein the joint which is now the. uppermost joint can be removed, etc. To remove the uppermost joint (i.e. drill string section  10 ) the following procedure is followed. Rotation of the drill string  1  by the top drive  18  is stopped while drilling fluid is continuously circulated through the drill string by operation of primary pump  19 . The fluid chamber  20  is moved along support column  22  to a position where the power tongues  34 , 36  are located at the level of the connector  9   a , 9   b , whereupon the tongues  34 , 36  are operated so as to break out and partly unscrew the connector  9   a , 9   b . The connector  9   a , 9   b  is unscrewed by the slips only to the extent that further unscrewing can be done by the top drive  18 . The fluid chamber  20  is then moved along support column  22  so as to position connector  9   a , 9   b  inside the lower fluid chamber portion  27 , and the seals  29   a , 29   b  are moved radially inward so as to seal against the respective upper and lower drill string sections  10 , 12 . The secondary pump  30  is operated to pressurise fluid camber  20 . The top drive is then rotated in counter clockwise direction thereby further unscrewing the connector  9   a , 9   b . Once the connector  9   a , 9   b  becomes disconnected the upper drill string section  10  is raised a short distance so as to position the upper connector half  9   a  in the upper portion  25  of the fluid chamber  20 . The valve  32  is closed so as to seal the upper fluid chamber portion  25  from the lower fluid chamber portion  27 . Simultaneously with closing the valve  32  the primary pump  19  is stopped and the secondary pump  30  is operated to pump drilling fluid through the fluid inlet  28  into the lower fluid chamber portion  27  and from there through lower drill string section  12  into the annular space  38 . The seal  29   a  is retracted to remove the upper drill string section, and the drill string joint which has now become the uppermost joint is connected to the top drive  18 . The procedure described heretofore is repeated in order to remove the now uppermost drill string joint. By the continued circulation of drilling fluid through the borehole  3  it is achieved that undesired settling of particles (e.g. drill cuttings) in the borehole occurs, and that the fluid pressure in the borehole can be controlled by controlling the pump rate of pump  30  and/or controlling the discharge means  50 . 
     Instead of using the secondary pump  30  to pump drilling fluid through the lower drill string section  12  when the connector  9   a , 9   b  is disconnected, the primary pump  19  can be used for this purpose in which case the primary pump  19  is connected to the fluid inlet  28  by suitable conduit means. 
     The above procedure relies on the use of the fluid chamber  20  to control the fluid pressure in the borehole by continued fluid circulation through the drill string  1  when the upper drill string section  10  is disconnected. In case it is impractical or impossible to use the fluid chamber an alternative procedure can be applied to connect or disconnect the upper drill string section  10  to or from the drill string  1 . In the alternative procedure, which can be applied in the absence of the fluid chamber, the tertiary pump  52  is operated so as to pump drilling fluid through the circuit formed by the pump  52 , the branch connection  54 , and the discharge means  50 . By controlling the pump rate of pump  52  and/or by controlling the discharge means  50  the fluid pressure in the annular space  38  can be controlled. The non-return valve in the drill string  1  prevents flow of drilling fluid from the annular space  38  into the drill string  1 . The alternative procedure can be used, for example, in case drill string stabilisers prevent passage of the drill string through the fluid chamber. 
     An advantage of continued fluid circulation through the drill string  1  using the fluid chamber  20  when the upper drill string joint are disconnected, is that the drilling fluid in the open part of the borehole  3  keeps flowing so that undesired settling of particles in the borehole is prevented. However once the drill string has been raised to a level whereby the drill bit  7  is located within the casing  42 , the drilling fluid which is pumped through the drill string  1  returns from the bit  7  through the annular space  38  to surface thereby leaving the drilling fluid in the open part of the borehole  3  stationary. It is therefore preferred that, once the drill bit  7  is within the casing  42 , pumping of drilling fluid by secondary pump  30  is stopped and pumping by tertiary pump  52  is commenced to control the fluid pressure in the borehole. This procedure has the advantage that the fluid chamber  20  then is no longer required and can be removed from the drill string. 
       FIGS. 2 and 3  show the discharge means  50  in more detail. The flow of drilling fluid to be discharged is supplied to the discharge means by discharge conduit  48 . 
     The discharge means comprise two pressure chambers  60 , 61 . Each pressure chamber is provided with a membrane  62 , 63  made out of flexible material, such as rubber. The membrane  62 , 63  divides each pressure chamber  60 , 61  in two compartments, a drilling fluid compartment  64 , 65  and an expel fluid compartment  66 , 67 . Both expel fluid compartments  66 , 67  are interconnected by an expel fluid conduit  68  passing a control valve  69 , which control valve  69  is a choke valve for controlling the flow of expel fluid through conduit  68  by throttling that flow. 
     The drilling fluid compartment  64 , 65  of each pressure chamber  60 , 61  is provided with inlet valve means ( 70 , 71 ) to direct the drilling fluid to be discharged to the drilling fluid compartment  64  or  65  respectively, and is provided with outlet valve means ( 72 , 73 ) to remove drilling fluid from the drilling fluid compartment  64  or  65  respectively. 
       FIG. 2  shows a first mode of the discharge means and  FIG. 3  shows a second mode. 
     In the first mode, as shown in  FIG. 2 , inlet valve  70  is open and inlet valve  71  is closed. Furthermore outlet valve  72  is closed and outlet valve  73  is open. The flow of drilling fluid is indicated with arrows  75 . From conduit  48  the drilling fluid flows to drilling fluid compartment  64 , whereby the membrane  62  is moved upwardly. Therefore expel fluid is expelled from compartment  66  through conduit  68  to expel fluid compartment  67 , thereby passing choke valve  69 . The flow of expel fluid is indicated with arrows  76 . The inflow of expel fluid into compartment  67  moves the membrane  63  downward, expelling the drilling fluid from compartment  65 , which drilling fluid can be further transported, for example to a filtering system (not shown). 
     The flow of drilling fluid to compartment  64  is controlled by controlling choke valve  69  up to the moment that drilling fluid compartment  64  is completely filled with drilling fluid. At that moment the discharge means are shifted to the second mode as shown in  FIG. 3 . 
     In the second mode, as shown in  FIG. 3 , inlet valve  70  is closed and inlet valve  71  is open. Furthermore outlet valve  72  is open and outlet valve  73  is closed. The flow of drilling fluid is indicated with arrows  75 . From conduit  48  the drilling fluid flows to drilling fluid compartment  65 , whereby the membrane  63  is moved upwardly. Therefore expel fluid is expelled from compartment  67  through conduit  68  to expel fluid compartment  67 , thereby passing choke valve  69 . The flow of expel fluid is indicated with arrows  76 . The inflow of expel fluid into compartment  66  moves the membrane  62  downward, expelling the drilling fluid from compartment  64 , which drilling fluid can be further transported, for example to a filtering system (not shown). 
     During operation of the discharge means the first and the second mode will alternate with each other, whereby the choke valve  69  may be maintained in the same position to achieve a predetermined resistance in expel conduit  68  in both modes. That will result in a constant resistance for the drilling fluid passing the discharge means. By changing the position of the choke valve  69  that resistance will be changed. 
     While the illustrative embodiments of the invention have been described with particularity, it will be understood that various other modifications will be readily apparent to, and can be easily made by one skilled in the art without departing from the spirit of the invention. Accordingly, it is not intended that the scope of the following claims be limited to the examples and descriptions set forth herein but rather that the claims be construed as encompassing all features which would be treated as equivalents thereof by those skilled in the art to which this invention pertains.