Summary
Modern-day oil exploration pushes operators into harsher and more-difficult
drilling environments in the search for hydrocarbon reserves. Eastern Canada is
one of those environments where deep water and the need to penetrate through
thick salt sheets greatly increases difficulties faced by drillers. This paper
describes a case history of deepwater subsalt drilling and examines the
requirements for success. This paper also details the challenges using seismic
data, of prewell planning for dealing with high pore-pressures and variable
fracture gradients. Experience shows that prewell engineering differs
considerably from conditions actually encountered that require rapid
adjustments based on actual well data. In the case reported here, a fluid
influx occurred at a depth where planning indicated a significantly lower
pore-pressure. This influx directly led to losing a bottomhole assembly (BHA),
sidetracking, and a re-evaluation of the well data and pore-pressure regimes
possible at that depth.
This paper also highlights the need for flexible well designs able to
respond to unanticipated drilling hazards and wellbore problems. In the case
history reported here, the 11¾-in. casing was set 511 m higher than originally
planned because of pore-pressure increases. This decision had a significant
effect later in the well construction program, requiring the use of expandable
casing not originally in the well program.
This paper illustrates on-the-fly modification of drilling designs to
rapidly deploy unplanned equipment, the use of unconventional borehole sizes,
and the use of newer technology such as rotary-steerable assemblies for
side-track kickoff. The paper will also discuss the optimized use of hole
openers and expandable casing and the potential effects of expandable casing on
subsequent hole-opener use. These dynamic modifications and immediate
implementation of lessons learned allowed successful drilling to a record depth
for eastern Canada.
Introduction
The Weymouth A-45 well is located approximately 160 miles south by southeast
from Halifax, Nova Scotia, Canada, in the deepwater area of the Scotian Shelf.
Prewell seismic analysis identified the presence of a subsalt anomaly, the
1507-m-thick Argo salt sequence. This sequence presented a potential major
drilling challenge and also a significant problem in prewell pore-pressure and
fracture gradient planning. The well incorporated a complex well design using
concentric hole openers and an unconventional casing designed to successfully
complete the well. The well design was planned in response to the challenges
posed by the 1685-m deepwater environment and the thick salt deposit, as well
as their combined effect on overburden and fracture pressure. The impact of
these factors combined with limited drilling-pressure margins (based on
expected pore-pressure increases) required a more complex borehole and casing
design. Despite the potential hazards and complex well design, the drilling
program allowed for flexibility in the decision processes and in well design
changes not only to deal with problems encountered, but also, to extend
drilling successes. Flexibility was particularly important when drilling
through the salt body with a point-the-bit rotary-steerable system. Although
the rotary-steerable system was not planned for use below the salt, the success
of the system in the shallower parts of the well led to its subsequent use
below the salt and highlighted the flexibility of rotary-steerable
technology.
Planning and Design
Three seismic lines and six offset wells were provided to perform an initial
analysis of the Weymouth Prospect in August 2002. These data were processed
using the Sperry Drilling Services formation-pressure estimation model to
generate overburden, pore-pressure, and fracture-pressure predictions.
The six offset wells (H-100 Shubenacadie, H-98 Evangeline, H-38 Glenelg,
J-48 Glenelg, N-49 Glenelg, and M-41 Tantallon) showed two different
pore-pressure regimes unrelated to water depth. H-100 and M-41 were both
deepwater wells.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
20 December 2005
- Revised manuscript received:
13 February 2007
- Manuscript approved:
26 April 2007
- Version of record:
20 September 2007
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