A mill assembly having a milling head which is releasable from the mill body, such as by shifting a ball clutch mechanism. The ball clutch mechanism can be shifted by dropping a pumpable plug through the work string to block fluid flow through the releasable milling head, or by increasing fluid flow through a constriction in the releasable milling head to increase the back pressure above the milling head. A check valve in the milling head can prevent uphole flow through the work string in the event of a pressure excursion. A fishing neck can be attached to the milling head.

CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of co-pending U.S. patent application Ser. No. 11/804,800, filed May 21, 2007, for “Releasable Mill”, which is a continuation of U.S. patent application Ser. No. 10/916,773, filed Aug. 11, 2004, for “Releasable Mill”, which claims the benefit of U.S. Provisional Pat. App. No. 60/495,021, filed Aug. 13, 2003, for “Releasable Bridge Plug Mill”. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0003]     1. Field of the Invention  
         [0004]     This invention is in the field of apparatus used to mill out downhole equipment in a well, such as in an oil or gas well.  
         [0005]     2. Background Art  
         [0006]     Some oil or gas wells are drilled into locations at which multiple oil or gas formations are found, at different depths. That is, one hydrocarbon formation may be above or below another, and there may be more than two such formations at different depths. It is common to produce hydrocarbons from only one selected formation at a time. One means used to assist in this type of production is a plug, which can be installed in the bore hole or casing, between two of the formations. Such a plug isolates one formation from another, while allowing access to the upper formation via the bore hole. It is also common to remove such a plug, in order to allow access to the lower formation, via the bore hole, for the purpose of producing hydrocarbons, or for other purposes.  
         [0007]     When such a plug is removed, it is often removed by lowering a mill into the bore hole or casing, attached to a work string. The mill is usually provided with some type of cutting structure on its lower face, and this cutting structure is often dressed with some type of cutting material, such as inserts or abrasives. The mill is lowered into contact with the upper end of the plug; then, the work string is rotated, thereby rotating the mill. Alternatively, a downhole motor can be used on the work string, as is commonly known in the art, and the mill can be rotated by operating the downhole motor. In either case, as the mill is rotated, the cutting structure cuts the plug into small cuttings, which are returned to the surface entrained in the drilling fluid which is pumped downhole through the work string. This operation is continued until the entire plug is removed, or until a sufficient portion of the plug is removed to allow the remaining portion to fall farther into the borehole.  
         [0008]     After this type of operation, it is necessary to remove the mill from the bore hole before access to the lower formation is available. This is because, although the mill may have passageways for drilling fluid, these fluid passageways are not sufficiently large to provide the desired degree of access to the lower formation. The mill body itself is typically a substantially solid, comparatively hard, metal body. Therefore, in order to complete the operation, the work string and the mill must be pulled from the bore hole to provide the desired access to the lower formation. As is well known, tripping a work string into or out of a well is a time consuming, expensive process. It is desirable to have a method and apparatus for removing such plugs, or other types of objects in a well bore, while eliminating the necessity for tripping the work string out of the bore hole, to remove the mill and provide access to the lower formation.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The present invention provides a mill assembly having a releasable milling head attached to a mill body with a ball clutch mechanism. The mill assembly can be lowered into a bore hole to mill out a plug, after which the milling head can be completely released from the work string, such as by shifting the ball clutch mechanism, and allowed to drop into the bore hole. Separation of the milling head from the mill body leaves a substantially open bore into and through the work string. The mill body and the work string can be left in the bore hole while production from the lower formation takes place, through this open bore. The milling head is provided with a check valve in the fluid path, to allow the downhole flow of drilling fluid during milling, but to prevent the uphole flow of fluids during a kick or pressure excursion. A fishing neck can also be provided on the milling head, to assist in the later removal of the milling head where desired.  
         [0010]     The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0011]      FIG. 1  is a longitudinal section view of a first embodiment of the apparatus;  
         [0012]      FIG. 2  is a lower end view of the milling head portion of the apparatus shown in  FIG. 1 , and showing the location of the line along which the section in  FIG. 1  is taken;  
         [0013]      FIG. 3  is an upper end view of the milling head portion of the apparatus shown in  FIG. 1 ;  
         [0014]      FIG. 4  is a lower end view of the mill body portion of the apparatus shown in  FIG. 1 ;  
         [0015]      FIG. 5  is a longitudinal section view of the apparatus shown in  FIG. 1 , after complete separation of the milling head from the mill body;  
         [0016]      FIG. 6  is a longitudinal section view of a second embodiment of the milling head of the present invention, with a ball check valve;  
         [0017]      FIG. 7  is an expanded longitudinal section view of a third embodiment of the apparatus of the present invention, with a flapper check valve and a fishing neck;  
         [0018]      FIG. 8  is an assembled longitudinal section view of the apparatus shown in  FIG. 7 ;  
         [0019]      FIG. 9  is a longitudinal section view of a ball clutch and fishing neck for use in a fourth embodiment of the apparatus of the present invention;  
         [0020]      FIG. 10  is a longitudinal section view of a collet for use in the fourth embodiment of the apparatus of the present invention, along with the ball clutch and fishing neck shown in  FIG. 9 ; and  
         [0021]      FIG. 11  is an assembled longitudinal section view of the fourth embodiment of the apparatus of the present invention, incorporating the ball clutch and fishing neck, and the collet, shown in  FIGS. 9 and 10 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]     As shown in  FIG. 1 , the mill assembly apparatus  10  of the present invention principally includes a mill body  12 , to which a milling head  14  is releasably attached, such as by one or more shear screws or pins  16 . The mill body  12  is adapted to be mounted on a work string (not shown) as is commonly known in the art, such as by threading thereto. A plurality of cutting inserts  18  can be provided on the lower face  34  of the milling head  14  to form a cutting structure. Alternatively, the cutting structure can include milled teeth, crushed carbide, or abrasives, without departing from the spirit of the present invention.  
         [0023]     One or more torque lugs  20 , better shown in  FIG. 3 , can be provided on an upwardly facing annular shoulder  46  of the milling head  14 . These torque lugs  20  can extend into one or more torque notches  28 , better shown in  FIG. 4 , formed on the lower end  48  of the mill body  12 . An axially oriented inner face or shoulder  42  in each torque notch  28  abuts an axially oriented outer face or shoulder  40  on each torque lug  20 . Rather than torque lugs and notches, mating shoulders could alternatively be used. When the milling head  14  is mounted to the mill body  12 , the upwardly facing annular shoulder  46  of the milling head  14  abuts the lower end  48  of the mill body  12 . Also, the upper end  36  of the milling head  14  can abut a downwardly facing annular shoulder  38  within the mill body  12 .  
         [0024]     The section shown in  FIG. 1  is taken along a broken section line as shown in  FIG. 2 , to better illustrate a possible placement of the torque lugs  20  and torque notches  28 , and the shear pins  16 .  
         [0025]     A fluid flow path can be provided through the mill body  12  and the milling head  14 , which can for example include the inner bore  44  in the mill body  12 , and a first conical surface  50 , a ball seat  30 , an inner bore  32 , a second conical surface  52 , an axial jet  24 , and a plurality of angled jets  26  on the milling head  14 . Drilling or milling fluid can be pumped down the work string (not shown) to flow through this fluid path in the mill body  12  and the milling head  14 , as indicated by the arrows. In addition to the mill assembly apparatus  10 , a pumpable ball or plug  22  can be provided for selectively restricting this fluid flow, as will be described below.  
         [0026]     The mill assembly apparatus  10 , assembled as shown in  FIG. 1 , is mounted to a work string (not shown) and lowered into a well bore, until the cutting structure on the lower face  34  of the milling head  14  contacts a plug or other item to be milled out of the bore hole. A rotatable work string or a downhole motor can be used, without departing from the spirit of the present invention. After contacting the plug to be milled, the mill body  12  is rotated in the clockwise direction, as viewed from the upper end, rotating the milling head  14  by virtue of the abutment of the axially oriented torque shoulders  40  and  42 , and causing the inserts  18  or other cutting structure to mill the plug away. Cuttings or fragments of the milled plug are removed from the bore hole entrained in the milling fluid which is pumped through the mill body  12  and the milling head  14  and returned up the annulus to the surface.  
         [0027]     After the plug has been milled away, the pumpable plug or ball  22  can be pumped downhole through the work string to land in the ball seat  30  in the milling head  14 . Alternatively, the fluid flow rate can simply be increased through the apparatus  10  by increasing the speed of the fluid pumps. Either action results in an increased hydraulic pressure at a location in the fluid flow path as it passes through the milling head. If the pumpable ball  22  is used, the increased hydraulic pressure occurs primarily on the first conical surface  50  and across the top of the ball  22 . If the increased pump speed is used, the increased hydraulic pressure occurs in the fluid flow path  50 ,  30 ,  32 ,  52 ,  24 ,  26 . This increased hydraulic pressure exerts an increased downward hydraulic force on the upwardly facing components of the surfaces of the milling head  14  which are exposed to the increased pressure. As this downward hydraulic force reaches a sufficient, predetermined, level, it causes the shear pins  16  to shear.  
         [0028]     When the shear pins  16  shear, the milling head  14  is completely released from the mill body  12  and completely separates therefrom, as shown in  FIG. 5 . This complete axial separation of the milling head  14  from the mill body  12  allows the milling head  14  to fall downhole, completely opening up the borehole at the previously plugged location. Since the torque shoulders  40  and  42  are axially oriented, they are adapted to separate from each other easily when the shear pins  16  shear, and they do not interfere with the shearing of the pins  16  or the complete axial separation of the milling head  14  from the mill body  12 .  
         [0029]     After complete separation of the milling head  14  from the mill body  12 , the inner bore of the mill body  12  is completely open to allow for flow of hydrocarbon fluids upwardly through the mill body  12  as shown by the arrows in  FIG. 5 . The separated mill body  12  thus performs thereafter as simply an extension of the work string, and the hydrocarbon fluid flow continues upwardly through the work string to the surface. Therefore, the complete separation of the milling head  14  from the mill body  12  allows for the efficient production of hydrocarbons from the bore hole, through the work string, without pulling and replacing the work string with a production tube.  
         [0030]     A second embodiment of the milling head is shown in  FIG. 6 . This embodiment of the milling head  140  can be fitted with a check valve comprising a ball seat  142 , a check ball  144 , and a spring  146 . It can be seen that, as milling fluid passes downhole through the fluid path in the milling head  140 , the check ball  144  can be lifted off its seat  142 , against the bias of the spring  146 , to allow flow out the lower end of the work string. A kick or pressure excursion sometimes occurs in the formation fluids, which could create an undesirable flow in the uphole direction through the work string. To prevent this, the spring  146  biases the check ball  144  toward engagement with its seat  142 . As pressure below the milling head  140  increases above the drilling fluid pressure, this causes the check ball  144  to seat more securely, thereby preventing flow in the uphole direction.  
         [0031]     A third embodiment of the apparatus  210  of the present invention is shown in  FIGS. 7 and 8 . In this embodiment, the mill body  212  is secured to the milling head  214  by shear pins  216  in shear pin bores  224  and  226  in the mill body  212  and the milling head  214 , respectively. Flow passages  228  are provided through the milling head  214 . However, in this embodiment, the check valve comprises a swing check type valve, with a check valve body  262  assembled in the milling head  214 , and with a flapper valve  264 , which is pivotably mounted to the check valve body  262  by a pivot pin  266 . The check valve body  262  can be retained in the milling head  214  by one or more snap rings or pins, as is known in the art. The flapper valve  264  is biased toward the closed position by a spring. Flow of fluid down through the apparatus can open the flapper valve  264  against the spring bias, but backflow through the check valve is prevented by shutting of the flapper valve  264 , which seats against the lower side of the check valve body  262 .  
         [0032]     Also provided in this embodiment is a fishing neck  260 , which is retained in the milling head  214 , above the check valve body  262 , by one or more snap rings or pins, as is known in the art. A ball seat  230  is provided in the upper side of the check valve body  262 . When milling has been completed, and it is desired to release the milling head  214  from the mill body  212 , a ball  222  is pumped downhole through the work string, to seat in the ball seat  230 . Increasing pressure above the pumpable ball  222  then shears the shear pins  216 , releasing the milling head  214  from the mill body  212 , as in the first embodiment. If it is desired to subsequently remove the milling head  214  from the well bore, known fishing techniques can be used to attach to the fishing neck  260  and pull the milling head  214 .  
         [0033]     A fourth embodiment of the apparatus  310  of the present invention is shown in  FIGS. 9, 10 , and  11 . In this embodiment, a ball clutch mechanism  360  is provided, incorporating a fishing neck, and including one or more ball clutch bores  324  through the wall of the ball clutch  360 . One or more clutch balls  326  are positioned in the clutch bores  324 , when the ball clutch  360  is assembled to the mill body  312 . The clutch balls  326  are forced outwardly in an inner annular groove within the mill body  312  by a collet  370  which is positioned in the inner bore of the ball clutch  360 . The fingers on the upper end of the collet  370  are outwardly biased to seat in an inner shoulder of the fishing neck and ball clutch  360 . This positioning of the collet  370  releasably retains the ball clutch  360  to the mill body  312 . The ball clutch  360  is, in turn, secured to the milling head  314  by one or more snap rings or pins, as is known in the art. So, the ball clutch mechanism  360  releasably retains the milling head  314  to the mill body  312 .  
         [0034]     Flow passages  328  are provided through the milling head  314 . This embodiment of the apparatus  310  can be fitted with a check valve comprising a ball seat  340  in the lower end of the ball clutch  260 , a check ball  344 , and a spring  346 . It can be seen that, as milling fluid passes downhole through the fluid path in the milling head  314 , the check ball  344  can be lifted off its seat  340 , against the bias of the spring  346 , to allow flow out the lower end of the work string. To prevent a kick or pressure excursion, the spring  346  biases the check ball  344  toward engagement with its seat  340 . As pressure below the milling head  314  increases above the drilling fluid pressure, this causes the check ball  344  to seat more securely, thereby preventing flow in the uphole direction.  
         [0035]     A ball seat  330  is provided in the upper side of the collet  370 . When milling has been completed, and it is desired to release the milling head  314  from the mill body  312 , a ball  322  is pumped downhole through the work string, to seat in the ball seat  330 . Increasing pressure above the pumpable ball  322  then forces the collet fingers inwardly, releasing the collet  370  from the inner shoulder in the ball clutch  360 . After the collet  370  is released in this fashion, it is forced further downwardly by fluid pressure. This downward movement of the collet  370  allows the clutch balls  326  to be released from the inner groove in the mill body  312 , releasing the ball clutch  360  and the milling head  314  from the mill body  312 . If it is desired to subsequently remove the milling head  314  from the well bore, known fishing techniques can be used to attach to the fishing neck on the ball clutch  360  and pull the milling head  314 .  
         [0036]     While the particular invention as herein disclosed is capable of obtaining the objects hereinbefore stated, it is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention.