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BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to wellbore completion. More particularly, the invention relates to downhole tools. More particularly still, the invention relates to an expandable bit with a pressure activated release member.  
         [0003]     2. Description of the Related Art  
         [0004]     In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. The drill bit generally includes a body portion for securing the drill bit to the drill string and a crown portion to form the wellbore. After drilling a predetermined depth, the drill string and the drill bit are removed, and the wellbore is lined with a string of steel pipe called casing. The casing typically includes a smaller outside diameter than the drill bit that formed the wellbore. The casing provides support to the wellbore and facilitates the isolation of certain areas of the wellbore adjacent hydrocarbon bearing formations. The casing typically extends down the wellbore from the surface of the well to a designated depth. An annular area is thus defined between the outside of the casing and the earth formation. This annular area is typically filled with cement to permanently set the casing in the wellbore and to facilitate the isolation of production zones and fluids at different depths within the wellbore.  
         [0005]     In a conventional completion operation, it is common to employ more than one string of casing in a wellbore. In this respect, the well is drilled to a second designated depth of a smaller diameter, and a second string of casing, or liner, is run into the drilled out portion of the wellbore. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing and then cemented in place. This process is typically repeated with additional casing strings until the well has been drilled to a total depth.  
         [0006]     The process of cementing a liner into a wellbore typically involves the use of wiper plugs and drill-pipe darts. Plugs typically define an elongated elastomeric body used to separate fluids pumped into a wellbore. A liner wiper plug is typically located inside the top of a liner, and is lowered into the wellbore with the liner at the bottom of a working string. The liner wiper plug has radial wipers to contact and wipe the inside of the liner as the plug travels down the liner. The liner wiper plug has a cylindrical bore through it to allow passage of fluids.  
         [0007]     Typically, the cementing operation requires the use of two plugs and two darts. When the cement is ready to be dispensed, a first dart is released into the working string. The cement is pumped behind the dart, thereby moving the first dart downhole. The first dart acts as a barrier between the cement and the drilling fluid to minimize the contamination of the cement. As the first dart travels downhole, it seats against a first liner wiper plug and closes off the internal bore through the first plug. Hydraulic pressure from the cement above the first dart dislodges the first dart and the first plug from the liner and pumped down the liner together. At the bottom of the liner, the first plug seats against a float collar, thereby closing off fluid flow through the float collar. The pressure builds above the first plug until it is sufficient to cause a membrane in the first plug to rupture. Thereafter, cement flows through the first plug, the float collar, a shoe track, and a float shoe and subsequently into the annular space between the liner and the wellbore.  
         [0008]     Generally, the shoe track is a space defined between the float collar and the float shoe. The shoe track is used to ensure that the float shoe is surrounded in high quality cement and that any contamination or lower quality cement that may bypass a second liner wiper plug is safely contained within the shoe track.  
         [0009]     After a sufficient volume of cement has been placed into the wellbore, a second dart is deployed. Drilling mud is pumped in behind the second dart to move the second dart down the working string into the liner. The second dart travels downhole and seats against a second liner wiper plug. Thereafter, hydraulic pressure above the second dart dislodges the second dart and the second plug from the liner and they are pumped together down the liner. In turn, cement ahead of the second plug is displaced through the shoe track and subsequently out of the liner into the annulus. After the cementing operation is complete, the shoe track is typically drilled out as the well is drilled to another designated depth.  
         [0010]     From time to time and for a variety of reasons it is necessary to form a portion of a wellbore that is at least as large as the section of the cased wellbore thereabove. For example, a monobore well consists of a sequence of expandable liners that are run through the existing casing, then expanded to achieve the same post-expansion through-bore. In forming the monobore well, the portion of the wellbore below the cased portion must be at least as large as the section of the cased wellbore thereabove.  
         [0011]     There are a variety of different methods of forming an enlarged wellbore. One such method is by positioning a conventional under-reamer behind a drill bit to cut the enlarged wellbore. In this drilling configuration, the drill bit acts as a pilot bit to cut the inner cross-sectional area while the under-reamer enlarges the cross-sectional area. Generally, the conventional under-reamer includes a number of expandable arms that move between a closed position and an open position. The ability of the conventional under-reamer to open and close the arms allows the under-reamer in the closed position and the pilot bit to travel though a smaller diameter casing. After passing through the casing and the shoe track the under-reamer may be opened to form an enlarged diameter bore below the casing shoe resulting in a wellbore equal to or larger than the original drilled hole. Thereafter, the enlarged wellbore may be lined with expandable liners. This procedure of forming the enlarged borehole, although effective may be time consuming and expensive.  
         [0012]     In recent years bi-center bits have been developed as an alternative to the conventional under-reamer. Generally, the bi-center bit includes offset cutting members mounted at irregular intervals around the crown of the bit. As the bi-center bit is rotated, the offset cutting members rotate to form an enlarged wellbore. Although, this method of forming an enlarged wellbore is becoming more common the bi-center bits are unstable due to their irregular structure and tend to be more difficult to control for directional purposes than ordinary drill bits. Additionally, the bi-center bits may not drill the expected swept diameter of the offset pads which ream the pilot hole created by the crown.  
         [0013]     More recently, an expandable bit has been used to form an enlarged portion of the wellbore. The expandable bit was introduced to overcome the deficiencies in the conventional under-reamer and the bi-center bit. An example of an expandable bit is disclosed in International Publication Number WO 01/81708 A1, which is incorporated herein in its entirety. Similar to the conventional under-reamer, the expandable bit includes a set of blades that move between an open position and a closed position. Generally, in operation hydraulic fluid is pumped through the center of the expandable bit to move the blades between the open and the closed position. A more detailed discussion of the expandable bit will be described in subsequent paragraphs.  
         [0014]     Even though the expandable bit overcomes many of the deficiencies in the conventional under-reamer and the bi-center bit, a problem still exists with the use of the expandable bit to drill out the shoetrack. The problem occurs when the blades in the expandable bit inadvertently move from the closed position to the open position and contact the ID of the casing while drilling out the shoetrack, thereby resulting in an increase of torque and damage to the blades.  
         [0015]     In view of the deficiency of the expandable drill bit, a need therefore exists for an expandable bit with a pressure activated release member to selectively allow the blades to move from the closed position to the open position. There is a further need for an improved expandable bit.  
       SUMMARY OF THE INVENTION  
       [0016]     The present invention generally relates to an apparatus and method of forming a wellbore. In one aspect, an expandable apparatus for use in a wellbore is provided. The apparatus includes a body and a blade assembly disposed on the body. The blade assembly is movable between a closed position whereby the expandable apparatus has a smaller outer diameter and an open position whereby the expandable apparatus has a larger outer diameter. The apparatus further includes a pressure activated member to selectively allow the blade assembly to move from the closed position to the open position.  
         [0017]     In another aspect, a method of forming a wellbore is provided. The method includes lowering a drill string with an expandable bit at the end thereof into the wellbore. The expandable bit includes a body, cutting members, and a pressure activated member for allowing the cutting members to move from a collapsed position to an expanded position. The method further includes operating the pressure activated member and activating the expandable bit, thereby causing the cutting members to move from the collapsed position to the expanded position. The method also includes rotating the expandable bit to form a portion of the wellbore. Additionally, the method includes deactivating the expandable bit, thereby causing the cutting members to move from the expanded position to the collapsed position and removing the drill string and the expandable bit from the wellbore.  
         [0018]     In yet another aspect, an expandable bit for use in a wellbore is provided. The expandable bit includes a body and at least one cutting member disposed on the body. The at least one cutting member is movable between a collapsed position and an expanded position. The expandable bit also includes a release member to selectively allow the at least one cutting member to move from the collapsed position to the expanded position. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0020]      FIG. 1  is a sectional view illustrating an expandable bit being lowered into a wellbore after a cementing operation.  
         [0021]      FIG. 2  is a sectional view illustrating a shoe track being drilled out by the expandable bit.  
         [0022]      FIG. 3  is a sectional view illustrating the expandable bit positioned at a lower end of a casing string.  
         [0023]      FIG. 4  is a sectional view illustrating the expandable bit forming a lower portion of a wellbore. 
     
    
     DETAILED DESCRIPTION  
       [0024]     In general, the present invention relates to an expandable bit with a pressure activated member. The expandable bit includes a set of blades that move between an open position and a closed position through the use of hydraulic pressure. As will be described herein, the expandable bit is employed to drill through a shoe track and then subsequently employed to drill the wellbore to a designated depth. It must be noted that aspects of the present invention are not limited to drilling through the shoe track, but are equally applicable to other types of wellbore operations requiring the use of the expandable bit. Additionally, the present invention will be described as it relates to a vertical wellbore. However, it should be understood that the invention may be employed in a horizontal or deviated wellbore without departing from the principles of the present invention. To better understand the novelty of the apparatus of the present invention and the methods of use thereof, reference is hereafter made to the accompanying drawings.  
         [0025]      FIG. 1  is a sectional view illustrating an expandable bit  100  being lowered into a wellbore  10  at the lower end of a drill string  145  after a cementing operation. As illustrated, the wellbore  10  is lined with a string of steel pipe called casing  15 . The casing  15  provides support to the wellbore  10  and facilitates the isolation of certain areas of the wellbore  10  adjacent hydrocarbon bearing formations. The casing  15  typically extends down the wellbore  10  from the surface of the well to a designated depth. An annular area  20  is thus defined between the outside of the casing  15  and the wellbore  10 . This annular area  20  is filled with cement  25  pumped through a cementing system  50  to permanently set the casing  15  in the wellbore  10  and to facilitate the isolation of production zones and fluids at different depths within the wellbore  10 .  
         [0026]     The cementing system  50  generally includes a float shoe  55  disposed at the lower end thereof to prevent reverse flow, or U-tubing, of the cement  25  from the annulus  20  into the casing  15 . The cementing system  50  also includes a shoe track  60  which is a space defined between the float shoe  55  and a float collar  65 . The principal function of the shoe track  60  is to ensure that the float shoe  55  is surrounded in high-quality cement and that any contamination that may bypass cement plugs  70 ,  80  is safely contained within the shoe track  60 .  
         [0027]     After the casing  15  is permanently set in the wellbore  10 , the expandable bit  100  is employed to drill through the cementing system  50  and to subsequently drill the wellbore  10  to another designated depth. As illustrated in  FIG. 2 , the shoe track  60  is being drilled out by the expandable bit  100 . During this operation, the expandable bit  100  remains in a closed position in order to prevent damage to a set of expandable blades. To this end, the expandable bit  100  is held in the closed position by a release member  200  as will be described herein.  
         [0028]      FIG. 3  is a sectional view illustrating the expandable bit  100  positioned at a lower end of the casing  15 . Generally, the expandable bit  100  may move between an open position and a closed position. In the open position, ( FIG. 4 ) arms  190  at the lower end of the expandable bit  100  are expanded outward while in the closed position the arms  190  are collapsed inward. The arms  190  are attached to a blade pivot housing  155  by a plurality of hinge pins  175 . The hinge pins  175  allow the arms  190  to swing out from a body  125  of the bit  100 . The arms  190  include a plurality of cutting elements  210  made of a hard material such as tungsten carbide or polycrystalline diamond. The arms  190  are constructed and arranged to permit the cutting elements  210  to contact and drill the earth when the arms  190  are expanded outward and not ream the wellbore or surrounding casing  135  when the arms  190  are collapsed inward. Each arm  190  may carry a single or double row of cutting elements  210  depending on the desired drilling configuration.  
         [0029]     As shown in  FIG. 3 , nozzles  185  are arranged at the lower end of the body  125 . The nozzles  185  are in fluid communication with a bore  205  defined in the body  125  to communicate fluid through the expandable bit  100  and allow jetting of the drilling fluid during the drilling operation to remove any cutting build up that may gather in front of the arms  190 . The nozzles  185  are also used to create a hydraulic pressure differential within the bore  205  of the expandable bit  100  in order to cause the arms  190  to expand outward as will be discussed herein.  
         [0030]     As briefly discussed above, the expandable bit  100  includes the release member  200  to ensure that the arms  190  remain in the collapsed position during the drilling operation of the shoe track. Generally, the release member  200  is a device that operates at a predetermined pressure or force. In one embodiment, the release member  200  is a shear pin disposed between a hydraulic cylinder  120  and a housing  135  as illustrated in  FIG. 3 . The shear pin is constructed and arranged to fail at a predetermined axial force. Generally, the shear pin is a short piece of brass or steel that is used to retain sliding components in a fixed position until sufficient force is applied to break the pin. Once the pin is sheared, the components may then move to operate or function the tool.  
         [0031]     Alternatively, other forms of shearable members may be employed in the release member  200 , as long as they are capable of shearing at a predetermined force. For example, a threaded connection (not shown) may be employed between the hydraulic cylinder  120  and the housing  135 . Generally, the threads machined on the hydraulic cylinder  120  are mated with threads machined on the housing  135  to form the threaded connection. The threads on the hydraulic cylinder  120  and the housing  135  are machined to a close fit tolerance. The threads are constructed and arranged to fail or shear when a predetermined axial force is applied to the hydraulic cylinder  120 . The desired axial force required to actuate the release member  200  determines the quantity of threads and the thread pitch. In another example, a collet assembly (not shown) may be employed between the hydraulic cylinder  120  and the housing  135 . The collet assembly is constructed and arranged to fail when a predetermined axial force is applied to the hydraulic cylinder  120 .  
         [0032]     As shown in  FIG. 3 , the release member  200  is disposed between the hydraulic cylinder  120  and the housing  135 . It should be understood, however, that the release member  200  may be disposed at any location along the hydraulic cylinder  120 , without departing from the principles of the present invention. For instance, the release member  200  may be disposed between the hydraulic cylinder  120  and the blade pivot housing  155 .  
         [0033]     After the expandable bit  100  is positioned at a desired location in the wellbore  10 , the release member  200  is actuated by pumping fluid from the surface of the wellbore  10  through the expandable bit  100 . Generally, as fluid is pumped through the expandable bit  100 , the nozzles  185  restrict the fluid flow causing a hydraulic pressure differential and urging a portion of fluid through a port  110  formed in the body  125  to fill a chamber  105  defined between the hydraulic cylinder  120  and an internal piston  115 . As the chamber  105  fills with fluid, the hydraulic cylinder  120  is urged axially upward creating a force on the pressure release member  200 . At a predetermined force, the release member  200  fails, thereby allowing the hydraulic cylinder  120  to move in relation to the housing  135 . As the chamber  105  fills with additional fluid, the volume of the chamber  105  increases, causing the hydraulic cylinder  120  to move axially upward compressing a biasing member  140 . At the same time, the hydraulic cylinder  120  draws the blade pivot housing  155  axially upward, thereby pulling the arms  190  over the head  180 . In this manner, the axial force created on the blade pivot housing  155  by the hydraulic cylinder  120  causes the arms  190  to pivot outwards at pins  175  to the expanded position and to remain in the expanded position as long as the hydraulic pressure differential is maintained in the body  125  of the expandable bit  100 . Additionally, guide pins  160  act on slots  170  machined in the arms  190  to ensure that the arms  190  return to the closed position upon removal of the hydraulic pressure differential.  
         [0034]      FIG. 4  is a sectional view illustrating the expandable bit  100  forming a lower portion of the wellbore  10 . After the expandable bit  100  is placed at a desired location in the wellbore  10 , the arms  190  are moved from the collapsed position and the expanded position by pumping fluid through the expandable bit  100 . Thereafter, the drill string  145  and the expandable bit  100  are rotated and urged axially downward to form the lower portion of the wellbore  10 .  
         [0035]     After the expandable bit  100  has drilled the wellbore  10  to a designated depth, the arms  190  are moved from the expanded position to the collapsed position by reducing the fluid flow through the expandable bit  100 . Generally, the reduction of fluid flow reduces the pressure differential created by the nozzles  185 , thereby causing the fluid pressure in the chamber  105  to be reduced to a hydrodynamic pressure below that required to compress the biasing member  140 . In other words, the reduction of the fluid flow allows the biasing member  140  to expand and urge the hydraulic cylinder  120  and the blade pivot housing  155  axially downward pushing the arms  190  over the head  180  and into the collapsed position. Thereafter, the expandable bit  100  may be removed from the wellbore  10  by pulling the drill string  145  and the bit  100  axially upward.  
         [0036]     In operation, the expandable bit is attached to a lower end of a drill string. Thereafter, the drill string and expandable bit are placed at a desired location in the wellbore proximate the cementing system. Next, the expandable bit and the drill string are rotated and urged axially downward through the cementing system to a location proximate the lower end of the casing. Subsequently, the release member is actuated and the arms are extended by pumping fluid from the surface of the wellbore through the expandable bit. As fluid is pumped through the expandable bit, the nozzles restrict the fluid flow causing a hydraulic pressure differential and urging a portion of fluid through a port formed in the body to fill a chamber defined between the hydraulic cylinder and an internal piston. As the chamber fills with fluid, a hydraulic pressure force is created in the chamber that urges the hydraulic cylinder axially upward against the pressure release member. At a predetermined hydraulic pressure, the release member fails, thereby allowing the hydraulic cylinder to move in relation to the housing. As the chamber fills with additional fluid, the volume of the chamber increases, causing the hydraulic cylinder to move axially upward compressing a biasing member. At the same time, the hydraulic cylinder draws the blade pivot housing axially upward, thereby pulling the arms over the head and into the expanded position. Thereafter, the drill string and the expandable bit are urged axially downward to form the lower portion of the wellbore.  
         [0037]     After the expandable bit has formed the wellbore to a designated depth, the expandable bit is typically closed hydraulically by reducing the fluid flow through the expandable bit. The reduction of the fluid flow allows the biasing member to expand and urge the hydraulic cylinder and the blade pivot housing axially downward pushing the arms over the head and into the collapsed position. Thereafter, the expandable bit may be removed from the wellbore by pulling the drill string and the expandable bit axially upward.  
         [0038]     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Summary:
An apparatus for use in a wellbore. The apparatus includes a body and a blade assembly. The blade assembly is movable between a collapsed position whereby the expandable apparatus has a smaller outer diameter and an expanded position whereby the expandable apparatus has a larger outer diameter. The apparatus further includes a pressure activated member for selectively allowing the blade assembly to move from the closed position to the open position. A method for using the apparatus, wherein a drill string with an expandable bit at the end thereof is lowered into the wellbore. Thereafter, the pressure activated member is activated to allow the cutting members to move from the collapsed position to the expanded position. Subsequently, the expandable bit is rotated to form a portion of the wellbore. Next, the expandable bit is deactivated and the cutting members are moved from the expanded position to the collapsed position.