| 摘要 |
An apparatus for drilling a wellbore (10) is disclosed, the apparatus comprising a lowermost pilot drill bit (16) having a diameter less than that of the diameter of wellbore (12) drilled by the apparatus, above which is a reamer, having an adjustable drill diameter from a first diameter less than that of the pilot drill to a second diameter greater than that of the pilot drill, above which is a body portion (24) having a lower diameter less than that of the pilot drill and an upper diameter greater than that of the pilot' drill but less than the second diameter of the reamer. |
