WELLBORE STABILITY ANALYSIS IN UNDERBALANCED DRILLING CONDITION IN OVERPRESSURE ZONE: CASE STUDY FROM WELLS DRILLED IN SOUTH SUMATRA BASIN

• Main Author: Christiano Baroek, Marino
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/53482
• Institution: Institut Teknologi Bandung

Overpressure was identified in the South Sumatra Basin at various depths starting at 2000 ft. There is an indication that in some fields, wells were drilled in underbalanced conditions with mud weight >1 ppg (pounds per gallon) lower than the overpressure. However, there was no evidence of well flowing or kick while drilling reported and those wells were able to be completed despite encountering numerous drilling problems. This study aims to predict the optimum lowest and safest mud weight to avoid and/or minimize borehole collapse while performing underbalanced drilling in overpressured formations in the South Sumatra Basin. Wellbore stability analysis consists of pore pressure and geomechanics analyses were conducted using the wireline log, cutting, and drilling data. Mohr Coulomb failure criterion is used to define the limit of collapse pressure using two cases with and without weak planes nearby the borehole. Pore pressure estimated using the Eaton’s method indicated an overpressure zone with the top of overpressure is between 2000-3500 ft, correlates with Gumai (Telisa) Formation and is dominated by fluvio-deltaic to marine shale sequence of Gumai (Telisa), Baturaja, Pendopo, and Talang Akar Formations. The overpressure is caused by loading mechanism. Wells were drilled in the overpressure zone in underbalanced conditions with mud weight 1-3 ppg lower than the pore pressure. Geomechanics analysis indicated that the stress regime is normal to strike-slip, where the change in stress regime is caused by a combination of rock rheology and overpressure effect. Shale dominant lithology within the overpressure zone is interpreted to cause a time delay in wellbore experiencing drilling problems in underbalanced condition. Wellbore stability analysis indicated that the maximum allowable underbalanced condition before encountering problems is 3-4 ppg difference between pore pressure and mud weight.