Kvitebjoern is a Statoil operated gas and condensate field that lies in block 34/11 in the Norwegian North Sea. The reservoir lies about 4,000 meters down, and features high pressure and temperature conditions (770 bar and 160 deg C). 34/11-A-9 T2 well was drilled as a gas producer by end of October 2009 and completed in February 2010. During the final completion phase, it was not possible to open the High Pressure/High Temperature (HP/HT) isolation ball valve set in the 9 7/8" liner at 6245.7 m MD / 3795.8 m TVD through pressure cycling.After several failed attempts to open the isolation ball valve with wireline using mechanical override tools, it was decided to punch above it to allow well production passing the outside of the valve through the annulus between 9 7/8" liner and the 5 ½" tail pipe. However, the production performance was poor. A feasibility study was carried out to evaluate available methods for opening or milling out the valve with the objective to improve the production characteristics. The well is an exploration well and an important objective for the operation was also to allow access for future production logging tools.The decision was then taken to mill the stuck-closed isolation ball valve using Coiled Tubing (CT).The operation presented several critical challenges. Statoil had not performed any HP/HT CT operations and the available experience was limited.To minimize uncertainty relating to depth determination during the milling operation, it was decided to use a telemetry system run at its operational pressure and temperature limits to provide real time Casing Collar Locator (CCL) reading in addition to downhole pressure and temperature data.