обсадная колонна из труб разного диаметра(There are numerous reasons why operators run tapered or mixed string of pipe. The most common tapered string of pipe run utilizing Frank's extended range tools is 13 5⁄8" by 14" intermediate casing strings. In some instances, the majority of the string of casing is made up of 13 5⁄8" O.D. casing, and then crosses over to 14" O.D. casing for the last 60'120'. This is done to mitigate excessive wear at the well head due to the rotation of the drill string as the well is being drilled. Another configuration that is often run consists of 13 5⁄8" casing on the bottom of the string, then 1,000' to 5,000' of 14" casing spaced out over the base of the salt canopy, with the remainder of the string comprising 13 5⁄8" casing. This configuration may be necessary due to two different scenarios that occur at the base of the salt canopy that can damage casing. Kenny Gray)
