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.


At the beginning of the underbalanced drilling campaign by PDO, fields and wells were targeted where the reservoir pressure was quite low. In a static condition, the well would neither flow to surface nor create a pressure at surface below the BOP's if the well was shut in. The wells drilled over a two years period were an IADC level 1-B-4 classification. In the rare case where a well did flow to surface after drilling, the well was killed with brine as the main driver for UBD was reservoir characterisation as opposed to productivity improvement.

In 2004, a carbonate field in the North of Oman conducted an underbalanced drilling trial on four wells. One of the requirements for underbalanced drilling in this field was for productivity improvement (PI) because the current stimulation treatments were not providing consistent results. For the well engineering team to meet this requirement, a completely new set of procedures and tools were required for the operations.

In 2004 there were two options for tripping the drilling assembly out of the hole with surface pressure. The most familiar method was a rig assist snubbing unit that can be mounted on the rig floor when the bit is near surface to manage the pipe light forces of a flowing or shut in well. The second method less familiar in the industry and in the trial phase offered a more operationally friendly alternative to snubbing operations. This option was a down hole valve similar to a subsurface safety valve giving a long lubricating length inside the hole. After drilling, the drill pipe is stripped to above the valve, the valve is closed then surface pressure is bled off to allow trips to surface similar to current practices. This down hole valve added a different level of complexity to the well construction process by requiring modifications to the well head equipment for control line penetrations. It also means that when the downhole valve is closed that the reservoir pressure balances with the well bore pressure.

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