This method of drilling for oil and gas entails keeping the bore pressure lower than the reservoir pressure during the drilling process. This means that fluids from the formation will enter the well bore and flow up to the surface, then these fluids are sealed off from escaping from the well and diverted to a holding tank by a special rotating head that is attached to the top of the blow out preventer.

There are several advantages to drilling over balanced wells. The elimination of formation damage, differential sticking, reduction in loss circulation problems and with the advantage of increased penetration rates.