Underbalanced (UBD) operations are being utilized with increasing frequency as a means of reducing invasive formation damage, minimizing problems with differential sticking, lost circulation, and increasing rates of penetration in many drilling operations. Additional to the operational benefits, the underbalanced drilling technology is nowadays utilized as a tool formation characterization associated with the reservoir fluids.

Tethys Oil Oman Ltd. has successfully drilled the first underbalanced well, Jebel Aswad 1 in Jebel Aswad field in Block 15. Two lateral re-entry wells were drilled, STK1 & STK2 into Shuaiba and Natih reservoir respectively, using Underbalanced drilling technique. The shuaiba reservoir is composed primarily of micritic lime mudstone, which generally has a moderate to good porosity with poor permeability. The Natih reservoir is a fractured chalky limestone with a predominantly low permeability matrix. The main objective was to drill 6-1/8" hole into the horizontal reservoir sections in underbalance condition to reduce/minimize formation damage and to evaluate production while drilling. Drilling the reservoir in underbalanced mode allowed for an early reservoir characterization.

This paper discusses the objectives, planning, drilling technology used, problems encountered and lessons learned during the first application of Underbalanced Drilling in Jebel Aswad field for Tethy Oil Oman Ltd.

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