The biggest 'underbalanced drilling' exercise outside North America has been successfully implemented in the Middle East by Petroleum Development Oman (PDO) with the help of the Frontier Group of the UK.

Frontier played a key role, under the auspices of the Shell UBD Global Implementation Team, in planning and implementing UBD operations in two of Oman's maturing oil fields, Nimr and Saih Rawl.

The UBD implementation project was described by PDO as the biggest of its type outside North America.

"UBD is one of the fastest-emerging technologies in the drilling industry and is capable of delivering substantial increases in initial well productivity and ultimate hydrocarbon recovery," explained Mark Clarke, chairman of Frontier.

"UBD differs from conventional drilling because the pressure in the wellbore is lower than the pressure in the reservoir being drilled. This avoids damage to the reservoir and results in hydrocarbon production while drilling," he added.

PDO was aware of the potential benefits of UBD and had, in fact, drilled some wells using this technique as early as 1992. However, single well trials of UBD in the past had been very expensive in terms of moving equipment to the Arabian Peninsula and the benefits had proved inconclusive. Therefore, the technique was not adopted for ongoing drilling operations.

PDO intends to continue applying UBD as their standard drilling method in Nimr and Saih Rawl and several other fields. Current plans are for UBD activity with PDO to increase from two to four rigs drilling UBD wells this year.

According to Clarke, the campaign was supported by Shell International through its regional office for the implementation of UBD technology, headed by Ian Davidson.

Nimr is a sandstone reservoir with high viscosity oil. Fourteen wells were drilled to evaluate the benefits of underbalanced drilling in improving productivity in this reservoir. These wells were drilled horizontally with crude oil and nitrogen as the drilling fluid.

Saih Rawl is a carbonate oil reservoir and four six-leg multilateral wells were drilled to provide an evaluation of productivity improvement. These wells were drilled horizontally with crude oil and natural gas as the drilling fluid.

According to Dr Paul Francis, Well Engineering Projects team leader with PDO: "From the initial results we are seeing, I am sure UBD will have a profound effect on the way we drill reservoirs in the future."