Technology from Europe
NORWAY / CASE STUDY

Anti-stall tool reduces destructive vibrations in simultaneous drilling, under-reaming operation

NILS REIMERS, Tomax AS

The combination of under-reamer technology with rotary steerable systems allows simultaneous drilling and under-reaming but also adds a significant challenge in terms of predicting bottomhole assembly stability. The arrangement of PDC cutters at different levels and the limited number of blades on the under-reamer increase the likelihood of vibration, as the two different levels of cutters might be working in two different layers of rock at the same time. This increased vibration raises the risk of downhole tool failures and costly round trips. In addition, the coupling of instabilities between the sets of cutters can produce an overall reduction in drilling efficiency that adds cost through reduced progress with the under-reamer.

Based on these concerns, the Norwegian operator Statoil added an anti-stall tool to the inventory for its Vigdis subsea field development, which took place from 2008 through mid-2009. The Anti-Stall Technology (AST) tool was developed by Stavanger, Norway-based Tomax AS, which was established in 2005 to address downhole vibration and stick-slip mitigation.

Technology description. The anti-stall tool consists of a mechanical-hydraulic converter placed in the lower part of the drillstring, over the under-reamer, and spaced out by a flexible sub or short collar. The concept is based on controlling drilling torque by regulating the weight on bit. Under normal, stable conditions, the unit will transfer torque and weight to the bit as a passive part of the BHA. However, if the transfer of energy to the bit is thrown out of balance, causing the bit to approach a stall condition, the quickly increasing torsion will activate the tool, causing it to contract. This contraction will offload the bit, which will then spin up to its original speed. With the bit back at full speed, the unit will gradually release accumulated torsion.

In practical terms, the tool’s momentary response/closed-loop function prevents disturbances caused by changes in formation or poor weight transfer to escalate into destructive vibrations.

Thousands of drilling hours since 2006 have demonstrated that the tool significantly reduces the risk of downhole failures while simultaneously increasing the penetration rate through improved efficiency of the cutters. The tool is applicable to a wide variety of operations ranging from offshore hole enlargement to onshore coiled-tubing well intervention and milling.

Drilling efficiency and stick-slip measured by MWD from the analogous well sections simultaneously drilled and under-reamed with and without aid of the anti-stall tool.

Statoil application. The anti-stall tool was run above the under-reamer in the deviated (70°–80°) 12¼-in. x 13½-in. tangent sections. The tool was also used for the 9½-in. reservoir laterals. For one section, in well D-2AH, the tool was left on deck to produce control data. The section drilled without the tool was compared to a nearly identical tangent section in well G-2H drilled from the same rig with the anti-stall tool.

The resultant data demonstrated that the anti-stall tool reduced the variation in bottomhole drillstring rotational velocity, or stick-slip (a proxy for vibration), measured by the MWD system by about half. Also, the interval drilled with the tool saw an increase of more than 100% in drilling efficiency compared to the interval drilled without the tool (using a common average rock compressive strength for the two intervals drilled to calculate drilling efficiency). Although drilling efficiency is usually directly related to penetration rate, in this case the penetration rate in the well interval drilled with the anti-stall tool was slightly less than predicted due to hole cleaning issues.

Subsequent applications. The result from this comparison led to continued use of the tool in Vigdis field for the duration of the development. The nominal data from the comparison was also used in implementations of the tool for other North Sea projects inside and outside Statoil.

For example, Marathon Oil used its interpretation of the results to drill and simultaneously under-ream long 8½-in. x 9½-in. laterals in Alvheim field. In this application, the BHA experienced minimal vibrations and unprecedented speeds, despite the limited rotary capacity of the fifth-generation semisubmersible used. Talisman used the tool in Gyda field to simultaneously drill and under-ream a record 13,136-ft, 12¼-in. x 13-in. extended reach section in one run.

As of spring 2011, the anti-stall tool is being used in BP’s Gunashli development in the Caspian Sea, Pioneer’s Oooguruk development on Alaska’s North Slope, Statoil’s Sleipner and Snorre fields and Dong Energy’s Trym field in the North Sea, and the new BP Skarv development in the Norwegian Sea. The tool is also being used increasingly in coiled tubing interventions. 

NILS REIMERS is the Operations Specialist for Tomax AS in Stavanger, Norway. He holds a BSc degree in mechanical engineering from the University of Stavanger and has served in various operational and management positions with Teleco, Baker Hughes and the Aker group.