Pressure Prediction and Drilling Challenges in a Deepwater Subsalt Well from Offshore Nova Scotia, Canada

Abstract

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.

Worldwide Pore Pressure Prediction: Case Studies and Methods

“…In 2006, Marland et al (2006) presented a case study on pore pressure prediction in a deepwater subsalt well offshore Nova Scotia, Canada. In this case three seismic lines and six offset wells were used to perform an initial analysis of pore pressure.…”

Cementing Casing Strings across Salt Zones: An Overview of Global Best Practices

“…Offshore Nova Scotia for example, wells have been drilled through salt sections that are greater than 1500 m in thickness (Marland and Nicholas 2007). Offshore Nova Scotia for example, wells have been drilled through salt sections that are greater than 1500 m in thickness (Marland and Nicholas 2007).…”