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

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
   The invention relates generally to multilateral well operations. More particularly, the invention relates to a method and apparatus for accessing a branch of a multilateral well. 
   A multilateral well, also known as a multi-branch well, is a well having one or more lateral boreholes branching off a single primary wellbore. The primary wellbore may be vertical, horizontal, or deviated. The lateral boreholes may branch off the primary wellbore in any number of directions to allow production from several target reservoirs or formations through the primary wellbore. Multilateral wells are advantageous in comparison to single wells in that their lateral boreholes can be brought into close contact with several target reservoirs, thereby allowing production from the reservoirs to be maximized. 
   Tubulars are often installed in lateral boreholes. For example, in unconsolidated or weakly consolidated formations, liners are often installed in lateral boreholes to prevent the boreholes from collapsing. After such installation, it is often desirable to re-enter the tubular in order to perform one or more operations in the lateral borehole. Such re-entry operations generally include inserting a downhole tool into the tubular. In some cases, there may be eccentricity between the tubular and the lateral borehole, for example, due to formation washout. In this case, there is the likelihood that a downhole tool inserted into the lateral borehole would be misaligned with the tubular and may not be able to enter the tubular or may even become stuck in between the tubular and the lateral borehole. 
   From the foregoing, a method of assuring entry of a downhole tool into a tubular in a lateral borehole would be useful. 
   SUMMARY OF THE INVENTION 
   In one aspect, the invention relates to a method of entering a previously-installed tubular in a lateral borehole from a primary borehole connected to the lateral borehole. The method comprises passing a downhole tool from the primary borehole into the lateral borehole, actuating an orientation mule shoe coupled to the downhole tool to lift a nose portion of the downhole tool into alignment with the tubular in the lateral borehole, and inserting the nose portion of the downhole tool into the tubular. In one embodiment, actuating the orientation mule shoe comprises bringing a leading face of the orientation mule shoe in contact with an opposing face of the tubular. In one embodiment, actuating the orientation mule shoe comprises rotating the orientation mule shoe along a helical path. 
   In another aspect, the invention relates to a downhole tool for entering a previously-installed tubular in a lateral borehole of a multilateral well. The downhole tool comprises a downhole tool body sized for insertion into the tubular and an orientation mule shoe coupled to a nose portion of the downhole tool body and rotatable to lift the nose portion into alignment with the tubular. In one embodiment, the orientation mule shoe is inserted in a sleeve having an internal helical groove. In one embodiment, the orientation mule shoe includes a pin which slidably engages the helical groove. In one embodiment, a leading face of the orientation mule shoe includes a flat portion and a tapered or arcuate portion for achieving different positions of the orientation mule shoe with respect to the tubular. 
   Other features and advantages of the invention will be apparent from the following description and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, described below, illustrate typical embodiments of the invention and are not to be considered limiting of the scope of the invention, for the invention may admit to other equally effective embodiments. The figures are not necessarily to scale, and certain features and certain view of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness. 
       FIG. 1  is a schematic of a multilateral well in which a tubular previously installed in a lateral borehole is now eccentric with the lateral borehole. 
       FIG. 2A  is an example of a downhole tool for re-entering a tubular that is eccentric with a lateral borehole. 
       FIG. 2B  shows the downhole tool of  FIG. 2A  aligned for entry with an eccentric tubular. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The invention will now be described in detail with reference to a few preferred embodiments, as illustrated in the accompanying drawings. In describing the preferred embodiments, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps have not been described in detail so as not to unnecessarily obscure the invention. In addition, like or identical reference numerals are used to identify common or similar elements. 
     FIG. 1  illustrates an example of a multilateral well  100  in which the method and apparatus of the invention may be employed. The multilateral well  100  includes a primary borehole  102  and a lateral borehole  104  branching off the primary borehole  102 . The multilateral well  100  may have one or more lateral boreholes. Casing  106  may be installed in the primary borehole  102  and may include a window  107  through which the lateral borehole  104  can be accessed, in a manner well known in the art. The lateral borehole may be an open or a cased hole. A tubular  108  has been installed in the lateral borehole  104 , in a manner well known in the art. For example, the tubular  108  may be a liner, such as a slotted or perforated liner, installed in the lateral borehole  104  to prevent the lateral borehole  108  from collapsing. The tubular  108  includes a bore (not shown) for receiving a downhole tool  110  at the end of a tool string  112 . In the illustrated example, the tubular  108  is eccentric with the lateral borehole  104 . This may be due, for example, to formation washout at  114 . In accordance with the invention, a downhole tool is provided with an aligning mechanism to facilitate entry of the downhole tool into an eccentric. In general, the aligning mechanism lifts the nose of the downhole tool into alignment with the tubular, thereby facilitating entry of the downhole tool into the tubular. 
     FIG. 2A  shows an example of a downhole tool  200  having an elongated downhole tool body  202  with a rear portion  201  and nose portion  203 . The rear portion  201  may or may not be formed contiguous with the nose portion  203 . The downhole tool  200  is adapted to facilitate entry into a tubular in a lateral borehole when the tubular is eccentric with the lateral borehole, more specifically when the nose portion  203  is not aligned for entry into the tubular. The tool body  202  is sized for insertion into the target tubular. In one example, the nose portion  203  of the tool body  202  includes a tubular sealing element  204  for sealing engagement with a sealing surface, such as a sealing bore of a tubular. The rear portion  201  may also include additional sealing elements  205  for sealing engagement with a sealing surface. Any suitable means of conveying the downhole tool  200  into the lateral borehole, such as a string of pipes or wireline, may be coupled to the rear portion  201  of the downhole tool  200 . 
   The downhole tool  200  includes a self-aligning mechanism  208  coupled to the nose portion  203 . The self-aligning mechanism  208  includes an orientation mule shoe assembly  210 . The orientation mule shoe assembly  210  includes an orientation mule shoe  212  having a generally cylindrical shape. The mule shoe  212  may also be hollow. The leading face  214  of the mule shoe  212  has a flat portion  216  and a tapered (or arcuate) portion  218 . A pin  220  is provided on the body of the mule shoe  212 . The mule shoe  212  is inserted in a sleeve  222 . The inner wall  224  of the sleeve  222  is provided with a helical groove  226 . The pin  220  on the body of the mule shoe  212  engages the helical groove  226  and can slide in the helical groove  226 . A spring member  228 , such as a compression spring, is disposed in the sleeve  222  and arranged to exert a biasing force against the mule shoe  212  such that the orientation mule shoe  212  extends from the sleeve  222 . In this example, the sleeve  222  is coupled to a terminal end of the nose portion  203 , for example, adjacent the sealing element  204 , such that the orientation mule shoe sleeve  210  becomes the leading end of the downhole tool  200 . 
   To actuate the self-aligning mechanism  208 , the leading face  214  of the mule shoe  212  is brought into contact with the opposing face of the tubular  108  and an end load is applied to the downhole tool  200  at a level sufficient to overcome the biasing force of the compression spring  228 . Once the spring force is overcome, the pin  220  slides in the helical groove  226 , causing the mule shoe  212  to rotate along a helical path and deflect off the face of the tubular  108 . With the spring force overcome, the mule shoe  212  retracts into the sleeve  222  as it rotates along the helical path. Rotation of the mule shoe  212  allows for several different positions of the mule shoe  212  relative to the face  108   a  of the tubular  108  to facilitate entry of the downhole tool  200  into the tubular  108 , as shown in  FIG. 2B . In an alternate embodiment, instead of compressing the spring  228  mechanically by application of an end load to the downhole tool  200 , the spring  228  could be compressed hydraulically. For example, a piston (not shown) could be coupled to the mule shoe  212 . When pressure differential across the piston overcomes the force of the compression spring  228 , the mule shoe  212  would be allowed to rotate along a helical path. Pressure build-up to drive the piston could be achieved using a variety of methods, including, but not necessarily limited to, ball drop and choke/flow restriction. In an alternate embodiment, hydraulic activation can be used to re-orient the mule shoe  212  prior to applying an end load to the downhole tool  200 . The mule shoe  212  could also be re-oriented manually by rotating the downhole tool  200 . For example, the mule shoe  212  may be re-oriented so that the flat surface  216  is in contact with the opposing face  108   a  of the tubular  108 , as shown in  FIG. 2A , prior to applying the end load to the downhole tool  200 . 
   In practice, the downhole tool  200  ( FIG. 2A ) is lowered into the primary borehole  102  ( FIG. 1 ) using any suitable means, such as a string of pipes or wireline. The downhole tool is then passed into the lateral borehole  104  ( FIG. 1 ). Any suitable method known in the art for locating the lateral borehole and orienting the downhole tool so that it can pass into the lateral borehole from the primary wellbore can be used. Once the downhole tool is in the lateral borehole, the orientation mule shoe  212  ( FIG. 2A ) can be actuated to align the nose portion of the downhole tool for entry into the tubular. To align the nose portion for entry, the orientation mule shoe is brought into contact with the tubular. The orientation mule shoe may be re-oriented if necessary so that the flat surface of the orientation mule shoe contacts the opposing face of the tubular when the orientation mule shoe is brought into contact with the tubular, as shown in  FIG. 2A . The orientation mule shoe is then rotated along a helical path as described above to align the nose portion of the downhole tool with the tubular, thereby allowing the nose portion to be inserted into the tubular. 
   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.

Summary:
A method of entering a previously-installed tubular in a lateral borehole from a primary borehole connected to the lateral borehole includes passing a downhole tool from the primary borehole into the lateral borehole, actuating an orientation mule shoe coupled to the downhole tool to lift the nose portion of the downhole tool into alignment with the tubular in the lateral borehole, and inserting the nose portion of the downhole tool into the tubular.