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CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND 
       [0003]    The disclosure relates generally to the field of dual drill pipe strings. More specifically, the disclosure relates to an actuator and a valve system and configurations of a valve system used with dual drill strings. 
         [0004]    It is known in the art of subsurface wellbore drilling to use a single pipe string, two parallel pipes or two nested or concentric pipe strings. Concentric or nested pipe strings refer to a string consisting of inner pipe joints arranged within outer pipe joints connected end to end. 
         [0005]    In concentric or nested drill strings, the inner pipe forms part of a flow bore extending from the surface to a drill bit at the lower end of the drill string. An annulus between the outer pipe and inner pipe forms part of a second flow bore extending from the surface to the drill bit. Further, it is known to provide barriers or valves (e.g., check valves) in the pipe string to prevent gas-kicks, blow-outs etc. to move to the surface during drilling operations. Drilling operations may refer to the drilling of a wellbore, including the connection and disconnection of pipe segments (joints or multiple joint “stands”) during drilling operations. The barriers may be in the form of valves in the flow bores, arranged to provide seals against uncontrolled flow, such as gas-kicks and blow-outs. The valves may be check valves allowing flow in one direction and preventing flow in the other direction. 
         [0006]    The term “drilling” as used herein should be understood to refer to creation of a hole in the subsurface by means of the pipe string. It particularly applies for drilling in the crust of the earth for petroleum recovery, tunnels, canals or for recovery of geothermal energy, both offshore and onshore. 
         [0007]    U.S. Patent Application Publication No. 2010/0116501 A1 discloses a backup safety flow control system for concentric drill strings. The &#39;501 publication shows a primary annulus shutoff valve assembly and a backup annulus shutoff valve assembly in the annular bore, and a primary inner bore shutoff valve assembly in the inner bore. In addition, in case the primary inner bore shutoff valve assembly fails, the flow control system includes a backup inner bore shutoff valve by means of a valve that may be dropped from the surface through the inner bore. When the wellbore pressure is brought under control, the drill string can be removed from the well so that the backup inner shutoff valve may be removed. 
         [0008]    Other dual drill string systems may include a valve to close both the inner pipe string proximate the bottom end thereof and the outer pipe proximate the bottom end thereof when fluid pumping from the surface is stopped. When such valves are affixed proximate the bottom end of a dual drill string, and the dual drill string is inserted into (“tripped” or “run”) the wellbore, the interior of both the outer drill string and the inner drill string will be void of drilling fluid. In other cases, the dual drill string may remain fluid filled, but pressures may be reduced therein when fluid circulation is stopped for any reason. Upon resumption of fluid flow (“circulation”), the one of the inner or outer drill string carrying the fluid under pressure from the surface will become charged with drilling fluid until an actuation pressure of the shutoff valve is exceeded. However, upon the pressure in the wellbore annulus exceeding the opening pressure of the other shutoff valve, that is, the valve which closes the fluid return path, the fluid will be exposed to an air-filled conduit or to a liquid filled conduit that has pressure lower than the wellbore pressure because on cessation of fluid circulation the shutoff valve will lock in a pressure in the return conduit that represents the wellbore pressure less fluid flow friction loss pressure. Thus, lower pressure will exist at the moment of opening the other shutoff valve. Such lower pressure may cause rapid drop in the pressure of fluid in the wellbore, which may lead to wellbore collapse and/or fluid influx from formations exposed to the wellbore. 
         [0009]    There exists a need for a method of operating a dual drill string in a wellbore and initiating circulation without exposing the wellbore to relatively low pressure upon opening of a drill string shutoff valve. 
       SUMMARY 
       [0010]    A method according to one aspect for operating a dual drill string in a wellbore, the drill string having a shutoff valve for at least a fluid return drill string therein proximate a bottom end thereof includes pumping fluid into the fluid return string such that a fluid pressure therein is substantially equal to a fluid pressure in the wellbore. Circulation of fluid in the wellbore is initiated by pumping fluid into a fluid supply string in the dual drill string such that the shutoff valve on the fluid return string opens to enable the fluid leaving the wellbore to enter the fluid return string. 
         [0011]    Other aspects and advantages will be apparent from the description and claims which follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows an example drilling arrangement using a nested or concentric drill pipe string and drill string valves. 
           [0013]      FIGS. 2A and 2B  show cut away views of a dual drill string actuator, in the closed and open position, respectively 
           [0014]      FIGS. 3A and 3B  show an example dual drill string rotary valve in the open (activated) position and closed position, respectively. 
           [0015]      FIGS. 4A and 4B  show an example dual drill string rotary valve in the open (activated) position and closed position, respectively. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    In  FIG. 1  a nested or concentric dual drill string  1  is shown inserted in a wellbore  17  being drilled through subsurface formations  33 . The wall of the wellbore  17  creates an annular space (well annulus  9 ) between the exterior of the dual drill string  1  and the wall of the wellbore  17 . The dual drill string  1  may comprise a dual bore drill pipe consisting of an inner pipe  3  arranged within an outer pipe  2 . A supply flow of drilling fluid (e.g., “drilling mud”), shown at A, is introduced through a suitable swivel  24  such as a top drive into an annular bore (“fluid supply flow passage”)  4  disposed between the inner pipe  3  and the outer pipe  2 . The supply flow of drilling fluid A may be ultimately directed to a drill bit  7  that cuts the formations  33 . A return flow of drilling fluid, shown at B is transported from the bottom of the wellbore  17  in an inner bore (“return fluid passage”)  5  within the inner pipe  3 . 
         [0017]    In the example shown in  FIG. 1 , the dual drill string  1  may be arranged with a piston  20  fixed to the dual drill string  1  and in sealing contact with the wall of the wellbore  17 . The top drive  24  may also rotate or drive the dual drill string  1 . A blow out preventer (BOP)  22  and a rotating control device (RCD)  23  may be arranged at the top of the wellbore  17 . By the arrangement of the RCD  23  and piston  20 , an isolated space is provided in the upper part of the wellbore  17 . In the present example, a fluid may be introduced through a fluid inlet  21  into the isolated space. The introduced fluid provides a pressure to the piston  20 , thereby forcing the piston  20  and the dual drill string  1  downwards when drilling is performed. As will be appreciated by those skilled in the art, other arrangements than the piston  20  shown in  FIG. 1  may be used for providing a driving force to the dual drill string  1 , or may be omitted, wherein the isolated space in the wellbore annulus  9  is closed by the BOP  22  and RCD  23 . Thus, the use of the piston  20  in the wellbore annulus  9  is not a limitation on the scope of the invention. 
         [0018]    The dual drill string  1  is typically arranged with a flow diverter  6  at a lowe4r end thereof connected to a bottom hole assembly (BHA)  8  holding the bit  7  at a lower end portion of the drill string. The bottom hole assembly (BHA)  8  may a standard type BHA that can be used with conventional (single flow bore) drill pipe and drilling tools, including, without limitation, hydraulic (mud) motors, drill collars, measurement and/or logging while drilling tools. The BHA may also be a reverse flow type such as used in air drilling mining operations. The flow diverter  6  has a flow passage assembly  10   a  providing a fluid connection between the fluid supply flow passage  4  of the dual drill string  1  and a channel  14  or channel assembly of the BHA  8 . The channel  14  of the BHA  8  is shown in the example of  FIG. 1  with the shape of an axial bore, and the flow passage assembly  10   a  is shown with essentially a Y-shape in an axial cross section. First diverging branches  30  of the Y fit in connection with the fluid supply flow passage  4 , and an axial passage part  31  corresponds to the stem portion of the Y and fits in connection with the axial shaped channel  14  of the BHA. The supply flow A exits from the channel  14  into the BHA  8  and thence into the cutting area of the drill bit  7 . 
         [0019]    From the drill bit  7 , the return fluid flow B moves in the well annulus  9  into a return flow passage assembly  10   b  arranged in the flow diverter  6 . The axial cross section of a return flow passage assembly  10   b  also has a Y shape with second diverging branches  41  opening at one end into the well annulus  9  and an axial passage part  40  connected with the fluid return flow passage  5 . The return flow B enters the inlet of the flow diverter return flow passage  10   b  and returns in the fluid return flow passage  5  of the dual string  1 . 
         [0020]    The dual drill string  1  may be arranged, for example, with a selected number of valve elements (four shown in the present example), although the number of such valves and their placement within the drill string is not intended to limit the scope of the invention. Two of the valve elements may be arranged for closing and opening of the fluid supply flow A, and two of the valve elements may be arranged for closing and opening of the fluid return flow B. By such arrangement of valve elements, a double barrier system may be provided both for the control of the fluid supply flow A and for control of the fluid return flow B. The closing of the valve elements may be performed, in some examples automatically if the drilling system needs to close down, and in case of emergency, for example, a kick or other unwanted well fluid control conditions. Other examples of valve elements, to be described in more detail below, may close both the fluid supply flow passage  4  and the return fluid passage  5 . 
         [0021]    In  FIG. 1  example locations of the four valve elements are shown schematically. Two bottom valves  11   c ,  11   d  provided for opening and closing the supply flow A, may be located in the bottom hole assembly  8 . The bottom valves  11   c ,  11   d  may be positioned to open and close the channel  14 , and one of the bottom valves, e.g.,  11   d , may be positioned to control the opening and closing of the outlet  15  of the channel  14 . The other bottom valve  11   c  may be positioned upstream along the channel  14  within the bottom hole assembly  8 . The bottom valves  11   c ,  11   d  may be conventional drill string check valves as are used with single bore drill string components. Upper valves  11   a ,  11   b  may be positioned in the dual drill string  1 . The upper valves  11   a ,  11   b , may be specifically configured to connect within a nested dual drill string, for example, one shown in U.S. Pat. No. 3,208,539 issued to Henderson, and the valves  11   a ,  11   b  may be referred to hereinafter for convenience as dual drill string valves. 
         [0022]    In the present example, the top drive  24  may include a shutoff valve B1 in the return fluid flow line B and a crossover valve A1 that selectively makes hydraulic connection between the supply fluid flow line A and the return fluid line B. The function of the foregoing valves B1, A1 will be further explained. 
         [0023]    The dual drill string actuators and associated valves  11   a ,  11   b  may be better understood with reference to  FIGS. 2A ,  2 B,  3 A,  3 B and  4 A,  4 B. An important component of a dual drill string valve according to the invention, and referring to  FIGS. 2A and 2B , is a dual drill string compatible valve actuator  100 . Referring to  FIG. 2A , an example dual drill string actuator  100  may be enclosed in a housing  110  that may have connections (not shown separately) at each longitudinal end for engaging the housing  110  to a segment of the dual drill string, e.g.,  1  in  FIG. 1 ) on one or both longitudinal ends thereof “Engagement” may include metal to metal or other form of sealing between the housing  110  and each connected segment of the outer pipe, as explained with reference to  FIG. 1 . “Engagement” may further include having an upper internal conduit  112  mounted in fixed longitudinal position within the housing  110 . Such mounting may include, without limitation, friction fit standoffs, welding, adhesive bonding, etc. The upper inner conduit  112  may be configured to sealingly engage the inner pipe ( 3  in  FIG. 1 ) to enable completion of the fluid return flow passage ( 5  in  FIG. 1 ) through the actuator  100 . A fluid return flow passage formed by the components of the actuator  100  is shown generally at  113  and  113 A. As will be further explained below, the actuator  100  also may provide a fluid flow passage between the interior of the housing  110  and the exterior of the upper internal conduit  112 , lower internal conduit  115  and additional components explained below. Thus, the actuator  100  may be configured so that its behavior with respect to the dual drill string ( 1  in  FIG. 1 ) is essentially “transparent”, that is, the drilling rig operator or user may handle the actuator  100  in essentially the same manner as any other segment of the dual drill string ( 1  in  FIG. 1 ). 
         [0024]    In the present example, a piston  114  may be disposed inside the housing  110  and may include at one longitudinal end a tube  114 A that may slidingly engage with an interior bore of the upper inner conduit  112 . The tube  114 A may be sealed to the upper inner conduit  112  using seals D1 of any type known in the art enabling longitudinal motion while maintaining a pressure tight seal, e.g., o-rings or the like. The lower inner conduit  115  may be mounted in the housing  110  at the opposite longitudinal end of the housing  110 . The lower inner conduit  115  may be configured at its longitudinal end to sealingly engage another segment of dual drill string such as shown in  FIG. 1 . The lower inner conduit  115  may be mounted inside the housing  110  in any manner as explained with reference to the upper inner conduit  112 . The piston  114  may also slidingly engage the lower inner conduit  115 . Such sliding engagement may include pressure tight sealing, for example, by using o-rings or similar seals such as shown at D2. Thus, the piston  114  may move longitudinally with respect to the upper  112  and lower  115  inner conduits while maintaining a sealed inner fluid passage, shown by the combination of elements  113 ,  114 B and  113 A. In the example shown in  FIGS. 2A and 2B , a spring or biasing device  116  may urge the piston  114  into its raised position ( FIG. 2A ) in the absence of any fluid flow through the actuator  100 . 
         [0025]    The mounting of both the upper inner conduit  112  and the lower inner conduit  115  within the housing  110  may be configured to enable fluid flow in a passage formed between the interior wall of the housing  110  and the exterior of the upper inner conduit  112 , the piston  114  and the lower inner conduit  115 . Thus, the actuator  100  may be substantially transparent with respect to the dual drill string as it concerns fluid flow therethrough; there is provided by the described structure both an inner flow passage and an outer flow passage corresponding to such passages in the dual drill string ( 1  in  FIG. 1 ). 
         [0026]    Specifically referring to  FIG. 2A , the actuator  100  is shown in its state that exists when the fluid supply flow (A in  FIG. 1 ) is stopped. The BHA  8  is shown schematically at a position below the actuator  100 . The BHA  8  may include a conventional float or check valve, shown at  8 A, and the lower part of the BHA  8 , which may include a “mud” drilling motor (not shown) and the drill bit ( 7  in  FIG. 1 ) is shown schematically at  8 B as a resistance to flow therethrough. In  FIG. 2A , the piston  114  is in its uppermost position. Referring to  FIG. 3A , when the fluid supply flow (A in  FIG. 1 ) is turned on, pressure P1 will exist in the passage between the interior wall of the housing  110  and the exterior of the upper inner conduit  112 , the piston  114  and the lower interior conduit  115 . Because of the resistance to flow provided by the BHA  8 , the pressure P1 will typically be greater than the pressure below the actuator  100 , shown by P2. The pressure P1 acts on the piston  114  to move it downwardly, as shown in  FIG. 2B . The float valve  8 A is shown open in  FIG. 2B , which results from flow leaving the actuator  100 . 
         [0027]    The actuator  100  shown in and explained with reference to  FIGS. 2A and 2B  may be used in conjunction with any other apparatus disposable in a drill string. For such use, it is only necessary to provide connection such that motion of the piston  114  causes operation of another device. 
         [0028]    It will be appreciated that the flow diverter ( 6  in  FIG. 1 ) and other detailed components of the BHA  8  have been omitted from  FIGS. 2A and 2B  for simplicity of the illustration. In actual drilling use, such components may be included in the dual drill string as required, for example, as shown in  FIG. 1 . 
         [0029]    Referring to  FIGS. 3A and 3B , one example of a drill string valve associated with the above described actuator will be explained. A drill string valve  111  using the actuator of  FIGS. 2A and 2B  may be formed by including within the piston structure ( FIGS. 2A and 2B ) a rotary valve. The rotary valve may be assembled from separate components, explained below, to form the piston ( 114  in  FIGS. 2A and 2B ), such that application of supply fluid flow (A in  FIG. 1 ) will cause downward motion of the rotary valve, thereby causing it to open. 
         [0030]    In the present example, the rotary valve may include a tube  114 A that sealingly, slidably engages the upper inner conduit  112 , as in the actuator shown in  FIGS. 2A and 2B . The tube  114 A may be sealed to the interior of the upper inner conduit using seals, D1 in  FIG. 2B . The tube  114 A may be affixed at its lower end to a rotatable valve disc  148 . The rotatable valve disc  148  may include an internal passage  148 A that is aligned with the passage  114 B in the tube  114  where the tube  114  and rotatable valve disc  148  contact each other, and is laterally displaced at the lower end of the rotatable valve disc  148 . The rotatable valve disc  148  may contact at its lower end a rotationally fixed valve plunger  146 . The rotationally fixed valve plunger  146  may include a corresponding passage  146 A ( FIG. 3B ) therein to provide fluid communication with passage  113 A in the lower inner conduit  115 . A helical guide  140  may be formed in the interior of the housing  110 , for example, as a groove or as a ridge. A groove may provide easier assembly and disassembly of the valve  111 , however this is not a limitation on the scope of the invention. A mating pin or groove, shown at  141 , may be provided on the tube  114  or the rotatable valve disc  148 . 
         [0031]    When the fluid supply flow (A in  FIG. 1 ) is turned on, and pressure P1 exists in the interior of the housing, but outside the tube  114 , rotatable valve disc  148  and rotationally fixed valve plunger  146 , the entire assembly of the foregoing components is urged downward by the differential pressure, essentially as explained with reference to the actuator described above. In the present example, however, engagement of the pin  141  with the groove  140  causes rotation of the rotatable valve disc  148 . In the “closed” position shown in  FIG. 3A , the passages  148 A,  146 A are misaligned, and the rotary valve is closed to flow. In  FIG. 3B , when the foregoing assembly of components is moved downwardly by pressure P1, the rotatable valve disc  148  rotates so that the passages  148 A,  146 A are aligned to enable flow therethrough. Thus, the interior passage of the valve  111 , consisting of upper inner conduit passage  113 , tube passage  114 B, valve disc/plunger passages  148 A,  146 A and lower inner conduit passage  113 A form an open passage to fluid flow. In this way, when the fluid supply flow (A in  FIG. 1 ) is stopped, the passage just described will close, thereby stopping flow from the well into the fluid return flow (B in  FIG. 1 ) part of the dual drill string (e.g.,  5  in  FIG. 1 ). Fluid flow into the fluid supply flow part of the dual drill string (e.g.,  4  in  FIG. 1 ) may be stopped by the float valve ( 8 A in  FIG. 2A ). 
         [0032]    A spring, such as shown at  116  in  FIGS. 2A and 2B  may be used in cooperation with the rotationally fixed valve plunger  146  to assist in closing the valve, substantially as explained with reference to  FIGS. 2A and 2B . 
         [0033]    An alternative valve  111  may be better understood with reference to  FIGS. 4A and 4B . The valve  11  shown in  FIG. 4A  (closed position) and  FIG. 4B  (open position) may include substantially all the components of the rotary valve shown in  FIGS. 3A and 3B , with the addition of a valve seat  137  ( FIG. 4A ) that cooperatively engages a seal seat  137  ( FIG. 4B ) when the rotary valve components are in the position shown in  FIG. 4A . Thus, a fluid flow passage formed inside the housing  110 , but outside the upper inner conduit  112 , tube  114 , rotatable valve disc  148 , rotationally fixed valve plunger  146  and lower inner conduit  115  will be closed to flow when the fluid supply flow (A in  FIG. 1 ) is turned off. 
         [0034]    Two or more of the valves shown in  FIGS. 4A and 4B  may be placed at selected longitudinal positions (e.g., as shown in  FIG. 1 ) to provide additional wellbore pressure control. 
         [0035]    The example actuators and valves shown in  FIGS. 2A ,  2 B,  3 A,  3 B,  4 A and  4 B are not intended to limit the scope of the present invention. In addition, the example dual drill string shown in  FIG. 1 , wherein one pipe string is nested within another pipe string, is also not intended to limit the scope of the invention. For example, side by side dual drill strings are known in the art and may be equally used with the example method to be described below. See, for example, U.S. Pat. No. 3,955,622 issued to Jones. 
         [0036]    Referring once again to  FIG. 1 , when the dual drill string  1  is run into the wellbore  17  to a selected depth, drilling may be resumed first by resuming circulation of the drilling fluid. It should be understood that running the dual drill string  1  into the wellbore  17  is not a prerequisite to performing the present example method. In other examples, the dual drill string  1  may already be at the selected depth, wherein circulation had been previously stopped for any other reason. In the present example, both the fluid supply flow passage (outer string flow path)  4  and return fluid passage (the inner string flow path)  5  may be initially dry because the shutoff valves  11   a ,  11   b ,  11   c ,  11   d  are closed, or the passages  4 ,  5  may be fluid filled and the shutoff valves  11   a ,  11   b ,  11   c ,  11   d  closed because fluid circulation had been previously stopped. Pumping of the supply fluid A may commence, and the crossover valve A1 in the top drive  24  or elsewhere may hydraulically connect the fluid supply flow passage  4  and fluid return flow passage  5  so that both are simultaneously filled with fluid, or if the passages  4 ,  5  are already fluid filled, may operate to pressurize the fluid passages  4 ,  5 . In some examples, the return flow path B may include a shutoff valve B1 that may be closed during the filling and/or pressurization of the dual drill string  1  with fluid. The foregoing valves A1, B1 in the top drive  24  are only examples of devices to fill and/or pressurize both passages simultaneously. Other flow control devices may be used to equal effect. When the both the return fluid passage  5  and the supply fluid passage  4  of the dual drill string  1  are filled with fluid and/or are pressurized, the bypass valve A1 may be closed, and fluid pumping may continue through the supply fluid flow passage  4 . As the fluid travels further, through the crossover  41  and out the drill bit  7 , the fluid enters the wellbore  17  and travels to the crossover  41 , where it is directed to the fluid return passage  5 . In some examples, only the fluid return passage  5  may be pressurized before resuming fluid circulation. In other examples, both passages  4 ,  5  may be pressurized substantially contemporaneously before fluid circulation is resumed. 
         [0037]    During drilling, when the drilling unit pumps are running, the return flow in the inner pipe will create a friction pressure loss. The total bottomhole pressure in the wellbore  17  will be the sum of the drilling fluid density multiplied by the vertical depth, plus the friction pressure loss in the return passage  5 . When drilling is stopped and the pressure in the dual drill string is bled off, (both passages  4 ,  5 ), e.g., for making pipe connections or for any other reason, the foregoing total bottomhole pressure less the friction loss pressure will be the pressure locked in, in particular in the return passage  5 , by the above described valves. 
         [0038]    To start or restart the drilling and/or circulation process and open the described valves again it is important (as it is desirable to maintain constant wellborefluid pressure) to equalize the differential pressure over the bottom hole inner pipe shutoff valve prior to opening thereof. Therefore in the present example the inner pipe channel may be pressurized prior to opening the valve therefor. The pressure may be increased so that the pressure in the return path  5  substantially matches the wellbore fluid pressure. 
         [0039]    As explained above with reference to  FIGS. 2 through 4 , the fluid flow through the supply flow passage will open the shutoff valves  11   a ,  11   b ,  11   c ,  11   d , enabling fluid to travel from the wellbore  17  into the return fluid passage  5 . Because the return fluid flow passage  5  has been or already is filled with fluid prior to opening the shutoff valves  11   a ,  11   b ,  11   c ,  11   d , by pressurizing the fluid return passage  5  so that its pressure is substantially equal to the wellbore fluid pressure, when the shutoff valves are opened, the fluid flowing from the wellbore  17  into the return fluid flow passage  5  will not be exposed to lower pressure as would be the case if the return fluid flow passage  5  were gas or air filled or were fluid filled and not otherwise previously pressurized to compensate for the friction pressure loss locked in by the respective shut off valve after circulation is stopped. By opening return flow into the return passage  5  only after substantially equalizing its pressure to the wellbore fluid pressure, rapid wellbore pressure drops and accompanying drilling hazards may be avoided. 
         [0040]    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.

Summary:
A method for operating a dual drill string disposed in a wellbore, the string having a shutoff valve for at least a fluid return drill string therein proximate a bottom end thereof includes pumping fluid into the fluid return string such that a pressure therein is substantially equal to a fluid pressure in the wellbore. Circulation of fluid in the wellbore is initiated by pumping fluid into a fluid supply string of the dual drill string such that the shutoff valve on the fluid return string opens to enable the fluid leaving the wellbore to enter the fluid return string.