Abstract:
A system and method for sensing at least one force on a downhole tool connected in a workstring, according to which a mandrel is connected in the workstring and is subjected to the force. Two or more sensors sense axial or torsional force on the mandrel and are connected in an electrical circuit to convert the sensed force to an output signal.

Description:
BACKGROUND  
         [0001]    This invention relates to a system and method for sensing and storing force and torque data on a downhole tool in a downhole oil and gas recovery system.  
           [0002]    Many tools are inserted downhole in a wellbore in an oil and gas recovery system to perform various functions in the recovery process. After many of these tools have been inserted downhole, they require the application of weight and/or torque to operate. For example, in order to “set” a typical downhole retrievable packer, the pipe, or workstring connected to the packer, must be picked up, rotated, and then set back down. After it is set in this manner, the packer is subjected to various other forces such as hydraulic forces, as well as forces caused by thermal expansion and contraction that occur during a cementing or stimulation treatment. Since these forces may change the setting force on the packer and may otherwise adversely affect its operation, it is important that the forces be sensed and their values either stored or transmitted to the surface in real time so as to permit remedial action. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]    [0003]FIG. 1 is an elevational view of a downhole tool including an embodiment of a system according to the invention.  
         [0004]    [0004]FIG. 2 is a partial schematic, enlarged, side view of a mandrel of the downhole tool of FIG. 1.  
         [0005]    [0005]FIG. 3 is a cross-sectional view taken along the line  3 - 3  of FIG. 2.  
         [0006]    [0006]FIG. 4 is a partial schematic, enlarged, view of the opposite side of the mandrel of FIG. 2.  
         [0007]    FIGS.  5 - 7  are views similar to those of FIGS.  2 - 4 , respectively, but depicting the mandrel rotated ninety degrees from the positions of FIGS.  2 - 4 , respectively.  
         [0008]    [0008]FIGS. 8 and 9 are electrical circuits employed in the above system. 
     
    
     DETAILED DESCRIPTION  
       [0009]    Referring to FIG. 1, a tubular mandrel is shown in general by the reference number  10  and forms part of a workstring that is inserted downhole in a wellbore, or the like. A retrievable packer  12 , or other downhole tool, is located immediately below the mandrel  10  with the corresponding ends of the mandrel  10  and the packer  12  being connected in any conventional manner. It will be assumed that, when inserted downhole and set, the packer  12 , and therefore the mandrel  10 , will be subjected to the forces discussed above. The mandrel  10  is thus a load-bearing member of the packer  12  subject to the forces experienced by the packer  12  when it is connected in the workstring. It is understood that other downhole tools (not shown) can also be connected in the workstring.  
         [0010]    A series of batteries  14  are angularly spaced in openings formed inside mandrel  10  and are attached to the mandrel  10  in any conventional manner. A printed circuit board  16  is mounted to the outer surface of the mandrel  10  in any conventional manner and is connected to the batteries  14  for receiving electrical power. An outer tubular case  18  extends over the mandrel  10  and the circuit board  16 . It is understood that one or more seal rings can be provided between the case  18  and the mandrel  10 .  
         [0011]    A plurality of strain sensors are located on the outer surface of the mandrel  10  and between the mandrel  10  and the case  18 . The sensors are not shown in FIG. 1 due to scale limitations, but are shown in detail in FIGS.  2 - 4 . In particular, a pair of axially-spaced sensors  20  and  22  (FIG. 2) are mounted to an exterior surface area of the mandrel  10 , and an additional pair of axially-spaced sensors  24  and  26  (FIG. 4) are mounted to an exterior surface area of the mandrel  10  which is diametrically opposite the first-mentioned surface area. The axes of the sensors  20  and  24  extend parallel to the axis of the mandrel  10  and the axes of the sensors  22  and  26  extend perpendicular to the axis of the mandrel  10 .  
         [0012]    As better shown in FIGS. 5 and 6, a pair of axially-spaced sensors  30  and  32  are mounted to an exterior surface area of the mandrel  10  and are angularly displaced approximately ninety degrees from the sensors  20  and  22  and from the sensors  24  and  26 . Also, as shown in FIGS. 6 and 7, a pair of axially-spaced sensors  34  and  36  are mounted to an exterior surface area of the mandrel  10  diametrically opposite the sensors  30  and  32 , and therefore also approximately ninety degrees from the sensors  20  and  22  and from the sensors  24  and  26 . The respective axes of the sensors  30 ,  32 ,  34 , and  36  extend at an angle to the longitudinal axis of the mandrel  10 , which, in the example shown, is approximately forty-five degrees. The sensor  30  extends perpendicular to the sensor  32  and the sensor  34  extends perpendicular to the sensor  36 .  
         [0013]    Each sensor  20 ,  22 ,  24 ,  26 ,  30 ,  32 ,  34 , and  36  can be in the form of a metal foil strain gauge whose resistance varies in response to various forces applied thereto, in a conventional manner.  
         [0014]    The disposition of the axes of the sensors  20  and  24  parallel to the axis of the mandrel  10 , and the disposition of the axes of the sensors  22  and  26  perpendicular to the axis of the mandrel  10  enables the sensors  20 ,  22 ,  24 , and  26  to respond to axial compression and tension along the mandrel  10 . Also, the angular disposition of the sensors  30 ,  32 ,  34 , and  36  enable them to respond to torsional forces on the mandrel  10 .  
         [0015]    The sensors  20 ,  22 ,  24 , and  26  are connected in an electrical circuit, shown in general by the reference numeral  40  in FIG. 8, which is configured in a conventional Wheatstone bridge configuration. The respective outputs of the sensors  20 ,  22 ,  24 , and  26  of the circuit of FIG. 8, are related to the applied tensile and compression loads on the mandrel  10  according to the following:  
           V   o     V     ≈         FP        (     1   +   v     )       ×     10   3         2      EA                             
 
         [0016]    whereby:  
         [0017]    V o  is the output voltage from the bridge  40   
         [0018]    V is the excitation voltage to the bridge  40   
         [0019]    v is Poisson&#39;s ratio  
         [0020]    P is the applied load  
         [0021]    F is a gauge factor for the strain gauge (usually =2)  
         [0022]    E is Young&#39;s modulus of the mandrel  10  material  
         [0023]    and  
         [0024]    A is the cross sectional area of the mandrel  10 .  
         [0025]    When the circuit  40  is provided with excitation voltage to the sensors  20 ,  22 ,  24 , and  26 , the measured output voltage is representative of the applied tension and compression to the mandrel  10 . In the event the mandrel  10  is subject to bending or torsional forces, the strains due to bending and torsion applied to the sensors  20  and  24  and to the sensors  22  and  26  cancel, thus rendering the circuit  40  insensitive to these forces. Also, the circuit  40  is insensitive to any changes in temperature since any temperature dependent changes in the resistance of the sensors  20 ,  22 ,  24 , and  26  are cancelled.  
         [0026]    The sensors  30 ,  32 ,  34 , and  36  are connected in an electrical circuit, shown in general by the reference numeral  42  in FIG. 6 which is also configured in a conventional Whetstone bridge configuration. The respective outputs of the sensors  30 ,  32 ,  34 , and  36  of the circuit of FIG. 9, are related to the applied torsional loads on the mandrel  10  according to the following:  
         V     V   o       =         2      FTR       π                   E        (       R   4     -     r   4       )                (     1   +   v     )                             
 
         [0027]    whereby, in addition to the variables defined above:  
         [0028]    T is the applied torque  
         [0029]    R is the outside radius of the mandrel  10   
         [0030]    r is the inside radius of the mandrel  10 .  
         [0031]    When the circuit  42  is provided with excitation voltage to the sensors  30 ,  32 ,  34 , and  36 , measurement of the output voltage is representative of the applied torsion to the mandrel  10 . Due to the angular disposition of the axes of the sensors  30 ,  32 ,  34 , and  36  relative to the axes of the mandrel  10 , and the design of the circuit  42 , the circuit  42  is insensitive to bending, tensile loads, and compressive loads on the mandrel  10 . Also, the circuit  42  is insensitive to any changes in temperature since any changes to the sensors  30 ,  32 ,  34 , and  36  corresponding to axial load, bending, or temperature effects are cancelled.  
         [0032]    It is understood that the circuit board  16  can include hardware and software to provide excitation voltage to circuits  40  and  42 , convert the measured output voltages to digital form, and store the measurements at predetermined intervals into nonvolatile memory. In addition the circuit board  16  can include recording devices that record the compression, tension, and torsion applied to the mandrel  10  as a function of time; as well as a telemetry system to transmit the measured output voltages corresponding to the measured values of forces on the mandrel  10  to the surface in real time for further processing.  
         [0033]    It is understood that variations may be made in the foregoing without departing from the scope of the invention. For example, the particular type and relative orientation of the sensors can be varied within the scope of the invention. Also, one or more sensors can be utilized for the purpose of sensing only tension, only compression, or only torque on the mandrel  10 , or any combination thereof. Further, the mandrel  10  can be located in a different location in the workstring relative to the packer  12  than described above, and can be located relative to other tools in the workstring so that the forces on the latter tools can be measured. Moreover, the angle that the axes of the sensors extend to the longitudinal axis of the mandrel  10  can be varied, and the above equations would be varied accordingly.  
         [0034]    Although only one exemplary embodiment of this invention has been described in detail above, those skilled in the art will readily appreciate that many other modifications are possible without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.