1. n. [Drilling]

A string of drillpipe or casing that consists of two or more sizes or weights. In most tapered strings, a larger diameter pipe or casing is placed at the top of the wellbore and the smaller size at the bottom. Note that since the pipe is put into the well bottom first, the smaller pipe is run into the hole first, followed by the larger diameter. Other than the different sizes, which are usually chosen to optimize well economics, there is nothing distinctive about the pipe sections. However, pipe-handling tools must be available for each pipe size, not just one size, as is the typical case.

Synonyms: combination string

2. n. [Well Completions]

A string comprising tubing or components of varying size or dimension. A tapered production string may be configured with larger OD tubing joints in the upper wellbore area to optimize the hydraulic performance of the string. Although a tapered coiled tubing string will have the same tubing outside diameter throughout, the upper portion of the string may have a greater wall thickness to support the load of the string below.

3. n. [Well Workover and Intervention]

A tubing string or work string that is made up from tubulars of different outside diameters (OD). In production applications, this may be used to improve the flow and production characteristics of a well. In drilling applications, a tapered string may be used to enable a small hole section to be drilled without changing the entire string. In coiled tubing operations, tapered strings are configured with a constant OD but with varying wall thickness.