Abstract:
An underreamer is provided in which the cutting elements are retracted into the tool body during run in, extensible for the performance of cutting operations, and retractable into the body of the tool afterwards.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to tools used in underreaming well bores, providing a underreaming tool with radially extendable and retractable cutting elements. 
       BACKGROUND OF THE INVENTION 
       [0002]    Underreaming of a well bore is a means of expanding a zone in a well bore to a greater diameter by cutting material away from the side of the well bore. Because the well bore is to be expanded in diameter, it is necessary to provide a tool that can run into the well bore in a “closed” position, then allow the cutting elements of the tool to extend radially outward until the desired bore diameter is reached. 
         [0003]    An example of such a tool is described in U.S. Pat. No. 4,282,942 to Longmore. That tool utilizes reverse circulation pressure to drive a cam to pivotally extend the cutting elements. Once the underreaming operation is complete, the reverse circulation pressure is removed and the cutting elements are allowed to collapse back to the tool body by gravity. 
         [0004]    Another type of underreamer is described in U.S. Pat. No. 5,853,054 to MaGarian, et al. That tool utilizes hydraulic pistons to pivotally extend and retract the cutting elements of the tool. However, it also anticipates that power to these pistons may be lost, and provides for biasing springs to retract the cutting elements if power is lost to the retracting piston. 
         [0005]    While useful, underreaming tools create a risk of having the tool stuck downhole if the cutting elements do not sufficiently or fully retract. Because underreamers are used to selectively increase the diameter of well bores, the nature of the operation will create a shoulder above the underreamed zone where the well bore narrows to its original diameter. Conversely, if the underreaming operation is performed below casing, the casing itself will present a narrow diameter through which the tool must fit to be retrieved from the well bore. Thus, an underreamer that fails to fully retract may become stuck, requiring time consuming efforts (and therefore expensive) efforts to fish it out of the well bore, or to grind through it. 
         [0006]    Other reliability problems can occur with current underreamers. For example, the cutting elements may fail to fully extend, preventing the underreaming operation from proceeding to the full desired diameter. Such a circumstance may require retrieving the underreamer from the well bore, repairing or replacing it, and running into the well bore again to complete the underreaming operation. Such instances are extremely undesirable due to the lost time and expense of running in and out of the well bore. 
         [0007]    Accordingly, it is an object of the invention to provide an underreamer with improved reliability. Further objects of the invention include further improving reliability by providing a positive indication at the surface when the cutting elements have been fully extended, and again when they have been fully retracted. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention is an underreamer comprising a cam-driven mechanism to extend and retract the cutting elements. When activated, the cam applies a radial force to the cutting elements essentially perpendicular to the longitudinal axis of the tool body. Thus, the cutting elements do not pivot relative to the tool body as they extend and retract, but move radially essentially perpendicularly to the longitudinal axis of the tool. 
         [0009]    The invention further provides a mechanism for selectively setting the cam in position to extend the cutting elements, and for selectively setting the cam in position to retract the cutting elements after underreaming operations are complete. This mechanism provides two different drop ball seats, the lower one of smaller diameter than the upper one. (As used herein, “upper” is intended to mean only “closer to the surface,” because the tool string may not be vertical.) 
         [0010]    When it is desired to set the cutting elements in the extended position, a first drop ball of a diameter small enough to pass through the upper drop ball seat is dropped through the tool string. The first drop ball travels though the tool body, past the upper drop ball seat, and seats in the lower drop ball seat, interrupting fluid flow through the tool body. Hydraulic pressure can then be used to shear a first set of shear screws. When the first set of shear screws shear, the hydraulic pressure shifts the cam and lower ball seat downward, moving the cutting elements into their extended position. Once the cutting elements are fully extended, a lower flow diverter opens to allow fluid flow around the lower ball seat. The resulting reduction in hydraulic pressure provides a positive indication at the surface that the cutting elements are fully extended. 
         [0011]    Similarly, when the underreaming operation is complete and it is desired to set the cutting elements in the retracted position, a second drop ball of a diameter sufficient to seat in the upper drop ball seat is dropped through the tool string. The second drop ball seats in the upper drop ball seat, again interrupting fluid flow through the tool body. Hydraulic pressure can then be used to shear a second set of shear screws. When the second set of shear screws shear, the hydraulic pressure shifts the cam and upper ball seat downward, moving the cutting elements into their retracted position. Once the cutting elements are fully retracted, an upper flow diverter opens to allow fluid flow around the upper ball seat. The resulting reduction in hydraulic pressure provides a positive indication at the surface that the cutting elements are fully retracted. 
         [0012]    In a preferred embodiment, upper and lower ratcheting followers are provided, so that as each shift of the cam is performed, the ratcheting followers prevent reverse shifting of the cam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a cross-sectional view of one embodiment of the underreamer of the present invention in the run-in position. 
           [0014]      FIG. 1A  is an expanded cross-sectional view of section A-A′ of  FIG. 1 . 
           [0015]      FIG. 1B  is an expanded cross-sectional view of section B-B′ of  FIG. 1 . 
           [0016]      FIG. 2  is a cross-sectional view of one embodiment of the underreamer of the present invention with the cutting elements in the extended position. 
           [0017]      FIG. 2A  is an expanded cross-sectional view of section C-C′ of  FIG. 2 . 
           [0018]      FIG. 2B  is an expanded cross-sectional view of section D-D′ of  FIG. 2 , reflecting the position of the lower ball seat before circulation resumes. 
           [0019]      FIG. 2C  is an expanded cross-sectional view of section D-D′ of  FIG. 2 , reflecting the position of the lower ball seat after circulation resumes. 
           [0020]      FIG. 3  is a cross-sectional view of one embodiment of the underreamer of the present invention with the cutting elements in the retracted position. 
           [0021]      FIG. 3A  is an expanded cross-sectional view of section E-E′ of  FIG. 3 , reflecting the position of the upper ball seat before circulation resumes. 
           [0022]      FIG. 3B  is an expanded cross-sectional view of section F-F′ of  FIG. 3 . 
           [0023]      FIG. 3C  is an expanded cross-sectional view of section E-E′ of  FIG. 3 , reflecting the position of the upper ball seat after circulation resumes. 
           [0024]      FIG. 3D  is an expanded cross-sectional view of section G-G′ of  FIG. 3C , provided to show greater detail. 
           [0025]      FIG. 4A  is a cross-sectional view of an embodiment of the cam section of an underreamer assembly during the process of extending the cutting elements in accordance with the present invention. 
           [0026]      FIG. 4B  is a cross-sectional view of an embodiment of the cam section of an underreamer assembly during the process of retracting the cutting elements in accordance with the present invention. 
           [0027]      FIG. 5  is a perspective view of an embodiment of an underreamer assembly in accordance with the present invention. 
           [0028]      FIG. 6  is a perspective view of an embodiment of a cutting element of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Referring to  FIGS. 1-3 , cross-sectional views of an embodiment of an underreamer of the present invention are shown, showing the tool positioned for run-in ( FIGS. 1 ,  1 A,  1 B), with the cutting elements extended ( FIGS. 2 ,  2 A,  2 B,  2 C), and with the cutting elements retracted for tool retrieval ( FIGS. 3 ,  3 A,  3 B,  3 C,  3 D). 
         [0030]    Underreamer  10  comprises a tool body  12 , cam  14 , cutting elements  16 , upper ball seat  18 , and lower ball seat  20 . Cutting elements  16  are aligned with openings  22  in tool body  12  (see also tool body  512  with openings  522  of  FIG. 5 ). Tapers  24  of cam  14  are preferably offset differently from their respective cutting elements  16 , so that when the cam  14  is shifted to displace cutting elements  16 , cutting elements  16  will not all shift at once, decreasing the amount of force necessary and reducing the strain on the tool. 
         [0031]    A more detailed view of this activation timing is shown in  FIGS. 4A and 4B . As shown in  FIG. 4A , when cam  14  is being shifted downward to extend cutting elements  16 A,  16 B,  16 C, and  16 D, the cam  14  engages these cutting elements with second leading edge  24 B′ of second taper  24 B, third leading edge  24 C′ of third taper  24 C, fourth leading edge  24 D′ of fourth taper  24 D, and fifth leading edge  24 E′ of fifth taper  24 E, respectively. As reflected in  FIG. 4A , cam  14  is preferably designed so that these leading edges first engage their respective cutting elements at different times. 
         [0032]    Similarly,  FIG. 4B  provides a detailed view of cam  14  as it is being shifted downward to retract cutting elements  16 A,  16 B,  16 C, and  16 D. In this case, cam  14  engages these cutting elements with first leading edge  24 A′ of first taper  24 A, second leading edge  24 B′ of second taper  24 B, third leading edge  24 C′ of third taper  24 C, and fourth leading edge  24 D′ of fourth taper  24 D, respectively. Again, cam  14  is preferably designed so that these leading edges first engage their respective cutting elements at different times. 
         [0033]    Referring again to  FIGS. 1-3 , those of skill in the art will recognize that the number and size of cutting elements  16  (and thus the number and size of tapers  24  and openings  22 ) is a matter of design choice, and may vary in accordance with the requirements of particular downhole environments. Additionally, one or more of cutting elements  16  may be replaced with a stabilizing element (not shown) that would open and close in the same manner as the cutting elements  16 , but would not provide any effective cutting action during underreaming. Rather such a stabilizing element would serve to stabilize the underreamer  10  while in operation. 
         [0034]    Once the underreamer  10  is positioned in a well bore (not shown) the cutting elements  16  must be opened to allow underreaming to begin. Referring more particularly to  FIGS. 2 ,  2 A,  2 B, and  2 C, a first drop ball  26  is circulated or allowed to fall to the underreamer  10 . First drop ball  26  is of sufficiently small diameter that it passes through upper ball seat  18 . (Upper ball seat  18  is of larger diameter than lower ball seat  20 ). First drop ball  26  continues through underreamer  10  until it seats on lower ball seat  20 , arresting fluid flow through underreamer  10 . 
         [0035]    Those of skill in the art will recognize that, while the use of ball seats and drop balls is embodied in the preferred embodiment of the invention, any seat capable of engaging a plugging device may be used in place of the ball seats described herein, and other plugging devices known to those of skill in the art may be used in place of drop balls. 
         [0036]    The increasing hydraulic pressure forces shears first lower shear screws  28 , allowing piston  19  to shift downward. This action causes first upper shear screws  62  to shear and releases lower ball seat  20 , lower seal sleeve  30 , cam  14 , and upper ball seat  18  to travel downward until lower seal sleeve  30  engages lower shoulder  32 . This displacement aligns the tapers  24  of cam  14  with cutting elements  16  so that cutting elements  16  are extended from the tool body  12 . This motion also moves upper retaining ring  66  into contact with upper shoulder  68 . 
         [0037]    During the course of the opening operation, upper ratchet  94  travels downward along upper ratchet platen  96 , providing a positive locking force prevent upward movement of piston  19 . 
         [0038]    Referring to  FIG. 2C , this downward displacement also moves lower locking balls  36  into a position where they are aligned with a lower ball release taper  35  in lower intermediate sleeve  34 . Lower locking balls  36  originally seated through lower sealing sleeve ball slots  31  into grooves  37  in lower ball seat  20 . With lower locking balls  36  allowed to move outward, lower ball seat  20  is forced downward until lower ball seat bypass ports  38  align with lower sealing sleeve bypass ports  40 . This alignment allows fluid to flow around first drop ball  26 , providing a positive pressure indication to the surface that cutting elements  16  are in the extended position. 
         [0039]    As part of the setting process, lower ratchet  42  (see  FIGS. 2B and 2C ) travels down-ward along lower ratchet platten  44 , providing a positive lock preventing lower sealing sleeve  30  and lower ball seat  20  from shifting upwards. 
         [0040]    When underreaming is complete, it is necessary to retract cutting elements  16  to allow retrieval of underreamer  10 . Referring to  FIGS. 3 ,  3 A,  3 B, and  3 C, second drop ball  46  is circulated or allowed to fall into underreamer  10  until it seats on upper ball seat  18 . Once second drop ball  46  is seated, fluid flow through the underreamer  10  is stopped. 
         [0041]    Fluid pressure shears second upper shear screws  70 , upper ball seat  18  shift downward, allowing first upper locking balls  48  drop into upper ball seat grooves  52 , locking upper ball seat  18  and upper sealing sleeve  50  together to close first gap  78 . Continued downward movement of upper ball seat  18  forces first upper locking balls  48  outward by the curvature of upper ball seat grooves  52 , releasing the lock between upper ball seat  18  and upper sealing sleeve  50 . 
         [0042]    Cam  14  can now move downward, shearing lower shear screws  54 . The shearing of second lower shear screws  54  allows lower inner sleeve  56  (and therefore cam  14  and upper ball seat  18 ) to move relative to lower sealing sleeve  30  and lower ball seat  20 . The down-ward shift of cam  14  draws cutting elements  16  inward to a fully retracted position. 
         [0043]    Third upper shear screws  76  also shear in this action, releasing seal bushing  64  into annulus  92  to prevent fluid flow through annulus  92  when circulation is reestablished. Seal bushing  92  is allowed to “float” to equalize pressures above and below it. 
         [0044]    Downward travel continues until lower retaining ring  67  engages inner shoulder  74 . Hydraulic pressure shears fourth upper shear screws  77 , allowing a continued downward shift of upper ball seat  18  and spacer  51 . As reflected in  FIGS. 3C and 3D , second upper locking balls  49  are now able to fall outward into lower retaining ring ball slots  69  due to the relative motion between spacer  51  and lower retaining ring  67 . 
         [0045]    Cam  14  is pushed downward until further movement is stopped by contact with intermediate shoulder  58 . 
         [0046]    Upper ball seat  18  and spacer  51  shift downward, closing second gap  80 , until the lower end of spacer  51  shoulders on torn end  81  of bottom push tube  83 . (See  FIG. 3C ). In this position, upper ball seat bypass ports  82  are aligned with upper sealing sleeve bypass ports  84 . This alignment allows fluid to flow around second drop ball  46 , providing a positive pressure indication to the surface that cutting elements  16  are in the retracted position. 
         [0047]    Referring to  FIG. 5 , additional features of a preferred embodiment of the invention are shown. Underreamer  510  comprises tool body  512 . Tool body  512  comprises a housing cover  524  that can be bolted in place through bolt holes  526 , allowing easy field disassembly of the underreamer and allowing expendable parts, such as shear screws and cutting elements, to be replaced in the field so that the underreamer may be ready for immediate re-use. Those of skill in the art will recognize that housing cover  524  is not limited in shape or form to that depicted, but may vary in accordance with engineering choice. Housing cover  524  also preferably provides openings  522  allowing extension of cutting elements through the housing cap  524 . Keys  528  fit into milled slots  530 ,  532  in tool body  512  and housing cover  524 , respectively. These keys improve the torsional strength of the tool. Alternatively, either the tool body  512  or the housing cover  524  may be provided with key extensions, and slots milled only into the other part to receive the key extensions. As those of skill in the art will recognize, such key extensions may take a variety of shapes and sizes. 
         [0048]    Referring to  FIG. 6 , a typical cutting element is shown. The cutting element  610  has a body  612  with an upper surface  614  and a lower surface  616  for engagement with the tapers of the cam. Cutters  618  provide the cutting action when cutting element  610  is extended from the underreamer. 
         [0049]    The above examples are included for demonstration purposes only and not as limitations on the scope of the invention. Other variations in the construction of the invention may be made without departing from the spirit of the invention, and those of skill in the art will recognize that these descriptions are provide by way of example only.