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.
Underbalanced drilling (UBD) is defined as drilling operations where the borehole pressure is intentionally maintained to be lower than formation pore pressure. The advantages of UBD include reduced formation damage, minimized loss of circulation, and increased rate of penetration. The reduced formation damage is attributed to the UBD pressure differential between the reservoir pressure and borehole pressure during drilling. This pressure differential reduces fluid filtration into the reservoir. It is believed that the higher the pressure differential, the less the formation damage, and thus the higher the well productivity. However, the pressure differential is also responsible for some drilling complications such as borehole collapse and excess formation fluid influx. It is highly desirable to have a guideline in selection of an optimum pressure differential to reach a balance between reducing formation damage and reducing drilling problems.
UBD is usually carried out using light drilling fluids such as oil, foam and aerated liquids. Selection of drilling fluids depends on the required underbalance pressure differential. The pressure differential is designed considering the following facts:
The pressure differential should be low enough to ensure that the entire section of open hole will not collapse during drilling.
The pressure differential should be low enough to ensure that separators and storage facilities can handle the formation fluid influx rate and total fluid volume.
The pressure differential should be high enough to counter the capillary pressure force that is responsible for liquid imbibition into the pay zone causing formation damage.