BRANCH BOREHOLES

摘要

A method of forming and lining a branch borehole comprises installing a packer in a main borehole, installing a whipstock on the packer, milling a window in the main borehole casing using the whipstock, recovering the whipstock, and then installing a deflector (20) to deflect a lining into the lateral. The external diameter (D) of the deflector is less than the internal diameter of the casing of the main borehole so as to permit subsequent milling away of cement and surplus lining material using a washover tool or thin walled mill. The preferred deflector includes a relatively steep deflector face (21) at the upper end of the deflector and a relatively shallow deflector face (22) which connects the lower end (23) of the face (21) to the full diameter (D) of the deflector. If it is desired to use the maximum possible diameter casing for the lateral, portions of the formation and/or the main borehole casing are removed by an appropriate tool after the deflector (20) has been positioned within the borehole.