Underbalanced drilling: Praises and perils

Underbalanced drilling (UBD) has been used with increasing frequency to min imize problems associated with invasive formation damage, which often great ly reduce the productivity of oil and gas reservoirs, particularly in openh ole horizontal well applications. UBD, when properly designed and executed, minimizes or eliminates problems associated with the invasion of particula te matter into the formation as well as a multitude of other problems such as adverse clay reactions, phase trapping, precipitation, and emulsificatio n, which can be caused by the invasion of incompatible mud filtrates in an overbalanced condition. In many UBD operations, additional benefits are see n because of a reduction in drilling time, greater rates of penetration, in creased bit hfe, a rapid indication of productive reservoir zones, and the potential for dynamic flow testing while drilling. UBD is not a solution for all formation damage problems. Damage caused by p oorly designed and/or executed UBD programs can rival or even greatly excee d that which may occur with a well-designed conventional overbalanced drill ing program. Potential downsides and damage mechanisms associated with UBD will be discussed. These include the following. 1. Increased cost and safety concerns. 2. Difficulty in maintaining a continuously underbalanced condition. 3. Spontaneous imbibition and countercurrent imbibition effects. 4. Glazing, mashing, and mechanically induced wellbore damage. 5. Macroporosity gravity-induced invasion. 6. Difficulty of application in zones of extreme pressure and permeability. 7. Political/career risk associated with championing a new and potentially risky technology. We discuss reservoir parameters required to design an eff ective underbalanced or overbalanced drilling program, laboratory screening procedures to ascertain the effectiveness of UBD in a specific application and review the types of reservoirs that often present good applications fo r UBD technology.