Abstract:
The present disclosure relates to a near-bit borehole opener (reamer) tool and a method of drilling a wellbore comprising: (a) disposing in the wellbore a tool string; (b) lowering and rotating the tool string and drilling a first portion of the wellbore having a first diameter with the drill bit, wherein the at least one cutter assembly of the borehole reamer is in a closed position; (c) reaming with the second borehole reamer a portion of the wellbore to a second diameter larger than the first diameter, wherein a rat hole portion of the wellbore is not reamed with the second borehole reamer, said rat hole portion of the wellbore having the first diameter; (d) extending the at least one cutter assembly of the first borehole reamer; and (e) concurrently rotating and moving the first borehole reamer up or down in the rat hole portion of the wellbore with the first diameter and abrading and cutting away the wellbore wall contacted by the at least one cutter assembly of the first borehole reamer assembly, thereby enlarging the diameter of a portion of the rat hole portion of the wellbore.

Description:
BACKGROUND 
       [0001]    During well drilling operations, a drill string is lowered into a wellbore. Typically, in conventional vertical drilling operations the drill string is rotated. The rotation of the drill string provides rotation to a drill bit affixed to the distal end of the drill string. If the wellbore is deviated from vertical, some prior art drilling systems use a downhole mud motor disposed in the drill string above the drill bit to rotate the bit instead of rotating the drill string to provide rotation to the drill bit. 
         [0002]      FIG. 1A  illustrates an example of a deviated wellbore including a bottom hole assembly (“BHA”) therein. A BHA  10  is shown drilling a borehole  2  in a subterranean formation  9 . Bottom hole assembly  10  includes a drill bit  6 , a first stabilizer  8 , a roller cone-type under reamer  17  and a drilling assembly  12  in the order shown. The drilling assembly  12  includes a bent housing directional mechanism  14  and a downhole motor  16  to directionally drill borehole  2  with bit  6  and roller-cone type under reamer  17 . Optionally, a second stabilizer  19  may be added to BHA  10  which may be located above (shown) or below drilling assembly  12 . BHA is illustrated creating a borehole  2  in a subterranean formation  9 . The borehole  2  includes a reduced diameter lower portion  3  sometimes referred to in the art as a “rat hole” or “pilot hole.” 
         [0003]      FIG. 1B  illustrates another example of a drilling assembly. A BHA  25  is shown drilling a borehole  2  in a subterranean formation  9 . Bottom hole assembly  25  includes bit  6 , a radial piston-type under reamer  18 , and a drilling assembly  12  including a downhole motor  16  and a bent housing directional mechanism  14 . The borehole  2  includes a reduced diameter lower portion  3 , sometimes referred to in the art as a “rat hole” or “pilot hole.” 
         [0004]    In recent years, rotary steerable systems (“RSS”) have been developed to provide downhole rotation to the drill bit. In a rotary steerable system, the BHA trajectory is deflected while the drill string continues to rotate. As such, rotary steerable systems include two types: push-the-bit systems and point-the-bit systems. In a push-the-bit RSS, a group of expandable thrust pads extend laterally from the BHA to thrust and bias the drill string into a desired trajectory. In order for this to occur while the drillstring is rotated, the expandable thrusters extend from what is known as a geostationary portion of the drilling assembly. Geostationary components do not rotate relative to the formation while the remainder of the drillstring is rotated. While the geostationary portion remains in a substantially consistent orientation, the operator at the surface may direct the remainder of the BHA into a desired trajectory relative to the position of the geostationary portion with the expandable thrusters. An alternative push-the-bit rotary steering system has lateral thrust pads mounted on a body, which is connected to and rotates at the same speed as that of the rest of the BHA and drill string. The pads are cyclically driven, controlled by a control module with a geostationary reference, to produce a net lateral thrust which is substantially in the desired direction. 
         [0005]    The rotary steerable tools are generally programmed by an engineer or directional driller who transmits commands using surface equipment (typically using either pressure fluctuations in the mud column or variations in the drill string rotation) which the RSS tools understand and gradually steer in the desired direction. 
         [0006]      FIGS. 2 and 2A  illustrate one example of a rotary steerable system such as Halliburton&#39;s Geo Pilot System. The system  50  may include a flex power assembly  34  attached to a drill string  31 . The system further includes in descending order a driver  32 , a stabilizer  30 , electronics module  28 , hydraulics module  26 , actuator  24 , compensator  22 , and extended gage bit  20 . 
         [0007]    In some implementations, a drill string using an RSS System, a lower portion of the drill string may include a measurement while drilling (MWD) and Logging While Drilling (LWD) telemetry tool section. MWD/LWD technology is well known in the prior art. In some implementations, the MWD/LWD system sends downhole data on the geologic formations penetrated by the wellbore and drilling performance data to the surface for evaluation. Transmission of information to or from the MWD tools typically uses mud pulse technology or, alternatively, other information transmission means. 
         [0008]    In order to pass through the inside diameter of upper strings of casing already in place in the wellbore, often times the drill bit will be of such a size as to drill a smaller gage hole than may be desired for later operations in the wellbore. It may be desirable to have a larger diameter wellbore to enable running further strings of casing and allowing adequate annulus space between the outside diameter of such subsequent casing strings and the borehole wall for a good cement sheath. A conventional borehole opener (reamer) may be included in the drill string above the MWD tools and the rotary steerable tools. Note as used herein the term “borehole opener” is interchangeable with “under reamer.” Because of the configuration of the tool string, it is not possible for the conventional reamer to reach the bottom of the wellbore. This leaves a smaller gage section of borehole that is referred to as a “rat hole” or alternatively a “pilot hole.” This under gage rat hole section may be 60 to 90 feet in length. 
     
    
     DETAILED DESCRIPTION 
       [0009]    The present disclosure includes a “near-bit reamer” disposed on the distal end of the tool string proximal to the drill bit. This near-bit cutting structure reamer may be less robust than the one of a primary conventional reamer (such as the reamers discussed in the above noted prior art references) because the near-bit reamer is only reaming the rat hole  3  portion of the wellbore that a conventional reamer cannot reach. 
         [0010]    Referring now to  FIG. 3 , wherein one implementation of a tool string  100  including a near-bit borehole enlargement tool  200  is illustrated. Note as used herein the terms “borehole enlargement tool” and “borehole opener tool” are used interchangeably. The tool string  100  is attached to a drill string  101  that is suspended from a drilling rig (not shown). The tool string may include a conventional under reaming tool  104 , e.g., a Halliburton model XR Reamer or UR type conventional under reamer. 
         [0011]    In some implementations, below the conventional reamer  104  is disposed a Measurement While Drilling (MWD) tool string and/or a Logging While Drilling (LWD) Tool string section generally denoted as element  120 . The MWD/LWD tool section  120  may include a HOC P4M Pulser  112  which is a communication device to receive RSS and MWD tool instructions and send data to a surface communication means. 
         [0012]    The MWD/LWD tool section  120  may include one or more in-line stabilizer elements  114 ,  118  and  122 . The MWD/LWD tool section  120  further includes elements  116  and  124  that receive information on downhole data of the geologic formations penetrated by the wellbore and drilling performance data and transmit that data to the surface for evaluation, typically using mud pulse technology or other data transmission means. 
         [0013]    Below the MWD tool section  120  is a flexible sub  130 . 
         [0014]    Disposed below the flexible sub  130  is the RSS tool string denoted generally as  140 . For an exemplary RSS tool string, see  FIG. 2 . 
         [0015]    In the present disclosure, below the RSS tool section  140  and the MWD/LWD tool section  120  is a near-bit reamer  200  which is disposed proximal to a conventional drill bit  150  that is disposed on the distal end of the tool string  100 . 
         [0016]    Referring to  FIG. 4 , therein is illustrated another embodiment of the present disclosure wherein the near-bit borehole enlargement tool (“NBR”) and drill bit are integrally combined as element  360 . In some implementations the fishing necks of an NBR tool are removed and the conventional pin and box connection of the NBR and drill bit are removed and the NBR tool is welded to the drill bit. (It will be understood that it is not necessary to modify actual existing NBR tools and drill bits to construct the combination tool  360 . The elements of such a combination tool  360  may be manufactured and constructed in accordance with the design elements disclosed herein.) It will be understood that welding is only one method of securing the bit body to the reamer body. The bodies may be integrally cast as a single body or machined from a single casting or forging. Alternatively, the two bodies may be secured by other conventional connection means. 
         [0017]    The alternative tool string  300  includes a conventional reamer  304 . Various crossover subs and stabilizers  318  and  322  are disposed above the MWD tool  320  (e.g., Halliburton Evader Gyro). The MWD/LWD tool section  120  may include a HOC P4M Pulser  312  which is a communication device to receive RSS and MWD tool instructions and send data to a surface communication means. 
         [0018]    Below the MWD tool section is a flexible sub  330 . 
         [0019]    Disposed below the flexible sub  330  is a RSS tool denoted generally as  340  (e.g., Halliburton Geo-pilot tool). Detailed information on Halliburton&#39;s Geo-pilot system is contained in Appendix A. 
         [0020]    In the present disclosure, below the RSS tool section  340  is an optional stabilizer sub  326 . The combination bit and reamer  360  includes a short near-bit borehole enlargement tool (NBR)  362  welded to the body of a roller cone or PDC bit  364  disposed on the distal end of the tool string  100 . 
         [0021]    It will be understood that the present disclosure is not limited to the Halliburton product elements described above. The Halliburton product elements included herein are exemplary products that may be used in the subject disclosure. However, other products of a similar nature manufactured by other manufacturers may be used as elements in the subject disclosure. 
         [0022]      FIGS. 5A and 5B  illustrate an enlarged perspective view of an exemplary combination near-bit hole opener (reamer) and drill bit  360 . The combination tool  360  includes a near-bit reamer  362  that includes a body section  361 . The overall length L 1  of the body section is 40 inches or smaller. The overall length of the NBR tool  360  is L 2  of 60 inches or smaller. The reamer  362  includes a plurality of cutter elements  368  disposed on radial pistons  369  disposed inside the body  361 . When the reamer  362  is actuated, the cutter elements  368  are moved radially outward from a central longitudinal axis  301  of the reamer  362  and contact the borehole wall. (It will be understood that other configurations of cutter elements may be used in the near-bit hole enlargement tool of the present disclosure.) As the reamer  362  is rotated by the rotation of the drill string  101 , the cutter elements  368  abrade and cut away the formation, thereby expanding the diameter of the borehole. 
         [0023]    Mechanical elements of a conventional near-bit reamer are illustrated in  FIG. 10  (e.g., Halliburton NBR tool). The conventional NBR includes a box connection  1001 , a pin connection  1002 , a body  1006  portion of approximately 16⅞ inches diameter and an overall length L 5  of 14.33 inches, a cutter  1010  is disposed on a piston that is adapted to move radially out of the body  1006  to engage the borehole wall. The overall diameter L 6  is about 16⅞ inches. 
         [0024]      FIGS. 6A ,  6 B and  6 C illustrate another embodiment of a combined near-bit borehole enlargement tool  460 . The combination tool  460  includes a drill bit  464  and a near-bit reamer  462  that includes a body section  461 . The reamer  462  includes a plurality of cutter elements  468  disposed on radial pistons disposed inside the body  461 . When the reamer  462  is actuated, the cutter elements  468  are moved radially outward from a central longitudinal axis  401  of the reamer  462  and contact the borehole wall. (It will be understood that other configurations of cutter elements may be used in the near-bit hole enlargement tool of the present disclosure.) As the reamer  462  is rotated by the rotation of the drill string  101 , the cutter elements  468  abrade and cut away the formation, thereby expanding the diameter of the borehole. For a combination tool  460  sized to ream a hole diameter of 17.5 inches to an opening of 20 inches, the overall length L 3  of the body section  461  is about 40 inches. The overall length of the combination tool  460  for reaming a 20 inch hole is L 4  about 60 inches or smaller. 
         [0025]      FIGS. 7 and 8  illustrate additional embodiments of the present disclosure wherein the body  761 ,  861  of the near-bit borehole enlargement tool  760  and  860  has spiral body design including spiral water courses  764  and  864  disposed in the outside of the body. The spiral water course provides the benefits over a linear longitudinal exterior water course of: 
         [0026]    a. Optimized stabilization 
         [0027]    b. Reduction of BHA vibrations, hence increasing cutter&#39;s lifetime 
         [0028]    c. Better cleaning performance 
         [0029]    The present disclosure further includes a method of using the near-bit borehole enlargement tool  200 ,  360 ,  460 ,  760  and  860  to open the reduced diameter portion rat hole  3  of the borehole  4 . 
         [0030]    It will be understood that other implementations of a combination bit and reamer may be used in the near-bit borehole enlargement tool of the present disclosure. 
         [0031]    It is important to note that it is not desirable to place a conventional reamer directly above the bit and below the RSS and MWD/LWD for several reasons. In some conventional reamers a ball (plug) is pumped down the drill string and landed in the under reamer which activates the reamer arms. Placement of a conventional reamer below the RSS and MWD/LWD may prevent the ball/plug from passing through the RSS and MWD/LWD tools and reaching the conventional reamer to activate it. Additionally, it is not desirable to place a conventional under reamer below the RSS and MWD tools because the conventional under reamer is too long to allow the RSS tool to steer and/or propel itself properly. 
         [0032]    Further, it is not desirable to place a conventional reamer below the RSS and LWD tools and ream as it is being drilled (to eliminate the creation of a rat hole) because the RSS and MWD/LWD tool strings need to be in contact with the wellbore walls in order to function. The RSS needs to contact the wellbore wall to direct the steering and it is desirable for the MWD/LWD tools to have the sensor elements of the tools in proximity to the borehole wall in order to obtain better quality formations data. 
         [0033]    Additionally, it is not feasible to use a larger gage bit on the bottom to drill an oversized hole (to eliminate the creation of a rat hole) because the RSS and MWD/LWD tool strings need to be in contact with the wellbore walls in order to function. The RSS needs to contact the wellbore wall to direct the steering and it is desirable for the MWD/LWD tools to have the sensor elements of the tools in proximity to the borehole wall in order to obtain better quality formations data (to eliminate the creation of a rat hole) because the RSS and MWD/LWD tool strings need to be in contact with the wellbore walls in order to function. The RSS needs to contact the wellbore wall to direct the steering and it is desirable for the MWD/LWD tools to have the sensor elements of the tools in proximity to the borehole wall in order to obtain better quality formation data. 
         [0034]    In prior art systems, in order to eliminate the rat hole, the entire drill string and tool string would have to be pulled from the wellbore and a trip would have to be made in the hole with a full gage bit or under reamer with a bull plug in the end and run to the bottom to drill/ream out the 60 to 90 foot rat hole section. This trip in and out of the wellbore with the drill string and an under reamer/larger gage bit to eliminate the rat hole costs many thousands of dollars of rig time. 
         [0035]    Referring now to  FIG. 9A , wherein is illustrated a simplified schematic of the tool string  100  and near-bit reamer  200 ,  360 ,  460 ,  760  and  860  of  FIGS. 5A ,  5 B,  6 A to  6 C,  7  and  8 .  FIG. 9A  illustrates a conventional reamer  104  with cutting arms extended and wherein the upper portion  5  of borehole  4  has been reamed out to a desired larger gage than the lower reduced diameter portion under gage rat hole portion  3 . The tool string  100  is disposed in the lower end of drill string  101 . The tool string includes a conventional under reamer  104 , an RSS section  140  and MWD section  120 , the near-bit reamer  200  and the bit  150 . As can be seen, the conventional reamer cannot reach the bottom of borehole  4  to enlarge the rat hole portion  3  of the hole because the reamer is disposed above the RSS section  140 , the MWD section  120 . 
         [0036]      FIG. 9B  illustrates the tool string  100  pulled up/back into the larger gage reamed portion of the borehole  4  and the conventional reamer&#39;s arms are closed and the near-bit reamer&#39;s cutting pads  208  are extended. (It will be understood that closing the conventional reamer&#39;s arms is optional because leaving the conventional reamer&#39;s arms open may provide stabilization for the bottom hole assembly as the near-bit borehole enlargement tool  200  is reaming down the rat hole in  FIG. 9C ). 
         [0037]      FIG. 9C  illustrates the tool string  100  after it has been rotated and moved back down the borehole  4  to enlarge the gage of a portion of the formerly reduced gage rat hole section  3 . 
         [0038]    It will be understood that the present disclosure can be implemented without pulling up the tool string  100  out of the rat hole and then lowering the near-bit reamer with extended cutters to ream the hole by moving downward into the rat hole while the reamer is being rotated. Instead, the cutters of the near-bit reamer may be extended while the near-bit reamer is in the rat hole portion of the hole and the tool string rotated and pulled upward to ream the rat hole in an upward direction. 
         [0039]    Note: The terms “raised” and “lowered” have been used herein to describe movement of the tool string; however, if the borehole  4  is deviated (e.g., horizontally, as wellbore  2  is illustrated in  FIGS. 1A and 1B ), raising the tool string  100  would be understood to mean moving the drill string away from the distal end of the borehole  4  and lowering would be understood to mean moving the tool string  100  toward the distal end of the borehole  4 . 
         [0040]    Note:  FIGS. 9A ,  9 B and  9 C are schematics that illustrate the cutters as pads on a piston; however, in other implementations the near-bit reamer may have arms with a reaming cone on each arm. Likewise, the upper conventional reamer  104  may have arms and roller cones or extendable pads. 
         [0041]    In reaming operations for the rat hole section  3  of borehole  4 , the tool string is pulled up out of the reduced diameter rat hole  3 , and in some implementations circulation of mud is increased from a first flow rate during drilling with the conventional under reamer to a second predetermined flow rate which shears pins in the NBR reamer and opens the NBR cutters  208 . The cutters  208  are extended away from the longitudinal axis  201  of the near-bit tool  200 . The drill string is rotated and lowered back into the under gage rat hole section  3  and the near-bit reamer enlarges the under gage rat hole section. See  FIGS. 9B and 9C . 
         [0042]    Advantages: 
         [0043]    The near-bit borehole enlargement tool (NBR)  200 ,  360 ,  460 ,  760  and  860  may be used in combination with a conventional under reamer (e.g., the Halliburton XR or UR reamer). When used in a tool string combination including a conventional under reamer, the “NBR” may remain dormant during reaming work performed by the conventional reamer disposed above the MWD/LWD tools and the RSS tool. The “NBR” may be activated when the total depth of a section of the wellbore is reached to ream the rat hole section. The NBR combination with the conventional reamer provides using the conventional reamer to ream long distances in the borehole due to its robust cutting structure. The NBR of the present disclosure is very compact (made much shorter than the original “NBR”) to reduce the rat hole distance at the bottom of a wellbore. The NBR tool of the present disclosure further provides: 
         [0044]    1. Easy steerability, as a function of the short length; 
         [0045]    2. Alternative helical stabilizing blades and helical mud ways instead of straight shape for:
       a. Optimized stabilization;   b. Reduction of BHA vibrations, hence increasing cutter&#39;s lifetime; and   c. Better cleaning performance.       
 
         [0049]    3. Monobloc product for:
       a. Stronger tool (possibility to eliminate the API connection between the Bit and the Reamer);   b. Cutting structure drilling performance optimization (combination of XR or UR and NBR concept);   c. Tool simplicity and parts count reduction; and   d. Stronger bit/reamer connection. Reduced loads (bending, side forces, and vibration) decrease connection failure risk.       
 
         [0054]    4. NBR structure better stabilized by means of the bit proximity to the reamer. 
         [0055]    5. NBR gets better well contact coverage (combination of the bit and the NBR might be close to 360°).
       a. Perfect pilot hole fitting;   b. Suppress kick-off risk;   c. Reduce BHA vibrations;   d. Increase cutting efficiency; and   e. Increase cutter lifetime.       
 
         [0061]    6. Eliminates rig time and saves money—in prior art systems in order to eliminate the rat hole, the entire drill string and tool string would have to be pulled from the wellbore and a trip would have to be made in the hole with a full gage bit or under reamer with a bull plug in the end and run to the bottom to drill/ream out the 60 to 90 foot rat hole section. This trip in and out of the wellbore with the drill string and an under reamer/larger gage bit to eliminate the rat hole costs many thousands of dollars of rig time.