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.


In early 2000 SIEP (Shell International Exploration and Production B.V.) identified UBD as one of four key technologies to be taken up within the Shell Group on a global basis. Global Implementation Teams were formed to assist Group Operating Companies to prepare implementation plans that included screening exercises, candidate selection, business case development, and execution. Several of PDO's assets were considered good candidates for UBD and following a ranking exercise; two fields in Nimr and Saih Rawl were selected. A Low Risk/High Reward strategy1 resulted in a ten well UBD campaign in the Nimr A field (Figure 4). The objective of the campaign (similar to that in the Saih Rawl campaign being executed concurrently2) was to quantify the value of UBD for PDO.

Nimr Project - Scene Setting

Nimr is actually a complex of six fields. UBD was implemented in the Nimr A field which is a large "turtle-back" structure primarily composed of two reservoirs beneath the Nahr Umr shale cap rock:

  1. the Haima Amin (Cambrian, Aeolian dune) which is the primary reservoir; and

  2. Al Khlata (Permian, glacial) which is eroded into the Amin and is present along the faulted flanks of the field.

The Amin and Al Khlata both have reservoir permeability of >1 D in the better quality sections. The Amin contains small cement streaks within the section, which may cause local baffling. The Amin also has 10–30+ m thick sections of weathered zone, which is of lower quality, permeability and oil saturation than the "good" Amin. The Al Khlata has been shown to have a high degree of lateral variability but is generally good quality over most of Nimr A field.

(Figure 2 and Figure 3). The crest of the structure is at about 680 m SS and the original OWC is at 747 m SS.

Oil density is 0.93 sg, viscosity is 300–500 cP at 50 °C reservoir temperature. Aquifer support is moderate to strong bottom water drive. The initial pressure gradient was 10.2 kPa/m and the current reservoir pressure gradients range from 6.5 to 9.5 kPa/m. The oil produces at a very low GOR (<1m3/m3) with intermittent traces of H2S.

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