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.

What is Underbalanced Drilling?

Underbalanced drilling, in its simplest definition, refers to a condition where the net pressure exerted by the circulating drilling fluid in the annular space between the drill string and the formation is less than the effective pore pressure in the formation adjacent to the wellbore. This results in a pressure imbalance situation where the flow of oil, water, or gas (which may be contained within the pore space) is induced into the wellbore and returns to the surface along with the circulating drilling fluid. Ideally, this condition is generated in every portion of the exposed viable reservoir pay during the complete drilling operation, but in some situations, due to circumstances which will be discussed in detail later in this paper, this may not be the case.

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