Review of Casing Design Practices for Abnormal Pressure Zones in the Deepwater Pre-Salt Sequences of OPL 321, Offshore Anambra State

Pre-salt deepwater exploration in Nigerian waters presents unique casing design challenges associated with overpressure transition zones, narrow drilling margins, and complex lithological sequences that differ markedly from the well-understood clastic sequences of the proven Niger Delta play. This study reviews casing design practices applicable to abnormal pressure zones expected in the deepwater pre-salt sequences of OPL 321, located in waters offshore the South East geopolitical zone. A review of pore pressure and fracture gradient prediction methodologies applicable to carbonate and evaporite sequences was conducted, drawing on analogues from the deepwater offshore Brazil Santos Basin and the Angolan Kwanza Basin. Equivalent circulating density management windows derived from overburden stress and minimum horizontal stress models were computed as a function of assumed formation temperature gradients. Three casing string design scenarios were evaluated: a conventional four-string design, a five-string design with dedicated salt casing, and a liner design strategy with full returns to surface. The five-string design with dedicated salt casing provides the widest operational safety margin across all evaluated pressure scenarios, albeit at the highest well cost. Risk-based casing seat selection criteria are presented, incorporating uncertainty ranges in pore pressure prediction. Recommendations for data acquisition priorities in an exploration well programme are provided to reduce pore pressure uncertainty. Keywords: casing design, abnormal pressure, pre-salt, deepwater, OPL 321.