Handling narrow margin and drilling problems in deepwater using geomechanics, well design, and process management

Abstract

This study presents an approach for managing narrow drilling margins (NDM) and associated challenges in deepwater environment, addressing gaps in unifying geomechanics, casing-seat design, and pressure management simultaneously. It predicts fracture pressure using geomechanical modeling and the Mogi-Coulomb criterion, reformulates pore pressure using a casing-seat equation, and estimates NM from fracture and pore pressures. Torque and buckling analyses are derived from tank agitation concept. The method was applied to ROSE 1-1 well in Kansas and validated with field data from Gulf Coast's deepwater project. At 4847 ft, the fracture gradient was 0.9010 psi/ft, approximating Excel Solver's result (0.903 psi/ft). Fracture gradient decreased with increasing inclination but increased with cohesion in both tensile and shear-based failure, and NM significantly reduced with greater water depth. Validation result showed an Annular Friction Pressure of 298.23 psi at 16,000 ft-subsea, aligning excellently with field value (300 psi). The predicted critical buckling forces for vertical (6297 lbf) and horizontal (117171.4 lbf) sections approximated Lubinski (6550 lbf) and Dawson and Paslay (116577.5 lbf) models, respectively, and the rotary torque (22623.6 lb-ft) matched industry model. Sensitivity analysis showed closure stress (82.1 %), maximum horizontal stress (6 %), and pore pressure (0.9 %) had the most effect on DM. Narrower margins—more stable with their casing shoe closer to vertical depth—were linked to higher tectonic stress, temperature, and horizontal well, while cohesive rocks yielded wider margins than rigid ones. Numerical results showed that underbalanced drilling poses higher risks of casing-string displacement than overbalanced drilling, which are preventable using the study insights.