Summary
The densely-faulted Njord reservoir in the Norwegian North Sea is considered
one of the most complex reservoirs in the world. The field is developed from a
semi-submersible platform with 15 subsea-completed wells drilled in a
pre-drilling campaign in 1996 to 1997 and two major platform drilling
campaigns, one in 1997 through 2000 and the other in 2002 to 2003. Drilling of
two conventional sidetracked oil producers in the last campaign was challenging
and costly. As the field matures, the need for a cheaper way of drilling
sparsely located smaller undrained compartments became essential. This led to
initiate an ambitious campaign called the low-cost infill targets (LIFT) for
identifying and drilling those targets using a cheaper drilling technique
called the through tubing rotary drilling (TTRD).
TTRD is a hugely demanding task especially, from a floating platform as any
economic rationale will be lost if completion accessories and well integrity
are compromised through TTRD. To the best of our knowledge, no TTRD operations
have previously been executed from a floater. The severity of depletion,
especially with depletion and re-pressurization (Huff’n Puff) of parts of the
reservoir provides a significant technical test and challenge for TTRD on
Njord. The relative movement of the floater also presents extra operational
challenges, which requires accurate measures to prevent damage to the tubing
hanger, Christmas tree (XMT), downhole-safety valve, and existing completion
string. Issues related to bottomhole assembly design to meet drilling and
production needs, mud rheology, equivalent circulating density (ECD)
management, rock mechanics, and completion techniques are critically analyzed
and risk-reducing or eliminating measures are put in place through extensive
research and development for each of the prospective targets.
This paper is intended to give a comprehensive description on the technological
challenges of the TTRD technology from a floating platform, research and
development activities to qualify the technology on Njord, screening of
drilling targets and the drilling experiences from two TTRD wells on Njord.
Introduction
The Njord Field is located in blocks 6407/7 and 10 in the Haltenbanken area
of the Norwegian Continental Shelf approximately 130 km northwest of the
operations base in Kristiansund. The field was discovered in late 1985 and went
on production on 30 September 1997. Considering deep water (330 m) and limited
area distribution of the reserves (6 km in diameter), the Njord Field was
developed by a semi-submersible platform with production, drilling, and living
quarters (PDQ) located directly above the subsea completed wells. The
subsea-completed wells are connected to the platform via flexible risers. The
produced oil is stored in a floating storage and offloading unit 2.5 km away
from the production platform (Fig. 1).
The commercial reservoir comprises the Lower Jurassic Tilje and Middle
Jurassic Ile Formations in the three main areas in block 6407/7 namely, the
East Flank and the Central- and Northern Areas (Fig. 2). However, the Tilje
Formations constitute the main reservoirs with 89% of the total in-place oil
volumes. The current in-place oil estimate for the Tilje reservoirs is 108.4
MSm3. A total of 17.9 MSm3 of oil has been produced by January 2005, which
constitutes an overall oil-recovery factor of only 16.5% for this formation.
The reasons for this kind of low-recovery factor are mainly two fold: depletion
drive is the preferred production mechanism for the Central- and the Northern
Areas, and the reservoir is heavily faulted leaving some of the fault
compartments undepleted. Because of this low recovery factor, the need for
improving the overall recovery factor is paramount.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
6 February 2006
- Meeting paper published:
1 May 2006
- Revised manuscript received:
15 May 2007
- Manuscript approved:
7 June 2007
- Version of record:
20 December 2007
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