Patent Publication Number: US-2005133267-A1

Title: [coring tool with retention device]

Description:
BACKGROUND OF INVENTION  
      Wells are generally drilled into the ground to recover natural deposits of oil and gas, as well as other desirable materials, that are trapped in geological formations in the Earth&#39;s crust. A well is drilled into the ground and directed to the targeted geological location from a drilling rig at the Earth&#39;s surface.  
      Once a formation of interest is reached, drillers often investigate the formation and its contents by taking samples of the formation rock and analyzing the rock samples. Typically, a sample is cored from the formation using a hollow coring bit, and the sample obtained using this method is generally referred to as a “core sample.” Once the core sample has been transported to the surface, it may be analyzed to assess, among other things, the reservoir storage capacity (porosity) and the flow potential (permeability) of the material that makes up the formation; the chemical and mineral composition of the fluids and mineral deposits contained in the pores of the formation; and the irreducible water content of the formation material. The information obtained from analysis of a sample is used to design and implement well completion and production facilities.  
      “Conventional coring,” or axial coring, involves taking a core sample from the bottom of the well. Typically, this is done after the drill string has been removed, or “tripped,” from the wellbore, and a rotary coring bit with a hollow interior for receiving the core sample is lowered into the well on the end of a drill string. Some drill bits include a coring bit near the center of the drill bit, and a core sample may be taken without having to trip the drill string. A core sample obtained in conventional coring is taken along the path of the wellbore; that is, the core is taken along the axis of the borehole from the rock below the drill bit.  
      A typical axial core is 4-6 inches (˜10-15 cm) in diameter and can be over 100 feet (˜30 m) long. The rotary motion is typically generated at the surface, and the coring bit is driven into the formation by the weight of the drill string that extends back to the surface. The core sample is broken away from the formation by simply pulling upward on the coring bit that contains the sample.  
      By contrast, in “sidewall coring,” a core sample is taken from the side wall of a drilled borehole. Sidewall coring is typically performed after the drill string has been removed from the borehole. A wireline coring tool that includes a coring bit is lowered into the borehole, and a small core sample is taken from the sidewall of the borehole.  
      In sidewall coring, the drill string cannot be used to rotate the coring bit, nor can it provide the weight required to drive the bit into the formation. Instead, the coring tool must generate both the rotary motion of the coring bit and the axial force necessary to drive the coring bit into the formation.  
      In sidewall coring, the available space is limited by the diameter of the borehole. There must be enough space to withdraw and store a sample. Because of this, a typical sidewall core sample is about 1 inch (˜2.5 cm) in diameter and less than about 2 inches long (˜5 cm). The small size of the sample does not permit enough frictional forces between the coring bit and the core sample for the core sample to be removed by simply withdrawing the coring bit. Instead, the coring bit is typically tilted to cause the core sample to fracture and break away from the formation.  
      An additional problem that may be encountered is that because of the short length of a side wall core sample, it may be difficult to retain the core sample in the coring bit. Thus, a coring bit may also include mechanisms to retain a core sample in the coring bit even after the sample has been fractured or broken from the formation.  
      Sidewall coring is beneficial in wells where the exact depth of the target zone is not well known. Well logging tools, including coring tools, can be lowered into the borehole to evaluate the formations through which the borehole passes. Multiple core samples may be taken at different depths in the borehole so that information may be gained about formations at different depths.  
       FIG. 1  shows an example of an existing sidewall coring tool  101  that is suspended in a borehole  113  by a wireline  107 , as disclosed in U.S. Pat. No. 6,412,575, which is assigned to the assignee of the present invention. A sample may be taken using a coring bit  103  that is extended from the coring tool  101  into the formation  105 . The coring tool  101  may be braced in the borehole by one or more support arms  111 . An example of a commercially available coring tool is further described in U.S. Pat. Nos. 4,714,119 and 5,667,025, both assigned to the assignee of the present invention.  
       FIG. 2  shows a perspective view of an existing coring device  201  taking a core sample  207  from a formation  203 . A coring bit  205  is connected to the coring device  201 , which may include a motor to extend the bit  205  and impart rotary motion to the coring bit  205 . The coring bit  205  extends into the formation  203 , and a core sample  207  is captured in the interior of the coring bit  205 . It is noted that the coring device  201  would typically be disposed in a coring tool (e.g.,  101  in  FIG. 1 ) for use downhole. The coring bit  205  would extend from the device  201  and tool (e.g.,  101  in  FIG. 1 ) and into the formation  203 .  
      Rotary coring tools typically use a hollow cylindrical coring bit with a formation cutter at a distal end of the coring bit. The coring bit is rotated and forced against the wall of the bore hole. As the coring bit penetrates the formation, the hollow interior of the bit receives the core sample. A rotary coring bit is extended from the tool using a shaft of mechanical linkage. The shaft is typically connected to a motor that imparts rotary motion to the coring bit and forces the bit against the formation wall. Rotary coring tools are generally braced against the opposite wall of the bore hole by a support arm. The cutting edge of the rotary coring bit is usually embedded with tungsten carbide, diamonds, or other hard materials for cutting into the formation.  
       FIG. 3  shows an example of a conventional rotary coring bit  301  that may be used with a sidewall coring tool, such as the coring tool  101  of  FIG. 1 . A similar coring bit is disclosed in U.S. Pat. No. 6,371,221, which is assigned to the assignee of the present invention. The coring bit  301  includes a shaft  303  that has a hollow interior  305 . A formation cutting element  307  for drilling is located at one end of the shaft  303 . As the coring bit  301  penetrates a formation (not shown) and a sample core (not shown) may be received in the hollow interior  305  of the bit  301 . After a sample is received in the hollow interior  305 , the core sample typically is broken from the formation by displacing or tilting the drill system. The coring bit  301  is then removed from the formation, with the core sample retained in the hollow interior  305  of the coring bit  301 . Other known formation cutting elements for a rotary coring bit may be used. Examples of such formation cutting elements are described in copending U.S. patent application Ser. No. 09/832,606, assigned to the assignee of the present invention.  
      While existing coring tools are useful, there is still a need for a coring tool that will more effectively ensure a good core sample can be retrieved for analysis.  
     SUMMARY OF INVENTION  
      In one or more embodiments, the invention relates to a sidewall coring tool that includes a tool body, a hollow coring shaft extendable from the tool body, a formation cutter disposed at a distal end of the hollow coring shaft, and a retention member segmented into a plurality of petals and disposed in the hollow coring shaft. In some embodiments, the plurality of petals comprises three petals.  
      In some embodiments, the invention relates to a method for taking a core sample that includes extending a coring bit into a formation, receiving the core sample in an internal sleeve having a retention member segmented into a plurality of petals proximate a distal end of the internal sleeve, and withdrawing the coring bit from the formation.  
      In some other embodiments, the invention related to a sidewall coring tool that includes a tool body, a hollow coring shaft extendable from the tool body, a formation cutter disposed at a distal end of the hollow coring shaft, an internal sleeve disposed inside the hollow coring shaft, and at least one retention mechanism selected the group consisting of a piston and a check valve, wherein the piston is disposed in the internal sleeve and moveable with respect to the internal sleeve, and the check valve is disposed in the internal sleeve.  
      In some embodiments, the intention relates to a method for taking a core sample that includes extending a coring bit into a formation, receiving the core sample in an internal sleeve having a piston disposed therein such that the piston is moveable with respect to the internal sleeve, and withdrawing the coring bit from the formation.  
      In some embodiments, the invention relates to a sidewall coring tool that includes a tool body, a hollow coring shaft extendable from the tool body, a formation cutter disposed at a distal end of the hollow coring shaft, and an internal sleeve disposed inside the hollow coring shaft. The internal sleeve may include a bladder configured to apply radial pressure to a core sample when the bladder is selectively filled with a fluid.  
      In some embodiments, the invention relates to a sidewall coring tool that includes a tool body, a hollow coring shaft extendable from the tool body, a formation cutter disposed at a distal end of the hollow coring shaft, and an elastic retention member disposed proximate a distal end of coring tool and having an aperture at its center.  
      Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       FIG. 1  is a cross-section of a wellbore with a prior art coring tool suspended in the wellbore.  
       FIG. 2  is a perspective view of a prior art coring device.  
       FIG. 3  is a perspective view of a prior art rotary coring bit.  
       FIG. 4A  is a cross section of a coring bit in accordance with one embodiment of the invention.  
       FIG. 4B  is a cross section of a coring bit in accordance with one embodiment of the invention.  
       FIG. 4C  is a cross section of a coring bit in accordance with one embodiment of the invention.  
       FIG. 5A  is a cross section of a coring bit with a retention device in accordance with one embodiment of the invention.  
       FIG. 5B  is a cross section of a coring bit with a retention device in accordance with one embodiment of the invention.  
       FIG. 6A  is a top view of a coring bit retention member in accordance with one embodiment of the invention.  
       FIG. 6B  is a top view of a coring bit retention member in accordance with one embodiment of the invention.  
       FIG. 6C  is a top view of a coring bit retention member in accordance with one embodiment of the invention.  
       FIG. 6D  is a top view of a coring bit retention member in accordance with one embodiment of the invention.  
       FIG. 7A  is a cross section of a coring bit retention member in accordance with one embodiment of the invention.  
       FIG. 7B  is a cross section of a coring bit retention member in accordance with one embodiment of the invention.  
       FIG. 7C  is a cross section of a coring bit retention member in accordance with one embodiment of the invention.  
       FIG. 7D  is a cross section of a coring bit retention member in accordance with one embodiment of the invention.  
       FIG. 7E  is a cross section of a coring bit retention member and internal sleeve in accordance with one embodiment of the invention.  
       FIG. 8A  is a cross section of a coring bit with a piston in accordance with one embodiment of the invention.  
       FIG. 8B  is a cross section of a coring bit with a piston in accordance with one embodiment of the invention.  
       FIG. 9  is a cross section of a coring bit with a cushion in accordance with one embodiment of the invention.  
       FIG. 10  is a cross section of a coring bit with a sample retention device in accordance with one embodiment of the invention. 
    
    
     DETAILED DESCRIPTION  
      In some embodiments, the invention relates to a coring bit with a retention member that retains a core sample in a coring bit. In other embodiments, the invention includes a piston or cushion that enables a core sample to be received and retained in a coring tool. In other embodiments, the invention relates to methods for retaining a core sample in a coring tool. The invention will now be described with reference to the attached drawings.  
       FIG. 4A  is a cross section of a coring bit  401  with a retention member  411  in accordance with one embodiment of the invention.  FIG. 4A  shows only the coring bit  401 , but those having skill in the art will understand that the coring bit  401  forms part of a coring tool (not shown) that is used to take core samples from a formation. By way of example, the coring bit may form part of a coring tool, such as the coring tool  101  in  FIG. 1 .  
      The coring bit  401  in  FIG. 4A  includes a hollow shaft  403  with a formation cutter  405  disposed at a distal end of the shaft  403 . The formation cutter  405 , or formation cutter, is formed of a material for drilling into the formation  402 . The formation cutter  405  may be formed of a strong material that is coated with a super hard material, such as polycrystalline diamond or tungsten carbide. In other embodiments, the formation cutter  405  may include other devices for cutting through soft formation, such as brushes. The term “distal end” is used to describe the end of the coring bit  401  that first contacts the formation. The distal end is the end of the shaft  403  that is the farthest from the center of the coring tool (not shown) while a sample is being taken. It is the first part of the coring bit  401  to penetrate a formation.  
      As shown in  FIG. 4A , a coring bit  401  may include an internal sleeve  407  that is disposed inside the hollow shaft  403 . The internal sleeve  407  is for receiving a core sample (not shown in  FIG. 4A ) as it enters the coring bit  401 . In some embodiments, the internal sleeve  407  is a “non-rotating” internal sleeve. A non-rotating internal sleeve is an internal sleeve that is free to rotate independent of the hollow shaft  403 . Thus, as the coring tool penetrates a formation  402 , friction between the internal sleeve and the core sample (e.g.,  410  in  FIGS. 4B and 4C ) prevents the internal sleeve from rotating with respect to the formation  402 . In some other embodiments, a mechanical stop, such as a key (not shown) may prevent the rotation of the internal sleeve. This reduces the erosion of the core sample by eliminating friction between the core sample and the internal sleeve during the sampling process. Examples of coring sleeves are disclosed in copending U.S. patent application Ser. No. 10/248,475, assigned to the assignee of the present invention.  
      A retention member  411  is disposed at the distal end of the internal sleeve  407 . The retention member  411 , as will be seen, enables a core sample to enter the coring bit  401  and the internal sleeve  407 , and it also retains the core sample  410  in the internal sleeve  407  once the core sample  410  has been received in the coring bit  401 .  
       FIG. 4B  shows a cross section of a coring bit  401  in the process of receiving a core sample  410 . As the formation cutter  405  penetrates the formation  402 , a core sample  410  enters the coring bit  401 . As the core sample  410  enters the internal sleeve  407 , it pushes the petals  411   a ,  411   b  of the retention member  411  out of the way so that the core sample  410  may enter the coring bit  401 . As the petals  411   a,    411   b  move, they apply a radially inward force to the core sample  410  that serves to guide the core sample  410  and hold it in place.  
       FIG. 4C  shows a cross section of a coring bit  401  that has received a core sample  410  in the internal sleeve  407  disposed inside the hollow shaft  403  of the coring bit  401 . The core sample  410  is retained in the coring bit  401  by the petals  411   a ,  411   b  of the retention member ( 411  in  FIG. 4A ) in at least two ways. First, the petals  411   a ,  411   b  press inward on the core sample  410  to stabilize it and hold it in place. Second, when the coring bit  401  retracts from the formation  402 , the petals  411   a ,  411   b  will tend to close and grip the core sample  410 . In hard rock, the additional friction between the core sample  410  and the petals  411   a ,  411   b  will act as a wedge gripper that retains the core sample  410  in the coring bit  401 .  
      In soft rock, the petals  411   a ,  411   b  may completely close and trap the core sample  410  in the coring bit  401 . This may be advantageous because of the tendency of unconsolidated or soft formations to fall out of the coring bit. Instead of losing ¾ inch (˜1.9 cm) to 1 inch (˜2.5 cm) of the core sample of an unconsolidated formation, the petals  411   a ,  411   b  may close to retain the core sample  410  in the coring bit  401 . The only core sample  410  that is lost is that part of the core sample that extends past the petals  511   a ,  511   b.  In some embodiments, the petals are about ¼ inch (˜0.6 cm) in length, and about ¼ inch of the core sample is lost in the closing of the petals. This assists in capturing and retaining core samples of a soft formation that can simply fall out of the coring bit when the sample is taken using a conventional coring bit.  
      The retention member  411  shown in  FIGS. 4A, 4B , and  4 C is preferably made of rubber, although it can be made of any material that is flexible and still has a memory. A material with a memory will “remember” its original position such that it will tend to move back to its original position whenever it is displaced. In some embodiments, material remains in the elastic deformation regime even when completely displaced by the core sample. Thus, when the petals of a retention member are pushed radially outward by a core sample, the petals are flexible enough to give way so that the core sample can easily enter the coring bit, but they also tend to push radially inward toward their original position. This tendency to move back to the original position is what creates the radial pressure against the core sample that will guide it into the coring bit and retain it there while the coring bit is being withdrawn from the formation.  
      In some embodiments, a retention member may not be attached at a distal end of an internal sleeve. For example,  FIG. 5A  shows a coring bit  501  with an internal sleeve  507  disposed inside a hollow coring shaft  503 . A formation cutter  505  is disposed at the distal end of the hollow coring shaft  503 . The retention member  511  is located near the mid-point along the axial length of the internal sleeve  507 . In this position, a retention member  511  provides guidance so that a core sample (not shown) will be maintained near the axial center of the internal sleeve  507 , while still offering the ability to retain the core sample in the coring bit  501  when the bit is withdrawn from the formation (not shown). For example, in a hard formation, the retention member  511  may act as a wedge gripper that retains the core sample (not shown) in the coring bit  501 .  
       FIG. 5B  shows another embodiment of a coring bit with a retention member  521  in accordance with the invention. The coring bit  521  includes a hollow coring shaft  523  with a formation cutter  525  at its distal end. A retention member  531  is held in the center opening of the formation cutter by a ring  533  in the formation cutter. In this position, the retention member  531  may enable a core sample (not shown) to enter the coring bit  521 , and it may also retain the core sample in the coring bit  521  once the sample is received.  
      It is noted that a coring bit in accordance with the invention may have various combinations of the described features. For example, may include a retention member located as shown in  FIG. 5A , but without an internal sleeve. In another example, a coring bit may include a ring (e.g., ring  533  in  FIG. 5B ) that is not disposed proximate the distal end of the coring bit. Those having ordinary skill in the art will be able to devise other embodiments of an coring bit that do not depart from the scope of the invention.  
       FIG. 6A  shows an end view of a retention member  601  in accordance with one embodiment of the invention. The retention member  601  has three petals  602   a ,  602   b ,  602   c  that are cut from the center of the retention member  601  out to an outer petal circumference  605 . In some embodiments, the petal circumference  605  is substantially the same size as the inner diameter of the formation cutter (e.g.,  505  in  FIGS. 5A, 5B , and  5 C). This enables the core sample to fit snugly through the retention member. In other embodiments, the petal circumference  605  may be larger than the inner diameter of the formation cutter (e.g.,  505  in  FIGS. 5A, 5B , and  5 C).  
      The petals  602   a ,  602   b ,  602   c  shown in  FIG. 6A  are located adjacent to one another. That is, the edges of one petal,  602   a  for example, are adjacent to edges on the other petals,  602   b ,  602   c , for example.  
      In some embodiments, a retention member  601  includes cuts or perforations  607 . The cuts  607  provide additional flexibility for the petals  602   a ,  602   b ,  602   c  when the retention member  601  is constructed of a stiff material or when there are only a small number of petals making each petal stiff.  
       FIG. 6B  shows an embodiment of a retention member  621  with petals  622   a ,  622   b ,  622   c  that are not adjacent to each other. In this embodiment, the petals  622   a ,  622   b ,  622   c  are separated from each other going back to the petal circumference  625 .  
       FIG. 6C  shows another embodiment of a retention member  631  in accordance with the invention. The petals  637 ,  638 ,  639  overlap with each other. For example, petal  637  has edge  637   a  that overlaps edge  639   b  of petal  639 . The other edge  637   b  of petal  637  is overlapped by edge  638   a  of petal  638 . Similarly, petal  639  has edge  639   a  that overlaps edge  638   b  of petal  638 .  
       FIG. 6D  shows another embodiment of a retention member  641  in accordance with the invention. The retention member includes an aperture  646  at its center. The aperture  646  is created because the retention member  641  extended inwardly only to an aperture circumference  647 . A core sample (not shown) may push its way through the aperture  646  by displacing the retention member  641 . The elasticity of the retention member  641  will cause the retention member  641  to exert an inward force on the core sample when it is received.  
       FIG. 6D  also shows some other optional features of a retention member. For example, a retention member  641  with an aperture  646  may not have any petals. A core sample may simply displace a solid retention member. In other embodiments, such as the one shown in  FIG. 6D , the retention member  641  may include one or more petals  642   a ,  642   b ,  642   c.  The petals  642   a ,  642   b ,  642   c  may be individual petals, or the petals  642   a ,  642   b ,  642   c  may be perforated with perforations  643  extending between the aperture circumference  647  to the petal circumference  645 . When a core sample (not shown) is taken, the core sample will break the perforations  643 , and the core sample may be received in the coring bit (not shown).  
      In fact, it is noted that the many of the above disclosed embodiments of a retention member may use radial perforations to segment the retention member into petals.  
      This would enable the retention member to serve as a cover that will prevent contaminants from entering the coring bit before a sample is taken and the perforations are broken.  
      It is noted that radial perforations are distinguished from circumferential perforations that may be used to increase the flexibility of the retention member.  
       FIGS. 7A-7E  show various embodiments of a retention member for use with a coring bit in accordance with the invention.  FIG. 7A  shows a retention member  711  with petals  711   a ,  711   b  that are tapered inwardly. The petals  711   a ,  711   b  have a petal circumference that is substantially the same as the inner diameter of the formation cutter  705 . A core sample will snugly pass through the petals  711   a ,  711   b  of the retention member  711 .  
       FIG. 7B  shows another embodiment of a retention member  721  where the petals  721   a ,  721   b  are tapered outwardly.  
      In this embodiment, when the petals  721   a ,  721   b  are displaced by a core sample (not shown), the pressure applied by the petals  721   a ,  721   b  will be slightly greater because they are displaced farther from their original position.  
       FIG. 7C  shows another embodiment of a retention member  731  in accordance with the invention. The petals  731   a ,  731   b  of the retention member  731  are rounded and extruding into the internal sleeve  707 . When a core sample (not shown)is received in the internal sleeve  707 , the petals  731   a ,  731   b  will be displaced inwardly.  
       FIG. 7D  shows another embodiment of a retention member  741  in accordance with the invention. The petals  741   a ,  741   b  of the retention member  741  are rounded and extruding outwardly from the internal sleeve  707 . When a core sample (not shown) is received in the internal sleeve  707 , the petals  741   a ,  741   b  will be displaced inwardly.  
       FIG. 7E  shows an embodiment of a retention member  751  that is similar to that shown in  FIG. 7B . In  FIG. 7E , the internal sleeve  757  has a notch  753  that provides space for the petals  751   a ,  751   b  in their displaced position. The inner diameter D 2  of the internal sleeve  757  in the notch  753  is larger than the nominal diameter D 1755  of the internal sleeve  757 . In the embodiment shown, the nominal diameter D 1755  of the internal sleeve  757  is substantially the same as the inner diameter of the formation cutter  705 . As a core sample (not shown) passes into the internal sleeve  757 , the petals  751   a ,  751   b  of the retention member  751  will be displaced into the notch  753 . The petals  751   a ,  751   b,  when displaced into the notch  753 , have substantially the same inner diameter as the nominal diameter D 1755  of the internal sleeve  757 . This enables the core sample  701  to fit snuggly at all points along the axis of the internal sleeve  757 , while still gaining the advantages of a retention member in accordance with embodiments of the invention.  
      The embodiment of an internal sleeve  757  that is shown in  FIG. 7E  may be used with various embodiments of a retention member. For example, an internal sleeve  757  with a notch  753  may be used with any of the embodiments of a retention member shown in  FIGS. 7A-7E .  
      A retention member in accordance with any of the embodiments of the invention may be designed specifically for a single use, or it may be designed to capture and retain multiple cores. For example, some coring bits are designed so that the core samples are stored in the internal sleeve. That is, the internal sleeve is moved from inside the coring bit into a storage area. In other embodiments, only the core sample is moved into a storage device, and the internal sleeve is used to capture another sample.  
      As will be understood by those having ordinary skill in the art,  FIGS. 7A-7E  show a cross section of particular embodiments of a coring bit and a retention member in accordance with the invention. As such, the figures show only two petals in each embodiment. This is simply a function of a cross section, and it is not intended to limit the invention. A retention member in accordance with the invention may have any number of petals. Optionally, the retention member may be uniform, solid, tapered, or have one or more apertures therethrough. Other configurations may be envisioned. The retention member may be adapted to tear and/or stretch as the core sample advances into the sleeve. Portions of the retention member that are stretched or torn may apply force to the core sample to grip the core sample. The retention member is preferably elastic so that it may retract to substantially its original configuration and close behind the core sample thereby restricting portions of the core sample from exiting the coring sleeve.  
       FIG. 8A  shows a cross section of a coring bit  800  with an internal sleeve  807  having a piston  802  in accordance with the invention. The piston  802  is axially moveable with respect to the internal sleeve  807 . The piston  802  is initially positioned proximate the distal end of the internal sleeve  807 . When a core sample is being collected from the formation  810 , the core sample will displace the piston  802  with respect to the internal sleeve  807 . The piston  802  may also include seals  812  or bearings to enable easier movement of the piston  802  within the internal sleeve  807 .  
      In the embodiment shown, the internal sleeve  807  has a diameter that is substantially the same as the inner diameter of the formation cutter  805 . In order to fit with the internal sleeve  807 , the piston  802  has a diameter that is substantially the same as the inner diameter of the internal sleeve  807  so that the piston seals  812  are able to form a seal between the internal sleeve  807  and the piston  802 .  
       FIG. 8B  shows a cross section of the coring bit  800  with a core sample  801  received inside the coring bit  800 . The core sample  801  has displaced the piston  802  to a position proximate the proximal end of the internal sleeve  807 . The piston  802  moves as the core sample  801  is received in the coring bit  800 . Thus, the piston  802  provides support for the core sample  801 . This may be advantageous in unconsolidated formations, where the formation core sample would fall apart as it came into the coring bit. The piston  802  may prevent the formation from falling apart.  
      Additionally, when the coring bit  800  is withdrawn from the formation  810 , the piston  802  helps to hold the core sample in the internal sleeve  807 . In some embodiments, the chamber  815  behind the piston  802  includes a check valve or other means (not shown) to allow air or fluid to be pushed out of the chamber  815 , but that will not allow the return flow. Thus, a vacuum behind the piston  802  will prevent the piston  802  from moving on the outward direction.  
      In some embodiments, the chamber  815  behind the piston is completely vented. Nonetheless, the core sample  801  may not be able to move out of the internal sleeve  807  without also moving the piston  802 . This may be caused by a vacuum created between the piston  802  and the core sample  801 . The friction between the piston  802  and the internal sleeve  807  will create additional resistance to the movement of the core sample  801 , which will help retain the core sample  801  in the coring bit  800 .  
      Further, in addition to a simple piston  802 , the internal sleeve  807  may also include a ratchet device or a locking device. Such a device would prevent the piston from moving in the outward direction.  
       FIG. 9  shows a cross section of a coring bit  900  that includes a cushion for receiving and retaining the core sample  901  in the bit  900 . A hollow outer shaft  903  penetrates a formation  910  using a formation cutter  905  disposed at the distal end of the shaft  903 . A core sample  901  is received in an internal sleeve  917  that is disposed inside the hollow shaft  903 .  
      The cavity (shown at  918 ) in the internal sleeve  917  behind the core sample  901  is filled with a fluid, such as water. The proximal end of the internal sleeve  917  includes a valve  921  for selectively permitting fluid to pass between the sleeve  917  and the rest of the tool. The valve  921  may be, for example, a check valve that enables the fluid to exit the cavity  918  as a core sample  901  moves into the internal sleeve  917 . When the coring bit  900  is withdrawn from the formation, the valve  921  may be used to prevent the reverse flow of fluid into the cavity  918 , and a vacuum is created behind the core sample  901  that retains the core sample  901  in the coring bit  900 .  
      In at least one embodiment, the check valve  921  in  FIG. 9  may be combined with the coring bit  800  in  FIGS. 8A and 8B . In such an embodiment, the piston ( 802  in  FIGS. 8A and 8B ) would force the fluid through the check valve ( 921  in  FIG. 9 ). The check valve ( 921  in  FIG. 9 ) would prevent the return flow of fluid and the vacuum behind the piston ( 802  in  FIGS. 8A and 8B ) and, thereby, retain the piston core sample in place.  
       FIG. 10  shows a cross section of a coring bit  1001  in accordance with another embodiment of the invention. A hollow shaft  1003  has a formation cutter  1005  at a distal end of the shaft  1003 . A bladder  1007  is used as an internal sleeve in the coring bit  1001  in  FIG. 10 . The bladder  1007 , when deflated, provides enough space to accept a core sample. The bladder  1007  may then be selectively inflated by filling it with fluid. The fluid may be stored hydraulic fluid, or it may be drilling mud that is pumped into the bladder. The type of fluid used is not intended to limit the invention.  
      When the bladder  1007  is filled, it will compress inwardly and exert a radial pressure on a core sample (not shown).  
      The pressure will apply an overburden to the core sample that will both stabilize and retain the core sample.  
      Embodiments of the invention may present one or more of the following advantages. A coring bit with a retention member or other retention device in accordance with the invention will retain the core sample in the coring bit while the coring bit is being withdrawn from the formation. This will prevent the core sample from being damaged or lost during this process.  
      Advantageously, a coring bit may include a retention member that will close completely when capturing a sample in soft or unconsolidated formation. When the retention member closes, the core sample will be completely enclosed in the coring bit and protected against further damage and loss.  
      Advantageously, a coring bit that includes a non-rotating internal sleeve will not degrade the core sample through friction between the core sample and the internal sleeve and the retention member. The internal sleeve and the retention member will not rotate with respect to the formation and the core sample as it is being captured.  
      Advantageously, embodiments of the invention that include a piston in the internal sleeve provide additional guidance for a core sample as it is being received. The piston is displaced by the core sample, and once the sample is fully received, the piston creates a vacuum or void behind the core sample that retains the core sample in the internal sleeve as the coring bit is withdrawn from the formation.  
      Advantageously, embodiments of the invention that include a cushion provide steady guidance for the core sample as it enters the coring bit. Once received in the coring bit, the core sample is retained by a vacuum or void behind the core sample.  
      While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.