Abstract: Underbalanced Drilling - Globally Practiced

HANNEGAN, DON, Williams Tool Co. Inc.

No one can deny that the applications of Underbalanced Drilling (UBD) technologies have reached significant levels of acceptance throughout the world. The problem solving benefits and documented reservoir production enhancements have served to proliferate the thought process of underbalanced technology, equipment and related applications. UBD has emerged as a "first thought" planning tool in many regions with complex, lower quality reservoirs or existing fields.

Underbalanced drilling is defined as drilling ahead with formation flow to the surface from a pressured reservoir; put differently, drilling ahead while the hydrostatic head pressure at the drill bit is less than pore pressure of the formation being encountered.

This methodology continues to broaden in application as technical, logistical, equipment and educational barriers are overcome. One of the latest areas to work toward utilizing underbalanced techniques is in what are considered "hostile formations" usually involving high temperature and high pressure (HT-HP). In these reservoirs there is often a relatively narrow margin between the fracture pressure and pore pressure gradients which requires a more complex planning process that may include a cross-disciplinary team of petroleum engineers as well as earth scientists.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California