You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND 
       [0001]    1. Field of the Disclosure 
         [0002]    The disclosure herein relates to mills for milling casings in a wellbore and specifically to mills that can be sheared when trapped (also referred to herein as stuck or lodged) in the wellbore to remove the service strings utilized to convey such mills in the wellbores. 
         [0003]    2. Background of the Art 
         [0004]    Wellbores are drilled in subsurface formations for the production of hydrocarbons (oil and gas). Modern wells can extend to great well depths, often more than 15,000 ft. A wellbore is typically lined with casing (a string of metal tubulars connected in series) along the length of the wellbore to prevent collapse of the formation (rocks) into the wellbore. A variety of devices are installed in the wellbore to produce the hydrocarbons from the formation surrounding the wellbore from one or more production zones. Sometimes it is necessary to mill a part of the casing to perform a downhole operation or for other reasons. The casing section remaining above the milled portion is sometimes removed from the wellbore. To perform a milling and retrieving operations, a tool (commonly referred to as a mill, with cutting members (also referred to as knives or blades) is typically conveyed into the casing by a service string to mill a certain length of the casing at a desired location. Sometimes the mill becomes trapped or lodged in the casing, such as in the case of cutting members not retracting after the milling operation or due to another downhole condition. Some mills include a shear mechanism on the upper section of or above the mill that allows an operator to over-pull the service string to cause the mill to separate from the service string and drop in the wellbore, which allows the operator to retrieve the service string to the surface. It is also desirable not to leave tools in the wellbore or minimize the size of such debris left or dropped in the wellbore so as to avoid performing secondary operations to remove tools left in the wellbore before performing operations needed at a later time or to not obstruct flow of fluids flowing through the wellbore or for other reasons. 
         [0005]    The disclosure herein provides apparatus that provides, among other things, mills with shear mechanism that allows disconnecting cutting members from the mill when such mills are trapped in the wellbore. 
       SUMMARY 
       [0006]    In one aspect, a mill for milling a casing is disclosed that in one non-limiting embodiment includes a plurality of radially extendable cutting members on a body, an activation device that extends the cutting members radially outward from the body upon application of a hydraulic pressure to the activation device and mechanically retracts the cutting members upon removal of the hydraulic pressure from the activation device, and wherein the cutting members are separable from the body. 
         [0007]    In another aspect, a method of milling a casing in a wellbore is provided that in one non-limiting embodiment includes: conveying a string in the wellbore that includes a mill that contains a plurality of radially extendable cutting members on a body that are separable from the body of the mill and an activation device that extends the cutting members radially outward from the body upon application of a hydraulic pressure to the activation device and mechanically retracts the blades upon removal of the hydraulic pressure from the activation device; extending the cutting members to engage with the casing; rotating the mill while applying a pull force on the mill to mill the casing; and applying a pull force on the mill to disconnect the cutting members when the mill is trapped in the wellbore. 
         [0008]    Examples of the more important features of an apparatus and methods have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features that will be described hereinafter and which will form the subject of the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0009]    For a detailed understanding of the apparatus and methods disclosed herein, reference should be made to the accompanying drawing and the detailed description thereof, wherein like elements are generally given same numerals and wherein: 
           [0010]      FIG. 1  shows a mill in the run-in position, according to one non-limiting embodiment of the disclosure; 
           [0011]      FIG. 2  shows the mill of  FIG. 1  while in the milling position; 
           [0012]      FIG. 3  shows the mill of  FIG. 1  in a trapped position in the wellbore; and 
           [0013]      FIG. 4  shows portion of the mill of  FIG. 3  after the cutting members have been disconnected or sheared off from the body of the mill. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  shows a mill  100  in the run-in position, according to a non-limiting embodiment of the disclosure. The mill  100  includes a body  110  having a number of cutting members or knives  120  radially arranged around the body  110 . Each such knife is radially expandable outward about its pivot  122 . In  FIG. 1 , the knives  120  are shown in the retracted or run-in position. The number of knives  120  varies depending on the application and is typically between six (6) and ten (10), wherein such knives are equally spaced around the body  110 . The mill includes a top sub  125  for connection to a service string (not shown) and includes a fluid passage  126  for supplying a suitable fluid to the mill for activating the mill, i.e. radially expanding the knives  120 . The mill  100  further includes a mill activation device or mechanism that includes a piston  130  that acts on a compression spring  135 . The piston  130  is supported or held by a retainer  132  on its downhole end  133 . An upper extension tube  136  moves upward and downward in a seal bearing  137  in the top sub  125  as the piston  130  moves upward and downward in its chamber  138 . A mandrel assembly  140  coupled to a lower end of the piston is configured to radially extend outward the knives  120 . The mandrel assembly  140  includes an extension rod  142  with its upper end  142   a  connected to the piston  130  and its lower end  142   b  connected to a knife opening device  144 . A keeper  150  and a knife holder  152  hold the knives  120  in the body  110 . The method of activating the blades  120  is described below. A knife tang  160  is provided to cause the knives to retract to their positions shown in  FIG. 1 . The knife tang  160  is connected on the bottom of the knives  120  and retracts as the piston  130  moves downward when the fluid pressure applied onto the piston  130  is removed. Thus, in the embodiment of  FIG. 1 , the knives  120  are hydraulically extended (pressure of fluid  101 ) and mechanically retraced (by spring activated knife tang  160 ). A shear member  170 , including, but not limited to a shear ring, pin and screw retained in the lower part  110   a  of the body  110  by a shear retainer  172 . 
         [0015]      FIG. 2  shows the mill of  FIG. 1  in the milling position. Referring to  FIGS. 1 and 2 , to activate the mill  100  to mill a casing  180  in a wellbore  103 , a fluid  101  under pressure is supplied to the mill  100 . The fluid  101  causes the piston  130  to move upward (left in  FIG. 2 ), which compresses spring  135 , which causes the upper extension tube  136 , the extension member, such as rod,  142  and the knife expansion device  144  to move upward. The upward movement of the knife extension device causes the knives  120  to expand radially outward about their respective pivots  122 , as shown in  FIG. 2 . As long as the fluid pressure is applied, the piston  120  remains in the upward position and the compression spring  135  remains compressed, as shown in  FIG. 2 . To mill the casing  180 , knives  120  are rotated by a motor (not shown) in the string or by rotating the string  190  with upward tension on the mill  100 . When the fluid pressure is removed, the piston  130  moves to the right and the knife tang  180  causes the knives  120  to retract as shown in  FIG. 1 . In the event one or more knives  120  do not retract, the string  190  cannot be moved upward to retrieve the mill  100  from the wellbore. The method of shearing a portion of the mill  100  to retrieve the string  190  are described below in reference to  FIGS. 3 and 4 . 
         [0016]      FIG. 3  shows the piston  130  in the downward position as in  FIG. 1  although the fluid pressure has been removed. The mandrel assembly  140 , including the extension rod  142  and expansion device  144 , also move downward with the piston  130  and attain the same position as in  FIG. 1 . When one or more knives  120  do not retract, such knives will remain extended and behind the casing  180  as shown in  FIG. 3 . To pull the string  190  out of the wellbore, the string  190  is pulled with a pull force that exceeds the shear strength of the shear member  170 , causing the shear member  170  to shear. Shearing of the shear member  170  causes the shear retainer  172  along with the knives  120  to separate or disconnect from the body  110 . The mill portion  188  after the blades  120  have been separated is shown in  FIG. 4 . The string  190  along with the mill portion  188  can now be retrieved from the wellbore by pulling up the string  190 . The mill  100  described herein may also be configured for use for milling the casing in the downward direction. In such a case, to mill the casing  104 , a push force is applied while rotating the cutting members  120  to mill the casing  104 . 
         [0017]    The foregoing disclosure is directed to the certain exemplary non-limiting embodiments. Various modifications will be apparent to those skilled in the art. It is intended that all such modifications within the scope of the appended claims be embraced by the foregoing disclosure. The words “comprising” and “comprises” as used in the claims are to be interpreted to mean “including but not limited to”. Also, the abstract is not to be used to limit the scope of the claims.

Summary:
A mill for milling a casing in a wellbore is disclosed that in one non-limiting embodiment includes a plurality of radially extendable cutting members on a body, an activation device that extends the cutting members radially outward from the body upon application of a hydraulic pressure to the activation device and mechanically retracts the blades upon removal of the hydraulic pressure from the activation device, and wherein the cutting members are separable from the body