ABSTRACT
The technique used worldwide while drilling is known as overbalanced drilling. Which is defined as the drilling process where the hydrostatic pressure used exceeds the formation pressure. This is done with the main purpose of “killing” the well. However, there are numerous problems that accompany overbalanced drilling. Such problems are differential pipe sticking, loss of circulation, formation damage and other problems. A relatively new technique was introduced, known as underbalanced drilling, where the well is being drilled with a hydrostatic pressure less than that of the formation. Correct and proper execution of such technique eliminates problems associated with overbalanced drilling. More precisely, underbalanced drilling solves the issues of formation damage, differential pressure sticking and loss of circulation. On the other hand, underbalanced drilling has its own problems as well. Additionally, underbalanced drilling increases the rate of penetration and provides other uses discussed in this project.  The objective of this paper was to compare between overbalanced and underbalanced drilling and ascertain which method is most suitable in all aspects. Finally, it can be concluded that UBD provides lower drilling costs, increased rate of penetration (ROP) and less hole problems when it was compared to overbalanced drilling of the same formations.