Underbalanced drilling technology.

Underbalanced drilling is one of the quickly developing technologies in the world. It enables to lower the development cost by increasing the oil production per well and by avoiding the possibility of drilling hazards such as lost circulation, differential sticking. Further, production impairment reduction is significant in underbalanced drilling because no fluid invasion occurs. This paper introduces the outline of underbalanced drilling in its history, advantages and disadvantages of the technologies, various fluid systems used with, etc.