In underbalanced drilling operations, the pressure of the drilling fluid in the borehole is intentionally maintained below the formation pore fluid pressure, in the openhole section of the well. This massive manual describes the different techniques that can be used to generate underbalanced conditions while drilling. The benefits of drilling underbalanced, which can be considerable in terms of reduced drilling cost and increased productivity, are discussed. However, underbalanced drilling is not suitable for all wells. Special equipment and procedures, as well as technical and economic factors limiting the application of underbalanced drilling, are examined. The manual addresses the selection of appropriate technique; well engineering (with calculation examples in the appendixes); and special considerations, such as safety, regulatory requirements, environmental issues, and formation evaluation. Twelve case studies are included. The manual is also available in CD-ROM format (see GRI-97/0236.1 and .2).