The National Oil Corporation (NOC) of Libya is considering adapting new technologies to ensure that strategic resources are optimally explored, developed and produced during the life cycle of their oil and gas fields. Among the various options available to achieve these objectives, Underbalanced Drilling (UBD) technology is considered one of the most effective methods.

Zueitina Oil Company, one of Libya's NOC affiliated companies, decided to pioneer the introduction of a new way of drilling horizontal wells in the depleted Sabah Field. Accordingly, UBD technology was utilized to drill the Zueitina Sabah G-55 well in the main Sabah field.

The main objective was to drill the horizontal reservoir section to TD @ 7628 ft MD eliminating/minimizing lost circulation such that no drilling time would be lost due to the need to cure losses or to mitigate other conventional overbalanced drilling problems.

The Sabah G55 well was the first underbalanced horizontal well to be drilled in Libya. The targeted formation is a mature reservoir that has been substantially depleted to a current estimated pressure of 1050 psi. The producing formation consists of two main reservoir zones; Zone I and Zone II, separated by a low porosity tight zone layer. Zone I is made up of dolomite and Zone II of limestone with good porosity development often up to 35%. Abundant fracturing is evidenced by mud losses while drilling, FMI, dip-meter, core description, pressure transient analysis, production performance and others.

This paper discusses the objectives, the planning, the drilling technology used, the performance of the horizontal UBD well compared to offset conventionally drilled wells, problems encountered and lessons learned during the first application of Underbalanced Drilling in Zueitina Oil Company, Libya.

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