Pressure Prediction and Drilling Challenges in a Deepwater Subsalt Well from Offshore Nova Scotia, Canada
- Christopher N. Marland (Halliburton) | Simon M. Nicholas (Sarawak Shell Berhad) | Wayne Cox (EnCana Corp.) | Chris Flannery (Murphy Oil Corp.) | Bruce Thistle (Nexen Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- September 2007
- Document Type
- Journal Paper
- 227 - 236
- 2007. Society of Petroleum Engineers
- 1.11 Drilling Fluids and Materials, 1.6.10 Running and Setting Casing, 2.1.7 Deepwater Completions Design, 1.14.1 Casing Design, 1.7 Pressure Management, 1.6.1 Drilling Operation Management, 1.14 Casing and Cementing, 4.1.2 Separation and Treating, 5.1.7 Seismic Processing and Interpretation, 3 Production and Well Operations, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.2.1 Wellbore integrity, 5.6.4 Drillstem/Well Testing, 1.6 Drilling Operations, 1.1 Well Planning, 4.1.5 Processing Equipment, 5.1.2 Faults and Fracture Characterisation, 1.10 Drilling Equipment, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6.2 Technical Limit Drilling, 1.1.6 Hole Openers & Under-reamers, 1.7.5 Well Control, 7.2.3 Decision-making Processes, 1.12.6 Drilling Data Management and Standards
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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.
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.
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