First Successful Application of Wellbore Stabilization Technology Eliminates the Need for Contingency Casing and Differential Sticking Risk While Drilling Through Carboniferous-Devonian Formations: A Case Study on an Exploratory Well in the Sub Andean Bolivian Basin

Abstract

Abstract The South Sub-Andean Bolivian Basin presents significant geological challenges. Carboniferous and Devonian sequences with anomalous pore pressure trends require significantly different wellbore designs. Fluid losses, absence of wellbore stability, low penetration rates, severe vibrations, wellbore tortuosity, and stuck pipe challenges are common non-productive events that can lead to costly side-tracking. This paper addresses the effective use of stabilizing technology to drill two formations with significantly different pressures in a single section. The integrity of the well was compromised by an unexpected fault in the Devonian at 3353 m (11,000 ft) in the 12.25-inch section that replicated Carboniferous formations. The abnormal upper zone pressure was controlled with 13.5 ppg mud. However, most fluid additives were incompatible with the wellbore and destabilized it. Excessive overbalance and wellbore instability caused fluid losses and differential sticking at 3505 m (11,500 ft), resulting in a stuck pipe event with costs and schedule consequences. The mud properties were improved by introducing novel cellulosic components and slightly reducing the drilling fluid density. The improvement enabled drilling a 975 m (3200 ft) deeper sidetrack section with zero stuck pipe incidents. Casing the troublesome section was not an option due to the limitations on the hole diameter. Consequently, a novel mud system tailored to the formation was used to stabilize the wellbore. The performance of the fluid was monitored using an HPHT filter press and permeability plugging tester at all times. No fluid losses were observed throughout the weak zone. The 12.25-inch section was drilled up to the depth of 4176 m (13,700 ft) utilizing a 13.2-ppg mud and maintaining a differential pressure of 3000 psi, avoiding a highly costly extra casing section. Two runs of wireline logs consisting of 90 static stations of 30 minutes each were conducted along 1981 m (6500 ft) of the open hole without detecting differential sticking. A 9.625-inch production casing string was run and cemented in place. The cement evaluation log obtained after 78 hours indicated good bonding and hydraulic seal, effective mud displacement, and a successful cement job. Drilling through abnormal pressure formations was exceptionally challenging due to an unexpected Carboniferous-Devonian fault. The paper describes the first successful application of fluid stabilization technology in Bolivia where 3000 psi of overbalanced pressure was managed while achieving well objectives and ensuring the successful completion of the project.