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TECHNICAL FIELD 
       [0001]    The present disclosure relates to systems, apparatus, and methods relating to tool string braking in a downhole drilling environment. 
       BACKGROUND 
       [0002]    Where downhole tools are used to accomplish stationary tasks (e.g., well-logging or well-completion tasks) via suspension lines (e.g., wirelines or slicklines) in a wellbore, the depth of the suspended tool string is of considerable importance. For example, in well-logging processes, it is often necessary to take corresponding measurements over multiple runs at the same depth position within the wellbore. Additionally, logs from different wellbores may be depth-matched for comparison. Thus, errors in depth measurement of the tool string are detrimental to data interpretation. Moreover, performing completion processes at the wrong depth can result in excessive fluid production in the wellbore and/or entirely bypassing a particular zone of interest in the wellbore. 
         [0003]    To locate the tool string in a substantially vertical wellbore, one conventional process is to initially drop the tool string below the intended depth and subsequently pull the tool string up to the target depth by a winch, so that the cable is held in tension. Yet, when the winch is stopped at the target depth, the tool string continues to move on the suspension line upward out of the wellbore. This phenomenon is known as “creep.” Failure to account for creep causes downhole tool operations to be conducted at an incorrect depth. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0004]      FIG. 1  is a schematic diagram of a tool conveyance system for use in a downhole environment of a wellbore. 
           [0005]      FIG. 2  is a side view of a tool string for a downhole tool conveyance system. 
           [0006]      FIG. 3A  is a cross-sectional side view of the braking apparatus of  FIG. 2  with the spring casing in a lowered position. 
           [0007]      FIG. 3B  is an enlarged view of a side slot of the braking apparatus of  FIG. 2 . 
           [0008]      FIG. 3C  is a cross-sectional side view of the braking apparatus of  FIG. 2  with the spring casing in a raised position. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]      FIG. 1  is schematic diagram of an exemplary tool conveyance system  10  for use in a downhole environment of a wellbore  12 . The tool conveyance system  10  includes a tool string  14 , a suspension line  15 , and a hoisting mechanism  16 . As shown, the tool string  14  is supported in the wellbore  12  by the suspension line  15 . In some examples, the suspension line  15  is an electrically conductive wireline that physically supports the tool string  14  and conveys electricity to the tool string. In other examples, however, the suspension line  15  is non-electrically conductive slickline that only provides physical support to the tool string  14 . The hoisting mechanism  16  provides motive force for moving the suspension line  15 , and thus the tool string  14 , through the wellbore  12 . In this example, the hoisting mechanism  16  is anchored to a ground surface  17  at the head of the wellbore  12 . However, other implementations may employ the hoisting mechanism  16  on a drilling rig, offshore platform, heavy-duty vehicle, etc. The hoisting mechanism  16  may include a motorized winch, crank, pulley or any other device suitable for anchoring and/or providing motive force to the suspension line  15 . 
         [0010]    The tool string  14  includes a cable head  18 , a downhole tool  20 , and a braking apparatus  100 . The cable head  18  securely couples the tool string  14  to the suspension line  15 . If the suspension line  15  is an electrical wireline, the cable head provides an electrical connection between the wireline and the downhole tool  20 . The downhole tool  20  may include one or more various types of downhole tools. The downhole tool(s) can be designed to accomplish well-logging tasks, such as measuring rock and fluid properties in a new wellbore and/or measuring pressures or flow rates in the wellbore, The downhole tool(s) can also be designed to accomplish well-completion tasks, such as perforating the wellbore casing to allow the inflow of gas and liquids. Downhole tools suitable for various other well-logging and/or well-completion operations can also be used. In some examples, the downhole tool  20  can include at least one well-logging tool and at least one well-completion tool. 
         [0011]    In the foregoing description of the tool conveyance system  10 , various items of conventional equipment may have been omitted to simplify the description. However, those skilled in the art will realize that such conventional equipment can be employed as desired. Those skilled in the art will further appreciate that various components described are recited as illustrative for contextual purposes and do not limit the scope of this disclosure. Further, while the tool conveyance system  10  is shown in an arrangement that facilitates deployment in a substantially vertical or straight wellbore, it will be appreciated that arrangements are also contemplated in a horizontal or highly deviated wellbore environment where the tool string may experience involuntary movement and therefore are within the scope of the present disclosure. The tool conveyance system  10  and other arrangements may also be used in wellbores drilled at an angle greater than  90  degrees to inhibit tool string movement due to gravitational forces. 
         [0012]      FIG. 2  is a side view of a tool string  14  that can, for example, be incorporated in the tool conveyance system  10  depicted in  FIG. 1 . In this example, the downhole tool  20  includes a casing collar locator  20   a  and a perforating gun  20   b . The casing collar locator  20   a  is an electrical well-logging tool used for depth correlation. The perforating gun  20   b  is a well-completion tool designed to create perforations (e.g., punched holes) in the casing of the wellbore, allowing oil and/or gas to flow through the casing into the wellbore. 
         [0013]    While the casing collar locator  20   a  and the perforating gun  20   b  are common downhole tools, their illustration in this example is not intended to be limiting. As discussed above, any suitable downhole tools are embraced by the present disclosure. Further, while in this example, the braking apparatus  100  is located between the casing collar locator  20   a  and the perforating gun  20   b , other arrangements are also contemplated. For example, the braking apparatus  100  can be located at the leading or trailing end of the tool string  14  without departing from the scope of this disclosure. 
         [0014]    Referring next to  FIGS. 3A-3C , the braking apparatus  100  includes a housing  102 , an actuating mechanism  104 , and a pair of braking arms  106 . As shown, components of the braking apparatus  100  are arranged about a central longitudinal axis  101 . The housing  102  is a hollow tubular body having an external cylindrical side wall outlining an internal cavity. The actuating mechanism  104  includes a wedge member  108  located at the floor  109  of the housing  102 . As shown, the wedge member  108  includes a cylindrical pedestal  110  projecting to a frustoconical tip  112  defined by a sloping outer conical surface  114 . 
         [0015]    The actuating mechanism  104  further includes a push-pull device  116  coupled to the housing  102 . The push-pull device  116  includes a biasing member casing  118  to house a biasing member (further discussed below) and a linkage member  120  attached to the upper end  119  of the biasing member casing. The linkage member  120  is connectable directly to the suspension line  120  or indirectly via other tool string elements to the suspension line. Similar to the housing  102 , the biasing member casing  118  is a hollow tubular body having a cylindrical side wall outlining an internal cavity. A guide rod  122  extends through the internal cavity of the biasing member casing  118  and through the floor  121  of the biasing member casing to reach the frustoconical tip  112  of the wedge member  108 . The distal end of the guide rod is attached to the tip  112  of the wedge member  108 . A biasing member  124  is disposed coaxially about the guide rod  122 . The biasing member  124  urges the biasing member casing  118  downward towards the wedge member  108 . The biasing member  124  is biasing to provide a downward biasing force at least as great as the weight of the tool string. In this example, the biasing member is an axial coil spring, in which the context of the casing  118  may alternatively be referred to as a spring casing  118 ; However, other types of biasing members (and corresponding casing for the biasing member) may also be employed as an alternative or supplementing biasing member (e.g., a disk spring, a resilient sleeve, and/or a compressible gas or fluid). 
         [0016]    The linkage member  120  is coupled, directly or indirectly, to the suspension line  15 . In either case, the coupling between the linkage member  120  and the suspension line  15  is such that at least a portion of the pulling force imparted on the suspension line by the hoisting mechanism  16  is conveyed to the linkage member  120 . So, when the hoisting mechanism  16  exerts a pulling force on the suspension line  15 , the spring casing  118  is pulled (e.g., with substantially equal pulling force) via its attachment to the linkage member  120 . When the pulling force on the linkage member  120  exceeds the biasing force of the biasing member  124 , the biasing member collapses, allowing the spring casing  118  to be moved upward in the housing  102 , away from the wedge member  108 . When the pulling force is reduced, or ceases, the biasing member  124  urges the spring casing back downward towards the wedge member  108 . 
         [0017]    The braking arms  106  are pivotally coupled to the floor  121  of the spring casing  118  and extend downward towards the wedge member  108 . As shown in  FIG. 3A , when the spring casing  118  is in the lowered position (e.g., when the pulling force exerted on the linkage member  120  is less than the biasing force of the biasing member  124 ), the braking arms  106  bear against the sloping conical surface  114  of the wedge member  108 , forcing the braking arms  106  to pivot radially outward. In this position, the braking arms  106  protrude through arm-bay openings  126  formed radially along a lower portion of the housing  102  (see  FIG. 3B ). With the braking arms  106  deployed through the arm-bay openings  126 , brake pads  128  formed on the distal ends of the braking arms  106  are designed to engage a casing wall of the wellbore  12 . Friction between the casing of the wellbore  12  and the brake pads  128  produce a braking force to hold the tool string  14  in place. Thus, the hoisting mechanism  16  is stopped when it is determined that the tool string  14  is at the target depth within the wellbore  12 , thereby eliminating the pulling force, the braking force from the deployed braking arms  106  counteracts the creep phenomenon. 
         [0018]      FIG. 3C  shows the spring casing  118  in a raised position (e.g., when the pulling force on the linkage member  120  is greater than the biasing force of the biasing member  124 ). In the raised position, the braking arms  106  pivot radially inward toward the central longitudinal axis  101  of the braking apparatus  100 . The inward pivoting motion of the braking arms  106  pulls the brake pads  128  away from the wellbore casing, lessening the friction braking force and allowing the pulling force of the hoisting mechanism  16  to move the tool string  14  upward through the wellbore  12 . 
         [0019]    In some embodiments, to reduce frictional drag as the tool string  14  is being lowered through the wellbore  12 , an electrical or mechanical device can be employed to hold the braking arms  106  in a retracted state until the lowest tool depth is reached. For example, a band can be used to hold the arms closed until a small charge is set off that would break a link in the band. The braking arms would then expand to the point allowed by the mechanism. As yet another example, a small motor could be used to hold the braking arms in place while the tool string is being lowered through the wellbore. 
         [0020]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various additions and modifications may be made without departing from the spirit and scope of the inventions.

Summary:
A braking apparatus for a tool string positionable in a wellbore and a method of braking a tool string in a wellbore is disclosed. The braking apparatus includes: a tubular housing having at least one radial arm-bay opening; an actuating mechanism including: a wedge member mounted in an internal cavity of the housing; an axial guide rod coupled at one end to the wedge member; and a push-pull device. The push pull device includes: a biasing member casing through which the guide rod extends to contact the wedge member, a biasing member; and at least one braking arm pivotably mounted to a lower portion of the biasing member casing, wherein when the biasing member casing of the push-pull device in in a lowered position, the braking arm bears on a sloped surface of the wedge member to project the braking arm into contact with a wellbore wall.