In May 2002, Petroleum Development Oman (PDO) embarked on a ten well, underbalanced drilling (UBD) trial campaign in the Nimr field using crude oil as the drilling fluid and membrane generated nitrogen as the lift gas. UBD was proposed as a productivity improvement technique for the Nimr field following a low risk/high reward analysis. The Nimr field is a complex of six fields. UBD was implemented in the Nimr A field consisting of two reservoirs: the Amin and Al Khlata, which are generally high permeability (±1Darcy) sandstone reservoirs containing medium gravity (21° API) viscous (300-500 cP) crude.

Horizontal wells are generally completed with a wire-wrap screen (WWS) across the reservoir section, due to sand production history in some wells, and are produced via artificial lift methods, primarily beam pump. Even though the predominant factor affecting net oil rate performance was the rate and behavior of water cut development it was suspected that drilling-induced skin, combined with mechanical skin from the completion, was a contributing factor to recent poor results from the horizontal wells.

The paper will demonstrate the value of a multi-well campaign to avoid eliminating a good candidate reservoir due to inconclusive start-up results associated introducing a new technology. It will describe some of these early start-up challenges, the equipment modifications and changes to operating procedures that have resulted in the uptake of this game-changing technology in the Nimr field. Additionally, it will emphasize the potential value of well inflow and reservoir characterization data gathered during UBD operations. This data indicated significant opportunities to improve well performance and increase ultimate recovery resulting in a potential value far exceeding those originally envisaged prior to initiating the UBD trial.

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