A GEOMECHANICAL APPROACH TO WELLBORE STABILITY IN THE SOUTH SUMATRA BASIN

Abstract

Wellbore instability continues to pose a major operational challenge in the South Sumatra Basin, largely due to the limited availability of integrated field-scale geomechanical modeling. This study develops a comprehensive one-dimensional (1D) geomechanical model and performs wellbore stability analysis for deviated wells in the S Field to support pre-drilling design and establish safe mud-weight requirements across varying well inclinations. Multiple data sources from nearby wells were integrated, including wireline logs, formation integrity and leak-off test results, pressure measurements, rock mechanical data, drilling reports, gamma-ray logs, and mud-logging records. Petrophysical interpretation, shear-wave velocity estimation, and application of the Mogi–Coulomb failure criterion were employed to define safe mud-weight windows for key stratigraphic intervals. Results indicate that in well S-9, pore pressure within the PRE-TERTIARY formation reflects an estimated pressure drop of approximately 1600 psi. The formation exhibits an unconfined compressive strength of about 3708 psi and a friction angle of 24.6°. Based on Mogi–Coulomb failure analysis, the safe mud-weight window for well S-9 ranges between 9.4 and 12.1 ppg. Model calibration using caliper data and drilling records confirmed predicted breakout behavior, improving confidence in stability predictions. Mechanical Earth Models developed for wells S-3 and S-4 were extended to wells S-8 and S-9, and sensitivity analyses were conducted to refine mud-weight recommendations and reduce shear-failure risk. The analysis further indicates that maintaining well inclination below 30° is essential for stable drilling. These findings provide practical geomechanical guidance for optimizing drilling performance in the South Sumatra Basin.