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BACKGROUND  
       [0001]     The invention generally relates to generating downhole power.  
         [0002]     A typical subterranean well includes various devices that are operated by mechanical motion, hydraulic power or electrical power. For devices that are operated by electrical or hydraulic power, control lines and/or electrical cables typically extend downhole for purposes of communicating power to these tools from a power source that is located at the surface. A potential challenge with this arrangement is that the space (inside the wellbore) that is available for routing various downhole cables and hydraulic control lines may be limited. Furthermore, the more hydraulic control lines and electrical cables that must be installed and routed downhole, the higher probability that some part of the power delivery infrastructure may fail.  
         [0003]     Thus, some subterranean wells have tools that are powered by downhole power sources. For example, a fuel cell is one such downhole power source that may be used to generate electricity downhole. The subterranean well may include other types of downhole power sources, such as batteries, for example.  
         [0004]     A typical subterranean well undergoes a significant amount of vibration (i.e., vibration on the order of Gs, for example) during the production of well fluid. In the past, the energy produced by this vibration has not been captured. However, an emerging trend in subterranean wells is the inclusion of devices to capture this vibrational energy for purposes of converting the energy into a suitable form for downhole power.  
         [0005]     Thus, there is a continuing need for better ways to generate power downhole in a subterranean well.  
       SUMMARY  
       [0006]     In an embodiment of the invention, a system that is usable with a subterranean well includes a first tubular member that is adapted to receive a flow of a first fluid. The system includes a second tubular member that is located in the flow and is substantially flexible to be moved by the flow to establish a pressure on a second fluid located inside the tubular member. A mechanism of the system uses this pressure to actuate a downhole tool.  
         [0007]     Advantages and other features of the invention will become apparent from the following description, drawing and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a schematic diagram of a well according to an embodiment of the invention.  
         [0009]      FIGS. 2, 3  and  4  depict a pump of  FIG. 1  for different positions of a flexible tube of the pump according to an embodiment of the invention.  
         [0010]      FIG. 5  is a block diagram of a hydraulic system according to an embodiment of the invention.  
         [0011]      FIG. 6  is a flow diagram depicting a technique to harness downhole energy according to an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0012]     Referring to  FIG. 1 , an embodiment  10  of a subterranean well in accordance with the invention includes a wellbore  12  that extends downhole through one or more subterranean formations. In the example depicted in  FIG. 1 , the system  10  may include a tubular string  14  (a production tubing, for example) that extends into the wellbore  12 . In the exemplary system  10  depicted in  FIG. 1 , the well is uncased. However, in other embodiments of the invention, the wellbore  12  may be lined by a casing string. A packer  30  may seal and anchor the tubular string  14  to the wellbore  12 .  
         [0013]     The tubular string  14 , in some embodiments of the invention, is a production tubing string that includes a central passageway  29  that receives the flow of production fluid from the well. For example, as depicted in  FIG. 1 , the tubular string  14  may receive the flow of well fluid (depicted generally by the arrows  27 ) from one or more zones, such as exemplary zone  32 .  
         [0014]     More specifically, the fluid flows from the zone  32  up through the central passageway  29  and returns to the surface of the well. Although  FIG. 1d epicts a vertical well, it is understood that in other embodiments of the invention, the well  10  may include various lateral, or horizontal, wellbores. Thus, the well  10  is merely depicted as an example to illustrate the harnessing of power, described below.  
         [0015]     In some embodiments of the invention, the tubular string  14  includes a pump  16  that harnesses energy that is generated or induced by the flow of production fluid through the tubular string  14 . More specifically, in some embodiments of the invention, the pump  16  is a “lymphatic pump,” in that the pump  16  directly converts energy induced by the flow or well fluid into hydraulic power that may be used to control one or more downhole tools of the string  14 .  
         [0016]     More specifically, in some embodiments of the invention, the pump  16  exerts hydraulic pressure on fluid that is stored in an accumulator  20  of a hydraulic system  18  of the string  14 . The pressure accumulated in the accumulator  20 , in turn, is used by the system  18  to drive, or actuate, one or more downhole tools  24  (one tool  24  being depicted in  FIG. 1 ) of the tubular string  14 . Depending on the particular embodiment of the invention, the tool  24  may be a sleeve, a valve, a packer, etc.  
         [0017]     In some embodiments of the invention, the pump  16  may have a form that is generally depicted in  FIG. 2 . In particular, the pump  16  includes a substantially flexible tubular member  50  that is located inside the central passageway  29  of the tubular member  14 . For example, in some embodiments of the invention, one end of the tubing  50  may be a free end  46 , in that the end  50  moves with the flow  27 . The opening at the end  46  is generally concentric with the longitudinal axis of the central passageway  29 . Thus, a portion  51  of the flow  27  is diverted into the tubing  50  to create a flowpath from the end  46  to a distal end  48  of the flow tube  50 . The pressure of this flow  51 , in turn, is affected by the movement of the flow tube  50 .  
         [0018]     More specifically, in some embodiments of the invention, the flow tube  50  moves due to the flow  27 , as depicted in  FIGS. 2, 3  and  4  for three different positions of the flow tube  50 . This waving action of the flow tube  50  serves to pump the flow  51 to pressurize fluid in the flow  51 . It is this pressure that may be used to actuate one or more downhole tools.  
         [0019]     Referring to  FIG. 5 , in some embodiments of the invention, the hydraulic system  18  may have a form like the one generally depicted in  FIG. 5 . In the system  18 , the end  48  of the flow tube  50  communicates the flow  51  to an accumulator  100 . The accumulator  100  may include, for example, a first chamber in communication with the flow  51  that is separated from a second chamber, containing a hydraulic control fluid, by a piston, for example. Thus, the accumulation of pressure from the flow  51  establishes a control pressure in a hydraulic output line  101  of the accumulator  100 . In some embodiments of the invention, the hydraulic output line  101  may be connected through a check valve  107  to a pressurized source  120 . Thus, the accumulator  100  may serve to pressurize a particular source  120  for purposes of forming a direct hydraulic power source. A hydraulic control circuit  102  is in communication with the pressurized source  120  for purposes of controlling when this pressurized source is applied to one or more downhole tools via hydraulic output lines  108 . Other variations are possible and are within the scope of the appended claims.  
         [0020]     Referring to  FIG. 5 , in some embodiments the hydraulic control also includes a maximum pressure relief valve  110  that provides an upper limit on the pressurized source. In some embodiments of the invention, the hydraulic system  18  may be a closed system in that the maximum pressure relief valve  110  is connected to a chamber to effectively “store” a maximum pressure in the well. This chamber may be used to power one or more downhole tools, for example.  
         [0021]     Thus, referring to  FIG. 6 , in some embodiments of the invention, a technique  200  may be used for purposes of performing a particular downhole function. Pursuant to the technique  200 , a flow of well fluid is directly converted into hydraulic pressure, as depicted in block  202 . The hydraulic pressure is then used (block  204 ) to perform some downhole function. For example, this downhole function may be the actuation of a valve, the movement of a sleeve, the setting of a packer, etc.  
         [0022]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.

Summary:
A system that is usable with a subterranean well includes a first tubular member that is adapted to receive a flow of a first fluid. The system includes a second tubular member that is located in the flow and is substantially flexible to be moved by the flow to establish a pressure on a second fluid located inside the tubular member. A mechanism of the system uses this pressure to actuate a downhole tool.