Abstract:
A system for controlling wellbore annulus pressure during the drilling of a subterranean formation includes a drill string extending into a wellbore. A primary pump is configured for selectively pumping a drilling fluid through the drill string and into an annular space created between the drill string and the wellbore. A fluid discharge conduit is in fluid communication with the annular space for discharging the drilling fluid to a reservoir. A fluid back pressure system is connected to the fluid discharge conduit configured to selectively maintain pressure therein. The fluid back pressure system includes a fluid choke, a back pressure source and an accumulator hydraulically coupled between the back pressure source and the discharge conduit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Priority is claimed from U.S. Provisional Application No. 61/148,083 filed Jan. 29, 2009. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OF DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The invention relates generally to the field of wellbore drilling using annular pressure control systems. More specifically, the invention relates to back pressure systems including an accumulator to maintain back pressure. 
         [0005]    2. Background Art 
         [0006]    U.S. Pat. No. 6,904,891 issued to van Riet and commonly owned with the present invention describes a system for controlling formation pressure during the drilling of a wellbore through subterranean formations. The system includes a drill string extending into the wellbore. The drill string typically includes a bottom hole assembly made up of a drill bit, sensors, and a telemetry system capable of receiving and transmitting sensor data. Sensors disposed in the bottom hole assembly may include pressure and temperature sensors. A surface telemetry system is included for receiving telemetry data from the bottom hole assembly sensors and for transmitting commands and data to the bottom hole assembly. A primary pump is used for selectively pumping a drilling fluid from a drilling fluid reservoir, through the drill string, out from the drill bit at the end of the drill string and into an annular space created as the drill string penetrates the subsurface formations. A fluid discharge conduit is in fluid communication with the annular space for discharging the drilling fluid to the reservoir to clean the drilling fluid for reuse. A fluid back pressure system is connected to the fluid discharge conduit. The fluid back pressure system includes a flow meter, a controllable orifice fluid choke, a back pressure pump and a fluid source coupled to the pump intake. The back pressure pump may be selectively activated to increase annular space drilling fluid pressure. 
         [0007]    The back pressure pump used in a system such as the one disclosed in the van Riet &#39;981 patent frequently has to have relatively large capacity to meet instantaneous fluid volume demands. It is desirable to modify the system shown in the van Riet &#39;981 patent to be able to use a much smaller back pressure pump. 
         [0008]    Other back pressure systems may use some of the flow from the drilling rig&#39;s primary mud pumps to charge the annulus for pressure maintenance. U.S. Pat. No. 7,185,719 issued to van Riet and commonly owned with the present invention discloses such a system. However, such systems require either increasing the duty cycle of the primary pump, or increasing the size of the primary pump. It is desirable to modify the system shown in the &#39;719 patent to reduce the amount of primary mud pump output that needs to be diverted to the annulus for pressure maintenance, and to reduce the size of the backpressure pump when such pumps are used. 
       SUMMARY OF THE INVENTION 
       [0009]    A system according to one aspect of the invention for controlling wellbore annulus pressure during the drilling of a subterranean formation includes a drill string extending into a wellbore. A primary pump is configured for selectively pumping a drilling fluid through the drill string and into an annular space created between the drill string and the wellbore. A fluid discharge conduit is in fluid communication with the annular space for discharging the drilling fluid to a reservoir. A fluid back pressure system is connected to the fluid discharge conduit configured to selectively maintain pressure therein. The fluid back pressure system includes a fluid choke, a back pressure source and an accumulator hydraulically coupled between the back pressure source and the discharge conduit. 
         [0010]    A method for controlling fluid pressure in a wellbore annulus according to another aspect of the invention includes pumping drilling fluid through a drill string extended into a wellbore, out the bottom of the drill string and into the wellbore annulus. Fluid is discharged from the annulus through a discharge conduit. The discharge conduit includes a fluid backpressure system. The fluid backpressure system including a first controllable fluid choke, a fluid backpressure source and an accumulator hydraulically coupled between the fluid backpressure source and the discharge conduit. The first controllable fluid choke is controlled to maintain a selected pressure in the annular space. The fluid backpressure source is operated to charge a fluid side of the accumulator to a selected pressure. Fluid is discharged from the accumulator into the discharge conduit to maintain the selected fluid pressure in the wellbore annulus. 
         [0011]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows a drilling system including an example back pressure system according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    An example drilling system including an example back pressure system according to the invention is shown schematically in  FIG. 1 . A drilling rig  14  or similar hoisting device suspends a drill string  10  in a wellbore  11  being drilled through subsurface rock formations  13 . A drill bit  12  is coupled to the lower end of the drill string  10 , and is rotated by the drill string  10 . Drill string rotation is enabled either by an hydraulic motor (not shown) coupled in the drill string  10  or by equipment such as a top drive  16  suspended in the drilling rig  14 . Application of some of the weight of the drill string  10  to the bit  12  and the rotation imparted to the bit  12  cause the bit  12  to drill through the formations  13 , thereby extending the length of the wellbore  11 . 
         [0014]    At the Earth&#39;s surface, a primary pump (“mud pumps”)  26  lifts drilling fluid (“mud”)  34  from a tank or pit  24  and discharges the mud  34  under pressure through a standpipe and flexible hose  31  to the top drive  16 . The top drive  16  includes internal rotary seals to enable the mud  34  to move through the top drive  16  to an internal conduit (not shown) in the interior of the drill string  10 . The drill string  10  may include a check valve  22  or similar device to prevent reverse movement of the mud  34  during times when the mud pumps  26  are not activated, and/or when the top drive  16  is disconnected from the upper end of the drill string  10 , e.g., during “connections” (adding or removing segments of pipe from the drill string  10 ). 
         [0015]    As the mud  34  travels through the drill string  10  it is eventually discharged from nozzles or courses (not shown separately) in the drill bit  12 . Upon leaving the drill bit  12 , the mud  34  enters the annular space between the exterior of the drill string  10  and the wall of the wellbore  11 . The mud  34  lifts drill cuttings from the wellbore  11  as it travels back to the surface. 
         [0016]    Discharge of the mud  34  from the annular space is controlled by a back pressure system. The back pressure system includes a rotating control head or rotating blowout preventer  18  coupled to the upper end of a surface pipe or casing  19 . The rotating control head  18  seals against the drill string  10  thereby preventing discharge of fluid from the wellbore except through a discharge line  20  The casing is typically cemented into the upper part of the wellbore  11 . Mud  34  leaves the annular space through the discharge line  20 . The discharge line  20  is coupled at one end to the rotating control head  18 . The discharge line  20  is coupled at its other end to a controllable orifice choke  30  that selectively controls the pressure at which the mud  34  leaves the discharge line  20 . After leaving the choke  30 , the mud  34  may be discharged into cleaning devices, shown collectively at  32 , such as a degasser to remove entrained gas from the mud  34  and a “shale shaker” to remove solid particles from the mud  34 . After leaving the cleaning devices,  32 , the mud  34  is returned to the reservoir  24 . Operation of the choke  30  may be related to measurements made by a pressure sensor  28  in hydraulic communication with the discharge line  20 . 
         [0017]    In annulus pressure control systems known in the art, such as the one shown in the van Riet &#39;981 patent cited in the Background section herein, pressure may be maintained in the back pressure system using a back pressure pump. As explained in the Background section herein, the back pressure pump in such systems may be relatively large to meet instantaneous volume demands in order to maintain back pressure. In the present invention, the back pressure system may include a much smaller back pressure pump  42  which may lift mud from the tank  24 . The discharge side of the back pressure pump  42  is hydraulically coupled to an accumulator  36 . A check valve  39  may be included in the foregoing connection to prevent the mud under pressure in the accumulator  36  from flowing back through the back pressure pump  42  when the pump  42  is not activated. A pressure sensor  40  may be included in the foregoing connection to automatically switch the back pressure pump  42  off when the accumulator  36  is charged to a predetermined pressure. The accumulator  36  is also hydraulically connected to the discharge line  20  through a controllable orifice choke  38  (which may be substituted by or include a valve). 
         [0018]    During operation of the back pressure system, the back pressure pump  42  operates to charge the accumulator  36 . As fluid volume is needed to maintain back pressure in the discharge line  20 , the accumulator choke  38  may be operated to enable flow from the accumulator  36  to the discharge line  20 . Concurrently, the discharge line choke  30  may be operated to substantially or entirely stop flow of mud  34 . 
         [0019]    In other examples, the back pressure pump  42  may be omitted, and some of the discharge from the mud pumps  26  may be used to charge the accumulator. One example is shown by the dotted line  43  in  FIG. 1  which indicates coupling some of the mud pumps&#39;  26  output to the accumulator  36 . 
         [0020]    The accumulator  36  may be any type known in the art, for example, types having a movable seal, diaphragm or piston to separate the accumulator  36  into two pressure chambers. Some accumulators can have the side of the diaphragm or piston opposite the fluid charged side prepressurized to a selected pressure such as with compressed gas, and/or may use a spring or other biasing device to provide a selected force to the diaphragm or piston. In some examples, the opposite side of the accumulator  36  may be charged with fluid under pressure using a separate fluid pump (not shown). In such examples, the back pressure exerted by the accumulator  36  may be changed by using the separate fluid pump, rather than by using a fixed charge (typically, by using compressed gas and/or a spring). The accumulator charge pressure may be increased under circumstances when it is necessary to discharge drilling fluid into the annulus to increase pressure. The charge pressure in the accumulator  36  may be relieved, for example, when the mud pumps  26  are restarted, or when the back pressure pump  42  is started. 
         [0021]    In the example of  FIG. 1 , the backpressure control system may be operated automatically by a dynamic annular pressure control system (DAPC)  50 . The DAPC system may include an operator control, such as a PC  52 , and programmable logic controller (PLC)  54 . The PLC  54  may accept as input signals from the various pressure sensors, including but not limited to  28  and  40  in  FIG. 1 . The PLC may also operate the variable, controllable orifice chokes  38 ,  30 , as well as the backpressure pump  42 . As explained in the van Riet &#39;891 patent referenced above, the DAPC system  50  may operate the various system components to maintain a selected fluid pressure in the discharge line  20 , and thus in the annular space between the wellbore wall and the drill string  10 . 
         [0022]    Back pressure systems according to the invention used in connection with wellbore drilling may use much smaller back pressure pumps and/or reduce the amount of draw from rig mud pumps than was possible using back pressure systems known in the art prior to the present invention. 
         [0023]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.