First TTRD Well Drilled From a Floating Platform - Concept Study to Reality
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 98880, "World's First TTRD Well Drilled From a Floating Platform in the Njord Field, North Sea - From Concept Study to Reality," by T. Flatekvaal, SPE, Hydro E&P; E. Saeverhagen, SPE, O.E. Eng, SPE, and N. Jepson, SPE, Baker Hughes Inteq; J. Oyovwevotu, SPE, Leading Edge Advantage; and J.F. Namtvedt, SPE, and M.M. Price, SPE, Hughes Christensen, prepared for the 2006 IADC/SPE Drilling Conference, Miami, Florida, 21–23 February.
The full-length paper describes the concept study, preplanning, and drilling of the world's first through-tubing rotary (TTR) -drilled well drilled from a floating platform. The full-length paper details the challenges of maintaining the integrity of the existing subsea well completion together with the specific bottomhole-assembly (BHA) requirements to drill in this highly depleted reservoir.

Introduction
The Hydro-operated Njord field is approximately 130 km northwest of Kristiansund and 30 km west of the Draugen field in 350 m of water in the North Sea, and is being developed from a floating steel platform (Njord A). Production began in 1997, but the field already is mature with a rapidly declining oil production. Oil production currently is approximately 35,000 B/D from eight oil producers; associated gas is injected into four gas injectors. Infill drilling is the most important (promising) measure to increase oil recovery. The reservoir is very complex with numerous faults and unconnected reservoir segments.
In May 2005, Well 6407/7-A-9 AH was drilled as a test well and technical-qualification well for a possible TTR-drilling (TTRD) campaign in Njord. Well A-9 AH proved that the concept of sidetracking from existing wells without retrieving the existing completion and horizontal Christmas tree was feasible from a floating platform with conventional rig equipment.

Feasibility Study
The Njord petroleum technology department established a task force to evaluate all wells for possible sidetrack candidates. Evaluation showed that drilling ordinary sidetrack wells would not be economical because abandonment of existing producing wells would leave behind unproduced reserves, and the cost of retrieving the existing completion string including the horizontal Christmas tree would be too high to be cost-effective because of the marginal additional reserves in each new target.
Compared to TTRD operations from fixed platforms, the following technical challenges had to be resolved to drill subsea TTRD wells from the floating Njord platformDamage to the existing completion string, including the horizontal Christmas tree, the seal areas for the tubing-hanger barrier plug, and the internal tree cap.The influence of rig dynamics, including heave, pitch, and roll on wear on the existing well completion.Damage to the tubing downhole safety valve (DHSV) at 850 m.Effect of rig heave on swab and surge in wells with small inside-diameter (ID)/outside-diameter tolerances and on depletion-induced weak formations.Required annular velocity in the marine riser to avoid critical accumulation of cuttings in the 19-in.-ID marine riser and subsea blowout preventer (BOP).Mud rheology effects resulting from mud cooling in the 360-m marine riser.Well-control issues because it was impractical to trip into the well through the subsea-BOP annular preventer.Increased focus on early kick detection because of the marginal well volume underneath the subsea-BOP stack.