The Full-length paper describes the technical basis for the design and implementation of a coiled-tubing (CT) underbalanced-drilling (UBD) campaign in the Sajaa field in Sharjah, UAE. The campaign calls for as many as three multilaterals per well to be drilled through tubing underbalanced from existing wells. The full-length paper describes the analysis, modeling, results, and their implication in the selection of the operational approach.