Results from the first five wells drilled with coiled tubing (CT) in the Yibal field have shown that through-tubing underbalanced-CT-drilling (CTD) technology can be applied safely and successfully. A CT unit can compete successfully with a conventional drilling rig in drilling ultraslim wells targeted at relatively small oil pockets, where well success depends on the accuracy of the reservoir model and geological knowledge of the target zone. Preliminary trials made it clear that this technology should be considered only for niche applications. Underbalanced drilling enables CTD to achieve rates of penetration (ROP＇s) as great as 80 m/h and 1000-m horizontal stepouts.