TOOL FOR MAKING AN OPENING IN A TUBULAR IN A WELLBORE

摘要

A tool for forming a window in a casing (C), with a starting mill (520) havi ng a nose which comprises cutting apparatus (525) covered by a protective layer (527). The protective layer acts as a bearing as the mill (520) travels down the whipstock (505) and helps urge th e milling material (522) into the casing to form the window. The protective layer (527) is eroded as the mill (520) travels down the whipstock and the cutting apparatus (525) is exposed to allow the cutting apparatus (525) to cut through the casing and into the formation beyond.