Summary
The search for energy self-sufficiency
With the exception of Russia, all other littoral countries of the Black
Sea have traditionally been dependent on energy imports. This
reality could soon change. Turkey has estimated that the Black Sea
holds recoverable reserves of ten billion barrels of crude oil and two
trillion cubic meters of natural gas.
As part of exploration plans to confirm the Black Sea’s reserves, an
ultra-deepwater well (one of the first in the region) was recently drilled
off the coast of Turkey. The well design specified a challenging, extreme
lean profile, whose successful execution depended on the combination
of strong, integral premium connections with high-collapse steel grades.
For these challenging conditions, the operator turned to Tenaris.
Challenges
An ultra-deepwater exploration
A drilling campaign has been launched off the Turkish coast. All of
the wells share similarly complex designs, with the wellheads set at
just over 2000 m BRT (below the rotary table) and a target depth in
excess of 3000 m BML (below the mudline) – for a total depth of
over 5000 m BRT.
Extreme lean profile
Due to the ultra-deep nature of this particular project – as well as the
need to reach the pay zone with a productive diameter – the operator
had specified an “extreme lean profile” design.
Lean profiles feature reduced clearances between the walls of the
open hole and the outer diameter of the casing string. In ultradeepwater
environments, the lean profile approach makes it possible
to reach the target depth with a productive diameter, a task very
difficult to attain with threaded and coupled strings. Additionally,
it allows for contingency strings, a highly valued alternative for such
demanding exploration wells. In practice, this meant that each of
the casing sections would be drilled with a diameter only slightly larger than that of the pipes used to case the well. With such tight
clearances, casing joints risk becoming stuck in the formation.
It was evident that the use of API steel grades would not be a good option
given the unexpected pressure and loading conditions at such depths.
Solution
End-to-end technical assistance
Tenaris was involved in every step of the project’s casing program, from
providing technical consulting support during the well design phase, to
supplying specialized OCTG and accessories, to providing running assistance
onboard the offshore rig with the most critical casing running jobs.
Streamlined coupling approach
To address the issue of extremely narrow clearances throughout the
well, integral flush connections were used for the 18", 16", 13 5/8"
and 9 5/8" casing sections, while the 7" liner was fitted with integral
semi-flush connections. The operator chose the Wedge Series 500™
line of premium connections (Wedge 511™, Wedge 513™ and
Wedge 523™) based on the individual characteristics and requirements
of each section.
At the heart of these products’ ability to meet the most demanding
lean profiles while simultaneously delivering unparalleled mechanical
strength is the unique thread design that all Wedge Series 500™ connections
share. Based on a dovetailed-wedge thread form, it creates
the largest possible contact surface area at make-up, which results in
superior compression efficiency and several times the torque strength
of most competing technologies.
High-collapse steel grade
Another key technical aspect for the successful execution of the
extreme lean profile design was the special, high-collapse steel
grade developed by Tenaris. The TN 125 HC proprietary grade was
used to manufacture the 16" through 7" casing joints run in the
well. The enhanced collapse resistance that this material allowed
was particularly important, given the exploratory nature of the
project and the envisioned downhole conditions.
Results & future outlook
Flawless Wedge™
Being able to combine an integral flush/semi-flush profile, robust
Wedge™ thread form and a high-collapse steel grade all in one product
line of premium connections afforded the operator the ability to
reliably design an ultra-deepwater well with extremely tight clearances.
For a flush and a semi-flush integral connection, Wedge Series
500™ products offer distinctively high compression and tension
efficiencies. This allowed the customer the possibility to push and
rotate casing strings as necessary.
All five casing strings were successfully run to their individual TD.
Total depth reached was 5225 m BRT. There were no rejections and
no breakouts, and the well was completed on time.
Offshore tubular services
Having a Tenaris field services specialist to assist the operator and casing
running crew onboard the offshore platform ensured that tubular
components were made up according to the company’s recommendations
for optimal performance.
Only the beginning
Should this and future exploration campaigns confirm the vast energy
resources that are believed to exist beneath the Black Sea, this emerging
oil & gas province might one day match the production rates currently
seen in established major markets such as Russia, the Caspian
Sea or the North Sea.
The challenging conditions posed by ultra-deepwater offshore
rigs require the support of sophisticated tubular products and
services, which in many cases make the difference between
failure and success.