A shifting tool and method of shifting a downhole device that requires only a minimal profile or no profile to engage and move the movable portion of the tool. The invention comprises a ported housing assembly and at least one friction pad alignable with said at least one port and radially movable through the port between a first pad position and a second pad position. In the second pad position, the friction pad extends outside said outer diameter of said housing assembly to engage the targeted downhole device. A mandrel positioned through the ported housing has a first section with a first outer diameter and a second section with a second outer diameter, said second outer diameter being greater than said first outer diameter. The mandrel is movable between a first mandrel position and a second mandrel position. In the second mandrel position, the second outer diameter supports the friction pads in the second pad position.

CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application Ser. No. 61/314,770 filed Mar. 17, 2010 and entitled Differential Shifting Tool and Method of Shifting, which is incorporated by reference herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to oil and/or gas production. More specifically, the invention is a differential shifting tool and method for selectively actuating a downhole device. 
     2. Description of the Related Art 
     In hydrocarbon wells, fracturing (or “fracing”) is a technique used by well operators to create or extend fractures from the wellbore deeper into the surrounding formation, thus increasing the surface area for formation fluids to flow into the well. Fracing is typically accomplished by either injecting fluids into the formation at high pressure (hydraulic fracturing) or injecting fluids laced with round granular material (proppant fracturing) into the formation. This requires selective actuation of downhole devices, such as fracing valves, to control fluid flow from the tubing string to the formation. 
     For example, U.S. Published Application No. 2008/0302538 (the '538 Publication), entitled Cemented Open Hole Selective Fracing System and which is incorporated by reference herein, describes one system for selectively actuating a fracing sleeve that incorporates a shifting tool. The tool is run into the tubing string and engages with a profile within the interior of the valve. An inner sleeve may then be moved to an open position to allow fracing or to a closed position to prevent fluid flow to or from the formation. 
     After the fracing process is complete and prior to the initiation of production operations, the ball and seat are typically milled out from each of the tools to allow a large flowpath through the producing string. After the milling process is complete, and as described in the '538 Publication, the shifting tool is disposed through the string and is caused to engage a profile within the downhole device, thus allowing the well operator to engage the moveable portion of the tool and close off the flow ports from the surrounding formation. 
     A common problem with conventional downhole devices during fracing and the milling process is the profile becomes damaged and/or destroyed. For example, it is not uncommon that the fracing process itself, which by its nature incorporates abrasive materials moving at high flow rates, erodes the engageable profile of the tool. To avoid this problem, well operators often limit the fracing flow rate to control erosion of the profile, which decreases the effectiveness of the fracing process and results in less than optimal results. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a shifting tool and method of shifting a downhole device that requires only a minimal profile or no profile to engage and move the movable portion of the tool. The invention comprises a ported housing assembly and at least one friction pad alignable with said at least one port and radially movable through the port between a first pad position and a second pad position. In the second pad position, the friction pad extends outside said outer diameter of said housing assembly to engage the targeted downhole device. A mandrel positioned through the ported housing has a first section with a first outer diameter and a second section with a second outer diameter, said second outer diameter being greater than said first outer diameter. The mandrel is movable between a first mandrel position and a second mandrel position. In the second mandrel position, the second outer diameter supports the friction pads in the second pad position. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a side elevation view of the preferred embodiment. 
         FIG. 2A  through  FIG. 2F  are various sectional views of the preferred embodiment of the present invention. 
         FIG. 3A  and  FIG. 3B  are a side sectional and front sectional elevation of the collet described with reference to  FIG. 2C . 
         FIG. 4A  through  FIG. 4D  are various views of a friction pad of the preferred embodiment of the invention. 
         FIG. 5A  through  FIG. 5C  describe operation of the preferred embodiment as it engages a profile of a downhole device. 
         FIG. 6A  and  FIG. 6B  show the collet friction pads, respectively, of the preferred embodiment when the shifting tool has engaged a downhole device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     When used with reference to the figures, unless otherwise specified, the terms “upwell,” “above,” “top,” “upper,” “downwell,” “below,” “bottom,” “lower,” and like terms are used relative to the direction of normal production through the tool and wellbore. Thus, normal production of hydrocarbons results in migration through the wellbore and production string from the downwell to upwell direction without regard to whether the tubing string is disposed in a vertical wellbore, a horizontal wellbore, or some combination of both. Similarly, during the fracing process, fracing fluids move from the surface in the downwell direction to the portion of the tubing string within the formation. 
       FIG. 1  shows a side elevation of a preferred embodiment  20  of the present invention. A top connection  22  is connected to a housing assembly  24 , which is connected to a bottom connection  26 . The housing assembly  24  comprises a release housing  28  fastened to the top connection  22  with a series of radially-aligned screws  30 . The upper end of a collet housing  32  having a series of collet ports  33  therethrough is threaded to and fixed to the bottom end of the release housing  28  with a series of radially-aligned screws  34 . A spacer tube  36  is connected to the lower end of the collet housing  32 . The top end of a pad housing  38  is threaded to and fixed to the bottom end of the spacer tube  36  with a series of radially-aligned screws  40 . The top end of a spring housing  42  is threaded to and fixed to the bottom end of the pad housing  38  with a series of radially-aligned screws  43 . The bottom connection  26  is threaded to and fixed to the bottom end of the spring housing  42 . 
       FIG. 2A  through  FIG. 2F  are sequential sectional elevations of the preferred embodiment  20  through section line  2 - 2  of  FIG. 1  showing the shifting tool in a disengaged, or “run in,” state. Referring to  FIG. 2A , a jet insert  46  is located within the top connection  22  and release housing  28 , and is threaded to the upper end of a jet receiver  48 . The jet insert  46  includes a tapering portion  52  that restricts the size of the flowpath through which fluids can move. The lower end of the jet receiver  48  is threaded to the upper end of an upper mandrel  50 . An annular backup ring  54  is circumferentially disposed around a groove formed in the outer surface of the jet receiver  48  to provide, along with a sealing element  56 , pressure isolation from the annular pressure of the wellbore, thus allowing for a differential pressure condition between the interior and exterior of the upper mandrel  50 . 
     Referring to  FIG. 2B , the release housing  28  is connected to a release nut  58  using screws  60 . The top connection  22  is threaded to the upper end of the release nut  58 . The upper mandrel  50  extends through, and is movable longitudinally within, the release nut  58  and into the collet housing  32 . A collet spring  62  is positioned in the annular space between the upper mandrel  50  and the collet housing  32 , and contacts the lower annular surface  64  of the release housing  28 . 
     Referring again to  FIG. 2B , a snap ring  66  is positioned around the upper mandrel  50  between first and second enlarged portions  68 ,  70 , and within a snap ring groove  67  formed in the inner surface of the release housing  28 . The snap ring  66  engages against the profile of the groove  67  to prevent longitudinal movement of the snap ring  66  and upper mandrel  50  until the pressure differential is sufficient to force the snap ring  66  out of the groove  67 . This allows circulation to be established through the shifting tool up to a certain pressure differential without extending the collet  82  (see  FIG. 2 ) so that the tool and the differential pressure can be freely moved up and down the tubing string. As flow rate is increased through the tool causing the differential pressure to increase past a first threshold, the snap ring  66  will be forced out of the groove  67  and allow the upper mandrel  50  to extend the friction pads  94  (see  FIG. 2D ) and engage the downhole device (e.g., the inner sleeve of a fracing valve). Thereafter, as long as the flow rate is maintained the valve can be opened or closed. When the flow rate is reduced, the differential pressure is reduced and the return spring  104  (see  FIG. 2F ) will cause the upper mandrel  50  and snap ring  66  to return to the run-in position, allowing the well operator to move to the next downhole tool or remove the shifting tool  20  from the tubing string. The upper end of the snap ring  66  is angled to minimize resistance when the snap ring  66  is moving upwell and returning to the run-in position shown in  FIG. 2B . 
     Referring to  FIG. 2C , the upper mandrel  50  has a collet engaging section  71  that includes first and second enlarged sections  72 ,  74 . The upper enlarged section  72  has an upper shoulder  76  angled at seventy-five degrees from the longitudinal axis  18  and a lower shoulder  78  angled at fifteen degrees from the longitudinal axis  18 . The lower enlarged portion  74  has upper and lower annular shoulders  79 ,  80  that are inclined at fifteen degrees from the longitudinal axis  18 . 
     A collet  82  is slidably positioned around the upper mandrel  50  proximal to the upper and lower enlarged sections  72 ,  74 . The lower end of the collet spring  62  is in contact with an upper ring  84  of the collet  82 . A lower ring  85  of the collet  32  is in contact with the spacer tube  36 . 
     The upper mandrel  50  has ports  83  positioned between the upper and lower enlarged portions  72 ,  74  that provide access to the interior of the upper mandrel  50 . The ports  83  allow the tool operator to establish circulation while running in the hole to wash out any debris that could prevent the shifting tool from getting downhole. The ports  83  allow this circulation and provide an exit path for fluid when the flow rate has created enough differential pressure to act against the spring  104  and extend the friction pads  94 , as will be described with reference to  FIG. 2D . 
       FIG. 3A  and  FIG. 3B  show the collet  82  in greater detail. The collet  82  includes six equally radially spaced fingers  86  extending between the upper ring  84  and lower ring  85  that are radially flexible toward and away from the longitudinal axis  18  of the tool. A key  87  is formed in each finger  86  approximately equidistantly from the upper and lower rings  84 ,  85 . Each key  87  includes a cylindrical outer portion  89  protruding radially outwardly of its corresponding finger  86  and an inner portion  91  having upper and lower shoulders  93 ,  95  that are angled at fifteen degrees and seventy-five degrees, respectively, from the longitudinal axis  18 . A concave support surface  97  connects the upper and lower shoulders  93 ,  95  of each key  87 . 
     Referring again to  FIG. 2C , the collet fingers  86  are radially expanded as the support surfaces  97  contact the lower enlarged section  74 , causing the outer portion  89  of the keys  87  to protrude through the collet ports  33  (see  FIG. 1 ) in the collet housing  32 . 
     Referring to  FIG. 2D , a spring ring  81  is fixed to the pad housing  38  adjacent the lower surface of the spacer tube  36 . The upper mandrel  50  is threaded to a lower mandrel  88  to form a piston section  90  with an enlarged diameter. A lower annular shoulder  92  of the piston section  90  is angled at fifteen degrees from the longitudinal axis  18 . Ports  83  are disposed through the lower mandrel  88  to allow the well operator to cause circulation between the lower mandrel  88  and the housing assembly  24 , as described with reference to  FIG. 2C . The friction pads  94  are spaced equally around the lower mandrel  88  downwell of the piston portion  90  and aligned with pad ports  99  disposed through the pad housing  38 . The lower end of the pad housing  38  is connected to the upper end of the spring housing  42 . The friction pads  94  has an upper inclined surface angled less than ten degrees relative to the lower shoulder  92 . 
     Referring to  FIG. 2E  and  FIG. 2F , an annular spring stop  96  is fastened to the lower mandrel  88  at flattened areas  100  thereof with three equally radially-spaced screws  100 . The lower mandrel  88  extends through a lower ring  102  integrally formed in the pad housing  38 . A return spring  104  is positioned around the lower mandrel  88  and abuts the spring stop  96 . The lower end of the spring housing  42  is threaded and fixed to the bottom connection  26 , which has three flow ports  107  therethrough. The lower end of the return spring  104  is in contact with the bottom connection  26 . 
       FIGS. 4A through 4C  depict a friction pad  94  of the preferred embodiment in greater detail. The friction pad  94  includes a plurality of gripping members  106  formed in an outer surface  108 . An inner surface  110  of the friction pad  94  corresponds in curvature to the piston portion  90  of the lower mandrel  88  (see  FIGS. 2C & 2D ). The inner surface  110  includes upper and lower inclined surfaces  112 ,  113  angled at twenty degrees from the longitudinal axis  18 . 
       FIG. 4D  is a side elevation of a portion of the preferred embodiment that more fully shows retention of the friction pads  94  within the pad housing  38 . In the “run-in” state, the friction pads  94  are held within pad housing  38  with slip springs  114  fastened to recessed portions  116  of the pad housing  38  with screws  118 . The slip springs  114  have a tapering end  120  that allows the slip springs  114  to bend outwardly as the corresponding friction pad  94  moves radially outwardly. 
       FIG. 5A  shows engagement of the shifting tool with a profile  124  left by the drilling out of a ball seat in an inner sleeve  126  (or another element of a downhole device). As the inner diameter of the inner sleeve  126  narrows, the outer portions  89  of the keys  87  will engage the inner sleeve  126 . 
     As shown in  FIG. 5B , as the shifting tool is run further downwell, the collet  82  resists downwell movement and remains stationary relative to the inner sleeve  126 , which causes compression of the collet spring  62  to urge the collet  82  downwell. The inner portions  91  of the collet  82  move upwell of the second enlarged portion  74 , which allows the upper portion  89  to recede into the collet housing  32 . 
     Thereafter, as shown in  FIG. 5C , the collet spring  62  urges the collet  82  downwell and back into the first position where keys  87  protrude past the outer diameter of the collet housing  32  and the lower ring  85  of the collet  82  is in contact with the spacer tube  36 . The collet  82  resists any upwell movement as the lower ring  85  of the collet  82  cannot move further downwell. In this manner, the collet  82  will “snap through” the profile  124  left after milling out, but will “land” downwell of the profile  124  as the tool is thereafter pulled upwell by the well operator. 
       FIGS. 6A and 6B  depict the shifting tool in the engaged state after a differential pressure condition has caused the upper and lower mandrels  50 ,  88  to move downwell to the second position. As shown in  FIG. 6A , at a first differential pressure, the upper mandrel  50  is in a second position downwell from the first position shown in  FIG. 2A  through  FIG. 2F . In the second position, the first and second enlarged sections  72 ,  74  of the upper mandrel  50  are downwell of the keys  87 . The upper shoulder  76  of the first enlarged section  72  is engaged with the lower shoulder  95  of the inner portions  91 . Outer portions  89  of the key  87  are within the outer diameter of the collet housing  32 . 
     As shown in  FIG. 6B , the piston section  90  has moved downwell to the second position within the pad housing  38 . In the second position, the friction pads  94  are supported by at least part of the piston section  90  of the lower mandrel  88 . When moving to this position, the upper inclined surface  112  of each friction pad  94  is engaged by the lower annular shoulder  92  of the piston section  90  to facilitate radial outward movement of the friction pads  94 . In this position, the slip springs  114  urge the frictions pads  94  radially inwardly such that, when the piston portion  90  no longer supports the friction pads  94  (i.e., when the differential pressure condition is overcome by the expansive force of the return spring  104 ), the friction pads  94  are moved radially inwardly by the slip springs  114 . 
     Thereafter, the sleeve of the downhole device can be shifted open/closed by application of tension or compression through the work string as long as flow is maintained in the shifting tool to support the friction pads  94  in the expanded position. Upon completion of the shifting of the inner sleeve into the open/closed position, fluid flow to the shifting tool is reduced, resulting in a decrease of differential pressure until the return spring  104  urges the spring stop  96  and connected lower mandrel  88  back to the first position shown in  FIG. 2D . As the friction pads  94  would no longer be supported by piston section  90 , the shifting tool is disengaged from the downhole device. 
     Because of engagement of the inner portion  91  of the keys  87  with the upper enlarged portion  72  of the upper mandrel  50 , the upper portions  89  of the keys  87  remain within the outer diameter of the collet housing  32 , and thus cannot engage the inner surface of the downhole device. This ensures that the shifting tool can be removed from the downhole device with engaging any profile  124  (see  FIG. 5A-5C ). After removal of the shifting tool, the well operator may reset the tool to the run-in state by inserting screws into the threaded holes (see  FIGS. 3A & 3B ) and expanding the collet  82  to allow repositioning relative to the upper mandrel  50 , as described with reference to  FIGS. 2A through 2F . 
     The present invention is described above in terms of a preferred illustrative embodiment of a specifically-described shifting tool and method. Those skilled in the art will recognize that alternative constructions of such an apparatus can be used in carrying out the present invention. Other aspects, features, and advantages of the present invention may be obtained from a study of this disclosure and the drawings, along with the appended claims.