Early efforts to drill wells in a gas field north of Bali Island, a tectonicregion in Indonesia, were hampered by high costs associated with wellbore instabilities. In the recently completed late-phase development program, drilling efficiencies and costs were improved with a strategy that included a rock mechanics program, mud optimization, and batch drilling. Wellbore instabilities occurred in the Ngimbang Shale, a hard, brittle, anisotropic formation, that is highly stressed. Field measurements of stresses, coupled with laboratory measurements of strength, demonstrated the presence of elevated horizontal stresses. The computed safe mud densities for wellbore elastic stability ranged from 10 to 15 ppg, depending on location, well azimuth, and deviation. High mud densities did not necessarily alleviate instability, as indicated by stability-days, a method of tracking the stable- time periods of each well. Drilling-time and costs were also improved with the adoption of asynthetic mud system, a batch drilling schedule, and sound field engineering. Cost savings were substantial, and the wells are now in production.

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