Underbalanced drilling is used with increasing frequency on a worldwide basis to reduce invasive formation damage effects and drilling problems associated with many horizontal wells in challenging reservoir exploitation situations. When the primary objective of the underbalanced drilling operation is to reduce or eliminate formation damage effects, the importance of maintaining a continuous underbalanced pressure condition during the complete operation is essential in obtaining the maximum benefit with respect to formation damage reduction. The importance of this has been emphasized in previous work, but this paper details some of the specific reservoir design and operational parameters which must be considered to ensure that the underbalanced pressure condition is maintained on a continuous basis. This includes such issues as pipe connection effects, various wellbore geometries, frictional flow and back pressure effects, localized depletion effects, gravity invasion and drainage effects, countercurrent imbibition effects, hole cleaning, bit jetting, and a number of other issues which can affect the ability to maintain a continuously underbalanced condition in a given reservoir situation. Examples of these situations will be presented, along with suggestions in certain operational circumstances which can be utilized to reduce the effect of these problems.

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