Underbalanced drilling (UBD) has been widely used as a means of reducing formation damage during drilling. The benefit of reduced formation damage results from the fact that the borehole pressure is intentionally maintained to be lower than formation pore pressure by a certain pressure differential during drilling the pay zone. It is generally believed that the higher the pressure differential, the less the formation damage, and thus the higher the well productivity. However, high-pressure differentials can cause problems in drilling operations such as wellbore damage due to borehole collapse and excess formation fluid influx to be handled at surface. Under given geological conditions it is highly desirable to find and use an optimum pressure differential to reach a balance between formation damage and wellbore damage.

The objective of this study was to develop a guideline to selection of the optimum UBD pressure differential. Borehole stability, formation fluid influx, and formation damage due to capillary-pressure-driven water imbibition have been considered in this study. The later was quantified on the basis of analysis of water imbibition into water-wet reservoirs. Result of analysis indicates that there exists a critical UBD pressure differential above which the formation damage due to imbibition of liquid from borehole is expected to be insignificant. The magnitude of the critical UBD pressure differential can be estimated using imbibition capillary pressure curve. A procedure for determining the critical UBD pressure differential is illustrated in this paper. This study provides petroleum engineers a guideline to the selection of optimum pressure differential for designing UBD operations.

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