Underbalanced (UB) drilling is becoming an important tool for developing producing horizons in underpressured reservoirs, partially depleted fields, new fields, and sensitive zones.  This technique is being used to maximize producing potential in fields similar to those found in some areas of the Permian Basin .  Reservoir types and applications where UB has been used successfully will be summarized. 

A important aspect of UB success is reservoir screening.   Basic screening criteria from readily available data, as well as results of applying this technique to the Permian Basin reservoirs, will be discussed.  After choosing a good UB reservoir candidate, it is essential to predict well productivity prior to drilling for the purpose of comparative economics.     A new technique has been developed to predict skin damage using drilling data and reservoir properties as inputs.  From the pivotal estimate of skin, a production forecast can be created.  

In addition to improving well productivity by reducing formation damage, there are many significant reservoir–related uses of UB emerging.   These uses include reservoir characterization, net pay evaluation, exploration and appraisal.