For many years underbalanced drilling technology has been commercially exploited in North America whilst international Underbalanced Drilling (UBD) projects have been sporadic at best. Many technical papers have been written regarding these operations with the recurring theme that although the technical benefits of UB technology were seen, the overall cost was too prohibitive to justify further campaigns.

The many benefits of UBD are widely publicized; however, the highest impact that UBD can make is on the reservoir and its productivity. A recent focus in UB operations has been to quantify this benefit through productivity increase comparisons, or productivity improvement factors (PIFs) against field averages. The increased PIFs provide justification to offset the higher costs associated with the technology.

Petroleum Development Oman (PDO) has undertaken an extensive UBD campaign aimed at increasing production through the elimination of reservoir damage1. To fully evaluate the technology, the PIFs of a one year campaign were analysed and measured against a pre-defined business case to determine if UBD were commercially viable. The UBD campaign was carried out in two fields in PDO. Although this paper focuses on the Saih Rawl campaign, the issues for the two fields were very similar.

Although early production results were very promising2, the commerciality, in PDO's low cost, land operation environment, was marginal at best. Re-assessment of the approach to all aspects of the UBD operation by the Project Team, including scope of work, equipment design, equipment utilization and UBD operations resulted in improved operational efficiency and an innovative contracting strategy mutually beneficial to both Operator and Contractor. This new approach has shifted UBD from a trial to a commercially viable technology in PDO.


For reasons stated in an earlier paper1, Petroleum Development Oman (PDO) undertook an extensive one year campaign to evaluate the benefits of UBD. Teething problems (associated with the start up of any project) were encountered, but distinct improvements in performance from well to well were also apparent. The one year technical trial to prove underbalanced drilling as a viable technology to increase productivity was successful, with PIFs typically greater than 2 observed (Figure 2).

The prime directive for the UBD campaign was to evaluate the benefit of UBD to PDO, i.e., in the campaign phase, reducing risk and information gathering were prime drivers, not cost. When the trial campaign was evaluated, however, cost became the major concern.

Campaign Costs

There were a number of factors that affected the initial operational performance (and hence cost) besides the usual learning curve effect. Throughout the first two wells, considerable emphasis was placed on obtaining information. From Figure 1 this is evident in the time taken for production testing during the drilling of each leg and at target depth of the 2nd well of the campaign. This information, although vital, added to the cost of the campaign, distorting the ‘real’ cost of UBD. In one instance rig time was dedicated to testing the maximum gas volume fraction in drillpipe that could be achieved yet still successfully transmit data via mud pulse MWD.

All of these factors resulted in wells exceeding the programmed drilling curve, despite high on bottom rates of penetration (ROPs)2.

In addition to the extra cost of the ‘learning curve effect’ and production testing was the fact that all associated UBD start-up costs (i.e., project costs, mobilization costs and rig conversion costs) were loaded on the first wells. Rig conversion costs (associated with using crude as the drilling fluid) and the cost of training crews were significant enough to make the association of the UBD package with just one rig the preferred mode of operation.

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