Often times reservoir and drilling engineers are faced with the decision on how best to construct a well to exploit zones of interest while seeking to avoid drilling problems that contribute to reservoir damage or cause loss of hole. The decision to pursue either Underbalanced Drilling (UBD) or Managed Pressure Drilling (MPD) operations is based on the intent of applying the most appropriate technology for the candidate well and entails either an acceptance of influx to the surface or avoidance of influx into the wellbore. Although there are some similarities in equipment utilization, it is a mistake to equate the two technologies as being the same. The merits of each technology will be described to aid in the decision-making process.

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