The history of UBD in Petroleum Development Oman stretches back to sporadic projects in the mid 1990's, however it has not been until recently that tangible gains have been realized using the technique.

Although the benefits of UBD are widely accepted within North America, internationally the technology has not been fully exploited for a number of reasons, namely the justification of increased drilling costs against perceived ‘intangible’ benefits. Such a justification is particularly hard for asset managers and well planners to make in the absence of any concrete industry data regarding production increases.

PDO embarked on a focused campaign to trial UB and evaluate its applicability as an enabling technology. A ‘Zero Cost Implementation’ approach was used to introduce the campaign, and drilling began in June 2002. Wells targeted in the Saih Rawl Field are under pressured oil wells, drilled as 5 legged producer-injector pairs off of a 7" backbone. The wells are normally completed by means of an Electric Submersible Pump.

Engineering for the UB project in Saih Rawl focused on the Asset Team's mandate of eliminating reservoir damage. Consequently, equipment was chosen and a program developed utilizing the injection of field natural gas via a concentric casing string to establish UB conditions.

As the technology was newly re-introduced to PDO and its local contractors, a stepwise approach to drilling underbalanced was adopted for the first well, and ultimately demonstrated the benefits of UBD. The first two legs of the first well were drilled conventionally overbalanced and stimulated, while the third leg was drilled overbalanced, stimulated and tested with the UB separation equipment. The remaining two legs were drilled underbalanced and tested.

The post drilling flow tests proved to be invaluable for evaluating the applicability of UBD, as the results showed a distinct increase in production from adjacent legs approximately 200 m apart in the same reservoir. This paper focuses on the design issues and the results of drilling SR 153 underbalanced.

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