Summary
Innovative geomechanical, drilling, logging, and seismic techniques have
been used to successfully develop a geologically complex discovery in the North
Sea, Tullich field. With individual reservoir-unit thicknesses below the limit
of seismic resolution, and a restricted depth window for completion, optimal
placement of horizontal production wells was vital to the successful and
economical development of the field. Wellbore instability was a key concern
after recent experiences while drilling similar formations in nearby fields.
These events had resulted in significant nonproductive time and cost overruns.
A geomechanical model was constructed from data acquired during the
appraisal-drilling phase and was used to make stability predictions for the
proposed horizontal production wells. Wellbore instability while drilling was
prevented through the application of a real-time wellbore-stability management
system that validated the stability predictions by monitoring surface and
downhole drilling parameters, produced solids, fluids, and log data. By
combining efficient teamwork and a variety of logging and drilling
technologies, the complex reservoir was successfully developed, both in terms
of cost savings during well construction and subsequent oil production that
exceeded expectation.
Introduction
The Tullich oil field is situated in Block 9/23a (Fig. 1) and operated 100%
by Kerr-McGee North Sea (U.K.) Ltd. It lies 5 km south of the Gryphon field and
came on stream in August 2002 as a subsea tieback from a central manifold to
the Gryphon “A” floating production, storage, and offloading facility.
Hamilton Brothers originally discovered the field in 1991 by drilling Wells
9/23a-27 and 9/23a-27Z, where thin oil- and gas-bearing sands were encountered
within the Eocene Balder formation. The sidetrack Well 9/23a-27Z was
successfully drillstem tested. However, development of the prospect was not
considered commercially viable at the time. Advances in seismic and drilling
technologies led Kerr-McGee to reconsider the prospect and acquire the block in
1999. In 2001 they implemented an exploration and appraisal program by drilling
two vertical Wells (9/23a-29A and 9/23a-31) and six sidetracks (9/23a-29Z
through 9/23a-29U).
Kerr-McGee recorded a 3D seismic survey in 1990, and in 1999 a 3D
ocean-bottom cable (OBC) seismic survey was acquired over the Gryphon field and
northern part of Block 9/23a. Inversion of the shear data from the OBC survey
indicated the potential for extensive development of the Lower Eocene Balder
sand, culminating in the exploration drilling program in 2001.
The reservoir is of excellent quality and is interpreted to be a complex of
turbidite sands, which lie in the Balder B2 zone directly above the massive
tuff (Balder B1 zone) shown in Fig. 2. While the individual sands are below
seismic resolution, the package could be mapped on the new seismic. An oil
column extends 200 ft between a gas cap above and an aquifer below.
The development program of four horizontal wells began in early 2002 with
the drilling of Well 9/23a-T2 and concluded in October the same year with the
completion of Well 9/23a-T3. The wells had horizontal sections between 3,852
and 5,606 ft.
This paper describes how the seismic, core, drilling, and logging data
acquired during the appraisal phase were used to evaluate wellbore-stability
hazards, and how that information was included in the successful drilling and
completion of the horizontal development wells. Details of how drilling and
well-placement difficulties in this structurally complex environment were
overcome are presented in recent publications by Greiss et al. (2003) and
McDonald and Tribe (2003).
© 2006. Society of Petroleum Engineers
View full textPDF
(
5,577 KB
)
History
- Original manuscript received:
27 January 2004
- Revised manuscript received:
1 September 2005
- Manuscript approved:
10 November 2005
- Version of record:
20 March 2006
View Cart
