Underbalanced drilling has been utilized with increasing frequency to minimize problems associated with invasive formation damage which often greatly reduce the productivity of oil and gas reservoirs, particularly in open hole horizontal well applications. Underbalanceddrilling, when properly designed and executed, minimizes or eliminates problems associated with the invasion of particulate matter into the formation as well as a multitude of other problems such as adverse clay reactions, phase trapping, precipitation and emulsification which can be caused by the invasion of incompatible mud filtrates in an overbalanced condition. In many underbalanceddrilling operations, additional benefits are seen due to a reduction in drilling time, greater rates of penetration, increased bit life, a rapid indication of productive reservoir zones and the potential for dynamic flow testing while drilling.

Underbalanced drilling is not a solution to all formation damage problems. Damage due to poorly designed and/or executed underbalanced drilling programs can rival or even greatly exceed that which may occur using a well-designed conventional overbalanced drilling program. Potential downsides and damage mechanisms associated with underbalanced drilling will be discussed. These include:

  1. Increased cost and safety concerns.

  2. Difficulty in maintaining a continuously underbalanced condition (due to such factors as pipe connections in rotary drilling, bit trips, mud pulsed logging, bit jetting and flushing effects, localized depletion and subsequent repressurization effects, poor knowledge of initial reservoir pressure, multiple zones with differing initial pressures, slug flow and liquid holdup in the vertical section of the wellbore, frictional pressure drop and mechanical problems with surface equipment or gas supply sources).

  3. Spontaneous imbibition and countercurrent imbibition effects.

  4. Glazing, mashing and cuttings induced 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.

This paper discusses reservoir parameters required to design an effective underbalanced or overbalanced drilling program, laboratory screening procedures to ascertain the effectiveness of underbalanced drilling in a specific application and presents screening criteria with respect to the types of reservoirs which present good applications for underbalanced drilling technology.

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