An apparatus and method for milling a window in a borehole

A tool for milling a window in the casing of a borehole, the tool comprising:

• a main body having a longitudinal axis wherein, when the tool is run into a borehole, the longitudinal axis is substantially parallel with the longitudinal axis of the borehole, the main body comprising a deflection surface which is disposed at an angle with respect to the longitudinal axis;
• a milling arrangement comprising a milling head and a guidance surface, wherein the guidance surface is disposed at an angle with respect to the longitudinal axis of the tool and is in contact with the deflection surface;
• and
• a drive arrangement which is operable to effect relative longitudinal movement between the main body and the milling arrangement.

A method of milling a window in the casing of a borehole, the method comprising the steps of:

• providing a tool according to any preceding claim;
• incorporating the tool into a drill string;
• lowering the drill string into a borehole having a casing;
• driving rotation of the milling head through the transmission arrangement;
• activating the drive arrangement to effect relative longitudinal movement between the main body and the milling arrangement, causing the milling arrangement to move laterally with respect to the main body, so that the milling head comes into contact with the casing;
• and
• raising and/or lowering the tool within the borehole to mill a window in the casing.