Abstract

Statoil ASA’s Visund project is operated from a floating installation located in the Tampen area of the Norwegian North Sea. The Visund field has been developed through 16 subsea-completed wells over the past six years.  Recently, Statoil ASA set a world record by drilling the longest reach well from a floating installation. The well A-6HT2 (A-6) was drilled to a measured depth of 9,082 meters, 819 meters deeper than the Visund A-22 AHT2 (A-22) well. A-22 previously held the record. In addition to setting a new world record, well A-6 was finished ahead of plan, within budget and set a Visund record for the fastest spud to total depth ERD well with an average rate of penetration (ROP) of 108 meters per day.

Compared to the A-22 well, a key technology used on A-6 was a full-length, intermediate sized 5 7/8-inch drill string. Well A-22 used a conventional tapered drill string with several different drill pipe sizes. Some of the achievements noted with the intermediate sized pipe were improved running and handling speed, reduced handling tool inventory and maintenance, reduced surface pump pressure and maintenance, reduced drill string weight and a large cost benefit from improved drilling speed. Achieving this record reach has enabled Statoil ASA to hit targets from the floating installation that were previously thought unreachable, reducing the need to develop the field with less economical mobile floating rigs and offset subsea developments.

Since its introduction in 1999, the design and operational benefits of intermediate sized 5 7/8-inch drill pipe have been presented in many publishing’s. 1,2 Few have focused on specific case histories with documented performance improvements. This paper will present a case history of 5 7/8-inch drill pipe in use and how it allowed a world record to be set. A novel method for equivalent circulating density (ECD) reduction in the troublesome 8 1/2-inch hole section will also be presented.

Visund Background

Statoil acquired the Visund Field from a major oil and gas operator in January 2003. The platform is semi-submersible and is used for both production and drilling. Odfjell Drilling, the drilling contractor, performs the drilling operations. Wells are tied back to the rig by flexible risers (Figure 1).  There are also two subsea wells that have been drilled in the northern section of the reservoir that are tied back to the platform. The Visund reservoir is primarily a gas reservoir, but as is common in Norway, oil is also produced from the field using the field’s gas reserves to drive and enhance oil production.3

Since the acquisition of Visund, there have been several changes made in the way drilling operations are performed. These changes were gradual through 2003 and 2004, with Statoil implementing its own drilling methods and philosophy. The cumulative result of these changes has been the ability to reach ever further into the reserves basin with increased overall drilling speeds. These improvements led to setting the world record reach from a floating installation on well A-22 in 2003. This record has only been superseded by Visund’s own new well A-6. Setting this kind of record relies on a methodical and meticulous well design and using the correct drilling techniques. Wells A-22 and A-6 have only been made possible via the evolution of technologies and operating practices used by Statoil on previous wells.

It is the eventual aim of the Visund team to drill 12 - 14 km wells from Visund, thus enabling them to drill into the field’s furthest extremities.  Looking at Figure 2, the reach required to exploit the most northerly reserves in this field is evident. It is important to realise the significance of reaching the entire reserves basin from a single platform. This paper will examine in detail the sizeable savings to be made when these wells are drilled from an existing structure, thus removing the need for subsea templates, as well as the vast cost of renting a floating drilling vessel to drill the wells. What this paper does not examine, although also highly significant, is the additional cost that would be incurred when performing any intervention work on these wells at a later date since this would require the rental of a mobile floating unit.