Patent Publication Number: US-6668945-B2

Title: Method and apparatus for milling a window in a well casing or liner

Description:
TECHNICAL FIELD 
     This invention relates to methods and apparatus for milling windows in well casings or liners. 
     BACKGROUND 
     Wellbores drilled through the earth&#39;s subsurface may be vertical, deviated or horizontal. Moreover, the wells may have one or more lateral branches that extend from a parent wellbore into the surrounding formation. After a wellbore has been drilled, it is typically lined with a casing and/or another liner. The casing extends from the well surface to some distance within the wellbore. Liners on the other hand may line other portions of the wellbore. The casing or liner is typically cemented in the wellbore. 
     In some cases, it may be desirable to change the trajectory of a wellbore after a casing or liner has been installed. Also, to form a multilateral well, one or more lateral branches are drilled and completed after a casing has been installed. 
     To change the trajectory of a well or to form a lateral branch from a cased or lined wellbore, a window is formed in the casing or liner to enable drilling of the surrounding formation. Generally, the casing is cut by one or more mills that are mounted on a mandrel at the bottom of a drill string. The mills may have abrasive elements made of sintered tungsten carbide brazed to their surface. When the drill string is lowered into the wellbore, it is deflected toward the casing by a deflection tool with a slanted surface, such as a whipstock. The whipstock may be set in the wellbore either during that run or a prior run. The whipstock is placed at a location in the well where the window will be formed. 
     Typically, as shown in FIG. 1, a milling assembly  10  includes a pilot mill  18  at the end of a mandrel  16  to provide an initial cut in the casing or liner  13 . One or more spaced apart gauge mills or reaming mills  20 ,  22 ,  24  may follow the pilot mill  18 . The peripheral surface of each mill has abrasive or cutting inserts (not shown) that are made of a hard material such as sintered tungsten carbide compounds. After the initial cut made by the pilot mill  18  in the casing or liner  13 , the mills  20 ,  22 , and  24  behind the pilot mill  18  enlarge the pilot window to form a full gauge window. 
     The mills  20 ,  22 ,  24  mounted on the mandrel  16  are able to ultimately form a continuous window in the casing or liner  13 . However, because of the arrangement of spaced apart mills on a conventional milling tool, this window is first formed in discrete zones. As shown in FIG. 2, the cuts  26 ,  28 ,  30 , and  32  formed by the mills  18 ,  20 ,  22 ,  24  at one point are discontinuous and will remain so until the milling process is near completion. That is, each mill  18 ,  20 ,  22 , and  24  enlarges a discrete opening  26 ,  28 ,  30 , and  32  in the casing  13  that lengthens and deepens over time. These openings are lengthened and widened until they eventually become one continuous full gauge window. This process may create large cuttings when the zones begin to overlap. The large debris may be difficult to remove from the well. 
     Moreover, milling operations may require different sized mandrels and mills to mill full gauge window. For example, a casing having a first size may require the use of a mandrel having a first diameter whereas a casing having a second size may require the use of a mandrel having a second larger diameter. Alternately, the same mandrel may be utilized in both casings; however, mills may need to be exchanged for differently sized casings. 
     Thus, a need for an improved milling apparatus and method continues to exist. 
     SUMMARY 
     In general, according to one embodiment, a method of milling a window in a liner comprises arranging a plurality of milling elements substantially continuously along a rotatable mandrel and actuating the mandrel to cut a window through the liner. The window is cut substantially continuously using the milling elements to a desired size. 
     Other or alternative features will become apparent from the following description, from the drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an example conventional milling assembly. 
     FIG. 2 illustrates openings in a casing or liner that are produced by the milling assembly of FIG. 1 during a milling operation. 
     FIG. 3A illustrates an embodiment of a milling assembly according to one embodiment of the present invention. 
     FIG. 3B illustrates another embodiment of a milling assembly. 
     FIG. 4 illustrates the opening in a casing or liner made by the milling assembly of FIG.  3 A. 
     FIG. 5 illustrates a milling assembly milling a window in surrounding casing. 
     FIG. 6 is a cross-sectional view of the milling assembly of FIG.  5 . 
     FIG. 7 illustrates a portion of the milling assembly of FIG.  5 . 
     FIG. 8 is a longitudinal sectional view of a milling element channel in the milling assembly of FIG.  5 . 
     FIG. 9 illustrates a continuous milling bar in accordance with an embodiment of the invention. 
     FIG. 10 is a cross-sectional view of a milling assembly according to another embodiment in a cased wellbore. 
     FIGS. 11 and 12 are partial cross-sectional views of the milling assemblies to illustrate the use of milling elements that protrude outwardly by different radial distances. 
     FIGS. 13 and 14 are cross-sectional views of milling assemblies according to other embodiments. 
    
    
     DETAILED DESCRIPTION 
     As used in this description, positional terms such as “up,” “down,” “upwardly,” “downwardly,” “upper,” and “lower,” and “above” and “below,” and other such terms that indicate position are used to describe some embodiments of this invention. These terms are for reference only and should not be considered as limiting. 
     As shown in FIG. 3A, a milling assembly  40  according to one embodiment, which may be disposed at the end of a drill string, includes a “continuous” milling tool  42  that may be used in combination with one or more mills  48  and  50  to create a window in a surrounding casing or liner  56 . As used here, a “liner” refers to a casing, liner, or any other downhole structure (tubular or otherwise) that is insertable into a wellbore to provide a flow path to the well surface. 
     The milling assembly  40  is driven by a rotary drive located at surface or by a downhole motor (not shown). The continuous milling tool  42  includes a rotatable mandrel  44  (rotatable by the rotary drive motor) with milling elements  46  disposed thereon. The mandrel  44  is a tubular structure that has threaded connections at each end (not shown). The threaded connection at one end may provide for the attachment of the mandrel  44  to a drill string via an articulated or flexible joint. This joint allows for the deflection of the milling tool  42  off of the well casing&#39;s longitudinal axis. Typically, the mandrel  44  is made from alloyed steel, although other materials can also be used. 
     The milling elements  46  may be disposed along the length of the mandrel  44  in a generally helical or any other desired arrangement. In this embodiment the milling elements  44  generally have a rectangular face  52 . However, any other suitable shape may be utilized, such as a square, diamond, or any other geometrical shape. The embodiment illustrated in FIG. 3A has generally a left-handed double helical arrangement of milling elements  46 . In other embodiments, a single-helical or a triple-helical (or other multi-helical) arrangement may be employed. In other embodiments, other predetermined patterns of milling elements  46  may be used. 
     Thus, generally, the milling tool according to some embodiments of the invention includes a rotatable mandrel having some length, with milling elements arranged substantially continuously along substantially the entire length of the rotatable mandrel. Moreover, milling elements typically encompass substantially less than the circumference of the mandrel. This is contrasted with conventional milling assemblies, such as the one shown in FIG. 1 that have discrete mills circumferentially mounted on a rotatable mandrel. 
     The term “substantially continuously” refers to an arrangement of milling elements that enables the milling elements to continuously mill a window in a portion of the surrounding liner, as opposed to milling discrete portions of a window, with further cuttings made to the discrete portions to form the final continuous window. Thus, the substantially continuous arrangement of milling elements enables the milling tool to continuously form a window in a portion of the liner. 
     The milling elements  46  may be fixedly or removeably attached to the mandrel  44 . For example, the elements  46  may be fixedly attached by brazing the elements  46  onto the outer surface of the mandrel  44 . In another embodiment, the elements  46  may be removeably attached to the mandrel  44  by using any one of a variety of attachment mechanisms. Although the elements  46  may be redressed regardless of how they are attached to the mandrel  44 , removable elements  46  advantageously enable redressing. 
     The milling elements are also referred to as “milling inserts.” The milling inserts are adapted to be arranged on a surface of the mandrel  44  (either directly on the surface or in a slot or channel formed in the surface). Each milling insert extends less than a fall circumference of the mandrel. 
     The milling elements are arranged along a “substantial length” of the milling tool. A substantial length refers to a length that is greater than that of a mill (such as a pilot mill, gauge mill, or reaming mill) used in conventional milling tools. 
     Removable elements  46  have the additional advantage of allowing the tool  42  to be adapted to mill casings or liners of various sizes and to mill windows of various gauges and lengths. Thus, the use of removable milling elements  46  may optimize the milling assembly  40  as a function of, but not limited to, milling conditions such as casing or liner material and hardness, hardness of the surrounding formation, cement characteristics, and the speed and torque of the work string. 
     In the embodiment of FIG. 3A, a pilot mill  48  and a gauge mill  50  are placed ahead of the continuous milling tool  42 . In other words, the pilot mill  48  and gauge mill  50  are more distally arranged on the milling assembly  40  than the continuous milling tool  42 . Other embodiments of the invention may include a pilot mill only (without a gauge mill) or more than two mills. 
     In yet another embodiment, as shown in FIG. 3B, a pilot mill  48  and gauge mill  50  may be placed ahead of the continuous milling tool  42  and one or more reaming mills  51  may be mounted on the milling tool  42 . Alternatively, one or more reaming mills  51  may be placed between adjacent milling tools  42 . In the arrangement of FIG. 3B, the continuous milling tool  42  is divided into two continuous milling tool portions. In each continuous milling tool portion, the milling elements  46  are arranged substantially continuously. 
     Typically, the pilot mill  48  has a diameter that is smaller than the diameter of the gauge mill  50 , as shown in FIGS. 3A and 3B. When the pilot mill  48  is engaged with the inner wall of the liner  56 , it provides a pilot opening through the downhole structure. 
     The gauge mill  50  may or may not be gauged at the full diameter of the desired opening in the casing. The diameter of the gauge mill  50  may be selected to be substantially identical to the inner diameter of the liner to cut a full gauge window. Typically, the gauge mill  50  is placed behind the pilot mill  48  and enlarges the pilot opening to the desired diameter. 
     The pilot mill  48  and gauge mill  50  may have tungsten carbide cutting inserts (not shown) brazed or otherwise attached to their outer surface to form a cutting surface. Other materials suitable for cutting through a casing may also be utilized. In addition to cutting an opening in the liner, the pilot mill  48  and gauge mill  50  may guide and stabilize the bottom end of the milling assembly on the face of a whipstock. 
     As shown in FIG. 4, the pilot mill  48  produces a pilot opening  54  through the casing or liner  56 , while the gauge mill  50  in conjunction with the milling tool  42  produce one substantially continuous cut  58  through the casing or liner  56 . Like the pilot mill in a conventional milling assembly, the pilot mill  48  in this assembly  40  provides a first cut  54  to initiate the window. Thereafter, the gauge mill  58 , if provided, and the continuous milling tool  42  are deflected to contact the wall of the liner  56  along the length of the milling tool  42 . As a result, a continuous opening  58  is cut in the liner  56  that may form a full gauge window. Moreover, the milling is concentrated on the liner  56  and not on the cement layer and surrounding formation. Thus, the size of milling debris and other particulate material may be reduced to reduce the amount of debris that needs to be removed. 
     Referring to FIG. 5, the milling assembly  40  with the continuous milling tool  42  is positioned in a cased wellbore  60 . An annular cement layer  62  is between the casing  56  and the wellbore  60 . A deflection tool  64 , such as a whipstock, may have been set in the wellbore  60  by conventional means in either a prior run or in the same run as the milling assembly  40 . The deflection tool  64  has an elongated body  66  and a slanted surface  68  to deflect the milling assembly  40  toward the wall of the liner  56  to be cut. Thus, the positioning of the deflection tool  64  will determine where the window will be formed in the liner  56 . Generally, as the milling assembly  40  comes in contact with the deflection tool  64 , a lateral force is placed on the milling assembly  40  that pushes or deflects the milling assembly  40  toward the liner  56  wall. As a result, the milling assembly  40  engages the liner  56  wall that is opposite the force to mill the window. Note that, in an alternative embodiment, the milling assembly may be a whipstock-less milling assembly that does not need the deflection tool  64 . Examples of whipstock-less milling tools are described in U.S. Ser. No. 09/713,048, filed Nov. 15, 2000. 
     The mandrel  44  may be in one or more sections to support the pilot mill  48 , gauge mill  50 , and the plurality of milling elements  46 . For example, one section may support the pilot mill  48  and gauge mill  50  whereas another section may support the milling elements  46 . In this embodiment, the mandrel  44  has a pair of milling element channels  70  (see FIGS. 6 and 7) and fluid circulation grooves  72 . The channels  70  and grooves  72  alternate and are separated by lands  74 . The channels  70  are adapted to receive the milling elements  46  and the circulation grooves  72  allow for the flow of fluid for cooling and/or removal of milling debris. As shown in FIG. 5, the milling elements  46  disposed in the channels  70 , the lands  74 , and the grooves  72  form generally parallel helices along the mandrel  44 . 
     The upper end of the mandrel  44 , as it is oriented in the vertical wellbore  60 , may be connected to a flexible section  76  that in turn connects to the work string. Additionally, the flexible section  76  may connect, either directly or indirectly to a power source such as a positive displacement motor, turbine, a rotary drive at the surface, or mud motor. The flexible section  76  has a pivoting portion to enable the mandrel  44  and its attached mills to be deflected towards the casing or liner wall. 
     The pilot mill  48  and gauge mill  50  are generally cylindrical and have lands  78  and fluid transfer channels  80 . Abrasive or cutting elements  82  of tungsten carbide may be brazed on the surface of the lands  78 . Fluid flows through the fluid transfer channels  80  to cool the mills  48  and  50  and/or to remove milling debris. 
     Generally, in operation, as the rotating milling assembly  40  encounters the deflecting tool  64 , it is forced laterally against the wall of the liner  56 . The pilot mill  48 , at the distal end of the assembly  40 , initiates the milling operation by cutting a pilot opening in the casing  56 . The gauge mill  50  and continuous milling tool  42 , behind the pilot mill  48 , engage the pilot opening to enlarge the opening to its desired diameter and length. The deflected gauge mill  50  and continuous milling tool  42  contacts the liner  56  wall along the length of the mill  50  and the tool  42 . Thus, one uninterrupted (or continuous) window is formed in the liner  56 . 
     FIG. 6 illustrates the cross-sectional view of one example embodiment of the milling tool  40 . The milling elements  46  are disposed within the channels  70  to provide the cutting surface of the continuous milling tool  42 . Each milling element  46  has a face  52 , a base  90 , and two sides  92 . Cutting inserts  94  are mounted on the face  52  of the milling elements  46 . The cutting inserts  94  may be brazed or otherwise embedded on the face  52  of the milling elements  46 . The cutting inserts  94  may be tungsten carbide or any other material suitable for milling a liner. 
     The sides  92  of the milling elements  46  have upper  96  and lower  98  segments that meet at about the midpoint  100  of each side  92 . The lower segment  98  slopes outwardly from the midpoint  100  to the base  90 . However, the lower segment  98  may take on any configuration that is complementary to the configuration of the milling element channels  70 . The upper segment  96  may also slope outwardly from the midpoint  100  to the face  52  of the element  46 . Alternately, the upper segments  96  may have a substantially straight wall from the midpoint  100  to the face  52  of the elements  46 . The milling element  46  is engaged in the channel  70  in a tongue and groove arrangement. 
     Once disposed within the channels  70 , individual milling elements  46  may be secured in place with a clamping element  102  such as a wedge. Generally, one side  92  of an element  46  abuts one wall  86  of the channel  70 . As a result, a gap is created between the opposite side  92  of the element  46  and the other complementary wall  86  of the channel  70 . The clamping element  102  is then positioned to fill the gap, securing the element  46  to prevent it from moving within the channel  70 . Because milling elements  46  may be positioned within the channels  70  as desired, the continuous milling tool  42  may be adapted to mill windows of various lengths. Moreover, the number of milling elements  46  per desired length may be varied. Thus, the desired number of milling elements  46  per length of mandrel  44  may be provided for a particular milling job. 
     In addition to a pair of opposed circulation grooves  72 , the mandrel  44  may also include a central bore  84  for the transport of fluid. The circulation grooves  72  may be generally U-shaped, or some variation thereof, and extend the length of the mandrel  44  in a generally helical arrangement. The circulation grooves  72  and the central bore  84  make up the drilling fluid circulation system. Thus, circulating fluid may flow through the central bore  84  to cool the milling tool  42  and/or transport the milling debris to the surface of the well. 
     The mandrel  44  also includes a pair of opposed milling element channels  70 . The channels  70  are adjacent to the circulation grooves  72  with the lands  74  between each channel  70  and groove  72 . The channels  70  also extend the length of the mandrel  44  as a helix. In this embodiment the walls  86  of the channels slope inwardly. Thus, the openings of the channels  70  narrow as they extend radially. In this embodiment, the configuration of the channels  70  and the milling elements  46  is complementary. In other embodiments, the channels  70  may take a different form to complement a differently shaped milling element  46 . 
     An enlarged view of how a series of milling elements  46  are arranged in the channel  70  is illustrated in FIG.  7 . As noted above, the milling elements  46  are secured in place by the clamping element  102 . In addition, spacers  104  are provided to control the density of the milling elements  46  in the channel  70 . 
     As shown in the longitudinal sectional view of FIG. 8, each clamping element  102  is generally L-shaped. A first portion  106  of the clamping elements  102  is disposed between one wall  86  of the channel  70  and one side  92  of the milling element  46  so that the opposite side  92  of the milling element  46  and the channel wall  86  are flush. A second portion  108  of the clamping element  102  extends the width of the channel  70  to fill in any gap between the channel  70  and the milling element  46 . 
     In another embodiment, individual milling elements  46  may be replaced by a bar  110 , as shown in FIG.  9 . In one embodiment, the bar  110  is formed of a soft iron. Like the milling elements  46 , the bar  110  has a face  112 , two sides  114  and a base  116 . The face  112  of the bar  110  includes a plurality of cutting inserts  94  brazed thereon. The cutting inserts  94  may be tungsten carbide or any other material suitable for milling a liner. The sides  114  and base  116  of the bar  110  are shaped to engage the channel  70  as described above. Thus, the bar  110  may take on a generally helical arrangement as defined by the channel  70 . One end of the bar  110  may have a receptacle  118  for a locking mechanism  120  that includes a locking pin. Therefore, the bar  110  may be inserted into a channel  70  to spiral around the mandrel  44 . Thereafter, the bar  110  may be secured within the channels  70  by positioning a pin  120  within the receptacle  118 . 
     In yet another embodiment of a milling assembly, shown in FIG. 10, a milling element  46 A is secured to a mandrel  44 A by a nut and bolt assembly  122 . In this embodiment, the mandrel  44 A includes a central bore  84 A and circulation grooves  72 A. In addition, the mandrel  44 A includes a channel  124  to receive the milling element  46 A, as well as a bolt bore  126  into which a bolt  130  can be inserted. The milling element  46 A is held in place by a nut  128  when the nut  128  is threaded onto one end of the bolt  130 . 
     The channel  124  includes a slanted surface  134  that receives the milling element  46 A. The milling element  46 A has a face  138 , two sides  140  and a base  142 . The face  138  of the milling element  46 A includes cutting inserts  94  brazed or otherwise attached thereto. 
     The bolt  130  may be any conventional bolt that has a threaded connection on one end. The nut  128  is adapted to engage the upwardly depending shoulder  146  of the milling element  46 A and a ridge  136  of the mandrel  44 A. 
     The continuous milling tools according to some embodiments are adapted to mill windows of various diameters. For example, as shown in FIG. 11, the same mandrel  44  may be adapted to have at least two different milling radii R 1  and R 2 . In this example, R 1  is smaller than R 2 . The milling radius of the milling tool  42  depends upon the size of the milling elements  46  that are disposed within the milling element channels  70 . In this example, the milling element  46  having the height H 1  is smaller than the milling element  46  having the height H 2 . Thus, when fitted with milling elements  46  of the height H 1 , the mandrel  44  will have the smaller milling radius R 1 . Additionally, when fitted with milling elements  46  of the height H 2 , the mandrel  44  will have a larger milling radius R 2 . 
     In an alternate embodiment, the milling radius may be increased by providing a shim  152  to increase the height of the elements  46 , as shown in FIG.  12 . In this embodiment, the elements  46  may all be of the same size. However, the height of a milling element  46  may be increased by positioning the shim  152  between the base  90  of the element  46  and the bottom of the channel  70 . Thus, by placement of the shim  152  the milling radius may be increased from R 1  to R 2 . 
     Referring to FIG. 13, a mandrel  44 B having a different shape (different than that of the mandrel  44  of FIG. 6) is shown. Like the mandrel  44 , two channels  70  are provided to carry two rows of milling elements  46  in a generally double-helix arrangement. 
     Alternatively, more than two channels  70  can be provided to carry more than two rows of milling elements. As shown in FIG. 14, three channels  70  are formed in a mandrel  44 C to provide a generally triple-helix arrangement (having three rows of milling elements  46  each arranged generally in a helix). 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.