Three years ago Petroleum Development Oman (PDO) implemented an "Underbalanced for Life" philosophy as a means to improve well and reservoir productivity. This philosophy requires not only drilling the well underbalanced but completing the well underbalanced so that only produced fluids are exposed to the reservoir.
From 2002 to 2004, the UBD campaign focused on wells and fields that had a reservoir pressure low enough to not flow to surface. This meant BHA trips and well completion operations were conducted conventionally and no special procedures and operations were required. At the end of 2004, PDO began drilling underbalanced in fields that had sufficient reservoir pressure to flow to surface.
Until recently, the only way for a drilling rig to trip pipe into and out of the well was to kill the well so that there was no surface pressure or to snub the pipe under pressure. A new alternative was identified where a down hole valve is used to lubricate pipe into and out of the well without surface pressure. To meet the philosophy of "underbalanced for life" the down hole valve was selected and new equipment, procedures and methods developed.
Early trials with the retrievable down hole valve provided sufficient success to continue drilling wells with natural flow to surface. More than 20 flowing wells have been completed underbalanced with a high rate of success.
This paper explains the well design, procedures and equipment used to implement the "Underbalanced for Life" philosophy.