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 rotarysteerable 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.